Study On Preparation And Properties Of Electrolytic Black Phosphate Coating

Electrolytic phosphating is a technology which can centralize energy on the surfaceof samples by means of additional current, to deposit a phosphating film with stabilityand insoluble.In this paper, ordinary carbon steel is taken as the experimental object, we haddeveloped a new zinc manganese system electrolytic phosphating bath and optimizedthe electrolytic phosphating solution formula and parameter by single factor experimentand orthogonal design experiment separately. The best compounding was obtained:Mn（H2PO₄）₂·2H₂O65g/L, Zn（NO₃）₂·6H₂O50g/L, NaF3.0g/L, CuSO₄1.0g/L, EDTA0.5g/L, Ni（NO₃）₂1.0g/L, right amount of OP-10. The process parameters are:phosphating time and temperature are8-13min and30-40℃respectively, currentdensity is0.7-1.3A/dm², total and free acidity are65-80and5-6points separately, andthe ratio of acidity is13-14.From the single factor experiment we could obtain that the additives could improvethe corrosion resistance and balck degree of the electrolytic phosphating film obviously.The additives promote the scanning electron microscope （SEM）morphology changedfrom unconstant needle-like to regular scaly structure, and the film was uniform anddense. Electrochemical tests shown that the corrosion current decreased significantly,the value of ac impedance was increased obviously, and the corrosion resistance time ofthe coating was more than300s evaluated by CuSO₄dripping test. X-ray diffraction（XRD）test shown that the composition of electrolytic phosphate coating mainlyincluded Zn₃（PO₄）2, Fe（H₂PO₃）₃,Mn（HPO₄）·3H₂O,Mn₃（PO₄）₂.We had added differentnano-powders （Ti,Fe,Al2O3,TiO2） in the electrolytic phosphating liquid to improve thecoatings corrosion resistance. Nano-powders had a significant impact on processparameters. The result of single factor experiment showed that the optimal technologyparameters of electrolytic phosphating process was as follows: electrochemical phosphating time of10-15min, electrochemical phosphating temperature for30-40℃,current density of1.5-2.0A/dm²; the total acidity control83-91point, the free acidity9-11point, and the ratio of acidity is8-9.The properties of electrolytic phosphate coating added different nano-powderswere analyzed by accelerated corrosion test, electrochemical test, SEM and XRD.Different nano-powdes such as Ti, Fe, Al₂O₃and TiO₂had different influences onthe electrochemical phosphating film’s crystal morphology and corrosion resistance.Especially the nano-Ti powder, it made the grains more dense and uniform, the acimpedance and corrosion current was increased and decreased separately. The corrosionresistiance time of the coating evaluated by CuSO₄dripping test was more than600s.XRD result shown that the compositions of electrolytic phosphating film mainly includeZn2PO₄（OH）,MnFe₂（PO₄）2, FePO₄·2H₂O, TiO₂, Mn3（PO₄）₂.