In marine environments, cathodic protection is applied to metallic structures to prevent corrosion through polarization. The cathodic current causes secondary mineral deposition reactions, whose composition, phase, and existence are dependent on the pH at the interface. These deposits can cause a significant increase in the interfacial impedance through electrolytic exclusion and obstructions to diffusion of cathodic reactants. In cathodic protection systems, the current distribution is affected by seawater conductivity, structure geometry, anode placement/shielding, and hydrodynamic effects. This non-uniform distribution can result is areas that are over-protected or under-protected. The impact of mineral deposition on the distribution of current on an electrode has not been rigorously evaluated. This work seeks to develop an understanding of how current distributions change on noble and passivated metals during mineral deposit formation.

This work examines the influence of mineral deposition on the current distribution on platinum and 316L stainless steel electrodes. A rotating disk electrode (RDE) method was used in conjunction with electrochemical impedance spectroscopy (EIS) to measure changes in electrode impedance during calcareous deposit formation. The measured charge transfer resistance was used to calculate the Wagner Number, which is a dimensionless quantity that can be used to evaluate the uniformity of current distribution on an electrode. The change in impedance and the calculated Wagner number was compared between the noble platinum electrode and the passivated 316L stainless steel electrode. Current distribution was also observed via electric field measurement in a concentric ring disk arrangement using macro scale scanning probe system. Electric field measurements were made under initial conditions, and after a deposit had formed by polarization in artificial seawater. The results from each of these methods suggests that mineral deposit formation results in an increase in current uniformity, regardless of electrode material.

The work presented here further develops our understanding of how cathodic protection is distributed across a platform in a marine environment as a function of the innate charge transfer characteristics of the protected material. Additionally, the impact of mineral deposition events on charge transfer, and therefore the degree of cathodic protection over an area, is characterized. This work indicates that mineral deposition processes causes current distributions across different conductive materials to be more similar.