Cleaning and corrosion control

Corrosion is metal turning back into a compound like an oxide. It needs an electrolyte (water, salt spray, battery acid, sweat) and a difference in electrical potential, either between two metals or between parts of one surface. The more anodic metal is the one that corrodes. From most anodic to least, the common ones run: magnesium, zinc, cadmium, aluminum, steel, then stainless steel, titanium and gold.

Kinds of corrosion

• Uniform (surface): a dull, rough, powdery area. On aluminum it's a white or gray powder; on steel, red rust.
• Pitting: small deep holes, worse than it looks.
• Galvanic: two different metals touching with an electrolyte, like a steel screw in aluminum skin. Separated with sealant, primer or a barrier tape.
• Intergranular: along the grain boundaries inside the metal, hard to see. Its severe form, exfoliation, lifts the metal in layers.
• Filiform: worm-like threads under paint, often from poor surface preparation.
• Fretting: two parts rubbing slightly under load, leaving dark powder.
• Stress corrosion cracking: cracks where a part under steady tension meets a corrosive environment.

Look hardest where water and chemicals collect: battery boxes, lavatories and galleys, wheel wells, bilges under floors, engine exhaust trails, and around fasteners and seams.

Treatment

Clean the area, then remove all the corrosion with the right tool for the metal: on aluminum, aluminum wool, nylon pads or approved abrasives, never steel wool or a steel brush, which leave steel particles that start new galvanic corrosion. Measure what's left against the limits in the structural repair manual, then treat the bare aluminum with a chemical conversion coating (like Alodine), which helps it resist corrosion and makes paint stick, and prime and paint.

Cleaning has its own hazards: some acid cleaners can cause hydrogen embrittlement in high-strength steel, so use only the cleaners the manual approves.