The effect of gas composition on the isothermal oxidation behaviour of PM chromium

The effect of small amounts of water vapour on the isothermal high-temperature oxidation behaviour of powder-metallurgical (PM) chromium in Ar/20% O-2 and N-2/20% O-2 has been studied. Additional oxidation experiments were performed with isotopically enriched gases, i.e. Ar/20% O-18(2) and N-15(2)/20% O-18(2) under controlled humidity using (H2O)-O-16 in combination with secondary ion mass spectrometry. In argon/oxygen, water vapour slightly increased the oxidation rate and was the main oxygen source for the oxidation process. The presence of water did not influence the type of kinetics and growth mechanism of oxide formation on pure chromium. In nitrogen/oxygen, the addition of water vapour reduced the oxide growth rate by about 50%. Under these conditions, nitrogen affects the reactivity of water molecules participating in the formation of an oxide layer. The mass transport mechanisms responsible for the formation of an oxide layer did not change, however. When nitrogen was present, a nitrogen-rich sub-layer formed near the scale/substrate interface, influencing the stress relaxation mechanism. Here, repetitive scale cracking occurred during isothermal exposure, which finally resulted in a more adherent and protective oxide layer with less internal stress and withstanding high thermal stresses accumulated during cooling from the test to ambient temperature. (C) 2002 Elsevier Science Ltd. All rights reserved.