Passivation of X65 steel in NaHCO3 solution saturated with CO2 using thin wire specimens

The passivation of X65 steel in NaHCO3 solution saturated with CO2 was investigated using electrochemical and immersion tests as well as surface morphology and composition analysis. The tests were conducted using thin wire specimens at atmospheric pressure in NaHCO3 solution with different concentrations and at different temperatures. The electrochemical properties of thin wire specimens and square specimens were compared. Thin wire specimens were immersed for a long period, during which the open circuit potential (OCP) was recorded. Anodic polarization was performed after the OCP was sufficiently high and stable. Electrochemical tests revealed that the electrochemical properties of the thin wire specimens were consistent with those of the square specimens. A sudden increase in the OCP of approximately 700mV occurred after the specimens were immersed for a long period in the tested concentration and temperature. The time at which the OCP increase occurred decreased when the NaHCO3 concentration or temperature were increased. After the OCP became stable, anodic polarization revealed favorable self-passivation. Immersion tests on X65 steel demonstrated that the OCP increase may have been due to the filling of pores between FeCO3 grains. Small tetrahedron grains formed in the pores of large hexahedral grains to lower the porosity of the film, thus making the film more compact. Consequently, the OCP increased and anodic polarization presented self-passivation.