Corrosion is one of the most persistent and dangerous threats to aircraft structural integrity. Unlike a dramatic in-flight failure, corrosion works slowly and silently, often hidden beneath paint, sealants, or tight-fitting assemblies. Left undetected or untreated, it can degrade load-bearing members to a fraction of their original strength. For Aviation Maintenance Technicians (AMTs), the ability to recognize every type of corrosion, understand its electrochemical roots, and assess its severity is not just a test requirement — it is a fundamental airworthiness responsibility that directly affects flight safety.
The FAA addresses corrosion control extensively in the Aviation Maintenance Technician Handbook — General (FAA-H-8083-30). This article works through each major corrosion type, explains how and why each forms, describes what it looks like in the field, and highlights the practical and exam-relevant points every AMT candidate must know.
The Electrochemical Basis of Corrosion
Nearly all corrosion is an electrochemical process. When two dissimilar metals are in contact in the presence of an electrolyte — any moisture that contains dissolved salts, acids, or other ions — electrons flow from the more chemically active (anodic) metal to the less active (cathodic) metal. The anodic metal is consumed in this process, converting the base metal into oxides, hydroxides, or salts that are structurally weak and often powdery or flaky. Even a single metal can corrode if different areas of its surface have different electrical potentials due to stress, grain structure variations, or surface contamination.
Four conditions must be present simultaneously for electrochemical corrosion to occur: an anode, a cathode, a metallic conductor connecting them, and an electrolyte. Remove any one of those four elements and the corrosion process stops. This principle drives every protective strategy — coatings, sealants, anodizing, and proper dissimilar-metal separation all work by eliminating one of these conditions.
Types of Corrosion and Their Identification
1. Surface (Uniform) Corrosion
Surface corrosion is the most straightforward type. It results from direct chemical or electrochemical attack across a broad, relatively uniform area of exposed metal. On steel, it appears as reddish-brown rust. On aluminum alloys — by far the most common aircraft structural material — it produces a gray or white powdery deposit and a dulling or pitting of the surface. Bare magnesium shows a white or grayish-white powdery deposit, sometimes with a snow-like appearance. Surface corrosion is typically the easiest to detect visually and, when caught early, is the most amenable to mechanical removal and surface treatment.
2. Dissimilar Metal (Galvanic) Corrosion
Galvanic corrosion occurs when two metals with different electrochemical potentials are in direct contact in the presence of an electrolyte. The more active metal in the galvanic series becomes the anode and is attacked. A classic aircraft example is aluminum in contact with steel fasteners in a wet environment — the aluminum around the fastener corrodes preferentially. Another common scenario is cadmium-plated steel hardware contacting aluminum structure. The galvanic series ranks metals from most active (anodic, like magnesium) to least active (cathodic, like gold). The further apart two metals are in this series, the more severe the galvanic attack. AMTs must use the correct fastener materials specified in maintenance manuals, use sealants and insulating washers to isolate dissimilar metals, and never substitute hardware without engineering authorization.
3. Concentration Cell (Crevice) Corrosion
Concentration cell corrosion develops where moisture is trapped in tight spaces — lap joints, under gaskets, beneath bolt heads, or between mating surfaces. Even a single metal can suffer this attack when different areas of its surface are exposed to electrolytes of different oxygen or ion concentrations. The area with lower oxygen concentration becomes anodic and corrodes. This type is treacherous because it is hidden by design: the very geometry that traps moisture also hides the damage. Inspecting lap joints in sheet metal skin, checking under sealant beads, and probing around rivet heads are critical habits for catching concentration cell corrosion before it becomes structural.
4. Intergranular Corrosion
Intergranular corrosion attacks the boundaries between the individual crystalline grains of a metal alloy rather than the surface. It is particularly dangerous in certain aluminum alloys (especially the 2000 and 7000 series) and stainless steels. In aluminum, the grain boundaries may be depleted of alloying elements during heat treatment, making them anodic relative to the grain interior. The surface of an affected part can look nearly normal while the interior is being eaten away along grain boundaries, dramatically reducing strength. Advanced intergranular corrosion in aluminum extrusions and forgings can produce a characteristic layered or flaking appearance called exfoliation corrosion — the metal literally peels apart in layers parallel to the surface. Exfoliation is a severe, visually alarming form of intergranular corrosion and typically requires part removal and replacement rather than simple cleaning.
5. Pitting Corrosion
Pitting corrosion creates small, localized cavities or pits in an otherwise relatively undamaged surface. It is common on aluminum, magnesium, and stainless steel. Pitting is initiated by local breakdown of the protective oxide layer, often triggered by surface contamination such as chloride ions from salt air or industrial pollutants. The pits themselves may be small and hard to see, but they act as stress concentration points under fatigue loading. A single small pit can be the initiation site for a fatigue crack that propagates to catastrophic failure under repeated flight loads. Pitting is often assessed with a magnifying glass and, when found, requires careful evaluation against allowable damage limits in the applicable structural repair manual.
6. Filiform Corrosion
Filiform corrosion appears as a network of fine, threadlike or worm-track lines running beneath a painted or coated surface. It is most common on aluminum and magnesium, and it is almost always associated with breakdown or inadequate application of a surface coating. Moisture and oxygen penetrate a defect in the coating, and the corrosion spreads laterally under the paint, creating the characteristic thread-like pattern. Filiform corrosion is primarily a cosmetic and early-warning concern — the underlying metal damage is usually shallow — but it signals coating system failure and must be treated before more serious attack develops.
7. Fretting Corrosion
Fretting corrosion is unique because it results from the combination of mechanical wear and electrochemical attack. It occurs when two surfaces in contact experience very slight, repetitive relative motion — typically micro-vibration during flight. The motion disrupts protective oxide films, exposing fresh metal that immediately re-oxidizes; the debris (fine metal oxide particles, often reddish-brown on steel or black on aluminum) accumulates in the joint and acts as an abrasive, accelerating wear. Common locations include bolt holes, spline couplings, and bearing seats. Fretting produces a roughened, pitted, and stained mating surface. Because fretting often occurs inside assemblies, it may only be discovered during scheduled disassembly inspections.
8. Stress Corrosion Cracking
Stress corrosion cracking (SCC) results from the simultaneous combination of sustained tensile stress and a corrosive environment. The stress may be applied (operational loads) or residual (from forming, welding, or improper heat treatment). SCC can cause sudden, brittle fracture of a part that appears otherwise undamaged. High-strength aluminum alloys, certain stainless steels, and titanium alloys can be susceptible. Cracks propagate along grain boundaries or through grains, often with branching patterns visible under magnification. SCC is particularly insidious because the part may carry normal operational loads for months before abrupt failure with little visible warning.
Why Corrosion Identification Matters
Correct identification drives correct treatment. Surface corrosion on aluminum may require only light mechanical abrasion and reapplication of chemical conversion coating. Intergranular or exfoliation corrosion in a primary structure spar or longeron may mandate part replacement with an FAA-approved repair scheme. Misidentifying one type as another — for example, treating exfoliation as simple surface corrosion — risks returning a structurally compromised part to service. Additionally, 14 CFR Part 43 and aircraft maintenance manuals establish the technician's legal obligation to return aircraft to an airworthy condition; inadequate corrosion treatment is a direct violation of that standard.
Key Numbers and Rules
- Galvanic series: Magnesium is the most anodic (most easily corroded); gold is among the most cathodic. The further apart two metals in the series, the greater the galvanic risk.
- Allowable damage limits: Always consult the Structural Repair Manual (SRM) or airframe manufacturer's data — there is no universal percentage; limits vary by location, material, and part criticality.
- Chemical conversion coating (Alodine/chromate treatment): Provides corrosion protection and promotes paint adhesion on aluminum; required after any mechanical removal of corrosion that disturbs the anodized or conversion coating.
- Exfoliation: Classified as a severe form of intergranular corrosion — laminar, layer-by-layer separation of material — and nearly always requires replacement of the affected structural member.
- Fretting identification clue: Reddish-brown powder (iron oxide) at a steel joint or black powder at an aluminum joint, combined with surface pitting at the mating surfaces.
- SCC susceptibility: High-strength, high-zinc aluminum alloys such as 7075 are generally more susceptible to stress corrosion cracking than alloys like 2024, though susceptibility depends heavily on temper — for example, 7075-T73 is far more SCC-resistant than 7075-T6.
Common Test Traps
- Exfoliation vs. surface corrosion: The FAA exam expects you to know that exfoliation is a severe, layered form of intergranular corrosion — not just heavy surface corrosion. Parts showing exfoliation in primary structure are typically non-repairable by cleaning alone.
- Galvanic series confusion: Students often confuse which metal corrodes. Remember: the anodic (more active, higher on the galvanic series) metal is attacked, not the cathodic one. Aluminum corrodes next to steel, not the other way around.
- Fretting is not pure corrosion: Fretting corrosion requires both mechanical motion and a corrosive environment. If the question emphasizes micro-vibration and debris at a joint, fretting is the answer — not galvanic or crevice corrosion.
- Concentration cell corrosion does not require dissimilar metals: It can occur with a single metal when different portions of the surface are exposed to electrolytes of different compositions or oxygen concentrations — a point the written test exploits.
- Magnesium identification: Magnesium corrodes to a white, chalky deposit similar in appearance to early aluminum corrosion, but magnesium is far more reactive and requires special care — never use steel wool on magnesium, as embedded iron particles will accelerate galvanic attack.