Aircraft Corrosion Control Guide for A&P Mechanics

Corrosion is the silent aircraft killer. It weakens structural components, causes system failures, and can render an airframe unairworthy. As an A&P mechanic, corrosion identification, assessment, and treatment are among the most critical skills you'll use throughout your career.

Types of Aircraft Corrosion

Type	Description	Where It Occurs
Surface (Uniform)	Even material loss across a surface	Any exposed metal surface
Pitting	Localized cavities in the metal surface	Aluminum, magnesium alloys
Intergranular	Corrosion along grain boundaries	High-strength aluminum alloys (2024, 7075)
Exfoliation	Layered lifting/flaking of metal layers	Extruded aluminum, wing skins
Galvanic (Dissimilar Metal)	Electrolytic reaction between different metals	Steel fasteners in aluminum structure
Filiform	Thread-like corrosion under coatings	Painted surfaces, improperly sealed areas
Stress Corrosion Cracking (SCC)	Cracks caused by tension stress + corrosive environment	Landing gear, fitting, highly stressed parts
Crevice	Corrosion in tight spaces where moisture collects	Lap joints, under fastener heads, faying surfaces
Fretting	Corrosion caused by slight movement between mating surfaces	Bushing interfaces, rivet holes

Corrosion-Prone Areas on Aircraft

Checklist of High-Risk Areas:

Wing spar caps and lower wing skins
Lap joints and circumferential/longitudinal skin joints
Landing gear components (especially struts and actuators)
Wheel wells and gear bays
Bilge areas and lower fuselage
Battery compartments
Lavatory and galley areas (exposure to waste and food acids)
Areas around drain holes and water traps
Under-insulation blankets (trapped moisture)
Fuel cell interiors
Engine nacelle and thrust reverser areas
Around fasteners and rivet heads
Dissimilar metal interfaces (steel-to-aluminum joints)
Cargo compartment floor structures
Horizontal stabilizer and elevator structure

Corrosion Identification

Visual Indicators:

Inspection Methods:

Method	Application
Visual Inspection	Primary method; uses magnification, borescope, mirror
NDT — Eddy Current	Detects subsurface corrosion and cracking
NDT — Ultrasonic	Measures material thickness loss
NDT — X-Ray	Detects internal corrosion in thick or inaccessible structures
Tap Testing	Detects disbonds and delamination in composites/bonded panels

Corrosion Treatment Procedures

Step-by-Step Treatment:

Identify and classify the corrosion type and severity

Clean the affected area — remove dirt, grease, and contaminants

Remove the corrosion — using appropriate methods:

- Mechanical removal: abrasive pads (Scotch-Brite), sandpaper, wire brushes

- Chemical treatment: approved corrosion removers (per manufacturer specifications)

— Never use steel wool on aluminum (causes galvanic corrosion)

Assess the remaining material — measure thickness; compare to minimum values in SRM

Apply protective treatment:

- Alodine (chromate conversion coating) for aluminum

- Primer (zinc chromate or epoxy primer)

- Topcoat paint

Document all findings and repairs in the maintenance record

Corrosion Severity Assessment

Level	Description	Action
Level 1	Light surface corrosion, easily removed	Clean, treat, protect
Level 2	Moderate corrosion, some material loss	Clean, measure, repair if within limits
Level 3	Severe corrosion, significant material loss	Structural repair or replacement required
Level 4	Extensive damage beyond repair limits	Component/skin replacement

Always refer to the aircraft Structural Repair Manual (SRM) and Corrosion Prevention and Control Program (CPCP) for allowable damage limits and approved repair methods.

Prevention Best Practices

✅ Keep drain holes clear — standing water is corrosion's best friend
✅ Ensure proper sealant application at lap joints and faying surfaces
✅ Use correct hardware — approved fasteners with proper plating
✅ Apply corrosion-inhibiting compounds (CIC) during heavy maintenance
✅ Re-protect any areas where paint or primer is damaged
✅ Use proper barriers between dissimilar metals
✅ Inspect bilge and wheel well areas regularly — these are corrosion hotspots
✅ Follow the aircraft CPCP schedule without deviation

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