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Understanding the Change of Thermomechanical Properties of Epoxy Coatings on AA 2024 T3 Aluminum During the Corrosion Process

Minoru Yada, Partha P. Sengupta, Michael D. Blanton, James W. Rawlins

Abstract


The changes in thermomechanical properties for a thermoplastic high molecular weight epoxy polymer blended with pyrene (fluorescence probe) and applied on 2024 T3 aluminum were characterized via fluorescence spectroscopy upon immersion in 5% sodium chloride solution for 96 hours. An image analysis was conducted to determine the site of corrosion initiation. The emission intensity ratio (I396nm/I374nm) of pyrene showed three successive chronological changes; 1) an initial increase, 2) subsequent decrease, and 3) increase again. The emission intensity ratio has been shown to be directly related to molecular relaxation properties i.e., polymer glass transition temperature (Tg). The ratioed changes quantified in terms of X and Y orientation were correlated to film phenomena as follows: 1) Gradual plasticization of the epoxy resin by water diffusion that initiates corrosion, 2) Chemical species like aluminum hydroxide generated by the corrosion process introduces pseudo crosslinking/ morphological stiffening that made the coating layer more rigid, and 3) the epoxy resins is swelled by water and plasticized as the aluminum ion is solubilized away in the salt water environment. To understand the difference between the Tg change behaviors at boundary and surface layer, z-axis depth analysis was also carried out with confocal laser microscopy.

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