Corrosion of High Chromium and Conventional Steels Embedded in Concrete

Corrosion rates of high chromium and conventional steel rebars were measured and compared by conducting two studies. One was on concrete blocks and the other was on bare steel rebars. In the former study, concrete blocks that had been made with two different steel rebars were placed in sodium chloride solutions, and air was blown through the solutions to accelerate corrosion of the embedded steel rebars. These blocks were taken out of the solution periodically, and the corrosion rates of the rebars were measured with a 3LP device. In the latter study, the bare rebars of the two steels were also corroded in sodium chloride solutions through which air was blown, withdrawn periodically, dried, and weighed after the corrosion products were removed. The corrosion rates were measured by the reduction of the weight of the rebars. In the study on concrete blocks, it was found that the corrosion rate increases for both steels as the concentration of sodium chloride in solution increases. It was also found that the corrosion rate of concrete blocks reinforced with conventional steel was about twice as much as that of the concrete blocks reinforced with high chromium steel after $132$ days of exposure. From the study on bare steel rebars, it was found that the rate of corrosion of conventional steel was 12 times as much as that of high chromium steel at 0.1% sodium chloride, and the ratio decreased to 2 times as much when the sodium chloride concentration was increased to 3%. It was also found that the corrosion rate of high chromium steel was very sensitive to sodium chloride concentrations whereas that of conventional steel was not sensitive. The corrosion products were analyzed using x-ray diffraction and atomic absorption spectroscopy to identify the minerals present in them. It was found that corrosion products produced on the high chromium steel were predominantly lepidocrocite $\left(\gamma \text{-}\mathrm{Fe}\mathrm{O}\mathrm{O}\mathrm{H}\right)$ and hematite $\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)$ , whereas that on the surface of conventional steel was predominantly magnetite $\left({\mathrm{Fe}}_3{\mathrm{O}}_4\right)$ . It appears that the former iron oxides form an adherent and nonporous protective layer while the latter iron oxides (magnetite) do not, which can explain the distinct difference in corrosion rates of the two steel rebars.