Morphologies and corrosion properties of PVD Zn–Al coatings

Physical vapour deposition (PVD) Zn alloy coatings are being considered as possible alternatives to conventional hot dip and electrochemically deposited coatings on steel substrates. Variation of the deposition temperature leads to very different microstructures being observed by SEM, primarily due to the low melting point of zinc. In the present study this phenomenon has been investigated in more detail. Zn+5 at.% Al coatings were deposited by magnetron sputtering using a homogeneously alloyed Zn–Al target. As the substrate temperature is increased, the coating morphology changed from a compact to an open sponge/woollen-like microstructure and the coating thickness increased by up to a factor of 10. The Zn–Al coatings displayed a hexagonal structure corresponding to that of Zn. The corrosion properties were investigated by potential–time and salt bath immersion tests. Interestingly, both the barrier and sacrificial protection afforded by the coating were independent of the microstructure.