Pulse Electrodeposition Cr/SiC Composite Coatings From Trivalent Chromium Baths In Ultrasonic Field

The technology and electrochemical behaviors of ultrasonic-pulseelectrodepositing Cr/SiC composite coatings from trivalent chromiumbath containing of compound carboxylate-urea were investigatedaccording to the SiC and chromium content and the thickness of thecomposite coatings.The effects of the bath and the operating conditions on the SiCcontent,thickness, surface appearance have been discussed, The optimumbath and operating conditions were obtained: CrCl₃·6H₂O0.65mol·L^-1,Na₃C₆H₅O₇0.4mol·L^-1, Na₂C₂O₄0.32mol·L^-1, CO（NH₂）₂2.05mol·L^-1,CH₃COONa0.35mol·L^-1, H₃BO₃0.72mol·L^-1, NaCl2.0mol·L^-1, SiC l.3mol·L^-1, CTAB0.013mol·L^-1, current density5.4A·dm-2, pH2.1,temperature36℃, electrodepositing times65min, work ratio0.35,switching times4ms, pulse frequency22Hz, ultrasonic frequency40kHz,ultrasonic power85W. Under the optimum conditions, the Cr/SiCcomposite coating with thickness13.83μm, SiC content7.2%andhardness1352.3Hv can be obtained.The surface morphology, microstructure and composition of Cr/SiCcomposite coatings were studied with the help of modern analyticalinstrument, such as scan electron microscope （SEM）, X-raydiffraction（XRD） and photo electron spectroscopy （EDS）. The results show that thecrystal grains with a ball-like crystal structure were obtained, the surfacemorphology was smooth, microcracks and pinholes could not be observed.SiC phase were found in Cr/SiC composite coatings, and the basic metalswere Cr with crystal structure. The results of corrosion and wearperformance testings indicated that the corrosion and wear resistanceincreased with increasing the SiC content in the Cr/SiC compositecoatings.The effect of ultrasonic and complex on the electrochemicalbehaviors of Cr/SiC composite coatings electrodeposition wereinverstigated by the modern electrochemical methods. The results show that single complexing agent and compound carboxylate-urea systemcould not only lead to the two steps of Cr³⁺ions reduction potentials shiftnegatively but aslo increase the hydrogen evolution potential, and anobvious synergistic effect was possibly produced between compoundcarboxylate-urea system. Meanwhile, due to the ultrasonic fieldpresenting in electrodeposition process, the two steps of of Cr³⁺reductionpotentials were shifted90mV towards more positive potentials, indicatingultrasonic could increase Cr³⁺deposition rate, it is benefit to obtain athick Cr/SiC composite coating. However, the hydrogen evolutionpotential also shifted positively in ultrasonic field, which was bad for theCr³⁺reduction.So ultrasonic power must be controlled to proper strength.