2006 Volume 55 Issue 10 Pages 458-465
Carbon steel was immersed in an aqueous solution of NaNO3 in a closed system for observing both the chemical interaction between metal and NO3-, and the effect of nitrate on the generation rate of H2 gas. The experimental pH range of the solution was 10.0-13.5 which corresponds to that of pore fluid in cementitious material.
The cathodic current density showed a “Tafel equation type” potential dependency in aqueous solution containing NO3- or NO2-. In spite of the accelerated cathodic reaction due to the existence of nitrate, the corrosion rate of carbon steel was not accelerated in the nitrate solutions. This fact suggests that the system is controlled by the anodic reaction. The nitrate reduction accompanied by the corrosion of carbon steel is considered to be a series reaction such as NO3-→NO2-→NH3.
The nitrate reduction reaction competes with the water reduction reaction (hydrogen evolution reaction) within the anodic controlled condition, therefore nitrate strongly reduces the hydrogen generation rate (1/100-1/500 of the cases without nitrate in 1.0 mol dm-3 NaNO3 cases). The generation rates of NH3 were independent of the concentration of NO3- over the range of 1.0×10-3∼1.0 mol dm-3.