1A3 - Chapter 4 - Corrosion Monitoring and Prevention Techniques

How is corrosion seen on the radiograph?

The photographic film becomes darker as it is exposed to increased amounts of radiation causing pits or corroded areas to shows up as dark spots on the radiograph.

Interior corrosion of a piping system is controlled by which 4 factors?

- The selection fo appropriate materials of construction - Wall thickness - Linings - The addition of treatment chemicals

What are disadvantages of amines?

- Their toxic health and environmental effects - The possibility that they can reduce heat transfer in condensers and heat exchangers by creating an overly thick film on the transfer surfaces

What are examples of chemical treatment?

- Anodizing - Nitriding - Phosphating

What are limitations of cathodic protection that must be considered when controlling corrosion?

- Attenuation - Many times one anode is not sufficient to reach all of the surfaces of the tanks and piping that need protective current - Internal and above ground surfaces are not protected by an underground cathodic protection system - Shielding and interference may result if the groundbed system is not place carefully - It is possible to cause corrosion my misplacement of grounded anodes. - It is possible to overprotect a buried tank and by doing so possible damage coatings or set up interference currents -Additional problems can result from stray DC or AC currents from nearby electrical systems

What are two important thing to avoid injury with electrical cathodic protection systems?

- Before maintaining any equipment, ensure that it has been de-energized and confirm that it is properly grounded by testing with a voltage meter - Do not work alone while making any adjustments or performing and maintenance

Where are cladding materials widely used?

- Chemical processes - Offshore oil production - Oil refining - Electric power generation industries

What are four classes of chemicals that passivation inhibitors include?

- Chromate, which is economical and effective, but which is also toxic and is only rarely used. Some jurisdictions have outlawed chromate use, others are expected to do thee same. - Nitrite, which is also economical and effective, but is not suited to systems with open exposure to air because it will oxidize itself. - Molybdate, which requires relatively high oxygen levels, must be used in conjunction with other chemicals which increase it's cost. - Orthophosphate also requires relatively high oxygen levels and is very economical. A disadvantage is that it will contribute significantly to the concentration of suspended solids as it reacts with calcium hardness to produce a precipitate.

What are 7 mitigative and monitoring measures for internal corrosion?

- Corrosion coupons - Electronic monitoring devices - Chemical inhibition (continuous and/or batch) - Lab analysis to determine causes of failure - Pipeline cleaning by piping or chemicals - Fluids analysis - Flow modelling

6 ways that metallic coatings applied?

- Electroplating - Electroless plating - Spraying - Hot dipping - Chemical vapour deposition - Ion vapour deposition

What are 6 applications of linear polarization meters?

- High pressure water system - Cooling water systems - Condensate systems - Potable water treatment and distribution systems - Pulp and paper manufacturing - Amine systems

What are three main areas that paints and coatings have been developed for?

- Impermeability - Corrosion inhibitor - Cathodic protection

What are six terms that are valuable in describing the degree of attack at a given location?

- No corrosion - Definite surface corrosion - Shallow metal attack - Pitted or grooved - Blistering or scaling - Cracking, checking, or crazing

What are 4 disadvantages of an impressed current system?

- Requires a power source which is subject to interruptions - May cause cathodic interference problems - Requires a higher level of maintenance - Overprotection can cause protective coating damage

What are three basic types of hydrogen probes?

- Simple hydrogen probe - Patch probe - Palladium foil type electrochemical cell

What are the five main factors when determining the location of an anode grounded?

- Soil resistivity - Soil moisture - Location of other structures - Availability of a power source - Accessibility and right of way

What are three things that corrosion coupons monitor?

- System corrosiveness - Material performance - Inhibitor performance

What are the 5 main important metallic coatings?

- cadmium - chromium - nickel - aluminum - zinc

What are the two type of penetrants in general use?

-One contains a red dye which gives good contrast with the white developer. -The other contains a dissolved material which makes the penetrant fluorescent under ultraviolet light and therefore readily visible.

The temperature of a VTER corrosion element may be directly measure to a resolution of _____ºC?

0.1ºC

What are 4 advantages of an impressed current system?

1. Large area can be protected by a single installation 2. Can be designed and adjusted for a wide range of voltage and current to suite the local site 3. High current flow (ampere-year output) is available from a single groundbed 4. Effective in protecting uncoated and poorly protectively coated pipelines

What are three criteria for cathodic protection that have Beene established by NACE?

1. A negative (cathodic) potential of at least 850 millivolts with the cathodic protection applied. This potential is measured with respect to a saturate copper/copper sulphate reference electrode contacting the electrolyte. Voltage drops other than those across the structure-to-electrolyte (structure-to-soil) boundary must be considered for valid interpretation of this voltage measurement. 2. A negative polarized potential of at least 850mV relative to a saturated copper/copper sulphate reference electrode is another criterion. Polarized potential is defined as the potential across the structure/ electrolyte interface that is the sum of the corrosion potential and the cathodic polarization. 3. Another criterion is defined as a minimum of 100mV of cathodic polarization between the structure surface and a stable reference electrode contacting the electrolyte. The formation of decay of polarization can be measured to satisfy this criterion.

What are four typical regulations regarding the protection of underground pipeline systems to minimize the risk of a gas or oil pipeline failure?

1. All buried metallic pipe must be properly coated and have a cathodic protection system designed to protect the pipe in its entirety. 2. Newly constructed metallic pipelines must be coated before installation and must have a cathodic protection system installed and placed in operation in its entirety, often within one year after construction of the pipeline 3. Cathodic protection requirements may not apply to electrically isolated, metal alloy fittings in plastic pipelines if the materials of the fittings provide corrosion control and is corrosion pitting will not cause leakage 4. A piping system that is under cathodic protection must be systematically monitored, tested for effectiveness of cathodic protection, often annually, and recorded for future reference

What are 6 typical coatings used in industry?

1. Epoxy resin coatings applied as a liquid or powder 2. Bituminous coatings which may be applied hot or cold and include coal tar enamels and asphalt mastics 3. Thermoplastic coatings such as polyethylene, vinyl and plastisols 4. Specialty coatings which include urethanes, fluorocarbons, phenolics and polyesters 5. Elastomeric coatings including natural rubber, butyl rubber, neoprene, hypalon, and nitrile rubber 6. Inorganic coatings and linings which also include glass

What are four different geometric shapes of coupons?

1. Flat plates are the most common type and are usually installed in pairs so duplicate measurements can be made. 2. Metal rods 6.4mm in diameter are in common use and may be installed singly but are more often mounted in groups of six or eight. 3. Banded coupons: a rubber band is sometimes wrapped around a conventional coupon to create an intentional crevice. This action is useful when oxygen corrosion is suspected. The crevice creates a concentration cell so attack occurs under the band. 4. Spools or nipples are mounted in the system in such a way that they can be removed for evaluation. They may be treated or flanged on both ends.

What are 5 disadvantages of a sacrificial anode system?

1. Low differential potential resulting in low current outputs. 2. Small area of coverage for per anode 3. Poorly protective coated pipelines may require many anodes 4. Effective protection is reduced in high-resistivity soils 5. Expensive if installed after pipeline construction

What are five advantages of a sacrificial anode system?

1. No external power required 2. No voltage regulation required 3. Easy to install and add anodes 4. Minimum maintenance required 5. Inexpensive if installed at time of pipeline construction

What 7 things are inert anodes made from?

1. Scrap iron, if available, although it has a high rate of consumption. 2. Graphite anodes for all types of groundbeds with a carbonaceous backfill 3. High silicon cast iron anodes can be used with seawater 4. Platinized titanium anodes that have a low consumption and a high current density and can be operated at a voltage of less than 10V 5. Platinized niobium/tantalum anodes have a low consumption, a high current density and can be operated at voltages up to 120V 6. Magnetite anodes which are expensive but have an extremely long life and may be an economical choice for some applications 7. Mixed metal oxide anodes which are basically a titanium plate coated with oxides which activate the titanium and enable it to function as a low resistance anode

What are three basic steps to metal surface pretreatment and plating operations?

1. Surface cleaning or preparation which involves the use of solvents, alkaline cleaners, acid cleaners, abrasive materials, and/or water 2. Surface modification which involves some change in surface properties such as an application of a metal layer or hardening 3. Rinsing or other workpiece finishing operations to produce the final product

What is the most common configuration of sacrificial anode?

17lbs magnesium anode with a length of 30 inches and a diameter of 6 inches. The anode is package with bentonite and gypsum, providing a low resistance environment around the anode metal which is maintained because the bentonite and gypsum absorb moisture.

What are common coupon exposure times?

4 to 6 weeks, but still depends on corrosiveness of the system

What is the optimum pH range for sodium sulphite?

8.5 - 10.0

Cladding can save up to _____% of the cost of using solid alloy?

80%

for the mm/year calculation: 87.6 = ? W = ? D = ? A = ? T = ?

87.6 = unitless factor to convert cm/hour to mm/year W = mass loss in milligrams D = Density of structure in grams/mL A = Original surface area in cm² T = Time in hours

What is the optimum pH range for hydrazine?

9.0 - 10.0

What are amines?

A complex group of hydrocarbons which have numerous industrial functions.

How does the slip-in rack work?

A coupon holding rod is contain in the retraction chamber which is flanged to the isolation gate valve. The other end contains a packing gland through which the coupon holder passes. Coupons are mounted on the rod in the extended position then drawn back into the retraction chamber. After bolting the chamber to the gate valve, the valve is opened and the coupons are slipped into the process stream.

What is the first item to be recorded during a visual inspection?

A description of the general nature and appearance of the corrosion. *Well defined terms are employed in the description so that the observations of different inspectors are comparable.

What are shallow vertical groundbeds?

A group of anodes installed vertically at a depth of less than 15m in a drilled hole, commonly used when space is very limited.

What are deep anode groundbeds?

A groups of anodes installed vertically at a depth of 15m or more in a drilled hole.

How can the principle of conductance be used to measure corrosion loss?

A loop of wire is inserted into the system and allowed to corrode. As it corrodes, its electrical resistance increases because its cross-sectional area decreases. As long as the wire is intact, its length remains constant.

How is dye penetrant inspection conducted?

A low viscosity, low surface tension liquid is applied to the clean metal surface. The liquid is drawn into any cracks or surface discontinuities by capillary action. The excess fluid is wiped from the surface and a developer is applied that acts like a blotter and draws the liquid back out of the defect.

What is dye penetrant inspection?

A method of detecting flaws or cracks which extends to the surface of a metal.

What is radiography?

A nondestructive tool used for inspection of welding and it can also be used for corrosion detection.

What is attenuation?

A reduction or lessening of protective current on the structure furthest away from the anode or groundbed.

What is corrosion monitoring?

A series of surveys planned and organized to obtain comprehensive information on corrosion conditions over time.

How do linear polarization meters work?

A source of electrical current is connected between two metal electrodes and a small amount of current is made to flow between them. An electrolyte, such as water, must be present to conduct currents between the probes. The current flow changes the potential of the "test" electrode. The corrosion rate (corrosion current) is proportional to the test current divided by the change in potential when the potential change is small (20mV or less). Sufficient current is applied to the test electrode to change its potential by a specific amount (for example, 10 mV), the current flow required to make the change is directly measured.

What kind of corrosion probe is used at medium and high-temperature locations?

A variable temperature electrical resistance (VTER) probe.

What do cathodic protection coatings usually contain?

A zinc-pigmented base paint which acts as an anode (Anti-rust paint)

What is the greater best limitation in corrosion coupon testing?

Accumulation of interior deposits in the piping system. *Once a layer of iron oxide or scale has formed on a pipe's interior surface, a different type of corrosion mechanism forms and is difficult to duplicate with coupons.

How are coupon results interpreted?

After being cleaned of scale, slime, or debris in the lab, the coupons are reweighed. The loss of mass is treated as though it were uniformly removed from the total surface area of the coupon and converted into an average penetration rate of thickness lost per year.

How is filtered iron testing done?

By filtering a sample across a 0.2 micron filter pad and then testing the filtrate for iron. If the filter sample is greater than 0.25ppm iron, then corrosion control is not optimized. *The filter paper can be saved as a qualitative measure of system cleanliness

How do precipitating (cathodic) inhibitors work?

By forming a chemical precipitate that adheres to the metal surface to isolate it from the water.

How does metal cladding improve the corrosion resistance of a material?

By metallurgically bonding to its susceptible core material a surface layer of a metal or an alloy with good corrosion resistance.

Metal cladding material is selected not only to have good corrosion resistance, but also to be what?

Anodic to the core alloy by about 80 to 100mV. *Therefore, if the cladding becomes damaged, or if the core alloy is exposed at drilled fastener holes, the cladding provides cathodic protection by corroding sacrificially

How do passivation (anodic) inhibitors work?

By oxidizing the metal surface to encourage development of a protective oxide coating.

How are protective coatings applied?

By painting or spraying onto the surface of the material to be protected.

What does 'cracking, checking, or crazing' mean?

Apparent size and numbers are noted. Note any tendency for cracks to follow a regular pattern. Any vessel having this type of damage is usually removed from service, but the records may be valuable for predicting the same type of damage to other equipment.

What is galvanizing?

Applying metallic zinc to carbon steel for corrosion control purposes, mainly for low temperature liquids.

How is the resistance of a VTER corrosion element temperature compensated?

By software, with the element thickness and corrosion ate calculated.

What is a galvanic probe assembly?

Brass and steel electrodes connected to an ammeter to measure current flow, which is proportional to the corrosiveness of the system.

Why is an electrical resistance corrosion rate meters temperature compensation system only effective at relatively low temperatures (below 100ºC)

Because at those temperatures no cooling is necessary.

How does the role of corrosion monitoring in defining changes in corrosion conditions over time represent a longer-term commitment than one time corrosion inspections?

Because corrosion monitoring represents a commitment to quantifying the rate of corrosion and determining underlying causes of corrosion damage.

How can a significant gain in scavenging efficiency with sodium sulphite be made?

By adding bivalent metal catalysts, such as iron, copper, cobalt, nickel, or manganese, to the sodium sulphite.

In a galvanic system, how can anode wastage, variations in soil resistance due to groundwater changes, and changes to the protected structure be compensated for?

By adjusting the rectifier which provides DC current to the system.

Why shouldn't coupons be touched with bare fingers?

Because grease from a finger print results in a false reading.

Why is a VTER probe removed and checked for pitting on a regular basis?

Because if does not measure pitting corrosion reliably.

Why do pure aluminum such as a sacrificial anode?

Because it forms a protective oxide coating with a very high resistance.

Why is sodium sulphite not used above 6900kPa?

Because it produces an unacceptably high level of dissolved solids, and it also breaks down into corrosive gases such as sulphur dioxide and hydrogen sulphide.

Why can the VTER probe be used in gas systems to measure corrosion from stack gases also?

Because it requires no electrolyte to carry current.

Why is the design and installation of a buried piping system without consideration of cathodic protection unacceptable?

Because leaks due to corrosion can cause environmental damage.

Why do most facilities apply a biocide treatment on a scheduled basis or visual basis?

Because microbiological testing is hard and inaccurate. There is no definitive means of determining the onset of a biofilm formation. *Improper dosage can be applied this way though because it's just a guess of what is needed

How does gamma radiography identify corrosion?

Because more radiation passes through a thinned, corroded area than through a thicker, undamaged area.

Why does the nickel plating process find wide application on items like pump parts and valves?

Because of its exceptional resistance and hardness.

Why is a standard method of reporting corrosion necessary?

Because of the various perceptions of the persons inspecting the equipment.

Why is it important that underground piping systems be isolated from a plants grounding grid?

Because the cathodic protection for the piping will be connected to the grid and will be sacrificed in protecting the grid components as well as the piping.

Why do zinc anodes have to be made with less than 0.0014% iron?

Because the iron in the zinc sets up local galvanic cells and seriously reduces the anode's efficiency.

Why is the corrosion rate generally useless in measuring pitting?

Because the mass loss occurs in a few isolated locations on the coupon. *The number, diameter, and depths of pits are noted.

Why are some water sources unsuited for sodium sulphite use?

Because the scavenging action is impeded by the presence of some organic materials in the water.

Why is the effectiveness of sodium sulphite as an oxygen scavenger affected by temperature?

Because the scavenging reaction happens much more quickly at higher temperatures.

Why does the voltmeter used to measure a galvanic system need to be quite sensitive?

Because the system voltage levels are very low. (0.85V ish)

Why has the use of corrosion recording meters proven to be extremely valuable?

Because they allow intermittent problems (that are very difficult to solve without a continuous record of corrosion rate) to be seen and then correlated with other events to find the cause of the problem.

Why do impressed current systems usually use graphite, cast iron with a high silicon content, and platinum alloys?

Because they have a slower rate of consumption (longer life).

How is total iron testing done?

By boiling a sample with hydrochloric acid and hydrogen peroxide to dryness, then reconstituting it with distilled water. *This procedure identifies all iron present in the water whether soluble of insoluble.

How is the sodium sulphate removed from the feedwater?

By continuous blowdown.

What are standard sizes of sacrificial anodes?

Between 1lbs and 48lbs

Which types of rays are used in radiography?

Both x-rays and gamma rays.

How is cathodic protection of the anodic material achieved?

By substituting an even more anodic material in a sacrificial anode or by using an external power source to impress a current on the cell which reverses the naturally occurring current flow between the two materials.

How is the efficiency of a zinc anode increased to greater than 90%?

By the use of alloys of aluminum, silicone, and cadmium.

How are ultrasonic waves generated?

By transducers which are made from piezoelectric materials such as quartz.

How is real value generated from corrosion monitoring initiatives?

By translating the information gathered over time into effective corrosion control programs.

Which areas are susceptible to HIC?

Carbon steel pressure vessels and piping contains process fluid with H2S, cyanide, or arsenic.

What is a corrosion coupon?

Carefully machined pieces of metal which are inserted into an external coupon rack system or directly into the process stream to evaluate either system corrosiveness, material performance, or inhibitor performance.

What is an impermeable coating designed to do?

Completely separate the metal from the corrosive environment and provide excellent adhesion to the metal.

What is the most widely used corrosion monitoring option?

Corrosion coupons.

What does 'definite surface corrosion' mean?

Describes the condition when the attack is deep enough to catch a knife blade. It may be further described as dulled, matte, or roughened.

What does 'shallow metal attack' mean?

Describes the removal of perceptible, although barely measurable, amount of metal from the surface.

What does 'blistering or scaling' mean?

Describes the sloughing off of thin layers from the metal surface which may be an indication of hydrogen diffusion. Blisters often indicate an earlier stage of the same or similar type of attack.

What does 'no corrosion' mean?

Describes the surface when it appears to be entirely unaffected.

What does 'pitted or grooved' mean?

Describes when metal is removed to a measurable depth. The size, depth, shape, and distribution of pits and grooves is described. The direction of grooving and the configuration of pits are also noted. *This information is critical because grooving and grouping of pits can weaken the structure more than random scattering. Also, the grouping may furnish valuable clues to the source of corrosion.

What the the inspection serve only to do?

Determine whether or not the equipment needs replacing, repairing or reconditioning. *Damage too slight to call for immediate remedial action may not even be recorded.

Why are cooling water system subjected to high corrosion rates?

Due to their continual contact with large volumes of untreated or minimally treated water.

What happens when two reactive metals are electrically connected to each other in an electrolyte (environments containing enough free ions to conduct electricity such as seawater and basically all natural waters)?

Electrons flow from the more active metal to the other less active metal which becomes the cathode due to the difference in the electrical potential.

Maintenance of a cathodic protection system is mainly a matter of what?

Ensuring that the anodes are replaced with new material when they are consumed to a point where they no longer provide adequate protection.

What is the insoluble, suspended iron removed during filtration a sample of?

Ferric iron which has usually been flaked off a corrosion site and transported in the water stream.

What is essential for chemical testing as corrosion monitoring?

Filtered and total iron testing. It gives a snapshot of corrosion tendencies because any iron that does not pass through a filter paper can be safely assumed to be the result of corrosion.

What are the most common type of coupons?

Flat plates.

What are linear polarization probes ideally suited to monitor?

Fluctuations that may occur within a system; for example, these probes can be used to monitor corrosion inhibitor effects on a continuous basis.

How are coupons oriented?

In such a way that they do not induce turbulence and cause erosion-corrosion, especially when mounted directly in the flow stream.

What is a non-process but important application of cathodic protection in a power plant?

For the reinforcing rebar used in concrete!!

How is a sacrificial anode installed near a pipeline?

From 1.5 to 3m from the pipeline with the top of the anode below the centreline of the pipeline.

What is the most widely used metallic coating method for corrosion protection?

Galvanizing.

What is the most widely used method of inspecting process equipment?

Gamma radiography.

Is a pressure test alone considered adequate proof of long-term pipeline integrity?

HELL NO

What oxygen scavenger is used above 6900kPA?

Hydrazine

What is a useful oxygen scavenger for condensate systems and boilers that are shut down and laid up?

Hydroquinone.

Who has published a number of standards of surface preparation methods and acceptability?

ISO (International Organization for Standardization)

How does a hydrogen patch probe work?

Identically to a regular probe except that the patch is sealed to the outside of the pipe or vessel and collects hydrogen atoms which penetrate the pipe wall.

How can the time and expense put into the initiation of a corrosion monitoring program be waste?

If proper records are not kept.

_________ iron is an indication of corrosion that has occurred. ________ iron is an indication of the corrosive potential of the water and the likelihood or iron deposition on downstream heat transfer surfaces.

Insoluble Soluble

How are nickel coatings applied?

In a chemical reduction process which depends upon the catalytic reduction process of nickel ions in an aqueous solution (containing a chemical reducing agent) and the subsequent deposition of nickel metal without the use of electrical energy (electroless plating).

Where are the metal test coupons kept before and after exposure?

In a special treated envelope. *It is air dried after removal, but not cleaned, before being placed into the envelope.

Where are zinc alloy anodes used?

In highly conductive electrolytes at ambient temperatures and in applications where sparks hazards need to be avoided such as in storage tanks containing flammable liquids.

Where are aluminum alloys anodes used?

In seawater environments.

Where are sacrificial anodes of magnesium ions primarily used?

In soils and areas which have high resistance to current flow.

How are linear polarization probes mounted?

In such a way that they can be removed from the system either by providing isolation valving or by using retractable probes. *They are removed periodically, cleaned, and inspected visually for signs of pitting.

Where can NACE's criteria for cathodic protection be found?

In their recommended practice (RP)-01-60 and RP-02-85

Coupon holders must be designed in such a way that they can be __________ from the fluid stream by ________ for removal.

Isolated Valving

What are the advantages of a VTER probe?

It allows accurate and direct monitoring of the resistance of a single corrosion element without the need for a reference element contained within the probe body. The single element may be cooled or heated to monitor corrosion and any desired temperature.

What is a disadvantage of sodium sulphite?

It increases the solids content of the water, so it is unsuitable for boilers that utilize spray type superheaters.

What happens to the resistance as the area of a conductor decreases?

It increases. *This principle can be used to measure corrosion loss.

What are two advantages of ultrasonic inspection?

It is nondestructive and can be used, in most cases, when the equipment is in service.

How does a palladium foil type electrochemical cell work?

It produces an electrical output proportional to the hydrogen evolution rate.

How does sodium sulphite scavenger oxygen?

It reacts with oxygen to form sodium sulphate.

How can the ammonia of hydrazine be beneficial?

It scavenges dissolved carbon dioxide.

What is carbonaceous backfill?

It serves as a sacrificial buffer between the anode and the reactive environment.

What is the difference between linear polarization meters and normal corrosion coupons?

Linear polarization measurements give a reading of the corrosion rate occurring on a coupon or electrode at the moment the measurement is taken. Regular coupons just give a direct measurement of the metal loss over some period of time.

From the galvanic series, what is the most logical choice for a sacrificial anode?

Magnesium *But in actual use tis electrochemical efficiency drops off rapidly to approximately 50%.

What is the direct observation corrosion monitoring technique?

Measurements are taken in quantitative terms when equipment is shutdown and the values are carefully recorded.

What are ultrasonic devices used to measure?

Metal thickness and to detect defects in metal.

Does a galvanic probe give a direct measurement of corrosion?

No, it detects changes in the corrosiveness of the system.

Is magnetic particle inspection applicable to all materials?

No, only to magnetic materials such as steel.

Is it economical to repair or removed corroded anodes?

Nah, rarely. They're usually replaced by adding new anodes to the groundbed.

What does hydrazine break down into at high temperatures?

Nitrogen and ammonia

What are solvent-free polymer epoxies?

Often water based so they are safe to work with and environmentally friendly. They can be made with very high adhesion to the base metal, and they provide excellent resistance to degradation due to ultraviolet radiation from sunlight and mechanical damage from impact. *Quick and inexpensive to apply

What does the gamma radiography process involve?

Passing gamma rays from a source, through the object being inspected, onto a piece of photographic film. When developed, the film is called a radiograph.

What are very useful for recording corrosion conditions?

Photographs. The comparison of photos from previous inspections often give an indication of the progression of the problem.

What are coupons examined for before being sent to the lab for evaluation?

Pitting and scale formation.

What type of corrosion is particularly adapted to measurement?

Pitting. The use of a micrometer pit gauge is recommended and the exact location of the pitting must be carefully specified.

What is magnetic particle inspection?

Primarily a method of locating cracks which extend to the surface of the metal part.

What are the most widely used products for the control of corrosion?

Protective coatings

What is the total iron a measurement of (once the filtered iron has been subtracted from it)?

Provides a measure of the soluble iron in ferrous form which usually occurs naturally in water.

Special ________ holders which can be withdrawn while the system is under full operating pressure are sometimes used.

Retractable

Why are galvanic systems installed with leads for a voltmeter, or with a dedicated voltmeter as part of the circuit?

So that routine readings (often done weekly) can provide data to determine if the system voltage is declining, an indication of anode wastage.

How are coupons mounted?

So that they are securely held and electrically isolated from contact with all other metals to prevent galvanic corrosion (except when the purpose of the test is to study galvanic corrosion).

What type of oxygen scavenger is commonly used for lower pressure applications?

Sodium Sulphite (up to 6900kPa)

Which oxygen scavenger is faster?

Sodium Sulphite. *Hydrazine requires longer retention time.

What is another type of protective non-metallic coating which has more recently become available?

Solvent-free polymer epoxy.

What is the main function fo protective coatings?

TO isolate the structural elements from environmental corrosives.

What happens when galvanic system voltage reaches a predetermined minimum?

The anode or groundbed must be replaced.

What is the primary method of corrosion control in an above grade piping system?

The application of protective coatings.

How are electrical resistance corrosion rate meters designed?

The commercial probes typically have two similar elements: an exposed corrosion element and a protected reference element. *The resistance of the corrosion element is highly temperature dependent; and the reference element, normally contained within the body of the probe, provides compensation for any temperature variation of the exposed element.

What are hydrogen probes useful in measuring?

The flow rate of atomic hydrogen generated from corrosion which passes through steel and gives rises to hydrogen induced corrosion (HIC) problems and blistering.

What is a groundbed?

The inert clusters of anodes connected together in a special backfill.

In a corrosion cell, which material becomes the cathode?

The material more negative in potential. Current flows to it from the more positively charged anode resulting in wastage of the anode.

How is magnetic particle inspection conducted?

The part to be inspected is magnetized using an electromagnet or a large permanent magnet. Cracks in the metal surface break the magnetic lines of force and produce local leakage fields. When finely divided magnetic particles, like iron powder, are sprinkled on the magnetized surface, they are attracted to the cracks by the localized fields making it possible to determine their size and exact location.

Why is zinc not the best sacrificial anode, even though it is second on the galvanic series?

The polarity of a zinc-iron couple could reverse as the temperature increases to greater than 60ºC.

What is the downside of an insert rack coupon holder?

The process must be shutdown to install or remove the rack from the pipeline.

How are pipeline spools and nipples weighed and evaluated?

The same way as coupons.

What happens to galvanized piping above 77ºC?

The steel would become the anode with respect to the zinc, rather than the zinc coating being the anode below 77ºC.

How are coupons used to monitor the performance of a corrosion inhibitor?

The uninhibited corrosion rate of a system is determine and used as a base corrosion rate. Once established, inhibitor feed is started and corrosion coupons are installed in the system. The performance of the inhibitor is usually expressed in terms of percent protection.

How are ultrasonic waves generated from piezoelectric materials?

They are capable of expanding and contracting when subject to a changing electrical field and will also produce an electrical potential when the material is place under mechanical stress. *The therefore, an ultrasonic transducer can be place on a metal surface and used both to create and detect ultrasonic energy.

How does a simple hydrogen probe work?

They consist of a thin-walled carbon steel tube inserted into the flow stream with a solid rod inside the tube forming a small annular space. Hydrogen atoms small enough to permeate the carbon steel collect in the annular space and combine to form hydrogen molecules which are too large to pass back into the process. As hydrogen gas collects, pressure builds in the annular space and registers on a pressure gauge located outside the piping.

What is one problem with linear polarization meters?

They do not detect pitting.

What are impressed current systems?

They employ inert (zero or low dissolution) anodes and use an external source of DC power to impress a current from the external anode onto the protected cathode surface.

What are distributed anode groundbeds?

They involve a group of anodes installed along the structure at less than 10, from the structure and spaced on 10m to 200m centres. *This type of groundbed is used to reduce interference from nearby structures and to protect sections of bare or poorly coated structures

What are metallic coatings?

They provide a layer that changes the parent metal's surface properties to those of the metal coating. The parent metal becomes a composite material exhibiting properties not generally achievable by either material if used alone. *The coatings provide a durable, corrosion resistant layer, and the parent metal provides the load bearing capability.

The amount of radiation that passes through a metal in a given length of time is inversely proportional to its ________?

Thickness

How should coupon mounting materials be selected?

To be fully resistant to the environment, since failure of these components will lead to a loss of data or loss of electrical isolation.

What is the objective of carbonaceous backfill?

To conduct the current discharging from the anode surface to the carbon/soil interface, where the electrochemical reaction may take occur, without impacting the anode material.

Why are only one manufacturer's coupons used, not multiple different manufacturers?

To ensure that the material used is consistent.

Why would radiographs be taken in the vertical plane?

To establish the pattern of attack on the bottom or top of piping or vessels.

What is the basic principle of ultrasonic inspection?

To generate high frequency sound waves and transmit them into the material being inspected. These waves travel through the object until they encounter the opposite surface or an internal discontinuity. Either an interface or a discontinuity will reflect the sounds waves back to the point of transmission of the ultrasonic wave and the arrival of reflected wave back to the point of transmission is directly proportional to metal thickness or the depth of the flaw.

Why are radiographs frequently taken in the horizontal plane?

To give a profile of pipe walls.

What is another alternative protective coating for vessels?

To install a pre-made liner within the vessel, with the liner itself being made of a corrosion resistant material.

Why does each plant have a grounding grid on which it is built?

To protect the plant from lightning and stray currents generated by the plant operation.

What are protective coatings used for?

To provide long-term protection under a wide range of operating conditions, extending from atmospheric pressure to the most demanding chemical processing conditions.

What is the basic principle of cathodic protection?

To reduce the corrosion rate of a metallic surface by reducing its corrosion potential by bringing the metal to a more negative potential state (as seen in the Galvanic series table)

What is one of the most efficient and cost-effective methods of controlling corrosion?

To regulate the chemistry of the fluid within a vessel or pipeline.

What is a corrosion cell?

Two dissimilar materials develop an electrical potential difference. As a result, current flows from an anode to a cathode.

How are corrosion reading plotted?

Versus time, and the slope of any portion of the resulting curve may be converted to a rate of corrosion.

What does the manufacturer do before the coupons are inserted into the system and after they are removed?

Weighs the coupons to an accuracy of four decimal places to determine the mass of metal lost to corrosion.

What is a conventional (remote) groundbed?

When a group of anodes is installed 100m from the structure and spaced on 5m to 10m centres to distribute protective current over a wide area of the structure being protected.

What is the different between two and three-electrode linear polarization probes?

When a two electrode probe is used in fresh water, the reading must be corrected for the solution resistivity. (The potential change of the test electrode is measured with reference to the second or working electrode) A three-electrode probe requires no correction for solution resistivity, because the third electrode is allowed to corrode freely (no current is applied to it), and it is used as the reference electrode.

What are non-metallic coatings?

When chemical treatment of the metal surface produces a layer of metallic oxide or compound which has a better corrosion resistance than the metal.

When are banded coupons used?

When oxygen corrosion is suspected, because the crevice created by wrapping a rubber band around a conventional coupon creates a concentration cell for attack to occur under the band.

How do hydrogen probes allow for rapid problem addressing?

When pressure accumulates at a certain rate per day the increases suddenly by a factor of 10 or something it indicated a significant change which must be address.

What are deep anode groundbeds for?

When space is not available for a conventional groundbed or when the soil has a high resistivity and the deeper strata has a low resistivity.

What is an important consideration when using hydrazine?

Whether or not there is copper in the system, because the ammonia will be corrosive to it.

How are readings taken from linear polarization meters?

With a small battery powered meter which can be carried around to various probe location.

What are two types of precipitating inhibitors?

Zinc and calcium carbonate

Which units for high and low corrosion rates are often used?

mm/year and µm/year (µm = micrometer)