Producing Data to Support the Detection and Understanding of Early On-Set Corrosion in Accelerated Weathering

The goal of the research reported herein was to generate data towards the development of methods and protocols for detecting, quantifying, and understanding early on-set corrosion events on and above aluminum 2024-T3. The studies focused on incorporating two main fluorescent probes into a few model epoxy and/or urethane coatings to detect X and Y axis specific pH changes occurring upon and above the substrate during pre-macroscopic corrosion. The primary fluorophore used was fluorescein, a pH sensitive probe capable of detecting the changes, mainly increases in pH that accompanies aluminum corrosion. The second molecular probe, rhodamine B, was utilized to better understand the direct and differential fluorescent behavior of fluorescein through fluorescence resonance energy transfer (FRET). The polymeric materials were selected to capture a broad range of material types, delivery methods, and yet mimic the matrix materials selected for use in aerospace and other applications to aluminum substrates. The 1 K and 2K epoxy coatings were based on diglycidyl ether of bisphenol-A (DGEBA) epoxide resins. Urethane coatings included a self-crosslinking polyurethane dispersion (PUD), solvent based 2K polyurethane (PU), and solvent based 2K polyurethane-urea (PUU) systems. The thin films were characterized for their water vapor transmission (WVT), water equilibrium content (WEC), leaching behavior, wet and dry adhesion, and hardness before, during and after simplified environmental exposure, i.e., 5% NaCl H20 solution. In summary of the data, comparing all of the polymeric material results, the epoxy systems outperformed the urethanes with lower WVT rates and WEC, preserved each of the fluorophores \u27 concentration to a greater extent within the polymer matrix, and retained their physical properties better upon accelerated weathering. In the leaching test, the 1 K epoxy system retained 100% of both fluorescent probes even after 21 days of immersion while the 2K epoxy system displayed comparable probe retention of rhodamine B. The 2K epoxy aliquots showed an increased fluorescence intensity of 18.2 times that of the blank test solution for fluorescein after three weeks of immersion, which indicates leaching of this probe. The urethane coatings each indicated a significant loss of both of the fluorescent probes. Compared to the blank samples, aliquots from the PU, PUD, and PUU samples indicated fluorescent intensity increases for fluorescein of 17.4, 148.2, and 21.2, respectively. Similar fluorescent intensity increases were also observed for the rhodamine B probe of2.8, 7.2, and 2.3, respectively. Another important observation was determined to occur in the pH sensitive probe. Fluorescein exhibits an isosbestic point at 460 nm and has been quantified and observed before in solution 1 • Investigations into fluorescein\u27s isosbestic point were made for use as an internal control for both concentration and differential pH phenomenon specifically induced via corrosion with lK epoxy thin films. Although the data hinted at a retention and shift of the isosbestic point when fluorescein was bound within the coating, this did not translate well into fluorescence intensity