AFM and XPS characterization of zinc-aluminum alloy coatings with attention to surface dross and flow lines

Surfaces of various zinc-aluminum alloy (Zn-Al) coated steel samples are st udied with attention to foreign surface dross by atomic force microscopy (A FM) and X-ray photoelectron spectroscopy (XPS/ESCA). AFM topographic maps o f zinc-aluminum alloy surfaces free of dross reveal the perfect nanoscale d etails of two kinds of dendrites: branched and globular. In all magnificati ons the dendrites appear smooth and, in general, very clean. XPS analysis o f the extreme surface of a Zn-Al sample reveals Al, Zn, Si and O as the mai n components. The XPS results show no segregation or separation of phases o ther than those indicated by the ternary AI-Zn-Si diagram. For surfaces of Zn-Al plagued with impurities, high resolution AFM topographic maps reveal three situations: (1) areas with well-defined dendrites, relatively free of dross; (2) areas with small, millimeter-sized black spots known as dross; and (3) areas with large black stains, known as flow lines. Dendrite deform ation and dross accumulation increase notably in the neighborhood, apparent ly clean to the naked eye, of dross or flow lines. XPS results of areas wit h dross and flow lines indicate unacceptable high concentration of Si and i mportant Si phase separation. These results, in the light of AFM work, reve al that dross and flow lines are a consequence of a high local concentratio n of Si from high melting point silica and silicate impurities in the Zn-Al alloy source.