The Influence of Cathodically Generated Surface Films on Corrosion and the Currently Accepted Criteria for Cathodic Protection

Cathodically generated surface films on steel specimens have been discovered to vary in composition and protective nature depending upon the electrolyte composition, pH and applied potential. Three electrolytes were studied, pure 3.5% NaCl, artificial seawater and an alkaline solution of composition similar to that found beneath a disbonded land-based pipeline coating. Specimen Potentials varied from free corrosion to -1400 mV (Ag/AgCl/3.5% NaCl). Films grown as a result of applying cathodic protection were analysed by X-Ray Diffraction and Scanning Electron Microscopy and corrosion rates were determined gravimetrically.

Films grown in 3.5% NaCl were composed of iron corrosion products and that were found to be more coherent and protective than calcareous deposits grown in artificial seawater. The ratio of magnesium to calcium influenced the film protective nature. Films grown in the land-based electrolyte were composed of a mixture of iron corrosion products and calcium carbonate with little magnesium present.

Current Criteria for Cathodic Protection used to control land- and sea-based systems will be assessed in view of a better understanding of what happens at the metal /electrolyte interface upon the application of Cathodic Protection.