The protective effect of H2S on steel corrosion arises from the formation of iron sulfide (FeS) passive films on steel surfaces. Various iron sulfides with different crystal structures, including mackinawite, cubic ferrous sulfide, pyrrhotite, and greigite, can develop as corrosion products for steel. The effect of those scales on corrosion damage is intricately linked to their physicochemical attributes and morphology. As multiphase streams move through pipelines, interplaying physical and chemical factors can lead to localized pitting attacks, especially where the iron sulfide film shows a weak crystalline structure. In this study, corrosion tests were performed under different H2S field environments. Scanning electron microscopy (SEM) and atomistic modeling were employed to understand the formation and disruption of protective scales at the atomic level.
In sour (H2S) corrosion systems, a small amount of H2S can retard the general CO2 corrosion rate of carbon steel by forming a passive iron sulfide (FeS) layer [1], [2]. Environmental factors dictate the formation of protective or partially protective FeS layers on carbon steel surfaces. High H2S levels often result in stable films that reduce the corrosion rate, contingent upon the maintenance of the sulfide layer [2]. Conversely, in slightly sour systems, which initially form mackinawite (FeS) [3], the system has the potential to cause pitting and extremely high localized corrosion rates [2].
It is widely accepted that different types of corrosion products, due to their different physicochemical properties, have different effects on steel corrosion in H2S environments [4]. As temperature variations occur between the inlet and outlet of a pipeline, the iron sulfide layer precipitation rate modulates, leading to heterogeneity in FeS film thickness on the steel surface. Not all forms of precipitated iron sulfide layer confer protection [5].
Different factors such as temperature, CO2 and H2S partial pressures, pH, and water chemistry significantly impact the type of corrosion products formed, thereby determining the stability, protectiveness, and adherence of the passive films [6]. Consequently, pitting attacks concentrate in areas where the iron sulfide film exhibits a weak, porous and non-protective structure. The occurrence and type of corrosion product that can form under the iron sulfide layer is a complex phenomenon that necessitates further research [7]. Most existing research primarily focus on corrosion behaviors at "low" temperatures (<25˚C) or "high" temperatures (>80˚C) [5], [8]. The effect of intermediate temperature ranges, specifically the 70-80˚C range commonly encountered in pipeline environments, remains underexplored, leading to an understanding gap regarding the temperature's influence on FeS layer formation and its subsequent impact on corrosion behavior.
