This chapter addresses one of the most critical aspects of aircraft maintenance: the prevention, detection, and treatment of corrosion, along with proper cleaning procedures. Corrosion is an electrochemical process that can compromise the structural integrity of aircraft components and is a leading cause of airframe deterioration. The information presented here aligns with FAA Advisory Circular AC 43.13-1B (Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair) and 14 CFR Part 43, which establish the regulatory framework for maintenance practices. Understanding corrosion mechanisms, proper cleaning techniques, and approved treatment procedures is essential for every aircraft maintenance engineer (AME) to ensure continued airworthiness and safety.
The Nature of Corrosion
Definition and Mechanism
Corrosion is the deterioration of a metal by chemical or electrochemical reaction with its environment. It is an electrochemical process that requires four elements:
9.Anode - the metal that corrodes (loses electrons)
10.Cathode - the metal that is protected (gains electrons)
11.Electrolyte - a conductive solution (typically moisture containing dissolved salts or acids)
12.Electrical path - metallic connection between anode and cathode
When these four elements are present, corrosion occurs. The rate of corrosion depends on environmental factors such as humidity, temperature, presence of contaminants (salt, acids, industrial pollutants), and the specific metals involved.
Types of Corrosion
Surface Corrosion
Surface corrosion appears as a general roughening or pitting of the metal surface. It may manifest as:
Aluminum: White or gray powdery deposit
Steel: Reddish-brown rust that is flaky and scaly
Magnesium: White, powdery, and voluminous corrosion product
Surface corrosion is often the first stage of deterioration and, if caught early, can be treated successfully without compromising structural integrity.
Pitting Corrosion
Pitting is a localized form of corrosion that creates small cavities or holes in the metal surface. It is particularly dangerous because:
The pits may be small on the surface but can extend deep into the material
Pitting creates stress concentrations that can lead to crack initiation
The actual depth of corrosion may be hidden beneath the surface
After removing corrosion products, pit depth must be measured to determine if the remaining material thickness is within the manufacturer's allowable limits.
Exfoliation Corrosion
Exfoliation corrosion is a severe form of intergranular corrosion that occurs in aluminum alloys. It appears as a layered or flaking surface, similar to the pages of a book. This type of corrosion:
Progresses along grain boundaries
Can cause significant structural weakening
Requires thorough evaluation to determine the extent of damage
May necessitate component replacement if deep or widespread
Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals are in contact in the presence of an electrolyte. The more active metal (anode) corrodes preferentially. Common examples include:
Steel fasteners in aluminum structures
Magnesium components in contact with aluminum
Dissimilar metal connections in landing gear assemblies
Filiform Corrosion
Filiform corrosion appears as thread-like filaments under paint films, typically on aluminum surfaces. It occurs when moisture penetrates beneath the protective coating and travels along the metal surface.
High-Temperature Corrosion
Components exposed to high temperatures, such as exhaust systems, experience accelerated oxidation and scaling. The primary factor is thermal oxidation, which causes the metal to react with oxygen at elevated temperatures, forming oxide layers that may flake off and expose fresh metal to further attack.
Corrosion Identification by Metal Type
Aluminum and Aluminum Alloys
Appearance: White or gray powder, sometimes with a chalky texture
Common locations: Lap joints, faying surfaces, areas around fasteners, battery compartments
Chapter 12: Cleaning and Corrosion Control (cleaning agents and procedures)
Documentation Requirements
All corrosion findings and treatments must be documented:
Location and extent of corrosion
Type of corrosion identified
Treatment performed
Remaining material thickness measurements
Any repairs or replacements made
Special Considerations
Oil-Soaked Wiring
Oil contamination of electrical wiring can lead to:
Insulation breakdown
Short circuits
Potential fire hazards
The correct action is to replace affected wiring and investigate/fix the source of the oil leak. Cleaning or coating oil-soaked wiring does not restore its protective qualities.
Cleaning Agent Surface Behavior
If a water-soluble cleaner beads up on a surface rather than wetting it:
This indicates a hydrophobic film (wax or polish) is present
The film prevents proper cleaning and inspection
A solvent wash should be used to remove the film before proceeding with water-soluble cleaners
Corrosion Under Paint
When corrosion is discovered under peeling paint:
Remove the paint to expose the full extent of corrosion
Remove all corrosion products
Verify remaining material thickness
Treat with corrosion inhibitor and protective finish
Simply applying inhibitor over corrosion does not address the underlying damage
Summary of Key Principles
267.Corrosion requires four elements: anode, cathode, electrolyte, and electrical path
268.Identification is visual: different metals produce characteristic corrosion products
270.Complete removal is essential: corrosion products must be fully removed to bare metal
271.Thickness verification is mandatory: remaining material must be within manufacturer's limits
272.Fasteners must be removed when corrosion is found beneath them
273.Magnesium requires specialized treatment: chemical methods and approved primers only
274.Cleaning agents must be compatible with the materials being cleaned
275.Thorough rinsing after chemical cleaning is critical
276.Environmental control (especially humidity) is the most effective corrosion prevention
277.Coastal environments require increased vigilance in high-risk areas
278.Approved data is required for repairs when corrosion removal exceeds allowable limits
Conclusion
Corrosion control is a fundamental responsibility of the aircraft maintenance engineer. The systematic approach of identification, neutralization, removal, evaluation, and protection ensures that corrosion is properly managed and the aircraft remains airworthy. Adherence to AC 43.13-1B procedures and 14 CFR Part 43 requirements, combined with a thorough understanding of corrosion mechanisms and treatment methods, enables the AME to effectively combat this pervasive threat to aircraft safety and longevity.
