Factors Affecting Corrosion
Many factors affect the type, speed, cause, and seriousness of metal corrosion. Some of these factors can be controlled and some cannot.
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.
Among the controllable factors which 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.
It is important that aircraft be kept clean. How often and to what extent an aircraft should be cleaned depends on several factors, including geographic location, model of aircraft, and type of operation.
Much has been done to improve the corrosion resistance of aircraft: improvements in materials, surface treatments, insulation, and in particular, modern protective finishes. All of these have been aimed at reducing the overall maintenance effort, as well as improving reliability. In spite of these improvements, corrosion and its control is a very real problem that requires continuous preventive maintenance. During any corrosion control maintenance, consult the Material Safety Data Sheet (MSDS) for information on any chemicals used in the process.
Corrosion preventive maintenance includes the following specific functions:
- Adequate cleaning
- Thorough periodic lubrication
- Detailed inspection for corrosion and failure of protective systems
- Prompt treatment of corrosion and touchup of damaged paint areas
- Keeping drain holes free of obstructions
- Daily draining of fuel cell sumps
- Daily wipe down of exposed critical areas
- Sealing of aircraft against water during foul weather and proper ventilation on warm, sunny days
- Maximum use of protective covers on parked aircraft
After any period during which regular corrosion preventive maintenance is interrupted, the amount of maintenance required to repair accumulated corrosion damage and bring the aircraft back up to standard will usually be quite high.
Inspection for corrosion is a continuing problem and should be handled on a daily basis. Overemphasizing a particular corrosion problem when it is discovered and then forgetting about corrosion until the next crisis is an unsafe, costly, and troublesome practice. Most scheduled maintenance checklists are complete enough to cover all parts of the aircraft or engine, and no part of the aircraft should go uninspected. Use these checklists as a general guide when an area is to be inspected for corrosion. Through experience it will be learned that most aircraft have trouble areas where, despite routine inspection and maintenance, corrosion will set in.
In addition to routine maintenance inspections, amphibians or seaplanes should be checked daily and critical areas cleaned or treated, as necessary.