Study On The Preparation And Properties Of Phosphating Coatings On Galvanized Steel

The phosphating application time of galvanized steel is shorter than that of steel,mainly used for coating and surface lubrication.In recent years,the increasingly application of galvanized steel in automobile production and household products,the type of the phosphating is thin film phosphating,and is mainly used for improving the bonding force and protective between organic coating and galvanized steel substrate to satisfied the requirements of modern production for its coating.In the 21st century,energy saving and environmental protection are advocated,while the traditional high and medium temperature zinc phosphating is high energy consumption,so the zinc phosphating of low temperature is the focus of current research.This paper discussed the main components of the low temperature zinc phosphating solution and its effect on the corrosion resistance of phosphating films,the optimum ratio of each component of phosphating solution was determined by orthogonal test,and the effects of three accelerators and nanoparticle TiO₂ on the surface morphology and corrosion resistance of phosphating films were studied.In this paper,by metallographic microscope,field emission scanning electron microscope,transmission electron microscope,X-ray energy spectrometer,X-ray diffractometer,open circuit potential-time curve,polarization curves,electrochemical impedance,coating weight-time curves,cross-cut test,salt water immersion test,copper sulfate spot test on galvanized steel surface to characterize and analyze the growth process and properties of phosphating films.The main contents and results as follows:1.With index of coating weight and time of copper sulfate spot test,according to the results of orthogonal test,the best formula and process parameters as follows:zinc dihydrogen phosphate 60 g/L,zinc nitrate 80 g/L,sodium fluoride 4 g/L,zinc oxide 6 g/L.Phosphating process conditions as follows:free acidity was 2.6,the total acidity was 48.6,acid ratio was 18.7,phosphating temperature was 35±1?,phosphating time was 5 minutes.2.According to the open circuit potential-time curve and surface morphology analysis of galvanized steel,the phosphating process can be divided into three stages.Stage 1:the dissolved of metal lead to potential positive shift observably,and the phosphating film began to form;stage 2:the polarization reaction occurred,the growth and crystallization of the film were accelerated;stage 3:the potential was slowly moving to a stable value,the film crystals were continuously and completely in sheet to accumulation.The results showed that the adhesion between the phosphating film layer and the organic coating was increased,the adhesion was enhanced and the corrosion resistance was improved.3.The addition of hydroxylamine sulfate,sodium chlorate and sodium nitrite promoted the corrosion resistance of the phosphating film.When the concentration was 4 g/L,1.5 g/L,and 2 g/L respectively,the film had the best corrosion resistance.New promoters of hydroxylamine sulfate,sodium chlorate and the traditional sodium had the same facilitated effect,electrochemical test,surface morphology,composition analysis and copper sulfate spot test made a consistent conclusion:the sequence of film corrosion resistance as follows,hydroxylamine sulfate>sodium chlorate>sodium nitrite.4.The addition of nano TiO₂ not only reduced the size of the phosphate crystal,porosity of the film,but also accelerated the phosphating reaction speed and acted as nucleating agent and accelerator in the phosphating reaction.The addition of nano TiO₂ made the phosphating film layer thicker and denser,thus improving the corrosion resistance of the film.