Hot dip galvanized steel darkening & chemical composition of the surface

The darkening process of hot dip galvanized steel has been studied by means of Auger electron spectroscopy, secondary electron microscopy and X-ray diffraction. This darkening effect appears about six months after coating and is characterized by the blackening of some spangles. Spangles have different morphologies -ridged with dull appearance and feathery that appears bright. It was found that the main difference between the bright and dull spangles formed during the galvanizing process rests on their crystallographic orientation. The larger impurity diffusion in dull spangles is responsible for both its lower resistance to corrosion attack and the blackening produced by diffusion and oxidation of antimony that is used as surfactant.