Conversion coatings for zinc electrodeposits from modified molybdate solutions

Among the alternatives which have been considered as potential replacements for chromate conversion coatings on zinc electrodeposits, molybdates have been the subject of extensive investigation. Whilst forming coatings of similar appearance to chromates, they are not thought to pose the same risks to human health and the environment. However, the corrosion protection of simple molybdates has been found to be inferior to chromates in chloride environments. The present research has sought to improve this aspect of the coatings by adding oxidising agents to the formulations. The composition and morphology of the coatings has been investigated, while the corrosion resistance was studied by salt spray corrosion testing and linear polarisation resistance. Calibrated line scans using the scanning vibrating electrode technique revealed that the addition of sodium persulphate to the molybdate solution reduced the corrosion current during the initial stage of immersion compared to the simple molybdate.