Failure of navy coating systems 2: failure pathways of artificially weathered navy coating systems applied to chromate conversion coated AA2024-T3 substrates

Dual layer coating systems consisting of volatile organic compound (VOC)-based aliphatic polyurethane topcoats and epoxy primers applied to AA2024-T3 substrates pretreated with chromate conversion coating (CCC) are the current mainstay of the U.S. Navy. Government mandates preclude their future use, however, and they are being phased out of service in favor of new VOC-free topcoat and primer high-solids reformulations. The durability of this new formulation to artificial weathering protocol GM9540P was evaluated using transmission-mode and diffuse reflectance Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) imaging, optical and scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) and laser-confocal topography. Extensive blistering of the coating system was observed. Depth-resolved FTIR spectra show a lack of chemical modification of the topcoat layer. Data collected show localized corrosion of the substrate material in the form of pitting, as well as chemical inhomogeneities within the primer layer manifest as regional undercuring of the epoxy matrix. It is hypothesized that barrier failure of the coating/pretreatment system is the result of a cascading series of component-specific failures, notably an apparent shift in the target polydispersity of the prepolymer components of the epoxy primer.