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

Abstract

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 MgCl 2 (due to its low deliquescence/efflorescence point of ∼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.

Utilization of chemical stability diagrams for improved understanding of electrochemical systems: evolution of solution chemistry towards equilibrium

“…3,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Some proven systems employed for this purpose include Znbased coatings for steel and Mg-based and Al-based coatings for aluminum alloys. 3,[14][15][16][18][19][20][21][22][23][24][25][26][27][28][29] All of these systems function primarily by galvanic coupling of the anodic metal pigments to another material in order to suppress corrosion degradation. The performance of the galvanic couple is influenced by pH, metal ion concentration, pigment passivation, pigment composition, and ionic strength.…”

“…The performance of the galvanic couple is influenced by pH, metal ion concentration, pigment passivation, pigment composition, and ionic strength. 23,26 These factors are considered in utilization of the chemical stability diagram, which predicts final equilibrium solution chemistry.…”

Performance of a Magnesium-Rich Primer on Pretreated AA2024-T351 in Full Immersion: a Galvanic Throwing Power Investigation Using a Scanning Vibrating Electrode Technique

“…9,10 This is an indication of increased throwing power under thicker electrolyte layers, when other conditions were held constant. 9,10 Regarding polymer resistance, it was found that the ionic resistance of the added polymer layer over the Mg electrode significantly mediated the galvanic current passing between anodes and cathodes and, when large enough, completely prevented the galvanic coupling of the electrodes altogether. 9,10 Galvanic systems have been studied previously using MEA, FEA and post-exposure corrosion volume loss characterization to study throwing power and defect protection.…”

“…[18][19][20][21][22][23][24] The galvanic protection potential is usually dictated by mixed potential theory and mediated by various electrical/ionic resistances between the anode and cathode such as: polymer barrier properties of the MgRP, pretreatment resistances, electrolyte chemistry, electrolyte thickness and geometry, and anode/cathode ratio. 9,10 Barrier protection is afforded by the MgRP itself and also the pretreatment layer. 11 Furthermore, the pretreatment may provide additional corrosion protection by the release of anionic species that inhibit both anodic and cathodic kinetics of AA2024-T351.…”

A Review of Modern Assessment Methods for Metal and Metal-Oxide Based Primers for Substrate Corrosion Protection

“…A key aspect of metal-rich primer (MRP) performance is the ability of the primer to protect latent or operational defects such as scratches as well as the substrate under the intact coating. Effective design of metal-rich primers to achieve such performance requires specific tuning for each chosen alloy substrate, and this tuning is informed by a variety of characterization methods (Bierwagen et al, 2007;Yan et al, 2010;King and Scully, 2011;King et al, 2014aKing et al, , 2015Bastos et al, 2016;Santucci et al, 2017aSantucci et al, , 2018a.…”