Interpretation of observations made using local electrochemical impedance mapping (LEIM) on organic coated aluminum alloy 2024-T3.

摘要

Local Electrochemical Impedance Mapping (LED4) was used to investigate local underfilm corrosion of organic coated (epoxy, polyurethane, vinyl) aluminum alloy 2024-T3 substrates immersed in chloride solutions. Several interesting features in LEIM were observed that would provide insight into the local breakdown processes of coated metals if they reflected actual electrochemical phenomena. Contribution to measurements from' artifact or quantities unrelated to breakdown, and the general effect of the dielectric layer on LEIM, were evaluated by comparison of analytical and numerical modeling to LEIM of fabricated electrodes. An equipotential disk was used to model underfilm corrosion. The fields calculated for these models were correlated with LEIM of both bare and coated fabricated electrodes (Au, Pt, Al, Cu). Numerical modeling predicted that a dielectric layer would dull edge effects and severely dampen the magnitude of the field emanating from the substrate surface. A salt film beneath the coating was predicted to have no significant effect on the field. LEIM of coated disk electrodes showed no evidence of the underfilm electrode with two exceptions: (1) underfilm corrosion occurring on pure aluminum, and (2) a copper electrode, which has a very active surface. The discrepancy between modeling and experimental results of coated systems prompted further experimental investigation to isolate the roles of current density and coating defects. Blisters were created on coated gold samples by placing NaCl and AlCl₃ salt islands beneath the coating for various coating and substrate configurations. LEIM recorded a peak in admittance only over an acidic blister in polyurethane where local hydrolysis had occurred. It was determined that one of two criteria is required to measure electrochemical activity through a film: (1) the substrate must be actively corroding to produce a current density sufficient to generate a measurable field, or (2) a low resistivity defect must exist or develop in the coating. Passive features, such as salt films or neutral blisters, were not measurable by LEIM. LEIM was found to be an effective and accurate method of measuring electrochemical quantities related to local breakdown of coated metals.