Abstract
The mechanism of the interfacial corrosion of painted steel sheets and the role of phosphate conversion and zinc coatings on paint performance were reviewed. XPS and EPMA analyses were successfully used to anaylyse the failured interfaces. The paint delamination process changes depending on the pretreatment and corrosion conditions. In the wet atmosphere such as salt spray test (SST), the delamination of the phosphated steel occurrs cathodically by corrosion-induced alkalline attack on the resins and phosphate films. Accordingly, the alkalline resistance of the phosphate films is of prime importance for improving paint performance. On the galvanized steel, however, anodic dissolution of zinc accelerates the paint creep back by forming a new corrosion cell between one anode and two cathodes which are localized at the scribe and the leading edge of delamination, respectively. On the other hand, in the cyclic corrosion test, anodic mining of galvanized steel is suppressed by a lack in water during drying and a reduction in potential difference of Zn/Fe due to higher temperature compared to the SST, resulting in a low corrosion rate. The effect of the minor elements such as Al and Pb present in zinc layer on the bond durability of the epoxy-bonded galvanized steel has been also discussed.
