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Effect of the breakdown time of a passive film on the electrochemical machining of rotating cylindrical electrode in NaNO3 solution
Accepted Manuscript Title: Effect of the breakdown time of a passive film on the electrochemical machining of rotating cylindrical electrode in NaNO3 solution Author: Dengyong Wang Zengwei Zhu Bin He Yongcheng Ge Di Zhu PII: DOI: Reference:
S0924-0136(16)30299-0 http://dx.doi.org/doi:10.1016/j.jmatprotec.2016.08.023 PROTEC 14935
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
29-2-2016 9-8-2016 24-8-2016
Please cite this article as: Wang, Dengyong, Zhu, Zengwei, He, Bin, Ge, Yongcheng, Zhu, Di, Effect of the breakdown time of a passive film on the electrochemical machining of rotating cylindrical electrode in NaNO3 solution.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2016.08.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of the breakdown time of a passive film on the electrochemical machining of rotating cylindrical electrode in NaNO3 solution Author names Dengyong Wang,a Zengwei Zhu,a,*Bin He,a Yongcheng Ge,a Di Zhua Affiliations Nanjing University of Aeronautics and Astronautics, Nanjing 210016, People’s Republic of China. *
Corresponding author Address: Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing
210016, People’s Republic of China; E-mail: [email protected]; Tel: +86 25 84896605; Fax: +86 25 84895912.
Abstract In electrochemical machining (ECM), a passive film is generally present on the anode surface at low current density but is broken down at high current density in a passivating electrolyte. This film can impede unwanted metal dissolution in a low-current-density region and thereby improve the accuracy of ECM. However, it may sometimes affect metal dissolution at high current density. In this paper, the breakdown time of the passive film is determined for different current densities by varying the applied anodic potential pulses. It is found that the breakdown time has a significant magnitude of a few seconds, which can be long enough to affect metal dissolution in ECM. To illustrate the effect of the breakdown time of a passive film, ECM tests with 1
cylindrical electrodes at different rotational speeds are conducted. The results indicate a correlation between the breakdown time and the rotational speed. At a high rotational speed, the surface of the anode workpiece is always in a passive state due to the existence of the breakdown time of a passive film, and hardly any material is removed. When a slower rotational speed is used, the steel passivity can be overcame, and a high material removal rate is obtained. A numerical simulation of the current density distribution on the cylindrical anode surface is performed, and the relationship between total metal dissolution time and breakdown time is found. It is seen that the breakdown time of the passive film can have a strong effect on the metal removal rate in ECM. Keywords: Electrochemical machining; Passive film; Breakdown time; Cylindrical electrode; Rotational speed 1. Introduction Electrochemical machining (ECM) is an anodic dissolution process using a high electric current density (>20 A cm–2) that can effectively remove materials from a metal, regardless of its hardness, reported by Rajurkar et al. (1999). Many applications have been successfully yielded in the aerospace, automotive, defense and medical industries by using ECM. To obtain high-quality products using ECM, neutral NaNO3 solution has become one of the most commonly used electrolytes because of its passivating characteristics. Béjar and Eterovich (1995) used a passivating electrolyte of NaNO3 in the wire-electrochemical cutting of mild steel. The results showed that by using NaNO3 2
solution the cuts are significantly more accurate than by when using an electrolyte of NaCl. In a super finishing process with ultrasonic and magnetic assistance, Pa (2009) used the NaNO3 electrolyte to obtain a smooth and bright surface of the workpiece. Xu et al. (2013) also used the NaNO3 electrolyte in an ECM method of blisk channels. The maximum of allowance difference between suction surface and pressure surface was controlled to be less than 0.4 mm. Wang et al. (2015) used an iron coating layer to reduce the stray current attack on the non-machined surface of the convex structure. The machining accuracy of the convex structure was improved significantly. De Silva et al. (2003) used a low electrolyte concentration to control the localization effects of the electrochemical dissolution in a precision ECM process. Early in the 1970s, Mao (1971) studied ECM of mild steel in NaNO3 solution in closed-cell systems. The results showed that the current efficiencies for material removal were significantly reduced at low current densities owing to the presence of a compact oxide film on the anode surface. As reported by Chin and Mao (1974), this oxide film was gradually broken down with increasing anode potential and current density, and the underlying metal surface was exposed to the electrolyte. Besides, scanning electron microscopy (SEM) of the breakdown of the film on nickel under transpassive dissolution conditions in NaNO3 solution was carried out by Datta and Landolt (1971). The results showed that transpassive dissolution was initiated by local breakdown of the passive film. Haisch et al. (2001) investigated the surface films of 100Cr6 steel in aqueous NaCl and NaNO3 solutions. It was found that in NaNO3 solution a strongly attached surface film formed by carbides and oxides hindered the 3
Fen+ diffusion. Lohrengel and Rosenkranz (2005) investigated the surface structures during ECM of iron in neutral NaNO3 solution. They expected a duplex structure: a solid oxide film of some nm and, above, a meta-stable, highly soluble supersaturated iron nitrate film. Taken together, these studies reveal that a passive film is generally present on the anode surface at low current density but is broken down at high current density in NaNO3 solution. However, there has been little investigation of the time taken for the passive film to break down at high current density. The passive film can prevent stray corrosion and improve machining accuracy to some extent at low current density. However, it may also sometimes impede material removal at high current density and reduce machining efficiency, as, for example, in the ECM of titanium and titanium alloys. The dissolution behavior of Ti90Al6V4 and Ti60Al40 studied by Weinmann et al. (2015) showed that higher titanium content of the alloy would impede the dissolution process, and an increase of chloride ions in the electrolyte would facilitate the dissolution. In particular, in some scheme of ECM, the inter-electrode gap changes dramatically due to the movements of the electrodes. For example, in rotate-print ECM proposed by Zhu et al. (2015), the revolving anode workpiece and cathode tool rotate relatively at the same angular speed, and the inter-electrode gap are changing constantly during the counter-rotating process. In the ECM assisted by low-frequency vibrations proposed by Hewidy et al. (2007), the inter-electrode gap also varies periodically with the tool vibration. In these ECM techniques, the current densities on the anode surface differ between the passive and transpassive regions. The breakdown 4
time of the passive film may have a strong effect on metal dissolution and processing efficiency. Thus, it is necessary to investigate the effect of the passive film on metal dissolution during ECM. In this paper, a specific phenomenon occurring in ECM using cylindrical electrodes is presented: when the rotational speed of the electrode is over 10 1/min, mild steel is hardly dissolved in NaNO3, even at a high current density of about 60 A cm–2. To investigate this phenomenon, the breakdown time of passive film is determined at different current densities by varying the applied anodic potential pulses, and a numerical simulation of the current density distribution on the cylindrical anode surface is conducted. It is found that the breakdown time is significant, with a value of a few seconds. From the relationship between the total metal dissolution time and the breakdown time, it is found that metal dissolution is strongly affected by the breakdown time. Within this paper it was focused on mild steel only, but the techniques presented can also be applied to other metals that are passivated in NaNO3 solution. 2. Current efficiency measurement Cubic electrodes of dimensions 10 mm × 10 mm × 10 mm were used for current efficiency measurement. Apart from some minor elements such as S and P, the main compositions of the mild steel were as shown in Table 1. Before each experiment, the mild steel specimen was degreased by acetone and rinsed by distilled water. Table 1. Main metal compositions of mild steel. 5
Element
Fe
C
Mn
Si
wt%
Balance
0.21
0.44
0.19
As shown in Wang et al. (2015), a specific mechanism was designed to obtain the current efficiencies at different current densities during ECM. It consisted of a rectangular flow channel through which the electrolyte was pumped at a constant high velocity, a removable cathode that could be adjusted to vary the inter-electrode gap, and an epoxy resin insulator with a square groove in which the cubic specimen was embedded. To maintain a constant current density in each run, the experiment was conducted in an equilibrium state, and the dissolution time was precisely controlled using a timer. Before and after each experiment, the specimen was rinsed, dried, and weighed carefully. The current efficiency was calculated as
M exp M theo M exp kIt
(1)
where Mexp is the actual experimental weight loss (g), Mtheo is the theoretical weight loss (g), k is the theoretical value of the mass electrochemical equivalent (g A-1s-1), I is the current (A), and t is the dissolution time in each run (s). The current efficiencies obtained for mild steel at current densities ranging from 7 to 80 A cm–2 in 106 g l–1 NaNO3 solution are shown in Fig. 1. It can be seen that the current efficiency exhibits a strong dependence on the current density. Up to 6.7 A cm–2, no metal removal can be observed, in accordance with a current efficiency of 0%. This means that exclusive oxygen evolution proceeds at low current density owing to the presence of a passive film on the specimen surface. When the current density is above the passive region, a rapid increase in current efficiency occurs. This 6
can be attributed to local breakdown of the passive film. At higher current density, above 40 A cm–2, the current efficiency for transpassive dissolution tends toward a constant value of about 73%.
–
Fig.1. Current efficiencies for ECM of mild steel in 106 g l 1 NaNO3 solution.
3. Breakdown time determination of a passive film As reported in Datta et al. (1984), the passive film formed at low current density is a few nanometers thick. It increases in thickness in the passive regime, and then decreases at the onset of the transpassive dissolution region. As shown by the current efficiency in Fig. 1, the specimen was pretreated at a low current density of about 6 A cm–2 for 30 s to obtain a compact passive film. Thereafter, the process of the breakdown of the passive film was studied at a higher current density. 3.1. Passive film formation To obtain a low current density of about 6 A cm–2, a large inter-electrode gap of a few millimeters was set. By applying an anodic potential pulse to the mild steel specimen, a passive film was formed on the surface. The variation of current with time was recorded using a Hall sensor with a high sampling frequency of 20 kHz. The recorded trace of current transient for a potentiostatic potential pulse of 30 V 7
in NaNO3 solution is shown in Fig. 2. It can be seen that the current first jumps to a large value and then falls. After this, the current reaches a relatively stable value owing to the formation of a passive film. Fig. 3(a) shows the SEM image of the initial specimen surface, it is seen that the surface is smooth. However, when an anodic pulse is applied, a compact surface film is formed on the whole specimen surface (Fig. 3(b)). From the SEM image shown in Fig. 3(b), the specimen surface is still flat, and no metal dissolution is observed.
Fig. 2. Current–time transient for the formation of a passive film at a potentiostatic pulse of 30 V.
(a)
(b)
Fig. 3. SEM images of the specimen surface: (a) Initial surface; (b) Surface with a passive film.
As the gas bubbles and heat in the electrolyte can be timely taken away, the 8
relation between the current I and potential U can be given approximately by Ohm’s law, so the initial current can be described as
Ii U Re
(2)
where Re is the electrolyte resistance in the inter-electrode gap. When a passive film layer with a certain resistance Rf forms on the specimen surface at low current density, the current becomes
I f U ( Re R f )
(3)
The resistance Rf increases with the formation of the passive film layer (Datta et al., 1984), and results in a decrease in the current. According to Fig. 2, the ratio Ii/ If is approximately 1.07. Using Eqs. (2) and (3), the ratio Rf/ Re is determined as 0.07. It can be concluded that the resistance of the passive film Rf is much smaller than the electrolyte resistance Re. 3.2. Passive film breakdown As the resistance of the passive film Rf is very small, the variation in current density in the transpassive dissolution region is mainly dominated by the electrolyte resistance. When local breakdown of the passive film occurs, the specimen surface is corroded locally, and the local inter-electrode gap Gx increases accordingly. This will result in a decrease in the average current density on the specimen surface. Thus, the decrease in the current can be regarded as indicating onset of breakdown of the passive film. A constant potential U of 15 V was applied to the specimen. To obtain different current densities, the initial inter-electrode gap G ranged from 0.2 to 1 mm. From the 9
current–time curves shown in Fig. 4, it can be seen that the plateau regions of the currents are found at different inter-electrode gaps. This indicates the existence of a breakdown time for the passive film.
(a)
(b)
(c)
(d)
Fig. 4. Current–time curves for the breakdown of the passive film at different inter-electrode gaps: (a) G = 1 mm; (b) 0.6 mm; (c) 0.4 mm; (d) 0.2 mm.
The breakdown times of the passive film at current densities ranging from 10 to 60 A cm–2 are shown in Fig. 5. It can be seen that the breakdown time decreases with increasing current density, and reaches a relatively steady value when the current density is over 40 A cm–2. For an initial current density of about 10 A cm–2, the breakdown time is as long as 3.5 s. Even at a high current density of 57 A cm–2, the 10
breakdown time is still significant, at 1.1 s. The specimens were corroded for different times at an inter-electrode gap of 0.2 mm (Fig. 4(d)). SEM images of the resulting surfaces are shown in Fig. 6. The surface structure after 1.3 s corrosion is shown in Fig. 6(a), from which it can be seen that the specimen surface is still flat and no obvious metal dissolution takes place. This is because of the presence of a compact passive film. When the corrosion time is extended to 2.6 s (Fig. 6(b)), bare metal areas are found on the specimen surface, indicating local breakdown of the passive film. After this, active metal dissolution occurs on the specimen surface, and bare metal areas and products (mainly oxides and
Breakdown time / (s)
carbides of iron) can be observed on the surface at 8 s, as shown in Fig. 6(c).
Current density / (A cm–2)
Fig. 5. Breakdown times of the passive film at different current densities.
Passive film Passive film
Bare metal area
(a)
(b) 11
Bare metal area Products attached
(c) Fig. 6. SEM images of surface structures corroded for different times: (a) 1.3 s; (b) 2.6 s; (c) 8 s.
4. ECM tests with cylindrical electrodes at different rotational speeds Cylindrical electrodes of dimensions 10 mm diameter × 5 mm length were used for ECM tests at different rotational speeds. The experimental set-up for the ECM process is shown in Fig. 7. The cylindrical anode workpiece and cathode tool were mounted on two shafts, which rotated relative to each other at the same rotational speed. The end faces of the electrodes were shielded by circular epoxy resin plates. To prevent a local lack of electrolyte in the narrow inter-electrode gap, the electrodes were immersed in a reactor cell, and the electrolyte was pumped through the working area with a flushing pressure of 0.5 MPa. As a result, the materials of the workpiece were gradually removed under electrolysis.
12
Electrolyte inlet Reactor cell
Rotating shaft Epoxy resin plate
Cathode tool Epoxy resin plate
Anode workpiece
Electrolyte outlet
Fig. 7. Schematic diagram of the experimental set-up for ECM with cylindrical electrodes.
The cylindrical anode workpieces were processed at rotational speeds ranging from 0.033 to 400 1/min in 106 g l–1 NaNO3 solution for 30 min. During each experiment, the initial gap between the cylindrical electrodes was controlled at a constant value of 0.25 mm, and a constant voltage of 15 V was applied to the electrodes. Removal of material from the anode surface can result in an increase in electrolyte resistance in the inter-electrode gap, with the current value consequently being reduced according to Ohm’s law. Thus, current measurements can be used to reflect the variation of the inter-electrode gap. The sampling frequencies of the current signals were set to correspond to the rotational speeds. As shown in Fig. 8, the current decreases rapidly at 0.033, 0.066 and 0.1 1/min, indicating an increasing inter-electrode gap. The current tends to decrease more slowly with higher rotational speed, but for rotational speeds greater than 10 1/min, the rates of decrease of the measure currents are nearly the same. Because there was no feed motion during the ECM process, the expansion of the inter-electrode gap can be attributed to the reduced diameter of the anode workpiece due to the material removal from the anode surface. 13
1/min 1/min 1/min 1/min 1/min 1/min 1/min
Fig. 8. Variations of current signals at different rotational speeds in 106 g l–1 NaNO3 solution for 30 min.
The diameters of the machined anode workpieces were measured, and the amounts of material removed along the radial direction were calculated (Fig. 9). It can be seen that the amount of material removed varies with rotational speed. Below 0.1 1/min, about 0.4 mm of material is removed. However, the amount drops rapidly with increasing rotational speed, tending finally to a much smaller value of about 0.04 mm. This means that very little mild steel is removed by ECM when the rotational speed is over 10 1/min.
14
Fig. 9. Amount of material removed at different rotational speeds in 106 g l–1 NaNO3 solution for 30 min.
It is well known that NaCl exhibits no passivation activity in ECM, owing to the activation of chloride ion (Chin and Mao, 1974 and Mao and Hoare, 1973). For comparison, the cylindrical anode workpieces were also tested in 106 g l–1 NaCl solution for 5 min. From the results shown in Figs. 10 and 11, it can be seen that both the time variation of the measured current and the amount of material removed tend to be independent of rotational speed. This indicates that the ECM process in NaCl solution is not affected by the rotational speed.
Fig. 10. Time variation of current signal at different rotational speeds in 106 g l–1 NaCl solution for 5 min.
15
Fig. 11. Amount of material removed at different rotational speeds in 106 g l–1 NaCl solution for 5 min.
5. Discussion To investigate the effect of the breakdown time of the passive film on material removal during ECM, the electric field distribution on the anode surface was calculated using the finite element method. The current lines between the cylindrical electrodes are shown in Fig. 12. The lengths of the red arrows represent the electric field intensity. It can be seen that the electric field intensity is strongest in the narrow middle area and declines gradually on each side.
16
Current line
Y / (mm)
Pt1 Anode workpiece： U=15 V Pt0
Pt2
Cathode tool: U=0 V
Pt3
Current line
X / (mm)
Fig. 12. Current line distribution between cylindrical electrodes.
The position of a given point P on the anode surface varies with the rotation of the anode workpiece. Figure 13 shows the current density at a point P at different rotation angles. It can be seen that the current density varies from near zero to a high value of nearly 60 A cm–2. According to the current efficiencies in Fig. 1, the anodic dissolution process at point P alternates between passive and transpassive regions during each rotation. From time t0 to t1, the material at point P is completely passivated owing to the formation of a passive film at low current densities. After time is t1, the current density is greater than 6.7 A cm–2, and the point P rotates into the transpassive region. This process lasts from time t1 to t3, after which point P moves back into the passive region.
17
Fig. 13. Current densities at point P at different rotation angles.
As discussed in Section 3.2, in the transpassive region (time t1 to t3), the passive film on the anode surface begins to break down before metal dissolution, with this process lasting for a few seconds. In a single rotation, because the current density distribution stays the same at different rotational speeds, the breakdown time of the passive film can be assumed to be a constant Tbreak for each run. So, the actual time for metal dissolution in each rotation can be calculated as Tdis = Ttrans – Tbreak
(4)
where Ttrans is the time in the transpassive region in each rotation. During the complete ECM process, the total time for metal dissolution is Ttotal = Tdis×N = Tdis×T×n
(5)
where N is the number of rotations during ECM, T is the complete processing time during ECM, and n is the rotational speed. As can be seen in Fig. 13, from the rotation angles at times t1 and t3, the transpassive region makes up a proportion of 0.199 of a single rotation. Therefore, the time spent in the transpassive region can be expressed as Ttrans = 0.199T1 = 0.199/n 18
(6)
where T1 is the processing time for a single rotation. Combining Eqs. (4) – (6), the total time for metal dissolution during ECM can be expressed as: Ttotal = (0.199/n– Tbreak ) ×T ×n = 0.199T– Tbreak ×T×n
(7)
As the complete processing time T and the breakdown time of the passive film, Tbreak, are constant for each run, the total time for metal dissolution, Ttotal, is in inverse proportion to the rotational speed n. This can explain why much more material is removed at low rotational speeds than at high rotational speeds. Because the experiments with cubic and cylindrical electrodes are all performed in a turbulent electrolyte flow, the stirring actions of the rotating electrodes can be ignored at a relative slow rotational speed. According to the experimental results described in Section 3.2, the breakdown times of the passive film, Tbreak, range from 1.1 to 3.5 s at different current densities. According to Eq. (7), the critical rotation speed for material dissolution is between 3.4 and 10.9 1/min. The theoretical total metal dissolution times Ttotal at different rotational speeds are plotted in Fig. 14. It is found that the trend of variation is in accord with that of the amount of material removed as shown in Fig. 9. The very small amount of material removed by ECM at rotational speeds above 10 1/min can be attributed to repetitive removal of passive film rather than transpassive metal dissolution.
19
Fig. 14. Theoretical total metal dissolution times at different rotational speeds.
6. Conclusions This paper has focused on the effect of the breakdown time of a passive film on the electrochemical dissolution of mild steel in NaNO3 solution. Based on experimentally determined current efficiencies, the processes of formation and breakdown of the passive film have been studied at different current densities. Breakdown times have been determined from recorded current traces. In addition, ECM tests with cylindrical electrodes and a numerical simulation of the current density distribution have been conducted to examine the effect of the breakdown time. The conclusions of this study can be summarized as follows: (1) According to the current efficiency obtained in NaNO3 solution, a passive film is present on mild steel specimen surface at current densities up to 6.7 A cm–2, but is broken down at higher current densities. (2) The breakdown time of the passive film formed at a low current density of about 6 A cm–2 is significant, with a value of a few seconds. 20
(3) The ECM tests with cylindrical electrodes in NaNO3 solution show that material removal rates depend strongly on rotational speed. In particular, when the rotational speed is above 10 1/min, there is virtually no dissolution of mild steel. However, this phenomenon is not observed in NaCl solution. (4) The current density distribution on cylindrical anode surfaces exhibits different trends of variation in the passive and transpassive regions. Investigation of the relationship between the total metal dissolution time and the breakdown time shows that the breakdown time has a strong effect on metal dissolution in ECM.
Acknowledgements The authors wish to acknowledge the financial support provided by the China Natural Science Foundation (51535006).
Reference Béjar, M.A., Eterovich, F., 1995. Wire-electrochemical cutting with a NaNO3 electrolyte. J. Mater. Process. Technol. 55, 417–420. Chin, D.T., Mao, K.W., 1974. Transpassive dissolution of mild steel in NaNO3 electrolytes. J. Applied electrochemistry 4, 155-161. Datta, M., Landolt, D., 1971. Film breakdown on nickel under transpassive dissolution conditions in sodium nitrate solutions. J. Electrochem. Soc. 124, 483-489. Datta, M., Mathieu, H.J., Landolt, D., 1984. AES/XPS study of transpassive films on iron in nitrate solution. J. Electrochem. Soc. 131, 2484–2489. 21
De Silva, A.K.M., Altena, H.S.J., McGeough, J.A., 2003. Influence of electrolyte concentration on copying accuracy of precision-ECM. CIRP Ann. 52, 165–168. Haisch, T., Mittemeijer, E., Schultze, J.W., 2001. Electrochemical machining of the steel 100Cr6 in aqueous NaCl and NaNO3 solutions: microstructure of surface films formed by carbids. Electrochim. Acta 47, 235-241. Hewidy, M.S., Ebeid, S.J., El- Taweel, T.A., Youssel, A.H., 2007. Modelling the performance of ECM assisted by low vibrations. J. Mater. Process. Technol. 189, 466–472. Lohrengel, M.M., Rosenkranz, C., 2005. Microelectrochemical surface and product investigations during electrochemical machining (ECM) in NaNO3. Corros. Sci. 47, 785-794. Mao, K.W., 1971. ECM study in a closed-cell system II. NaCl, NaClO4, and NaNO3. J. Electrochem. Soc. 118, 1876–1879. Mao K.W., Hoare, J.P., 1973. The anodic dissolution of mild steel in solutions containing both Cl- and NaNO3- ions. Corros. Sci. 13,799–803. Pa P.S., 2009. Super finishing with ultrasonic and magnetic assistance in electrochemical micro-machining. Electrochim. Acta 54, 6022–6027. Rajurkar, K.P., Zhu, D., Mcgeough, J.A., Kozak, J., DeSilva, A., 1999. New developments in electro-chemical machining. CIRP Ann. 48, 567–579. Wang, D.Y., Zhu, Z.W., Bao, J., Zhu, D., 2015. Reduction of stray corroison by using iron coating in NaNO3 solution during electrochemical machining. Int. J. Adv. Manuf. Technol. 76, 1365–1370. 22
Wang, D.Y., Zhu, Z.W., Wang, N.F., Zhu, D., Wang, H.R., 2015. Investigation of the electrochemical dissolution behavior of Inconel 718 and 304 stainless steel at low current density in NaNO3 solution. Electrochim. Acta 156, 301-307. Weinmann, M., Stolpe, M., Weber, O., Busch, R., Natter, H., 2015. Electrochemical dissolution behavior of Ti90Al6V4 and Ti60Al40 used for ECM applications. J. Solid State Electrochem. 19, 485–495. Xu, Z.Y., Xu, Q., Zhu, D., Gong, T., 2013. A high efficiency electrochemical machining method of blisk channels. CIRP Ann. 62, 187–190. Zhu, Z.W., Wang, D.Y., Bao, J., Wang, N. F., Zhu, D., 2015. Cathode design and experimental study on the rotate-print electrochemical machining of revolving parts. Int. J. Adv. Manuf. Technol. 80, 1957–1963.
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S0924-0136(16)30299-0 http://dx.doi.org/doi:10.1016/j.jmatprotec.2016.08.023 PROTEC 14935
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
29-2-2016 9-8-2016 24-8-2016
Please cite this article as: Wang, Dengyong, Zhu, Zengwei, He, Bin, Ge, Yongcheng, Zhu, Di, Effect of the breakdown time of a passive film on the electrochemical machining of rotating cylindrical electrode in NaNO3 solution.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2016.08.023 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of the breakdown time of a passive film on the electrochemical machining of rotating cylindrical electrode in NaNO3 solution Author names Dengyong Wang,a Zengwei Zhu,a,*Bin He,a Yongcheng Ge,a Di Zhua Affiliations Nanjing University of Aeronautics and Astronautics, Nanjing 210016, People’s Republic of China. *
Corresponding author Address: Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing
210016, People’s Republic of China; E-mail: [email protected]; Tel: +86 25 84896605; Fax: +86 25 84895912.
Abstract In electrochemical machining (ECM), a passive film is generally present on the anode surface at low current density but is broken down at high current density in a passivating electrolyte. This film can impede unwanted metal dissolution in a low-current-density region and thereby improve the accuracy of ECM. However, it may sometimes affect metal dissolution at high current density. In this paper, the breakdown time of the passive film is determined for different current densities by varying the applied anodic potential pulses. It is found that the breakdown time has a significant magnitude of a few seconds, which can be long enough to affect metal dissolution in ECM. To illustrate the effect of the breakdown time of a passive film, ECM tests with 1
cylindrical electrodes at different rotational speeds are conducted. The results indicate a correlation between the breakdown time and the rotational speed. At a high rotational speed, the surface of the anode workpiece is always in a passive state due to the existence of the breakdown time of a passive film, and hardly any material is removed. When a slower rotational speed is used, the steel passivity can be overcame, and a high material removal rate is obtained. A numerical simulation of the current density distribution on the cylindrical anode surface is performed, and the relationship between total metal dissolution time and breakdown time is found. It is seen that the breakdown time of the passive film can have a strong effect on the metal removal rate in ECM. Keywords: Electrochemical machining; Passive film; Breakdown time; Cylindrical electrode; Rotational speed 1. Introduction Electrochemical machining (ECM) is an anodic dissolution process using a high electric current density (>20 A cm–2) that can effectively remove materials from a metal, regardless of its hardness, reported by Rajurkar et al. (1999). Many applications have been successfully yielded in the aerospace, automotive, defense and medical industries by using ECM. To obtain high-quality products using ECM, neutral NaNO3 solution has become one of the most commonly used electrolytes because of its passivating characteristics. Béjar and Eterovich (1995) used a passivating electrolyte of NaNO3 in the wire-electrochemical cutting of mild steel. The results showed that by using NaNO3 2
solution the cuts are significantly more accurate than by when using an electrolyte of NaCl. In a super finishing process with ultrasonic and magnetic assistance, Pa (2009) used the NaNO3 electrolyte to obtain a smooth and bright surface of the workpiece. Xu et al. (2013) also used the NaNO3 electrolyte in an ECM method of blisk channels. The maximum of allowance difference between suction surface and pressure surface was controlled to be less than 0.4 mm. Wang et al. (2015) used an iron coating layer to reduce the stray current attack on the non-machined surface of the convex structure. The machining accuracy of the convex structure was improved significantly. De Silva et al. (2003) used a low electrolyte concentration to control the localization effects of the electrochemical dissolution in a precision ECM process. Early in the 1970s, Mao (1971) studied ECM of mild steel in NaNO3 solution in closed-cell systems. The results showed that the current efficiencies for material removal were significantly reduced at low current densities owing to the presence of a compact oxide film on the anode surface. As reported by Chin and Mao (1974), this oxide film was gradually broken down with increasing anode potential and current density, and the underlying metal surface was exposed to the electrolyte. Besides, scanning electron microscopy (SEM) of the breakdown of the film on nickel under transpassive dissolution conditions in NaNO3 solution was carried out by Datta and Landolt (1971). The results showed that transpassive dissolution was initiated by local breakdown of the passive film. Haisch et al. (2001) investigated the surface films of 100Cr6 steel in aqueous NaCl and NaNO3 solutions. It was found that in NaNO3 solution a strongly attached surface film formed by carbides and oxides hindered the 3
Fen+ diffusion. Lohrengel and Rosenkranz (2005) investigated the surface structures during ECM of iron in neutral NaNO3 solution. They expected a duplex structure: a solid oxide film of some nm and, above, a meta-stable, highly soluble supersaturated iron nitrate film. Taken together, these studies reveal that a passive film is generally present on the anode surface at low current density but is broken down at high current density in NaNO3 solution. However, there has been little investigation of the time taken for the passive film to break down at high current density. The passive film can prevent stray corrosion and improve machining accuracy to some extent at low current density. However, it may also sometimes impede material removal at high current density and reduce machining efficiency, as, for example, in the ECM of titanium and titanium alloys. The dissolution behavior of Ti90Al6V4 and Ti60Al40 studied by Weinmann et al. (2015) showed that higher titanium content of the alloy would impede the dissolution process, and an increase of chloride ions in the electrolyte would facilitate the dissolution. In particular, in some scheme of ECM, the inter-electrode gap changes dramatically due to the movements of the electrodes. For example, in rotate-print ECM proposed by Zhu et al. (2015), the revolving anode workpiece and cathode tool rotate relatively at the same angular speed, and the inter-electrode gap are changing constantly during the counter-rotating process. In the ECM assisted by low-frequency vibrations proposed by Hewidy et al. (2007), the inter-electrode gap also varies periodically with the tool vibration. In these ECM techniques, the current densities on the anode surface differ between the passive and transpassive regions. The breakdown 4
time of the passive film may have a strong effect on metal dissolution and processing efficiency. Thus, it is necessary to investigate the effect of the passive film on metal dissolution during ECM. In this paper, a specific phenomenon occurring in ECM using cylindrical electrodes is presented: when the rotational speed of the electrode is over 10 1/min, mild steel is hardly dissolved in NaNO3, even at a high current density of about 60 A cm–2. To investigate this phenomenon, the breakdown time of passive film is determined at different current densities by varying the applied anodic potential pulses, and a numerical simulation of the current density distribution on the cylindrical anode surface is conducted. It is found that the breakdown time is significant, with a value of a few seconds. From the relationship between the total metal dissolution time and the breakdown time, it is found that metal dissolution is strongly affected by the breakdown time. Within this paper it was focused on mild steel only, but the techniques presented can also be applied to other metals that are passivated in NaNO3 solution. 2. Current efficiency measurement Cubic electrodes of dimensions 10 mm × 10 mm × 10 mm were used for current efficiency measurement. Apart from some minor elements such as S and P, the main compositions of the mild steel were as shown in Table 1. Before each experiment, the mild steel specimen was degreased by acetone and rinsed by distilled water. Table 1. Main metal compositions of mild steel. 5
Element
Fe
C
Mn
Si
wt%
Balance
0.21
0.44
0.19
As shown in Wang et al. (2015), a specific mechanism was designed to obtain the current efficiencies at different current densities during ECM. It consisted of a rectangular flow channel through which the electrolyte was pumped at a constant high velocity, a removable cathode that could be adjusted to vary the inter-electrode gap, and an epoxy resin insulator with a square groove in which the cubic specimen was embedded. To maintain a constant current density in each run, the experiment was conducted in an equilibrium state, and the dissolution time was precisely controlled using a timer. Before and after each experiment, the specimen was rinsed, dried, and weighed carefully. The current efficiency was calculated as
M exp M theo M exp kIt
(1)
where Mexp is the actual experimental weight loss (g), Mtheo is the theoretical weight loss (g), k is the theoretical value of the mass electrochemical equivalent (g A-1s-1), I is the current (A), and t is the dissolution time in each run (s). The current efficiencies obtained for mild steel at current densities ranging from 7 to 80 A cm–2 in 106 g l–1 NaNO3 solution are shown in Fig. 1. It can be seen that the current efficiency exhibits a strong dependence on the current density. Up to 6.7 A cm–2, no metal removal can be observed, in accordance with a current efficiency of 0%. This means that exclusive oxygen evolution proceeds at low current density owing to the presence of a passive film on the specimen surface. When the current density is above the passive region, a rapid increase in current efficiency occurs. This 6
can be attributed to local breakdown of the passive film. At higher current density, above 40 A cm–2, the current efficiency for transpassive dissolution tends toward a constant value of about 73%.
–
Fig.1. Current efficiencies for ECM of mild steel in 106 g l 1 NaNO3 solution.
3. Breakdown time determination of a passive film As reported in Datta et al. (1984), the passive film formed at low current density is a few nanometers thick. It increases in thickness in the passive regime, and then decreases at the onset of the transpassive dissolution region. As shown by the current efficiency in Fig. 1, the specimen was pretreated at a low current density of about 6 A cm–2 for 30 s to obtain a compact passive film. Thereafter, the process of the breakdown of the passive film was studied at a higher current density. 3.1. Passive film formation To obtain a low current density of about 6 A cm–2, a large inter-electrode gap of a few millimeters was set. By applying an anodic potential pulse to the mild steel specimen, a passive film was formed on the surface. The variation of current with time was recorded using a Hall sensor with a high sampling frequency of 20 kHz. The recorded trace of current transient for a potentiostatic potential pulse of 30 V 7
in NaNO3 solution is shown in Fig. 2. It can be seen that the current first jumps to a large value and then falls. After this, the current reaches a relatively stable value owing to the formation of a passive film. Fig. 3(a) shows the SEM image of the initial specimen surface, it is seen that the surface is smooth. However, when an anodic pulse is applied, a compact surface film is formed on the whole specimen surface (Fig. 3(b)). From the SEM image shown in Fig. 3(b), the specimen surface is still flat, and no metal dissolution is observed.
Fig. 2. Current–time transient for the formation of a passive film at a potentiostatic pulse of 30 V.
(a)
(b)
Fig. 3. SEM images of the specimen surface: (a) Initial surface; (b) Surface with a passive film.
As the gas bubbles and heat in the electrolyte can be timely taken away, the 8
relation between the current I and potential U can be given approximately by Ohm’s law, so the initial current can be described as
Ii U Re
(2)
where Re is the electrolyte resistance in the inter-electrode gap. When a passive film layer with a certain resistance Rf forms on the specimen surface at low current density, the current becomes
I f U ( Re R f )
(3)
The resistance Rf increases with the formation of the passive film layer (Datta et al., 1984), and results in a decrease in the current. According to Fig. 2, the ratio Ii/ If is approximately 1.07. Using Eqs. (2) and (3), the ratio Rf/ Re is determined as 0.07. It can be concluded that the resistance of the passive film Rf is much smaller than the electrolyte resistance Re. 3.2. Passive film breakdown As the resistance of the passive film Rf is very small, the variation in current density in the transpassive dissolution region is mainly dominated by the electrolyte resistance. When local breakdown of the passive film occurs, the specimen surface is corroded locally, and the local inter-electrode gap Gx increases accordingly. This will result in a decrease in the average current density on the specimen surface. Thus, the decrease in the current can be regarded as indicating onset of breakdown of the passive film. A constant potential U of 15 V was applied to the specimen. To obtain different current densities, the initial inter-electrode gap G ranged from 0.2 to 1 mm. From the 9
current–time curves shown in Fig. 4, it can be seen that the plateau regions of the currents are found at different inter-electrode gaps. This indicates the existence of a breakdown time for the passive film.
(a)
(b)
(c)
(d)
Fig. 4. Current–time curves for the breakdown of the passive film at different inter-electrode gaps: (a) G = 1 mm; (b) 0.6 mm; (c) 0.4 mm; (d) 0.2 mm.
The breakdown times of the passive film at current densities ranging from 10 to 60 A cm–2 are shown in Fig. 5. It can be seen that the breakdown time decreases with increasing current density, and reaches a relatively steady value when the current density is over 40 A cm–2. For an initial current density of about 10 A cm–2, the breakdown time is as long as 3.5 s. Even at a high current density of 57 A cm–2, the 10
breakdown time is still significant, at 1.1 s. The specimens were corroded for different times at an inter-electrode gap of 0.2 mm (Fig. 4(d)). SEM images of the resulting surfaces are shown in Fig. 6. The surface structure after 1.3 s corrosion is shown in Fig. 6(a), from which it can be seen that the specimen surface is still flat and no obvious metal dissolution takes place. This is because of the presence of a compact passive film. When the corrosion time is extended to 2.6 s (Fig. 6(b)), bare metal areas are found on the specimen surface, indicating local breakdown of the passive film. After this, active metal dissolution occurs on the specimen surface, and bare metal areas and products (mainly oxides and
Breakdown time / (s)
carbides of iron) can be observed on the surface at 8 s, as shown in Fig. 6(c).
Current density / (A cm–2)
Fig. 5. Breakdown times of the passive film at different current densities.
Passive film Passive film
Bare metal area
(a)
(b) 11
Bare metal area Products attached
(c) Fig. 6. SEM images of surface structures corroded for different times: (a) 1.3 s; (b) 2.6 s; (c) 8 s.
4. ECM tests with cylindrical electrodes at different rotational speeds Cylindrical electrodes of dimensions 10 mm diameter × 5 mm length were used for ECM tests at different rotational speeds. The experimental set-up for the ECM process is shown in Fig. 7. The cylindrical anode workpiece and cathode tool were mounted on two shafts, which rotated relative to each other at the same rotational speed. The end faces of the electrodes were shielded by circular epoxy resin plates. To prevent a local lack of electrolyte in the narrow inter-electrode gap, the electrodes were immersed in a reactor cell, and the electrolyte was pumped through the working area with a flushing pressure of 0.5 MPa. As a result, the materials of the workpiece were gradually removed under electrolysis.
12
Electrolyte inlet Reactor cell
Rotating shaft Epoxy resin plate
Cathode tool Epoxy resin plate
Anode workpiece
Electrolyte outlet
Fig. 7. Schematic diagram of the experimental set-up for ECM with cylindrical electrodes.
The cylindrical anode workpieces were processed at rotational speeds ranging from 0.033 to 400 1/min in 106 g l–1 NaNO3 solution for 30 min. During each experiment, the initial gap between the cylindrical electrodes was controlled at a constant value of 0.25 mm, and a constant voltage of 15 V was applied to the electrodes. Removal of material from the anode surface can result in an increase in electrolyte resistance in the inter-electrode gap, with the current value consequently being reduced according to Ohm’s law. Thus, current measurements can be used to reflect the variation of the inter-electrode gap. The sampling frequencies of the current signals were set to correspond to the rotational speeds. As shown in Fig. 8, the current decreases rapidly at 0.033, 0.066 and 0.1 1/min, indicating an increasing inter-electrode gap. The current tends to decrease more slowly with higher rotational speed, but for rotational speeds greater than 10 1/min, the rates of decrease of the measure currents are nearly the same. Because there was no feed motion during the ECM process, the expansion of the inter-electrode gap can be attributed to the reduced diameter of the anode workpiece due to the material removal from the anode surface. 13
1/min 1/min 1/min 1/min 1/min 1/min 1/min
Fig. 8. Variations of current signals at different rotational speeds in 106 g l–1 NaNO3 solution for 30 min.
The diameters of the machined anode workpieces were measured, and the amounts of material removed along the radial direction were calculated (Fig. 9). It can be seen that the amount of material removed varies with rotational speed. Below 0.1 1/min, about 0.4 mm of material is removed. However, the amount drops rapidly with increasing rotational speed, tending finally to a much smaller value of about 0.04 mm. This means that very little mild steel is removed by ECM when the rotational speed is over 10 1/min.
14
Fig. 9. Amount of material removed at different rotational speeds in 106 g l–1 NaNO3 solution for 30 min.
It is well known that NaCl exhibits no passivation activity in ECM, owing to the activation of chloride ion (Chin and Mao, 1974 and Mao and Hoare, 1973). For comparison, the cylindrical anode workpieces were also tested in 106 g l–1 NaCl solution for 5 min. From the results shown in Figs. 10 and 11, it can be seen that both the time variation of the measured current and the amount of material removed tend to be independent of rotational speed. This indicates that the ECM process in NaCl solution is not affected by the rotational speed.
Fig. 10. Time variation of current signal at different rotational speeds in 106 g l–1 NaCl solution for 5 min.
15
Fig. 11. Amount of material removed at different rotational speeds in 106 g l–1 NaCl solution for 5 min.
5. Discussion To investigate the effect of the breakdown time of the passive film on material removal during ECM, the electric field distribution on the anode surface was calculated using the finite element method. The current lines between the cylindrical electrodes are shown in Fig. 12. The lengths of the red arrows represent the electric field intensity. It can be seen that the electric field intensity is strongest in the narrow middle area and declines gradually on each side.
16
Current line
Y / (mm)
Pt1 Anode workpiece： U=15 V Pt0
Pt2
Cathode tool: U=0 V
Pt3
Current line
X / (mm)
Fig. 12. Current line distribution between cylindrical electrodes.
The position of a given point P on the anode surface varies with the rotation of the anode workpiece. Figure 13 shows the current density at a point P at different rotation angles. It can be seen that the current density varies from near zero to a high value of nearly 60 A cm–2. According to the current efficiencies in Fig. 1, the anodic dissolution process at point P alternates between passive and transpassive regions during each rotation. From time t0 to t1, the material at point P is completely passivated owing to the formation of a passive film at low current densities. After time is t1, the current density is greater than 6.7 A cm–2, and the point P rotates into the transpassive region. This process lasts from time t1 to t3, after which point P moves back into the passive region.
17
Fig. 13. Current densities at point P at different rotation angles.
As discussed in Section 3.2, in the transpassive region (time t1 to t3), the passive film on the anode surface begins to break down before metal dissolution, with this process lasting for a few seconds. In a single rotation, because the current density distribution stays the same at different rotational speeds, the breakdown time of the passive film can be assumed to be a constant Tbreak for each run. So, the actual time for metal dissolution in each rotation can be calculated as Tdis = Ttrans – Tbreak
(4)
where Ttrans is the time in the transpassive region in each rotation. During the complete ECM process, the total time for metal dissolution is Ttotal = Tdis×N = Tdis×T×n
(5)
where N is the number of rotations during ECM, T is the complete processing time during ECM, and n is the rotational speed. As can be seen in Fig. 13, from the rotation angles at times t1 and t3, the transpassive region makes up a proportion of 0.199 of a single rotation. Therefore, the time spent in the transpassive region can be expressed as Ttrans = 0.199T1 = 0.199/n 18
(6)
where T1 is the processing time for a single rotation. Combining Eqs. (4) – (6), the total time for metal dissolution during ECM can be expressed as: Ttotal = (0.199/n– Tbreak ) ×T ×n = 0.199T– Tbreak ×T×n
(7)
As the complete processing time T and the breakdown time of the passive film, Tbreak, are constant for each run, the total time for metal dissolution, Ttotal, is in inverse proportion to the rotational speed n. This can explain why much more material is removed at low rotational speeds than at high rotational speeds. Because the experiments with cubic and cylindrical electrodes are all performed in a turbulent electrolyte flow, the stirring actions of the rotating electrodes can be ignored at a relative slow rotational speed. According to the experimental results described in Section 3.2, the breakdown times of the passive film, Tbreak, range from 1.1 to 3.5 s at different current densities. According to Eq. (7), the critical rotation speed for material dissolution is between 3.4 and 10.9 1/min. The theoretical total metal dissolution times Ttotal at different rotational speeds are plotted in Fig. 14. It is found that the trend of variation is in accord with that of the amount of material removed as shown in Fig. 9. The very small amount of material removed by ECM at rotational speeds above 10 1/min can be attributed to repetitive removal of passive film rather than transpassive metal dissolution.
19
Fig. 14. Theoretical total metal dissolution times at different rotational speeds.
6. Conclusions This paper has focused on the effect of the breakdown time of a passive film on the electrochemical dissolution of mild steel in NaNO3 solution. Based on experimentally determined current efficiencies, the processes of formation and breakdown of the passive film have been studied at different current densities. Breakdown times have been determined from recorded current traces. In addition, ECM tests with cylindrical electrodes and a numerical simulation of the current density distribution have been conducted to examine the effect of the breakdown time. The conclusions of this study can be summarized as follows: (1) According to the current efficiency obtained in NaNO3 solution, a passive film is present on mild steel specimen surface at current densities up to 6.7 A cm–2, but is broken down at higher current densities. (2) The breakdown time of the passive film formed at a low current density of about 6 A cm–2 is significant, with a value of a few seconds. 20
(3) The ECM tests with cylindrical electrodes in NaNO3 solution show that material removal rates depend strongly on rotational speed. In particular, when the rotational speed is above 10 1/min, there is virtually no dissolution of mild steel. However, this phenomenon is not observed in NaCl solution. (4) The current density distribution on cylindrical anode surfaces exhibits different trends of variation in the passive and transpassive regions. Investigation of the relationship between the total metal dissolution time and the breakdown time shows that the breakdown time has a strong effect on metal dissolution in ECM.
Acknowledgements The authors wish to acknowledge the financial support provided by the China Natural Science Foundation (51535006).
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