FeS Corrosion Products Formation and Hydrogen Uptake in a Sour Environment for Quenched & Tempered Steel

Abstract

Surface corrosion product formation is one of the important factors affecting the corrosion rate and hydrogen uptake in a H 2 S environment. However, it is still unclear how the base material composition will affect the corrosion products that are generated, and consequently their impact on the corrosion rate. In this paper, corrosion product formation and the impact of the Mo content of the base material on the composition of the corrosion products and hydrogen absorption in a sour environment was investigated. The corrosion layer was composed of a double layered mackinawite (FeS 1−x ) structure, which was enriched with molybdenum and chromium. The layers were formed via two different mechanisms, i.e., the inner layer was created via a general oxide film formation corrosion mechanism, whereas the upper layer was formed by a precipitation mechanism. The presence of this double corrosion layer had a large influence on the amount of diffusible hydrogen in the materials. This amount decreased as a function of contact time with the H 2 S saturated solution, while the corrosion rate of the materials shows no significant reduction. Therefore, the corrosion products are assumed to act as a physical barrier against hydrogen uptake. Mo addition caused a decrease in the maximal amount of diffusible hydrogen.

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“…Huang et al [ 96 ] observed that when a mixture of crystalline FeS and mackinawite is formed as corrosion layer, this layer acts as an n‐type semiconductor that attracts the S 2− and HS − anions while repelling H + cations, reducing the diffusion of H + ions to the surface of steel which in turn reduces the reduction of H + to form adsorbed H atoms. This further confirms the observations from Wallaert et al, [ 103 ] where the amount of diffusible hydrogen measured from the sample immersed in NACE solution A saturated with H 2 S at pH 4 and room temperature rapidly increases to a peak before decreasing less rapidly even though the corrosion rate increases continuously over time. Zhou et al [ 104 ] also observed a similar trend in steel sample‐immersed partial pressure of H 2 S (pH 2 S) of 1 MPa but when compared to that of 0.1 MPa, the peak value for hydrogen permeability was lower peak and a plateau was observed instead of a decay.…”