Types of Corrosion and How to Prevent Them

Corrosion is a natural process where a material (usually a metal) degrades due to chemical and/or electrochemical reaction with their environment. Like any metal object, aircraft can fall victim to corrosion. Just a few years of corrosion can leave a plane unairworthy and cost a huge sum of money to fix. As such, aircraft operators employ a number of corrosion prevention methods. This blog will cover the types of corrosion aircraft face, as well as a few ways to prevent it from occurring.

The first type of corrosion experienced by aircraft is surface corrosion, which appears as a general roughening, etching, or pitting of the metal’s surface. It is usually accompanied by powder-like deposits of corrosion products and can be caused by either chemical or electrochemical stress. At times, corrosion will spread under the surface coating and be largely unrecognizable. However, upon closer inspection, paint or plaiting will begin to lift off the surface in small blisters resulting from the pressure of the underlying accumulation.

The second type of corrosion, dissimilar metal corrosion, is a result of contact between dissimilar metal parts in the presence of a conductor. While surface corrosion may not be taking place, a galvanic action occurs at the points of contact where the insulation between the two surfaces has broken down. This can be very serious because in many instances the action is taking place out of sight, and the only way to detect it prior to structural failure is by disassembly and inspection. The contamination of a metal’s surface by mechanical means can also induce dissimilar metal corrosion. The improper use of steel cleaning products, such as steel wool or a steel wire brush on aluminum or magnesium, can force small pieces of steel into the metal being cleaned, which will then further corrode and ruin the surface.

Intergranular corrosion is another type of corrosion that attacks the grain boundaries of an alloy, usually as a result of a lack of uniformity in the alloy structure. Aluminum alloys, as well as certain stainless steels, are especially susceptible to this type of corrosion. The lack of uniformity results from changes that occur within the alloy during heating and cooling throughout the material’s manufacturing process. Intergranular corrosion can exist without visible surface evidence, though severe cases often result in the surface metal flaking due to delamination of the grain boundaries caused by pressure of corrosion residual product buildup. It is difficult to detect in its early stages and often affects components such as spars.

The final two types of corrosion are stress and fretting corrosion. Stress corrosion is the result of the combination of sustained tensile stresses and a corrosive environment. It is found in most metal systems but particularly affects aluminum, copper, certain stainless steels, and high strength alloy steels (those over 240,000 psi). Stress corrosion frequently occurs along lines of cold working and can be transgranular or intergranular in nature. Fretting corrosion is a type of corrosive attack that occurs when two mating surfaces are subject to slight relative motion. It is characterized by pitting of the surfaces and the generation of fine debris. Because the restricted movements of the two surfaces prevent the debris from easily escaping, a highly-localized abrasion occurs. The presence of water vapor further increases this type of deterioration. If the contact areas are small and sharp, deep grooves resembling brinell markings or pressure indentations may be worn in the rubbing surface.

There are four ways to prevent corrosion. The first is to have regular inspections to check for evidence of corrosion. Whenever paint begins to chip, it should be fully stripped and sanded before repainting. Second, lubricants and other corrosion-prevention materials should be applied to the aircraft to seal it from moisture. Thirdly, only the right cleaning products should be used during cleaning. The wrong products can contaminate the surface and cause corrosion. Finally, mild corrosion can be handled through abrasion, followed by primer and paint. The removal must be done carefully as stainless and common steel brushes can ruin the aluminum as well as the integrity of the paint.

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