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 (CP) 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 were found to be
more coherent and protective than calcareous deposits grown in artificial seawater. The
ratio of magnesium to calcium of specimens exposed to artificial seawater 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 CP 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.
Keywords: Cathodic Protection (CP), electrochemistry, potentiostatic method, scanning
electron microscopy, silver/silver chloride reference electrodes, surface films,
weight loss measurements, x-ray diffraction analysis, surface films, calcareous
deposits and protection criteria