Heterogeneous dissolution behaviour of a polycrystalline pure iron in sulphuric acid (pH 1 or 2.3) was investigated by a simple polarization technique and scanning electrochemical microscopy (SECM). Both corrosion potential and corrosion current on a single grain were found to be dependent on crystallographic orientation of the iron grain. A high corrosion current flowed on a grain showing a relatively noble corrosion potential, although both cathodic and anodic Tafel slopes were independent of the orientation. It was shown that cathodic hydrogen evolution reaction (HER) governed the corrosion reaction on the iron grain. On the other hand, an SECM image visualizing the generation of Fe~(2+) ions from the polycrystalline iron revealed that the iron dissolved heterogeneously corresponding to the grain orientation. The dissolution reactivity of each grain was different from the corrosion reactivity obtained by the simple polarization of the single grain. It was suggested from the comparison that the heterogeneous dissolution was due to the difference in HER reactivity of grains. Covalent bonding of Fe and H and coverage of H on Fe seemed to play important roles in the local HER leading to the heterogeneous dissolution of polycrystalline pure iron.
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