Corrosion is one of the most persistent and dangerous enemies of aircraft structure. Unlike a visible crack or dent, corrosion can lurk beneath paint, inside crevices, and deep within metal grain boundaries — silently destroying the strength of primary structure while the aircraft looks perfectly airworthy from a distance. For the Aviation Maintenance Technician (AMT), understanding exactly what corrosion looks like, where it tends to form, and why it develops is not just exam knowledge — it is a critical safety skill exercised at every inspection.
The FAA's Aviation Maintenance Technician Handbook — General (FAA-H-8083-30) dedicates significant attention to corrosion because its consequences range from cosmetic blemishes to catastrophic structural failure. This article walks through each major type of corrosion recognized in FAA guidance, explaining the electrochemical principles behind each, the visual clues that distinguish one from another, and the aircraft areas where each is most likely to be found.
The Electrochemical Foundation
Almost all metallic corrosion is electrochemical in nature. For corrosion to occur, four elements must be present simultaneously: an anode (the metal that is attacked and oxidized), a cathode (a dissimilar or less active metal that is protected), an electrolyte (a conductive fluid — often moisture with dissolved salts or acids), and a metallic path connecting anode to cathode. Remove any one of these four elements and the corrosion cell cannot function. This is the underlying logic behind every corrosion-prevention strategy an AMT employs, from anodizing aluminum to applying sealant in lap joints.
Types of Corrosion and How to Identify Them
Surface (Uniform) Corrosion
Surface corrosion is the most straightforward type: the metal is attacked more or less uniformly across an exposed area. On aluminum alloys, it typically appears as a gray or white powdery deposit — aluminum oxide — that dulls the naturally bright surface. On steel, it produces the familiar reddish-brown iron oxide (rust). Surface corrosion usually begins when protective coatings fail, allowing moisture and oxygen to reach bare metal. Because the attack is spread over a large area, material loss per unit area is relatively slow, but if ignored, pitting begins and the damage accelerates. Surface corrosion is the easiest type to detect and, when caught early, the easiest to treat.
Pitting Corrosion
Pitting corrosion is a localized, aggressive form of attack in which small pits or craters form on the metal surface. It is particularly common on aluminum and stainless steels and is especially dangerous because the visible surface pit may be only a fraction of the total damage — the corrosion can undercut beneath the surface far more than the visible opening suggests. Pits often appear as small white or gray dots on aluminum with powdery residue around them. On steel, pitting shows as small rust-filled craters. Because pits concentrate stress, they are prime initiation sites for fatigue cracks, making pitting corrosion a serious structural concern even when the pits appear small.
Dissimilar Metal Corrosion (Galvanic Corrosion)
Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence of an electrolyte. Every metal has a different electrochemical potential; when two metals with significantly different potentials are coupled, the more active (anodic) metal corrodes preferentially while the less active (cathodic) metal is protected. A classic aircraft example is an aluminum structure fastened with steel or copper hardware without proper insulation — the aluminum corrodes at the joint. FAA-H-8083-30 references a general galvanic series table that lists metals from most active (anodic, least noble — such as magnesium) to least active (cathodic, most noble — such as gold and platinum); exact ordering can vary somewhat by reference source. The farther apart two metals are on this series, the more severe the galvanic attack will be. Identification: corrosion concentrated at and around fastener holes, along bond lines, or at interfaces between different metals, often with white or gray powdery buildup on the aluminum side.
Concentration Cell Corrosion (Crevice Corrosion)
Concentration cell corrosion develops when the same metal is exposed to different concentrations of oxygen or ions in the electrolyte — typically in a tight crevice or under a deposit where oxygen cannot freely circulate. The area with less oxygen becomes anodic and corrodes. This is why lap joints, under fastener heads, beneath paint blisters, under gaskets, and inside control cable conduits are prime corrosion locations. An AMT will often see surface paint that appears sound but when probed or removed reveals significant metal loss underneath. Active corrosion under paint frequently manifests as paint bubbling or blistering. A related but distinct form, filiform corrosion, produces thread-like, worm-track patterns visible under the topcoat and is generally treated in FAA guidance as its own corrosion type, driven by high humidity and thin-film breakdown, though it shares the underlying oxygen-concentration-cell mechanism.
Intergranular Corrosion
Intergranular corrosion attacks the grain boundaries of a metal rather than the grain faces. It is particularly treacherous because the metal surface can look entirely normal while the internal structure is severely compromised — the metal may literally crumble when stressed. It is most commonly associated with certain aluminum alloys (especially those improperly heat-treated or sensitized) and some stainless steels. In advanced cases, intergranular corrosion in aluminum causes exfoliation: layers of metal peel or flake away in a leafing or blistering pattern, exposing fresh layers to further attack. Exfoliation is a late-stage visual indicator; by that point, significant structural strength has already been lost. Detection in early stages may require eddy current or other NDT methods because visual inspection alone is insufficient.
Stress Corrosion Cracking
Stress corrosion cracking (SCC) is the result of the combined and simultaneous action of tensile stress (either applied load or residual stress from manufacturing) and a corrosive environment. The cracks propagate along grain boundaries or through grains depending on the alloy and environment, and they can advance rapidly — often with little or no visible corrosion product to warn the inspector. High-strength aluminum alloys, certain stainless steels, and titanium alloys are susceptible. SCC cracks tend to branch and follow irregular paths. Identification is difficult visually; dye penetrant or other NDT methods are typically required for confirmation. High-stress areas such as wheel well fittings, landing gear attach points, and engine mounts deserve particular vigilance.
Fretting Corrosion
Fretting corrosion occurs when two surfaces in contact experience slight, repetitive relative motion (micro-slip) in the presence of oxygen. The motion abrades the protective oxide layer, exposing fresh metal that immediately re-oxidizes, producing fine, reddish-brown or dark oxide powder (on steel) or grayish-black powder (on aluminum). Over time, pitting and surface damage accumulate, and the resulting surface roughness can initiate fatigue cracks. It is commonly found between closely fitted parts that vibrate: under bolt heads, between splines, in press-fit assemblies, and between control cable and fairlead contact points. The tell-tale sign is a ring or patch of dark powdery oxide around a joint that should be stationary.
Why Correct Identification Matters
Each corrosion type demands a specific treatment approach. Surface corrosion on aluminum may be removable by mechanical cleaning and chemical treatment followed by reapplication of primer and topcoat. Intergranular or stress corrosion cracking, however, may require component replacement because no surface treatment restores lost grain-boundary integrity. Misidentifying exfoliation as minor surface corrosion and simply sanding it smooth is a dangerous error — the underlying structure may be unairworthy. Similarly, treating galvanic corrosion without correcting the dissimilar-metal contact (by applying proper insulating coatings, using compatible fasteners, or installing isolation tape) ensures the corrosion will return.
Key Numbers and Rules
- Four elements of a corrosion cell: anode, cathode, electrolyte, and metallic path — eliminate one to stop corrosion.
- Galvanic series: magnesium is near the most anodic (most susceptible) end; platinum and gold are near the most cathodic (most protected) end. The greater the separation in the series, the more severe the galvanic attack.
- Exfoliation is a late-stage visual sign of intergranular corrosion; by the time it is visible, structural integrity is already seriously diminished.
- Fretting corrosion signature: reddish or grayish powdery oxide at a joint that should not be moving.
- Stress corrosion cracking often has no obvious corrosion product visible — NDT methods are required for reliable detection.
- Concentration cell corrosion is a primary type found in lap joints, under fastener heads, and beneath paint blisters; filiform corrosion is a related but distinct thread-like pattern often discussed separately.
Common Test Traps
- Galvanic vs. concentration cell confusion: Galvanic corrosion requires two different metals; concentration cell corrosion involves the same metal exposed to different electrolyte concentrations. The FAA exam tests whether you know the distinction.
- Exfoliation identification: Exam questions may describe a layered, blistering, or leafing appearance on aluminum — the correct answer is intergranular/exfoliation corrosion, not surface corrosion.
- Fretting vs. wear: Fretting corrosion involves oxidation of abraded particles and produces oxide powder; simple mechanical wear does not necessarily involve a corrosive chemical reaction. The combination of micro-motion plus oxidation is the defining characteristic.
- Stress corrosion cracking appearance: A common trap is assuming SCC will look like obvious corrosion. It often does not — the surface may appear clean while cracks run deep. The answer is that visual inspection alone is insufficient; NDT is required.
- Paint blistering as a corrosion sign: The exam may describe bubbling or filiform paint. This can indicate active concentration cell (crevice) or filiform corrosion underneath, but blistering can also result from simple moisture entrapment or paint adhesion failure without active corrosion — the correct approach is always to investigate before assuming corrosion is present and repainting over it.