Effect of chromium on the oxidation of steels used in the construction of petroleum refinery heaters

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Tezin Türü: Doktora

Tezin Yürütüldüğü Kurum: Orta Doğu Teknik Üniversitesi, Mühendislik Fakültesi, Metalurji ve Malzeme Mühendisliği Bölümü, Türkiye

Tezin Onay Tarihi: 2013

Öğrenci: ABDELRAHMAN SULTAN

Danışman: İSHAK KARAKAYA

Özet:

Low chromium steels used in the construction of petroleum refinery heaters work in flue gas environments containing water vapor. On the other hand, the higher oxidation rates of chromium containing steels in moist atmospheres have been reported. P-5, P-22, P-11(containing 5, 2.25 and 1.25 %Cr) and C-5 steels, used in petroleum refinery construction, were subjected to isothermal oxidation tests. These steels were tested in two different environments; air and flue gas of natural gas combustion -CO2+2H2O+7.52N2 gas mixture- by using a thermogravimetric analysis (TGA) technique. They were tested at 450 and 500oC to cover the temperature range used in the operation of heaters. From the analysis it was found that P-5 steel retained the best oxidation resistance among all steels in air while in CO2+N2+H2O environment, C-5 was superior to P-5 and P-22. To analyze the oxides and to correlate the TGA results to oxide composition and morphology; optical microscopy, SEM-EDX examinations and XRD measurement were performed. The lower oxidation rates of P-5 and P-22 steels in air were explained with reference to an oxide layer with higher chromium at the surface. On the other hand, the higher oxidation rate in CO2+2H2O+7.52N2, was explained by the loss of the protective property of the oxide due to internal oxidation of chromium. The higher corrosivity of water vapor containing environments on Cr containing steels was attributed to the increase in permeability of oxidants to steel due to expansion of lattice parameters by dissolution of hydrogen which was formed by the combustion reaction and/or decomposition of water vapor. It was shown that addition of free oxygen to H2O containing environments reduces the rate of oxidation. R value representing the ratio of H2O to O2 mole fraction was found to be an important parameter to determine the behavior of Cr containing steels in moist atmosphere in the presence of free oxygen.