Microstructure of zinc hot-dip galvanized coatings used for corrosion protection

Low carbon steel substrates were galvanized in molten zinc containing 1 wt.% manganese. The as-cast coatings were examined with optical microscopy, scanning electron microscopy and X-ray diffraction. From the above investigation it was deduced that, although Mn concentration in the coating cross-section is very low (below the EDS sensitivity), Mn-rich crystals are deposited at the lateral surface of the hot-dip galvanized coating. These Mn “islands” are very beneficial with regard to the corrosion performance of the coating, because they act as sacrificial anodes that protect Zn.