Analysis of current spikes associated with pitting events was performed on high purity A1 loop wire electrodes over a range of C1-concentrations (10 -5 to 1M) and potentiostatieally applied potentials. A distribution of pitting and repassivation potentials was observed at each C1 concentration. Factors controlling the transition from metastable to stable pitting were identified by comparing the electrochemical behaviors of stable pits at elapsed times equal to the mean lifetime of metastable pits. This comparison also provided insight on the origins of statistical distributions of pitting potentials. The key differences were that (i) stable pits had a faster rate of rise in pit current which implies a faster growth rate and, subsequently, larger pit radii at times equal to metastable pit lifetimes, and (if) stable pits satisfied the criterion IpJrpi t > 10 -2 A/em, at all times during growth, indicating that a concentrated A1C13 solution must be maintained for pit survival. However, the pit growth rates were ohmically limited near the pitting potential. Chromate inhibitor decreased the metastable pit nucleation rate and minimized pit growth rates so that the 10-~A/cm criterion was difficult to achieve; hence the chance for pit stabilization was reduced.

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The mechanism of trench formation next to cathodic intermetallic compound particles ͑IMC͒ on bare polished AA 2024-T3 was studied in situ using confocal laser scanning microscopy. Trenches formed at the interface between the matrix and some IMC of the Al-Cu-Mn-Fe and Al-Cu types in all electrolyte solutions studied, including 0.1 M Na 2 SO 4 ϩ 0.005 M NaCl with pH adjusted to 3, 6, and 10 as well as in near-neutral 0.5 M NaCl ͑pH 6͒. The trenches in the acidic solution were narrower, and not every cathodic IMC examined developed trenches. Two models for trench formation are compared in their ability to rationalize the experimental observations. The alkaline model ascribes the trenching completely to the effects of local pH increase and is unable to rationalize trenching at low pH, the dependence of trenching on particle type, and the effect of chloride ion content on trenching rate. An anodic trench model combines the influences of IMC cathodic kinetics, galvanic coupling, and anodic dissolution of the matrix, and particle/matrix metallurgy on the trenching behavior. The latter model is able to rationalize the effects of chloride content, bulk pH, as well as the observation that not all particles of a given type undergo trenching.

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.

The distance over which a Mg-Rich Primer (MgRP) coating galvanically protected exposed AA2024-T351 by sacrificial anode based cathodic protection (termed galvanic throwing power) was studied via finite element analysis (FEA) modeling. FEA enabled prediction of the spatial distribution of the galvanic current density over the surface of a simulated bare AA2024-T351 scratch. The galvanic current density in various full immersion and thin layer electrolyte geometries relevant to field service was investigated. Current and potential distributions extended across the simulated bare AA2024-T351 scratch when the electrolyte layer was thick, continuous and more conductive (higher salt concentration) and in the absence of a resistive coating separating the Mg from the electrolyte. 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 coating over the Mg. Additionally, galvanic protection intensified during a short period of time associated with drying and re-wetting cycles due to changing electrolyte conductivity and E-i behavior. These results qualitatively predict and agree with the experimental behavior of previously reported microelectrode investigations, of a similar Mg/AA2024-T351 couple configuration, to a first order approximation.