Electrochemical and Microscopic Investigation of the Passivation and Depassivation Processes of Iron and Steel in Simulated Concrete Pore Solutions.

摘要

This study involved the investigation of the kinetics of the passivation and chloride-induced depassivation of iron and carbon steel in saturated calcium hydroxide solution (CH) and simulated concrete pore solution (CP). The investigation consisted of electrochemical (Electrochemical Quartz Crystal Nanobalance, Electrochemical Impedance Spectroscopy, and Free Corrosion Potential) and microscopic/spectral (Transmission Electron Microscopy and Electron Energy Loss Spectroscopy) studies.;Microscopic and spectral studies showed that the chemical composition and ionic state of iron change within the oxide film The inner oxide film resembles FeO in both passivating solutions, which is known to be protective in alkaline media. The outer oxide film resembles alpha-Fe2O 3 in the CP solution and FeOOH in the CH solution. The transition between these layers is indistinct and, most likely, in the form of Fe3O 4 (FeIIO.Fe2IIIO3). In presence of chloride inner protective layer experiences the most deterioration, leaving steel depassivated after adequate amount of chloride penetrates through the unprotective outer porous layer.;The electrochemical studies showed that the thickness of passive film formed on iron was on the scale of nanometres and composed of two parts: a thin protective oxide/hydroxide layer on the steel surface and a thick porous oxide/hydroxide layer above it. The protective layer formed rapidly within 10-20 minutes of exposure to the passivating solutions, while the unprotective layer continued to grow for days albeit at a decreasing rate. The chloride-induced depassivation of iron was explained with three successive stages: chloride ingress through the porous layer, dissolution of the protective layer, and passivity breakdown.