Pitting Corrosion of Supermartensitic Stainless Steel in Chloride Solutions Containing Thiosulfate or H 2 S
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Pitting Corrosion of Supermartensitic Stainless Steel in Chloride Solutions Containing Thiosulfate or H2S Moˆnica M.A.M. Schvartzman, D.R. Lopes, L. Esteves, W.R.C. Campos, and V.F.C. Lins (Submitted October 23, 2017; in revised form March 12, 2018) Deepwater oil and gas extraction environment have high salinity, low oxygen content, and the presence of carbon dioxide and hydrogen sulfide (H2S). Handling of H2S-containing gaseous mixtures should be carried out in laboratories with special installations, owing to the high toxicity of this gas. An alternative is the use of non-toxic substances that can generate H2S in the medium, such as sodium thiosulfate. The objective of this work is to study the viability of substituting H2S by thiosulfate for the evaluation of the susceptibility to pitting corrosion in sour environments of the supermartensitic stainless steel (SMSS). The electrochemical behavior of SMSS in NACE TM0177-2016 solution C solution containing different concentrations of H2S or thiosulfate ions at 1023 mol/L was performed by using cyclic potentiodynamic polarization (CPP). The condition of shorter OCP time (300 s) prevents the formation of a passive layer and allows the generation of H2S due to the reaction of the solution with the metal surface. The CPP curves of SMSS in the media containing 3% H2S/Ar (2.96 kPa) and 1023 M of thiosulfate ions (OCP 300 s) were similar, the pitting potential was about 2 322.5 ± 8.7 and 2 338.8 ± 11.4 mVAg/AgCl, respectively, and the pit morphology showed several small pits on the steel surface in both conditions, suggesting the viability of the use of thiosulfate-containing brines to simulate H2S solutions. Keywords
hydrogen sulfide, pitting corrosion, supermartensitic stainless steel, thiosulfate
1. Introduction Supermartensitic stainless steels (SMSS) are a class of martensitic steels that is based on 13 wt.% Cr steel. They contain less than 0.015 wt.% carbon; 11-13 wt.% chromium; 26.5 wt.% nickel; and 0-2.5 wt.% molybdenum. It has been used in flowlines for transporting unprocessed fluid in the oil and gas industry due to their reasonable corrosion resistance and good mechanical properties and weldability (Ref 1-4). SMSS exhibits a higher corrosion resistance in comparison with martensitic stainless steels. Generally, these materials are an economical alternative against uncoated carbon steels, carbon steel used without inhibitors or duplex stainless steels in some environments (Ref 3-6). In the oil and gas industry, the materials are used in mild (CO2 corrosion) and slightly sour (H2S corrosion) environments. The corrosion of metal due to interaction with hydrogen sulfide (H2S) dissolved in water is called sour corrosion. Therefore, H2S becomes a highly corrosive agent and can accelerate the corrosion process leading to pipeline embrittlement. The main forms of sour corrosion are uniform, pitting,
Moˆnica M. A. M. Schvartzman, D. R. Lopes, L. Esteves, and W. R. C. Campos, Corrosion, Centro de Desenvolvimento da Tecnologia Nuclear –
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