The passive films formed on the 9%Cr, 0.1%C, microcomposite steels (ASTM A1035) in highly alkaline environments, such as concrete pore solutions, were studied. In-situ surface-enhanced Raman spectroscopy (SERS) was conducted during cyclic potentiodynamic polarization experiments to characterize the formation and breakdown mechanisms of the passive films formed. A solution of 0.55 M KOH + 0.16 M NaOH, with and without chlorides (0 % and 3.5 % NaCl), was used for the SERS experiments. In addition, the corrosion performance of microcomposite steels as a function of pH (9–13) and ionic strength of the solution (from 10-5 to 10-1 M) was evaluated via polarization resistance (Rp) experiments, and compared to the performance of regular carbon steel. Results show that the passive film formed on the microcomposite steel seems to be characterized mainly by the presence of Cr(OH)3 and minor amounts of Fe species; more specifically Fe3O4 and a Fe(III) phase. The film is considerably more protective than that formed on conventional carbon steel under the same test conditions. Microcomposite steel has not shown a reduction of polarization resistance as a function of pH drop when the ionic strength of the medium was kept constant, in a stark contrast to the behavior verified for the regular carbon steel rebar.

Partial replacement of Portland cement by supplementary cementitious materials results in decreased concentration of calcium hydroxide, leading to a reduction in the pH of the pore solution and influencing the formation of passive films on steel. Decreases in pH are often equated to lower-quality passive films, but, as has been recently shown, that is not always the case. This study evaluates the effect of partial replacement of cement Portland by silica fume, both on the pH of pore solution and corrosion resistance of reinforced concrete. Three concrete mixtures with variations of cement replacement by silica fume (0, 5 and 10% by volume) were produced. Pore solution chemical composition, corrosion current density and polarization resistance were measured up to 91 days. Results show that, especially in early ages—when concrete resistivity remains low and unchanged among the different mixtures tested—partial replacement of cement by silica fume led to an improved passivation process and increased corrosion resistance, even with a related decrease in calcium hydroxide concentration and pH.