The behaviour of mechanically polished iridium surfaces was studied in oxygenated aqueous sulphuric acid solutions at 30°C by the open circuit potential technique. The conclusions derived from these measurements were confirmed by determining the dc characteristics of the system. In sulphuric acid solutions more concentrated than 0·5M the pre immersion oxide film grows to become multilayered. The resultant oxide film behaves as a compact film and does not exhibit a reduction peak. In more dilute solutions the preimmersion oxide film appears to hydrate and dissolve. The order of this hydration-dissolution reaction is ~ 1 with respect to the hydroxyl ion.
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