Silane based pretreatments for automotive steels

Previous papers have reported on various silane based pretreatments which can replace the phosphate and chromate based systems currently used for corrosion protection of metals in applications where such metals are used in the unpainted or powder painted state. The present paper reports for the first time on the performance of silane based pretreatments in automotive systems. On electrogalvanised steel, hot dip galvanised steel, and aluminium alloys (Al 6061, Al 6111), the silanes bis-(trimethoxysilylpropyl)amine (bis-amino silane) and bis-( triethoxysilylpropyl) tetrasulphide (bis-sulphur silane) were found to provide excellent corrosion protection in conjunction with the electrocoat ED-5000 (PPG Industries Inc.). On cold rolled steel, a mixture of the above two silanes provided corrosion protection comparable to the currently used zinc phosphate system. The corrosion performance of automotive steels pretreated with silanes has been compared with that of zinc phosphated steels using a variety of tests including the GM scab test, electrochemical impedance spectroscopy, and N-methyl pyrrolidone adhesion tests. The silane films have also been characterised by reflection-absorption infrared spectroscopy.