The Effect of Corrosion Product Formation Mode at the Early Stage of Sour Corrosion of Pipeline Steel

The rapid process at the initial stage of sour corrosion on carbon steels and the relatively low uniform corrosion rate show the change of corrosion mechanism and the effect of corrosion products over time. In this paper, the scaling mode at the early stage of carbon steel exposed to H₂S environments with short exposure time were investigated. Corrosion exposure tests and linear polarization resistance (LPR) techniques were performed. The corrosion products of pipeline steel formed under sour corrosion conditions were analyzed by scanning electron microscopy (SEM), and X-ray diffraction (XRD). Based on corrosion rate and the parameters of water chemistry, solubility limits of iron sulfide (FeS) were calculated. A preferred orientation of mackinawite appeared after 30 min of immersion. After exposure for one hour, the solution supersaturated and the corrosion product began to precipitate on the steel surface. Assuming the corrosion film covered the matrix layer by layer, the thickness of FeS were calculated based on water chemistry input. There was a significant difference between the calculated result and the film thickness of the cross-section. The corrosion product layers were loose and covered the matrix incompletely at the early stage of H₂S corrosion. The rapidly scaling mode derived by the experiment and theoretical works is expected to improve the understanding of the mechanism of H₂S corrosion at early stage and the possibility of the nucleation of the pitting under sour conditions.