Corrosion Mechanism of J55 Steel Casing in Block ContainingHydrogen Sulfide in Changqing Oilfield

The corrosion product composition and morphology on the surface of J55 steel casing in H₂S containing block in Changqing Oilfield and in different corrosive enviroments (pure CO₂, pure H₂S, different partial pressure CO₂+H₂S enviroments) were analyzed by energy spectrum analyzer, scanning electron microscope and X-ray diffractometer. The corrosion rate was calculated, and the corrosion mechanism of J55 steel casing in H₂S containing block was studied. The results show that the coarse-grained FeCO₃ corrosion product film with low compactness was formed in a single CO₂ environment for J55 steel, whose corrosion rate was high. The addition of H₂S made J55 steel form a fine-grained and dense FeS corrosion product film in CO₂+H₂S environments, which could protect the base metal and significantly reduce the corrosion rate of J55 steel. The corrosion mechanism of J55 steel in sulfur-containing block of Changqing Oilfield was CO₂+H₂S corrosion dominated by H₂S corrosion.