Improved testing methodologies for evaluating the corrosion resistance of paint systems and materials for aging military aircraft

The prediction of in-service performance of military aircraft materials in corrosive environments has long relied on atmospheric exposure test sites. There are established accelerated test methods for assessing the performance of aerospace coatings however, these are often far removed from the environments experienced by aircraft during typical operations and do not provide predictive information regarding expected service life. Often tests incorporate salt spray and ultraviolet radiation whilst exposing the metal to continuous or cyclic conditions dissimilar to those of the corrosive environment. Consequently, coatings which pass these standard tests sometimes prematurely fail in-service, leading to corrosion and failure of structural components. The divergence between laboratory and real world performance is exacerbated when comparing chromate containing to chromate free technologies. In general, salt spray and other accelerated environmental tests provide a ranking of materials in terms of their resistance to a particular environment. For example, the salt spray test provides a comparison between chromate levels present in chromated systems but is not necessarily valid to non-chromated systems. The validity of accelerated tests when evaluating materials and coatings for use in conditions which are not directly related is, therefore, of doubtful benefit in the prediction of service life. It is however, necessary to implement screening tests to enable down-selection and ranking of the best performing systems. The quantitative link between real world exposure and laboratory based accelerated testing was explored through the implementation of a large scale time-optimised four factorial experimental design. The design encompassed two base materials, aircraft aluminium alloy (AA) 2024-T3 and AA7075-T6. The base materials were used in conjunction with 6 coating systems, each including; a pre-treatment, a primer and a topcoat, this resulted in twelve distinct coupon types, each consisting of three sections; primer, primer-topcoat and topcoat. The twelve coupon types were subjected to nine experiments, four 18 month external exposure tests and five accelerated laboratory tests. The four external tests were defined as either tropical or temperate in climate and coastal or inland in location. The first of the accelerated laboratory tests was the standard neutral BS 9227 - Corrosion tests in artificial atmospheres, salt spray test [1]. The remaining four accelerated tests utilise a cyclic design that aims to replicate the weather patterns associated with the tropical and temperate locations used for exposure testing. Accelerated test protocols 1 and 2 utilised controlled temperature and humidity cycles in a chamber, whilst accelerated tests 3 and 4 were a variant on a Scab test [2]. Time remained a key factor linking all experiments with all tests split by seven equal time points, allowing for interrogation throughout the process of exposure and enabling the pursuit of similarities between failure mechanisms of different coupon types.