Passivation behavior of Cr-modified rebar in simulated concrete pore solutions with different pH

In this study, the enhanced passivation effect of Cr-modified rebar compared with carbon rebar has been explored with regard to the formation process of the passive film, which could provide a theoretical basis for the development and application of concrete materials in harsh environments. X-ray photoelectron spectroscopy and capacitance-potential method were used to investigate the composition, structural depth distribution, and semiconductive properties of passive films formed by Cr-modified rebar and carbon rebar in simulated concrete pore solutions with different pH. The kinetics of passive film growth and formation were analyzed by tracking tests of potentiostatic current transients and open circuit potential under natural aeration. The results show that the activation energy of Cr-modified rebar is lower at 14.59 kJ/mol, compared to carbon rebar's 25.36 kJ/mol. In the early stage, Cr-modified rebar can achieve a thinner, more stable initial film in a shorter time than carbon rebar. The rate of reduction of the initial thickening rate of the passive film on Cr-modified rebar is five times than that of carbon rebar per unit pH. The film growth rate after early stage of Cr-modified rebar decreases, whereas that of carbon steel increases, with decreasing pH. The passive film containing Cr oxide or hydroxide enhances the barrier effect of the oxide film to ion migration, and the re-dissociation of Fe oxide hinders the passivation process. This grants the thickness of the stable passive film of the two rebars changing oppositely and the Cr-modified rebar compared to carbon rebar enhance passivation with pH falling.