Effect of simulated pore solution on passivation characteristic of P110 steel

Cement was used in oil industry to fix well. After hydration of cement, a porous material with overbased pore solution will be formed. Passivation analysis will be conducted on the passivation of the outer wall of downhole P110 casing steel under overbased conditions. Due to the difficulty of obtaining cement pore solution, two kinds of simulated concrete pore solutions (SP solution, CF solution) were used to carry out experiments. The different properties of passive film in two different solutions were obtained, which provided reliable data to support for the research of passive film properties of casing steel. Therefore, the electrochemical properties were characterized by electrochemical impedance spectroscopy (EIS), polarization curves method and Mott-Schottky curves method during the formation of the passive film. Moreover, X-ray photoelectron spectroscopy (XPS) was measured to qualitatively analyze the composition of the passive film on the casing surface, and scanning electron microscope (SEM) was used to observe surface morphology of the P110 casing steel. The result showed that the passive film was enhanced with time increasing and the time for complete passivation of P110 steel in two simulated concrete pore solutions was different, requiring longer passivation in CF solution. The passive film in the CF solution had a larger film resistance and the film layer was more stable. The semiconductor characteristics of the passive film were n-type and the donor density of the passive film in CF solution was lower than that of the SP solution. The passive film formed in two simulated concrete pore solutions was all Fe oxides, but not completely the same. Also the steady-state passive film formed in the two solutions had a different microscopic morphology.