Aircraft corrosion types for the A&P General: how to tell them apart

Corrosion identification and inspection (ACS code AM.I.G.K6) was the FAA's single most-missed General code from April 2024 to March 2025, missed 77% of the time. The questions turn on a few distinctions: where each form shows up, what it looks like, and which metal corrodes when two are coupled.

Questions and answers, explained

All 12 questions above, with the correct answer and why it is correct. Everything here is on aircraft corrosion types for the a&p general: how to tell them apart.

1. Which form of corrosion is the most common, appearing as a general roughening, etching or pitting of a metal surface?

   • AExfoliation
   • BIntergranular corrosion
   • CUniform surface corrosionCorrect
   • DStress corrosion cracking

   Why: The FAA handbook calls general surface corrosion, also called uniform etch or uniform attack, the most common form. It shows as a general roughening, etching or pitting, often with a powdery deposit of corrosion products.

   Reference AM.I.G.K3 · FAA-H-8083-30B, Ch. 8 (Surface Corrosion)

2. Worm-like traces of corrosion products under a polyurethane paint film are characteristic of

   • Afiliform corrosionCorrect
   • Bfretting corrosion
   • Cgalvanic corrosion
   • Dexfoliation corrosion

   Why: Filiform corrosion is a special oxygen concentration cell under organic coatings. The handbook notes polyurethane finishes are especially susceptible, and that it forms when relative humidity is about 78 to 90 percent and the surface is slightly acidic.

   Reference AM.I.G.K3 · FAA-H-8083-30B, Ch. 8 (Filiform Corrosion)

3. According to the FAA handbook, filiform corrosion can be prevented by storing aircraft where the relative humidity is below

   • A90 percent
   • B85 percent
   • C95 percent
   • D70 percentCorrect

   Why: The handbook lists storage below 70 percent relative humidity, coatings with a low rate of oxygen and water-vapor diffusion, and washing off acidic pollutants as the ways to prevent filiform corrosion. It forms at roughly 78 to 90 percent humidity.

   Reference AM.I.G.K5 · FAA-H-8083-30B, Ch. 8 (Filiform Corrosion)

4. Exfoliation corrosion is most often found on

   • Afabric-covered control surfaces
   • Bextruded sections such as sparsCorrect
   • Ccast magnesium wheels
   • Dstainless-steel exhaust stacks

   Why: Exfoliation is an advanced form of intergranular corrosion that lifts the surface grains as corrosion products expand along the grain boundaries. The handbook says it is most often seen on extruded sections, where the grain is thinner than in rolled forms.

   Reference AM.I.G.K3 · FAA-H-8083-30B, Ch. 8 (Exfoliation Corrosion)

5. Why is intergranular corrosion in a high-strength aluminum alloy part especially dangerous?

   • AIt can exist with no visible evidence on the surfaceCorrect
   • BIt always leaves a bright red residue on the surface
   • CIt occurs only on painted surfaces
   • DIt spreads only along the rows of rivets in a skin

   Why: Intergranular corrosion attacks along the grain boundaries and may exist without visible surface evidence. The handbook notes alloys such as 2014 and 7075 are more susceptible if they were improperly heat-treated and then exposed to a corrosive environment.

   Reference AM.I.G.K6 · FAA-H-8083-30B, Ch. 8 (Intergranular Corrosion)

6. In an oxygen concentration cell under a rubber gasket, corrosion occurs at the area of

   • Ahighest oxygen concentration
   • Bhighest metal ion concentration
   • Clowest oxygen concentrationCorrect
   • Dgreatest surface stress only

   Why: Where air cannot reach the solution, under gaskets, wood or rubber, a difference in dissolved oxygen sets up a cell. The low-oxygen area becomes the anode and corrodes, which is why corrosion hides under seals and in crevices.

   Reference AM.I.G.K2 · FAA-H-8083-30B, Ch. 8 (Oxygen Concentration Cells)

7. An aluminum skin is fastened with stainless-steel screws and moisture gets between them. Which metal corrodes?

   • AThe stainless steel, because it is harder
   • BBoth corrode at the same rate
   • CNeither, because stainless steel is passive
   • DThe aluminum, because it is more anodicCorrect

   Why: In a galvanic couple the more active (anodic) metal corrodes and the more noble (cathodic) metal is protected. Aluminum sits well above passive stainless steel in the galvanic series, so the aluminum around the screws corrodes. An insulating barrier prevents it.

   Reference AM.I.G.K12 · FAA-H-8083-30B, Ch. 8 (Galvanic Corrosion; Figure 8-10)

8. Of the following metals, which is the most anodic (most likely to corrode) in the galvanic series?

   • ATitanium
   • BMagnesiumCorrect
   • CCadmium
   • DNickel

   Why: Magnesium is at the active end of the galvanic series shown in the FAA handbook, ahead of zinc, aluminum and cadmium. Magnesium parts corrode quickly and need prompt, careful treatment. Titanium and passive nickel are near the noble end.

   Reference AM.I.G.K12 · FAA-H-8083-30B, Ch. 8 (Galvanic Series)

9. Why should steel wool or a steel wire brush never be used to clean corrosion from aluminum?

   • AIt leaves the surface too smooth for paint
   • BIt magnetizes the aluminum
   • CEmbedded steel particles cause further corrosionCorrect
   • DSteel tools are too soft to remove the corrosion

   Why: The handbook warns that steel wool or steel wire brushes on aluminum or magnesium can force small pieces of steel into the metal, setting up dissimilar-metal corrosion and ruining the surface. Abrasive pads used on one metal should not be reused on another.

   Reference AM.I.G.K7 · FAA-H-8083-30B, Ch. 8 (Galvanic Corrosion; Corrosion Removal)

10. How does shot peening increase a part's resistance to stress corrosion cracking?

    • AIt removes all the oxide film from the surface
    • BIt creates compressive stresses in the surfaceCorrect
    • CIt plates the surface with cadmium
    • DIt anneals the metal so it is softer

    Why: Stress corrosion cracking needs sustained tensile stress. Shot peening leaves compressive stress in the surface layer, which applied tension must overcome before the surface is in tension at all, so the threshold stress level rises.

    Reference AM.I.G.K5 · FAA-H-8083-30B, Ch. 8 (Stress Corrosion Cracking)

11. What is the main way to prevent galvanic corrosion where dissimilar metals must be joined?

    • ATighten the fasteners to a higher torque
    • BUse a fastener of the more anodic metal
    • CWash the joint with fresh water weekly
    • DPlace an insulating barrier between the metalsCorrect

    Why: Galvanic corrosion needs an electrical path between the metals and an electrolyte. Separating them with a barrier such as sealant, primer, tape or an insulating washer breaks the circuit. The handbook notes eliminating any one of the conditions stops electrochemical corrosion.

    Reference AM.I.G.K12 · FAA-H-8083-30B, Ch. 8 (Electrochemical Attack)

12. Which condition is NOT required for electrochemical corrosion to occur?

    • AA metal that corrodes (anode)
    • BA continuous liquid path (electrolyte)
    • CAn electrical path between anode and cathode
    • DA temperature above 100 °FCorrect

    Why: The handbook's simplified corrosion cell needs an anode, a cathode, a conductor between them and a continuous electrolyte. Remove any one and electrochemical corrosion stops. Heat speeds corrosion but is not a requirement.

    Reference AM.I.G.K2 · FAA-H-8083-30B, Ch. 8 (Electrochemical Attack; Simplified Corrosion Cell)

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