Corrosion of Metals by Acid River Water. I : Theoretical Consideration of Corrosion of Iron and Steel

The mechanism of corrosion of iron and steels by acid river water was theoretically studied. The corrosion was found to be either a catnodic or a mixed control. In the stagnant acid water, the rate-determining step of the corrosion reaction was the diffusion of dissolved oxygen (diffusion control) at higher pH than 3.6, whereas it was the discharge of hydrogen ions (activation control) at lower pH than 3.6. The corrosion rate at the activation control was calculated from the heat of adtivation of the rate-determining step, the pH and the natural electrode potential. The corrosion rate in the mixed control was calculated from the heat of activation, the pH, the standard single electrode potential, the diffusion constant of metal ions and the thickness of diffusion layer. The heat of activation was calculated from the cathodic current density and the overvoltage. In the flowing acid water, the rate-determining step was the diffusion of hydrogen ions at higher pH than 4.1, whereas it was the diffusion of dissolved oxygen at lower pH than 4.1. The corrosion rate was proportional to seven-eighth power of the flow velocity. Stainless steels might be passivated in the high speed flow. It was shown that the "moto-electrode effect" was important on the corrosion in the acid river water