XPS study of coating delamination from non-rinse chromate treated steel

Abstract

The mechanisms of delamination of an epoxy coated steel system with various substrate non-rinse chromate treatments has been investigated by XPS and TOF-SIMS. First, the chromate surface before epoxy coating was investigated to establish the layer structure produced by the chromate treatments. The pre-treatment layer was found to be a complex material consisting of trivalent and hexavalent chromium, and a cross-linked structure of these ions within chromium oxide. A non-rinse chromate with colloidal SiO₂ added has a similar structure of trivalent and hexavalent chromium oxide at the outer surface, but within the inner layer Cr oxide and SiO₂ are bonded to each other. This leads to a more complicated structure compared to the chromate treatment. Adsorption isotherms of epoxy resin and curing agent on chromate layers were established by using XPS, in order to evaluate the adsorption characteristics of these molecules, to gain an insight into the initial stages of adhesion. The chromate surface was found to have more bonding sites than the chromate with SiO₂ surface.

Steel substrates treated in this manner were coated with an epoxy coating and exposed to a saline environment at both the free corrosion potential and a cathodic potential The delamination kinetics of the epoxy coating were recorded. Interfacial failure surfaces from the delamination studies were examined by XPS and TOF-SIMS. By obtaining Cr 2p spectra at high resolution and peak-fitting to resolve the chemical states present, it has proved possible to determine the layer structure produced by the chromate treatment and elucidate their chemical behaviour during the electrochemical tests. In the case of the chromate treatment, failure following cathodic polarisation is within the conversion coating itself and associated with the inner layer, whilst an interfacial failure was observed for the untreated bare steel. All failure surfaces of the chromate treated steel sample were rich in Cr³⁺ , compared with the initial value obtained before the delamination experiments, and rich in sodium. It is shown that cathodic conditions exist under epoxy coating which have brought about the reduction of Cr⁶⁺ to Cr³⁺ within the chromate layer. This cathodic reduction, in the case of the chromate treated sample, leads to failure by the destruction of the cross-linked structure incorporating Cr⁶⁺ and Cr³⁺ .