Lifetime Prediction for Organic Coatings on Steel and a Magnesium Alloy Using Electrochemical Impedance Methods

Electrochemical impedance studies were conducted on an epoxy polyamide-coated AISI 1010 steel (UNS G10100) and an epoxy/chromate conversion-coated magnesium (Mg) alloy ZE41A-T5 (UNS M16410). Results were compared to evaluate the general applicability of various impedance-derived measures of coating performance for radically different metal substrates. Both coating systems were immersed in room-temperature aqueous sodium chloride (NaCl) solution of near neutral pH. Correlation of impedance parameters obtained early in exposure with long-term visual appearance demonstrated that low-frequency impedance (< 10 mHz), coating resistance, breakpoint frequency, phase angle minimum, and saddle frequency associated with the phase angle minimum forecasted the long-term performance of both systems. Impedance-based parameters adequately estimated the defect areas associated with corrosion sites that penetrated the organic coating but were unable to estimate the areas associated with coating blisters. More conservative impedance-based criteria had to be used for the coated Mg alloy than for the coated steel in predicting long-term performance. It was hypothesized that differences in the critical impedance thresholds resulted from rapid Mg oxidation and reduction of water reaction rates that occurred at even the smallest defect sites and from the greater solubility of magnesium hydroxide (Mg[OH]2). These findings imply that more conservative coating performance criteria than traditionally envisioned for steel in neutral salt water might be necessary for Mg.