Effect of H2/H2O ratio on thermally grown chromia scales formed on Ni25Cr alloy in Ar–H2–H2O atmospheres at 1000°C

Abstract

The oxidation behaviour of a NiCr-base model alloy with a 25% chromium content is investigated at 1000°C in different low p(O2) environments (Ar –H2–H2O) with the emphasis of how hydrogen andyor water vapour affects the oxidation behaviour of chromia forming alloys \\in low p(O2) environments containing H2 and H2O. After equilibrium conditions are achieved, chromia growth on Ni25Cr in low p(O2) type test gases containing H2 and H2O is ruled by the concentration of Cr interstitials available for Cr outward transport. The chromia microstructure, therefore, consists largely of columnar shaped oxide grains. The chromia scales are growing predominantly from oxygen coming from the water vapour in the test gas. H-species may facilitate oxide growth in pores at the metal – oxide interface. H-defects affect the transport properties of chromia scales formed on Ni25Cr. At low p(O2) the H-defects in the chromia scale are compensated by a lower concentration of Cr interstitials. Depending on the nature of the test gas, i.e. p(O2), p(H2) and p(H2O), as well as on the Cr activity in the metal substrate, the growth of chromia scales on NiCr alloys may be featured and shaped differently.