The Effect Of NO₂^-on The Pitting Corrosion Of Q235Carbon Steel In Synthetic Carbonnated Concrete Pore Solution

Reinforced concrete is the most widely used structural materials in the world, and its service life is mainly affected by the rebars corrosion. Generally, the pH of the concrete is12.5-13.5and the oxide film formed on steel surface can protect the rebars from corrosion. Carbonization can lead to the decrease in concrete pH, the Cl-in the concrete will cause the breakdown of the oxide film and lead to rebar corrosion. Inhibitors can effectively reduce the corrosion rate of rebars and prolong the service life of concrete structures.Many inhibitors have been developed for reinforced concrete constructures, and the calcium nitrite is used widely. A lot of studies have been done on the calcium nitrite inhibitor and most works focused on the effect of calcium nitrite on uniform corrosion. However, there are very few study about the influence of calcium nitrite on the localized corrosion and the inhibition mechanism, especially on early stage of pitting corrosion. Understanding the inhibition mechanism clearly will benefit the development and practical use of inhibitors in concrete.In this paper, the pitting corrosion behaviors of Q235carbon steel in the slightly carbonated and chloride-contaminated simulated concrete pore solution were studied by slow potentiodynamic polarization measurement, potentiodynamic polarization measurement, X-ray photoelectron spectroscopy (XPS), Mott-Schottky plots, scanning electron microscope (SEM) and Scanning microelectrode technique methods. The effects of NO2-and Cl-on Em and Eb of steel were studied. The effect of NO2-concentration on the passive film on the steel surface under different Cl-concentrations and the effect of polarization potential and NO2-concentration on the nucleation and growth of metastable pitting were also studied. The mechanism of NO2-on the initiation and growth process of the metastable pitting was analyzed. The main conclusions are as follows:In the carbonized simulation of concrete pore solution (pH11) with Cl-, NO2-can inhibit the metastable pitting and stable pitting of carbon steel and the corrosion inhibition effect increase with the increasing concentration of NO2-. When the concentration of NO2-is below a threshold, the variation of concentration will significantly affect the Em and Eb values, while above the threshold the variation of concentration will affect Em and Eb slightly. The threshold increases with the increase in Cl-concentration.In the synthetic carbonized concrete pore solution, with the increasing of the concentration of NO2-,the nucleating rate of metastable pittings decreases, the growth rate of metastable pitting decreases also. The NO2-concentration has no noticeable effect on the life of metastable pitting. The metastable pitting with a moderate growth rate transform to stable pitting easily.The donor concentration and the stability of passivation film on the surface of carbon steel are affected by the concentration of NO2-. The higher concentration of NO2-, the lower donor concentration and the higher Fe3+/Fe2+ratio. NO2-can promote a more complete and stable passivation film on the carbon steel samples which will inhibit the occurring of metastable pitting and reduce nucleation rate.The metastable pittings on the surface of carbon steel after potentiostatic polarization measurement is the result of the current fluctuation.Metastable pitting is nucleated on MnS inclusions preferently. Metastable pitting’s morphology on the surface of carbon steel is related to the shape and depth of inclusions.The metastable pitting nucleating around the shallow inclusions are open.The metastable pittings with residual oxide film around it’s mouth, were nucleated around the deep inclusions.The metastable pittings tend to nucleating around the nucleated pittings, and expand horizontally easily, form the shallow stable pits.