Effect of microstructure on sulfide scale formation and corrosion behavior of pressure vessel steel in sour environment

• Effect of microstructure on sour corrosion resistance of steel was investigated.
• Polarization curves for 2 kinds of microstructure were obtained within 72 h.
• Precipitation property of sulfide scale depends greatly on the distribution of Fe3C.
• Fe3C network of pearlite promotes better adhesion of sulfide scale for ferritic steel.
• Much finer Fe3C particles of bainitic steel lead to easy detachment of sulfide scale.

To elucidate the influence of steel microstructure on sulfide scale (FeSx) formation and corrosion behavior of pressure vessel steel in sour environment, two kinds of microstructure of ferrite/pearlite and bainite are evaluated. This study clearly reveals that the morphology of sulfide film formed on the steel is remarkably similar to that of the microstructure of the steel substrate, and sulfide film is preferentially precipitated on the cementite (Fe3C) and pearlite with low hydrogen overvoltage and acts as a cathode with respect to ferrite. Moreover, it is evident from electrochemical corrosion evaluation that measured current density of bainite is higher than that of ferrite/pearlite, and the polarization resistance is the other way around, suggesting that, it could be easier for aggressive ionic species to be penetrated into the sulfide film formed on the steel substrate with bainite. The precipitation property of sulfide scale depends greatly on the steel microstructure characterized by the distribution of Fe3C, which in turn has an influence on the corrosion behavior within a given specific period of 72 h. The differences in the shapes and distributions of Fe3C could affect the mechanical attachment of the sulfide scale with the steel substrate. Long range of Fe3C network of pearlitic grains could promote better adhesion of the scale for the steel with ferrite/pearlite, while much finer Fe3C particles of the bainitic microstructure could lead to easy detachment of the scale from the steel with bainite.