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

Abstract

The behaviour of molybdate conversion coatings on zinc coated mild steel in corrosive chloride environments was investigated using electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and neutral salt fog exposure. It was found that the presence of a simple molybdate coating initially increased the rate of corrosion of zinc. Molybdenum species were initially present in the conversion coating in either the +V or +VI oxidation states. Exposure to neutral salt fog reduced molybdenum to either Mo(IV) or Mo(III). This reduction of molybdenum, an additional cathodic process, may result in the activation of zinc observed in these studies. For molybdate-passivated surfaces in the early stages of exposure to neutral salt fog, corrosion products were found to be less voluminous than those observed 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 24 h exposure to salt fog, no molybdenum could be detected. This implies that 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 fog, by approximately 12 to 24 h. This behaviour may be attributable to corrosion inhibition by Mo(III) and Mo(IV) species while they are present at the surface.