An electrochemical evaluation of possible non-chromate conversion coating treatments for electrodeposited zinc-nickel alloys

A number of possible non-chromate conversion coating formulation shave been studied to examine their efficacy as replacements for chromate-based coati ng treatments on electrodeposited zinc-nickel alloys. Data have been collec ted from both electrochemicals and conventional salt fog corrosion tests. I n the later tests all the alternative non-chromate systems studied delayed the onset of the corrosion of the zinc-nickel coating itself but only the s imple molybdate and the permanganate/molybdate dual treatment brought about improvements in times to red rusting of the steel substrate. The electroch emical evaluations suggested that simple molybdate-tread surfaces act as on ly moderate barriers to the chloride environments. Of the two dual treatmen ts, permanganate/molybdate behaved similarly to the simple molybdate treate d surfaces in the electrochemical tests, but the molybdate/permanganate was more protective, although this was not exhibited in the salt fog corrosion tests.