Adhesion promotion and corrosion resistance of mixed phosphonic acid monolayers on AA 2024

Mixed monolayers were used to prepare surfaces of AA 2024 with precisely controlled corrosion protective and adhesive properties. In this study, the corrosion inhibition and adhesion promotion as a function of the composition of mixed phosphonic acid monolayers on AA 2024 was investigated. Mixed monolayers were formed by competitive adsorption of n-dodecylphosphonic acid (DPA) and 12-aminododecylphosphonic acid (ADPA) from ethanolic solution. Their compositions were determined by X-ray photoelectron spectroscopy. Atomic force microscopy studies and polarization modulation infrared reflection–absorption spectroscopy as well as contact angle studies proved the formation of monolayers with varying surface chemistry. The wet-adhesion strength was tested by 90° peel tests after application of an epoxy-amine adhesive. Linear sweep voltammetry was performed to evaluate the corrosion inhibition of the monolayers, and the corrosion resistance of the metal/adhesive composite was investigated by means of its corrosive delamination behavior. The electrochemical measurements demonstrated that reducing the surface concentration of ADPA improved the corrosion inhibition of the monolayer. However, the corrosive delamination experiments revealed that the improved corrosion inhibition by the self-organized DPA is of minor importance for the corrosion behavior of the metal/adhesive composite, as the adhesive properties dominate.