9 Corrosion

Specify the effects of the following corrosive agents on aircraft structure. a. Acids and alkalis

Acids—moderately strong acids severely corrode most of the alloys used in airframes. The most destructive are sulfuric acid (battery acid), halogen acids (hydrochloric, hydrofluoric, and hydrobromic), nitrous oxide compounds, and organic acids found in the wastes of humans and animals. • Alkalies—as a group, alkalies are not as corrosive as acids. Aluminum and magnesium alloys are exceedingly prone to corrosive attack by many alkaline solutions unless the solutions contain a corrosion inhibitor. Substances particularly corrosive to aluminum are washing soda, potash (wood ashes), and lime (cement dust). Ammonia, an alkali, is an exception because aluminum alloys are highly resistant to it.

d. Air (moisture, humidity, acid rain, volcanic fallout)

Atmosphere—the major atmospheric corrosive agents are oxygen and airborne moisture. Corrosion often results from the direct action of atmospheric oxygen and moisture on metal, and the presence of additional moisture often accelerates corrosive attack, particularly on ferrous alloys. However, the atmosphere may also contain other corrosive gases and contaminants, particularly industrial and marine salt spray.

d. Brass

Blue or blue-green powdery deposit

o. Chlorinated rubber compounds

Chlorinated rubber is an elastomer (natural rubber or a polyolefin) to which 65% of chlorine has been added to give a solid film-forming resin. Chlorinated rubber is used for coatings, maintenance paints, traffic marking, swimming pool paint, adhesives and fire retardants

Identify what substances metals are converted into during the corrosion process

Corrosion is a natural process that converts a refined metal into a more chemically-stable form such as oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment.

b. Battery compartments and vents

Despite improvements in protective paint finishes and in methods of sealing and venting, battery compartments continue to be corrosion prone areas. Fumes from overheated electrolyte are difficult to contain and spread to adjacent cavities, causing a rapid corrosive attack on all unprotected metal surfaces. Battery vent openings on the aircraft skin should be included in the battery compartment inspection and maintenance procedure. If aircraft batteries with electrolytes, sulfuric acid, or potassium hydroxide are in use, their leakage will cause corrosion. Regular cleaning and neutralization of acid deposits minimizes corrosion from this cause. Consult the applicable maintenance manuals for the particular aircraft to determine the type of battery installed and the recommended maintenance. uniform surface corrosion

Describe the chemistry behind the various forms of corrosion found on aircraft.

Direct chemical attack direct exposure of a bare surface to caustic liquid or gaseous agents common causes are spilled acid battery or fumes. residual flux deposits from inadequately cleaned, welded brazed or soldered joints, entrapped caustic cleaning fluid Electrochemical corrosion Corrosion is a natural occurrence that attacks metal by chemical or electrochemical action, converting it back to a metallic compound. The following four conditions must exist before electrochemical corrosion can occur. [Figure 8-5] 1. A metal subject to corrosion (anode) 2. A dissimilar conductive material (cathode) that has less tendency to corrode 3. Presence of a continuous, conductive liquid path (electrolyte) 4. Electrical contact between the anode and the cathode (usually in the form of metal to metal contact, such as rivets, bolts, and corrosion)

i. Metal spraying

E.G. aircraft cylinders are somtimes sprayed with molten aluminium on their surface. to acomplish this a steel aircraft cylinders are sand blasted. the aluminium wire is feed into an acetylene flame where the wire is melted and blown onto the surface by high pressure compressed air. this is also a sacrificial corrosion

k. Exfoliation corrosion

Exfoliation corrosion is an advanced form of intergranular corrosion and shows itself by lifting up the surface grains of a metal by the force of expanding corrosion products occurring at the grain boundaries just below the surface. [Figure 8-15] It is visible evidence of intergranular corrosion and is most often seen on extruded sections where grain thickness is usually less than in rolled forms. This type of corrosion is difficult to detect in its initial stage. Extruded components, such as spars, can be subject to this type of corrosion. Ultrasonic and eddy current inspection methods are being used with a great deal of success.

e. External skin areas

External aircraft surfaces are readily visible and accessible for inspection and maintenance. Even here, certain types of configurations or combinations of materials become troublesome under certain operating conditions and require special attention.

m. Corrosion fatigue

Fatigue corrosion involves cyclic stress and a corrosive environment. Metals may withstand cyclic stress for an infinite number of cycles so long as the stress is below the endurance limit of the metal. Once the limit has been exceeded, the metal eventually cracks and fails from metal fatigue. However, when the part or structure undergoing cyclic stress is also exposed to a corrosive environment, the stress level for failure may be reduced many times. Thus, failure occurs at stress levels that can be dangerously low depending on the number of cycles assigned to the lifelimited part. Fatigue corrosion failure occurs in two stages. During the first stage, the combined action of corrosion and cyclic stress damages the metal by pitting and crack formations to such a degree that fracture by cyclic stress occurs, even if the corrosive environment is completely removed. The second stage is essentially a fatigue stage where failure proceeds by propagation of the crack (often from a corrosion pit or pits). It is controlled primarily by stress concentration effects and the physical properties of the metal. Fracture of a metal part due to fatigue corrosion generally occurs at a stress level far below the fatigue limit of an uncorroded part, even though the amount of corrosion is relatively small.

i. Filiform corrosion

Filiform Corrosion Filiform corrosion is a special form of oxygen concentration cell that occurs on metal surfaces having an organic coating system. It is recognized by its characteristic wormlike trace of corrosion products beneath the paint film. [Figure 8-8] Polyurethane finishes are especially susceptible to filiform corrosion. Filiform occurs when the relative humidity of the air is between 78-90 percent, and the surface is slightly acidic. This corrosion usually attacks steel and aluminum surfaces. The traces never cross on steel, but they cross under one another on aluminum, making the damage deeper and more severe for aluminum. If the corrosion is not removed, the area treated, and a protective finish applied, the corrosion can lead to intergranular corrosion, especially around fasteners and at seams. Filiform corrosion can be removed using glass bead blasting material with portable abrasive blasting equipment or sanding. Filiform corrosion can be prevented by storing aircraft in an environment with a relative humidity below 70 percent, using coating systems having a low rate of diffusion for oxygen and water vapors, and by washing the aircraft to remove acidic contaminants from the surface, such as those created by pollutants in the air.

n. Fretting corrosion

Fretting corrosion is a particularly damaging form of corrosive attack that occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion. It is characterized by pitting of the surfaces and the generation of considerable quantities of finely divided debris. Since the restricted movements of the two surfaces prevent the debris from escaping very easily, an extremely localized abrasion occurs. [Figure 8-17] The presence of water vapor greatly increases this type of

Describe corrosion proofing methods for seaplanes.

In addition to routine maintenance inspections, amphibians or seaplanes must be checked daily and critical areas cleaned or treated, as necessary.

Detail corrosion detection methods and explain how results may be interpreted.

In addition to visual inspection, there are several NDI methods, such as liquid penetrant, magnetic particle, eddy current, x-ray, ultrasonic, and acoustical emission, that may be of value in the detection of corrosion. These methods have limitations and must be performed only by qualified and certified NDI personnel. Eddy current, x-ray, and ultrasonic inspection methods require properly calibrated (each time used) equipment and a controlling reference standard to obtain reliable results.

c. Lavatories and food service areas

Lavatories, Buffets, and Galleys These areas, particularly deck areas behind lavatories, sinks, and ranges, where spilled food and waste products may collect if not kept clean, are potential trouble spots. Even if some contaminants are not corrosive in themselves, they attract and retain moisture and, in turn, cause corrosive attack. Pay attention to bilge areas located under galleys and lavatories. Clean these areas frequently and maintain the protective sealant and paint finishes.

Explain the following treatments relating to aluminium alloys: a. Mechanical corrosion removal

Mild corrosion can be removed using a neutral houshold cleaner bon ami. make sure it does not contain chlorine. Scotch-brite can also be used. More severe corrosion can be removed using aluminium wool or a aluminium wire brush. Blasting with glass beads smaller the 500 mesh can be used to remove corrosion from pits. After removing corrosion magnify to make sure all traces are removed Sever corrosion rotary files or power grinders using rubbing wheels impregnated aluminium oxide are used to grind out corrosion damage. after using 5X to 10X remove another .002" to be sure the ends of the intergranular cracks have been reached. finish with 280 then 400 grit and clean and neutralize

f. Titanium

No visible corrosion products at low temperature. Colored surface oxides develop above 700 °F (370 °C)

c. Aluminium

On the surface of aluminum alloys and magnesium, it appears as pitting and etching and is often combined with a gray or white powdery deposit

m. Parkerising

Parkerizing is a method of protecting a steel surface from corrosion and increasing its resistance to wear through the application of a chemical phosphate conversion coating. Parkerizing is usually considered to be an improved zinc or manganese phosphating process, and not to be an improved iron phosphating process, although some use the term parkerizing as a generic term for applying phosphating (or phosphatizing) coatings that does include the iron phosphating process.

i. Piano wire hinges

Piano-type hinges are prime spots for corrosion due to the dissimilar metal contact between the steel pin and aluminum hinge. They are also natural traps for dirt, salt, and moisture. Inspection of hinges must include lubrication and actuation through several cycles to ensure complete lubricant penetration. Use water-displacing lubricants when servicing piano hinges

Specify why pure aluminium is considered to be corrosion resistant.

The corrosion resistance of aluminum is due to its tendency to form a compact oxide layer over the surface. The oxide formed offers a high resistance to corrosion. The normal surface film present in air is about 1 nm thick. The film thickness increases at the elevated temperature

Identify common aircraft metals in order of their electrical potential. Show how anodes and cathodes develop and produce electron flow.

The reaction in this corrosive attack requires a medium, usually water, that is capable of conducting a tiny current of electricity. When a metal comes in contact with a corrosive agent and is also connected by a liquid or gaseous path that electrons flow through, corrosion begins as the metal decays by oxidation. [Figure 8-5] During the attack, the quantity of corrosive agent is reduced and, if not renewed or removed, may completely react with the metal becoming neutralized. Different areas of the same metal surface have varying levels of electrical potential and, if connected by a conductor such as salt water, sets up a series of corrosion cells and corrosion will commence.

f. Engine inlet and frontal areas

These areas are being constantly abraded with airborne dirt and dust, bits of gravel from runways, and rain erosion, leading to removal of the protective finish. Furthermore, cores of radiator coolers, reciprocating engine cylinder fins, etc., may not be painted due to the requirement for heat dissipation. Engine accessory mounting bases usually have small area of unpainted magnesium or aluminum on the machined-mounted surfaces. Inspection of these areas must include all sections in the cooling air path, with special attention to places where salt deposits may be built up during marine operations. It is imperative that incipient corrosion be inhibited and that paint touchup and hard film preservative coatings are maintained on seaplane and amphibian engine surfaces at all times.

f. Stress

This form of corrosion involves a constant or cyclic stress acting in conjunction with a damaging chemical environment

l. Stress corrosion

This form of corrosion involves a constant or cyclic stress acting in conjunction with a damaging chemical environment. The stress may be caused by internal or external loading. [Figure 8-16] Internal stress may be trapped in a part of structure during manufacturing processes, such as cold working or by unequal cooling from high temperatures. Most manufacturers follow these processes with a stressrelief operation. Even so, sometimes stress remains trapped. The stress may be externally introduced in part structure by riveting, welding, bolting, clamping, press fit, etc. If a slight mismatch occurs or a fastener is over-torqued, internal stress is present. Internal stress is more important than design stress, because stress corrosion is difficult to recognize before it has overcome the design safety factor. The level of stress varies from point to point within the metal. Stresses near the yield strength are generally necessary to promote stress corrosion cracking. However, failures may occur at lower stresses. Specific environments have been identified that cause stress corrosion cracking of certain alloys. 1. Salt solutions and sea water cause stress corrosion cracking of high-strength, heat-treated steel and aluminum alloys. 2. Methyl alcohol-hydrochloric acid solutions cause stress corrosion cracking of some titanium alloys. 3. Magnesium alloys may stress corrode in moist air. Stress corrosion may be reduced by applying protective coatings, stress relief heat treatments, using corrosion inhibitors, or controlling the environment. Shot peening a metal surface increases resistance to stress corrosion cracking by creating compressive stresses on the surface which should be overcome by applied tensile stress before the surface sees any tension load. Therefore, the threshold stress level is increased.

l. Sherardising

This process involves heating the steel parts up to c. 500 °C in a closed rotating drum that contains metallic zinc dust and possibly an inert filler, such as sand.[5] At temperatures above 300 °C, zinc evaporates and diffuses into the steel substrate forming diffusion bonded Zn-Fe-phases.

State how titanium alloy is protected after light corrosion removal.

Treatment of Titanium and Titanium Alloys Attack on titanium surfaces is generally difficult to detect. Titanium is, by nature, highly corrosion resistant, but it may show deterioration from the presence of salt deposits and metal impurities, particularly at high temperatures. Therefore, the use of steel wool, iron scrapers, or steel brushes for cleaning or for the removal of corrosion from titanium parts is prohibited. If titanium surfaces require cleaning, hand polishing with aluminum polish or a mild abrasive is permissible if fiber brushes only are used and if the surface is treated following cleaning with a suitable solution of sodium dichromate. Wipe the treated surface with dry cloths to remove excess solution, but do not use a water rinse.

e. Organic growths

Water—the corrosiveness of water depends on the type and quantity of dissolved mineral and organic impurities and dissolved gasses (particularly oxygen) in the water. One characteristic of water that determines its corrosiveness is the conductivity or ability to act as an electrolyte and conduct a current. Physical factors, such as water temperature and velocity, also have a direct bearing on its corrosiveness.

d. Wheel wells and landing gear

Wheel Well and Landing Gear More than any other area on the aircraft, this area probably receives more punishment due to mud, water, salt, gravel, and other flying debris. 1. Magnesium wheels, especially around bolt heads, lugs, and wheel web areas, for the presence of entrapped water or its effects 2. Exposed rigid tubing, especially at B-nuts and ferrules, under clamps and tubing identification tapes 3. Exposed position indicator switches and other electrical equipment 4. Crevices between stiffeners, ribs, and lower skin surfaces that are typical water and debris traps 5. Axle interiors 6. Exposed surfaces of struts, oleos, arms, links, and attaching hardware (bolts, pins, etc.)

e. Magnesium

White, powdery, snow-like mounds and white spots on surface

g. Sacrificial corrosion

a method of corrosion protection in which the surface is plated with the metal less noble than itself. any corrosion will attack the plating rather than the base metal. e.g. cadmium plating

j. Control surface recesses: wing flap and spoiler recesses

a thin film of water displacement lubricant can protect the skin lap joints in these areas

Explain the following treatments for ferrous metals: a. Mechanical corrosion removal

abrasive sanding and wire brush. a dry clean surface is an ideal setting for corrosion so once corrosion is removed it must be primed asap zinc chromate primer is used manly

b. Chemical neutralisation

after removal of the corrosion treat the surface with a 5% chromic acid solution to neatralize any remaining corrosion salts. leave for 5 minutes then wash off with water and allowed to dry. alodine conforming to MIL-C5541 will also neautralize the corrosion as well as form a protective film on the metals surface

b. powerplant external preservation.

all oil absorbs contaminants such as carbon sulphur and hydrocarbons as a engine operates which can attack internal parts of the engine. drain and refill with a preservative. some times you may need to run the engine to het the preservative and coat all internal surfaces. when the engine is stopped the spark plug is removed and the engine is turned so the piston is at the bottom and a preservative is sprayed into the cylinder, do this to all cylinders then install desiccant plugs to absorb moisture. do not move prop and break the film of oil. wipe prop with a oily cloth never use acid or alkaline cleaners on metal props

f. Cadmium plating

almost all steel hardware is cad plated. it is electroplated onto the steel to a min thickness of 0.005 inch . when the cad plating is scratched through galvanic action takes place and the cadmium corrodes. it is also similar to aluminium and once the oxide forms on the surface of the cadmium it is air and water tight meaning no further corrosion can take place.

e. Concentration cell corrosion

also known as cressive corrosion. is corrosion of metals in a metal to metal joint even though the metals are identical. or corrosion of a spot on a metals surface covered by a foreign material. oxygen concentrated cells. metal ion concentrated cells and active passive cells are the three general types of concintration cell corrosion

f. Organic films including; zinc chromate, chromic acid etch, wash primer and epoxy primers

an aluminium surface is usually roughened with a mild chromic acid etch. zinc chromate is a paint primer. the film is slightly porous and water can enter it causing chromate ions to be released and held to the surface of the metal. the ionised surface prevents electrolytic action and inhibits the formation of corrosion a wash primer is used in aircraft factories for priming new aircraft before they are painted. 2 part that consists of a resin and alchol-phosphoric acid catalyst expoxy primer provide max corrosion protection. can be used on ally, magnesium and steel. it can aslo be applied over a wash primer

j. Intergranular corrosion and intergranular stress corrosion

an attack along the grain boundries of a material. every grain has a clearly defined boundary and from a chemical point of view differs from the metal within the grain centre. the grain boundary and the grain center can react with each other as anode and cathode when in contact with an electrolyte. oin the metal forming process if if quenching is delayed these grain boundries can become large enough to be effective anodes and cathodes. spot seam welding can also enlarge these grain. eddy-current and ultrasonic can be used to detect and once found it must be replaced

n. Enamels

an opaque or semi-transparent glossy substance that is a type of glass, applied by vitrification to metallic or other hard surfaces for ornament or as a protective coating.

e. Chromic acid pickling

can also be made of 50 sulferic acid to a gallon 10 percent chromic acid solution. apply and leave to stand for 10 - 15 minutes then rinse with hot water

b. Abrasive blasting

can be done using sand, aluminium oxide or glass beads. if it has been plated protect that area on highly stressed areas only use a fine grit abrasive or glass bead

c. protection of light alloys from steel fasteners.

drill holeds and counter sink treat with conversion coating material such as alodine . prime with zinc chromate also coat the fastner and install wet. does not completly insulate the joint but does exclude moisture

j. Organic coatings

e.g. paint. cad plated parts need to be etched to get proper adhesion

g. Metal ion concentration cell corrosion

e.g. some metal to metal joints rub against each other creating high concintration of metal ions adjacent to a low concintration of metal ions. when water absorns oxygen it attracts electrons. these electrons leave the aluminium to form negative hydroxide ions. positive aluminium ions are formed. the water can move freely on the surface of the metal and this movement continually carries the aluminium ions away. the water between the two pieces does not move and has a higher comncintration of poitive metal ions than there is in the open. the metal area between the faying surfaces where the concintration of positive metal ions is highest becomes the cathode and attracts electrons from the skin in an open area which is the anode as electrons flow from the anode to the cathode they leave positive aluminium ions on the surface near the supply of negative hydroxide ions. these hydroxide ions join the aluminium ions and form hydroxide which is a corrosion salt

f. Dichromate conversion

forms a more protected film than. apply to the material and leave to stand until golden brown oxide film forms. rinse with cold water and dry with compressed air. as the oxide film is incredibly soft when it is wet. the film is continues and protects the magnesium by excluding the electrolyte from the surface

g. Lead

gray patina

e. Hard chroming

hard chrome is used to form a wear resistant surface on piston rods cylinder walls and other parts subject to abrasion. parts to be hard chrome are generally under size then plated back to dimensions. cylinders are often plated with porus chrome which surface has thousand of little cracks which hold oil and aid in lubrication

State how the following factors contribute to corrosion: a. Size and type of metal

in galvanic corrosion If the surface area of the corroding metal is smaller than the surface area of the less active metal, corrosion is rapid and severe. When the corroding metal is larger than the less active metal, corrosion is slow and superficial.

d. Anodising

is an electrolytic method. produces a hard surface oxide layer

e. Alodine solution

it is a chemical process. all corrosion must be removed. the surface should then be cleaned with a metal cleaner until it supports an unbroken water film. any break in this film of the rinse indicates the presence of wax, grease or oil. while still wet brush or spray on the chemical and allow it to stand for 2-5 min. flush with fresh water. yellow brown or invisible depending on the chemical used. if a powder forms it is an indication of poor rinsing or failure in keeping the the surface wet during the time the chemical was working.

d. Surface oxide film; anodising

it is a electrolytic treatment. a part is bathed in a lead vat containing a solution of chronic acid and water. the process forms an oxide film on the part and prevents it from further corrosion

c. Pitting corrosion

likely a result of uniform corrosion. pits form as localised anodic areas. and as corrosive action contiues untill an appreciable percentage of metal is converted salts. usually detected by the appearence of clumps of white powder on the surface

q. Control cables

made from either carbon steel or corrosion resistant steel. if carbon steel is left unprotected and water is allowed to get between the cable strands it will corrode. if corrosion is suspected release the tension and open by untwisting. spray with water displacing fluid to protect

Describe the following treatments for Magnesium alloys: a. Mechanical removal of corrosion

magnesium corrosion occupies more space than the metal therfore so it raises pain and swells joints. all traces must be removed. anodic to almost all aircraft metals so never remove with metal tools. use stiff non metalic bristle brush or nylon scrapper. if deep pits exist cur out with carbide tip cutting tool or scraper. if you are to use abrasive blasting only use glass beed used only by magnesium

h. Active-passive cells

metals which depend on a tightly adhering passive film for corrosion protection are prone. the corrosion usually starts as a concintrated cell. e.g,. salt deposits on the surface of a metal in the presence of water containing oxygen can create the oxygen cell. if the passive film is brocken beneath a salt deposit the active metal beneath the film will be exposed to corrosion attack. an electrical potential devalops between the large area of passive film and the small area of the anode the active metal

h. Fuel tanks

microbial growth is the primary cause of corrosion in these areas as it can grow to water holding scum which will attach to the alluminium. also the sealant used traps fuel but does not fully trap water which can seep through and cause oxygen concintrated cell corrosion

Describe the general identifying characteristics of corrosion on the following metals: a. Steel

on steel, a reddish corrosion byproduct commonly referred to as rust

c. Cladding

once aluminium is cladded it is possible the alloying agent can cause dissimilar metal corrosion. this can be protected by coating them with a layer of pure aluminium. it acounts for five to ten percent of the total sheets thicknes. the cladding material is anodic any as compared to the core material and any corrosion that takes place attacks the cladding instead of the core

difference between oxygen concentration cell corrosion and metal ion concentrated cell corrosion

oxygen concentration cell corrosion forms in the closed areas between the faying surfaces where metal ion concentration cell corrosion forms in an open surface

Describe the following corrosion prone areas, the likely types of corrosion found in these areas, and the probable causes: a. Engine exhaust trail areas

particular trouble where gaps, seams, hinges, and fairings are located downstream from the exhaust pipes or nozzles. [Figure 8-19] Deposits may be trapped and not reached by normal cleaning methods. Pay special attention to areas around rivet heads and in skin lap joints and other crevices. Remove and inspect fairings and access plates in the exhaust areas. Do not overlook exhaust deposit buildup in remote areas, such as the empennage surfaces. Buildup in these areas is slower and may not be noticed until corrosive damage has begun.

d. Nickel or chrome plating

produces a air tight coating over the surface can be decorative or hard chrome. decorative is used for appearance and surface protection

s. Electronic equipment

seal with a transparent film which excludes the oxygen and moisture

g. Cooling air vents

see above

Describe corrosion prevention methods such as: a. dissimilar metal insulation.

spray with 2 coats of zinc chromate primer and a strip of pressure sensitive vinyl tape placed between the surfaces before the are assembled

b. Copper

the corrosion forms a greenish film

r. Welded areas

the flux can contain compounds that are corrosive towards aluminium. remove all traces of flux after welding

k. Zinc chromate priming

the primer is slightly porous and water can enter causing chromate ion to be realesed and held on the surface of the material. prevents electrolytic action and inhibits the formation of corrosion

c. Corrosion removal from highly stressed parts

use a fine stone, fine abrasive paper or pumice and only fine grit blasting material. remove only minimum of material. do NOT use a wire brush

d. Galvanic corrosion or dissimilar metal corrosion

when 2 dissimilar metals make electrical contact in the presence of an electrolyte. e.g aluminium inspetion plates are held on by steel screws. 2 metals of they same group show little tendency to corrode. the further apart the groups the greater the corrosion. corrosion is much more rapid when anodic metal is smaller than that of the cathodic metal. the greater the area of the cathode allows for higher electron flow. if the other way around the corrosion is slow and superficial.

Describe the following types of corrosion, the causal factors, how each affects the base metal, and how each type of corrosion is treated or neutralised: a. Oxidation (ferrous oxide)

when a metal such as aluminum is exposed to gas containing oxygen a chemical reaction takes place on the surface between the metal and the gas. two aluminium atoms join 3 oxygen atoms and to form aluminium oxide. if the metal is iron or steel the same takes place but there is a formation of iron oxide. aluminium oxide forms a layer and the reaction slows dramatically where iron forms a porus film so it is not air tight and will continue to corrode

b. Decarbonisation

when carbon deposits have been baked on they are to be removed with the process know as decarbonising. a heated tank with a decarbinizing agent either water soluble or a hydrocarbon base. parts are immersed and do not mix the metals

n. Engine mount structure

when reciprocating engines are started the heavy current from the starter must return to the battery via engine mounts. this corrosion forms a potential difference for corrosion to form. to protect welded steel areas from corrosion they should be periodically filled with hot lindseed oil or another type of tubing oil. the oil then should be drained and the hole plugged

f. Oxygen concentration cell corrosion

when water covers the surface of an aluminium surface and seeps into the cracks between lap joints. since water in an open area readily absorbs oxygen from the air. it attracts ecectrons from the metal to form negative hydrogen ions. the electrons required to form these negative ions comes from the metal itself. the area between the skin does not give up electrons to the water on its surface because there is not enough oxygen there to form hydroxide ions. insteadd its electrons flow to the cathodic surface the open area. the area betwwen the skin having lost eletrons now contains positive aluminium ions and this area becomes the anode. electrons flow within the the metal from the anode to the cathode, leaving positive metal ions in the area between the sheets. these positive aluminium attract negative hydroxide ions from the open water and the aluminium corrodes forming aluminium hydroxide. it forms in areas where there is a deficiency of oxygen forms in aluminium, magnesium or on ferrous metals

b. Uniform surface corrosion

where a area of unprotected metal is exposed to the atmosphere containing battery fumes, exhaust gasses or industrial contaminations. a uniform attack over the entire surface. the dulling of the surface is caused by the microscopic amounts of metal being converted into corrosion salts. if not removed pits can form

h. Galvanising

where steel parts are treated with a coating of zinc. similar to cad plating in which it is a sacrificial corrosion. steel is galvenised by passing it through vats of moltent zinc and then rolling it smooth through a series of rollers

b. Salts

• Salts—most salt solutions are good electrolytes and can promote corrosive attack. Some stainless-steel alloys are resistant to attack by salt solutions but aluminum alloy, magnesium alloys, and other steels are extremely vulnerable. Exposure of airframe materials to salts or their solutions is extremely undesirable