Met. Mater. Int., Vol. 19, No. 4 (2013), pp. 731~740 doi: 10.1007/s12540-013-4012-8 Imitating Seasonal Temperature Fluctuations for the H 2 S Corrosion of 304L and 316L Austenitic Stainless Steels A. Davoodi 1, * , M. Babaiee 2 , and M. Pakshir 3 1 Hakim Sabzevari University, Faculty of Engineering, Materials Science and Polymer Engineering Department, Sabzevar, 391, Iran 2 Shiraz University, School of Engineering, Department of Materials Science and Engineering, Shiraz, 713451154, Iran 3 Shiraz University, School of Engineering, Department of Materials Science and Engineering, Shiraz, 713451154, Iran (received date: 6 January 2012 / accepted date: 3 September 2012) Temperature fluctuations are inevitable in sour oil and gas production. In this study, the H2S corrosion of 304L and 316L alloys was investigated at pH 3 and temperatures of 20-60 ° C using DC and AC electro- chemical techniques. Two-fold increases in the corrosion rates of both alloys were reported with increases in temperature to 60 ° C. In the 304L alloy, the surface layer was observed to be 3% rougher and 34% thicker than that of the 316L alloy. The two alloys exhibited different corrosion behaviors in the tempera- ture ranges of 20-40 ° C and 40-60 ° C. Although the 316L alloy revealed a greater corrosion resistance at the free potential condition, the passivation on the 304L alloy was significantly greater than that of the 316L alloy at 40 ° C and 15 ppm H2S. The FeS2 and combined FeS2-MoS2 compounds contributed to the surface layer constituents in the 304L and 316L alloys, respectively. The increase in temperature kinetically provided more favorable conditions for FeS2 than MoS2 formation, i.e. it had a relatively constructive effect on the 304L alloy passivation. Key words: alloys, corrosion, electrochemistry, microstructure, scanning electron microscopy (SEM), X-ray diffraction 1. INTRODUCTION Stainless steel grades 304L and 316L are commonly used construction materials for piping, stationary, and rotary equip- ment that come in contact with corrosive environments during the production of hydrocarbon fluids [1,2]. Their corrosion resistance is attributed to the passive film formation on the alloy surface and the difference observed in the corrosion behavior of the 304L and 316L alloys has been associated with the additional molybdenum alloying element in the 316L alloy. It has been demonstrated that molybdenum partici- pates in the surface reactions and enhances the protective- ness of the surface layer [3]. Nevertheless, the presence of hydrogen sulfide, carbon dioxide, dissolved salt, and water in the produced hydrocar- bon fluids deteriorates the alloy passivity [4-6].The presence of the abovementioned contaminations becomes even more frequent as the exploitation of deep reservoirs increases and unconventional resources are increasingly used [2,7]. Despite several attempts to remove these contaminations [2,8-11], their presence is inevitable due to practical process restriction. Among these impurities, the presence of hydrogen sulfide in produced oil and gas results in operational, environmental, and treatment problems. The procedure of selecting either the 304L or 316L alloy as the paramount alloy, from both an economical and performance viewpoint, is affected by the simultaneous influence of two kinetics parameters: the H 2 S concentration and temperature [12]. High temperature H 2 S corrosion on stainless steel has been reported recently and it was stated that the high concentration of sulfur in the sulfu- rated compounds favors the formation of extremely aggres- sive acids, such as H 2 SO 4 , which contribute to an accelerated and intense corrosion process of the stainless steel [13]. Three natural processes are proposed in order to explain the H 2 S accumulations in the reservoirs: bacterial sulfate reduction (BSR), thermal cracking or thermal decomposition of sulfides (TDS), and thermochemical sulfate reduction (TSR) [7,14-17]. In addition, secondary H 2 S could also be induced during the steam injection recovery process of heavy oil and gas [7]. In the southern Persian Gulf, the proposed TSR is a significant mechanism for obtaining high H 2 S content, whereas the other processes, such as BSR and organic sulfur origi- nated from the TDS of kerogen and/or oil, are also possible *Corresponding author: adavoodi@kth.se ©KIM and Springer, Published 10 July 2013