| The traditional chromium plating technology is mainly based on hexavalent chromium plating,but this process is strictly prohibited because of the extreme toxicity of hexavalent chromium.Trivalent chromium plating is an excellent alternative technology for environmental protection and high efficiency.The selection of anode material is vital for improving the quality of cathode and optimizing the plating process.However,the poor catalytic activity of traditional lead-alloy materials limits the ability of anodic reaction to a certain extent,and the electrode itself is easy to dissolve and affect the quality of chromium plating.There is also a practical problem that the trivalent chromium is oxidized into toxic hexavalent chromium in the plating solution.Lead dioxide(PbO2)electrode the advantages of remarkable conductivity,good stability,high electrocatalytic activity,as well as low cost,and can be used as a major anode in the electroplating of copper,zinc,chromium,and other metals.There is great room for improvement in grain size control and electrocatalytic performance.Therefore,the PbO2 electrode can be modified to improve the anode reaction capability and mitigate lead leakage caused by trivalent chromium oxidation and self dissolving.The surface active layer of PbO2 electrode was modified by doping active particles.In this study,there were three parts:The preparation and process optimization of PbO2SiO2-ZrO2 electrode,the performance test of series anodes,and the exploration of the essential reasons for improving the application performance of modified anodes.The main research contents and conclusions of the study are as follows:(1)Preparation and optimum process optimization of the PbO2-SiO2-ZrO2 electrodeThe PbO2-SiO2-ZrO2 composite anode material was prepared by adding nano-silica(SiO2)and nano-zirconia(ZrO2)into the lead nitrate system with stainless steel sheet as matrix by constant-current electrodeposition method,and was applied to the simulation of trivalent chromium plating in the sulfate system.Through various physical and electrochemical characterization methods,the factors affecting the preparation of the electrode were investigated and analyzed,and the optimum preparation conditions were determined:SiO2 concentration of 20 g·L-1,ZrO2 concentration of 20 g·L-1,PEG content of 2 g·L-1,current density of 20 mA·cm-2,plating time of 40min.The PbO2-SiO2-ZrO2 electrode prepared under optimal conditions has compact structure,uniform grain distribution,low coating roughness,and shows excellent electrocatalytic activity and corrosion resistance.(2)Characterization and structure-activity relationship analysis of series anodesAt the same time,a series of PbO2 electrodes were prepared,including three modified electrodes(PbO2-SiO2,PbO2-ZrO2 and PbO2-SiO2-ZrO2)and unmodified pure PbO2 electrode.The results show that SiO2 and ZrO2 in the electroplating solution can be successfully doped into the active layer of the electrode.The grain size and roughness of the three modified electrodes are reduced,and the electrocatalytic activity and corrosion resistance of them are improved.Among them,the surface of PbO2-SiO2-ZrO2 electrode is uniform and compact,with the smallest grain spacing,the lowest average grain size(15.548 nm),and the smallest surface roughness coefficient Ra(1.32 μm)and Rz(446 nm),which are mainly attributed to the influence of the successful doping of SiO2 and ZrO2 on the nucleation process of PbO2.Thus,it promotes the increase of the number of crystal nuclei and inhibits the growth of crystal nuclei,thus refining the grains and making the coating smooth.The PbO2-SiO2-ZrO2 electrode has the highest electrochemically active surface area(10.83 cm2),the smallest oxygen evolution potential(1.533 V),the lowest apparent activation energy(27.67 kJ·mol-1),and the smallest charge-transfer resistance(0.482 Ω·cm2),which shows excellent electrocatalytic activity of oxygen evolution.And this is mainly due to the refined grain and the increased active surface area,which promote the contact between solid and liquid interface and reduce the energy barrier of oxygen evolution reaction.In addition,the PbO2-SiO2-ZrO2 electrode has the smallest self-corrosion current density(8.954×10-6 A·cm-2)and the largest polarization resistance(3799.2 Ω),showing good corrosion resistance in the trivalent chromium plating system.A smooth and compact coating structure with a low roughness helps to slow the penetration and corrosion of the active electrode layer by an acidic chrome plating solution,as well as improving the corrosion resistance of the electrode.(3)Research on the essential reasons for improving the application performance of modified anodeIn the simulation process of trivalent chromium plating,the energy consumption,hexavalent chromium formation and lead leakage of a series of PbO2 electrodes during 90 min chromium plating were investigated.Compared with the pure PbO2 electrode,the cell voltage of the three modified electrodes decreased.Among them,the PbO2-SiO2ZrO2 electrode has the lowest initial cell voltage(5.93 V)and the lowest enhancement(0.1 V),and the hexavalent chromium concentration in the vicinity of the electrode is lowest(0.065 mg·L-1)after 90 min of chrome plating,and the relative lead content of the chrome plating layer is low,showing excellent stability and energy conservation.Reason analysis,fine grain and high active area,helps to improve the contact area of the modified electrode and the electrolyte,improving the reaction capability of the anode,electrode reaction occurred voltage reduction,reducing power consumption.For the modified anode material with high electrocatalytic activity,the easier the positive reaction of oxygen precipitation is,the greater the degree of inhibition is for the side reaction of oxidation of trivalent chromium to hexavalent chromium,thus effectively slowing down the generation of hexavalent chromium.In addition,the dense structure and excellent corrosion resistance of the PbO2-SiO2-ZrO2 electrode helps to maintain its stability in the plating solution and is not easy to dissolve.Therefore,the pollution caused by lead leakage of the modified anode in the process of chromium plating is light. |
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