Corrosion Prevention and Treatment Methods for Aircraft

Corrosion is one of the most persistent and dangerous threats to aircraft structural integrity. Unlike a dramatic accident, corrosion works silently — eating away at metal from the inside of joints, beneath paint, and in hidden recesses — until a component loses so much cross-sectional area or surface integrity that it can no longer carry its design load. For the Aviation Maintenance Technician (AMT), understanding corrosion is not optional; it is a core airworthiness responsibility backed by the manufacturer's structural repair manuals, FAA Advisory Circulars, and the fundamental principles found in the Aviation Maintenance Technician Handbook—General (FAA-H-8083-30). This article covers the electrochemical basis of corrosion, the most common types found on aircraft, prevention strategies, and the step-by-step treatment methods every AMT must master.

Corrosion on aircraft is broadly defined as the deterioration of a metal due to a chemical or electrochemical reaction with its environment. Almost every metal used in aviation — aluminum alloy, steel, magnesium, titanium — is susceptible to at least one form of corrosion under the right conditions. The rate and severity depend on the metal involved, the presence of an electrolyte (moisture), and the environment in which the aircraft operates. Coastal, high-humidity, and de-icing-salt environments accelerate corrosion dramatically compared to hot, dry climates.

How Corrosion Works: The Electrochemical Basis

Corrosion is fundamentally an electrochemical reaction. For corrosion to occur, four elements must be present simultaneously: an anode (the metal that corrodes), a cathode (a dissimilar or more noble metal that is protected), an electrolyte (a conductive solution such as salt water, condensed moisture, or battery acid), and a metallic path connecting the anode and cathode. Remove any one of these four elements and the corrosion reaction stops. This is the foundational principle behind every prevention strategy.

In practice, the anode and cathode do not have to be two entirely different pieces of metal. Within a single aluminum alloy panel, variations in grain structure, heat-treatment state, or surface contamination can create microscopic anodic and cathodic areas adjacent to each other. Add a thin film of moisture and corrosion begins immediately at the anodic sites.

Types of Corrosion Found on Aircraft

Surface (Uniform) Corrosion

Surface corrosion appears as a general etching, pitting, or roughening of the metal surface and is typically the easiest type to detect visually. On aluminum, it shows as gray or white powdery deposits. On steel, it appears as reddish-brown rust. Because it is spread over a wide area rather than concentrated at one point, surface corrosion may remove material relatively uniformly, but it can still reduce structural margins if left untreated.

Pitting Corrosion

Pitting is an extremely localized form of corrosion that creates small, deep cavities in the metal surface. It is especially dangerous because its depth can be disproportionately large compared to the small pit visible at the surface, and pits act as stress concentrators that initiate fatigue cracks. Aluminum alloys are particularly susceptible to pitting in chloride environments.

Intergranular Corrosion

Intergranular corrosion attacks along the grain boundaries of the metal rather than across the grain faces. It can penetrate deeply into a part with little or no visible surface indication, making it one of the most insidious types. High-strength aluminum alloys such as 7075-T6 are especially vulnerable when improperly heat-treated or when the alloy's grain boundaries become anodic relative to the grain interiors. In extreme cases, the metal will exfoliate — layers of the metal literally lift and separate — which is called exfoliation corrosion.

Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte. The less noble metal (lower on the galvanic series) becomes the anode and corrodes preferentially. A classic aircraft example is aluminum structure in contact with steel fasteners without proper insulating sealant or cadmium plating. The further apart the two metals are on the galvanic series, the more severe the attack. Magnesium alloys sit at the extreme anodic end of the galvanic series alongside zinc, among the least noble structural metals in common use, and must be carefully isolated from other metals per the galvanic series table in AC 43.13-1B.

Stress Corrosion Cracking

Stress corrosion cracking (SCC) results from the combined effect of sustained tensile stress and a corrosive environment. Neither the stress alone nor the corrosion alone would cause rapid failure, but together they produce cracks that propagate at stress levels far below the material's nominal tensile strength. High-strength steel landing gear components, wing attachment fittings, and highly stressed aluminum forgings are typical candidates. SCC cracks are often transgranular or intergranular depending on the alloy and are very difficult to detect without dye penetrant or eddy current inspection.

Fretting Corrosion

Fretting corrosion occurs at the interface of two tightly fitted surfaces that experience slight relative motion under load — for example, at bolt holes, press fits, and under clamps. The micro-motion abrades the protective oxide film, exposes fresh metal, and the debris oxidizes, creating a reddish or black powder. It is commonly found around wing attachment bolts and control surface hinges.

Corrosion Prevention Methods

Prevention is always preferable to treatment, and every aircraft design incorporates multiple layers of corrosion protection.

• Surface treatments: Aluminum alloys are commonly anodized — an electrochemical process that thickens the natural aluminum oxide layer — providing both corrosion resistance and a good base for paint. Steel parts may be cadmium-plated, phosphate-coated (Parkerized), or chrome-plated.
• Conversion coatings: Chemical film treatments such as Alodine (chromate conversion coating) create a chemically bonded, corrosion-resistant layer on aluminum and provide excellent adhesion for primer and topcoat.
• Primers and topcoats: Zinc chromate primer (or its modern non-chromate equivalents) is applied to bare metal before topcoat. The primer sacrificially inhibits corrosion at any scratched area. Topcoats seal the surface from moisture and oxygen.
• Sealants: Polysulfide and other aerospace sealants are applied to faying surfaces (mating structural surfaces), around fastener holes, and along seams to exclude moisture from joints where galvanic and crevice corrosion would otherwise develop.
• Dissimilar metal isolation: Zinc chromate tape, sealant, anodized aluminum washers, or cadmium-plated steel fasteners are used wherever dissimilar metals must contact each other.
• Drainage and ventilation: Aircraft structures include drain holes at low points of bays, bilges, and fairings to prevent water accumulation. AMTs must verify these remain open during maintenance.
• Corrosion inhibiting compounds (CICs): Products such as LPS-3, ACF-50, or similar water-displacing compounds are applied to internal cavities, control cables, and unpainted steel components to displace moisture and leave a protective film.

Corrosion Treatment Methods

When corrosion is discovered during inspection, the AMT must assess its severity, remove it completely, restore the protective coating, and document the work. The process follows a logical sequence.

1. Identify and assess: Determine the type, depth, and extent of corrosion. Consult the applicable manufacturer's structural repair manual (SRM) and FAA Advisory Circular AC 43.13-1B for allowable limits. Corrosion that exceeds limits requires engineering disposition before the aircraft can return to service.
2. Remove corrosion products: For aluminum, use non-metallic abrasive pads (Scotch-Brite type), aluminum wool, or aluminum oxide abrasive paper. Steel wire brushes or steel wool must never be used on aluminum because steel particles embed in the softer metal and initiate new galvanic cells. For steel structure, steel wire brushes and abrasive discs are appropriate. All corrosion products and oxides must be removed until only sound metal remains.
3. Blend smooth: After mechanical removal, any pits or rough areas should be blended to a smooth contour using fine abrasive to minimize stress concentration. The resulting depression must be within the allowable thickness limits defined in the SRM.
4. Apply conversion coating: Bare aluminum must be treated with a chemical conversion coating (such as Alodine) as soon as practicable after corrosion removal to restore the passive oxide layer and provide primer adhesion; specific timing requirements, where specified, come from the applicable SRM or process specification rather than a fixed universal interval.
5. Restore protective finish: Apply the appropriate primer (typically zinc chromate or approved equivalent) followed by topcoat to match the surrounding finish and restore environmental protection.
6. Inspect and document: Record the location, type, extent, treatment, and materials used in the aircraft maintenance records as required by 14 CFR Part 43.

Key Numbers and Rules

• Galvanic series: metals farther apart on the series produce more severe galvanic corrosion when coupled.
• AC 43.13-1B, Chapter 6 provides the primary guidance for corrosion removal and treatment on certificated aircraft not covered by a specific SRM.
• Allowable corrosion depth limits for structural members are defined by the manufacturer's SRM; typical allowances for aluminum skin are often expressed as a percentage of original thickness.
• Steel wire brushes and steel abrasives are prohibited on aluminum — only non-metallic or aluminum-compatible abrasives are acceptable.
• Conversion coatings should be applied to bare aluminum as soon as practicable after final cleaning and abrasive treatment to prevent re-oxidation and contamination; exact intervals, if specified, are set by the SRM or process specification.
• Drain holes must never be filled or covered during repairs — blocked drainage is a primary cause of hidden internal corrosion.

Common Test Traps

• Steel tools on aluminum: Many test questions ask which abrasive is acceptable for aluminum corrosion removal. Only non-metallic abrasives or aluminum-compatible materials are correct; steel wire brushes are specifically prohibited.
• Galvanic series direction: Remember that the less noble (more anodic) metal is the one that corrodes — not the more noble cathode. A common trap reverses this relationship.
• Intergranular vs. surface corrosion: Questions often describe a part that sounds fine on the surface but has reduced strength; this scenario points to intergranular or exfoliation corrosion, not simple surface etching.
• SCC conditions: Stress corrosion cracking requires both sustained tensile stress AND a corrosive environment simultaneously. Either condition alone is not sufficient.
• Documentation requirement: Corrosion treatment is a maintenance action and must be recorded in the aircraft maintenance records per 14 CFR Part 43 — failure to document is an airworthiness violation, not just an administrative oversight.