IN-SITU RAMAN-STUDY OF THE CORROSION OF ZINC-COATED STEEL IN THE PRESENCE OF CHLORIDE .2. MECHANISMS OF UNDERPAINT CORROSION AND ROLE OF THE CONVERSION LAYERS

The underpaint corrosion of zinc-coated steel exposed to chloride containing solutions was studied by Raman spectroscopy; the selection of an appropriate resin (without fluorescence) allows us to perform the analysis directly through the paint. The influence of a conversion layer (chromatation or phosphatation) was investigated. On chromated zinc-coated steel as well as on the specimens without a conversion layer, two kinds of blisters were found: the ones containing ZnO, identified as cathodic blisters, and the others containing hydroxychloride, identified as anodic blisters. On the phosphated specimens, the growth of the cathodic corrosion products seems hindered. The role of the local pH variation related to the confinement is particularly emphasized.