Growth Law And Corrosion Behavior Of Ce-P Conversion Coating On NdFeB Zinc Coating

In this paper,Cerium sulfate and Sodium Polyphosphate were added to improve the corrosion resistance of the composite coating.The optimum process parameters for the preparation of conversion film were determined by an orthogonal test.The changes in micromorphology,surface roughness,surface defects,microstructure,and corrosion resistance of Ce-P conversion film during the growth process were systematically studied by SEM,EDS,AFM,XPS,and electrochemical analysis techniques.The effects of cerium salt concentration,film-forming time,and water-soluble additives on the properties of the conversion film were analyzed.Combined with the neutral salt spray test,the growth law and corrosion resistance mechanism of Ce-P conversion film and the improvement mechanism of water-soluble additives on the corrosion resistance of conversion film were deeply discussed.Ce-P conversion film was prepared on the surface of Nd Fe B zinc coating by a simple immersion method.The feasibility of cerium deposition on the surface of a zinc coating was analyzed by theoretical calculation.The composition and process parameters of the conversion solution were optimized by an orthogonal test.The influence of process parameters on film quality was analyzed by a neutral salt spray test.The optimized process parameters of the conversion solution were 5g/L cerium sulfate,5g/L sodium nitrate,and 2% sodium polyphosphate.The film was formed for 2 minutes,and the temperature was 30℃.The orthogonal test results show that the concentration of cerium sulfate has the greatest influence on the corrosion resistance of the conversion film.With the increase of the concentration of cerium sulfate,the transformation of the appearance of the conversion film is colorless,bluewhite,and gray-white,among which the corrosion resistance of the blue-white conversion film is the best.The effect of cerium sulfate concentration on the growth process of Ce-P conversion film was studied.SEM and AFM images showed that the conversion film was rough and porous when cerium sulfate was not added to the conversion solution.With the introduction of cerium sulfate in the solution,nano Ce particles filled the pores of the conversion film,and gradually gathered and grew with the increase of cerium sulfate concentration.The conversion film is mainly composed of Zn,O,P,and Ce elements.The composition of the film changes with the concentration of cerium,but the main components of the film are cerium and zinc oxides,hydroxides,and phosphates.The formation mechanism of Ce-P conversion film believes that the formation of the film is the common result of the dissolution of zinc coating and the deposition of polymerized phosphate ion and cerium ion.The electrochemical test results show that the semiconductor properties and surface defects of the conversion film are different with the concentration of cerium salt;Compared with zinc coating,Ce-P conversion film shows lower corrosion current density,higher film resistance,and charge transfer resistance.Therefore,Ce-P conversion film is a good physical barrier on the surface of zinc coating,which can effectively improve the corrosion resistance of neodymium iron boron zinc coating.In view of the synergy between different corrosion inhibitors,water-soluble accelerators were introduced into the Ce-P conversion solution system,and an improved Ce-P conversion coating was constructed on the surface of Nd Fe B zinc coating by a one-step immersion method.Through the open circuit potential time curve,the potential change in the growth process of the film was determined,and the changes in micromorphology,electrochemical behavior,and corrosion resistance of the film were studied.The results show that compared with the untreated Nd Fe B zinc coating,the improved Ce-P conversion film is dense and uniform without cracks,and its self-corrosion current density is reduced by nearly three orders of magnitude.The introduction of an accelerator makes the cerium salt particles adsorbed on the coating surface more refined,shortens the film-forming time,and increases the film-forming thickness,so the corrosion resistance becomes better.