Why Corrosion Is a Certification Topic, Not a Detail

A corroded spar cap or a wing attach fitting can reduce a structure below its design ultimate load without any visible deformation, which is why 14 CFR 43.13(a) requires every repair to restore the original strength and why corrosion removal is treated as a repair, not as cleaning. The two documents you are expected to know are AC 43.13-1B (Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair) and AC 43-4B (Corrosion Control for Aircraft), which replaced AC 43-4A. The FAA-H-8083-31 Airframe handbook covers the mechanisms and the Airframe Airman Certification Standards list corrosion identification and treatment under the structural knowledge elements of AM.II.

The exam tests three things: can you name the corrosion type from a description or a photograph, can you say where it is likely to be found, and do you know the accepted order of operations for removing and treating it. Most candidates lose points on the third question.

The Corrosion Types You Must Recognize

Type	What it looks like	Where it starts
Surface / uniform	Even dulling, etching or powder over a broad area	Exposed skins, unpainted panels, high-humidity environments
Pitting	Pinpoint craters, white or gray deposits in the bottom	Bare aluminum with a broken oxide film, salt spray, acid spills
Intergranular	Blistered or lifted grain structure, subsurface attack along grain boundaries	2024 and 7075 alloys, heat-affected zones, exfoliation edge
Exfoliation	Flaking, layered leaf-like swelling that looks puffy	Extruded stringers, spar caps, thick rolled plate of high-strength alloy
Galvanic	Corrosion concentrated on the less noble metal of a pair	Where dissimilar metals touch with an electrolyte present
Filiform	Thread-like filaments that tunnel under the paint film	Painted surfaces at 60 to 95 percent relative humidity
Fretting	Dark powder and shallow pitting at close-tolerance joints	Vibrating riveted joints, bolted lugs, bearing housings
Crevice	Attack in gaps where stagnant electrolyte is trapped	Faying surfaces, under gaskets, around fasteners
Stress corrosion cracking	Branched cracking that crosses grains, often with no warning	High-strength 7075 and magnesium parts under sustained tensile load

Where Airframes Corrode First

The formula has not changed in fifty years: moisture plus oxygen plus a susceptible alloy plus a broken protective coating. That combination is predictable, so the exam expects you to know the classic locations.

Battery compartments and vent paths — electrolyte runoff attacks aluminum aggressively and spreads below the box floor where it lands.
Lavatories and galleys — spilled liquids and urine create alkaline conditions that dissolve the naturally protective aluminum oxide film.
Wheel wells and flap recesses — water, mud and de-icing fluid are thrown into enclosed bays where air movement is poor.
Exhaust and engine aft areas — heat degrades coatings, and exhaust deposits hold moisture against the skin.
Magnesium components — gearbox housings, brackets and castings are the most reactive metals on the aircraft. Magnesium corrosion appears as white powder and can be catastrophic if untreated.
Lap joints, faying surfaces and fastener heads — crevice corrosion begins where the sealant or primer has failed.
Under seaplane and float-equipped aircraft — continuous salt exposure turns every scratch in a coating into an initiation site.

Inspection: Finding What You Cannot See

Visual inspection with good light and a 10-power magnifier finds most surface and pitting corrosion, and a borescope is the only way to look inside pressure boxes, tail cones, and box beam interiors. But intergranular and exfoliation corrosion frequently hide behind intact paint, which is where non-destructive testing earns its place: eddy current inspection detects subsurface corrosion and thinning in aluminum structure without removing the surface, ultrasonic inspection measures remaining thickness in thick sections, and dye penetrant or fluorescent penetrant reveals surface-breaking cracks near corrosion sites. On high-strength steel parts, magnetic particle inspection is the method of choice.

Two practical rules are tested repeatedly. First, when corrosion is found, look for the source: an unsealed fastener, a leaking drain, a failed seal or an improperly reinstalled gasket. Removing the corrosion without fixing the cause means it comes back. Second, remove and inspect the parts that touch the corroded area, because galvanic attack damages both members of a couple.

Removal and Treatment: The Correct Sequence

The accepted flow under AC 43.13-1B and AC 43-4B is straightforward, and the exam tests whether you apply the steps in order.

1. Clean and neutralize. Remove dirt, salt and any electrolyte source first. Alkaline spills are neutralized with a mild acid solution, acid spills with a mild alkaline solution, then the area is flushed with clean water and dried. Cleaning before removal prevents spreading contaminants into fresh metal.
2. Remove the corrosion. Mechanical removal is the standard method — abrasive paper or a fine stainless steel wire brush used in a direction that will not damage the surrounding coating edge. Chemical removal is used where abrasive work would damage the structure, and removal stops when the alloy surface is bright. Never remove more material than the Structural Repair Manual permits; SRMs publish allowable limits, and exceeding them turns a corrosion repair into a major structural repair.
3. Restore the protective film. Bare aluminum must be treated immediately, usually with a chemical conversion coating. Chromate conversion coatings under MIL-DTL-5541 are the traditional finish, with Type I covering chromate conversion compounds and Type II covering the non-hexavalent alternatives.
4. Prime and topcoat. Conversion coating alone is not an exterior finish. The area gets an epoxy primer, typically to MIL-PRF-23377 or the water-reducing MIL-PRF-85582, then the exterior topcoat. Faying surfaces and joints that are reassembled are sealed, and fastener holes are treated before the fastener goes in.
5. Protect dissimilar metal joints. Where two different metals must touch, the joint is separated with primer, sealant or a gasket, or the fastener is chosen so the galvanic couple is small.
6. Record it. Every corrosion repair is a maintenance record entry under 14 CFR 43.9, including a description of the work, the date, and the signature and certificate number of the person who approved it for return to service.

Prevention Is a Maintenance Habit

Corrosion control programs work because they are routine: frequent washing to remove salt and pollutants, keeping drains and vents clear, touching up paint breaks before the oxide film is breached, applying corrosion-inhibiting compounds to structure in humid or marine environments, and inspecting the known trouble spots at every phase inspection rather than waiting for a scheduled heavy check. An aircraft in a coastal or tropical environment corrodes years faster than the same aircraft in a dry climate, and its inspection intervals should reflect that.

How to Drill Corrosion for the Exam

Corrosion questions reward pattern recognition more than memorization. Practice identifying types from descriptions, then practice the treatment sequence as a chain rather than as separate facts: clean, remove, treated with conversion coating, prime, topcoat, seal, record. SkyLicense includes corrosion questions across the General and Airframe banks with references to the specific AC or handbook section, so each miss teaches you where the source material lives.

For the wider airframe syllabus, pair this article with our FAA A&P study guide and our NDI/NDT guide for A&P mechanics, then test yourself against the Airframe practice test.

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