Unraveling the CO 2 /H 2 S Ratio on the Corrosion Performance of Ni‐Sn‐P Coating in Ultra‐High Pressure

ABSTRACT The effect of CO 2 /H 2 S pressure ratio on the corrosion behavior of Ni‐Sn‐P coating was investigated using weight loss measurements and surface chemical analysis. The results demonstrate that the corrosion mechanism and the product films are closely related to the ratio variations. At a lower ratio (CO 2 /H 2 S pressure ratio≈2.37), the corrosion process is dominated by H 2 S. The interfacial pH remains higher, allowing the controlled formation of a dense and protective film consisting of Ni 3 S 2 , NiS and SnO 2 , resulting in a lower corrosion rate (0.082 mm/a). When the ratio was increased to 11.28, CO 2 decreases the pH value, creating a lower S 2− and higher Ni 2+ concentrations, promoting the formation of Ni 3 S 2 with porous structures, and the corrosion rate rises to 0.1103 mm/a. The main reason was that the lower pH suppresses free S 2− formation while accelerating Ni anodic dissolution. This favors the excessive generation of sulfur‐deficient Ni 3 S 2 with a porous, needle‐like structure that provides pathways for corrosive ions, leading to localized corrosion and a significantly higher corrosion rate. Throughout the process, Sn exists as SnO 2 , providing partial protection that proves insufficient to offset the negative effects of excessive porous Ni 3 S 2 . A model is established to clarify the related mechanism.