Electrochemical and Microscopic Study of Pitting Corrosion of Ultra-pure Iron

Abstract

Pitting corrosion of ultra-pure iron has been studied by both electrochemical and microscopic methods in borate buffer solution with various additions of potassium chloride.

Breakdown potential (E_b) has been determined by galvanostatic, potentiostatic and potentiokinetic measurements. It has been found that the effect of increasing Cl⁻ ion concentration is to shift E_b in the active direction by about 100 mV for a 10-fold change in concentration.

The pits were of a polyhedral shape independent of either the Cl⁻ ion concentration (0·01−0·5 N-Cl⁻) or potential (E_b−+800 m V_S.C.E.). Only {110} planes were exposed within the pits.

Pit number was independent of time but increased with potential. At a constant potential the pits did not develop at the same rate and the number of larger pits increased with time. It has been also found that the pits grew in width only after they had approached the depth limit of ∼ 0·1 mm.