Corrosion Resistance and Structure of Thiol Self-assembly Super Thin Film on Zinc Coated Steel Sheet

Abstract

Thiol self-assembly super-thin films have attracted considerable attention for molecular-level surface design as in chemical sensors, molecule-based devices and corrosion resistant films. Meanwhile, chromate coatings on zinc coated steel sheets are widely used as an economical method of preventing corrosion. In response to recent environmental regulations, such as the RoHS Directive, chromate-free coatings have been developed and applied to electrical appliances. However, a thinner coating is required to obtain better electroconductivity, which influences the electromagnetic shielding performance of digital electrical appliances. In this study, we investigated the corrosion resistance and film structure of alkanethiol and triazinethiol self-assembly super thin layers on zinc coated steel sheets. The corrosion inhibition mechanism of the thiol layers was discussed in order to develop a new design concept for 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 molecule showed relatively poor water repellency and excellent corrosion resistance despite the very small thickness (several monolayers), as a result of suppressed oxygen reduction reaction determined by electrochemical measurement. XPS analysis revealed that alkanethiol molecules adhered to the zinc and formed layers with no decomposition, and triazinethiol molecules adhered to the zinc and then partially decomposed and reorganized to form a new network structure.