Studies On Technology And Basic Mechanism Of Plating Ni-Fe,Ni-Fe-Cr Alloy Foil And Ni-Cr Alloy

Ni-Fe alloy foil and Ni-Fe-Cr alloy foil are not only excellentelectromagnetic memory materials and magnetic shielding materials, butalso good decorative and protective materials. Ni-Cr alloy deposit hasfavorable wear and corrosive resistance. With the rapid development ofmodern science and technology, the demands for these materials havebeen increasingly extending, and electrodeposition is one of the mosteffective approaches to obtain these functional materials. In China, theelectrodepositions of Ni-Fe alloy foil and Ni-Fe-Cr alloy foil are in thestage of laboratory research, while the electrodeposition of Ni-Cr alloy onaluminum and its alloys has not been investigated yet, and the researchescorresponding about their basic theories are only few. Owing to the status,the electroplating technologies and the fundamental theories of Ni-Fealloy foil, Ni-Fe-Cr alloy foil and Ni-Cr alloy plating on Al and Al alloysubstrate were studied in this dissertation.The respective electrochemical rules of Ni-Fe alloy, Ni-Fe-Cr alloyand Ni-Cr alloy codeposition, under various conditions were studiedsystematically by means of linear potential sweep, cyclic voltammetry andA.C. impedance.The electrodeposition of Ni-Fe alloy is an anomalous codeposition, itsreactive process is controlled by the electrochemical reaction. Withincreasing the concentration of the Fe²⁺ ions, adding brightener in theelectrolyte and prolonging the time for olation of the electrolyte, thecathodic polarization increased, the deposits became more fine andcompact, and their brightness was improved. Citric acid favored thecodeposition of Ni-Fe alloy and the formation of bright deposits.The codeposition of Ni-Fe-Cr alloy was controlled by electrochemicalprocess. The depositing rate of Ni-Fe-Cr alloys was mainly affected by therate of Fe and Cr deposition. The complexing agent played the role ofdepolarizing in the course of Ni-Fe-Cr alloy codeposition. The increase oftemperature favored the rate of Ni-Fe-Cr alloy deposition, but the Crcontent in the deposit decreased. The brightener increased the cathodicpolarization of Ni-Fe-Cr codeposition and made the deposit compact and with fine structure. The electrodeposition of Ni-Fe-Cr alloy was moredifficult than that of Ni-Fe alloy.The codeposition of Ni-Cr alloy is an irreversible process. Ni and Crplay the role of decreasing the overpotential of hydrogen evolution, whichmakes the cathodic current efficiency of Ni-Cr alloy codeposition lower.When the electrode potential is more negative, the rate of Ni-Crcodeposition is faster than that of the single metal. The influences oftemperature, pH value, complexing agent, brightener and olation on theelectrochemical performance of Ni-Cr alloy codeposition have beenexplained. The buffering capacity of the Ni-Cr electrolyte is steady.Using citric acid as a complexing agent and adding proper brightener,stabilizer and additives, nano crystalline Ni-Fe alloy foil, Ni-Fe-Cr alloyfoil and Ni-Cr alloy deposit with bright, smooth, compact, supertoughness, high electrical resistance and good corrosion resistance havebeen obtained from the chloride-sulphate aqueous solution system.The electroplating technology of magnetic Ni-Fe alloy foil was studiedsystematically. The influences of electrolyte compositions and operatingconditions on the iron content of Ni-Fe alloy foil, cathodic currentefficiency, thickness and hardness of the deposits were analyzed. In orderto obtain Ni-Fe alloy foil with the compositions of Ni_50～60Fe_40～50, theoptimum electrolyte compositions and process parameters were as follows:The mole ratio of Ni²⁺/Fe₂₊ was 4.5～5.5, and concentrations of theconstituents were Cl^-(13.5～15g/L), H₃BO₃(45～50g/L), citricacid(45g/L), brightener(10g/L), wetting agent(0.4～0.6g/L) and stabilizer(5～10g/L), respectively. The cathodic current density was 10～14A/dm²,temperature range was 50～55℃and pH value was 3.1～3.5. Operatingaccording the above conditions, the cathodic current efficiency was 81.3～97.5%, the thickness of the deposited foil was up to 22.3～40.7μm, andthe electrical resistance was 22～62.99μΩ·cm。Adopting the electrolyte system containing NiSO₄, NiCl₂, FeSO₄ andCrCl₃, the influence of electrolyte compositions and technical parameterson the contents of Ni, Fe and Cr of the Ni-Fe-Cr alloy foil deposits,current efficiency, thickness and the surface morphology of the depositswere investigated systematically. The chromium content in Ni-Fe-Cr deposits was difficult to increase and the reasons for this were analyzed.The optimum technical conditions for electroplating Ni-Fe-Cr alloy foilswere as follows. The current density was 14～20A/dm², temperature rangewas 20～30℃, pH value was 1.6～2.7. The concentrations of theconstituents were: NiSO₄·6H₂O(120～140g/L), CrCl₃·6H₂O(25～65g/L),FeSO₄·7H₂O(70g/L), citric acid(50～70g/L), H₃BO₃(50g/L), multipleadditive(6～14g/L) and stabilizer(10g/L), respectively. The compositionof Ni-Fe-Cr alloy foils is Fe31.1%～53.6%, Ni44.5%～65.4%, Cr1.2%～3.9%, respectively. The Ni-Fe-Cr alloy foils obtained are fineelectromagnetic shielding materials with good toughness, thickness (about30μm), tensile intensity (900～996MPa) and electrical resistivity (48.7～95.4μΩ·cm).The difficulty of poor adherence between Ni-Cr alloy deposit and thesubstrate of aluminium and its alloys was resolved firstly by the followingpre-treatment: corruption, chemical polishing and immersing in zinc twotimes. The technology of electroplating Ni-Cr alloy from the electrolytesystem containing CrCl₃ and NiSO₄ was studied systematically. Thehardness and the corrosion resistance of aluminium increased greatly. Theeffects of solution compositions and operating parameters on the Crcontent of Ni-Cr alloy deposits, current efficiency, thickness and hardnesswere also investigated systematically. The optimum conditions forelectroplating Ni-Cr alloy deposits with composition of Ni_75～95Cr_5～25 wereas follows: The concentrations of constituents-NiSO₄·6H₂O, CrCl₃·6H₂Oand H₃BO₃ were 30～100g/L, 50～115g/L and 40～50g/L, respectively;the mole ratio of citric acid to Cr³⁺ was 1.5～2.0; the cathodic currentdensity was 10～20A/dm²; temperature range was 24～40℃; pH valuewas 1.6～3.5. The thickness of the deposit was 10～30μm.X-ray diffraction (XRD), scanning electron microscope (SEM),micro-hardness test and emergency cooling and heating test have beenused to examine the structure, morphology, hardness of the deposits andthe adherence between the deposits and the substrates. The results showthat Ni-Fe, Ni-Cr and Ni-Fe-Cr alloy deposits are all FCC solid solutionwith crystallite size less than 10nm, and they are typical nano crystallite.The appearance of these deposits is bright and smooth and without pits and microcracks on the surface. The Ni-Cr and Ni-Fe-Cr alloy depositshad the typical characteristics of Cr alloy with uniform and compactstructure on the surface. The adherence between Ni-Cr alloy deposit andthe aluminium substrate was excellent.By means of chemical immersion and examining the anodicpassivation curves, the corrosive performance of Ni-Fe alloy foil,Ni-Fe-Cr alloy foil and Ni-Cr alloy deposits in the solution of 10% H₂SO₄and 5% NaCl have been researched. The results indicated that they wereall of excellent corrosion resistance.The mechanism of trivalent chromium deposition in the citric acidsolution system was studied by using several electrochemical methods.The apparent active energy of Cr³⁺ reduction was 37.19～78.37kJ/mol,indicating that the cathodic reduction of Cr³⁺ was a process controlled byelectrochemical step. The results of cyclic voltammetry and A.C.impedance indicated that the discharge of Cr³⁺ ions on the cathode wascompleted in two steps: In the first step, the Cr³⁺ ion obtained twoelectrons and formed an intermediate electrochemically active adsorptiveproduct, in the second step, the adsorptive intermediate product obtainedan electron and was reduced to Cr. The reaction mechanism can beexpressed as follows. Cr³⁺ + 2e→Cr_ad⁺ (â… ) Cr_ad⁺ + e(?)Cr (â…¡)Stepâ… is a rate-determining step. The kinetic parameters such asapparent transfer coefficient and reactive series have been determined bymeans of examining steady polarization curves.The kinetic equation of the electrode reaction rate derived from themechanism can be expressed as the following. i=3Fk₁a_Cr3+exp(-2α₁Fφ/RT)-3Fk_-1K_-2exp[(3-2α₁)Fφ/RT]The kinetic parameters calculated from the above equation are shownas follows: The cathodic Tafel slope is 0.118, the cathodic apparenttransfer coefficient (?) is 0.5, the cathodic reactive series of Cr³⁺ is 1.The anodic Tafel slope is 0.024, the anodic apparent transfer coefficient(?) is 2.5, the anodic reactive series of Cr³⁺ is 0, the chemical metric numberνof the electrode reaction is 1, and the apparent diffusioncoefficient D₀ is 1.606×10^-5 cm²/s.The theoretical values of the kineticparameters are consistent with the experimental values, which illustratesthe hypothesized depositing mechanism of trivalent chromium is correct.