The inhibiting effect of underpotential metal deposits on iron corrosion

The polarization behaviour of iron in perchlorate, chloride and sulphate base electrolytes at pH values ranging from 0.3 ≤ pH ≤ 3.5 was found to be changed drastically by the addition of small amounts of the heavy metal ions Mez+ = Pb2+, Tl+ to the electrolyte solutions. At potentials positive to the corresponding Me/Mez+‐electrode a strong inhibition of the active iron dissolution process as well as the charge transfer controlled hydrogen evolution was observed. This effect can be explained by an underpotential deposition of Mez+ leading to a complete coverage of Me‐adsorbate on the iron surface. In deaerated solution an inhibition factor of 98% can be obtained, whereas in aerated solutions especially under rotating conditions the corrosion rate was not decreased. In the latter case the corrosion process is dominantly determined by the limiting diffusion controlled oxygen reduction reaction which was found to be not markedly inhibited by the metal adsorbates. In acid chloride solutions a competetive adsorption of Cl− must be taken into account.