MSE 202

What are the two reactions that describe the breakdown of the passive layer by chloride ions?

1) FeO-OH +Cl- -> FeO-Cl + OH- and 2) FeO-Cl + H2O -> Fe3+ + Cl- + 2OH-. The FeO-OH is the passive layer in stainless steel, although it may be CrO3 or another FexOx but these both are still susceptible to pitting corrosion.

What are some applications of polylactides?

Applications include bone plugs, screws and fracture fixation plates. Applications are limited as a result of rapid reduction in strength in vivo. The whoosh effect can cause abnormal bone formation and/or resorption.

How does pressure affect crystallisation?

Applying pressure allows entropy to be energetically reduced so more crystallisation can occur.

How is the potential changed in non-standard conditions?

As described by the Nernst equation

Sketch and label curves for stainless steel and what happens as the chromium content is changed?

As the chromium content is decreased, the critical current, ipp and Epp are all lowered.

What are polymer superstructures?

Depending on the crystallisation mechanism, different superstructures with the same crystal width can be made. These have different properties. The Mw and no. of entanglements must be known to predict it. The concentration of the polymer solution can be changed to get the required superstructure.

Describe gels?

Gels have small amounts of entanglements so can be used to make fibres. There are small crystals that hold it together.

How is the growth rate affected by the molar mass?

G=KM_w^α where a is negative replace M with c (concentration) for crystallisation. Then a->b where b is positive. Essentially larger Mw means less diffusion and so slower growth.

How does a polymer melt cool?

Irregular chains become rubbery/amorphous so the repeat units are not ordered. Regular chains crystallize below Tc so that the repeating units are ordered in a crystalline lattice (overall semicrystalline). The uncrystallised regions turn glassy below Tg.

Describe polyethylene?

It consists of 1000-40,000 units of ethylene and is a waxy solid at RT as its Tm is around 100C

Describe the Avrami exponent?

It depends on the particular nucleation growth mechanism (so does Z). Spheres have an n of 4-3, discs of 2-3 and rods of 1-2.

Describe polyglycolide?

It has a structure of (O-CO-CH2) n. It has a high Mw and is therefore a hard, tough, crystalline polymer with a melting point of 228 and a Tg of 37C. The chains pack into sheets that zig-zag. It has a close packing and a close approach of ester groups (which give it a high melting point).

What kind of surface is silica glass when corroding?

It is a type 2 surface, but it can change to a type V.

How can PLA with an Mw > 100 kDa be made?

Lactides can be obtained from the condensation of two lactic acid molecules. Lactic acid can be made from the fermentation of glucose or other sugars. If a specific stereo-orientation is required, i.e. L-lactide, then it can be made from two L-lactic acid molecules. Ring opening polymerisation can then be made from the lactides to make PLA.

What is polymorphism?

Occurrence of more than one type of crystalline order for a polymer i.e. the polymer can be solid I or solid II etc. Larger effect on the mechanic properties.

What defines an oligomer and a polymer in terms of n?

Oligomers have 5-10 monomers whilst polymers have to have at least 100, but usually have 10,000 to 100,000.

What is the equation for Tm¬?

T_m=(∆H_f)/(∆S_f )

What are the uses of copolymers?

The Mw and linearity can be controlled.

How do concentrated polymer solutions differ from dilute solutions?

The number of entanglements decrease in the dilute system so more amorphous phase,smaller crystals are made from the concemtrated system. The chains are separated from the solvent. The number of entanglements in concentrated solutions is higher.

How fast does the liquid to solid transition occur?

The overall transformation has a reduction in volume during crystallisation which is measured by dilatometry. This is expressed through the Avrami equation.

What is anodic polarization?

ηa electrons are removed from the metal and the deficiency results in a positive potential change (due to a slow liberation of e- by the surface reaction). It is positive by definition.

What is the Gibbs free energy (from electrochemical potential)?

∆G= -nFE

What is the relationship between the Gibbs energy and the equilibrium constant?

∆G^0= -RT ln⁡(K_eq )

What is the oxygen reduction equation? [acidic]

O2+4H+ + 4e- ↔ 2H2O

What is the pourbaix diagram temperature?

25 C

How is Mw related to N?

They are proportional i.e. M_w/M_e -1=n

What kind of crystals form under strain?

"Shish-kebab' crystals. The fibrils have exceptional mechanical properties. This is due to the bonds between the discs which produce a very strong orientation and thus strength in that direction.

What is the Avrami equation?

((V_t-V_∞))/((V_0-V_∞))=e^(-Zt^n )where V0 is the initial volume, Vt is the volume at time t, Vinfiity¬ is the final volume, Z is the prefactor, t is time and n is the Avrami exponent.

Once the pit is initiated, it becomes anodic and the rest of the material becomes cathodic. A galvanic cell is set up. Metals ions from stainless steel undergo oxidation reactions from M -> Mn+ + ne-. These metal ions build up in the hole and could precipitate out of the hole by diffusion due to the concentration gradient, but this is extremely slow due to the pit geometry. The dissolved metal ions undergo hydrolysis by Fe2+ + 2H2O -> Fe(OH)2 + 2H+ and Fe2+ + 2H2O -> FeOH+ + H+ (i.e. Mn+ + H2O <-> M(n-1) OH + H+). This causes H+ to build up within the pit, reducing the pH to 0. From the anodic reaction, electrons are released from metal oxidation and facilitate the O2 + 2H2O + 4e

-> 4OH- cathodic reaction. As H+ builds up inside the cell and OH atoms build up in the bulk electrolyte, the pit becomes positively charged and the electrolyte become negatively charged. This has an autocatalytic processing as pitting is accelerated. The current density eventually reaches Icritical so it can grow stably by itself.

What are the three main differences between anode sacrificial protection and ICCP?

1) The anode if isolated: when mounted on the surface it protects, the anode must be shielded. This protects the metal from excessive current densities close by the anode. 2) Non-consumable undoes are used (in water the anode reaction is water oxidation and in sea water it is chloride oxidation). 3) Control electronics: the system can be regulated and the current can be controlled. They may be separate control units for different areas being protected. Bonus: 4) Risk: if the power is cut, the system is left unprotected.

What is the Flory Huggins formula?

1/T_(m,2) +1/(T_(m,2)^* )= -R/(∆H_1 ) [ln⁡〖v_1 〗+v_2+χv_2^2 ]

How many units are required for solids?

1000 to 10,000 units produce solids that can be used for containers. >100,000 units are required for tough solids used for implants or Kevlar.

What is the optimal Ca percentage in phosphate glasses?

25% Ca is optimal for corrosion resistance, as it does not uncecessarily break P-O-P bonds. As the CaO content increases, Tg increases and durability increases. Ca2+ ions act as ionic cross links between polymeric phosphate anions. No further increase in durability above 25% CaO is observed as no more cross links can be formed: adjacent cross links are difficult to form and chaines in chain configuration would be necessary. After 25%, corrosion is accelerated again as the nework Q structure is disrupted.

What is the hydrogen reduction (evolution) reaction? [acidic]

2H+ + 2e- -> H2

What is the effect of Si content on bioactivity?

A Q2 structure is needed for bioactivity. At 60 mol% SiO2, a network connectivity of 2.4 is reached and bioactivity is lost as it is too similar to Q3. At higher silica content, there is increased corrosion resistance due to more network connectivity so it does not corrode slowly enough. Higher silica contents reduce the rates of ion exchange, network dissolution, silica repolymerisation and HCA crystallisation.

What is the atomic structure of glass?

A amorphous network with no long range order (only local order).

What is passivity?

A condition of corrosion resistance due to the formation of thin surface films under oxidizing conditions with high anodic polarization. The corrosion rate is substantially reduced, even though there is a strong driving force for oxidization, as a passive film begin to form, despite the fact that there is no visible evidence on the surface. The system is kinetically restricted.

What are corrosion inhibitors?

A corrosion inhibitor is a chemical compound when added in small concentrations can stop or slow down the corrosion of metals and alloys by forming an effective protective coating in situ. A good inhibitor can give 95% inhibition at 80 ppm (i.e. use low concentrations). Some mechanisms of its effect are the formation of a passivation layer and inhibiting either oxidation or reduction reactions. It can be described anodic or cathodic depending on which reaction is affected. The suitability depends on the material it acts on and the nature of the substance in the operating environment (temperature, pH etc). Inhibitors are most useful in recirculating systems and their effectiveness is diminished by increasing solution corrositivity, concentration and temperature. An inhibitor for one metal may be corrosive to others.

What is the rule for eutectic systems?

A deviation in polymer size of >20% will mean a eutectic system forms instead of a solid solution

What are gels?

A gel is an interconnected, rigid network with pores of submicrometer dimensions and polymeric chain whose average length is greater than a micrometre.

Describe the growth graph?

A high temperature means low nucleation and high diffusion. A low temperature means high nucleation but low diffusion. Both are needed for nuclei to be made and grow so there is a midpoint.

What is the definition of corrosion numerically?

A metal is considered to be corroding when the ions in solution exceed a user-defined concentration of 10-6 M (less than thus and the metal is deemed to be immune)

What determines a polymer's size?

A molecule of a polymer is a chain. The molecular weight is a guide to the size of the chain.

How was Kevlar created?

A rigid polymer was created using aromatic units reducing entropy and increasing Tm¬.

Describe a silica tetrahedron?

A silicon atom bonded to four oxygen atoms in tetrahedral coordination. It has a charge of 4-.

What are the different copolymer isomers?

Alternating: ABABAB, block: AAABBB, triblock: AAABBBAAA, trimonomer: AAABBBCCC, irregular linear: AABAABBBA.

What are the drying methods for sol-gels and what kind of material does each produce?

Ambient oven drying produces Xerogel and critical point drying (sublimation) produces Aerogel.

What effect does the catalyst have on sol-gel glasses?

Acidic catalysis promotes hydrolysis (so it makes sol-gel monoliths and normal powders/glasses). Basic catalysis promotes condensation (so it's used for nanoparticles synthesis). Acidic and basic are relative to the isoelectric point of the sol.

How do acids attack glasses?

Acids dissolve the alkali components of the glass. It leaves behind a porous surface of holes where the alkali has been removed however the porous network slows the rate of attack as the acid must penetrate the surface layer to find more alkali to dissolve.

Describe Aerogel?

Aerogel is an excellent insulator. It has 98% porosity and is thus filled with air at 98%. It is very light. It is however expensive, and large sheets cannot be made easily.

What is the mechanism for crevice corrosion?

After the oxygen has been depleted within the crevice, oxidation of the metal occurs. Electrons from this reaction are conducted to external adjacent regions, where they are consumed in a reduction reaction. H+ and Cl- ion concentrations are high in this region.

How do alkalis attack glasses?

Alkalis dissolve the glass surface, and as long as there is a continuous supply of alkali, the corrosion occurs at a uniform rate.

How are Pourbaix diagrams constructed?

All the necessary electrochemical reactions must be written out. Reactions that involve charge transfer (electrons) are influenced by variations in electrode potentials (E) - i.e. horizontal lines. Reactions involving H+ will be influenced by pH - i.e. a vertical line at the equilibrium pH. Reactions involving charge transfer (e-) and H+ will be influenced by both pH and potential - i.e. slopes. Calculate the Nernst equation E values for equations containing electron transfers - if log[H+] comes up, convert it to pH. For equation without electrons, use the equilibrium constant K and then log both sides to get a log[H+] term and then covert that to pH and rearrange to get a straight pH line.

Describe polymer optical properties?

Amorphous and crystalline polymers are transparent. Semicrystalline polymers are only transparent if their crystals are smaller than the wavelength of light.

When do polymers lose their structural integrity?

Amorphous polymers lose structural integrity at Tg whilst crystalline polymers lose it at Tm. Semicrystalline polymers slowly lose their mechanical properties between Tg and Tm.

What is the difference between amorphous polymers and crystalline polymers?

Amorphous polymers only have short range order, no long range order. Fully crystallised polymers have long range order.

What are the three types of polymer macrostructures?

Amorphous, semicrystalline and crystalline. The amount of ordering they have separates them.

What is the relationship between the repassivation rate and the pitting rate?

As the passive layer is active, it could grow back. Only if the rate of pitting is faster than the rate of repassivation will the pit grow.

How does temperature affect corrosion?

As the temperature increases, the corrosion rate increases. For example, boiling water may cause the breakdown of aggregates present in glass leaving pits an evaporation can cause abrasion of protective films leaving an unreacted surface exposed. The rate of both ion exchange and network dissolution are described by Arrhenius equations: rate=A e^(-E_a/RT) which means they are strongly dependent on temperature (also the composition of the glass as well as its structure affect the A and Ea term in the equation). In effect, at normal temperatures its a type II surface where a silica rich layer forms but at higher temperatures, especially above 120C, the silica is more soluble in water and thus the SiO2 dissolves directly into the water at higher temperatures (i.e. especially in superheated water).

Does the Tafel plot hold true at small overpotentials?

At low net currents near the potential Ecorr, a mixed current (part anodic and part cathodic) will flow so the Tafel relationship no longer holds and a linear logarithmic behavior is seen. The linear part is however a good estimate of the isolated anodic and cathodic currents.

How does pH affect corrosion?

At pH < 5, ion exchange is dominant. At pH 5-9 there is little corrosion. At a pH > 9 there is total dissolution of the silica network. Cation release can increase he pH.

What is crystallisation temperature?

At the Tc, the atoms move to a point where crystals start to form.

Describe the polarization curve?

At the most negative potentials, the metal is cathodically protected: a negative current will flow by the reduction of O2 or H+ (i.e. the immune/protected region of the Pourbaix diagram). As the potential is increased the current will reach zero. In practice a metal isolated on its own will have an average potential of Ecorr but corrosion cells will be set up leaving some parts more positive or negative than Ecorr. As a result, parts of the metal will corrode at Ecorr on an isolated piece of metal. Positive of Ecorr and the current increases exponentially, and corrosion occurs faster. As the potential is further increased, the metal oxidation process changes to the formation of an insoluble oxide which partially or largely protects the metal from further corrosion (i.e. a passive oxide layer). Formation of the passive layer is accompanied by a rapid fall in the corrosion current of many orders of magnitude. Once this is created, the passivation potential is created. The maximum corrosion current flowing before this fall is the critical current for passivation is the current needed to form the passive layer. When formed the current rapidly forms and the metal reaches the passive zone, where a passive current flows. Icrit can be up to 20 mA cm-2, whereas the passive layer protects nearly cancels this out. The behavior of the metal is bounded by water reactions for most corrosion systems. At higher potentials some materials may oxidize to higher valence states (if an anions formed then further corrosion can occur - in stainless steel Cr3+ -> Cr6+ (which is toxic) i.e. transpassivity, which occurs in systems with oxidizing solutions i.e. bleach paint). At higher potentials, further anodic reactions are viable through solution species.

How do isomers affect polymer properties?

Atactic polymers will not crystallise and will remain amorphous. Head to head coupling causes steric hindrance and reduces crystallinity.

How can biodegradable polymers be sterilised?

Autoclaving or dry heating cannot be used because it would cause degradation. The devices must therefore by sterilised by gamma radiation, ethylene oxide, plasma etching or electron beam irradiation. Radiation can result in significant degradation of the polymer chain. Polyglycolide, polylactide and polydioxanone are especially sensitive to gamma radiation and are usually sterilised for device applications by exposure to ethylene oxide (but they must be stored for 2 weeks in a vacuum to fully residual EtO gas). Temperature must be kept low below the Tg of the polymer to prevent dimension change sterilisation.

What are bioborates?

B2O3 based glasses (or Na2O-CaO-B2O3) which have been observed to rapidly convert to hydroxyapatite when placed in an aquous phosphate solution. The similar Li2O-CaO-B2O3 degrades more slowly.

Can microbes affect corrosion?

Bacterial colonization of surfaces can cause severe corrosion problems. Deposits can act as crevices and the products of bacterial metabolism can be very aggressive (acids, suphides etc). Bacterial corrosion often occurs in natural water under stagnant conditions.

What is the glass transition temperature?

Below Tg, the glass is solid glass (amorphous). Above the Tg flow occurs within the glass and the atoms move.

Compare bioborates and silciates?

Bioactive borate glasses, unlike silica glasses, form HCA directly on the surface of the udnerlying unreacted glass without forming a borate rich layer. This is because borate is readily soluble in body fluids, similar to phosphate glass. The HCA forms on the outside of the borate fibres and forms inwards, the lack of a borate rich self passivating layer means corrosion continously occurs at the same rate. This means borate glasses can react to completion (total HCA conversion) without a significant reduction in dissolution kinetics, so the HCA is formed faster. Bioborate is a type V surface like phosphate glass.

What are some clinical applications of bioglass?

Bioglass particles are used for orthopaedic applications (repairing broken bones). They can be used in toothpaste to protect and repair sensitive teeth (tubules become remineralised).

How do copolymer isomers affect their properties?

Block copolymers usually form two or more separate phases i.e. micro phase separation. Alternating and irregular copolymers cannot separate once connected.

Describe polyglycolide and polylactide degradation?

Both polymers are polyesters and possess an ester group in the polymer backbone that can be hydrolysed. The degradation products of these two polymers are glycolic acid and lactic acid, both of which occur naturally in the body.

How does branching affect Tm?

Branching increases entropy and thus increases Tm.

Why is polymer chain branching induced?

Branching lowers the Tm¬ and crystallinity (increasing transparency). It also decreases the Young's modulus.

What other factors affect Tm¬?

Chain length increase entropy. Chain conformation can affect entropy i.e. rods increase enthalpy and decrease entropy. Thicker/larger crystals increase enthalpy. Pressure/stress elongate polymers to give thicker crystals. Thermal history: controls lamellar crystal thickness. The chemical composition affects enthalpy. Chain rigidity affects entropy. Additives and solvents like water act as plasticizers to reduce Tm. The measuring technique also affects measured Tm.

What does the fold length depend on?

Chain length, crystallisation kinetic, pressure, annealing and chain stiffness. However above a certain molar mass, the lamellar thickness becomes independent of chain length. This is because of folding (which occurs to negate entropy by introducing entanglements). Once this happens the fold length is influenced by crystallisation conditions like supercooling.

What affects the conformations adopted?

Chemical structure and processing

How is corrosion prevented (summary)?

Choose the right material, change the environment (i.e. reduce the temperature, stress, fluid flow etc), use inhibitors (either anodic or cathode which work by interfering with surface redox reactions), change the design of the system (i.e. be able to drain a liquid system and wash it, prevent air coming in, avoid crevices & galvanic couples etc). Use physical barriers with good adhesion (coatings/films/paint). use pourbaix diagrams

What are some common diene homopolymer isomers?

Cis polymers have the linking bond opposite the X and trans polymers have the linking bond parallel to the X group.

Why is it difficult to dispose of nuclear waste?

Containers must be safe over geological timescales, space is limited, waste must be stored at least 1km underground, tanks can leak and the public must be convinced.

What is corrosion fatigue?

Corrosion fatigue is caused by cyclic stress which causes cracks to initiate and grow in areas of localized slip within grains (i.e. it's usually transgranular), and the crack propagation is assisted by corrosion processes. CF cracks normally form slowly, so they can be characterized by the presence of corrosion products within their cracks. It does not require a specific corrosive environment-material combination.

What is stress corrosion cracking?

Cracks formed by static fatigue act as stress multipliers and coupled with atmospheric water corrosion can cause failure. Moisture can get trapped at these cracked and this can lead to static weathering. It is common in silicate glasses where Si-O-Si strained bonds are hydrolysed into silanol groups and SiOH; a gel-like region forms where the crack is able to propagate under sub-critical regions).

What is crevice corrosion?

Crevice corrosion is similar to pitting but occurs in areas of restricted geometry such as seals and gaskets or under surface deposits. These areas have restricted diffusion. Crevice corrosion is easier to establish than pitting and is still very hard to detect until failure has occurred. There is a critical dependence on potential, temperature and aggressive species, however a lower Ecritical, Tcritical and [X-] than for pitting on a free surface. A smaller current density is also generated. As moisture can be trapped in crevices, it allows corrosion to occur even when the atmosphere is otherwise dry. Crevice corrosion occurs in small gaps where solution can become stagnate and locally depleted of oxygen. Crevice corrosion can be avoided by welding properly, designing properly, stopping moisture stagnation and removing dirt frequently.

What factors affect the strength of the cross links?

Cross linking would increase in the order of Na+ -> Ca2+ -> Fe3+ -> Ti4+. Other intermediates like Mg, Zn and Ti can also be used.

Describe crystallisation at surfaces?

Crystallisation is affected by surfaces because there are temperature gradients inside the mould, so anisotropic crystals form at the surfaces with isotropic (i.e. spherulites) growing at the centre.

What is quiescent crystallisation?

Crystallisation with no external contact so that the radius of gyration is constant.

How does morphology affect dissolution rate?

Deceasing dissolution rate is caused by decreased SA: V ratios. A monolith disk dissolves slower than a porous foam and a lot slower than a powder.

What is the equation for dispersity?

D = Mw/Mn¬

How can the shape of the polymer be changed to slow degradation?

Degradation of thick sections can occur faster than thin sections due to the build-up of a localised low pH accompanying the degradation within the section. This can result in the rapid release of lactic acid and polylactide oligomers resulting in a toxic response (whoosh effect). This is overcome using basic fillers which neutralise the acidic carboxyl groups produced on hydrolysis.

Describe density measurement?

Denser polymers are more crystalline. It allows you to compare which sample is more crystalline. You can compare it to a 100% crystalline polymer for a rough estimate. The density is precise to 0.01 g/cm3. However it is not directly quantitative (you need to compare). Pores, voids etc. will affect the density measurements. Some techniques are dilatometry, flotation and density gradient columns which allow calibration curves to be made (floating height -> density).

Describe the Hoffmann weeks plot?

Depending on the Tc, the Tm changes. As you change lamellar thickness, a function of Tc, the Tm changes (higher Tc = thicker). The plot allows the equilibrium melting temperature to be identified by extrapolation.

What are some applications of bioborates?

Diabetics with wounds often have vascular deficient tissue and the body has no effective way of supplying natural growth factors and nourishment required to heal it. Bioactive borate glass nanofibres in the form of a pad can heal chronic wounds. Also it can treat veneous ulcers. The procedure is: apply bioborate glass nanofibres t the wound. The fibre is flexible and similar to cotton to touch. The fibre is kept in place for 3-4 days and changed with perioidic dressings. The remaining fibres are washed away with sterile saline.

What are the methods to determine molecular order?

Dilatometry, DSC or WAXD (wide angle x-ray diffraction).

What are the consquences of corrosion failure>

Direct cost of replacement Downtime (loss of product) Human safety Liability Environmental impact Impact on future activity (public perception) economics/cost

How does temperature affect the corrosion rate of phosphate glasses?

Dissolution rate increases with temeprature regardless of pH and composition. Activation energy for dissolution (16 kcl / mol) is independent of pH and composition. Increased temperature also increases the glass's solubility. Its similar to silicate systems that it undergoes total dissolution.

What is the Nernst equation?

E=E0-2.303RT/nF log⁡((∏ox)/(∏red)): you do this on the reduction reaction i.e. when electrons are added on the LHS. The LHS is on the bottom, RHS on top.

What is erosion corrosion?

EC is the synergistic action of chemical dissolution due to corrosive media and mechanical abrasion or wear as a consequence of fluid motion which results in the removal of material. Impingement can cause the disruption or thinning of the passive-protective film allowing the dissolution rate to increase. It is especially harmful to alloys that passivate by forming a protective surface film; the abrasive action may erode away the film leaving a bare metal surface exposed. If the coating is not capable of continuously and rapidly reforming as a protective barrier, the corrosion that ensues may be severe. It is also called flow-assisted corrosion or impingement attack. It occurs in liquid streams, often where there is turbulence in the system. The presence of bubbles or solid particles aggravate the problem. Increasing the fluid flow rate increases the corrosion rate. Low strength, low corrosion resistant materials are the most susceptible (stainless steel, Cu, Al etc). This can be used to polish surfaces however (beneficial) in chemical mechanical planarization (CMP).

What is environmentally induced cracking?

EIC describes the failure of a material by brittle mechanical cracking that is caused by the synergistic effect of corrosive media and tensile stress. There are three types: corrosion fatigue (CF), stress corrosion cracking (SSC) and hydrogen induced cracking (HIC or HE).

Describe electrochemical corrosion due to concentration differences?

Electrochemical corrosion may occur as a consequence of concentration differences of ions or dissolves gases in the electrolyte solution and between two regions of the same metal piece. The corrosion occurs in the locale with the lower concentration.

What is the fundamental thermodynamic parameter of an electrochemical reaction?

Electrode potential E.

What are the 3 mechanisms of glass corrosion?

Glass in an aqueous solution results in the chemical and structural change of the glass surface. The accumulation of corrosion products on the glass surface results in a change in the chemical composition of the glass and the pH of the solution. Glass in the presence of microbials results in chemical and structural changes within the glass surface.

How is iron incorporated into phosphate glasses?

Fe2O3 goes into phosphate backbone in an octahedral format forming hydration resistant bonds. Up to 5 mol% Fe2O3 reduces degredation by 3-4 orders of magnitude (Boron, Ti and Al also).

What are the key parameters of erosion-corrosion?

Flow rate/ turbulence, the volume fraction, hardness and shape of dispersed solids, the material resistance (surface hardness and electrochemical behavior) and the environment (potential, current, pH and corrosive species concentration).

How do folded crystals compare with non-folded crystals for Tm?

Folded polymers of the same lamellar thickness have a higher Tm¬ because at the same thickness the folded one has a higher Mw and thus a higher Tm¬.

Compare 30Na2O-70SiO2 and 20Na2O-10CaO-70SiO2 glass?

For 30Na2O-70SiO2, the initial corrosion is on exchange, which leads to the formation of a SiO2 passive layer in. After a few hours, stage 2 dissolution occurs. The binary 30Na2O-70SiO2 glass has initially a type IV behaviour under these corrosion conditions. Since the SiO2 film which forms has a low density and is not protective, it quickly degrades to type V behaviour. The ternary 20Na2O-10CaO-70SiO2 is more corrosion resistant. The SiO2 layer which forms is thicker and develops over a longer period of time. It is a protective type II surface.

How does pit geometry influence pitting?

For a pit to grow, a critical local chemistry must be established within the occluded volume. The initial pit must be big enough for metastable growth: small pits won't have enough exposed metal to dissolve to start the autocatalytic process. The distribution of pits is also important, i.e. small new pits near big pits cannot survive as they will be cathodically protected.

What is a type II silicate glass surface reaction?

For a soda-lime-silica glass in contact with water with a pH of under 9, stage 1 ion exchange is the dominant corrosion reaction. A silica-rich protective layer is formed (0.01-1μm) due to selective alkali ion remove. The glass is then durable for pH values of less than 9.

What is the effect of composition on the dissolution rate of phosphate glasses?

For dissolution of vitreous P2O5 and ultraphosphates (Q>2), cleavageof P-O-P bonds is necessary. Hydrolysis of P-O-P bonds in vitreous P2O5 occurs readily but also ultraphosphate glasses are prone to hydrloysis which results in their degredation in the prescence of atmospheric humudity. For meta and polyphosphate glasses no P-O-P cleavage is necessary: the phosphate chains stay intact during glass dissolution. These glasses dissolve by the hydration of entire phosphate chains and subsuequent chain disentanglement and dissolution. Hydrolysis of polymeric phosphate chains cocurs in solution after intiail hydration and dissolution but at a slower rate. Solubility increases as phophate content increases i.e. invert->polyphosphate->metaphosphate.

What is a type I silicate glass surface reaction?

For fused silica (only SiO2) glass exposed to water with a pH of under 9: no corrosion occurs.

What is impressed current cathodic protection?

For larger structures, sacrificial anodes cannot economically deliver enough current to provide complete protection. The system is moved to an immune system using an impressed current and anode. Impressed current cathodic protection systems use anodes connected to a DC power source. Anodes for ICCP systems are tubular and solid rod shapes or continuous ribbons of various specialized materials. These include high silicon cast iron, graphite, mixed metal oxide, platinum and niobium coated wires (and others). A typical ICCP system for a pipeline would have a positive DC output terminal connected via cables to the array of anodes buried in the ground.

What is the simple model of polymer backbone hydrolysis?

For semicrystalline polymers there are several stages. Water first diffuses into the polymer (the maximum concentration is reached in a few hours). Water attacks the amorphous regions first (hydrolysis) and the Mw falls without the loss of the mechanical properties (as the crystalline region holds the matrix together). The chains become forced apart. Oligomers are created. Carboxylic acids groups are formed at each ester bond division. The pH locally decreases. A critical Mw is reached and oligomers start to leave each other. Space is created into which water molecules diffuse, and this in turn facilitates the creation and removal of more oligomers; water uptake and swelling also increases.

What is a type V silicate glass surface reaction?

For soda-lime-silica glasses exposed to water at a pH of above 9, there is a complete dissolution of the silica layers. Cation exchange occurs, but this is irrelevant as the silica bonds are continuously broken down.

How does demixing affect properties?

For solid solutions the properties mix, but for eutectic systems the different phases have different properties (bad).

Where do inaccuracies arise from in XRD/spectroscopy?

From distinguishing the amorphous phase (background scattering) to the crystalline phase (diffraction). This produces an amorphous halo.

What is the ideal final state of crystallisation polymers?

Fully disentangled, extended and crystographically arranged macromolecules. However this does not have great mechanical properties.

What are examples of glasses with a higher corrosion resistance?

Fused silica glasses, aluminosilicates and borosilicates.

What is the name of Q4 amorphous silica?

Fused silica.

What is the peak growth rate equation?

G_m=1/2 (T_m+T_g )

What is intergranular corrosion?

Grain boundaries tend to be more reactive than the rest of the alloy (they may be chemically or structurally inhomogeneous - impurities concentrate and alloy components deplete at grain boundaries). For active grain boundaries, dissolution follows the path of the boundary and the alloy disintegrates - the grains can fall out. Galvanic coupling between the grain boundaries and the grains can accelerate corrosion. This means some materials are more likely to disintegrate along the grain boundaries.

What are Evans diagrams?

Graphs of standard potential versus log current density. It essentially describes the coupling of mixed anodic and cathodic reactions (they are each different reactions). The anode has a more negative E0/Ecorr than the cathode. Reactions are plotted starting from their E0 and i0 values if it simply one element corroding, or Ecorr/icorr values if they are galvanic couples. The intersection between the cathodic reaction of the highest reaction and the anodic reaction of the lowest is the Ecorr an icorr of the intermediate cell. At the point, the cathodic reaction of the highest and the anodic reaction of the lowest occur. The separation in E between the intersection and the E0 of either the anodic or cathodic reaction is the anodic/cathodic polarization. The slopes are formed from anodic and cathodic Tafel equations. It can be used for sacrificial protection.

What is the structure of ethylene?

H2C=CH2. It is a gas at RT at its Tm¬ is -169C

What is the hydroxide reduction (evolution) reaction? [basic]

H2O + 2e- ↔ H2 + 2OH-

Can acidic corrosion be useful?

HF baths can be used to remove scratches on the glass surface.

What is hydrogen induced cracking?

HIC is the degradation of the mechanical properties of a metal as a result of hydrogen penetration and diffusion into the metal (it can do this because the hydrogen atoms are very small). Hydrogen comes from the cathodic reactions that accompany corrosion. Atomic hydrogen resides on the surface before recombining to form H2 has which can be absorbed into the metal where it can weaken metallic bonds or recombine to form gaseous voids. The hydrogen production is governed by two steps: 1) 2H+ + 2e- -> 2H0 and 2) 2H0 -> H2. Overprotection from galvanic coupling (i.e. in a Zn-Fe system) can also produce hydrogen which can cause HIC (i.e. in iron) i.e. the effect is accelerated by cathodic polarization.

What is predetermined nucleation?

Heterogeneous. Sporadic is homogenous.

What factors counteract flow for crystallisation under strain?

High rates of deformation, elongational flow, high molar masses and high rates of cooling.

Which materials are more susceptible to HIC?

High strength metals and especially BCC metals/alloys (iron) are more susceptible to HIC than FCC metals /alloys (Cu, Al, SS). Reactive metals (Ti, Zr, Nb etc) are embrittled by the formation of hydrides within the alloy matrix.

How does platelet width change with pressure?

Higher pressure eliminates folding, yielding extended chain crystals like oligomers.

What are the possible mechanisms of corrosion for phosphate glasses?

Hydration, where the phosphate chains disentangle from the surface of the glass and float off into the solution. Hydrolysis, which is cleavage of the chain in solution. Acid/base hydrolysis which accelerates dissolution in especially acidic conditions (ion exhange more rapid in acidic conditions, which means the ions holding the cross links are removed and at high pH, there it toal dissolution faster: this is slower than tha acidic method though).

What is the problem with PLA degradation?

Hydrolysis causes random chain scission. Each scission causes the Mw to halve. The process is catalysed if the pH moves from 7, and carboxylic acid groups are formed within the polymer after each scission from the remnants of the ester bond. Degradation is therefore autocatalytic and an implant's mechanical properties can be lost faster than expected.

Describe hydrolysis?

Hydrolysis is a reaction with water involving the splitting of a bond and the addition of H+ and OH- from water. Polyesters are particularly susceptible to hydrolysis.

Describe IR/NMR & Raman spectroscopy techniques?

IR/NMR are not quantitative so not very useful. RAMAN gives you coherence along chain. Because polymers are anisotropic (spectroscopy perpendicular to chain, XRD parallel to chain) you cannot compare XRD data to spectroscopy data.

How does icorr differ from I0 of any one element?

Icorr is the corrosion current density of the coupled reaction. Both the anodic and cathodic reactions are acceleration at icorr.

How is polyglycolide synthesized?

If made by condensation, then the water by-product would immediately start hydrolysing the polymer and ultimately this would result in a low Mw polyester. It if formed by ring-opening polymerisation of cyclic dimers of glycolide using a catalyst with heat. The Mw is determined by temperature, time and the concentration of the catalyst used.

What is cathodic protection (sacrificial anodes)?

In a corrosion cell it is the anode which corrodes (the cathode does not). To protect a metal, we can simply make it the cathode in the cell (cathodes can be damaged if the current density is too great i.e. HIC may occur). Sacrificial anodes are designed and selected to have to have a more negative electrochemical potential than the metal of the structure. For effect corrosion protection, the potential of the steel surface is polarized more negative until surface has a uniform potential (i.e. pushed to immunity). The anode continues to corrode, consuming the material until eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode. The driving force for the corrosion protection current flow is the difference in electrochemical potential between the anode and cathode. Sacrificial anodes are made in various shapes and are often alloys of Zn, Mg and Al. The electrochemical potential, current capacity and consumption rate of these alloys are chosen to provide optimum protection. As anodic protection is limited by distance, these blocks must be placed on the hull in many different locations. You must calculate the corrosion rate of the sacrificial blocks to figure out how often to replace them. It is inexpensive, easy to install and requires no external power supply but the blocks must be frequently replaced and large numbers of blocks are used for large structures and may lead to addition weight/stress on the structure. They are often used for small ships, marine pipelines and offshore platforms

Where is erosion-corrosion commonly found?

In piping, especially at bends and elbows and abrupt changes in pipe-diameter positions where the fluid changes direction or flow suddenly become turbulent.

What is an example of intergranular corrosion?

In stainless steel, when heated to 500-800 °C, sensitizing occurs and C rapidly diffuses to the GBs. The chromium and carbon form precipitates in the grain boundaries, which deplete the nearby regions of Cr and C, weakening them. Also, in Al-Fe alloys, Fe has a low solubility and segregates at GBs.

What are the anodic polarisation curve equations?

In the active zone Fe->Fe2+ + 2e-. In the passive zone Fe->Fe2+ + 2e-. In the transpassive zone FexOy->Fe4+/Fe3+

What is the polymer crystallisation problem?

In the melt, the entropy is high due to flexible polymer chains. When crystallised the entropy decreases, this must be made up by the enthalpy i.e. T∆S<∆H.

What do nucleating agents do?

Increase Gmax¬¬ so that growth is quicker.

What does chain length do to Tm?

Increasing the chain length increases the Tm until it levels off due to folding. More chains means more secondary bonds so a higher enthalpy. However the folding eventually becomes independent of chain length so it levels off. The folded chains have more surface energy so this is what causes levelling off. T_m=T_m^* (1-2γ/(L∆H_V )) where T*m is the equilibrium melting temperature for infinitely long extended crystals (ideal).

Describe nylon 6, 6?

Increasing the number of CH2 units will lower the enthalpy and increase the entropy, reducing Tm¬.

What is the rate controlling mechanism in stage 2?

Interface controlled i.e. how quickly the silica network interface is broken down. Ions still diffuse through the silica rich layer, but much more slowly so it does not affect the overall rate.

Why are pits hard to detect?

Ions may precipitate over the hole as a salt film so it makes detecting the pits hard i.e. Fe(OH)3 produces a cap. This further stabilizes growth which is now diffusion controlled.

What is weld-decay?

It is a form corrosion that is caused by heating a region in sensitizing temperatures during welding. It is normally found a few mm away from the actual weld and is characterized by a sugary appearance (the grains are falling out). The key factor is the time spent at the temperature, heat input must be controlled.

Describe monocryl (Ethicon)?

It is a segmented block copolmer of 75% glycolide and 25% ε-caprolactone. It is pliable and has a low Mw, but has hard segments for strength. It loses all stregnth in 28 days (in vivo). It can be used for bladder sutures. It is synthesised by polymeriseing pre-exsitng soft segments with glycolides to give hard segments.

What kind of surface is the apatite surface layer?

It is a type 3 surface. Amorphous calcium phosphate nucleates on the surface (at silanol bonds) and a surface layer of bone like (non-stoichiometric) apatite forms. The stoichiometric HA ratio is 1.67 for Ca: P. In apatite layers forming on bioactive glasses, some CO32- ions are exchanged for PO43- ions to form a carbonated apatite similar to that in bone. CaF2 can be incorporated for a fluorapatite film where F- substitutes for OH-.

What is galvanic corrosion?

It is dissimilar metal corrosion that occurs when two metals with different potentials are coupled electrochemically to form a cell. They can be described with Evans diagrams. The main driving force is the difference in electrode potential. The electrolyte allows ionic conduction whilst the bulk metals allow electron conduction.

How is polylactide synthesized?

It is formed by the ring opening polymerisation of cyclic dimers of lactide with a catalyst and heat.

How is phosphate glass made?

It is melt derived. it is made by mixing metal oxides and carbonate powder in platinum crucibles at 800-1000C. It has a composition of 50P2O5.(50-X)M2O.(X)CaO where M is Li or Na. It has a high thermal expansion coefficient so it is good for coatings. It has a low melting point, good optical properties (which makes it superior to silica glasses). However it has poor corrosion resistance and is compeltely soluble in water.

How does the radius of gyration change during solidification?

It is retained unless there is slow crystallisation or applied stress/pressure.

What does the intersection point of the anodic and cathodic Tafel plots signify?

It is the position of icorr and Ecorr. Tafel plots ignore the signs

Why is sodium added to SiO2 in glass?

It makes Q3 glass and reduces the melting point. It also lowers the viscosity

Describe X-ray diffraction analysis?

It tells you how ordered x/y are in the [100] and [010] directions. It can only show if crystallinity is present (the XRD diffraction patterns) but not the degree of crystallinity. The orientation and crystal structure can be derived from the patterns.

What is the structure of phosphate glass?

Its basic structure is PO43- which can attach to up yo three neighbouring groups such as P2O5. Modifiers such as sodium oxide and calcia cleave the P-O-P bonds and make non-bridging oxygen bonds. Due to the the valency of +5, phosphorous atoms have a double bonded oxygen atom, which reduces the maximum possible network connectivity to Q3. This means they cannot be as well connected (as therefore less corrosion resistant) than silica glasses. They can incorporate strucutal water during melting (P-OH) and each chain terminates in a P-OH, reducing the cross link densit from infinity. Phosphate glasses tend to form long chains.

How do lamellae affect light interactions?

Lamellae (i.e. inside spherulites) cause light to interact anistroptically so that parallel to the chain it is transparent and perpendicular to the chain it is perpendicular. This causes the Maltese cross.

How does crystal distribution affect the melting temperature?

Larger, fewer crystals decrease entropy. Smaller more crystals increases enthalpy. Both increase Tm¬.

What are the important parameters for the mechanical properties?

Length i.e. long vs short. Dispersity (distribution of chain length) i.e. broad vs monodisperse. Note, the graph is required, not just the number because e.g. 2 could mean a broad peak or two separate sharper peaks, in this case the single broader peak is better. The spatial arrangement (conformation) i.e. coiled or rods.

What are the different chain geometries?

Linear-coiled i.e. HDPE, nylons, linear rod-like, branched-short-chain i.e. LLDPE, branched long chain i.e. LDPE, graft copolymers, ladders i.e. very stiff and used for electronics, 3D crosslinked tenuous i.e. rubber, 3D crosslinked dense i.e. epoxies and other copolymers.

How do different chain lengths crystallise?

Longer chains crystallise into platelets whilst short chains form extended chain crystals.

What are the physical meaning of the passivation parameters (Icrit, Ecrit, Ipassivation)?

Lower values indicate a more resistant material (adding Cr to Fe lowers these).

What is stress corrosion cracking?

Material failure which occurs below the maximum design set stress due to a susceptible material being in a specific corrosive environment with an applied tensile stress. Small cracks form and propagate in the direction perpendicular to stress and can result in failure. It can be transgranular or intergranular and is often branched with secondary cracks. The corrosion rate must be low (otherwise the failure will occur by dissolution). The stress that produces stress corrosion cracking doesn't need to be applied externally, it can be residual from rapid temperature changes and uneven contraction or for two phase alloys in which each phase has a different coefficient of expansion, or it can come from the work done in forming the material (anneal to stop this).

What is the difference in the structure of melt-derived glass and sol-gel glass?

Melt derived glass has a constant Q-structure throughout the sample and a low specific surface area (there are no nanopores). Sol-gel glass has a very variable and random Q structure and a very high specific surface area (due to nanopores). Sol-gel glasses have a high hydroxyl group content which acts like a network modifier, like sodium ions do in melt glasses.

What does the ability to phase separate depend on?

Miscibility (block copolymers increase the miscibility of the blend), length and composition.

Describe how biodegradable polymers must be stored?

Moisture can degrade polymers in storage, during processing and after device fabrication. The polymers are quickly packaged after manufacturing and generally double bagged under an inert atmosphere or vacuum. They are stored in a freezer to minimise the effects of moisture present. They are opened at room temperature to minimise condensation and should be handled as little as possible in ambient atmospheric conditions. They remain stable at room temperature for over two years in desiccated moisture proof bags. The final packaging consists of placing the suture or device in an airtight moisture-prof container. A desiccant can be added (sutures wrapped around specially dried paper holder that acts as a desiccant).

What is chemical toughening?

Na+ is exchanged for K+ at the surface using a K+ solution. K+ is larger than Na+ and causes compressive forces in the surface of the glass. This slows the ion exchange in water and gives the glass scratch resistance by reducing the damage zone in a scratch. The amount of scratch resistance is determined by the depth of the chemically strengthened layer (deeper=better).

What are the components of Soda-lime-silica glass?

Na2O, CaO, SiO2.

What factors affect order?

Not crosslinks, but the conformation in the liquid/melt state

Describe the results from WAXD?

Note: they tell you about the crystalline presence not quantity.

How does heterogeneous nucleation affect crystal structure?

Nucleation is made easier: the graph is shifted to the right.

What is the oxygen reduction equation? [basic]

O2 + 2H2O + 4e- -> 4OH-

Describe water stability

O2 stable at high E, i.e. water oxidation to oxygen. H20 stable in between i.e. hydrogen to protons/hydroxide to water. Lowest E means H2 is stable i.e. hydrogens go to protons or hydroxide is generated.

What is contaminant surface attack?

Organic and inorganic contaminants can fill holes in the glass surface network and can chemically react with the glass becoming bonded to the surface. This results in discolouration. The surface of the glass is hydrophilic and is subjected to a corrosion process that will make its surface rougher and more damaged.

How is commercial glass different to bulk glass?

Other oxides such as alumina and magnesia are added. The surface composition may be different. Treatments with sulphur, chlorine and fluorine at high temperatures can remove the basic cations leaving a durable Si rich layer. 12-17 mol% Na2O is enough to allow for melting and forming whilst being low enough for decent corrosion resistance. More than one alkali earth metal may be used.

What occurs at the anode/cathode?

Oxidation at anode, reduction at cathode (AO)

What are oxygen scavengers?

Oxygen scavengers are distinct from corrosion inhibitors in that they affect the bulk solution. Hydrazine and vitamin C both help reduce the rate of corrosion in boilers by removing the dissolved oxygen from the water (hydrazine is toxic though). Oxygen scavenger example: N2H4 + O2 -> N2 + 2H2O. An alternative to removing oxygen is to remove the conductive medium by arranging a completely dry atmosphere.

How does cooling rate affect demixing?

Phase segregation becomes negated [the samples all solidify at the same temperature not at two distinct temperatures]. This is because the chains are often longer, and the temperature falls faster so diffusion rate decreases quickly.

How can degradation rates be changed?

PGA-PLLA copolymers can be made. However, 50:50 ratios are not ideal because the PGA would be degraded first and the chain integrity would be lost, and also the melt-processing ability is lost. A 25-75% glycloide polymer results in a amorhpus copolymer. A 90:10 ratio of PLA to PGA is better. This polymer is called Vicryl (poly(glycolide-L-lactide)) and is used for surgical mesh for body walls. Different ratios result in different degredation times, which can be used for different body parts.

Describe polyhydroxylalkanoates?

PHAs are produced by microbes and their structures can be manipulated by genetic strategies. Their biodegradabiity can be regulaed by blending with other polymers. he high abundance of microbes that produce PHA-degrading enzymes mean they can easily be disposed of. They are used to make shampoo bottles. Genetically altered plants like tabacco have been devlped that contain the necessary metabolic pathways to accumulate PHAs.

Describe poly-3-hydroxybutyrate?

PHB is a polyester made from bacterium through biological synthesis. It has a chiral centre and the biosyntheis gives it an ideal sterochemsitry which leads to a crystallinty of >95%. It degredades into a normal metabolite: hydroxybutyric acid. It has a Tg of 10C and degredase by hydrolysis but much more slowly than PGA or PLA as its more hydrophobic. It can undergoes degredation in soil by bacteria. However it degrades too slowly for most biomedical applications and it can only be produced very slowly.

What is Floreon?

PLA has a low toughness at room temperature and can deform or fail in warm climates. This is because its Tg is between 40-60C. Additives can be used to improve toughness and thermal resistance as they created energy absorbing nanoglobules in the PLA matrix. The globules are a combination of 3 polymers of specific Mw. This creates Floreon. Floreon softens above 70C but still degrades after 6 months in contact with soil at >30C. However we don't know its gas barrier properties.

Why do PGA and PLLA degrade differently?

PLLA has a longer lag time before degradation. This is because of the extra methyl group which increases hydrophobicity and thus also increases Tg. Thus water uptake is slower. However, they are both autocatalytic and the Mw halves after each hydrolysis so their degradation profiles once water uptake has occurred is quite similar. The lag time occurs as water must be taken in before hydrolysis can occur, and also because the polymers are still highly entangled, so it takes time and several hydrolysis cuts before there is a real effect on the mechanical properties of the polymer. Resorption is initially slow, then autocatalytic. Once enough cuts have been made and the polymer is past the critical entanglement value, the mechanical properties are rapidly lost.

What is the difference between PLLA and PDLLA?

PLLA is semicrystalline (37%) and has a melting point of 170C, with a Tg of 66C. It is Poly L lactic acid. It has a high Tg which gives it a low degradation rate (sutures take 2 years to resorb). PDLLA is an amorphous polymer having a random distribution of both isomeric forms of lactic acid and thus is unable to arrange into a crystalline organized structure. This means it has a lower tensile strength and more elongation and a much more rapid degradation time (making it more useful as a drug delivery system). It has a Tg of 55C.

How can degradation by slowed through ether bonds?

PPD is a flexible polymer similar to PGA, but with a lower Tg. In poly-p-dioxanone, ether linkages spread out the ester linkages. This reduces the Tg to around -10 to 0C however the reduction in Tg is far outweighed by the lower density of ester units, which slows the degradation rate. It is made through polymerisation of ether containing lactones. It is semicrystalline at around 55% crystallinity.

How does chain length affect demixing?

Phase segregation becomes negated for longer overall chains because longer chains are heavier with more tangle and so diffuse more slowly. So they are harder to separate and easier to quench into a solid solution with no demixing.

How do phenyl rings affect Tm?

Phenyl rings

What is the effect of phenyl rings?

Phenyl rings and long unbranched chains reduce conformation entropy and increase Tm

Talk about polymer coatings?

Plastics offer increased lifetimes over paint coatings but are not immune to chemical attack and degradation i.e. by thermal/UV mechanisms. The same environmental considerations as with painted surfaces apply. Common polymers used are nylon, polyethylene, PVC and PTFE (PTFE offers protection up to around 250C).

Describe long chain platelets?

Platelets have an almost constant width across materials of around 10 nm because the chains fold. The platelets are usually interconnected, regular and brittle.

Why does polyester clothing shrink in the wash?

Polyester clothing are made of aromatic polyester, with a high Tg and so have little water uptake/degradation normally. Polyester often has a Tg lower than 40C so when heated above this, the diffusion coefficient increases by 104 times. Also, carboxylic acid groups form, which autocatalyses degradation.

What are common polymers broken down by hydrolysis?

Polylactide, polyglycolide, polydioxanone, poly(-3-hydroxybutyrate) and copolymers of these.

What kind of ordering is required for crystals?

Polymer chains often only have 1D or 2D orientation, 3D orientation is needed for crystals.

What are the polymer degradation mechanisms?

Polymers can be digested by enzymes (biodegradation) or can be hydrolysed with water.

Talk about biodegradable polymers?

Polymes like starch, cellulose and PLA can be used for consumer package i.e. for food or as packaging for other products, rubbish bags, disposable plates etc. Biodegredable polymers can also be used for down-the-drain products like detergents and cosmetics.

What are some limitation of Pourbaix diagrams?

Pourbaix diagrams can only be defined for pure metals, not alloys. They offer no kinetic information. They relate only to the stable species, but do not describe the metastable species often present. They do not fully describe passivity.

How does the growth rate change with supercooling for primary and secondary nucleation?

Primary nucleation is g=e^(-1/(∆T^2 )) and secondary is g=e^(-1/(∆T^ ))

What is the name of Q4 crystalline silica?

Quartz.

What is the rate surface reaction equation?

R = -k1t0.5-k2t1.0, where the first term is stage 1 and the second term is stage 2.

What effects do composition have on bioborate degredation?

Rate of degredation increases with higher B2O3 contents in the glass. The rate o conversion to HCA was found to increase remarkably with increase B2O3 content of the glass. B(OH)3, released during network dissolution, is a weak acid but stronger than silicic acid. This means there is a more rapid pH increase but the resulting pH is lower as allows for a buffering effect. Biomedical borate glass have a 20% Na2O, 20% CaO and 60% B2O3 composition.

Describe the different crystal growth regimes?

Regime I: the growth direction is much slower than the spreading direction. Each new nucleus fills out before another nucleus is added to the crystal. Regime II: the growth and spreading directions are the same. Multiple nucleation occurs within one layer. Regime III: the growth direction is slightly slower than the spreading direction. Multiple nucleation occurs on more than one layers.

Describe the stability of different chain length distributions?

Solid solutions are kinetically stable (the metastable state). The eutectic demixed state is thermodynamically stable (heating a quenched polymer causes demixing).

What are the common vinyl homopolymer isomers?

Regular: isotactic and syndiotactic. Isotactic: the X groups are on the same side. Syndiotactic: the X groups alternate. Irregular: atactic and irregular coupling. Atactic: X groups randomly distributed. Irregular coupling: X groups randomly distributed but sometimes two X groups on adjacent carbons align the same way (head to head coupling). If two hydrogen align the same way it is tail to tail coupling.

How are rods created?

Rods i.e. liquid crystals are formed by inserting aromatic elements into the chain which make them much more rigid.

How does the SA: V ratio affect phosphate glass corrosion?

SA: V is important for silicates but is unimportant for phosphate glass corrosion rates because the uniform dissolution of the glass does not result in a large pH increase that can be caused by the selective leaching of cations. pH dpes chainge but inly within the rand that dissolution is unaffected.

What is SO2 treatment?

SO2 gas treatment improves weathering resistance: the sodium in the surface layers react with SO2 forming sodium sulphate which is washed off prior to packaging. So reaction 1 is stopped as there is less alkali near the surface (a type II surface is artificially formed beforehand). This is often used for flat/window glass.

What are sols?

Sols are dispersion of colloidal particles in a liquid which each have a diameter of 1-100 nm.

How do short polymers/oligomers crystallise?

Short polymers have fewer possible arrangements and tend not to entangle and so a lower entropy. Therefore the entropic effects of crystallisation are very small, and they tend to crystallise a lot.

How do spherulites grow?

Spherulites grow at constant temperature linearly with time. This indicates polymer crystallisation is controlled by nucleation and not diffusion (which scales t1/2). The growth rate on spherulites strongly depends on the crystallisation temperatures, reaching 0 at the Tm/Tg and a maximum in between.

What are the stages of bone bonding in the short term with respect to the glass surface?

Si-OH (silanol) bonds form via cation exchange with protons in the solution (type II). A silica rich layer is formed. The silica network breaks up and more Si-OH bonds are formed at the glass-solution interface as the pH locally is high (whilst the bulk pH is still 7.4 due to the high volume) i.e. stage 1 and stage 2 both occur. As the local pH > 9, the OH- ions attach the O-Si-O bonds and release silicic acid (soluble silica); further silanol bonds are made (type IV). The high concertation of silanol bonds in the silica gel layer means the SiO2 condenses and repolymerises into a silica rich layer on the surface depleted in alkalis and alkali-earth cations (even when the pH >7). Ca2+ and PO43- groups migrate to the surface from the solution forming a CaO-P2O5 rich film on top of the SiO2-rich layer (type III). This is because the high concentration of Si-OH bonds gives the surface a negative charge and provides nucleation sites for calcium phosphate precipitation. The crystallisation of the amorphous film by the incorporation of OH- and CO32- anions from the solution form a mixed hydroxyl carbonate apatite layer (HCA). This process is about 2 hours long.

What is the chemical formula of silica?

SiO2

What experiments were done between bioborate glasses and silica glasses?

Silica glasses were tested with increase borate content until the glass was a full bioborate. It was found that the pH change was minimized (boric acid is also released as a buffer) and there was complete (weight) loss of th bioborate glass much more quickly. These observations occur because there is no self-passivating borate rich layer. So bioborate is more effective.

Why are mixed network former glasses corrosion resistant?

Silicates are less resistant to alkali and HF, but aluminosilicates/borosilicates are less resistant to acids.

How does slower cooling affect crystallisation direction?

Slower cooling means more orientation.

How does supercoiling affect platelet width?

Small undercooling means thicker crystals because there is more energy for movement. Larger undercooling means thinner platelets develop.

Describe fractionation?

Smaller chains are effectively small particles and different chain lengths tend to crystallise separately. This can occur as either a solid solution (indistinguishable) or more commonly a eutectic system (clear phase separation). Usually polymers with more polydispersity demix. Oligomers (lower polydispersity) are less likely to demix.

What are the advantages of sol gel bioactive glass versus melt derived glass?

Sol gel glasses have a lower processing temperature, they can have more simple compositions, they can have faster controllable degradation, they form HCA layers faster at similar silica contents, they are bioactive up till 80mol% silica, their nanoporosity provides anchor sites for bone cells / protein adsorption and scaffolds can easily be made.

What is the difference between sol gel and melt derived bioactive compositions?

Sol gels can have a 58S composition of 60% SiO2, 36% CaO and 4% P2O5 or a 70S30C composition of 70% SiO2 and 30% CaO (all in mol%). They can be bioactive up to 80mol% SiO¬2 as they have a nanoporous structure (so dissolution still occurs quickly). They do not require Na2O as they are no melted. They have a mix of Q structures, where the Q units may be distributed unevenly. Ca ions are found in their octahedral sites.

What are the pros and cons of sol-gel derived glasses?

Sol-gel glasses can be made at a lower temperature, and sodium is not required as there is no melting stage. It can have any composition and the glasses has a nanoporous texture which gives it a high specific surface area. It therefore has enhanced bioactivity and resobability but also corrodes more easily. It is difficult to make large monoliths using the sol-gel method.

Describe the nanoporosity of sol-gel glasses?

Sol-gel glasses have 2-500nm pores which are interconnected into a tortuous network. They form interstitial sites and give the glass a high specific surface area.

What is the degredation mechanism of bioborates in a phosphate containing solution?

Stage 1 is the dissolution of ions such as Na+ and B(OH)3 into the solution via ion exchange. Stage 2 is the reaction of Ca2+ cations of the glass with the phosphate ions of the solution to form hydroxyapatite. Regardless of composition, all Na and B ions in the glass particles can dissolveinto aqueous compositions. Borate glasses convert compltely to HA.

What are some examples of material-environmental combinations for EIC?

Stainless steel and seawater, carbon steels and caustic NaOH solutions, Ni-Cu alloys and acidic fluoride solutions.

Describe starch?

Starch is inexpensive, annually renewable and derived from corn. It biodegrades naturally and recycles atmospheric CO2 trapped by starch produing plants in the process. All starches contain amylose and amylopectin. Starch based biopolymers can be produced by blending or mixing them with a synthetic polymer like polycaprolactone to increase flexibility and resistance to moisture. These materials can be used for form films and sheets. It is water sentive and belends with >85% starch can be used for foaming.

What are the 4 typical exposure conditions?

Static aqueous corrosion, dynamic aqueous corrosion, static weathering and dynamic weathering.

How can concrete be used for corrosion prevention?

Steel reinforcing bars (rebars) are used in the construction industry. The high alkaline environment of concrete inhibits Fe corrosion by pushing it into a passive region. CO2/Cl- ingress corrosion may still occur, so epoxy-coated rebars or stainless steel rebars may be used in more aggressive environments at initial greater expense but lower expense in the long term. Fibre-reinforced polymer rebars are now also being used in high corrosion environments.

What are the steps for a risk analysis

Step 1: Complete a corrosion risk assessment to assign a high, medium or low risk classification to all operating units, equipment, systems etc. Step 2: Carry out a detailed corrosion assessment for all high or medium risk items Step 3: Estimate the overall risk rating Step 4: Prioritise the items in accordance to their risk rating Step 5: Design the RBI Programme to monitor corrosion more actively Step 6: Systematically monitor the inspection activities and performance of equipment Step 7: Repeat the process as necessary.

What is crystallisation under strain?

Strain causes flow, causing the radius of gyration to increase. Crystallisation is faster because the polymer chains align, extend and disentangle, and as the entropy decreases (configurations decreases during extension), the driving force for crystallisation increases.

Which factors affect crystallisation under non-ambient conditions?

Strain, trans-crystallisation (temperature dependence), epitaxial crystallisation (surface effects) and pressure effects.

How does chemical composition affect Tm?

Stronger bonds means more enthalpy. Even number of carbons in chain melt higher than odd number because of symmetry. Increasing the number of carbons decreases hydrogen bond frequency, decreasing the Tm¬.

What is the Flory-Huggins parameter, χ?

Stronger interactions mean that smaller particles will be closer to polymers and so other polymers' interactions will be reduced (i.e. hydrogen bonds) reducing overall Tm

What is the requirements for crystallisation?

Sufficient regularity of the macromolecule. This mainly depends on the chemical structure, regularity of coupling, regularity of copolymerisation, randomness vs regularity (irregular will not crystallise) and the number of branches. Entanglements hinder crystallisation. Processing, such as cooling rate, stress and pressure also affect crystallinity so that it ranges from 0 to 100%.

Describe the connectivity of semicrystalline materials?

Switch board connectivity gives better isotropic mechanical properties than the anisotropic regular folded polymers.

What are homologous systems?

Systems of chemically identical species, but for their chain length. The Flory-Huggins parameter applies again to reduce Tm¬. Note: if the second polymer is much smaller you get a homologous system, but if it is only slightly smaller you get a demixed system.

Describe environmental factors?

Temperature, exposure time (continuous or cycled), relative humidity, solution pH, presence of inhibitors in the corrosion solution, external stresses upon the specimen, radiation, solution composition and the SA: V ratio.

What is the effect of alumina of glass durability?

The Al3+ ions create in a molecular complex with Na+ ions and the Na-O-Al complex serves as a tetrahedral network former that creates Na-O-Al-O-Si bridging bonds. The molecular complex binds the Na+ ions tightly to the glass structure and inhibits dealkalisation. When the SiO2 layer forms on the glass surface, it contains Al ions. The alumina-silica film is much denser and very protective of the glass surface (it's a type II surface).

What is the Tafel equation?

The Tafel equation relates the rate of an electrochemical reaction to its overpotential. There are two equivalent ways of writing it. Anodic Tafel is η_a=β_a log⁡(i_anodic/i_0 ) where βa = 2.303RT/αnF. Cathodic Tafel is η_c=β_c log⁡(i_cathodic/i_0 ) where βc is 2.303RT/(1-α)nF . α can be approximated to 0.5 for a single step reaction. A plot of η versus log(i) should be linear with a slope of β. It can also be extrapolated from the Butler-Volmer equation for a large overpotential.

Describe thermal analysis i.e. DSC?

The Tm¬ is independent of crystallinity. The area under the Tm¬ hump is ΔHf and will increase for more crystallinity. The first hump is the Tg (midpoint of increase) and soon as you are above this the amorphous regions crystallise and so there is an endothermic response. At the Tm¬ the latent heat given off is proportional to the amount of crystallinity. If Tg enthalpy change = ΔHf then you know the material was fully amorphous initially but if ΔHf is bigger than you know there was some crystallinity before. At a given temperature, you know how much nucleation and diffusion there is and thus the crystal sizes i.e. in between ΔH1and ΔHf there will be lots of nucleation but little diffusion so there will be small crystals. Thus DSC can tell you the initial state of the material. It is not as accurate as density measurements however.

What are the stages of bone bonding in the long term with respect to biology?

The adsorption and desorption of biological growth factors, in the HCA layer (HCA formation is continuous through this process) activates the differentiation of stem cells. The action of macrophages remove debris from the site allowing cells to occupy the space. The stem cells attach to the bioactive surface after 20h. Stem cells differentiate into osteoblasts. An extracellular matrix forms from the osteoblasts to form bone. Inorganic calcium phosphate matrices crystallise to enclose bone cells in a living composite structure (takes about 100 hours).

How is semicrystallinity measured?

The amorphous regions don't give any signals, and multiple techniques must be used.

What areas of the polarization curve allow for SCC?

The areas with a slow corrosion rate, which mean the cracks are very small and the stress can concentrate on one point. At higher corrosion rates, the holes/cracks are much bigger and the stress cannot concentrate on one area.

What are ionic x-links in metaphosphates?

The chains and rings in metaphosphate glasses can be ionically connected via ionic bonds via modifier cations (di- or higher valent cations). This can be calvium or titanium ions. Thecations can form ionic cross links between the NBO of individual chains. Cross linking inceases with increasing valency and increases for ions wih the same charge butdecreasing ionic radius. Other intermediates like magnesium can also be used.

Describe the anisotropic properties of polymer chains?

The chains are much stronger than the stress applied is parallel to the chains rather than perpendicular to the. However ductility is higher when perpendicular.

How does mechanical deformation affect crystallisation?

The chains are physically extended beforehand, reducing the entanglements.

How does ion composition affect the dissolution rate of phosphate glasses?

The charge to size ratio of the modifier cation also affects phosphate glass solubility. An increase in the charge to size ratio results in more effective charge linking of the phosphate units, and the stronger the cross linking, the lower the glass solubility. The glass solubility also increases with increasing ionic radius by constant charge as in Li+ -> Na+ -> K+. Cross linking would be expected to increase for cations of the same charge but with decreasing ionic radius as for the series Ba2+ -> Sr2+ -> Ca2+ -> Mg2+.

What variables affect galvanic corrosion?

The corrosion is most severe near the junctions and is also affected by the electrolyte conductivity. The relative areas of the anodic and cathodic materials: as Aianode = Aicathode, the anodic area is equal to the cathodic area (avoid large cathodes/small anodes and if used screws, make sure it's the same material or cathodic to the rest of the structure). Also, differential aeration effects, heat-affected zones in welds and grain boundaries also affect it.

How does crystallinity affect degradation?

The degree of crystallinity influences strength, fracture toughness and degradation behaviour. The crystalline regions do not take up much water and are more resistant to degradation. Crystalline region can form crystallites which cannot degrade, which form particulate debris following degradation.

What is weathering?

The deterioration of a glass surface by atmospheric conditions.

What is the rate controlling mechanism in stage 1?

The diffusion of ions in ion exchange. It is diffusion controlled. The rate of alkali extraction is parabolic in character, assuming the area undergoing glass corrosion remains constant.

Why are larger chains stronger?

The end group effects are minimised.

What is the graph of lamellar thickness versus Mw/chain length?

The first graph occurs if chains infinitely extend. Note: there is only a small region where chain length actually affects platelet width

Describe soda-lime-silica glass corrosion by water?

The first stage is ion exchange between alkali (metal/earth metal - Na+) ions and H+ ions from the solution, during which the rest of the glass network is unaltered. The H+ ions displace the alkali metal ions to form -OH groups off the surface Si atoms. The loss of alkali ions at the surface layer creates a silica-rich passive layer. This means the ion exchange mechanism is self-passivating as this silica rich layer protect the alkali ions deeper down the glass network from the H+ ions. The rate of ion exchange is slowed, unless stage 2 occurs. and The loss of H+ ions raises the pH of water. As the pH rises to above 9 and of the alkali solution remains in contact with the glass surface, stage 2: total dissolution of the silica surface layer occurs. The OH- ions attack the O-Si-O bonds and release Si atoms from the glass in the form of SiO44- ions. Ion exchange occurs simultaneously with the total dissolution of the silica network. The rate controlling reaction is determined by the magnitude of the rate constants for each mechanism as a function of time. Draw the diagram?

How are polymer platelets characterised?

The fold length I is the height of a platelets. Since platelets stack, the long period L (measured by SAXS) is the distance between two points on adjacent platelets. The degree of crystallinity is given by X. I = XL.

What are the geometrical constraints on crevice corrosion?

The gap has to be wide enough for the solution to get in but small enough for stagnation.

How can glass be used to store nuclear waste?

The glass is mixed with up to 30% nuclear waste and then melted at 1350C and caste into blocks. The glass immobilises radionuclides into a solid glass product. The glass is stored beneath many different layers for protection (such as steel). Borosilicates (Si and B as network formers) are used. The components of this glass are SiO2, B2O3 Na2O and LiO2. The Cs and Sr found in the holes in the network can act as modifiers and intermediates. It can accommodate over 40 elements in an open random network. Borosilicates are often used for outstanding chemical durability in a wide range of environmental conditions (stale in a variety of fluids in 100 temperature range over thousands of years). The borosilicate glass can immobilise all waste components, it has a high mechanical integrity, a high thermal stability, a high radiation stability and a high corrosion resistance. There is a compromise between the solubility of the waste in the glass and corrosion resistance.

How does nuclear waste glass change over time?

The glass may be cracked from initial processing. The glass will eventually become saturated with water. In the first few hundred years, significant radiogenic heat will maintain the waste at several hundred degrees Celsius causing significant radiation damage and more cracking. Ionisation and electronic excitations produce point defects in the glass structure as electron-hole pairs. This can result in covalent and ionic bond rupture and enhanced diffusion in the long term. Once the metal canister has corroded, water in contact with the glass will cause the radionuclides to escape. Once the water and canister corrosion products reach the glass, rapid changes will occur to its surface which will lead to the formation of surface layers. Initially this forms a gel layer which layer may form alteration products or secondary phases like clay-like aluminosilicates.

How does composition affect a glass's corrosion resistance?

The higher the proportion of bridging oxygen bonds, the higher the durability. The higher the cross-linking of the Si network, the more resistant the glass is to hydrolysis: this can be achieved by adding nitrogen. As the Na2O content increases, corrosion resistance decreases as the number of bridging oxygen bonds is decreased. Adding CaO up to 20mol% increases corrosion resistance (10% is ideal). CaO decreases the rate of dealkalisation by increasing the molecular coupling of the network modifier ions and increasing resistance to the water attack of alkali ions. However too much CaO can cause crystallisation and small amount of magnesia can be added to prevent this.

Describe nuclear waste glass?

The leaching rate decreases with time. It is subject to static corrosion conditions where there are three stages: ion exchange (cations out, water diffusion in), matrix dissolution and the forming of surface layers from salts and silicates. It must be kept away from groundwater. Degradation increases with temperature and is low between pH 5-9.

What is the melt-glass process?

The melt is mixed at high temperatures and then quenched and then annealed.

What are the different types of phosphate glasses?

The most common phosphate glasses are metaphosphate glasses of 50 mol% P2O5. They have chains of inifite length, or rings. This means they have entangled pmoelcular/polymeric chains rather than a full silica-like network. The long chain polymeric phosphate anions are connected by ionic bonds to modifier cations which hold the networks together. Ultraphosphates have >50 mol% P2O5 and have 2D or 3D phosphate networks. Polyphosphates have <50 mol% P2O5 and they are built of phosphate chains and rings, with the chain length decreasing with decreasing phosphate content. Invert glasses (Q0) have lower phosphate contents, and isolated orthophsopahte groups are present: the glassy state is present due to the interaction of cations and phosphate groups.

What is Q structure?

The number of Si-O-Si bridging oxygen bonds associated with a particular silicon atom.

How does pH affect phosphate glass corrosion?

The pH of the surrounding solution also affects glass dissolution. In an acidic envinroment, phosphate glass increases dramatically (autocatalyitc) whole a basic pH increases glass totaldissolution (at a slower rate than acidic corrosion though). The pH pf the solution near the surface of the phosphate glass does not increase so much as it does when a silicate glass starts to dissolve. In silicates, cation release causes a pH increase which can damage cells if it is too high: when a phosphate glass degreades, cations are still released but so is phosphoric acid which buffer the pH. Total dissolution still occurs at all pH values, but the glass is still most stable between 5-9 pH. The rate of degredation can increase by 100 times from pH 1-5. Under acidic degredation the corrosion rate is almost independent from composition but under alkaline composition, degredation is hghly dependent on composition.

Can glass be corroded by water?

The pH of water is often acidic. Glass is hydrophilic so it attracts and holds moisture, and the water corrodes the alkali in the glass like an acid. The rate slows down as the passive porous layer is formed. At higher temperatures, and higher acidic concentrations / higher rates of agitation, the rate of corrosion increases.

What external factors affect the degradation rate?

The pH, the temperature, enzymes, sterilisation techniques, surface treatments, external stresses and the shape of the polymer.

How is pitting related to the passive surface layer?

The passive layer is a very thin protective film which is formed dynamically (i.e. it is continuously broken down and rebuilt) and is self-recovering. The passive layer inhibits corrosion but does not stop it: there is still some diffusion of ions through the passive layer. Pitting corrosion is where defects or faults in the passive layer are attacked by aggressive ions, which can result in the rapid corrosion of the nearby passive layer as well as the base metal underneath.

What is the lifecycle of a pit (short)?

The pit nucleates at an initiation site and then undergoes metastable growth. If the Icritical is reached, then there can be stable propagation of the pit (or the pit terminates and the metal repassivates).

What were to the theories on the structure of platelets?

The polymer chains could lie parallel and extended to form the platelets or fold. The folded polymers have a much smaller end to end distance. Electron diffraction parallel and perpendicular to the chain discovered the crystal structure. The chains indeed fold, and under favourable kinetic conditions, in a regular manner.

How do physical barriers work?

The presence of a physical barrier between the electrode and electrolyte is a simple and effective method of controlling corrosion by forming a barrier to O2 and H2O. However, to paint or coating forms a complete barrier (i.e. thermal expansion of the substrate may cause cracking in the paint film, or changes in humidity can cause cracking due to water uptake). The exposed, small area leads to an unfavorable anode-cathode reaction that local increases corrosion. An effective pain film thickness of <300 μm is required. The coating can be from metallic (e.g. galvanizing), paints, plastics or concrete. Check how strong the adhesion is. It is often the cheapest and simplest method of preventing corrosion, but may be more expensive in the long term.

What critical factors influence whether pitting occurs or not?

The presence of aggressive species in a critical concentration (Cl-1 or suphides). A critical electrochemical potential, the pitting potential (which is linked to the concentration of the aggressive species). The temperature, i.e. the critical pitting temperature (CPT). The existence of initiation sites i.e. secondary phase particles, intermetallics, surface defects, clusters etc (i.e. MnS inclusion in stainless steel).

How is nanoporosity developed?

The primary particles condense into secondary particles which polycondense past the gelling point to form a wet gel. The wet gel aggregates into tertiary particles and into tertiary particle fusions.

What factors influence the corrosion rates and mechanisms of phosphate glasses?

The rate is controlled by the surface pH and/or charge that develops at the glass-solution interface. The rate is sensitive to glass composition, and higher alkali ion contents mean the glass degrades faster. Durability decreases in the order of K<Na<Li. Increases the calcium content reduces degredation. A 'mixed-alkali' effect also plays a role and affects the leach rate. Increasing the calcium content improves corrosion resistance up to a point, beyond this it isn't as stable.

What facts affect the rate of hydrolysis?

The rate of hydrolysis is dependent on the water absorption rate, and is often limited by the diffusion of water through the polymer. The diffusion of water in polymers is often related to their solubility, Tg, Mw and degree of crystallinity. It is also pH dependent: esters are hydrolysed faster under acidic conditions (or alkaline conditions). Acid is produced upon the hydrolysis of an ester, so pH falls during hydrolysis accelerating the degradation process. The polymer type also affects the rate, particularly: hydrophobicity, the density of the ester bonds, the amount of cross linking, the Mw, the %crystallinity, the Tg, the % of each polymer in a copolymer, random versus blocky copolymers and the size of side groups.

What is the float glass process?

The raw materials are weighed and mixed, before being melted by heaters and spread over a molten tin surface. They are then annealed and cut.

What is diffusion controlled polarization?

The reaction rate is controlled by the supply a species to the cathodic site (once there the charge transfer is fast), for example oxygen reduction. The current is independent of potential when oxygen becomes limited. The Evans curve for the oxygen reduction forward reaction curve downwards at a limited current described by Fick's first law i¬_lim=(nFDC^0)/(δ (distance)). As the oxygen concentration increases, the limiting current density increases.

What is the reaction rate of simple redox?

The reaction rate is i0, the exchange current density under equilibrium. It is the fundamental kinetic parameter of an electrochemical reaction. In mixed redox, the reaction rate is icorr (corrosion current density) and the rest potential is Ecorr¬¬. Note = the net current density is zero (the anodic and cathodic currents are equal and opposite).

How does solubility affect phosphate glass corrosion?

The saturation of thr glass by a solution can occur in static conditions. Crystalline precipitates can then form. In alkaline solutions, hydroxyapatite can form on the surface of the glass (better at higher pH). The layer does not inhibit dissolution. This means it can be used for bioglass. Solutions containing Ca2+ will inhibit dissolution, but those containing orthophosphate will not (phosphorus in solution is present as long chain polymeric phosphates rather than monomeric orthophosphates).

What is Q4 structure?

The silicon atom has four bridging oxygen bonds. 100% network former.

What is Q0 structure?

The silicon atom has four non-bridging oxygen bonds. Almost no network former.

What is Q1 structure?

The silicon atom has one bridging oxygen bonds and is bonded to three oxygens which are ionically attracted to network modifiers (three non-bridging oxygen bond). 25% network former.

What is Q3 structure?

The silicon atom has three bridging oxygen bonds and is bonded to one oxygen which is ionically attracted to a network modifier (one non-bridging oxygen bond). 75% network former.

What is Q2 structure?

The silicon atom has two bridging oxygen bonds and is bonded to two oxygens which are ionically attracted to network modifiers (two non-bridging oxygen bond). 50% network former.

How can the length of the polymer be expressed?

The size can be expressed as the contour length (i.e. the radius of a coil or half the total length of a rod), or the extended chain length (unravel the polymer and measure the total length). The molecular weight is given by M_w=n ×M_monomer^molecular. The length degree of polymerisation is given by the number of monomer units. The mechanism of polymerisation affects these variables.

What is the process of sol-gel glass formation?

The sol is prepared by mixing an alkoxide chemical precursor, nitrate/chloride precursors, water and a catalyst. The alkoxide undergoes hydrolysis and gains hydroxyl ions. The sol is transferred to a mould. The intermediate then undergoes polycondensation to form a siloxane (Si-O-Si) network. The polycondensation increases the sol viscosity until it becomes a gel (gelation occurs). After gelation, the gel is aged at 80°C where new crosslinks are formed, the interparticle nicks increase in thickness and porosity decreases. The strength increases with aging (an aged gel must develop enough strength to overcome cracking when drying). The gel sits in a liquid called pore-liquor (by-products of gelation), so drying is performed to remove the liquid. During drying, water evaporates creating outward capillary forces whilst condensation produces inward shrinkage forces. This results in cracks and fractures. A slow rate of 1°C/min is used. Less tortuous porosity or larger pore size glasses are more likely to survive. Aerogel glasses used sublimation. The glass then undergoes thermal stabilisation at 500-900°C where the density, strength and hardness of the glass are improved, as OH groups are removed and the number of bridging oxygen bonds are increased. Other by-products are removed. Further sintering can be done to produce denser films, and densification here happens by viscous flow above the Tg. The number of OH groups are reduced further and pore size decreases.

How do polymers behave in solutions?

The solvent molecules expand the polymer reducing the number of entanglements whilst keep the entropy constant. Thus crystallisation is easier and the chains become more rigid.

What is E0?

The standard electrochemical potential defined for a unit solution activity for the reduction reaction. The more negative the reduction potential, the more reactive the metal.

How does orientation affect properties?

The strong intrinsic dependence of macroscopic properties of polymer solids on their degree of perfection and orientation is true for the Young's modulus, thermal conductivity, electrical conductivity, optical qualities and tensile strength. Oriented polymers are much better at these qualities along their chains than isotropic polymers.

Describe polylactides?

The structure of PLA is (O-CO-CHCH3). It more hydrophobic than PGA as there is a methyl group present. This means the Tg is higher at 60C (the higher hydrophobicity means there is a lower water uptake and lower hydrolysis rates). The methyl group restricts the rotation of the chain. The methyl group means each carbon in the backbone acts as a chiral centre, and this means there are two (L and D) enantiomers. The methyl group also increases its molar mass above PGA. The methyl group pushes the chain further apart so they are not packed as close as PGA, and thus not as tough.

Describe dip-coating?

The substrate is withdrawn vertically from a coating bath at a constant speed. The moving substrate is coated in the sol bath. Since the solvent is evaporating and draining, the entrained film acquires a wedge shape. Within the thinning film, the inorganic species are progressively concentrated by evaporation leading to aggregation, gelation and drying to form a dry gel or Xerogel. Once the film dries, it shrinks in volume. Once the film is attached to the substrate it cannot shrink in that direction so the reduction in volume is followed by a reduction in thickness. When the film has solidified and stresses cannot be relieved by flow, the stress develop in the plane of the substrate. The stress in the film is nearly equal to the tension in the liquid. Despite the large stress, the film does not crack if the film thickness is below a certain critical thickness.

What factors affect HCS formation?

The surface area (sol gel vs melt derived), surface morphology (also affects surface area), concentration, agitation rate and flow rate/path.

What is the graph for growth rate?

The temperature must be above the Tg for diffusion but mist have undercooling below the Tm i.e. at Tc for crystallisation.

What physical factors affect the corrosion rate?

The type of corrosion i.e. aqueous vs weathering. Whether the corrosion is dynamic or static. The exposed surface area to solution volume ratio (SA: V). Porosity. The corrosion behaviour of bulk glass versus fibres versus powdered glass.

Describe the specimen state variables?

The type of network formed (or mixtures of network formers), the mol% of network former (affects the Q structure), the mol% and type of network modifiers (which affect network connectivity) and which intermediates are used (i.e. magnesium covalently bonds into the network unlike calcium which ionically bonds. This increases corrosion resistance due to stronger bonds). The thermal history: the degree of annealing for stress, the amount of phase separation (i.e. calcium rich areas will degrade faster) and the % crystallisation (any composition of glass can be made, and then ceramic can be formed from it; semi-crystalline glass ceramics are not good however as the glass will corrode before the ceramic parts). Prior corrosion exposure history, surface features such as SA: V ratio and surface composition (coatings), as well as the homogeneity of the glass and other surface treatments.

What are the issues with biodegredable polymers?

There is no consumer value for biodegredability and so industry competes on cost effectiveness. There is no suitable for large scale biodegredable poylmer disposal yet.

Describe helices?

These are very common conformations for polymer crystals. The notation is A*u/t where A is the class i.e. the number of skeletal atoms [carbons] in the unit of the chain, u is the number of units on the helix per crystallographic repeat and t is the number of turns of the helix per crystallographic repeat.

Describe biodegradable medical polymers?

These polymers are designed to break down within the body after performing their function, leaving normal metabolites of the body. They are either broken down under the action of enzymes or cells or they are broken down by chain scission (by the hydrolysis of ester bonds).

Describe passive films?

They are 2-6 nm in thickness and can also form under high electric fields. They can be crystalline, nano-crystalline or amorphous, but are not identical to the bulk oxide. For alloys, the film has heterogeneous chemistry. Passive surfaces are dynamic (they are continuously broken down and self-repaired) and there is a small measureable current running through the metals with them (nano/micro amperes). All engineering metals exhibit passive behavior: this is why they don't revert to their mineral states all the time.

How do you read a Pourbaix diagram?

They are diagrams of equilibrium potential vs pH. The regions where soluble ions are the stable species are the corrosive regions. The regions with solids are stable passivity regions. The region of the original metal is the immune region. Pourbaix diagrams do not contain kinetic information, only thermodynamic information. Above each line, the reduced species [species gaining electrons] is stable.

Describe polyamides?

They are strong due to the secondary hydrogen bonds. They have good creep resistance. However they can be improved if they could be formed with higher Mw (i.e. this became Kevlar). Nylon is a coiled polyamide

What environments are soda-lime-silica glasses unstable in?

They are unstable in aqueous environments especially at high temperatures.

How can biodegredable consumer plastics be disposed of?

They are usually broken down by enzyme degredation via microorganisms.This can either be in a landfill (anaerobic) or by composting (aerobic). They can also be broken down by water: the water uptake causes swelling and chain scission (hydrlosyss). It is often assisted by an increased temperature of 60C in factories. A combination of both mechanisms can also be used, water uptake at an increased heat along with microorganisms.

How does the Q structure of phosphate glasses change?

They behave normally: glasses with 45 mol% P2O5 have a mixture of Q1 and Q2 species, whereas from 50-55 mol% P2O5 there is only a Q2 species.

What are the key characteristics of bioglass?

They bond to bone, they are osteoinductive (start osteogenesis) and they stimulate genes in osteoblasts.

What is the use of nano-particle sols?

They can be used for coatings - their nanoporosity allows them to be functionalized.

How are bioactive glasses sterilised and packaged?

They cannot be used in an autoclave (hot water will degrade them). They must be sterilised by gamma radiation. They must be stored in a moisture-free environment.

How do rigid molecules behave during melt?

They cannot coil and the entropy change between the melt and solid is negligible.

What is the definition of bioactivity?

They form a bond with tissue and may stimulate a positive biological response from the body. A glass's bioactivity depends on its rate of corrosion.

What are the uses of Xerogel?

They have a high optical quality and can be functionalized easily. Their composition allows control of their refractive index. They are often used for coating as they crack. Liquid crystals align along their function groups. They can be made into inorganic/organic network hybrids.

Describe melt-derived bioactive glasses?

They have a quaternary composition of 46.1% SiO2, 24.4% Na2O, 26.9% CaO and 2.6% P2O5 in mol%. They are made by melting and mixing the oxide compositional components in a platinum-gold crucible (inert), before being poured in water for quenching or poured into a preheated graphite mould and annealed before being cooled in a furnace (to prevent cracking).

How do side chains affect polymers?

They increase configurational entropy and thus increase Tm¬

How do secondary bonds affect Tm?

They increase enthalpy and thus increase Tm.

How do polymers usually crystallise?

They partially disentangle, extend and crystallise so the final structure is semi crystalline. The entanglements are pushed to the amorphous regions.

How do cross links affect the system?

They reduce the configurational entropy.

How do phosphate glasses corrode?

They use H+ and OH- ions during corrosion, like silica glasses. The surfaces uniformly dissolve due to acid and alkali catalysed hydrolysis of the phospahte network i.e. it is a type V surface. Increasing the phoshate content past Q2, without extra titanium x-links, causes faster corrosion. Corrosion occurs much faster as ion exchange occurs so fast that phosphates are lost from the chains quite quickly. The chains get cut as the phosphates are removed. The water, by this mechanism, seperates the chains from the glass, and the chains float off and degrade somewehere else. However, mechanial integrity is lost in the bulk glass quickly.

How does crystal thickness affect its melting temperature?

Thicker crystals have a higher system enthalpy so their Tm increases.

What is a type III silicate glass surface reaction?

This is a rare type of corrosion which occurs with soda-lime-silica and soda-lime-silica-alumina glasses. CaO or Al2O3-SiO2 protective precipitates form on the surface above the silica rich layer, which are opaque.

What is condensation evaporation?

This is a type of dynamic weathering which is very harmful. Moisture condenses on the surface of the glass and stage 1 cation exchange occurs. The moisture, rich in cations, quickly evaporates before it forms droplets. This leaves behind the cations (i.e. Na+ or Ca2+) in a crystalline layer on top of the glass i.e. a non-protective layer of discrete particles is formed on top of the silica rich layer. The thin Na/Ca-rich layer reacts with CO2 in air forming Na2CO3 which you can't see through. Furthermore, these crystalline deposits may not form a complete seal, and instead create an uneven surface that increase the SA: V ratio when further moisture condenses onto the surface, i.e. increasing the rate of corrosion. This is particularly a problem in humid, warm countries and in bathrooms.

Describe static aqueous corrosion?

This is caused by the entrapment of moisture on the surface of the glass (i.e. even moisture trapped inside a glass container). It is characterised by a long exposure time, a small SA: V ratio. Stage 1 occurs much faster than stage 2. Stage 1 occurs quickly and a silica rich layer is formed, which means further corrosion is inhibited. As there is a low ratio between the glass surface area and the volume of the liquid, the pH of the liquid is not raised enough for stage 2 to occur.

What is Mesolution?

This is the chain length in between polymer entanglements. It is much higher than Memelt as the water molecules expand out the polymer chains.

What is the critical current density for pitting?

This is the required current density required to maintain the electrolyte chemistry in the face of diffusion out of the cavity.

Describe dynamic aqueous corrosion?

This is usually caused by moisture particles running across the surface of the glass. It can also be caused by gaseous attack where water vapour condenses on the surface of the glass and can either evaporate off or run off. It is characterised by a short contact time, a high SA: V ratio. Stage 1 occurs much faster than stage 2. The volume of the droplet is very small and due to the high SA: V ratio, the pH of the droplet is increased a lot. A silica rich layer is formed. If the runoff is fast, then stage 2 doesn't occurs. If the droplet sits on the surface because of slow runoff, there is very localised dissolution which causes pitting.

What is dynamic weathering?

This is where moisture condenses onto the glass surface. The contact time is short and the SA: V ratio is high. Unless the volume condensing is high, or the contact time is longer, stage 1 occurs faster than stage 2 and a silica rich layer is formed. If the contact time is longer then as the pH of the droplets increases a lot, there can be localised dissolution from stage 2 (pitting). The pits can attract more H2O condensation. There are two possibilities in condensation-runoff: where moisture collects on the glass surface triggering dissolution (stage 2 occurs) or natural runoff occurs carrying away dissolution products (stage 2 doesn't occur).

What is transpassivity?

This occurs when oxides dissolve as the oxide is further oxidized to an anion

Why is calcium added to SiO2 in glass?

To improve chemical durability and corrosion resistance. Too much calcium causes crystallisation however.

What is the compositional dependence of bioglass?

Too much SiO2 results in an inert glass. Not enough CaO forms an inert glass as well. Too much CaO or Na2O means glass doesn't form. There is a sweet spot for bioactive glass formation.

How does pressure affect the growth rate graph?

Under pressure the Tm¬ increases so crystallisation can occur at a higher temperature where there is more diffusion (so thicker crystals can be made). Essentially the curve is shifted to the right so the max growth rate occurs at a higher temperature.

How can a system be thermodynamically protected from corrosion?

Use a Pourbaix diagram: the pH can be increased or decreased and the applied potential can be changed to be more negative or positive (anodic protection if a more positive potential pushes it into a passive region and cathodic protection if a more negative potential is used to push the metal into an immune region). If this pushes the species into an immune or passive region, corrosion is prevented. Note, sometimes for passive areas, the passive layer may be non-uniform, conducting or permeable to ions or may be damaged by the flow of electrons along its surface. This may still allow corrosion (i.e. by pitting) and may accelerate the local corrosion rate owing to small anode areas exposed -> passive areas may prevent corrosion. Note: in the passive regions corrosion reactions may still occur but the more negative the potential the smaller the anodic reaction and the greater the cathodic reaction (i.e. the metal is more cathodic, and thus protected). Note: the corrosion engineer often has no control over the electrolyte (i.e. offshore oil/gas platforms).

Describe copolymer melting?

Use a second monomer to reduce the overall Tm i.e. crystallisable with non-crystallisable additive monomers.

How can the corrosion rate be measured?

Use a three electrode system of a counter electron (platinum wire) for oxidation, a working electrode (sample to be measured) for reduction and a reference electrode (i.e. SHE) to calculate the electrode potential difference. The solution it is in affects the working conditions. Connect all three to a potentiostat, apply potentials above and below the rest potential Ecorr and sweep slowly. At each voltage, the reaction is allowed to reach equilibrium. Measure the resultant current. As the polarization increases, Ia increases and ic decreases so imeas -> ia.

What is activation polarization?

When a charge transfer step in the half-cell reaction controls the rate of charge flow, the reaction is under activation control and activation polarization occurs. Essentially there is an activation potential required to control enough electrons to allow the reaction to occur. The reaction rate is limited by charge transfer at the surface. An example is hydrogen evolution. Below the half-cell electrode potential, the reduction forward reaction is favoured, and above the oxidation reverse reaction is favoured. The rate, measured by the current density created, increases by an order of magnitude for a polarization change of 0.1 V.

What is static weathering?

When condensate gets trapped onto the glass surface (i.e. between two sheets of glass) then there is a very sever attack on the glass. Due to the long exposure time and the extremely high SA: V ratio, stage 1 occurs quickly and dramatically raises the pH of the condensate. Stage 2 then occurs such that stage 2 occurs much faster than stage 1. There is a total glass breakdown around the condensate which forms a corrosion pit.

What is pitting corrosion?

When corrosion is geometrically localized on an otherwise passive surface, pitting can occur. It leads to severe material damage, loss of sterility and can as a stress intensifier for cracks. They are random and difficult to monitor as the net currents generated are small. One needs to use critical parameters for material selection to avoid it as much as possible.

Describe paraffin arrangement?

When polyethylene reaches 10,000 g/m there are around 2000 units in between entanglements. This is because there are roughly 5 knots per 10000 units. 3 knots per chain is required for plasticity so at this point, the polymer can withstand plastic deformation. If shorter than 10,000 g/m then the oligomers will form fully extended chain crystals (perfect crystallisation) when crystallised.

What occurs when polymers dissolve?

When polymers dissolve, the molecules separate as they take up water.

Why is higher polydispersity bad?

When smaller chains are present, Tm¬ is lowered even at very small fractions (most polymers shouldn't even have slightly large polydispersity).

When does an oligomer become a polymer?

When the chains are long enough to sustain plastic deformation.

What is a type IV silicate glass surface reaction?

When there is a lower SI content (<60%) i.e. in bioactive glasses, the silica rich film is produced very slowly and is very thin. The silica concentration is too low to form a protective layer thick enough and fast enough to protect the glass. Both stage 1 and stage 2 corrosion occur at the same time. This is bioglass.

Describe how the mould affects crystallisation?

With a heated mould, the temperature is the lowest at the centre of the melt so that is where there is the smallest temperature i.e. largest undercooling. This means there will be smaller grains in the centre than at the outer edges. Pressure/heterogeneous nucleation shifts the curve to the right so that even at low undercooling there is still decent nucleation at the edges.

How can the unit cell be define?

Within the crystalline region.

What is the current-time curve for erosion corrosion?

X

What is the degree of crystallinity?

X = fraction of volume occupied by the crystalline phase. It is not the crystalline quality.

Can PLA be used in packaging?

Yes, it can replace PET. 90% PLA and 10% additives can lead to a Floreon nanocomposite which can be produced from plant feedstock. In a 2 stage process it can be broekn down by hydrolysis and then microorganism digestion. The bacteria to do this are in the soil and active at >30C. It can be used to grow new plants.

Why is a higher Tm¬ advantageous?

You can undercool at a higher temperature so diffusion is higher at the same undercooling and this overall nucleation and growth is faster. Also higher Tm prevents creep, but you do not want it so high that processing is difficult.

How is sacrificial protection used on a ship?

Zn blocks are used to protect steel hulls. The protection offered would be excellent if the rate of metal dissolution was reasonably constant. Zn corrodes in sea water by forming a dense skin which severely limits its current output. By alloying Zn, a uniform and sustained corrosion rate can be maintained (C-Sentry alloy).

How do branch points affect Tm?

branch points and cross links represent non-crystallisable defects reducing crystal size and crystallinity overall so lower Tm. Also they increase entropy and lower Tm.

What is the Butler-Volmer equation?

i=i_0 [e^((α_A nF)/RT η)-e^(-(α_c nF)/RT η) ] , where i is the electrode current density, i0 is the exchange current density, n is the number of electrons involved in the electrode reaction, α are the respective charge transfer coefficients and η is the activation overpotential

What is the chemistry of the bulk solution in pitting taken as?

pH 7 with 0.1M Cl-.

Whats the risk equation

risk = probability of failure times consequences of failure

What factors affect the rate of glass attack (glass durability)?

specimen state/composition, environmental factors and physical factors.

What is Faraday's law?

the mass of a substance liberated by an electrochemical process is proportional to the charge passed m=(i_corr tz)/nf where i is the current, t is time, z is the atomic weight, n is the number of electrons exchanged and F is the faraday constant.

How do small particles affect Tm¬?

they act as plasticizers, increasing entropy, decreasing Tm. Increasing small particle/copolymer concentration decreases Tm progressively (look at quasi phase diagram).

What are the forms of metallic corrosion?

uniform, galvanic, pitting, crevice, intergranular, corrosion fatigue, erosion corrosion, stress corrosion cracking, hydrogen induced cracking and selective leaching/dealloying.

What is the equation for thickness loss?

x/t=(Mi_corr)/nFρ, where x/t is the corrosion rate (meters per year etc), M is the atomic weight of the metal, ρ is the density, n is the number of electrons exchanged, F is the Faraday constant and i is the corrosion rate.

What is the equation for the overpotential?

η=E-E_eq

What is cathodic polarization?

ηc electrons are supplied to the surface, so the surface becomes more negative relative to the equilibrium. It is negative by definition.