Electrochemical Characteristics of Iron During Corrosion: Effect of Heat Treatment and Purity

The electrochemical behavior of unannealed and annealed Armco iron in deaerated 1N H₂SO₄ during corrosion has been investigated. In addition, studies of iron of higher purity, Ferrovac E (99.91%) and zone refined iron (99.997%), were made. Throughout each experiment, the corrosion potential, open circuit differential capacitance, anodic and cathodic polarization, anodic transients, and the corrosion rate were determined as a function of immersion time. For immersion times of less than 30 hours, there were considerable differences between the electrochemical behavior of unannealed and annealed Armco iron. The corrosion rate and the open circuit differential capacitances were greater for the unannealed iron. Furthermore, the capacitances of the latter were not nearly as reproducible, but for all runs the capacitance increased monotonically with immersion time and appeared to approach a constant value at long times. On the other hand, the capacitance of the annealed iron was reproducible and increased linearly with immersion time up to the end of the experiments. Zone refined iron exhibited a steady state capacitance of 45 μf/cm² which was considerably lower than values observed for Armco iron after extended immersion. All annealed iron samples had initial open circuit capacitances of approximately 35 μf/cm². The corrosion rate of iron increased with increase in the purity of the iron. Anodic Tafel slopes of 30, 38, 45 and 43 mv were obtained for unannealed and annealed Armco iron, Ferrovac E and zone refined iron, respectively. Tafel slopes for the hydrogen evolution reaction of 70, 80 and 130 mv were observed for Armco iron, Ferrovac E, and zone refined iron, respectively.