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The corrosion and passivation of iron in the presence of halide ions in aqueous solution

Corrosion Science
Publication Date
DOI: 10.1016/s0010-938x(79)80113-4


Abstract The anodic dissolution behaviour of iron in halide solutions has been studied with both stationary and rotating electrodes. With stationary electrodes active dissolution kinetics are observed, whereas with rotation a pronounced active/passive transition occurs. A distinct pitting potential ( E c ) was noted in each solution, the value of E c increasing in the order I>Br>Cl>F. Halide ion concentration and electrode velocity did not have any effect on the value of E c , indicating that the kinetics of pit initiation are independent of mass-transfer effects. During anodic dissolution at potentials more regative than E c , an inhibiting effect was noted, the degree of which depended on the atomic radius of the anion. A model is suggested which involves three electrode reactions of iron with the electrolyte: (1) Active dissolution involving the well-known FeOH + (ads) rate-determining step. (2) Above the passivation potential, increased reaction of the metal surface with hydroxyl ions causes passivation due to the enhanced access of OH − to the surface and accelerated removal of solvated protons caused by rotation and a thinning of the diffusion layer. (3) At the pitting potential, direct reaction of the metal with electro-adsorbed halide ions produces pit initiation and growth by a complex ion formation reaction not possible at lower electrode potentials.

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