Progress in Organic Coatings 51 (2004) 91–102 Utilizing the structural memory effect of layered double hydroxides for sensing water uptake in organic coatings F. Wong, R.G. Buchheit ∗ Fontana Corrosion Center, Ohio State University, Columbus, OH 43210, USA Received 29 March 2004; received in revised form 29 June 2004; accepted 6 July 2004 Abstract In this paper, we report results demonstrating the structural memory effect of synthetic calcined layered double hydroxide (LDH) Li 2 [Al 2 (OH) 6 ] 2 CO 3 ·nH 2 O powder immersed in a bulk electrolyte, exposed to humid air, and embedded in a water-permeable epoxy ma- trix. 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 Li 2 [Al 2 (OH) 6 ] 2 CO 3 ·nH 2 O 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 10 vol. %), 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. © 2004 Published by Elsevier B.V. 1. Introduction Synthetic layered double hydroxide (LDH) compounds are important industrial materials used as absorbents, poly- mer stabilizers, alkaline catalysts, acid neutralizing agents, and precursors in the formation of spinel ceramics [1,2]. LDHs are manufactured on an industrial scale with a man- ufacturing capacity that increased by at least 20,000 metric tons in 1999 alone [3,4]. LDH compounds consist of positively charged layers of mixed metal hydroxides separated by negatively charged lay- ers of anions and water. The prototypical LDH compound ∗ Corresponding author. Tel.: +1 614 292 6085; fax: +1 614 292 9857. E-mail address: butchheit.8@osu.edu (R.G. Buchheit). is the naturally occurring hydromagnesite, whose formula is Mg 6 Al 2 (OH) 16 ·CO 3 ·4H 2 O [5,6]. Structurally, the com- pound consists of Mg(OH) 2 layers, which carry a net positive charge due to the periodic substitution of Al 3+ on Mg 2+ sites. The positive charge on the Mg(OH) 2 layers is offset by net negative charge on interleaving layers consisting of carbon- ate ions and water molecules. The artificial LDH used in this study, Li 2 [Al 2 (OH) 6 ] 2 CO 3 ·nH 2 O, is slightly different in that the positive charge on the metal hydroxide layer arises due to the presence of Li + cations situated in normally unoccupied octahedral sites in an Al(OH) 3 layer (Fig. 1). LDH compounds are usually synthesized as powders by titration of one metal salt solution with another to induce pre- cipitation [7,8,9]. LDH coatings may be formed by sol–gel methods using metal alkoxides [10] and by reaction of a metal 0300-9440/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.porgcoat.2004.07.001