Cleaning & Corrosion Control
Why must rust be removed completely from a steel part, whereas the gray-white oxide that forms on bare aluminum is far less urgent to remove?
Answer
Rust is a porous, non-protective oxide that holds moisture against the metal and promotes further corrosion, while aluminum oxide forms a tight, adherent film that tends to protect the metal beneath it
Iron oxide (rust) is loose and porous, retains moisture against the base metal, and keeps the corrosion reaction alive, so it must be removed entirely; aluminum's oxide is a thin, tightly adhering film that slows further attack. The distractors misstate the mechanism—the key difference is porosity and moisture retention, not electrical conductivity, temperature dependence, or added weight.
Source: FAA-H-8083-30B AMT Handbook—General, Ch. 'Cleaning and Corrosion Control' (ferrous corrosion / rust); AC 43.13-1B, Ch. 6
Not the answer
- Rust is electrically nonconductive and blocks bonding, while aluminum oxide conducts current normally
- Rust forms only at high temperature and spreads through the part, while aluminum oxide is unaffected by heat
- Rust adds significant weight to the structure, while aluminum oxide is essentially weightless
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