Novel insight to aluminum compounds in the outermost layers of hot dip galvanized steel and how they affect the reactivity of the zinc surface

Hot dip galvanized zinc coatings (containing about 0.2% of aluminum) were studied with surface analytical techniques. A few nanometer thick aluminum oxide (Al2O3) layer dominated the very outermost surface of such zinc coatings. The inert Al2O3 layer formed a continuous impermeable barrier on zinc, providing short term humidity resistance. However, this barrier effect was largely lost during temper rolling. Residues of Al2O3 were shown to prevent titanium hexafluoride pretreatment chemical from reacting with the zinc surface. Segregated aluminum/iron (Al/Fe) intermetallic compounds were also detected at the surface of the zinc coating. The Al/Fe compounds populated the surface immediately beneath the Al2O3 layer. The Al/Fe precipitates were found predominantly in the dendritic valleys. Repulsion between the Al/Fe precipitates and the pretreatment chemical was observed.