Exposure to temperature and relative humidity changes, deposition of pollutants and other corrosive ions on a coating surface, along with water from rain and condensation, stress fields, and ultraviolet (UV) radiation from sunlight, eventually degrade aircraft coating systems. This deterioration is characterized by a sequential progression of behaviors, including: diffusion of water and dissolved oxygen; transport of electrolyte ions; development of anodic and cathodic regions at the coating substrate interface; onset of electrochemical reactions; chemical and hydrolysis reactions; displacement of polymer-surface bonds by water-surface bonds, and accumulation of water to form blisters or the formation of corrosion products at the coating-oxide interface, both of which result in loss of coating-to-surface adhesion. This degradation eventually leads to surface corrosion.

Coating system removal for inspections and mitigation actions are planned prior to the onset of corrosion damage for many commercial and military aircraft. These removal and repainting operations are performed on a time-based, rather than condition-based, schedule and these processes generate large quantities of waste that must be properly handled and can expose workers to toxic materials. Thus, from a maintenance and environmental perspective, there is much interest in reducing the number of aircraft paint removal coating operations to the minimum necessary, preferably basing these decisions on the condition of the coating.

The approach of this project focused on developing an improved mechanistic understanding of the roles that environmental exposure and stress play in degrading the corrosion barrier properties of the aircraft topcoat and the entire coating system. The principal means used for this included analysis of electrochemical impedance spectroscopy data, scanning electron microscopy imaging and compositional mapping, and numerical modeling of electrolyte transport through the coating. Finite element analysis was used to both determine stress fields in coating systems near rivet and fastener connections and simulate crack propagation in coatings due to fatigue loading.

The principal results obtained from this program include a mathematical description for saturation of organic polymer that predicts dielectric property changes over very long duration exposures, a method for deriving impedance matrices of equivalent circuit models for electrochemical impedance spectroscopy (EIS) analysis, and an algorithm for determining the frequency-dependent impedance of an equivalent circuit model. Results also included a finite element model (FEM) of crack propagation in multi-layer coating systems in response to external flight loads. Finally, the project team proposed methods for measuring fatigue properties of unsupported organic coatings and measured and reported changes in mechanical and barrier properties of aircraft coating systems as a result of environmental stressors.

Synthesis of environmental exposure data is, as yet, incomplete. This exposure data includes: particular water and pollutant accumulation, and exposure to UV radiation; with the new models for the rate of water saturation of organic polymers; and the effect of cyclic and high stress loads on the coating system. Such a full-state model is necessary to be able to accurately predict the state of the coating along with the underlying substrate.