Corrosion products formed on API 5L X70 steels exposed to hydrogen sulfide environments

• E.M.A. Alves, R.B. Ribeiro, S.M. Ribeiro, H.A. Acciari and E.N. Codaro

São Paulo State University (Unesp), School of Engineering, 12516-410, Guaratinguetá, Brazil

Abstract: In this study, the corrosion behavior of API 5L X70 MS and API 5L X70 MO steels exposed to liquid and vapor phases of the NACE 177 B test solution was evaluated. The corroded surfaces were characterized by SEM and EDS and the corrosion products were identified by XRD for different exposure times (12, 24, 96 and 720 h). Thermodynamic and kinetic aspects were also studied to predict the formation of stable and metastable products. Results revealed no significant effect of the steel chemical composition and microstructure on its corrosion behavior in both environments. Mackinawite was the only corrosion product found. Initially, a thin film was formed and grew in a non-uniform manner. As the exposure time increased, a double-layer film was formed, the inner layer of which was coarser and less compact than the outer layer. The surrounding environment influenced growth rate and film structure. The film formed from the vapor phase was thicker and more crystalline than that from the liquid phase. Despite these differences, the films significantly reduced the corrosion rates of both steels to values below 17 mpy, probably because the double-layer structure acts as an effective diffusion barrier.

Keywords: API X70, sour corrosion, iron monosulfide, mackinawite

Int. J. Corros. Scale Inhib., 2025, 14, no. 4, 2334-2361
doi: 10.17675/2305-6894-2025-14-4-33

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