SURFACE CHARACTERIZATION OF HOT-DIP GALFAN COATINGS

The surface of a hot dipped Galfan (Zn-5wt.%Al-mischmetal) coating on sheet steel was characterized with the use of various microscopy techn iques. Surface depressions, or dents, were found to occur at eutectic nodule boundaries and triple points, and were typically 10-15mum deep. The surface characteristics of the Galfan coating were reproduced by the solidification of Zn-5%Al-mischmetal alloy samples on an inert sub strate, implying that surface depressions are not caused by substrate interactions. Chemical analyses of both the coating and the alloy samp les indicate that impurities, particularly lead, are strongly segregat ed to eutectic nodule boundaries and triple points. Based on these obs ervations, a mechanism for denting and cracking in Galfan coatings is suggested.