Corrosion-resistant ion-plated Al–Zn films

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Abstract
Microcrystalline, thin, passivated, metal films can surpass similar bulk material in corrosion resistance. If these films form a spinel oxide in situ, corrosion protection might be further enhanced over simple oxides. We extend our previous work on Al–Mg and Al–Cu films to Al1−xZnx, 0<x<1. We deposited Al–Zn films, 1–2 μm thick, onto liquid-nitrogen-cooled substrates at ∼0.1 μm/min by dc ion plating. Films were smooth, fine grained, and columnar. Potentiodynamic data were taken at 20 mV/min in aerated 3.5% NaCl solutions at room temperature. Corrosion (rest) potentials were ∼−1.05 V (saturated calomel electrode) for all x. Anodically scanned specimens had a short passive region, then one to three active–passive peaks followed by a long passive region that ended in transpassive behavior due to crevice corrosion. The total protection range (rest to transpassive potential) was ∼2.2 V for all x. Galvalume (55Al–Zn) and Galfan (5Al–Zn), commercial hot-dipped alloys, have ranges of 0.4 V. Thus, ion-plated Al–Zn alloys seem markedly superior to hot-dipped coatings. Auger electron analysis of corroded films indicated the presence of aluminum and oxygen with only small amounts of zinc. No evidence of spinel oxide was found by reflection electron diffraction (RED). RED results best fit aluminum oxides. The role of zinc is unclear. It may have formed a very thin spinel layer, or it may have facilitated the formation of the aluminum oxides that are good barriers.

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