Cleaning & Corrosion Control
Why must rust be removed completely from a steel part, whereas the smooth surface oxidation that forms naturally on bare aluminum is not considered harmful?
Answer
Rust is porous and holds moisture against the steel, while aluminum oxide is a tight, protective film
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 natural oxide is a thin, tightly adhering film that slows further attack. The difference is porosity and moisture retention, not electrical conductivity (aluminum oxide is itself a poor conductor) or added weight. Do not confuse that smooth oxide with white or gray powdery deposits on aluminum, which are corrosion products and must be removed.
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 adds significant weight to the structure, while aluminum oxide is essentially weightless
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