Abstract
AlSi12Cu1(Fe) alloy was subjected to the conversion bath containing: Cr2(SO4)3, HF and 200–800 ppm Zr (as H2ZrF6). Coatings composed mostly of ZrO2 and to a smaller extent by Cr(OH)3 with granular morphology were obtained. Concentration of Zr in the conversion bath did not affect the coatings thickness, which was non-uniform and ranged from 35 to 90 nm. Cracking and detachment were observed, which are due to the presence of possible phases like: Si, Cu0.01Al0.97Si0.02, FeAl3Si2 and Al2Cu, except the α–Al phase. These Cu- and Fe-rich intermetallic particles serve as the cathodic areas that are responsible for the formation of micrometric sizes numerous spheroids in the coating due to intensive hydrogen evolution. Longer protective effect, up to 240 hrs in a neutral salt spray and the lowest corrosion currents after 5 days in 0.05 M NaCl were obtained at 600 and 800 ppm Zr in the bath, respectively. It was due to higher content of oxidized chromium and aluminum compounds in these TCC coatings.
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