| For heavy pollution, high energy consumption and other important drawbacks, conventional hexavalent chromium plating has been replaced by trivalent chromium plating gradually. Currently, lead-based alloys (e.g. lead-silver alloy or the like) as anode materials are widely used in trivalent chromium plating. However, lead-based alloys have many inherent defects, such as poor corrosion resistance, low catalytic activity, high oxygen evolution potential and ineffective in restraining poor stability of plating bath caused by hexavalent chromium. In recent years, the research and development of new trivalent chromium plating anode materials that can improve the stability of plating bath, inhibit the formation of hexavalent chromium, decrease oxygen evolution overpotential of anode and energy consumption significantly have become one of the hottest issue.Aiming at eliminating inherent defects of lead-based alloys, stainless steel-based/SnO2-Sb2O3/β-PbO2-CeO2-ZrO2inert composite electrode materials used for trivalent chromium plating were prepared by direct-current and pulse-current composite electrodeposition methods with anodic oxidation. Through a series of electrochemical measurement, the laws and action mechanism between influence factors of electrode preparation and its electrochemical properties were studied systematically; analyzing corrosion behavior and deposition process; exploring the mechanism of corrosion resistance and composite electrodeposition of the electrode. The comprehensive performance of the new anode materials were investigated while they were applied to trivalent chromium plating in sulfate bath. The essential reason of inert composite anode in reducing cell voltage and restraining the formation of hexavalent chromium were explored. The mainly work and conclusions are as follows:(1) Preparation and performance study of stainless steel-based/SnO2-Sb2O3/β-PbO2-CeO2-ZrO2composite materialsThe process conditions for preparing SnO2-Sb2O3intermediate with thermal decomposition method:thermal decomposition temperature is500℃, thermal decomposition number is4times. The bonding force, corrosion resistance, electrocatalytic activity and energy conservation of SnO2-Sb2O3intermediate are improved significantly. The studies on electrochemical properties of β-PbO2-CeO2-ZrO2coating under different concentration of modified substances showed that:CeO2particles can refine grains and reduce lattice defects of electrode materials; ZrO2particles can refine grains and increase electrical conductivity of electrode material; meanwhile, oxygen vacancies produced by ZrO2particles can form protective oxidation film; the dispersant agent with steric-hinerance effect accelerate dispersion of CeO2and ZrO2uniformly forming a stable colloidal solution and promoting co-deposition of solid particles. When the modified substances of CeO2,ZrO2and dispersant agent concentration is16g·L-1,30g·L-1,30mL·L-1respectively, the electrochemical properties of β-PbO2-CeO2-ZrO2electrode is improved effectively and its grain size is refined. The influence of direct-current(DC) process on electrochemical properties of β-PbO2-CeO2-ZrO2electrode are mainly reflected in three aspects: appropriate plating temperature and stirring rate can increase kinetic energy of solid particles that promoting the particles to migrate and adsorb on anode surface; appropriate current density can accelerate metal oxide to coat nano-particles effectively forming a compact and smooth coating with a certain thickness. It follows that the optimum conditions for preparing β-PbO2-CeO2-ZrO2electrode materials by DC electrodeposition:current density is20mA·cm-2, temperature is80℃, stirring rate is350r·min-1. The researches on electrochemical properties of P-PbO2-CeO2-ZrO2electrode materials prepared under different pulse conditions showed that appropriate pulse average current density, duty cycle and pulse work time can promote the formation of crystal nucleus effectively, prevent the grains growing excessively and eliminate concentration polarization around the anode. Thereby the optimum conditions for preparing β-PbO2-CeO2-ZrO2electrode materials by pulse electrodeposition:pulse average current density is0.3A-cm-2, duty cycle is30%, pulse work time is100ms.Combined with the surface morphology and electrochemical performance analysis, it is obvious that the stainless steel-based/SnO2-Sb2O3/β-PbO2-CeO2-ZrO2materials have many advantages, such as grain refinement, compact and uniform structure, excellent corrosion resistance and energy conservation, high catalytic activity and even distribution of the micro activity, which can improve comprehensive properties of electrode materials significantly. Compared to direct-current technology, the electrode prepared by pulse technology with lower porosity, better corrosion resistance and energy saving is more smooth and orderly arrangement of surface particles. Through AC impedance spectroscopy analysis of corrosion process of composite electrode, it showed that:dispersion effect was existed in corrosion process of composite electrode; the corrosion process of composite electrode prepared under different concentration of dispersant agent, current density, temperature, stirring rate, pulse average current density, duty cycle and pulse work time was mainly controlled by charge transfer; However, that of electrode prepared under different concentration of CeO2and ZrO2was controlled by charge transfer in early stage, the late stage was controlled by diffusion step. Combining scanning electrochemistry microscope (SECM) with SEM and energy spectrum testing showed that:the composite electrode materials prepared under optimum conditions contained more nanometer particles, the surface morphology of them were level and electrocatalatic activity and its uniformity distribution were improved.(2) Investigating of deposition mechanism of inert composite electrodeBy analyzing CV curves, the deposition process of composite electrode was investigated. The deposition mechanism is proposed:Pb2+and hydroxyl radicals formed intermediate product Pb(OHads)2+and then it transformed into PbO2. The process of intermediate product Pb(OHads)2+transformed into PbO2was rate-determining step. The modified substances can accelerate grain nucleation, delay grain growth and refine grain size. It is favorable toward promoting co-deposition of β-PbO2-CeO2-ZrO2when plating bath component:CeO2concentration is16g·L-1, ZrO2concentration is30g·L-1and dispersant agent concentration is50mL·L-1. The formation reaction of β-PbO2-CeO2-ZrO2was steady and irreversible process.(3) The study of anode properties for trivalent chromium platingThrough electrochemical studies, it is easy to conclude that the p-PbO2-CeO2-ZrO2composite electrode showed excellent corrosion resistance and energy conservation compared to P-PbO2and Pb-Ag alloy. The addition of CeO2and ZrO2changed growth orientation of PbO2grains and improved texture structure and grain size of coating. Through the comprehensive performance investigation of electrode in trivalent chromium plating, it was found that β-PbO2-CeO2-ZrO2composite electrode can effectively reduce cell voltage, improve corrosion resistance and inhibit formation of hexavalent chromium. When the concentration of ZrO2is30g·L-1in bath, the Zr concentration in prepared electrode materials is high, which can reduce and stabilize cell voltage, reduce hexavalent chromium concentration effectively and ensure energy conservation and the stability of plating bath. |
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