Effect of Pulsed Polarization Parameters on the Nickel ... - Springer Link

2 downloads 0 Views 266KB Size Report
It is advisable to use the dimensional electrochemical machining (DECM) based on high-rate local anodic dissolution of metals to solve a number of practically.
ISSN 1070-4272, Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 2, pp. 247−252. © Pleiades Publishing, Ltd., 2010. Original Russian Text © N.G. Dem’yantseva, S.M. Kuz’min, M.A. Solunin, A.M. Solunin, S.A. Lilin, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 2, pp. 249−254.

APPLIED ELECTROCHEMISTRY AND CORROSION PROTECTION OF METALS

Effect of Pulsed Polarization Parameters on the Nickel Shaping N. G. Dem’yantseva, S. M. Kuz’min, M. A. Solunin, A. M. Solunin, and S. A. Lilin Ivanovo State University of Power Engineering, Ivanovo, Russia Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia Ivanovo State University of Chemical Engineering, Ivanovo, Russia Received March 19, 2009

Abstract—Electrochemical shaping of nickel under pulsed polarization with various pulse shapes and repetition frequencies was studied. The effect of parameters of the pulsed dimensional electrochemical machining on the process of anodic dissolution of the metal was examined. DOI: 10.1134/S1070427210020114

theoretical models have been developed for this case [1–4]. At the same time, electrode processes occurring under pulsed polarization have been insufficiently analyzed. Most of studies of DECM in pulsed modes have been carried out at frequencies of up to 50 kHz and, as a rule, with rectangular pulses. The main body of evidence concerning pulsed anodic DECM was summarized in [1, 4–7]. The topicality of the problem under consideration is evidenced by results of an experimental study of microDECM with ±5-V pulses with repetition frequencies of up to 100 MHz and widths of 20–500 ns [8]. The goal of the present study was to examine processes of nickel shaping with pulses of various shapes and frequencies applied to the electrochemical cell, including a superposition of a sinusoidal alternating current and a dc current.

It is advisable to use the dimensional electrochemical machining (DECM) based on high-rate local anodic dissolution of metals to solve a number of practically important problems: fabrication of articles from hardto-machine metals and alloys whose cutting is rather labor consuming and, occasionally, hardly possible; fabrication of irregularly shaped articles (including various molds and turbine and compressor blades); and processing of articles intolerant against noticeable mechanical impacts. When performing DECM processes, it is necessary to provide a high-speed shaping process (up to millimeters per minute), with the required precision in both shape and size (as good as tenths of a micrometer) and low roughness of the resulting surface (Rz = 0.05–0.25 μm). The above characteristics depend on numerous factors, among which the most important are the nature and concentration of solution components, interelectrode gap between the instrument-electrode (IE) and a working electrode (WE) being processed, flow rate of the working solution through the interelectrode gap, and anodic current density. Together with the factors mentioned above, rather important is the shape of the applied polarizing signal (dc, sinusoidal, pulsed voltage). The DECM process under a dc polarizing voltage has been studied in ample detail and commonly accepted

EXPERIMENTAL For carrying out the study, an original electrochemical cell was fabricated and a setup described in detail in [9] (Fig. 1) was assembled. The setup had the form of a cell 1, made of methyl methacrylate and equipped with a support in which WE 2, a 0.1-mm-thick nickel foil, was fixed. As a cathode served a mobile IE 3, a stainless 247

248

DEM’YANTSEVA et al. Electrolyte

Electrolyte

Fig. 1. Block diagram of the experimental setup. For explanations, see text.

steel tube with outer and inner diameters of 2 and 1 mm, respectively, directed at the foil edge. The IE was moved by dc electric motor 4 via gear unit 5. To preclude a short circuit between the WE and IE, the motion velocity of the latter was controlled by varying the feed voltage of the electric motor from power source 6. The DECM

process was performed in a 1 M aqueous solution of NaNO3. The electrolyte composition in the interelectrode gap was maintained constant by passing a flow of the electrolyte from vessel 7 via flexible tubing 8 and IE 3 into the zone of the electrochemical reaction, with the spent electrolyte discharged into receiver 9. The IE was polarized using various power sources 10 providing necessary pulse shapes. Pulsed current parameters were recorded in 3 min, with an appropriate sweep range chosen for oscilloscope 11. For each polarization mode, different ordinate axis scales were chosen. The current and voltage scales were 0.9 A div–1 and 10 V div–1 for rectangular pulses, 1.8 A div–1 and 10 V div–1 for halfwave sinusoidal pulses, and 0.9 A div–1 and 0.5 V div–1 for a sinusoidal polarization superimposed on a dc anodic current. The amplitude characteristics of the pulses were recorded from a preliminarily calibrated oscilloscope screen with an accuracy not worse than 10%. The schematic circuits of the power sources providing IE polarization with rectangular pulses, half-wave

(a) (b)

Rl Rl (c)

Rl Fig. 2. Schematic circuits of power sources for EUI polarization with (a) rectangular pulses, (b) half-wave sinusoid, and (c) harmonic signal superimposed on a dc bias potential. (a) (1) Transformer, (2) diode, (3) capacitor, and (4) transistor; (b) (1) high-frequency generator, (2) power amplifier, and (3) diode; (c) (1) dc power source, (2) transformer, and (3) diode; Rl is the load (electrochemical cell) for all the three power sources. RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 2 2010

EFFECT OF PULSED POLARIZATION PARAMETERS ON THE NICKEL SHAPING

sinusoid, and harmonic signal superimposed on a dc bias potential are shown in Figs. 2a–2c. The rectangular pulses (Fig. 2a) were formed with a KT827A transistor operating in the switching mode. The transistor was controlled via a decoupling capacitor by rectangular pulses from a G4-117 low-frequency signal generator at a pulse repetition frequency of 0–200 kHz. The use of a 15000-μF storage capacitor provided a constant amplitude of the voltage during a pulse. Pulses in the form of a unipolar half-wave sinusoid were formed by connecting a 2D213A diode in series with a load (Fig. 2b). The sinusoidal polarization (Figs. 2b and 2c) was provided by a G4-117 generator and an LV103 power amplifier. The sinusoidal signal was superimposed on a dc bias potential (Fig. 2c) with a ferrite-core transformer, which protected the output circuits of the power amplifier from a dc voltage. To protect the output of the RFT 3217 source of a controllable dc voltage from the c component, it was shunted with a 2D213A high-frequency diode. The anodic current efficiency η (%) was found as the ratio between the mass of nickel m that passed into solution and the theoretical value:

The theoretical mass mth was found from the Faraday law:

249

Fig. 3. Photograph of the cavity formed in the nickel foil in its anodic electrochemical machining and illustration of how the characteristic dimensions of the cavity, a and b, are determined.

calculate the amount q of charge passed through the electrochemical cell in DECM processes performed in various modes by mathematical processing of the current oscillograms. In polarization with rectangular unipolar pulses, the amount of charge was calculated by the formula T

T

mth = qkel,

where q is charge passed through the cell, and kel is the electrochemical equivalent of nickel on the assumption that the metal passes into solution in the oxidation state Ni(II). The mass m was found as the difference between the electrode masses before and after an experiment. The sample was weighed on an analytical balance with an accuracy of 0.001 g. The dependences of the current and voltage, observed in the course of electrochemical shaping, suggest that, in the parameter range under study, current pulses repeat the shape of polarization pulses and the amplitude of current pulses linearly increases in the course of time. This occurs because the true surface area of the WE grows in the course of experiments because of its dissolution to give a cavity whose profile is shown in Fig. 3. The dependences found in this way make it possible to

where t is the period of the signal; I1 and I2, amplitudes of rectangular current pulses at the beginning and end of an experiment; τ, experiment duration; and N, number of pulses during a time τ. In the case of polarization with a half-wave sinusoid, the amount of charge was calculated by the formula T

The amount of charge passed upon superposition of a sinusoidal alternating current on a dc current was calculated by the formula q = Iavτ = 0.5(Imin + Imax)τ,

where Imin and Imax are the minimum and maximum

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 2 2010

250

DEM’YANTSEVA et al. (a)

(b)

k, arb. units

η, %

f, kHz

f, kHz

Fig. 4. (a) Form factor k and (b) current efficiency η vs. the frequency f at various types of IE polarization. (1–3) Half-wave sinusoid at IE motion velocities of (1) 0.25, (2) 0.36, and (3) 0.48 mm min–1; (4) rectangular unipolar pulses at a motion velocity of 0.48 mm min–1; and (5) range of k, obtained with a sinusoidal alternating current superimposed on dc current (at various Z) at the IE motion velocity of 0.87 mm min–1.

values of the current averaged over the time of the electrochemical treatment. The precision of the electrochemical shaping was evaluated by the form factor k = b/a (Fig. 3), whose increase points to a better shaping precision. To calculate the coefficient, the sample was photographed. The data in Fig. 4a indicate that the manner in which the form factor k depends on the pulse repetition frequency f is determined by parameters of polarization pulses. These curves have peaks at 7–8 kHz for the halfwave sinusoid (Fig. 4a, curves 1–3) and 0.5–1 kHz in the case of rectangular unipolar pulses (curve 4). The peaks in curves 1–3 are the more pronounced, the higher the velocity at which the IE moves. A similar behavior was observed in the case of DECM of nickel with rectangular pulses [10]. The form factor increases on making faster the velocity of IE motion because the distance between the IE and WE decreases in the given case. As a consequence, the electric field strength becomes higher and, accordingly, so does the anodic current density, which substantially improves the precision of the electrochemical shaping [2]. In addition, the shaping precision depends on the manner in which ions move in the interelectrode gap. According to the theoretical model developed in [9– 12], a charged particle experiences in a nonuniform electric field (occurs in electrochemical systems under

DECM conditions) on which a high-frequency (>100 Hz) polarization is superimposed a quasi-electric force pushing charged particles of any charge to the region with a weaker field. This result was obtained on averaging the particle motion during the oscillation period of the external field. In this case, a time-symmetric oscillation under nonuniform field conditions does not return a particle to its initial conditions, but causes its drift. A description of this phenomenon in terms of the action of some additional force, considered by Landau [13], was generalized in this study to the case of motion of a charged particle in a viscous medium for various time dependences of the periodic field. The effect of the additional force on the electrochemical shaping process can be understood as follows. Regions of a “stronger” field appear at sharp edges and corners of the cathode and working electrode, and regions of a “weaker” field, at depressions (Fig. 5). In this case, the smaller the radii of curvature of corners, projections, and depressions, the stronger the action of the quasi-electric force on any charged particles in the interelectrode gap. In the “ideal” anodic shaping mode, a cavity almost precisely corresponding to the IE shape must be produced (i.e., the angles ACA' and aca' must be close to 90°). The effective carryover of both anions and cations from strong field regions, which leads to slowing-down of the metal dissolution in regions 3 and acceleration of this process in regions 4, must lead to

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 2 2010

EFFECT OF PULSED POLARIZATION PARAMETERS ON THE NICKEL SHAPING

Fig. 5. The electrochemical machining of a nickel foil (2) with a tubular instrument-electrode (1) at a high-frequency ac field applied to the system. The schematic illustrates specific features of the electric field distribution in the interelectrode gap. (3, 4) Regions of strong and weak fields respectively; (5) “technological” projection. The arrows show direction of the electrolyte solution flow.

IE polarization with both a half-wave sinusoid and rectangular unipolar pulses, a minimum is observed in the η–f dependences (Fig. 4b) in the frequency range in which the k–f dependences have a maximum (Fig. 4a). However, in contrast to the form factor, the η, %

k, arb. units

a stronger localization of the DECM process (to a better precision of the electrochemical shaping process) [2]. The aforesaid is confirmed by the appearance of a “technological projection” on the anode (Fig. 5, position 5). In polarization of the IE with rectangular pulses, the shaping precision can be made 2–3 times better as compared with the polarization by a half-wave sinusoidal current (Fig. 4a, curve 4). At the same time, at pulsed current frequencies higher than 2 kHz, the DECM precision is almost the same in both cases and independent of the current shape (Fig. 4a, curves 3 and 4). The comparison of rectangular pulses with halfwave sinusoidal pulses at the same IE motion velocities is justified by the fact that, in both cases, the amplitude of polarization pulses is chosen so that almost the same quantities of electricity are passed through the cell. Upon superposition of a harmonic sinusoidal current on a dc current, the amplitude of the latter was also chosen equal to the amplitude of the rectangular pulsed current, with the effective anodic current increasing by approximately a factor of 2. As a result, the metal dissolution rate increased to the same extent and it became possible to raise the IE motion velocity to 0.87 mm min–1. Just at this IE motion velocity the variation range of the form factor k was determined at various ratios between the amplitudes of the sinusoidal component (Ua) and the dc polarization U (Fig. 4a, dependence 5) (henceforth this ratio Ua/U is denoted by letter Z). According to the theoretical concepts being developed, the quasi-electric force depends not only on the perturbation frequency and type of applied polarization, but also on Z. As can be seen in Fig. 6, the form factor strongly depends on Z. The range of values of the form factor at various Z (Fig. 4a, dependence 5) covers the corresponding values obtained for shaping both with rectangular pulses and under half-wave sinusoidal polarization (Fig. 4a, curves 1–4). It should be emphasized that, in the case of superposition of the sinusoidal polarization and a dc component, the dependences of the form factor, k–f(Z), and current efficiency, η–f(Z), show maxima at close values of Z, with the optimal value of Z falling within the range 0.4– 0.6 (Fig. 6). Comparison of the data in Figs. 4a and 4b shows that the dependences are correlated. In the case of

251

Fig. 6. (1) Form factor k and (2) current efficiency η vs. the ratio Z between the amplitudes of the ac and dc components of the polarization at an ac frequency of 10 kHz.

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 2 2010

252

DEM’YANTSEVA et al.

current efficiency weakly depends on the IE motion velocity (Fig. 4b, curves 1–3). Presumably, changes in the configuration of the interelectrode space do not lead under the conditions studied to any significant changes in the efficiency of the overall process of electrochemical dissolution of nickel. It should be noted that the experimental results reported in [8] can be interpreted in terms of the concepts being developed for understanding the effect of pulse DECM parameters (pulse shape and repetition frequency) on the precision of the electrochemical shaping; these concepts are, presumably, of rather general nature. In addition, DECM with pulses of various shapes requires a variety of power sources. In this case, DECM with pulsed polarization sources having different characteristics (different designs) may yield dissimilar results even at rather close pulse shapes. Effects of this kind may occur because the electrochemical cell is an open nonlinear dissipative system. The interaction of the electrochemical cell with a power sources in such systems is described by self-organization laws [14]. CONCLUSIONS (1) Frequency ranges in which the metal dissolution process can be performed with the maximum precision or output capacity were determined for various pulsed polarization types. (2) It was shown that, in the instrument-electrode polarization with a half-wave sinusoid or rectangular unipolar pulses, the maxima in the frequency dependences of the form factor coincide with the minima in the frequency dependences of the current efficiency. (3) It was demonstrated that, with a sinusoidal polarization superimposed on the dc component, a ratio Z between the amplitude of the ac voltage and the dc voltage can be chosen so that the dependences k–f(Z) and η–f(Z) are symbate. This simultaneously enables an electrochemical machining with the maximum possible shaping precision and a high rate of the anodic dissolution of nickel. REFERENCES 1. Petrov, Yu.N., Korchagin, G.N., Zaidman, G.N., et al., Osnovy povysheniya tochnosti elektrokhimicheskogo formoobrazovaniya (Fundamentals of Improving the

Precision of Electrochemical Shaping), Moroz, I.I., Ed., Kishinev: Shtinitsa, 1977. 2. Davydov, A.D. and Kozak, E., Vysokoskorostnoe elektrokhimicheskoe formoobrazovanie (High-Rate Electrochemical Shaping), Moscow: Nauka, 1990. 3. Dikusar, A.I., Engel’gardt, G.R., and Molin, A.N., Termokineticheskie yavleniya pri vysokoskorostnykh elektrodnykh protsessakh (Thermochemical Phenomena in High-Rate Electrode Processes), Kishinev: Shtinitsa, 1989. 4. Eliseev, Yu.S., Krymov, V.V., Saushkin, B.P., and Mitrofanov, A.A., Fiziko-khimicheskie metody obrbotki v proizvodstve turbinnykh dvigatelei (Physicochemical Machining Methods in Manufacture of Turbine Engines), Saushkin, B.P., Ed., Moscow: Drofa, 2002. 5. Rybalko, A.V., Development of Processes of Dimensional Electrochemical Machining with Microsecond Current Pulses and Apparatus for Their Implementation, Doctoral Dissertation, Voronezh, 1997. 6. Galanin, S.I., Elektrokhimicheskaya obrabotka metallov i splavov mikrosekundnymi impul’sami toka (Electrochemical Machining of Metals and Alloys with Microsecond Current Pulses), Kostroma: Kostrom. Gos. Tekh. Univ., 2001. 7. Smirnov, M.S., Improving the Precision and Surface Quality in Electrochemical Machining via Application of Ultrahigh Density Pulses, Cand. SCi. Dissertation, Ufa, 2004. 8. Zhan, Zh. and Zhou, D., Elektrokhimiyaб 2008, vol. 44, no. 8, pp. 998–1003. 9. Dem’yantseva, N.G., Solunin, M.A., Kuz’min, S.M., et al., Izv. Vyssh.Uchebn. Zaved., Khim. Khim. Tekhnol., 2009, vol. 52, no. 2, pp. 78–84. 10. Kuz’min, S.M., Solunin, M.A., Dem’yantseva, N.G., et al., Elektron. Obrab. Mater., 2006, no. 4, pp.53–59. 11. Dem’yantseva, N.G., Solunin, M.A., Kuz’min, S.M., et al., Aktual’nye problemy elektrokhimicheskoi tekhnologii, Saratov, 21–24 aprelya 2008 (Topical Problems of Electrochemical Engineering, Saratov, April 21–24, 2008), pp. 163–170. 12. Solunin, A.M., Solunin, M.A., and Solunin, S.A., Izv. Vyssh.Uchebn. Zaved., Fiz., 2003, no.10, pp. 48–52. 13. Landau, L.D. and Lifshits, E.M., Teoreticheskaya fizika, vol. 1. Mekhanika (Theoretical Physics, vol. 1, Mechanics), Moscow: Nauka, 1988. 14. Trubetskov, D.I., Mchedlova, E.S., and Krasichkov, L.V., Vvedenie v teoriyu samoorganizatsii otkrytykh sistem (Introduction to the Theory of Self-Organization of Open Systems), Moscow: Izd. Fiz.-Mat. Lit., 2005.

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 2 2010