Utilizing the structural memory effect of layered double hydroxides for sensing water uptake in organic coatings

In this paper, we report results demonstrating the structural memory effect of synthetic calcined layered double hydroxide (LDH) Li₂[Al ₂(OH)₆]₂CO₃·nH₂O powder immersed in a bulk electrolyte, exposed to humid air, and embedded in a water-permeable epoxy matrix. Reconstruction of calcined LDH by the structural memory effect can be detected by X-ray diffraction (XRD) leading to a novel approach for remotely and non-destructively detecting water uptake in optically opaque organic coatings. The LDH Li₂[Al₂(OH) ₆]₂CO₃·nH₂O was synthesized by aqueous co-precipitation then calcined in air at temperatures in excess of 220°C to form a Li-Al mixed hydrated oxide powder. Reconstruction in a matter of days was observed when the calcined mixed oxide was immersed in 0.5M NaCl solution. During exposure to humid air, LDH reconstruction was slower occurring over a matter of weeks, perhaps in a deliquescent electrolyte. Paint-like coatings were made and applied to aluminum alloy 2024-T3 (Al-4.4Cu-1.5Mg-0.6Mn) substrates by adding the calcined LDH at a rate of 10 wt.% to a commercial epoxy. Coated substrates were then exposed to 0.5M NaCl solution and LDH reconstruction progressed over tens of days as the coating absorbed water. During these exposure experiments, XRD and electrochemical impedance spectroscopy measurements were made periodically to track LDH reconstruction and measure uptake of water in the coating via capacitance measurements. LDH reconstruction was tracked using the ratio of the {003} LDH diffraction peak to the {111} Al diffraction peak. Using the Brasher-Kingsbury equation, the volume fraction of water in the coating was estimated from capacitance data. Up to the point of apparent coating saturation (about 10vol. %), the XRD peak height ratio varied linearly with the estimated coating water content. This result suggests that additions of calcined LDH to organic coating may lead to methods for sensing early-stage coating degradation due to water uptake and may give an advance warning of substrate corrosion.