The impact of mechanical strain environment on aircraft protective coatings and corrosion protection

Aircraft paint schemes (paint and sealant) are a key element of corrosion preventive measures, and ensuring that such schemes and other coatings are effective and durable under service conditions is essential if corrosion costs and maintenance costs are to be minimized. Coatings can degrade under the influence of environmental factors such as exposure to moisture, high temperature, ultraviolet radiation, and so forth. Coating failure is often particularly evident at joints, and while failure may be accelerated in some regions by local variations in coating thickness and geometry at features such as edges, or by erosion, it is likely that the displacements which occur at these locations under service loads will be a contributing factor. The impact of in-service mechanical loading on coating degradation has so far received little attention, despite clear evidence that coatings tend to fail first at specific sites such as sheet ends and around fastener heads. This paper argues that the magnitude of the applied service loads and the nature of the load history should be considered in predicting and assessing rates of coating degradation, and that development of a thermomechanical history is a more appropriate approach. The likely impact of joint displacements on the protection of aging aircraft is also discussed.