Electrochemical Cell Study of Cut Edge Corrosion Inhibition on Coil-coated Steel Sheet

SUMMARY

Corrosion and corrosion inhibition of coil-coated galvanised steel has been studied using an electrochemical sandwich cell which models the geometric situation at sheet steel cut edges. The galvanic currents flowing during immersion in a solution simulating typical acid rainwater were measured. Also, potentio-dynamic polarisation was carried out on the galvanic cell where the individual currents on the steel and zinc, as well as the total current, were determined. Where no inhibitor is present in solution, the corrosion rate is controlled by oxygen diffusion to the steel cathode. In the presence of strontium Chromate the cathodic reduction of oxygen on the steel cathode is substantially reduced in addition to the expected anodic passivation of zinc. This is presumably due to reduction of Chromate at the cathode forming a protective chromium oxide film. Some alternatives to Chromate were also tested. Molybdate/phosphate pigments and calcium ion-exchange pigments alone gave poor inhibition. However, they were found to act synergistically in combination giving an inhibitive efficiency closely approaching that of Chromate.