Improved Durability of Aluminum Adhesive Bonds with Phosphonic Acid Inhibitors

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Abstract
In this paper, we review our work on the use of hydration inhibitors to improve the durability of adhesively-bonded aluminum structures. Treatment of adherends given the Forest Products Laboratory (FPL) surface preparation in a solution of nitrilotris methylene phosphonic acid (NTMP) increases bond durability so that hydration of the adherend is no longer the limiting factor on crack propagation. In bonds with phosphoric-acid-anodized (PAA) adherends treated with NTMP, the transition to failure in the adhesive occurs at a shorter crack length. The initial strength of epoxy adhesive bonds is not changed by such pretreatments. Adsorption of NTMP onto FPL surfaces displaces the water initially present and forms P—O—Al bonds. Saturation coverage is approximately one monolayer. NTMP adsorbed onto PAA surfaces, which have little initial water, occupies residual active sites. Hydration of a PAA surface or an inhibited FPL surface proceeds in three steps: 1) reversible physisorption of water, 2) slow dissolution of the inhibitor-aluminum complex followed by rapid transformation of the freshly-exposed amorphous aluminum oxide to boehmite, and 3) formation of bayerite on the surface. We have identified several important criteria for inhibitors designed to improve bond durability; i.e. they should 1) displace water and occupy all active sites on the Al2O3 surface, 2) form strong inhibitor-surface bonds, 3) form a water-insoluble complex with aluminum, 4) be chemically compatible with the adhesive or primer, and 5) couple chemically or mechanically to the adhesive.