Study on Corrosion by Determination of Oxygen Consumption Rate

Self-Polarization Curves of a Steel in a Neutral Solution

Abstract

Concentrations of dissolved oxygen in 3% NaCl solution filled in a closed vessel containing a corroding steel specimen were determined periodically by the polarographic technique. The specimen had been heated at 200°C for 10hr to grow oxide film. As the concentration of oxygen decreased, the rate of oxygen consumption and the corrosion potential of the steel decreased. When oxygen concentration fell to about 1ml/l, the solution was renewed by a fresh one containing oxygen of the predetermined concentration. This operation was repeated at 25°C until a steady state was observed.
Under the experimental conditions, the amount of oxygen consumed was found equivalent to that necessary for oxidizing iron to ferric state. Then corrosion rate of the steel can be derived from the rate of oxygen consumption.
Plots showing the relation between corrosion potential and corrosion current at varied oxygen concentrations give, in the steady state, a straight line on E vs. log i diagram. This is the anodic polarization curve. A line connecting the plots corresponding to the same oxygen concentration gives a cathodic polarization curve, since plots are on the locus of the change of corrosion potential with immersion time of the specimen which corrodes at that oxygen concentration accompanied by breakdown of the oxide film.