Study On Electrochemical Noise Energy Generated During Ni And Zn Electroplating Processes

Electrochemical noise(EN)measurement is a significant technology which has been widely used to investigate the material corrosion and its protection,battery discharge and plating processes.EN technology is in situ and nondestructive compared with other traditional electrochemical technology.The choice of sampling frequency is crucial issue because sampling frequency has a significant influence on EN data analysis.Nowadays,there are two main methods for choosing sampling frequency.Most of researches believe that the sampling frequency is sensible when no diffuse hump appears in the high frequency part of power density spectrum(PSD)obtained by fast fourier transform(FFT)and maximum entropy method(MEM).Besides,some studies confirm the optimum sampling frequency through comparing spectral noise impedance,derived from FFT and MEM,with polarization resistance.The sampling frequency is best when the spectral noise impedance close to the polarization resistance.However,there are still some defects in EN technology.The traditional method for the selection of EN sampling frequency still has some shortcomings.First,DC drift should be eliminated prior to using FFT and MEM resulting in introduction of errors.Meanwhile,FFT is based on hypothesis that time series is periodic or the time series is zero outside the finite time series.And the length of the prediction error filter M is an uncertain empirical value which will dramatically affect the result in MEM.Moreover,the principle of EN technology is incomplete needing to be improved.The physical significance or role of certain parameters is ambiguous(for example the crystal and the information in energy distribution spectrum EDP).Typically,everything has energy and any process will cause energy exchange between the system and the environment.However,the energy of EN,its main influencing factors and their common laws,especially the physical significance of EN energy,are still unclear.Electrodeposition is an important method for material preparation.The electrochemical noise energy,ED,generated in nickel and zinc electroplating processes have been investigated using electrochemical noise technique.The main factors which influence the ED and its corresponding variation have been discussed.Then,the physical significance of ED is clarified.The main research contents are as follows:(1)Study on the selection of reasonable sampling frequency.Based on fast wavelet analysis(FWT)and Nyquist sampling theorem,the influence of different sampling frequencies on RP-EDP spectra is investigated.And according to the characteristics of RP-EDP spectra,the optimal sampling frequency adopt 40 Hz during nickel electrodeposition process under experimental conditions.(2)Study on the main influencing factors on ED and its corresponding variation rule.EN origins widely,but in a particular electrochemical process the source is limited.Furthermore,EN generated by different factors has a unique frequency.The change rule of ED has been investigated by changing the values of the main parameters which can affect the electrode process.The common rules of all the main influencing factors are found out by comparing the magnitude of the impact and particularly the reason why cause the differences.The mechanism of main factors(such as Current density jk,temperature,concentration of main salt,conductivity of substrate,size of particles of deposit)effecting on ED has been discussed.It finds that:(ⅰ)the substrate and number average particle size djk affect charge transfer rate which can impact on ED;(ⅱ)jk affects the charge transfer rate by changing the size of particles of deposit and Femi energy EF of the electrode surface and then influences on ED;(iii)the concentration of impacts on the charge transfer rate by changing the size of particles of deposit and then influences on ED.(3)Study on the physical significance of ED.The EN generated from different processes has different level of energy which reflects the amount of energy exchanged between electrode and environment during electrode process.The main influence factors and corresponding common regularity for ED has been ascertained.On the basis of common regularity and electrochemical noise principle,physical significance of ED has been theoretically explained.ED mainly depends on the difficulty for forming the initial deposit monolayer,and the greater the difficulty,the greater the ED.Hence,ED can be used as a new parameter to characterize the difficulty level of electrode process which involves the formation of initial monolayer of crystallites,pits or other forms of localized attacks.The factors(such as j_k,substrate,T and c)that affect the charge transfer rate may affect ED by influencing the difficulty of forming the initial deposit monolayer.The kinetic information of electrode process reflected by the crystal in RPEDP can been explained using the physical significance of ED.(4)Application of ED in the study of Zn electroplating.In order to verify the universality of ED,common zinc electroplating system has been chosen to study the influence of the main parameters on ED.The initial nucleation/growth mechanism has been elucidated by ED,cyclic voltammetry and Chronoamperometry technology.It finds that ED weakly influences the concentration of ZnCl2 and ED decreases with the increase of jk,but ED changes little with jk compared with the variation range of jk.In other words,the regulation is similar with Ni electroplating process.Meanwhile,it is found that the electrodeposition of zinc on highly-pure Cu is more difficult than that of Ni.ED generated by zinc electroplating is significantly larger than that of nickel electroplating under the same deposition time and similar j_k.