Prolonged protection, by zirconium conversion coatings, of AlSi7Mg0.3 aluminium alloy in chloride solution

Zirconium conversion coatings (ZrCC) were prepared on aluminium alloy AlSi7Mg0.3 by immersion of the latter in 100-500 ppm baths of H2ZrF6. Their electrochemical characterization was carried out while immersed in 0.5 M NaCl for up to 10 days. Immersion tests were performed for up to 28 days. Detailed microstructure characterization of the coatings was carried out prior to and following immersion. The ZrCC-200 coating had a bi-layer structure that was thicker round the intermetallic particles (130-150 nm) than at the matrix (50-60 nm). The corrosion resistance of the ZrCC-200 coating improved during immersion, which was ascribed to transformation of ZrF4/ZrOxFy to ZrO2 center dot xH(2)O((s)) and to formation of Al(OH)(3) in the outermost part of the coating.