Use of surface behavior diagrams to study hydration/corrosion of aluminum and steel surfaces

Abstract

The use of surface behavior diagrams can frequently enhance the understanding of quantitative x-ray photoelectron and Auger electron spectroscopic measurements. These diagrams have been used to study (1) the adsorption of nitrilotris methylene phosphonic acid (NTMP), a hydration inhibitor, from aqueous solutions onto aluminum surfaces prepared for adhesive bonding and onto steel surfaces, and (2) the subsequent hydration/corrosion of the oxide or metal when exposed to moist environments. The results show that NTMP coverage increases with inhibitor solution concentration until saturation (monolayer) coverage is achieved. At the same time, NTMP displaces the water initially adsorbed on the oxidized surface. Subsequent exposure of the treated surfaces to moisture results in a slow dissolution of the NTMP, exposing amorphous Al2O3 or Fe2O3, which rapidly hydrates to AlOOH or FeOOH. Further hydration of the aluminum samples occurs as Al(OH)3 grows on the surface. These findings indicate that the dissolution of the adsorbed inhibitor is the rate-limiting step in the hydration/corrosion of oxide-covered metal surfaces which, in the case of aluminum, is the mechanism of failure of adhesive bonds in a humid environment.