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Atmospheric corrosion is characterized by thin film and discontinuous electrolytes that behave in fundamentally different ways than bulk electrolytes. These differences arise due to their finite volumes that limit availability of ionic species and permit the accumulation of corrosion products that can then impact reaction kinetics. To explore these differences between bulk and droplet electrolytes, two different cells were conceived, tested, and used to study galvanic couples in atmospheric environments. The first examined galvanic kinetics of droplets with two discrete half cells. This configuration, shown in Figure 1, allows the time resolved galvanic current and potential of the zero-resistance-ammeter (ZRA) connected anode and cathode to be examined during humidity cycling in the absence of reaction product cross-talk. The galvanic corrosion current and potential were examined during humidity cycling between 95% and 20%. This test configuration was shown to be sensitive to the deliquescence/efflorescence points. The second galvanic atmospheric corrosion cell is that depicted in Figure 2. In this configuration the anode and cathode are brought into intimate contact with a single microliter droplet, in which the droplet is placed on the cathode and the anode is polished to a fine tip. The anode is then translated to pierce the droplet surface with a three axis micromanipulator and the aid of a microscope camera. This configuration differentiates itself from Figure 3 in that a fixed quantity of electrolyte utilized in this system and reaction products from both electrodes are free to interact with one another and either electrode. The galvanic corrosion test cell from Figure 2 displayed a precipitous decay in corrosion current that was found to be dependent upon electrolyte composition. ZRA measurements were compared with SEM and EDS mapping of corrosion products and electrolyte residue to understand the conditions that diminish the galvanic current. These results support significant anion consumption; Cl-, in this case; with a clear dependence on pH that suggests ambient CO2 plays a role as well. This talk will also discuss how the two test configurations compare with one another and use polarization curves to understand the information they provide in a broader context and how they should be applied. Figure 1 |
