Influence of water vapour on high temperature oxidation of steels used in petroleum refinery heaters A. Sultan, I ˙ . Karakaya * and M. Erdog ˘an 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 CO 2 þ 2H 2 O þ 7.52N 2 , a gas composition which simulates the combustion products of natural gas, at 450 and 500 8C. P-22 steel had the best oxidation resistance at both temperatures in air. In CO 2 þ 2H 2 O þ 7.52N 2 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 CO 2 þ 2H 2 O þ 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 8C. 1 Introduction Heaters (furnaces) in petroleum refineries are used to preheat feedstock to reaction or distillation temperatures. They are designed to raise the temperature of the fluid to a maximum of about 510 8C (9508F). The fuel burned may be residual fuel oils, refinery gas, natural gas or combinations, depending on economics, operating conditions and emission requirements. Steel pipes used in heaters are surrounded by gaseous combustion products, which are mixtures of mainly; carbon dioxide, water vapour and nitrogen. Selection of heater tubes and their thicknesses require, in addition to other specifications, corrosion allowance [1], because the thicknesses are expected to be lost due to corrosion during operation. Many high-temperature materials, for example Ni–Cr alloys, rely on the formation of Cr 2 O 3 scales for protection against high- temperature oxidation. On the other hand, materials that form iron oxide scales, e.g. low chromium Fe–Cr alloys, exhibit much poorer oxidation properties. Depending on temperature and chemical environment, these steels may develop protective chromium-rich oxide scales or non-protective iron oxide scales [2, 3]. The oxidation kinetics of chromium and chromium containing alloys that form Cr 2 O 3 when oxidized in dry gases is known to be influenced by CrO 3 (g) formation at the scale/gas interface at high temperatures around 1000 8C [4]. The presence of water vapour in the oxidizing gases results in higher oxidation rates than in dry gases which cannot be explained by CrO 3 (g) formation. In the literature, this behaviour is known as ‘breakaway oxidation’, which is associated with the rapid formation of a continuous non-protective Fe-rich oxide scale instead of the protective chromia-based oxide scale in H 2 O containing atmospheres. The influence of water vapour on the high temperature oxidation of Fe, Cr, Fe–Cr and Fe–Cr–Ni alloys, has received considerable attention [5–17]. Several attempts have been made to explain the apparent higher corrosivity of water vapour. The explanations put forward can be classified according to the role proposed for water vapour: (i) dissociation processes as a result of H/H 2 O bridges providing hydrogen to the scale in the form of H þ or OH  [6, 7], Materials and Corrosion 2012, 63, No. 2 DOI: 10.1002/maco.200905429 119 A. Sultan, I ˙ . Karakaya, M. Erdog ˘an Department of Metallurgical and Materials Engineering, Middle East Technical University, I ˙ no ¨nu ¨ Bulvarı, 06531 Ankara (Turkey) E-mail: kkaya@metu.edu.tr www.matcorr.com ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim