Effect of skin pass rolling on the primer adhesion and corrosion resistance of hot-dip galvanized (HDG) steel

By using three different skin pass reductions, 0%, 0.75%, and 1.5%, the influence of skin pass rolling on the primer adhesion and corrosion resistance of primed hot-dip galvanized (HDG) steel has been studied. The corrosion resistance of primed panels was determined by a cyclic prohesion test, and the primer adhesion was examined with a combined cross-cut and impact test. Surface roughness was determined for untreated and pretreated skin passed panels and the samples were also studied using an optical microscope and a scanning electron microscope. Electron spectroscopy for chemical analysis (ESCA) and glow discharge optical emission spectroscopy (GD-OES) were used to characterize the chemical surface composition of the panels. According to the prohesion test results, the roughest samples showed the best corrosion resistance and also slightly improved adhesion test results. The ESCA and GD-OES results showed that the outermost surface was enriched by aluminium in the zinc coating. During skin pass rolling, the aluminium oxide-rich surface is broken and zinc is revealed to the surface. An increase in the skin pass reduction resulted in an almost linear increase in the surface roughness. Mechanical removal of the surface aluminium also affected the amount of aluminium dissolved during the chemical pretreatment. The best results obtained for the roughest samples are mainly due to the most homogeneous skin pass pattern obtained with the highest skin pass reduction.