Influence of Electrolyte Layer Thickness and pH on the Initial Stage of the Atmospheric Corrosion of Iron

Cathodic polarization curves for the reduction of oxygen and protons on platinum and iron electrodes under thin electrolyte layers (X1: 1 mm to 10 p.m) were measured to elucidate the effects of the thickness and the pH of the electrolyte layer on the initial atmospheric corrosion stage of iron. A limiting current was exhibited on the cathodic polarization curves under neutral electrolyte layers containing 0.1 to 2 M NaC1, which was inversely proportional to the thickness of the electrolyte layer down to approximately 20 p.m. This result indicates that oxygen diffusion through the electrolyte layer is the rate-determining step (rds) for the oxygen reduction process. Meanwhile the limiting current under still thin-ner layers (X6: 10 to 20 p.m) was independent of the thickness, where an oxygen dissolution step at the air/electrolyte inter-face is the rds because of the considerably higher diffusion rate under such very thin electrolyte layers. AC impedance corrosion monitoring of iron for the period of 4 h was performed under electrolyte layers of different thickness (X1: 1 mm to 10 p.m) and pH (pH 3.0, 4.0, and 5.7). The results indicated that the corrosion rate showed a maximum at an electrolyte thickness of 20 to 30 p.m in all cases, and the effect of pH on the atmospheric corrosion rate was negligible, except for cases wherein a relatively thick electrolyte layer (>1 mm) was present for a short exposure period (<1 h). This can be attributed to a neutralization of the electrolyte due to corrosion reaction because of the extremely limited amount of elec-trolyte present.