The effect of Al and Cr additions on pack cementation zinc coatings

Zinc is widely used as a protective coating material due to its corrosion resistant properties. The structure and oxidation resistance of Al and Cr mixed zinc coatings, deposited by pack cementation process, is thoroughly examined in this work. The morphology and chemical composition of the as-deposited and oxidized samples was accomplished by electron microscopy while the phase identification was performed by XRD diffraction analysis. The experimental results showed that the addition of aluminum or chromium in the pack mixture forms only Al and Cr rich phases on the surface of the specimens without affecting significantly the phase composition of the rest zinc coatings. In the case of Zn-Al coatings, the overlying layer contains high concentrations of Al together with lower amounts of zinc and iron and in Zn-Cr coatings this layer contains Cr, Fe and Zn atoms and has much smaller thickness. The presence of these additional layers promotes significantly the oxidation resistance of the zinc pack coatings and they preserve most of their initial thickness and chemical content when exposed to an aggressive environment while their oxidation mass gain was measured at low levels during the oxidation tests.