Characterizing the Impact of the Oxygen Reduction Reaction Mechanism and Current Distribution on Mineral Deposit Structure and Composition

Abstract

In marine environments, cathodic protection is applied to metallic structures to prevent corrosion through polarization. The cathodic current causes secondary mineral deposition reactions, whose makeup, 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. Two factors contributing to the pH at a surface are current distributions across a surface and the reaction mechanism mediating the current. This work looks at the influence of these two factors on the make-up, phase and surface impedance impacts of mineral deposits on electrode surfaces. The role of Oxygen Reduction Reaction (ORR) mechanism and current distribution on the formation of Ca and Mg deposits on platinum and 316L stainless steel were assessed using a combination of rotating disc electrode (RDE), electrochemical impedance spectroscopy (EIS), and surface analysis techniques.In order to verify the material and potential dependent ORR mechanisms on platinum, glassy carbon, and 316L stainless steel, a series of RDE experiments were conducted. The platinum electrode and glassy carbon electrodes were shown to have excellent agreement with the predicted Levich behavior for a 4-electron ORR mechanism and a 2-electron ORR mechanism respectively, for all of the potentials in the mass transport limiting current range for ORR. However, the 316L stainless steels samples exhibited a deviation in the predicted Levich response at lower cathodic overpotentials in the ORR limiting current region. This behavior has been hypothesized to be caused by a reduction in the iron oxide layer present on the stainless-steel surface, which in turn results in an increase in ORR efficiency at more cathodic overpotentials. This transition in ORR efficiency may result in a change in the interfacial pH gradient, and impact the resultant mineral deposit structure and composition. In this work, four different ORR mechanism conditions were evaluated in terms of mineral deposit formation potential: polished platinum electrodes, polished glassy carbon electrodes, polished 316L stainless steel electrodes, and pre-reduced 316L stainless steel electrodes.Current distribution was evaluated in a fabricated, macro scale scanning probe system. These experiments assessed Pt and 316L as a 7 cm diameter disk working electrode, surrounded by a platinized ring counter electrode under quiescent conditions. Current distribution was observed via electric field measurement and surface potential mapping. The electrodes were polarized in ASTM D1141 artificial seawater, and the growth of the deposit was monitored in situ using EIS. The structure of the deposits on each material was evaluated by confocal profilometry and scanning electron microscopy with energy dispersive x-ray spectroscopy to determine structure and chemical composition. Mineral deposits were evaluated spatially and correlated to RDE work via an equipotential model. The work presented here provides insight into the contribution of ORR mechanism and current distribution across several size scales, on mineral deposit formation on materials with both ideal and hybrid ORR activity.