PVD Al-Ti and Al-Mn coatings for high temperature corrosion protection of sheet steel

Aluminum, aluminum-titanium and aluminum-manganese coatings with varying Ti and Mn content were deposited on low carbon steel substrates by ion bombardment assisted magnetron sputtering using substrate bias voltages between -50 and -200 V. The structure, chemical composition and morphology of the coatings have been analyzed and related to functional properties such as adhesion, friction and most important the oxidation and corrosion resistance, especially at high temperatures. For pure Al coatings the oxidation resistance, as measured by the mass increase during isothermal annealing at 600 degreesC, showed a clear dependence on substrate bias voltage, with a better resistance at high bias. Al-Ti coatings behaved the same but showed a higher corrosion resistance. Al-Mn coatings showed no dependence on bias voltage and their mass increase by oxidation had a rather large value. During annealing inter-metallic Al-Mn phases were formed and the oxidation of the steel substrate under the layer was noticed. Coatings consisting of an Al/Ti bi-layer or multi-layer gave the best results, showing an excellent preservation of their appearance after annealing. The corrosion tests, consisting of the exposure to a combination of humidity and salt or to a synthetic concentrated exhaust condensate during several weeks, indicated that before annealing the corrosion resistance was good, but after annealing the corrosion resistance was insufficient, except for the bi-or multi-layer coatings. The results are explained by the formation of brittle interface phases during annealing when Al is in contact with the steel substrate. The promising properties of these coating systems are especially illustrated by the fact that many of the PVD coatings with a thickness of 3-4 mum showed a higher oxidation and corrosion resistance than a classical 20 mum thick Al-Si coating applied by the hot dip process. (C) 2001 Elsevier Science B.V. All rights reserved.