The effect of water vapour on the nitridation behaviour of pure Cr in atmospheres with more than one oxidising species at high temperatures

摘要

Oxidation of chromium becomes more complex when more than one oxidant is present in the reacting gas. The chemical reaction between chromium and the various oxidising species of a gas mixture is very complex and is governed by a number of factors. One of the key parameters is the ability of the scale to prevent gas phase transport via cracks and pores. In the present studies oxidation tests with pure chromium were carried out in two types of test gases containing N_2, O_2, H_2O or N_2, H_2, H_2O representing low and high p(O_2)gases. All experiments were carried out at 1000°C. Specimens were exposed to test atmospheres for various times. After exposure the corrosion products were characterized by optical metallography and scanning electron microscopy (SEM) with energy and wave length dispersive x-ray analyisis (EDX/WDX). The studies of the oxidation behaviour of pure chromium in the mentioned multi-component gas mixtures with variations in the concentrations of O_2, H_2O and/or H_2O respectively revealed that the scale growth and the permeability of the scales for nitrogen was affected by the water vapour content as well as the oxygen partial pressure of the test gas.