Behaviour of molybdate-passivated zinc coated steel exposed to corrosive chloride environments

The behaviour of molybdate conversion coatings on zinc coated mild steel in corrosive chloride environments was investigated using electrochemical imp edance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and neutr al salt fog exposure. It was found that the presence of a simple molybdate coating initially increased the rate of corrosion of zinc. Molybdenum speci es were initially present in the conversion coating in either the +V or +VI oxidation states. Exposure to neutral salt fog reduced molybdenum to eithe r Mo(IV) or Mo(III). This reduction of molybdenum, an additional cathodic p rocess, may result in the activation of zinc observed in these studies. For molybdate-passivated surfaces in the early stages of exposure to neutral s alt fog, corrosion products were found to be less voluminous than those obs erved on untreated surfaces. This may be due to the presence of inhibiting Mo(IV) or Mo(III) species in the corrosion product layers. However, after 2 4 h exposure to salt fog, no molybdenum could be detected. This implies tha t the lower oxidation state molybdenum species formed are soluble. However, surfaces passivated from molybdate solutions appear to forestall the onset of red rust, during immersion in chloride solutions and exposure to salt f og, by approximately 12 to 24 h. This behaviour may be attributable to corr osion inhibition by Mo(III) and Mo(IV) species while they are present at th e surface.