Anodic oxidation of both particulate and compact iron in chloride-containing acetate buffer electrolytes

The voltammetric records of Fe powder and compact iron electrode in chloride-free and chloride-containing acetate buffer electrolytes of various pH values were compared in the range 4.0 to 6.0. Owing to the consumption of the powder, the voltammetric current at the powder electrode decreases nearly to zero. Full passivation of the iron surface was observed in the absence of chloride on both electrodes. The differences become clear in chloride-containing electrolytes: with the compact electrode, the dissolution proceeds continuously while with the powder electrode the rate of the process is limited by both the degree of iron conversion and potential applied. Chloride ions not only incorporate into the intermediate species but also form soluble species in the passivation layer, leading to its degradation. The result is an observable anodic dissolution reaction of further Fe powder. The composition of the layer differs in dependence on pH. At lower pH, dissolution proceeds faster and to a larger extent. At higher pH, the thickness of the passive layer is larger and it contains more Fe hydroxides/oxides with higher resistance to the action of chloride. Scanning electron microscopy confirmed deep breakdown of the crystal structure of Fe grains.