| Composite electrodeposition with nano particles is a rapid developing surface technology in recent years, which has received a wide range of applications in the project. Nanoparticles agglomerated very easily in the plating bath because of its high surface energy, which would result in the unique properties of nano composite coatings reduce. Therefore, dispersion of these particles in the plating bath is one of the key steps toward achieving optimal mechanical properties for electroplating nano composite coatings. Because of trivalent chromium lower toxicity, only1%of hexavalent chromium, and sewage treated simply, the requirements meeted environmental protection. In addition, relative to hexavalent chromium plating, the trivalent chromium plating also has other features:the higher sediment velocity, the better capacity of the bath dispersed and plating deeply, plating without the impact of current interrupted, etc. In recent years, trivalent chromium is awared by more and more scholars. Owing to its superior properties such as high strength, high melting point, desirable heat stability and wear resistance, nano-Al2O3particles have been widely used to modify the hardness, abrasive and wear resistance of metal deposits currently. In this work, nano-Al2O3particles were added to the trivalent chromium plating bath to prepare the Cr-Al2O3composite coatings with desirable properties by means of co-electrodeposition. The study included the dispersion of nano-Al2O3in aqueous solution and trivalent chromium plating bath and the effects of bath component and electrodeposition process on Cr-Al2O3coating performance. Properties of Cr-Al2O3coatings such as surface morphology, composite, organizational structure, corrosion resistance and hardness were characterized by means of SEM, EDS, XRD, Tafel plots and electrochemical impedance spectroscopy(EIS) separately. At last, The mechanism of trivalent chromium electrocrystallization from trivalent chromium plating bath has been explored by using several kinds of electrochemical methods:cyclic voltammogram curves and cathodic polarization curves. Analyses of the results are as follows:(1) The agglomeration of nano-Al2O3particles in aqueous solution is studied. The results are revealed that when the pH is about9, the Zeta potential of nano-Al2O3is zero; with the addition of dispersant ammonium citrate, its Zeta potential changes very little, and it will get higher Zeta potential when under acidic conditions, then the nano-Al2O3particles disperses very well. Meanwhile, an nonionic surfactant ammonium citrate is chosen from different dispersants through20minutes ultrasonic by the sedimentation experiment. The nano-Al2O3particles are dispersed stably in aqueous solution when the appropriate dosage of ammonium citrate is2%. The dispersion of the nano-Al2O3particles with ammonium citrate in trivalent chromium plating bath is also discussed. The results show that it disperses well but is not better than in aqueous solution, we also find that the dispersant agent can make the nano-Al2O3particles more uniformly distribute throughout the composite coating.(2) Through single-factor experiment, effects of HCOONH4, auxiliary complexing agent, wetting agent, dispersant, AlC13·6H2O, nano-Al2O3concentration, current density, pH value, temperature, stirring speed, deposition time on Cr-Al2O3coatings were studied to obtain a suitable scopes. Orthogonal experiment L9(34) was designed to ascertain the influence of HCOONH4, auxiliary complexing agent, dispersant and AICl3·6H2O on the thickness, appearance and the content of Al. Results revealed that under the optimized process condition and the optimized bath recipes namely:HCOONH418.9g/L, auxiliary complexing agent3.85g/L, dispersant2%and AlCl3·6H2O12g/L, the Cr-Al2O3composite coatings are bright and possess the thickness of13.93μm. Results of SEM and EDS show that nano-Al2O3particles are well distributed in the coatings and the surface morphology of Cr-Al2O3composite coatings is homogeneous and freely crystallized. However, some Al2O3particles in the coatings have lager diameter than before, given some of them agglomerate. The main element in the coating are as follows:Cr, O and Al. Among them, the average content of Al is10.90%. The nano-Al2O3andCr achieved co-deposition.(3) Surface morphology, composition, crystal structure of the coatings were characterized through Using SEM, EDS, XRD. The results showed that:pure Cr coating cracks were obviously coarser and deeper; chromium composite coating has smaller number and shallower cracks. Nano-Al2O3particles deposited can obviously improve the structure of coating. Analysis of XRD demonstrates that structure of Cr-Al2O3composite coatings is amorphous. Kinds of performance test show that Cr-Al2O3composite coatings possess good adhesion and high hardness(1362.3HV). Tafel plots and EIS demonstrate that Cr-Al2O3composite coatings have much better corrosion resistance than Cr coatings in neutral, acid and alkaline corrosive medium.(4) Analyses of cyclic voltammogram curves show that the potential of Cr3+move toward to negative and the cathodic polarization increase with the increasing of scanning speed. Electroplating is an irreversible process. Cathodic polarization cures show that trivalent chromium ions are deposited by a four-step reduction and there are two reduction peaks between-0.85V—1.00V and between-1.00V—1.20V respectively. Conclusions are made by analyzing cathodic polarization as follows:addition of auxiliary complexing agent is good for dispersion of the plating bath; wetting agent in the bath refines grains of the coatings and brightens the coatings; the addition of nano-Al2O3particles strengthens the cathodic polarization of trivalent chromium and boosting the deposition rate. |
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