Ultra thin layers as new concepts for corrosion inhibition and adhesion promotion

Abstract

The adsorption and self-organization process as well as the surface reactions of several bifunctional adhesion promoters on different oxide surfaces have been investigated. The aim was to improve the adhesion between metal oxides and different organic coatings. We developed a large number of bifunctional compounds, which are able to adsorb spontaneously on different pre-treated metal (oxide) surfaces. The second group can be designed for grafting different otherwise incompatible layers. Therefore, a special two-step procedure has been developed: (1) adsorption of the designed bifunctional molecules on the substrate and (2) surface reaction of the terminal reactive group with a polymeric top coating or with further monomers resulting in a strongly bond composite. For this purpose substances were chosen having a surface reactive group, an aliphatic spacer and a reactive group for a suitable top layer. Phosphonic acids forming strong bonds with several metal surfaces were chosen as surface-active groups on metal oxide substrates. The termination of these compounds with further reactive groups opened a wide range of possible applications. Functionalities like amino or carboxylic groups allowed reactions with commercial lacquers (e.g., polyurethane) for improving adhesion promotion and corrosion inhibition of the metal substrates. By using polymerizable groups like thiophene and pyrrole an in situ surface polymerization with further monomers is possible directly on the substrate. The adsorbed films of bifunctional phosphonic acids on metal (oxide) substrates were characterized by contact angle measurements, X-ray photoelectron spectroscopy (XPS), the surface polymerized films were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), cyclovoltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results showed that monolayers were formed, which were correctly oriented on the surfaces: the phosphonic acid group was attached to the substrate whereas the terminal group was free standing for further reactions. Surface polymerization with additional monomer was possible either chemically or electrochemically resulting in smooth polymer layers of adjustable thickness. The conducting polymers were found to be p-conductive with a doping level of about 30%. Conductivity measurements revealed a conductivity of about 0.13 S/cm for the best films. Based on this principle two possible applications are given: firstly, a corrosion protecting system for steel, and secondly, a model release system for protecting steel after damage of the coating.