The Anodic Behavior of Iron-Carbon Alloys in Moving Acid Media

The anodic behavior of pure iron, mild steel (0.055% C) and iron‐1.3% C was studied in flowing oxygenated and deaerated solutions of sulfuric and hydrochloric acids, the results being presented mainly as apparent anodic polarization curves at low current densities and as plots of potential against Reynolds number for a fixed current density at high current densities. The data were obtained largely by conventional galvanostatic methods for plate electrodes situated in a cell of square cross section, designed to allow the complete development of the hydrodynamic and mass transfer boundary layers. In solutions containing oxygen, the rest potentials and apparent polarization curves were shifted in the positive direction on increasing the flow rate, the effect being greater the lower the carbon content of the alloy. In deaerated acid the curves were insensitive to flow changes up to a critical current density, which was lower for more dilute acids and higher carbon contents, since the reactions involved are entirely activation controlled. Influences of carbon content, heat‐treatment, and acid and cation concentrations on the position and length of the Tafel region of the anodic polarization curve are considered. At high current densities ( e.g. , 100 ma/cm 2 ) the behavior of all three alloys departed from Tafel behavior, and potentiostatic and galvanostatic results were observed to be sensitive to the liquid flow rate in both hydrochloric acid and sulfuric acid solutions. From galvanostatic results obtained at a current density of 440 ma/cm 2 it was deduced that a film was formed at the anode surface. The presence of this film was shown to present an additional impediment to convective mass transfer of anolyte, thus occasioning a redistribution of the potential difference between metal surface and bulk electrolyte. While changes in liquid flow rate could be expected to control the rate of mass transfer in the liquid boundary layer, their effect on the thickness and nature of the porous surface layer was complex.