Molybdate based conversion coatings for zinc and zinc alloy surfaces: a review

The replacement of chromate conversion coatings for Zn surfaces has been necessitated by industry due to the toxicity issues associated with Cr(VI) and the enforcement of 'WEEE' and 'ELV' directives by the European Union. Molybdate conversion coatings have been investigated by many researchers as possible 'non-chromate' alternatives due to their low toxicity. A significant amount of work has been undertaken in this area in the past 20 years and this review tries to bring together findings from this work. Although a direct replacement for Cr(VI) conversion coatings using molybdates does not appear to have been found, limited success has been gained with the addition of synergistic chemical species such as PO43- and Al3+. Immersion time, pH, temperature and additives have been investigated by many researchers and the effects of these on molybdate systems have been contrasted. X-ray photoelectron spectroscopy characterisation data are included and shows coatings to contain Mo in the +2, +4, +5 and +6 oxidation states. ASTM B-117 neutral salt spray corrosion data suggests that the best performance of molybdate based coatings is around 24 h without white rust on electroplated Zn surfaces. Molybdate treatments on Zn-Ni alloy surfaces have given times to white rust of up to 360 h. In conclusion, the outlook for molybdate based conversion coatings on Zn surfaces is a promising one; with synergistic additions giving increased efficacy in corrosion tests.