Corrosion Protection Mechanism of Pr-based Inhibitors

The mechanisms by which rare-earth (RE) compounds inhibit corrosion of aluminum alloys are being investigated. The corrosion protection provided by an epoxy-polyamide primer containing a Pr-based corrosion inhibitor was investigated.  Polarization experiments, x-ray diffraction, and scanning electron microscopy-energy dispersive spectroscopy were performed on specific areas of coated test panels prior to and following salt spray exposure to quantify changes that occurred during corrosion.  Species thought to be involved in dissolution and precipitation were studied using thermodynamic calculations and precipitation studies conducted under conditions representative of the corrosion environment.  The results from characterization and analysis showed that corrosion protection requires that the appropriate phase of a PrO_x compound be incorporated into the proper type of coating.  The addition of either Pr₂O₃ or Pr₆O₁₁ species to the primer matrix inhibited the corrosion of Al 2024-T3 by decreasing corrosion current and increasing passivation, depending on the Pr phase present and pH of the electrolyte.  Analysis of Pr-based primers indicated that Pr-species migrated from the coating to the scribe during salt spray exposure.  Prior to corrosion testing, no Pr-was detected in scribed areas.  After testing, Pr-rich precipitates were observed in localized clusters. Pr-based primers afford corrosion protection through the dissolution of Pr-rich species from the coating matrix and are influenced by electrolyte pH and initial PrO_x phase. Understanding the mechanisms by which rare-earth compounds inhibit corrosion will reduce the risk of implementing environmentally benign coating systems as chromate replacements.