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In this study, the role of Nb addition on the corrosion behavior of low-alloy steel exposed to H 2 S solution is studied by means of microstructure, macro/microsurface morphology characteristics, type of corrosion product, cross-sectional structure and elemental distribution. The addition of Nb is a route to refine grains and improve corrosion resistance. The surface formation process of iron sulfides changes from homogeneity to dispersion at the initial corrosion stage when Nb is added to steel. However, the transformation process and crystal structure of iron sulfides are not affected by the addition of Nb. The elemental distribution of corrosion products is not affected by Nb addition although thick and compact corrosion products are formed on the specimen surface of Nb steel. A schematic model is proposed to clarify the difference of the corrosion mechanism in atomic and molecular level. • Niobium addition improves corrosion resistance of low alloy steel in H 2 S solution. • Role of niobium addition on the characteristics of iron sulfides is weak. • A schematic model of corrosion mechanism is fundamentally proposed. [ABSTRACT FROM AUTHOR]

The corrosion behavior of low‐alloy steel exposed to saturated H2S solution is systematically studied using electron backscattered diffraction, scanning electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, and electron probe microanalysis. The experimental results suggest that the corrosion rate decreases with prolonged immersion time because more corrosion products are formed on the steel surface. The corrosion products consist of mackinawite, greigite, and molybdenite regardless of the corrosion time. The structure of the corrosion products transforms from a single layer into two layers with increasing corrosion time. The Cr content in the corrosion products is similar to that in the steel substrate. The corrosion products are rich in molybdenum owing to the formation of molybdenite. [ABSTRACT FROM AUTHOR]