Many factors affect the type, speed, cause, and seriousness of metal corrosion. Some of these factors that influence metal corrosion and the rate of corrosion are:
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- Type of metal
- Heat treatment and grain direction
- Presence of a dissimilar, less corrodible metal
- Anodic and cathodic surface areas (in galvanic corrosion)
- Temperature
- Presence of electrolytes (hard water, salt water, battery fluids, etc.)
- Availability of oxygen
- Presence of biological organisms
- Mechanical stress on the corroding metal
- Time of exposure to a corrosive environment
- Lead/graphite pencil marks on aircraft surface metals
Pure Metals
Most pure metals are not suitable for aircraft construction and are used only in combination with other metals to form alloys. Most alloys are made up entirely of small crystalline regions called grains. Corrosion can occur on surfaces of those regions that are less resistant and also at boundaries between regions, resulting in the formation of pits and intergranular corrosion. Metals have a wide range of corrosion resistance. The most active metals (those that lose electrons easily), such as magnesium and aluminum, corrode easily. The most noble metals (those that do not lose electrons easily), such as gold and silver, do not corrode easily.
Climate
The environmental conditions that an aircraft is maintained and operated under greatly affects 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.
Geographical Location
The flight routes and bases of operation expose some airplanes to more corrosive conditions than others. The operational environment of an aircraft may be categorized as mild, moderate, or severe with respect to the corrosion severity of the operational environment. The corrosion severity of the operational environments in North America are identified in Figure. Additional maps for other locations around the world are published in AC 43-4.
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| North America corrosion severity chart |
The corrosion severity of any particular area may be increased by many factors, including airborne industrial pollutants, chemicals used on runways and taxiways to prevent ice formation, humidity, temperatures, prevailing winds from a corrosive environment, etc. Suggested intervals for cleaning, inspection, lubrication, and preservation when located in mild zones are every 90 days, moderate zones every 45 days, and severe zones every 15 days.
Foreign Material
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:
- Soil and atmospheric dust
- Oil, grease, and engine exhaust residues
- Salt water and salt moisture condensation
- Spilled battery acids and caustic cleaning solutions
- Welding and brazing flux residues
Micro-organisms
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
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.