Kinetics of the spontaneous dissolution of iron in concentrated ionic media

Abstract

The kinetics of the dissolution of polycrystalline iron in acid solutions has been studied. Perchloric acid in sodium perchlorate, hydrochloric acid in sodium chloride and nitric acid in sodium nitrate solutions were used, covering the range of pH between 0 and 6 at temperatures from 40 to 70°C.The rate of dissolution was followed by recording the activity of hydrogen ion in the solution, the corrosion potential being constant during the run.The kinetic constant for hydrogen-ion disappearance was used to calculate the corrosion current-density assuming that hydrogen-ion discharge on an iron surface follows a Tafel equation with a slope of 2·3(2RT/F). For sodium perchlorate solutions the corrosion current increases with pH (in the range 1–5) with δ log icorr/δpH = −1. For sodium chloride solutions the slope is close to −0·50.In the same range of pH the corrosion rate is greater in the order ClO4−, Cl−, NO3−. The results are interpreted in terms of recent information about the kinetics of the iron electrode.