The effect of halide ions on the anodic passivation of titanium in acid, neutral and alkaline solutions has been studied with the following results: (1) In the presence of fluoride, a severe attack of titanium occurs in an acid solution owing to the chemical dissolution of the oxide film by undissociated HF. However, titanium is not attacked in neutral and alkaline solutions. (2) Chloride ion does not prevent the passivation of titanium owing to a very high discharge overvoltage of chloride ion on titanium. (3) Bromide ion discharges from acid and neutral solutions at a high overvoltage, causing a pitting corrosion in a neutral solution. (4) Iodide ion also shows a high discharge overvoltage in acid and neutral solutions, but not in an alkaline solution below the discharge potential of hydroxyl ion. No pitting corrosion occurs by the discharge of iodide ion. (5) The corrosion resistivity of titanum in halide-bearing solutions is due to its larger affinity with oxygen than with halogen. Therefore, in aqueous solutions the amount of the dissolved oxygen, hydroxyl ion, or oxidizing agent has a significant effect on the corrosion resistivity of titanium. No pitting corrosion takes place unless a considerable external electrolytic current is applied, and the pitting corrosion is usually prevented under natural conditions.
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