Silane based metal pretreatments as alternatives to chromating

Novel silane based metal pretreatments have been developed as cost effective alternatives to the chromating process. The new process is a simple solution dip process, is non-carcinogenic, and has outperformed chromated systems under different test conditions. The system provides superior corrosion and adhesion performance. The process has been developed by optimising parameters such as choice of silane, silane concentration in the solution, extent of hydrolysis, pH of application, coating method, solution contact time, and nature and extent of curing. The pretreatments are versatile and can be modified to accommodate different top coat systems including paints. The performance of this new pretreatment has been tested on a number of metal and alloy substrates including aluminium and Al alloys, various steels, copper and Cu alloys, brass, zinc, and several automotive and aircraft alloys. The superior performance is confirmed by using accelerated a.c. and d.c. electrochemical techniques, ASTM B-117 salt spray tests, and a variety of other corrosion and adhesion tests. Protection against forms of corrosion such as general, pitting, galvanic, stress corrosion cracking, and dezincification is established. Characterisation and modelling were performed using advanced techniques including SEM, EDX, XPS, AES, SIMS, RA-IR, Raman spectroscopy, spectroscopic ellipsometry, EIS, AFM, and contact angle measurements. The unique nature of these chemicals and the adopted methodology are explained and the possible impact of the new development assessed.