Galvanic Couple Current and Potential Distribution between a Mg Electrode and 2024-T351 under Droplets Analyzed by Microelectrode Arrays.

摘要

The galvanic throwing power of bare and polymer coated Mg over a simulated bare AA2024 scribe was studied directly with diagnostic multi-electrode arrays, which enable the spatial distribution of cathodic current density to be elucidated. The galvanic current density over the AA2024-T351 coupled to Mg in various full immersion, thin layer, and droplet electrolyte geometries relevant to atmospheric field exposures was investigated during simulated atmospheric exposures. In these microelectrode studies, current and potential distributions extended somewhat more uniformly across a 5.75 mm long, simulated bare 2024-T351 scratch when the electrolyte layer was thick, continuous and more ionically conductive (i.e., higher salt concentration) in the absence of a polymer coating over the Mg. Current and potential distributions did not extend across simulated defects when the electrolyte became discontinuous or the ionic path became tortuous due to drying or the addition of a resistive polymer coating over the Mg. Additionally, galvanic protection is shown to intensify for short period of time during drying and re-wetting cycles at close distances between Mg and 2024-T351 rationalized to be caused by changing electrolyte conductivity, E-i behavior, and electrode area effects. The drying characteristics of individual salts were also shown to have an effect on the current and potential distribution as MgCl2 (due to its low deliquescence/efflorescence point of approximately 35% RH at STP) was shown to be less susceptible to drying at low RH, thus extending the time into the drying cycle where the galvanic couple was active compared to pure NaCl or ASTM Substitute Ocean Water.