Aqueous Solution In The Electrodeposition Of Zn-ni-re Alloy Coating Process And Mechanism

In this thesis, the process of electrodeposition of Zn-Ni-RE alloy from aqueous solution and its mechanism of co-deposition have been discussed in detail. It covered the following aspects:The author confirms the electrodeposition technology of Zn-Ni-RE alloy coatings, which is by using Hull trough experiment and rectangle alveus experiment to choose the bright area of the coatings. The technology conditions are: ZnCl₂ 15-35g/L, NiCl₂·6H₂O 30～70g/L, LaCl₃-7H₂O 2～10g/L,NH₄Cl 80～120g/L, C₆H₈O₇·H₂O 15～35g/L, C₆H₅Na₃O₇·2H₂O 25～45g/L, H₃BO₃ 25g/L, proper addition, PH is 2.6 to 3.5,30minutes of electroplate time, D_k=2～4A/dm²of electric current density and 15～30℃temperature of electroplating solution.Researches on the process of electrodeposited Zn-Ni-RE alloy coatings indicated that alloy coatings with different contents could be obtained by means of appropriate parameters. The concentration of main-salt was the primary influence on the content of nickel in the alloy coatings. The adding of NiCl₂·6H₂O and LaCl₃·7H₂O could help to increase the content of nickel, while the adding of ZnCl₂ helping to reduce the content of nickel. The adding of LaCl₃·7H₂O could help to increase the content of lanthanum, while the adding of NiCl₂·6H₂O and ZnCl₂ have little effect to the content of lanthanum. The adding of NH₄Cl could help to increase the content of nickel. The adding of C₆H₈O₇·H₂O and C₆H₅Na₃O₇·2H₂O helping to reduce the content of nickel, both of the two influence were weak; When the concentration was 20g/L and 30 g/L, the content of lanthanum was at the highest level. Increasing temperature could help to increase the content of nickel, but helping to reduce the content of lanthanum. Increasing the value of pH could increase the content of nickel, When the value of pH was 2.8 the content of lanthanum was maximal. Current density influenced the component of alloy coatings greatly. Increasing current density could help to increase the content of nickel, When current density was 3A/dm² the content of lanthanum was maximal.Zn-Ni-RE's anti-corrosion performance has been studied. The result is that the content of nickel in Zn-Ni(2%)-RE alloy coating's is lesser than Zn-Ni(13%) alloy coating with equal anti-corrosion performance. The cost will be saved greatly.The cathodic polarization of Zn-Ni-RE alloy coatings obviously lower to the Zn-Ni alloy coatings, the curves of cathodic polarization were determined in the cell of three electrodes, the cathodic polarization of the electrodepositing process is increased due to occurring of LaCl₃ special adsorption on electrode surface. Zn-Ni alloy coatings' codeposition belonged to abnormal codeposition, which is proved by the method of cathodic polarization decomposition. The various characters of cathode polarization curve have been discussed while adding different quantity rare earth salt. The cathodic polarization of Zn-Ni-RE alloy coatings diminishingly when the adding of rare earth at a certain quantity. Mechanisms of Zn-Ni-RE's co-deposition have been discussed.The mechanism of rare-earth conversion coatings was eondueted to study the corrosive resistance of the coating, as well as polarization resistance and corrosion current of coatings in 5% NaCl solutionhave been determined by electrochemical method, and XRD was used to analyze the products of corrosion.