Experimental Determination of the Passive‐Active Transition for Iron in 1M Sulfuric Acid

Abstract

Current‐voltage curves from three iron rotating disks are obtained using a polarization device which permits determination of the transition region between the active and passive states. Diameters of the three electrodes are 0.494, 0.298 and 0.0986 cm, respectively. Results from the smallest electrode have the least amount of ohmic distortion. Trends in the data, which are observed as the electrode size is decreased, form the basis for the conclusions that are drawn from this work. Analysis of these trends indicates that the active‐passive transition is characterized by a nearly vertical line on a current vs.potential plot. The Z‐shaped nature of the current‐voltage curves, reported by Epelboin et al., is entirely due to ohmic potential drop in the solution. Analysis of the data, within the framework of a mathematical model presented by Law and Newman, indicates that the average current density on the active portion of the disk increases as the size of the active area decreases during the passivation process.