Use of hydration inhibitors to improve bond durability of aluminium adhesive joints

Abstract

The adsorption of selected organic hydration inhibitors onto Forest Products Laboratory (FPL)-etched aluminium surfaces and the subsequent hydration of the treated surfaces have been studied using X-ray photoelectron spectroscopy (XPS) and surface behaviour diagrams (SBDs) supplemented by Fourier Transform Infra-red Spectroscopy (FTIR). Wedge tests were used to evaluate performance of these inhibitors in improving bond durability and the locus of failure was identified by XPS and high resolution scanning electron microscopy (X-SEM). The results indicate that nitrilotris methylene phosphonic acid (NTMP) and related compounds adsorb to the alumina surface via the POH bonds of the phosphonic acid groups, resulting in a displacement of water normally adsorbed onto the surface. A model of adsorption was developed which suggests that after treatment with very low concentrations of inhibitor (∼1 ppm), only one leg of the NTMP molecule adsorbs onto the surface although at higher concentrations (∼100 ppm) all three legs adsorb. Hydration is a three-step process: (i) reversible physisorption of water; (ii) slow dissolution of the inhibitor followed by rapid hydration of the freshly exposed Al₂O₃ to boehmite (AlOOH); and (iii) further hydration of the AlOOH to bayerite [Al(OH)₃]. Analysis of the adsorption, hydration, and wedge test results using different inhibitors suggests the following five inhibitor characteristics that promote good bond performance: (i) displacement of water and occupation of all active sites on the Al₂O₃ surface; (ii) formation of strong inhibitor surface bonds; (iii) insolubility of the resulting inhibitor-aluminium complex in aqueous solutions; (iv) compatibility with the adhesive or primer; (v) coupling of the inhibitor to the adhesive.