常温磷化过程中的开路电位-时间曲线以及成膜规律

Measurement of Open Circuit Potential-Time Curve for Room Temperature Phosphating and Analysis of FilmForming Pattern

目前,对常温磷化成膜机理及规律的认识十分有限,进而影响了常温磷化膜的开发和应用。通过测量常温磷化过程中磷化液的开路电位-时间曲线分析了成膜规律,采用扫描电镜(SEM)考察了成膜过程中磷化膜的形貌变化,测定了膜层的耐蚀性、孔隙率随成膜时间的变化,通过X射线衍射(XRD)分析了磷化膜的相结构。结果表明:常温磷化成膜过程主要由8个阶段构成,即氧化层或其他腐蚀抑制物的溶解、形核与早期成长、晶核或初生晶粒的再溶解、快速成膜、形成完整单层覆盖层后磷化膜的增厚、再结晶、稳定的H+腐蚀扩散通道的建立、膜的生长与溶解平衡;开路电位随时间的变化很好地反映了常温磷化膜的生长规律,可用开路电位-时间曲线来监控磷化膜生长,也可将其作为筛选添加剂或改进磷化工艺的判据;改进的粘贴K3Fe(CN)6滤纸法可用于磷化膜的孔隙率测定,其测量结果与硫酸铜点滴试验结果一致;磷化膜主要由Zn3(PO4)2.4H2O,Zn2Fe(PO4)2.4H2O,Fe3(PO4)2.4H2O组成。

The open circuit potential-time curve for room temperature phosphating was measured,and the film-forming pattern was analyzed.Besides,the morphology and phase structure of the phosphating film were analyzed by means of scanning electron microscopy and X-ray diffraction.The corrosion resistance and porosity of the phosphating film were also examined.Results indicate that as-deposited phosphating film consists of Zn3(PO4)2· 4H2O,Zn2Fe(PO4)2·4H2O and Fe3(PO4)2·4H2O.The film-forming process mainly involved 8 steps including dissolution of oxide layer or corroded layer,nucleation and early stage of growth,redissolution of crystal nucleus or primary grains,fast forming of film,thickening after formation of complete phosphating monolayer,recrystallization,establishment of stable channels for H+ corrosion,and balanced growth and dissolution of phosphating film.Besides,the variation of open circuit potential with time could well reflect the growth pattern of the room temperature phosphating film.Thus open circuit potential-time curve could be adopted to monitor the growth of the phosphating film or used as a criterion to screen additives or modify phosphating process.In addition,the porosity of the phosphating film could be measured using modified route of pasting K3 Fe(CN)6 filter paper,and relevant results well conformed to those determined by dipping test of aqueous copper sulfate.