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The electrochemical response of Pr-containing epoxy-polyamide primers on aluminum alloy 2024-T3 substrates was investigated. The effect of electrolyte pHs on corrosion behavior was studied before and after salt spray exposure. X-ray diffraction analysis confirmed that the praseodymium phase that was added to the primer converted into Pr(OH)3 in the as-deposited coating and remained present in the primer up to 3000 hours of salt spray exposure. Examination of panels prior to salt spray testing indicated that Pr was not detected in the scribes; however, following salt spray testing, Pr-rich species were found in localized clusters in scribes. Electrochemical tests were performed on test panels with machined defects using electrolyte pHs ranging from 5 to 8. The largest change in passivation range occurred at pH=8, and was two times higher than the change observed for bare Al 2024-T3. Corrosion protection of Pr-based primers is due to the ability of Pr-rich species to dissolve from coatings during exposure to corrosive environment and is influenced by pH.

The corrosion protection of epoxy-polyamide primers containing either hexagonal Pr2O3 or cubic Pr6O11 inhibitors deposited on chromate conversion coated Al 2024-T3 substrates was studied. Electrochemical evaluations, scanning electron microscopy with energy dispersive spectroscopy, and x-ray diffraction were used to characterize primers. The electrochemical response of coated panels prior to and following ASTM B117 salt spray exposure was also studied. The addition of PrOx species to the primer matrix improved the corrosion protection of artificial defects by decreasing icorr values from ~10-7 to ~10-9 A/cm2, increasing the passivation range (> 300 mV), and shifting Ecorr values to more negative potentials. After salt spray testing, Pr-hydroxide/hydroxycarbonate species were present on the surfaces of the primers. The addition of Pr2O3 afforded enhanced corrosion protection compared to the addition of Pr6O11 because Pr2O3 is a more reactive phase that provided more Pr3+ species for transport and reaction than Pr6O11.

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