2009 Volume 95 Issue 7 Pages 557-564
Thiol self-assembly super thin films have attracted attention from the viewpoint of surface design, e.g. chemical sensors, molecular devices and corrosion preventive films at a molecular level. On the other hand, chromate coatings on zinc coated steel sheets have been widely applied as an economical method of corrosion prevention. In response to recent environmental regulation, e.g., RoHS Directives, chromate-free coatings have been developed and applied to electrical appliances. However, a thinner chromate-free coating is required for the purpose of obtaining a better electro-conductivity, which influences the electromagnetic shielding performance of digital electrical appliances. In this paper, the corrosion resistance and film structure of alkane- and triazine- thiol self-assembly super thin layers on zinc coated steel sheets were investigated. The mechanism of corrosion prevention by the thiol layers was discussed in order to obtain a new design concept of thinner chromate-free coatings with high corrosion resistance. The alkanethiol layers showed high water-repellency and poor corrosion resistance, and the triazinethiol layers with three thiol groups per molecular showed relatively poor water-repellency and excellent corrosion resistance in spite of very small thickness such as a few monolayer thickness, which result from suppressing oxygen reduction reaction according to electrochemical measurement. It was found from XPS analysis that alkanethiol molecules combined to Zn form layers without decomposition, and triazinethiol molecules combined to Zn and the part of these molecules decomposed to form a new network structure.