(V) Study of the Passive Film of 316LN Stainless Steel Rebars in Synthetic Concrete Pore Solution Under Stress Corrosion Cracking

In this work, the passivity breakdown of the passive film of the austenitic AISI 316 LN stainless steel (SS) rebar is studied. This corrosion process experiences a dynamic evolution from charge transfer to mass transport dominating process, due to the development and rupture of the passive film. In order to see the changes in the passive film of the samples exposed to stress corrosion cracking, uniaxial tensile test by slow strain rate test (SSRT) and synthetic concrete pore solution (high alkaline, pH = 12.6) contaminated with chloride ions (4 wt.% Cl^–) were combined. The study of the formation and breakdown of the passive film is done by electrochemical tests including Mott-Schottky, impedance (EIS), polarization curves (CPP) and current monitoring (current transient). The current density monitoring shows multiple peaks related to the formation of metastable pits on the surface when the tensile load gets close to the yield stress (f_y), followed by passivity breakdown of the passive film as the combine effect of residual stresses, in between the rebar and the passive film, and the pit nucleation breaks the protective layer. The activation of dislocations dynamics induces high stress concentration in the passive film after reaching f_y, increasing the oxygen diffusion due to the development of oxygen vacancies on the film. This development of the density of oxygen vacancies is reflected on the Mott-Schottky as the donor carries density increases, making the passive film more doped, and hence more unstable. The addition of 4 wt.% Cl^– to the synthetic concrete pore solution enhances the pit initiation and propagation process leading to film rupture, which is also seen in the development of a mixed fracture mechanism, characteristic of SCC.