Many factors affect the type, speed, cause, and seriousness of metal corrosion. Some of these factors can be controlled and some cannot.

Climate

The environmental conditions under which an aircraft is maintained and operated greatly affect corrosion characteristics. In a predominately marine environment (with exposure to sea water and salt air), moisture laden air is considerably more detrimental to an aircraft than it would be if all operations were conducted in a dry climate. Temperature considerations are important because the speed of electrochemical attack is increased in a hot, moist climate.
 

Size and Type of Metal It is a well known fact that some metals will corrode faster than others. It is a less known fact that variations in size and shape of a metal can indirectly affect its corrosion resistance. Thick structural sections are more susceptible to corrosive attack than thin sections because variations in physical characteristics are greater. When large pieces are machined or chemically milled after heat treatment, the thinner areas will have different physical characteristics than the thicker areas. (See figure 6-59.) From a corrosion control standpoint, the best approach is to recognize the critical nature of the integrity and strength of major structural parts and to maintain permanent protection over such areas at all times to prevent the onset of deterioration.

Foreign Material

Among the controllable factors which affect the onset and spread of corrosive attack is foreign material which adheres to the metal surfaces. Such foreign material includes:

(1) Soil and atmospheric dust.
(2) Oil, grease, and engine exhaust residues.
(3) Salt water and salt moisture condensation.
(4) Spilled battery acids and caustic cleaning solutions.
(5) Welding and brazing flux residues.

It is important that aircraft be kept clean. How often and to what extent an aircraft should be cleaned depends on several factors, such as location, model of aircraft, and type of operation.