Cut Edge Corrosion Mechanism of Prepainted Zn Coated and 55%Al-Zn Coated Steels and the Role of Chromate in the Primer Coating

Electrochemical study on cut edge corrosion of prepainted Zn coated (GI) and 55%Al-Zn coated (GL) steels has been performed in wet-dry cyclic conditions in order to clarify the difference of their degradation mechanisms. In addition, the role of chromate in the primer coating to the cut edge corrosion has been also investigated. Maximum width of delaminated polymer coating from the cut edge for GI and GL specimens was evaluated under wet-dry cyclic conditions. The cyclic tests were carried out for 1000 h by change of relative humidity, where the salt of NaCl or MgCl2 was deposited on the specimen every 48 It. The cut edge corrosion test under NaCl deposit indicated that the delamination of the GL specimen progressed at a higher rate than the GI. This is attributed to difference of corrosion morphology of their coatings from cut edge. The delamination was inhibited under MgCl2 deposit, especially for the GI specimen. The chromate in the primer coating inhibited the delamination for the GL specimen. The electrochemical corrosion monitoring was also performed under condition of alternate exposure to immersion in NaCl solution and drying at 60% RH and 25 degrees C. On the basis of the results of the delamination tests and electrochemical measurements, the mechanism of cut edge corrosion for GI and GL and role of chromate in the primer coating were discussed.