Effects of minor elements in Al alloy on zincate pretreatment

The effects of minor elements in Al alloys on Zn deposition during double zincate pretreatment were investigated using four Al alloys: Al foil containing ca. 55 ppm Cu (Al(Cu)), Al–1%Si (Al(Si)), 99.99% Al (Al(4N)) and 99.999% Al (Al(5N)). SEM images showed that a uniform Zn layer was deposited on Al(Cu) and Al(Si) but not on Al(4N) or Al(5N) after the double zincate process. Immersion potential during the zincate process indicated that the duration of substitution reaction of Zn deposition and Al dissolution on Al(Cu) was shorter than that on the other alloys. These results suggest that a small amount of Cu increases the efficiency of the zincate process and the density of Zn deposition. Etching pretreatment of alloys in sulfuric acid containing Cu2+ ions before zincate pretreatment dramatically improved the uniformity and efficiency of Zn deposition on Al(Si) but not on Al(4N) or Al(5N), indicating that small and dispersed Cu particles were deposited efficiently on the Al(Si) alloy and provided Zn nucleation sites. From the results, it was concluded that the activity level of native Al for the zincate process is considerable low and is thus dramatically affected by alloying or surface modification with a small amount of electrochemically active elements.