Cleaning, Corrosion and Finishes
14% of the exam 2 min
Fourteen percent of the weighting, home of the FAA's most-missed General code from April 2024 to March 2025 (corrosion identification and inspection, missed 77% of the time). How corrosion works, its forms, galvanic action, treatment, coatings and finish defects.
How corrosion works
Electrochemical corrosion needs four things: a metal that corrodes (the anode), a cathode, an electrical path between them and a continuous electrolyte. Remove any one and it stops. In a galvanic couple the more anodic metal corrodes and the more noble one is protected; magnesium sits at the active end of the galvanic series, and aluminum corrodes when fastened to passive stainless steel. Concentration cells corrode where oxygen, or metal ion concentration, is lowest, which is why corrosion hides under gaskets, wood, rubber and faying surfaces. Direct chemical attack comes from agents such as spilled battery acid, flux residue and trapped caustic cleaners.
The forms you must tell apart
Uniform surface corrosion is the most common form. Filiform leaves worm-like traces under paint, especially polyurethane, and forms at roughly 78 to 90 percent humidity; storage below 70 percent prevents it. Intergranular corrosion attacks grain boundaries, especially in 2014 and 7075 alloys that were improperly heat-treated, and may show nothing on the surface. Exfoliation is advanced intergranular corrosion that lifts the surface grains, most often on extruded sections such as spars. Stress corrosion cracking needs sustained tensile stress; shot peening raises resistance by leaving compressive stress in the surface.
Removal, treatment and protection
Treatment follows one sequence: remove as much of the corrosion products as practicable, neutralize residue in pits and crevices, restore protective surface films, then apply temporary or permanent coatings or paint. In critical, highly stressed areas the remaining pits are blended out to prevent stress risers. Never use steel wool or steel wire brushes on aluminum or magnesium; embedded steel starts new corrosion, and abrasive pads are not moved from one metal to another. Alclad sheet has pure aluminum layers rolled onto an alloy core; anodizing grows a hard oxide film electrolytically and is easily damaged by severe abrasives. Piano hinges are serviced with water-displacing lubricants.
Finishes and markings
Fisheyes are small craters where paint will not wet a contaminated spot, typically silicone; a clean surface, checked with a water break test, prevents them. Blushing is the cloudy, flat look of dope that dried in high humidity; a retarder such as butyl alcohol slows drying and prevents it. Paint adds weight aft of control surface hinge lines, so after repainting the balance of the control surfaces must be checked. Nationality and registration marks on a standard-category fixed-wing airplane must be at least 12 inches high (14 CFR 45.29).
Where people lose points
✗ "Corrosion starts where the most oxygen reaches the metal."
✓ In an oxygen concentration cell, it is the LOW-oxygen area that corrodes.
✗ "The harder metal in a galvanic couple corrodes."
✓ Hardness is irrelevant. The more anodic metal corrodes.
✗ "Leave pits in a stressed part alone so it stays thick."
✓ Pits are stress risers. In critical areas they are blended out within limits.
✗ "No surface evidence means no intergranular corrosion."
✓ It can exist with no visible sign; that is what makes it dangerous.
✗ "Fisheyes come from painting too thin."
✓ They come from contamination such as silicone.
Numbers to memorize
| Most common form | Uniform surface corrosion |
| Filiform humidity range | About 78-90%; store below 70% |
| Exfoliation | Most often on extruded sections |
| Most anodic common metal | Magnesium |
| Concentration cell corrodes at | Lowest oxygen / metal ion concentration |
| Treatment order | Remove, neutralize, restore film, coat |
| Shot peening | Compressive surface stress resists stress corrosion |
| Fisheyes | Contamination such as silicone |
| Blushing | High humidity; butyl alcohol retarder |
| Registration marks, fixed-wing | At least 12 inches high (45.29) |
Test yourself
No answers here on purpose — retrieving them is the practice. Drill this domain if any of these stall you.
- Name the four conditions for electrochemical corrosion.
- Describe filiform corrosion and how to prevent it.
- Explain why steel wool is banned on aluminum.
- Compare Alclad and anodizing as protection for aluminum.
- Say what must be checked after repainting a control surface, and why.