Oxidation of Low Chromium Steels in Dry and Moist Atmospheres

P-5 (5%Cr) steel is used in the construction of high temperature industrial equipment that works in the flue gas environment because of its high corrosion resistance. However, there are studies reporting higher oxidation rates of Cr containing steels in moist environments when compared to oxidation in air. In order to understand the oxidation behaviour of P-5 steel, isothermal oxidation tests were performed in two different environments; air and flue gas composition of natural gas combustion (CO2+2H2O+7.52N2) using thermogravimetric analysis (TGA) at 450 and 500°C. SEM-EDX and XRD analyses were used in the characterisation of the oxide scales. The results revealed that P-5 possessed better resistance to oxidation in air than in water vapour-containing environment at both temperatures. TGA results of P-5 were used together with the previous data obtained from P-11 (1.25% Cr) and P-22 (2.25% Cr) steels to find a correlation between the oxidation behaviour and Cr content. From the analysis of oxidation data, it was deduced that the corrosion rate in air decreased as the Cr content in the steel increased. However, the corrosion rate increased with the increase in Cr% in the other environment. Formation of chromium rich protective oxide layers on the P-5 and P-22 steel surfaces were used to explain the lower oxidation rates in air. On the other hand, the higher weight gain in the CO2+ 2H2O +7.52N2 environment was attributed to the presence of hydrogen provided by water vapour. It was proposed that hydrogen diffusion into the steel increased the permeability of the oxide scale to oxygen which caused internal oxidation of chromium. As a result, chromium is entrapped inside the steel and the protective nature of the oxide scale is lost