Chromate-free corrosion resistant conversion coatings for aluminum alloys

Inorganic polycrystalline hydrotalcite, Li₂[Al₂(OH)₆]₂·CO₃·3H₂O, coatings can be formed on aluminum and aluminum alloys by exposure to alkaline lithium carbonate solutions. This process is conducted using methods similar to traditional chromate conversion coating procedures, but does not use or produce toxic chemicals. The coating provides anodic protection and delays the onset of pitting during anodic polarization. Cathodic reactions are also inhibited which may also contribute to corrosion protection. Recent studies have shown that corrosion resistance can be increased by sealing hydrotalcite coated surfaces to transition metal salt solutions including Ce(NO₃)₃, KMnO₄ and Li₂MoO₄. Results from these studies are also reported.