Passivity Breakdown of Passive Film for G3 Alloy in Severe Sour Environment at High Temperature

The high temperature and high H₂S/CO₂partial pressure corrosion environment was simulated in a high temperature and high pressure (HTHP) autoclave, and the passivity breakdown of passive film for G3 alloy in severe sour environment at high temperature was investigated by scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that in the simulated environment of 150 ℃, 3 MPa H₂S, 2 MPa CO₂, and 30 000 mg/L NaCl, the compositions of the passive films formed on the surface of G3 alloy was transformed to metal sulfides from metal oxides and hydroxides with the increase of exposure time. After the exposure of 672 h, the passive film brokedown and some pits appeared on the surface of G3 alloy. The sulifides of Cr, Ni and Fe appeared in the passivation film. The susceptibility to pitting of G3 alloy should be high in severe sour environment at high temperature. The passivity breakdown of passive film and the initiation of pitting were related to the surfuration effect of S^2- on the passivation film.