Abstract
The kinetics of anodic iron dissolution in deaerated and oxygenated distilled water has been studied using a galvanostatic method. In oxygenated water an indirect method was also used. This indirect method is based on determination of the anodic partial potential-current curve from the dependence of the open circuit potential on the concentration of dissolved oxygen. In deaerated water and in oxygenated water containing 0.25m moles O2/liter or less, an anodic Tafel slope of about 2.3 RT/F was observed; the slope becomes about 2.3×(2/3) RT/F in water containing higher concentrations of dissolved oxygen. The experimental results in deoxygenated water or water with CO2-0.25m moles O2/liter are in agreement with a postulated consecutive dissolution mechanism in which OH- and adsorbed H2O molecules participate in the dissolution process via formation of an adsorbed Fe(OH)ad intermediate. In water containing more than 0.25m moles O2/liter, the data are explained by assuming that the dissolution reaction is in competition with formation of a “passivating” film formed via reaction Fe(OH)ad+OH-→[Fe(OH)2]ad+e.
