ABSTRACT:

Corrosion scale composed of iron-sulfur compounds can significantly affect the corrosion process. However, the transition of different crystals along with time and the protection effect of different corrosion products are not well understood. In this paper, the H2S/CO2 corrosion of pipeline steel under 1.5 MPa H2S and 0.2 MPa CO2 partial pressures at 90 oC was studied. SEM and XRD were used to investigate the characteristics and evolution of corrosion product. Results indicated that the corrosion rate of pipeline steel decreased and then tended to steady with time. At the initial corrosion stage, the corrosion scale was made mainly of mackinawite crystals with a small amount of FeS crystals. After a certain period, pyrrhotite crystals appeared on the surface of the corrosion scale and then the corrosion process was controlled by the growth of pyrrhotite. During oil and gas production and transportation, H2S/CO2 corrosion problem is one the most important issues. General corrosion is the main type encountered in H2S/CO2 environments, and corrosion scale composed of iron-sulfur compounds can significantly affect the corrosion process 1~5. Several studies have shown that the corrosion products could be Fe9S8, Fe3S4, FeS2 and FeS, which varied along with the value of pH, corrosion exposure time and the content of H2S. Fe9S8 has the poorest protection performance while FeS2 and FeS have relatively good protection ability because of their integrated crystal structure 6. Wikjord 7 suggested that the sequence of corrosion products’ transformation was mackinawite, cubic FeS, troilite, pyrrhotite and pyrite. The transition of different crystals along with time and the protection effect of multiplayers films with different sulfide ferric on metal surface are not well understood. The objective of this study is to investigate the formation of iron sulfide layer under high content CO2/H2S environments.

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