Passivation performance of Nb microalloyed rebar in concrete carbonation environments with different pH

In this study, XPS sputtering depth, SEM and electrochemical tests (CV, EIS, M-S, i-t, DPP) were used to study the structural composition and formation mechanism of surface passive film of Nb microalloyed rebar in SCPS with different pH. The results showed that after passivation for 10 d in SCPS with different pH, compared with CS rebar, the stability and compactness of surface passive film of 34Nb rebar gradually increased with the decreases of pH. Firstly, with the decreases of pH, the outer layer of surface passive film of 34Nb rebar was composed of Fe oxides and Fe hydroxides, and the inner layer was composed of Fe oxides and Nb oxides, thus increasing the mass ratio of Fe³⁺/Fe²⁺ and Nb₂O₅/(NbO₂ + NbO). Secondly, with the decreases of pH, the addition of Nb promoted the formation of Fe oxides in 34Nb rebar, obtaining the excellent R_ct, E_corr and E_p. Finally, with the decreases of pH, the addition of Nb promoted the enrichment of Nb oxides in 34Nb rebar, inhibiting the degradation of Fe oxides, and the passive film exhibited the P-N type semiconductor, thus significantly decreasing the carrier density and enhancing the passivation rate of 34Nb rebar.