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Materials Science at Oregon State University

Investigation of Passivity of Iron and Carbon Steel in Simulated Concrete Pore Solutions

Date: Thursday, Oct 4th
Presenter: Prof. O. Burkan Isgor, School of Civil and Construction Engineering, Oregon State University

Abstract


Kinetics of passivation and chloride-induced depassivation of iron and carbon steel in saturated calcium hydroxide solution and simulated concrete pore solution was studied. The investigation consisted of several electrochemical (EQCN, EIS and OCP monitoring), microscopic (FIB/TEM) and spectral (XPS and EELS) techniques. 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 decreasing rates. Microsocpic and spectral studies revealed that passive films of carbon steel in concrete were typically 5-10 nm thick and contained a layered structure as well. The inner oxide film had a structure similar to that of FeO, which is known to be unstable in the presence of chlorides. The outer oxide film was typically in the form of alpha-Fe2IIIO3 / Fe3O4 (FeIIO-Fe2IIIO3). The transition layer between the inner and outer layers of passive film was mainly composed of Fe3O4 (FeO-Fe2O3). The chloride-induced depassivation is explained with three successive stages: chloride ingress through the porous layer, dissolution of the protective layer, and passivity breakdown. 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. During this process the relative amount of the FeIII-oxides to FeII-oxides increases as a result of valence state transformation of oxides from FeII to FeIII.