The dissatisfactory corrosion rate of high-Mn steel in an aqueous acid environment extremely restricted its new application such as expandable casing and oil well pipe, although it possessed excellent mechanical property. In our study, high-Mn steel alloyed with Ni and Cr could depress its uniform corrosion. However, pitting corrosion occurred. The maximum pitting rate was 18.08 mm/a. In this research, the pitting corrosion of these steel in high temperature, high CO2, low H2S, and high Cl- environment was investigated by scanning electron microscope (SEM), energy dispersive detector (EDS) and X-ray diffraction (XRD). The results showed that in some local places the corrosion scale was bulged like hillocks which were the representation of pitting. The surface morphology and the cross section morphology were observed by SEM which showed that the corrosion scale had a bedded structure. According to the EDS and XRD results, the corrosion products of the uniform surface and the unbroken part of the hillock were sulfide which contained Fe7S8 and (Fe,Ni)9S8. The other part of the hillock was only Fe7S8 and the core was (Fe, Mn)CO3. The element Cl appeared only on the bottom of the pit. The accumulation of Cl was the incentive of pitting.
Keywords: high-Mn steel, H2S/CO2, alloying agent, pitting corrosion, corrosion product, scale structure.