Chapter: Cleaning and Corrosion Control

Overview

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:

Anode - the metal that corrodes (loses electrons)
Cathode - the metal that is protected (gains electrons)
Electrolyte - a conductive solution (typically moisture containing dissolved salts or acids)
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
Contributing factors: Battery acid fumes, salt-laden air, trapped moisture, improper cleaning residues

Steel and Alloy Steels

Appearance: Reddish-brown rust, flaky and scaly
Common locations: Landing gear components, engine mounts, control cables, exhaust systems
Contributing factors: Moisture, salt, high temperatures, lack of protective coating

Magnesium Alloys

Appearance: White, powdery, and voluminous
Common locations: Engine accessory gearboxes, wheel assemblies, transmission housings
Special concern: Magnesium is highly reactive and requires specialized treatment; certain chemicals can cause violent reactions

Copper and Copper Alloys

Appearance: Greenish-blue or green corrosion products
Common locations: Electrical connections, battery terminals, hydraulic fittings

Corrosion Prevention

Environmental Control

The most effective way to prevent corrosion is to control the environment in which the aircraft operates or is stored:

Humidity Control

Moisture is the primary electrolyte that enables corrosion. During storage:

Use dehumidifiers to maintain low relative humidity in hangars
Ensure adequate ventilation to prevent condensation
Avoid plastic tarps that can trap condensation against the aircraft surface
Heating alone is insufficient; humidity must be actively controlled

Coastal Environment Considerations

Aircraft operating in coastal environments are exposed to salt-laden air, which accelerates corrosion. High-risk areas include:

Lower wing skins and lap joints where moisture accumulates
Control surface hinges and exposed mechanisms
Landing gear components
Any area where water can collect and remain trapped

Design Considerations

Certain design features help prevent corrosion:

Drain holes in low points of structures to allow water egress
Sealants at faying surfaces to prevent moisture ingress
Corrosion-inhibiting compounds applied during assembly
Protective coatings (paint, primer, anodizing, conversion coatings)

Protective Coatings

Primers

Zinc chromate primer is the standard corrosion-inhibiting primer for aluminum and magnesium structures
Primer must be applied to clean, dry, corrosion-free surfaces
Only approved primers should be used on magnesium components

Conversion Coatings

Chemical treatments that produce a protective oxide layer on the metal surface
Particularly important for magnesium alloys
Applied before priming

Corrosion-Inhibiting Compounds (CICs)

Materials such as MIL-PRF-81309 that displace moisture and provide a protective film
Used in areas prone to corrosion, such as lap joints and faying surfaces
Applied after cleaning and corrosion removal

Cleaning Procedures

General Principles

Proper cleaning is essential for both corrosion prevention and detection. Key principles include:

Material compatibility: Cleaning agents must be compatible with the materials being cleaned
Complete removal: All cleaning residues must be thoroughly removed
Inspection opportunity: Cleaning should facilitate inspection, not hide defects

Exterior Cleaning

The standard method for cleaning aircraft exteriors is:

Use a mild detergent (non-abrasive, non-corrosive) with water
Apply with soft brushes or cloths
Rinse thoroughly with clean water
Allow to dry completely

Avoid:

High-pressure washers that force water into seams and electrical connections
Chemical strippers that remove paint and can attack the base metal
Scrapers or abrasive tools that can scratch the skin
Strong alkaline or acidic cleaners on aluminum surfaces

Cleaning Aluminum Surfaces

Use mild soap solutions or approved water-soluble cleaners
Avoid highly alkaline cleaners that can etch aluminum
Isopropyl alcohol and TSP (trisodium phosphate) solutions are acceptable if used correctly
After using any chemical cleaner, rinse thoroughly with clean water to remove corrosive residues

Cleaning Acrylic Windshields

Acrylic windshields require special care:

Clean with mild soap (non-abrasive) and water
Use a soft cloth
Never use petroleum-based solvents that cause crazing and clouding
Never use dry cloths that can scratch the surface
Never use high-pressure air that can force debris into edges

Cleaning Magnesium Alloys

Magnesium requires specialized handling:

Use only cleaners specifically designed for magnesium
Never use acids or strong alkalis that can cause violent reactions
Avoid MEK (methyl ethyl ketone) and similar aggressive solvents
Follow manufacturer-approved procedures

Cleaning Landing Gear Components

Use water-soluble cleaners approved for the specific metals involved
Verify material compatibility (aluminum, steel, or combination)
If the cleaner beads up on the surface, it indicates a hydrophobic film (wax or polish) that must be removed with a solvent wash before proceeding
Full-strength or high-pH cleaners can damage protective coatings

Post-Cleaning Procedures

The most critical step after using any chemical cleaner is thorough rinsing with clean water. This is essential because:

Alkaline cleaners leave corrosive residues that attack aluminum
Concentrated residues can form if the surface is simply air-dried
Wiping may not remove residue from crevices and faying surfaces
Wax or other coatings are not substitutes for proper rinsing

Corrosion Treatment Procedures

General Treatment Process

The treatment of corrosion follows a systematic process:

Identify the type of corrosion and affected metal
Neutralize any corrosive agents
Remove corrosion products completely
Evaluate remaining material thickness
Treat the surface with corrosion-inhibiting compounds
Protect with appropriate primer and finish
Document the finding and treatment

Neutralizing Corrosive Agents

Acidic Contamination (Battery Acid)

Lead-acid batteries produce sulfuric acid that can leak and cause severe corrosion:

Neutralize with a mild alkaline solution: baking soda (sodium bicarbonate) and water
Apply the solution until fizzing stops, indicating neutralization
Rinse thoroughly with clean water
Dry completely before further treatment

Alkaline Contamination

Neutralize with a mild acid solution (such as diluted vinegar)
Rinse thoroughly with clean water

Corrosion Removal by Metal Type

Aluminum

Remove corrosion products using mechanical methods (aluminum oxide paper, Scotch-Brite pads) or chemical methods
Use mild abrasives to avoid removing base metal
After removal, verify remaining thickness is within limits
Apply corrosion-inhibiting compound and primer

Steel

Light surface rust: remove mechanically with a wire brush to bare metal
Inspect for pitting or deeper corrosion after removal
If pitting is found, assess depth and remaining material thickness
Apply corrosion-inhibiting primer

Magnesium

Chemical treatment is required - mechanical methods alone are insufficient
Use approved conversion coatings
Apply only approved primers (zinc chromate)
Never use steel brushes that can cause galvanic corrosion
Never use sandblasting that can remove base material

Fastener Corrosion

When corrosion is found around or under fasteners:

The fastener must be removed to fully assess the damage
Hidden corrosion can progress beneath the fastener head
Clean and treat the area after fastener removal
Inspect the fastener itself for corrosion damage
Reinstall with corrosion-inhibiting compound

Evaluating Corrosion Damage

Material Thickness Measurement

After corrosion removal:

Measure the remaining material thickness
Compare to the manufacturer's minimum allowable dimension
If below limits, the component must be replaced or repaired using FAA-approved data (STC, field approval, or manufacturer's data)

Pitting Assessment

Measure pit depth after corrosion removal
Determine if remaining material is within allowable limits
Pits that exceed limits require repair or replacement
Filling pits with filler or primer is not a structural repair

Control Cable Corrosion

Any corrosion causing pitting or reducing cross-sectional area is unacceptable
Corroded cables must be replaced - cleaning and oiling will not restore strength
Pitting creates stress concentrations that can lead to cable failure

Treatment of Specific Components

Battery Compartments

Neutralize acid with baking soda solution
Remove corrosion products
Rinse and dry thoroughly
Apply corrosion-inhibiting compound
Replace any damaged wiring or components

Engine Mounts

Clean the area
Mechanically remove corrosion using aluminum oxide paper (not wire brush)
Apply corrosion-inhibiting primer
Sandblasting is too aggressive for thin structural parts

Landing Gear Torque Links

Light corrosion that has not affected function can be cleaned and protected
Remove corrosion and apply protective finish
Replacement is not necessary for minor corrosion within limits

Exhaust Systems

High-temperature corrosion (oxidation and scaling) is the primary concern
Salt-laden air can contribute but is not the primary factor
Galvanic corrosion is less common in exhaust systems

Regulatory Requirements and Standards

14 CFR Part 43

14 CFR 43.13(a) requires that all maintenance be performed using methods, techniques, and practices acceptable to the Administrator. This means:

Work must be performed in a manner that restores the aircraft to its original or properly altered condition
Corrosion treatment must follow approved procedures (AC 43.13-1B)
Components that cannot be restored to airworthy condition must be replaced or repaired with approved data

AC 43.13-1B

Advisory Circular 43.13-1B provides acceptable methods, techniques, and practices for:

Chapter 6: Corrosion Prevention and Control
Chapter 7: Cable Assemblies (control cable corrosion limits)
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

Corrosion requires four elements: anode, cathode, electrolyte, and electrical path
Identification is visual: different metals produce characteristic corrosion products
Neutralize before removal: acid corrosion requires alkaline neutralization (baking soda); alkaline corrosion requires acid neutralization
Complete removal is essential: corrosion products must be fully removed to bare metal
Thickness verification is mandatory: remaining material must be within manufacturer's limits
Fasteners must be removed when corrosion is found beneath them
Magnesium requires specialized treatment: chemical methods and approved primers only
Cleaning agents must be compatible with the materials being cleaned
Thorough rinsing after chemical cleaning is critical
Environmental control (especially humidity) is the most effective corrosion prevention
Coastal environments require increased vigilance in high-risk areas
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.