Non-chromate conversion coating treatments for zinc-alloy electrodeposits
In recent years, the suspected carcinogenic and toxic effects of hexavalent chromium have caused great concern, resulting in the imposition of stringent environmental and workplace regulations in the metal finishing industry. This has lead to the search for suitable alternatives to the chromate formulations. In this investigation molybdate, cerium and permanganate-based solutions have been examined as to their suitability as possible replacements for chromate conversion coatings on zinc-alloy electrodeposits. Investigations have primarily concentrated on producing passive surfaces from molybdate-based immersion processes. The novel conversion coatings have been examined using scanning electron microscopy to elucidate the coating morphologies. Analysis of the molybdate-based coatings has been undertaken to characterise and determine the composition using molybdenum K-edge EXAFS (extended X -ray absorption fine structure) studies. The corrosion behaviours of the treated surfaces have been assessed using neutral salt spray (fog) corrosion tests and electrochemical methods utilising the linear polarisation technique. Findings indicate that simple molybdate, cerium and permanganate-basecdo ating treatments only achieve, at best, corrosion protection by means of a barrier effect, as a result hindering the dissolution processes, thus providing modest improvements to the formation of white and red corrosion products compared with the 'as-plated' zinc-alloy deposits. However, certain molybdate-based treatments, utilising a permolybdate species, appeared to possess additional corrosion resistant behaviours, possibly associated with an inhibition or retarding of the cathodic reactions. Consequently, superior corrosion performances were apparent for a number of the permolybdate coatings in comparison with the simple molybdate treatments, achieving performance characteristics more comparable to a chromate coating, although still at a reduced level of protection. Specific permolybdate coatings were deemed to be capable of providing enhanced corrosion performances, other than by acting solely as barriers to corrosion.