Effect of surface modification of zinc powders with organosilanes on the corrosion resistance of thin zinc-rich coatings

摘要

There are increasing demands to use a pre-coated steel sheet in the automotive manufacturing industry in order to lower the production cost by making the coating process simple. However, because the coated films are mostly non-conductive, a proper method to make the film to be weldable has to be achieved. Zinc rich coatings (ZRCs) were applied onto the carbon steel panels to impart weldability with different pigment volume concentrations (PVCs) of metal powders. The thin films loaded with the zinc powders can provide spot weldability to the coated steel sheet due to electrical connections between the welding electrode and substrate, while they cannot provide sufficient anti-corrosion performance. In this work, surface modification of zinc powders was carried out to improve the corrosion resistance of coatings containing zinc powders. Various organosilanes were chosen to study the effect of organic groups with regard to different chain length and functional groups. The corrosion resistance of coated steel panels was compared by using salt spray test, scanning electron microscopy, scanning vibrating electrode technique and electrochemical impedance spectroscopy. ZRCs with non-treated zinc powders showed low performance in the corrosion resistance at PVCs lower than that required for cathodic protection. Moreover, the thin ZRCs had low barrier properties. Thus surface modification of metal powders was needed to increase barrier properties of powders, itself. The corrosion resistance of ZRCs using modified metal powders showed better anti-corrosion resistance compared to ZRCs with non-treated powders. In addition, the corrosion resistance of coated panels was strongly dependent on the type of organosilanes used to modify powders.