Hexavalent chromium containing coatings are widely used to protect U.S. military weapons systems from corrosion, yet present acute health and environmental hazards. This project focused on determining corrosion protection mechanisms for coatings that utilize rare-earth (RE) compounds instead of hexavalent chromium. Understanding the mechanisms by which RE compounds inhibit corrosion would reduce the risk of implementing environmentally friendly coating systems as chromate replacements.
Two specific coating systems were examined; cerium-based conversion coatings (CeCCs) and epoxy polyamide primers containing praseodymium-based inhibitors. Prior to the start of the project, both coating systems had demonstrated corrosion protection that could meet current U.S. military requirements for aircraft. The approach taken in this project assumed that RE compounds are not inherently protective. Instead, the appropriate phase of a RE compound has to be incorporated into the proper type of coating to provide corrosion protection in specific environments. As part of the project, mechanistic models for corrosion protection were devised by fully characterizing the phases that were present in as-deposited coatings, the transport processes that occurred during corrosive attack, and the species that formed to passivate the substrates.
Discovery of sub-surface crevices in substrates with CeCCs led to a significant change in the emphasis of the project. The initial concept for the model was that CeCCs were primarily barriers between the corrosive species (i.e., chloride ions) and the underlying high strength aluminum alloys. After the discovery of the sub-surface crevices, more research was required to develop and validate models that explained how CeCCs could not only protect the substrate directly beneath the coating, but also inhibit corrosion in sub-surface crevices that were connected to the surface through large (>1 μm wide) cracks visible through the coating surface. The final models identified an altered layer between the coating and the substrate that inhibited corrosion through the coating and showed how growth of a hydrated aluminum oxide layer inhibited corrosion in the crevices.
For epoxy-polyamide primers containing a praseodymium-based inhibitor package, elucidation of a corrosion protection mechanism first required investigation of the phase stability of praseodymium species in aqueous and ambient air environments. At the beginning of the project, little was known about the phase stability of praseodymium species compared to cerium species. After the initial phase stability studies, the emphasis switched to examining the corrosion response of model primers that were formulated with specific praseodymium compounds or other components of the inhibitor package to isolate and identify the corrosion response of each component as a function of exposure time and exposure conditions (e.g., pH). At the same time, characterization studies provided evidence of the precipitation of praseodymium-rich crystallites in areas of exposed substrate during corrosive attack. Through the understanding of praseodymium phase stability and studying the corrosion response as a function of pH, a model for the corrosion protection provided by the praseodymium-based inhibitor package was developed. Protection relies on the dissolution of praseodymium species from the primer matrix that occurs at a relatively low pH, which is promoted by the incorporation of an acid extender into the primer. The dissolved praseodymium species then transports to the site of attack and precipitate due to a rise in local pH, which is promoted by the cathodic response of intermetallic particles in the alloy matrix near the site of corrosive attack.
The results of this project will lead to an improved fundamental understanding of the corrosion inhibition mechanisms of rare-earth compounds and lead to improvements in the performance of existing systems, design of new inhibitors, and extension of protection to other metals. The reduction of hexavalent chromium in protective coatings will help reduce the Department of Defense’s environmental liabilities associated with the maintenance of weapons systems.