Q1: How does aluminum's natural oxide layer protect it?
Upon air exposure, aluminum forms a 2-10nm amorphous Al₂O₃ layer. This barrier self-repairs if scratched, preventing further oxidation. Unlike iron oxide (rust), aluminum oxide is dense and adherent. Protection works in pH 4-9 environments.
Q2: When does galvanic corrosion occur?
When aluminum contacts more noble metals (e.g., copper/steel) in electrolytes (e.g., rainwater), it corrodes sacrificially. Prevention: Use insulating gaskets, apply sealants, or select compatible metals like stainless steel. Coatings isolate dissimilar metals.
Q3: How does alloy choice affect corrosion resistance?
Pure aluminum (1xxx) resists corrosion best. Copper-containing alloys (2xxx) are vulnerable. Magnesium alloys (5xxx) excel in marine settings. Avoid pairing 7xxx alloys with carbon steel. Silicon-rich alloys (4xxx) resist acidic environments.
Q4: What is pitting corrosion and how is it managed?
Chlorides (e.g., de-icing salts) penetrate oxide layers, creating microscopic pits. Control methods: Alloy selection (e.g., 5052 over 2024), thicker anodizing (≥25μm), cathodic protection, or protective coatings like chromate conversion.
Q5: How does stress corrosion cracking (SCC) develop?
SCC occurs in high-strength alloys (e.g., 7075) under tensile stress in corrosive environments. Preventing SCC involves:
Using SCC-resistant tempers (e.g., T73 instead of T6)
Reducing residual stresses via heat treatment
Avoiding designs with sustained stress concentrations
Applying protective coatings
