Electrochemical Test Configurations to Study Atmospheric Corrosion

Abstract

Atmospheric corrosion is one of the most difficult forms of corrosion to forecast. Although standardized exposures and accelerated tests exist for material characterization and prediction purposes, the premise of these approaches can be oversimplified to a fault that precludes utility. For instance, the concept of time of wetness is now recognized as a broad stroke parameter that distinguishes little about the rate of corrosion.1 Furthermore, many off-the-shelf systems and public weather services allow streaming of environment parameters such as relative humidity, temperature, wind speed/direction, solar radiation and precipitation. This deeper understanding of atmospheric corrosion processes and litany of meteorological information brings to question the best electrochemical approach(es) to characterize materials for atmospheric corrosion modeling. In this work we examined three different electrochemical test configurations, show in Figure 1, to study atmospheric corrosion with polarization experiments that cover comparable parameter space. These experiments were tailored to the oxygen reduction reaction (ORR), the primary cathodic reaction in many corrosion systems, as literature data permits facile calculation of diffusion limited current under thin film electrolytes and forced convection. The first configuration consisted of an electrode stack with an oxygen permeable counter electrode placed in an environmental chamber, in which a wetted membrane fixed the electrolyte thickness. The second approach also required an environmental chamber to control film thickness but instead utilized microelectrodes to enable polarization with minimal disruption to the boldly exposed electrolyte surface. The third configuration deviated from the environmental chamber approach with a bulk electrolyte and rotating disk electrode to set the diffusion layer thickness with rotation rate. These systems were all evaluated in chloride concentration relevant to atmospheric conditions. Polarization curves for different test configurations are examined in 2.7 M NaCl. While comparable, these configurations display distinct differences in iR drop, reproducibility and charge transfer resistance, which give rise to different open circuit potentials, Tafel slopes and diffusion limited behavior. These results will be discussed in terms of test configuration features to better understand applicability of different configurations, including practical limitations and potential for improvements/optimization. 1 Schindelholz, E., Risteen, B. E. & Kelly, R. G. Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies: II. MgCl2, Artificial Seawater. J Electrochem Soc 161, C460-C470, doi:10.1149/2.0231410jes (2014). Figure 1: Photographs of each test configuration; membrane configuration (left), microelectrode configuration (center) and rotating disc electrode (right). Figure 1