Study On Trivalent Chromium Cathodic Electrodeposition Process In Chromium Sulfate/Sodium Oxalate System

Chromium electroplating has been widely used in various fields of modern industry and daily life due to its excellent physical,mechanical and electrochemical characteristics.Conventional Cr plating is frequently carried out in the hexavalent Cr electrolytes.However,the chromium electrolytes containing chromic acid are so toxic,carcinogenic and hazardous that hexavalent chromium electroplating has been restricted in US,Japan,China,Europe and other areas.Recently,the chromium deposition is known to be possible from solutions based on much less harmful Cr?III?compounds.Trivalent chromium electroplating,lower toxic and operating at room temperature,has drawn a great attention as one of the most challengeable processes to replace the conventional hexavalent Cr plating.However,the electrochemical reactions taking place by chromium electroplating from solutions of Cr?III?salts arecomplicated.It is difficult to electro-chemically reduce the trivalent chromium ions.The comprehensive investigation on trivalent chromium electroplating is necessary in order to improve this process.It may lead the chromium plating process towards the green,low-power development,with broad market prospect.The trivalent chromium plating has been proposed for a long time.It has some distinct advantages over hexavalent chromium on decorative application.There are potential problems on its application,and its available condition is limited.Thus,hard chromium plating has not yet been practically used.In order to accelerate industrialization of trivalent chromium plating,depth studies on trivalent chromium ion coordination mechanism and cathodic reduction mechanism are necessary,which has important theoretical and practical significance.Because there are not harmful chlorine produced on the anode in sulfate trivalent chromium system,coupled with its chrome plating color closer to that of hexavalent chromium plating,trivalent chromium electroplating has become a hotspot.In this thesis,trivalent chromium cathodic electrodeposition process and the effect of the additive,polyethylene glycol?PEG?,poly dipropyl sulfonate?SPS?have studied in chromium sulfate/sodium oxalate system.Some meaningful results are obtained.It has a guiding role for excellent trivalent chromium plating additives and process development.The main contents and results of this paper are as follows:1.The cathodic electrodeposition process in chromium sulfate/ sodium oxalate system and the effects of temperature,scan rate,electrode potential are studied by usingelectrochemical quartz crystal microbalance?EQCM?,cyclic voltammetry,linear scanning,chronoamperometry and other methods.According to Cr³⁺ complex features and EQCM results,the electrochemical reduction mechanism of the trivalent chromium is clarified deeply.2.PEG effects on trivalent chromium cathodic electrodeposition process in chromium sulfate/sodium oxalate system are studied by using EQCM,cyclic voltammetry,linear scanning,chronoamperometry.The structure and morphology of chrome plating are characterized by scanning electron microscopy?SEM?,atomic force microscopy?AFM?and X-ray diffraction?XRD?.PEG effects on trivalent chromium electrodeposition mechanism are investigated.The results show PEG can be adsorbed on the electrode surface,occupied part of the electrode surface and the rate of hydrogen evolution reaction is reduced;PEG reduces simultaneously the critical potential in the trivalent chromium electrodeposition process and reduces the difference between the electrode surface pHsf and bulk solution pH0,affecting the composition and structure of intermediate complexes generated in trivalent chromium electrodeposition process,thus changing the properties and deposition rate of chrome plating,conduciving to the formation of chromium plating;PEG alsochanges nucleation mechanisms,from continuous nucleation to instantaneous nucleation in the trivalent chromium electrodeposition and the fine,uniform chrome plating can be obtained.3.By using similar methods,SPS and PEG+SPS effects on trivalent chromium electrodeposition process in chromium sulfate/sodium oxalate system are studied.The results show the higher bath temperature can improve the speed of trivalent chromium electrodeposition;SPS effects on trivalent chromium electrodeposition process and themechanism of action are similar with PEG;SPS can increase the degree of cathodic polarization and change trivalent chromium nucleation mechanism,suppress the hydrogen evolution reaction,conducive to the electrodeposition of chromium;Chromium plating structure is related with the bath temperature and additives;SPS and PEG can refine the surface of the coating,and the SPS thinning is higher than PEG;SPS and PEG have synergistic effects for trivalent chromium electrodeposition process.