Pulse Plating Of Zn-Fe Alloy

Zn-Fe alloy with high corrosion resistance,low hydrogenous brittleness,facile jointing and good mechanical properties has been recently developed as a kind of new and protective coatings,and shows a promising future.The capability of electrolytic solution, the electrodeposition mechanism of coating and the effect of process parameters on coating properties were studied.The corrosion resistance,components and microstructure of Zn-Fe alloy coatings were studied,including passivated coating.The results showed that with the duty cycle and average current density increasing, the content of iron decreased,and as the frequency increasing,the content of iron decreased at the beginning and then increased.With the increasing of content of KC1 and C₆H₅Na₃O₇·2H₂O and H₃BO₃ in electrolytic solution,the content of iron increased at first and then decreased,and the content of iron decreased with the increasing of the content of ZnCl₂ in electrolytic solution,however,the content of iron increased with the increasing of the content of FeCl₂ in electrolytic solution.The electrodeposition process was controlled by activation polarization,and the electrode reaction occurred steeply,at the same time there was electroactivative medial production.The electrodeposition process had the characteristic of 3-D growth type.With the duty cycle and frequency increasing,the corrosion resistance of Zn-Fe alloy coating decreased.And as the average current density increasing,the corrosion resistance increased at first and then decreased.With the content of KCl and FeCl₂ in electrolytic solution increased,the corrosion resistance increased,and the corrosion resistance decreased with the increasing of the content of ZnCl₂,while the corrosion resistance increased at first and then decreased with the increasing of the content of C₆H₅Na₃O₇·2H₂O and H₃BO₃.The corrosion resistance of coating obtains by pulse plating was better than those obtained by direct current plating.The corrosion resistance of the passivated coatings was enhanced significantly,and the chromium element became one component of the passivated film.In comparison,the corrosion resistance of black passivated coating was the best,and colour passivated followed by and blue passivated was the worst.The phase structure of Zn-Fe alloy coating was Fe₄Zn₉,and it didn't change by using pulse plating or passivating.The SEM results showed that the coating was smoother and more even by pulse plating.The component of Fe-Zn alloy electrolytic solution and process parameters were optimized as follows:KC1 170～180 g/L,ZnCl₂ 70～80 g/L,FeCl₂ 7～8 g/L, C₆H₅Na₃O₇·2H₂O 13～17 g/L,H₃BO₃ 14～18 g/L,SF-522 softener 25～35mL/L,SF-522 brightener 1～2mL/L,and duty cycle 30%,frequency 125 Hz,average current density 1.6A/dm²,pH 4.0～4.8,anode was a zinc sheet and experiment temperature was room temperature.The experiment results showed that the iron content of Zn-Fe alloy coating was between 0.14%and 0.15%,and the brighter and finer coating with better appearance and stronger corrosion resistance could be obtained.