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

Slimes, molds, fungi and other living organisms (some microscopic) can grow on damp surfaces. Once they are established, the area tends to remain damp, increasing the possibility of corrosion.

Manufacturing processes, such as machining, forming, welding, or heat-treatment, can leave stresses in aircraft parts. The residual stress can cause cracking in a corrosive environment when the threshold for stress corrosion is exceeded.

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

There are two general classifications of corrosion that cover most of the specific forms: direct chemical attack and electrochemical attack. In both types of corrosion, the metal is converted into a metallic compound, such as an oxide, hydroxide, or sulfate. The corrosion process involves two simultaneous changes: the metal that is attacked or oxidized suffers what is called anodic change, and the corrosive agent is reduced and is considered as undergoing cathodic change.

Direct chemical attack, or pure chemical corrosion, is an attack resulting from direct exposure of a bare surface to caustic liquid or gaseous agents. Unlike electrochemical attack where anodic and cathodic changes take place a measurable distance apart, the changes in direct chemical attack occur simultaneously at the same point. The most common agents causing direct chemical attack on aircraft are: spilled battery acid or fumes from batteries; residual flux deposits resulting from inadequately cleaned, welded, brazed, or soldered joints; and entrapped caustic cleaning solutions. [Figure 8-4]

With the introduction of sealed lead-acid batteries and the use of nickel-cadmium batteries, spilled battery acid is becoming less of a problem. The use of these closed units lessens the hazards of acid spillage and battery fumes.

Many types of fluxes used in brazing, soldering, and welding joining operation. Flux residues are hygroscopic in nature, absorbing moisture, and unless carefully removed, tend to cause severe pitting.

Caustic cleaning solutions in concentrated form are kept tightly capped and as far from aircraft as possible. Some cleaning solutions used in corrosion removal are, in themselves, potentially corrosive agents. Therefore, particular attention must be directed toward their complete removal after use on aircraft. Where entrapment of the cleaning solution is likely to occur, use a noncorrosive cleaning agent, even though it is less efficient.

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]