Abstract
Plasma deposits of two high-alloy steels, nickel and a Ni-Cr alloy were studied from the point of view of their isothermal oxidation at elevated temperatures. Deposits separated from the substrate were measured by a gravimetric method after mechanical treatment of both faces of each sample followed by oxidation at 1000–1150 K for various periods of time up to 144 hours. In spite of the irregular growth of the oxide layers due to the defect structure of plasma sprayed deposits as well as of the oxide layers, the oxidation kinetics could be taken for approximately parabolic. The study of the composition of oxide layers on all alloys showed that they contained very low amounts of chromium. This is in a strong contrast to oxides growing during plasma spraying, which are significantly enriched in chromium as compared to the alloy composition. The preferential oxidation of chromium during plasma spraying results in chromium depletion of the alloy phases in plasma deposits. This depletion is the main reason for rapid oxidation of steel deposits during isothermal oxidation tests, which was shown also by comparing one of the plasma deposited high-alloy steels with bulk steel of similar starting composition. The plasma deposits of nickel and the Ni-Cr alloy exhibited significantly better oxidation resistance.

Key words
plasma spraying, metallic coatings, oxidation tests, oxidation kinetics, oxidestructure, element distribution

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