Corrosion mitigation of aerospace alloys using rare earth diphenyl phosphates

Abstract

The corrosion protection of aluminium alloys is of high importance, particularly in the aerospace industry. The most widely used technologies utilise Chromium(VI) compounds for conversion coatings and primer additives in paint systems to provide corrosion protection to these alloys. These compounds are highly toxic, carcinogenic and detrimental to the environment, therefore the identification of alternative systems that are safe and environmentally benign, that meet or exceed the current levels of corrosion protection is vital. This research program examines the corrosion inhibition effectiveness of selected rare earth diphenyl phosphates (RE(dpp)3). These compounds incorporate known inhibitor species, namely rare earth metals, organics and phosphates into a single complex, with the aim of achieving synergistic inhibition in corrosive environments. A screening study utilising immersion and weight loss experiments identified Cerium diphenyl phosphate (Ce(dpp)3) and Mischmetal diphenyl phosphate (Mm(dpp)3) as the most effective inhibitors of corrosion for AA2024-T3. The inhibiting efficiency, mechanism of inhibition and surface interaction of these complexes on aluminium alloy AA2024-T3 was characterised using a range of electrochemical and surface techniques. A similar study was carried out using AA7075-T6 to assess the adaptability of the RE(dpp)3 compounds to protect different alloy compositions. The complexes were effective in significantly reducing the corrosion rate of the alloys, with both the cathodic and anodic corrosion processes being suppressed. This mixed inhibition was not attained with the constituent rare earth and diphenyl phosphate ions individually, indicating the need for the complex to remain intact in solution to achieve the high level of corrosion protection observed. The initiation and propagation of surface pits was effectively suppressed by the RE(dpp)3 complexes. The combination of electrochemical and surface characterisation techniques has for the first time allowed insights into the mechanism of action of these compounds on aluminium alloys, and indicated deposition was initiated at electrochemically active intermetallic particles. The mixed rare earth phases present in Mm(dpp)3 produced a synergistic effect, providing a greater degree of corrosion protection compared with Ce(dpp)3, particularly on AA2024-T3. In the final phase of this research project the RE(dpp)3 inhibitor compounds were incorporated into an epoxy coating system, and demonstrated that the initiation of filiform corrosion on AA2024-T3 could be reduced by up to a factor of 3 by their addition. The growth rate of filaments was also impeded.