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

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.