An Electrochemical Evaluation of Possible Non-Chromate Conversion Coating Treatments for Electrodeposited Zinc-Nickel Alloys

A number of possible non-chromate conversion coating formulations have been studied to examine their efficacy as replacements for chromate-based coating treatments on electrodeposited zinc-nickel alloys. Data have been collected from both electrochemical and conventional salt fog corrosion tests. In the latter tests all the alternative non-chromate systems studied delayed the onset of the corrosion of the zinc-nickel coating itself but only the simple molybdate and the permanganate/molybdate dual treatment brought about improvements in times to red rusting of the steel substrate. The electrochemical evaluations suggested that simple molybdate-treated surfaces act as only moderate barriers to the chloride environments. Of the two dual treatments, permanganate/molybdate behaved similarly to the simple molybdate treated surfaces in the electrochemical tests, but the molybdate/permanganate was more protective, although this was not exhibited in the salt fog corrosion tests.