Chromates used as pretreatments for metals and as anti-corrosive pigments in primers have been found to be a strong carcinogen and hence there is considerable pressure from environmentalists and government agencies for its replacement. Dr. Van Ooij and his research group have developed an array of environmentally friendly hydrophobic silane based pretreatments for corrosion protection of metals. However these coatings are extremely thin, of the order of a few hundred nanometers and hence unable to provide long term corrosion protection like a paint film which has thickness in microns. To eliminate this problem, the idea of Superprimer was conceptualized. “Superprimers” are self-priming coatings containing a mixture of polymer resin and silane as the binder and chromate-free, environmentally friendly corrosion inhibitor make a complete primer formulation. Thus, by adding the right silane to a coating resin, the chromate pretreatment step would be eliminated and the chromate pigment would be replaced by an environmentally friendly anti-corrosive pigment.
This research deals with the evaluation of polyurethane-silane superprimer coatings as potential replacement of chromate pretreatment and chromat-containing high VOC primers for corrosion protection of AA 2024-T3 aluminum alloy. The silane chosen was BTSE because of its dipodal structure, hydrophobicity and ability to hydrolyze in water. EIS revealed that the hydrophobicity of the coatings increased considerably due to addition of 4 % BTSE silane improving its impedance and water barrier properties. RAIR and NMR spectroscopy showed hydrolysis and self condensation to siloxanes of the silane molecules. The formation of the siloxane network enhances the tensile strength, Tg and thermal degradation behavior of the coating, as detected by tensile testing, DSC and TGA, respectively. ToF_SIMS and EDXS studies of the interface indicated the presence of siloxane layer at the interface between the metal and coating. It indicated that the silane added to the polyurethane formulation segregated at the interface to form a dense siloxane network below the polyurethane coating. Four different, environmentally friendly corrosion inhibitive pigments were studied by incorporating them separately in the optimized polyurethane-silane formulation and applied them directly over AA2024-T3 without the use of a chromate conversion coating. Corrosion tests such as EIS and salt spray test (ASTM-B117) indicated that calcium zinc molybdate can be a promising replacement to chromates for the system under study.