电镀锌板无铬耐指纹涂层的破坏机制

Failure Mechanism of Chrome-Free Anti-Fingerprint Coating on Galvanized Steel Sheet

耐指纹涂层的破坏机制不同于较厚的有机涂层,目前对此研究较少。以聚氨酯乳液、纳米二氧化硅水性分散液、钝化剂(草酸钛钾、碳酸锆铵)及各种助剂制成无铬耐指纹涂料,将其涂覆于电镀锌板表面制成涂层,采用X射线光电子能谱(XPS)分析其表面及100 nm深处的成分与结构;采用数码显微镜、电化学阻抗谱研究了涂层在3.5%NaCl溶液中浸泡不同时间的腐蚀行为,并分析了其破坏机制。结果表明:涂层的表层主要由树脂和纳米二氧化硅组成,越靠近基体,金属元素含量越大;涂层中钛元素主要以四价的二氧化钛形式存在,部分为三价钛的氧化物,锆为四价,主要以二氧化锆形式存在,少部分以磷酸氢盐或聚氨酯的键合物形式存在,硅主要以二氧化硅形式存在;涂层腐蚀过程为微孔腐蚀,腐蚀液首先通过微孔进入涂层,然后逐渐渗透,最后缓慢破坏锌层,在此过程中生成了化学转化膜来延缓腐蚀。

Polyurethane emulsion,water-borne dispersion of nano-silica,passivation agents( potassium titanium oxalate and ammonium zirconium carbonate) and various auxiliary agents were used as the starting materials to prepare Cr-free anti-fingerprint paint.Resultant paint was then coated onto the surface of galvanized steel sheet to afford anti-fingerprint coating thereon.The composition and structure of the coating surface and subsurface at a depth of 100 nm were analyzed by means of X-ray photoelectron spectroscopy( XPS).Moreover,a digital camera and an electrochemical test rig were performed to evaluate the corrosion resistance of as-prepared Cr-free anti-fingerprint coating in 3.5% NaCl solution,and the failure mechanism of the coating was discussed.Results indicated that the coating surface was mainly composed of resin and nano-silica,and the content of metal tended to rise towards the steel substrate.Titanium mainly existed in the form of titanium dioxide,accompanied by a small amount of trivalent titanium oxide.Zirconium mainly existed in the form of zirconium dioxide,accompanied by a small amount of hydrophosphate or compounds bonded with polyurethane.Moreover,silicon mainly existed in the form of silica.The corrosion process of as-prepared coating was dominated by micro-porous corrosion.Namely,the corrosive medium penetrated into the coating through the surface micro-pore,then gradually permeated and led to slow destruction of the coating.Chemical conversion coating was generated during the corrosion process to slow down the corrosion.