Effect of Temperature on the Corrosion Behavior of API X120 Pipeline Steel in H 2 S Environment
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JMEPEG DOI: 10.1007/s11665-017-2834-0
Effect of Temperature on the Corrosion Behavior of API X120 Pipeline Steel in H2S Environment Paul C. Okonkwo, Mostafa H. Sliem, R.A. Shakoor, A.M.A. Mohamed, and Aboubakr M. Abdullah (Submitted November 9, 2016; in revised form June 9, 2017) The corrosion behavior of newly developed API X120 C-steel that is commenced to be used for oil pipelines was studied in a H2S saturated 3.5 wt.% NaCl solution between 20 and 60 °C using potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The corrosion products formed on the surface of the alloy were characterized using x-ray diffraction and scanning electron microscopy. It has been noticed that the formation of corrosion product layer takes place at both lower and higher temperatures which is mainly comprised of iron oxides and sulfides. The electrochemical results confirmed that the corrosion rate decreases with increasing temperature up to 60 °C. This decrease in corrosion rate with increasing temperature can be attributed to the formation of a protective layer of mackinawite layer. However, cracking in the formed mackinawite layer may not be responsible for the increase in the corrosion rate. More specifically, developed pourbaix diagrams at different temperatures showed that the formed protective layer belongs to mackinawite (FeS), a group of classified polymorphous iron sulfide, which is in good agreement with the experimental results. It is also noticed that the thickness of corrosion products layer increases significantly with decrease in the corrosion rate of API X120 steel exposed to H2S environment. These findings indicate that API X120 C-steel is susceptible to sour corrosion under the above stated experimental conditions. Keywords
API X120 steel, hydrogen sulfide, impedance spectroscopy, sour corrosion, temperature
1. Introduction Pipelines are one of the most convenient means of transporting petroleum and its products from one region to another. Carbon steels (C-steel) are commonly used as a material for the transportation pipelines because of their economic advantages and ability to withstand the operating pressure (Ref 1, 2). However, the steel surface of the pipelines is always exposed to corrosive environments during the transportation of the petroleum and its products, and the integrity of the pipeline system was found to be always affected (Ref 3-5). The sulfur content in the petroleum and its products has shown to play a critical role in the internal corrosion of the C-steel pipelines (Ref 6). However, other parameters such as temperature, fluid, velocity and microstructure are contributing to the increase in the corrosion rate of C-steels in the petroleum industry (Ref 5, 7-10). The sour corrosion of C-steel has been receiving more attention in recent years because the mechanism of sulfide corrosion absorption is not yet clearly understood. Many researchers have shown that the formed iron sulfide on the steel
Paul C. Okonkwo, Mostafa H. Sliem, R.A. Shakoor, and Aboubakr M. Abdullah, Center of A
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