An aircraft, like any metal object, is inherently prone to corrosion. All corrosive attacks begin on the surface of the metal making the classification of corrosion by physical appearance a convenient means of identification. Much time and money may be spent on corrosion protection, but inevitably, nature will prevail.
Usually the development of corrosion will depend on how old the aircraft is, what type of environment it is based in, whether or not it is hangared, and how often it is cleaned.
Corrosion on an aircraft is nothing more than rust of the metal parts, although aluminium corrosion doesn’t produce the reddish colour most people think of as rust. Rather, it usually first shows as a ‘dulling’ of the aluminium surface, then progresses to more severe pitting and eventual destruction of the metal. Left untreated, corrosion can render an aircraft unserviceable within a few years.
TYPES OF CORROSION
General surface corrosion is the most common form of corrosion and results from direct chemical attack on a metal surface. It involves only the metal surface and usually occurs over a wide area which is more or less equal in section. It is simply caused by exposing the metal to oxygen in the air such as when paint is worn off the skin of an aircraft.
The pitting corrosion aircraft are most vulnerable to is one of the most destructive and intense forms of corrosion. It can occur in any metal, but is most common on metals that form protective oxide films, such as aluminium and magnesium alloys. It is first noticeable as a white or grey powdery deposit, similar to dust, which blotches the surface. When the deposit is cleaned away, tiny holes or pits can be seen on the surface. These small openings may penetrate deeply into structural members and cause damage completely out of proportion to its surface appearance.
Filiform corrosion is a special form of oxygen concentration cell which occurs on metal surfaces having an organic coating system, particularly on aluminium surfaces poorly prepared for polyurethane paints. This type of corrosion has fine worm-like lines of corrosion under the paint that will eventually lead to bubbling and flaking.
Crevice or deposit corrosion can occur wherever moisture or other pollutants are trapped. Lapped skin joints or rivets on an oil-stained belly are examples of spine corrosion spots.
Intergranular corrosion is an attack on the grain boundaries of a metal. This type of corrosion is not frequently found, but is a particularly nasty type of corrosion. It is usually found in high strength type parts like wing spars, stringers, etc, and when detected it is usually too late.
Stress corrosion is found in highly stressed parts like landing gear components or crankshafts, this type may develop from a scratch nick or surface corrosion.
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 this limit has been exceeded, the metal will eventually crack and fail from metal fatigue.
CORROSION AND YOUR PLANE
As our general aviation fleet ages, corrosion is becoming an increasingly common problem. Since moisture is the common culprit in most types of corrosion, aircraft that have been based in coastal areas are more susceptible, however inland based aircraft are not immune against this at all.
Hangaring, frequent washing, thorough inspection cleaning and treatment while your aircraft is undergoing a MPI could save the day.
I have found many aircraft that have been repainted with the old paint and paint remover residue lying in corners of the interior panels, hidden away and eating the aircraft from the inside like cancer. So it pays to get a paint job done professionally.