Influence of water vapour on high temperature oxidation of steels used in petroleum refinery heaters

The oxidation behaviours of three different steels used in the construction of petroleum refinery heaters were investigated by thermogravimetric analysis (TGA) technique. C-5, P-11 and P-22 steel samples were tested in two different environments: air and CO2+2H2O+7.52N2, a gas composition which simulates the combustion products of natural gas, at 450 and 500 degrees C. P-22 steel had the best oxidation resistance at both temperatures in air. In CO2+2H2O+7.52N2 environment, the oxidations of all the steels were accelerated and C-5 exhibited better oxidation resistance than P-22 and P-11. Analyses of oxidation products by optical microscopy, SEM-EDX and XRD were carried out to correlate TGA results to oxide composition and morphology. The lower oxidation rate of P-22 in air was explained with reference to the formation of a protective Cr-containing oxide layer between the steel and the iron oxide scale. The higher oxidation rates of chromium containing steels in CO2 + 2H(2)O + 7.52N(2) environment were attributed to the depletion of protective Cr-containing oxide scale, which was deduced from the lower Cr content of this layer than that formed in air oxidation, as a result of probably faster oxidation of Cr even inside the steel. Therefore, the oxidation mechanisms of Fe-Cr alloys with intermediate Cr contents at higher temperatures could also be valid for steels with low chromium contents such as P-22 (2.25%) even at 450 and 500 degrees C.