Hydrogen in Chromium: Influence on the High-Temperature Oxidation Kinetics in O2, Oxide-Growth Mechanisms, and Scale Adherence

Abstract

Oxidation of chromium in 20 mbar isotopicallylabeled molecular oxygen was studied at 900°C. Thesamples comprised specimens with as-received contents ofhydrogen and specimens where this hydrogen had been eliminated by vacuum annealing. Two-stageoxidation was performed by exposure of the samples to16,16O2 and 18,18O2. Reaction rates were studied bymeasuring the change in oxygen pressure in a closedreaction-chamber with a mass spectrometer during oxidation. Theoxide formed was characterized by secondaryion massspectrometry (SIMS) and scanning electron microscopy(SEM). The adherence of the oxide scale to theunderlying metal substrate is poor and the oxidation rateis initially slightly higher when the metal containshydrogen prior to oxidation. It is shown by 18O depthprofiles that chromium diffusion within theCr₂O₃ scale is increased and oxygen diffusion isdecreased for metal containing hydrogen. It is believedthat the poor adherence of the oxide scale, in thiscase, is due to low anion diffusion relative to cation diffusion, which leads to an insufficientportion of oxide growth at the metal-oxideinterface.