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Current efficiency during the electrochemical machining of iron and nickel
Corrosion Science, 1975, Vol. 15, pp. 435 to 440. Pergamon Press. Printed in Great Britain
C U R R E N T EFFICIENCY D U R I N G THE ELECTROCHEMICAL M A C H I N I N G OF IRON A N D NICKEL* JAMES P. HOARE and CHARLES R. WIESE Electrochemistry Department, Research Laboratories, General Motors Corporation, Warren, Michigan, U.S.A. Abstract--Current efficiency determinations from weight-loss measurements were made on pure iron and pure nickel anodes in 4M NaCIO3 solution in a flow cell at flow rates between 500 and 3000 cm/s in a current range from 5 to 50 A/cm~. The current efficiency for metal removal was virtually independent of current density and flow rate on iron anodes. On nickel anodes the current efficiency increased strongly with current density. In the high current density region, the current efficiency decreases with flow rate up to 2000 cm/s and then increases with higher flow rates. This behavior was accounted for by differences in the nature and properties of the anodic films formed on iron and nickel anodes. INTRODUCTION FROM various polarization studies, it has been concluded 1-e that the electrochemical machining (ECM) process takes place in the transpassive region and that good dimensional control is obtained 2.~-9 in electrolytes in which a potential-dependent, protective film is formed on the anode surface. Recently, the use of the term, "throwing power", borrowed from the plating industry, has been suggested 9-12 as a possible means for determining the ECM properties of a given metal-electrolyte system. This, of course, is possible because the underlying principle which governs the throwing power of an ECM electrolyte for the machining of a given metal is the nature and properties of the potential-dependent film formed on the surface of that metal anode in the given electrolyte. As noted by Landolt, 9 the throwing power of various electrolytes is due to the different dependency of metal dissolution on current density in the given electrolyte. Since the current density is a function of the applied potential which controls the nature and properties of the anodic film formed on the metal surface, the underlying factors which determine this dependency of the dissolution efficiency on current density are the nature and properties of this anodie film. In the transpassive region, electropolishing of the anode surface takes place if the anodic film is reduced to a thin, uniform, porous oxide layer *s about 50 ,~ thick. .4 This is the case for the machining of steel in NaCIOs solutions. *s Although Mao and Chin .5 have reported that the oxide film is completely removed from a steel anode in the transpassive region in NaCIOa solution because the film is not detected on scanning electron micrographs, the following points should be taken into consideration. First, steel pieces machined in NaCIOs solutions remain mirror bright for years during exposure to the atmosphere (surface protected by a thin uniform oxide film) whereas those machined in NaCl solutions (no oxide films present) begin to rust within a few days when exposed to atmospheric conditions. Secondly, a film o f ~/-FegOa was detected .8 on steel surfaces machined in NaCIOa from the results of vacuum fusion, *Manuscript received 6 November 1974. 435
436
JAMES P. HOARE and CHARLES R. WIESE
electron probe and electron diffraction analyses but not on steel surfaces machined in NaCI solutions. In the third place, a thin ( ~ 50 A) uniform oxide film cannot be detected on a SEM micrograph of the machined steel surface in NaCIO3 solution because of the high degree of surface smoothness. The hope that the ECM behavior of a given metal in a series of electrolytes should be the same for other metals in the same electrolytes has been expressed in the literature? Since the ECM behavior of a given metal in a given electrolyte depends on the nature and properties of the anodic film formed on that metal 17.]s and since the nature and properties of the anodic film are very likely to be different for various metals in the same electrolyte, this expressed hope most probably cannot be realized. In fact, there is some evidence in the literature that the ECM behavior of Ni 6,%]"02° in electrolytes such as solutions of NaCi, NaCIO.~, NaNOa is very different from that on Fe or steel ~.21,~2 in these electrolytes. It is the purpose of this report to describe some current efficiency measurements for metal removal of nickel and iron anodes in NaC103 solutions under the same ECM conditions in the same flow cells as a function of current density and electrolyte flow rate. The results show that the ECM behavior of Ni in NaC10.~ solutions is very different from that of Fe in these solutions. EXPERIMENTAL Anodes were made from electroformed iron (99.99~ pure) and nickel (99.9+ ~o pure) ingots which were machined on a lathe to rods about 13 cm long. These rods were potted in epoxy in a mold such that the outside diameter was 6'4 cm. The ends of the rods were polished on a polishing wheel and the diameter of the polished face, from which the'area was calculated, was determined with a microscope fitted with a Filar eyepiece. After the rod was washed with distilled water, rinsed with acetone and dried in a vacuum oven at l l5°C and 620 Torr for ~h, its weight was determined on an analytical balance. To get an accurate weight determination, the electrode was rebaked and reweighed until no further weight change was detected. This cross-sectional area of the iron anode was 0.121 cm ~ and that of the nickel anode 0.0733 cm 2. A brass rod 6.4 cm in diameter served as the cathode. The test anode and the brass cathode were mounted with the polished ends flush with the walls of the channel of a flow cell which is described in greater detail elsewhere. ~3 The rectangular flow channel 0.0508 cm between the walls containing the electrodes by 0.714 cm deep by 16.5 cm long was formed between two acrylic plates clamped together with toggle clamps and sealed with O-rings. A solution of 4 M NaCIOs was pumped through the cell by a stainless steel turbine pump and the flow rate was determined with a stainless steel rotameter. From a calibration chart, the flow rate was recorded as linear cm/s. Current efficiency determinations for the removal of iron and nickel were carried out at solution flow rates between 500 and 3000 cm/s and in the current range from 5 to 50 A/cm 2. During each run the temperature was maintained between 25 and 30°C. Since the surface of the anode recesses from the wall of the channel during the run, the current was allowed to flow so that the anode was recessed no deeper than 0"5 mm. At the end of each run the anode was washed with distilled water, rinsed in acetone, dried in the vacuum oven and reweighed. RESULTS Weight-loss measurements were recorded as the average of 5 independent runs ( r e p r o d u c i b l e w i t h i n ± 2.5 m g ) a n d t h e c u r r e n t efficiency f o r m e t a l r e m o v e d w a s c a l c u l a t e d o n t h e b a s i s o f 300 c o u l o m b s p a s s e d . C u r r e n t efficiency d a t a a r e p l o t t e d i n F i g . 1 as a f u n c t i o n o f c u r r e n t d e n s i t y f o r p u r e i r o n ( o p e n s y m b o l s ) a n d p u r e n i c k e l (filled s y m b o l s ) f o r t h e f o u r flow r a t e s (500, 1000, 2 0 0 0 a n d 300 c m / s ) i n t h e c u r r e n t d e n s i t y r a n g e f r o m 5 t o 50 A / c m ~. A b o v e 5 A / c m 2, t h e c u r r e n t efficiency f o r t h e r e m o v a l o f p u r e i r o n i n 4 M N a C l O a
Current efficiency during the electrochemical machining of iron and nickel
437
140
[20 -- o o~
/
[q •
~
_
~
~,-
I00
.u
~
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L-/=
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IO
'
20
;
30
Current density,
I
40
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50
A/cm 2
FIG. 1. Current efficiencydeterminations for the anodic removal of 99.99 % pure iron (open symbols) and of 99.9 + Yopure nickel (filled symbols) in 4M NaCIOs solution as a function of current density and solution flow rate; 500 (<3), 1000 ( A ), 2000 ( A ), and 3000 cm/s (ra). Broken lines show behavior at very low current densities based on data reported in the literature)." is virtually 100% and virtually independent of flow rate and current density. Values higher than 100% may be accounted for by minute grains of metal dropping out o f the electrode surface. These tiny grains which may be aggregates of only a 100 atoms or less can react chemically with the electrolyte so that unreacted metal is not found in the sludge in agreement with the observations of Datta and Landolt. 6 Because of the small size of these grains, the metal surface of the pure iron anode remained bright and smooth. Above 40 A / c m , the number of grains lost to chemical oxidation increases greatly as noted by the upswing of the curves at the highest current densities. At low flow rates the corrosion product is not removed from the reaction zone rapidly enough which may permit a local cell corrosion of the metal similar to crevice corrosion attack. Such behavior would result in the high current efficiency values at 500 em/s as seen in Fig. 1. In contrast, the current efficiency for the removal of pure Ni is highly dependent on both the flow rate and the current density. The current etficieney, in general, increases with current density but decreases with flow rate up to 2000 em/s. At 3000 em/s the current efficiency rises once more. The dotted lines in Fig. 1 show the behavior at low current densities as reported in the l i t e r a t u r e ) , " DISCUSSION Although the chief constituent of the conducting film formed on Ni anodes x4 is [3-NiOOH (hydrated Ni~Os), the film may be considered, depending on the potential, to be a mixture of Ni(OH)~ and I~-NiOOH~5 or a mixture of [~-NiOOH and NiO~. =6
438
JAMESP. HOAREand CHARLESR. WIESE
Since the Ni(III) and Ni(IV) oxides are formed anodically from the poorly conducting Ni(OH)s according to equations (I) and (2), Ni(OH)2 ~ 13-NiOOH + H + + e
(1)
{3-NiOOH -+ NiO2 + H + + e,
(2)
and
the composition of the film should be sensitive to the pH of the environment in which the anode finds itself. Consider, now, the data in Fig. 1 for Ni. At low current densities, a major portion of the current is consumed in the evolution of 02, 2H20 "+ 02 + 4H + + 4e,
(3)
because the potential is high enough so that the anodic film is highly conducting (containing only Ni(IIl) and Ni(IV) ions in the lattice). In tiffs region even at the lowest flow rate studied (500 cm/s), the flow rate is large enough to remove the H+ ions generated according to equation (3) so that significant changes in pH are not realized in the vicinity of the anode. Consequently, the current efficiency is virtually independent of the electrolyte flow rate between 500 and 2000 cm/s in the current density range from 5 to 20 A/cm 2. In the region of high current densities ( ~ 40 A/cm2), the vicinity of the anode becomes very acid due to the reaction described by equation (3). Either equations (1 and 2) are shifted to the left by this acidic environment causing a reduction in film conductivity because of an increase in the Ni(OH)z concentration in the film, or the film is removed chemically, such as, 2[3NiOOH -4- Ni + 6H + --* 3Ni +2 + 4HzO
(4)
exposing the underlying Ni to attack. In either case, the current efficiency for metal removal is increased with increasing current density in agreement with the data of Fig. 1. However, in this region, the H + ions are more efficiently removed from the neighborhood of the anode with increasing flow of electrolyte. As a result, the conducting film present at low current densities is less modified in the high current density range at the higher flow rate. According to this viewpoint, the current efficiency for metal removal should fall with increasing rate of solution flow in accord with the data of Fig. 1 in the range of solution flow between 500 and 200 cm/s. This behavior of the nickel oxides is responsible for the sigmoid curves found on Ni anode, Fig. 1, at flow rates between 500 and 2000 cm/s. Landolt9 also observed this behavior. At the highest flow rate studied, 3000 cm/s, a new mechanism has taken over which manifests itself as a change from the sigmoid behavior to a linear increase of the current efficiency for metal removal in the current density range from 5 to 40 A/cm 2. In general, the current efficiency is higher at 3000 cm/s than at lower flow rates. The curve at 2000 cm/s (inverted triangles in Fig. 1) between 40 and
Current efficiency during the electrochemical machining of iron and nickel
439
50 A/cm 2 appears to display a transition from the sigmoid to the linear behavior. At 3000 cm/s, such extreme hydrodynamic conditions cause chunks of the anodic film to be mechanically removed from the anode surface, thus exposing the underlying metal to attack. Undercutting of the film at these exposed sites will cause more chunks of film to flake off. With increasing current density, this undercutting effect would worsen causing an increase in the current efficiency for metal removal. The complex combination of events occurring at the anode surface produces, fortuitously, a linear increase in current efficiency with current density for this cell geometry. In the case of iron, the thick anodic film is removed by an ion exchange mechanism 17 at low current densities before any p H effects can become important. Since the thin uniformporous oxide film formed on iron in NaCIOa solutions at high current densities is extremely tenacious, little or no effect of current efficiency with flow rate is found. Consideration of the cell geometry in an ECM fixture turns out to be an important factor in studies designed to compare machining rates and current efficiencies of various metals or alloys in various electrolytes. When the ECM properties of steel were compared with those of Ni in various electrolytes during the machining in the flow cell described in Ref. 22, in the tube-end ECM test rig described in Ref. 13, and in the plunge-cut machine described in Ref. 18, somewhat different results were obtained. It is, therefore, important to describe in some detail the type of test rig on which the ECM measurements were carried out and only those data obtained from the same machining operation can be compared rigorously. These data, we believe, demonstrate undoubtedly that the ECM behavior (e.g. metal removal rate, current efficiency for metal removal) of a given metal in a given electrolyte may be directly related to the nature and properties of the anodic film formed on the metal surface in that electrolyte. Since, apparently, a universal electrolyte for the machining of all metals does not exist, it is necessary to understand the properties of the anodic films formed on a given metal so that a good E C M electrolyte can be tailor-made for the machining of that metal. Powerful tools for obtaining this understanding include the data recorded from potentiostatic polarization curves, constant current film stripping studies and mass balance or current efficiency determinations.
Acknowledgements--The authors are pleased to express their gratitude to Prof. Ernest Yeager of Case-Western Reserve University for his informative discussion and to Dr. Roger L. Saur of the Electrochemistry Department of General Motors Research Laboratories for making the electrode area measurements. They are also indebted to S. M. Dobrash of the Electrochemistry Department of General Motors Research Laboratories for making the electroformed ingots. REFERENCES J. P. HOARE, Nature, Loml. 219, 1034 (1968). J. P. HOARE, J. electrochem. Soc. 117, 142 (1970). K. CHIKAMORI and S. ITo, Denki Kagaku Oyobi Butsuri Kagaku 37, 602 (1969). D. LANDOLT, R. M. MULLER and C. W. ToaxAS, J. electrochem. Soc. U 8 , 36 (1971). A. D. DAVYDOV, R. A. MIRZOEV, V. D. KASHCrlEEVand B. N. KABANOV, Elektrok#nya 8, 1468 (1972). 6. ]~. DATTA and D. LANDOLT, Corros. Sei. 13, 187 (1973). 7. J. P. HOARE, M. A. LABODA, M. L. McMILLAN and A. J. WALLACE, J. electrochem. Soc. 116, 199 0969).
I. 2. 3. 4. 5.
440
JAMES P. H O A ~ and CI-IAgLESR. WIESE
8. M. A. LABODAand J. P. HOA~, Coll. Czech. Chem. Comm. 36, 680 (1970). 9. D. LAlqDOLT,J. electrochem. Soc. 119, 708 (1972). 10. D-T. ~ and A. J. WALLACE,J. electrochem. Soc. 118, 831 (1971). 11. P. A. BROOKand Q. IQnAL, J. electrochem. Soc. 116, 1458 (1969). 12. D-T. C3-mqand A. J. WALLACE,J. electrochem. Soc. 120, 1487 (1973). 13. J. P. HOARE, K.-W. MAO and A. J. WALLACE, Corrosion 27, 211 (1971). 14. M. NOVAK, A. K. N. REDDY and H. WROnUAWA,J. electrochem. Soc. 117, 733 (1970). 15. K.-W. MAo and D.-T. CHIN, J. electrochem. Soc. 121, 191 (1974). 16. M. L. McMILLAN, unpublished reults. 17. K.-W. MAO, IV[. A. LABODAand J. P. HOARE,J. electrochem. Soc. 119, 419 (1972). 18. J. P. HOARE, A. J. CHAgTRAhq)and M. A. LABODA,J. electrochem. Soc. 120, 1071 (1973). 19. P. J. BOWDENand J. M. EvANs, Nature 222, 377 (1969); J. electroclwm. Soc. 116, 1715 (1969). 20. P. J. BOWDENand J. M. EvANs, Electrochim. Acta 16, 1071 (1971). 21. J. P. HOARE, M. A. LABODAand A. J. WALLACE,./'.electrochem. Soc. 116, 1715 (1969). 22. J. P. HO~tE and K.-W. MAO, J. electrochem. Soc. 120, 1452 (1973). 23. K.-W. MAo, J. electrochem. Soc. 120, 1056 (1973). 24. G. W. D. BRIOGSand M. FLEISCHMANN,Trans. Faraday Soc. 62, 3217 (1966). 25. G. W. D. BRIOOSand W. F. WVNNE-JONES,Electrochim. Acta 7, 241 (1962). 26. D. E. DAVIESand W. BARKER, Corrosion 20, 47t 0964).
C U R R E N T EFFICIENCY D U R I N G THE ELECTROCHEMICAL M A C H I N I N G OF IRON A N D NICKEL* JAMES P. HOARE and CHARLES R. WIESE Electrochemistry Department, Research Laboratories, General Motors Corporation, Warren, Michigan, U.S.A. Abstract--Current efficiency determinations from weight-loss measurements were made on pure iron and pure nickel anodes in 4M NaCIO3 solution in a flow cell at flow rates between 500 and 3000 cm/s in a current range from 5 to 50 A/cm~. The current efficiency for metal removal was virtually independent of current density and flow rate on iron anodes. On nickel anodes the current efficiency increased strongly with current density. In the high current density region, the current efficiency decreases with flow rate up to 2000 cm/s and then increases with higher flow rates. This behavior was accounted for by differences in the nature and properties of the anodic films formed on iron and nickel anodes. INTRODUCTION FROM various polarization studies, it has been concluded 1-e that the electrochemical machining (ECM) process takes place in the transpassive region and that good dimensional control is obtained 2.~-9 in electrolytes in which a potential-dependent, protective film is formed on the anode surface. Recently, the use of the term, "throwing power", borrowed from the plating industry, has been suggested 9-12 as a possible means for determining the ECM properties of a given metal-electrolyte system. This, of course, is possible because the underlying principle which governs the throwing power of an ECM electrolyte for the machining of a given metal is the nature and properties of the potential-dependent film formed on the surface of that metal anode in the given electrolyte. As noted by Landolt, 9 the throwing power of various electrolytes is due to the different dependency of metal dissolution on current density in the given electrolyte. Since the current density is a function of the applied potential which controls the nature and properties of the anodic film formed on the metal surface, the underlying factors which determine this dependency of the dissolution efficiency on current density are the nature and properties of this anodie film. In the transpassive region, electropolishing of the anode surface takes place if the anodic film is reduced to a thin, uniform, porous oxide layer *s about 50 ,~ thick. .4 This is the case for the machining of steel in NaCIOs solutions. *s Although Mao and Chin .5 have reported that the oxide film is completely removed from a steel anode in the transpassive region in NaCIOa solution because the film is not detected on scanning electron micrographs, the following points should be taken into consideration. First, steel pieces machined in NaCIOs solutions remain mirror bright for years during exposure to the atmosphere (surface protected by a thin uniform oxide film) whereas those machined in NaCl solutions (no oxide films present) begin to rust within a few days when exposed to atmospheric conditions. Secondly, a film o f ~/-FegOa was detected .8 on steel surfaces machined in NaCIOa from the results of vacuum fusion, *Manuscript received 6 November 1974. 435
436
JAMES P. HOARE and CHARLES R. WIESE
electron probe and electron diffraction analyses but not on steel surfaces machined in NaCI solutions. In the third place, a thin ( ~ 50 A) uniform oxide film cannot be detected on a SEM micrograph of the machined steel surface in NaCIO3 solution because of the high degree of surface smoothness. The hope that the ECM behavior of a given metal in a series of electrolytes should be the same for other metals in the same electrolytes has been expressed in the literature? Since the ECM behavior of a given metal in a given electrolyte depends on the nature and properties of the anodic film formed on that metal 17.]s and since the nature and properties of the anodic film are very likely to be different for various metals in the same electrolyte, this expressed hope most probably cannot be realized. In fact, there is some evidence in the literature that the ECM behavior of Ni 6,%]"02° in electrolytes such as solutions of NaCi, NaCIO.~, NaNOa is very different from that on Fe or steel ~.21,~2 in these electrolytes. It is the purpose of this report to describe some current efficiency measurements for metal removal of nickel and iron anodes in NaC103 solutions under the same ECM conditions in the same flow cells as a function of current density and electrolyte flow rate. The results show that the ECM behavior of Ni in NaC10.~ solutions is very different from that of Fe in these solutions. EXPERIMENTAL Anodes were made from electroformed iron (99.99~ pure) and nickel (99.9+ ~o pure) ingots which were machined on a lathe to rods about 13 cm long. These rods were potted in epoxy in a mold such that the outside diameter was 6'4 cm. The ends of the rods were polished on a polishing wheel and the diameter of the polished face, from which the'area was calculated, was determined with a microscope fitted with a Filar eyepiece. After the rod was washed with distilled water, rinsed with acetone and dried in a vacuum oven at l l5°C and 620 Torr for ~h, its weight was determined on an analytical balance. To get an accurate weight determination, the electrode was rebaked and reweighed until no further weight change was detected. This cross-sectional area of the iron anode was 0.121 cm ~ and that of the nickel anode 0.0733 cm 2. A brass rod 6.4 cm in diameter served as the cathode. The test anode and the brass cathode were mounted with the polished ends flush with the walls of the channel of a flow cell which is described in greater detail elsewhere. ~3 The rectangular flow channel 0.0508 cm between the walls containing the electrodes by 0.714 cm deep by 16.5 cm long was formed between two acrylic plates clamped together with toggle clamps and sealed with O-rings. A solution of 4 M NaCIOs was pumped through the cell by a stainless steel turbine pump and the flow rate was determined with a stainless steel rotameter. From a calibration chart, the flow rate was recorded as linear cm/s. Current efficiency determinations for the removal of iron and nickel were carried out at solution flow rates between 500 and 3000 cm/s and in the current range from 5 to 50 A/cm 2. During each run the temperature was maintained between 25 and 30°C. Since the surface of the anode recesses from the wall of the channel during the run, the current was allowed to flow so that the anode was recessed no deeper than 0"5 mm. At the end of each run the anode was washed with distilled water, rinsed in acetone, dried in the vacuum oven and reweighed. RESULTS Weight-loss measurements were recorded as the average of 5 independent runs ( r e p r o d u c i b l e w i t h i n ± 2.5 m g ) a n d t h e c u r r e n t efficiency f o r m e t a l r e m o v e d w a s c a l c u l a t e d o n t h e b a s i s o f 300 c o u l o m b s p a s s e d . C u r r e n t efficiency d a t a a r e p l o t t e d i n F i g . 1 as a f u n c t i o n o f c u r r e n t d e n s i t y f o r p u r e i r o n ( o p e n s y m b o l s ) a n d p u r e n i c k e l (filled s y m b o l s ) f o r t h e f o u r flow r a t e s (500, 1000, 2 0 0 0 a n d 300 c m / s ) i n t h e c u r r e n t d e n s i t y r a n g e f r o m 5 t o 50 A / c m ~. A b o v e 5 A / c m 2, t h e c u r r e n t efficiency f o r t h e r e m o v a l o f p u r e i r o n i n 4 M N a C l O a
Current efficiency during the electrochemical machining of iron and nickel
437
140
[20 -- o o~
/
[q •
~
_
~
~,-
I00
.u
~
1
40
0
L-/=
r
IO
'
20
;
30
Current density,
I
40
l
50
A/cm 2
FIG. 1. Current efficiencydeterminations for the anodic removal of 99.99 % pure iron (open symbols) and of 99.9 + Yopure nickel (filled symbols) in 4M NaCIOs solution as a function of current density and solution flow rate; 500 (<3), 1000 ( A ), 2000 ( A ), and 3000 cm/s (ra). Broken lines show behavior at very low current densities based on data reported in the literature)." is virtually 100% and virtually independent of flow rate and current density. Values higher than 100% may be accounted for by minute grains of metal dropping out o f the electrode surface. These tiny grains which may be aggregates of only a 100 atoms or less can react chemically with the electrolyte so that unreacted metal is not found in the sludge in agreement with the observations of Datta and Landolt. 6 Because of the small size of these grains, the metal surface of the pure iron anode remained bright and smooth. Above 40 A / c m , the number of grains lost to chemical oxidation increases greatly as noted by the upswing of the curves at the highest current densities. At low flow rates the corrosion product is not removed from the reaction zone rapidly enough which may permit a local cell corrosion of the metal similar to crevice corrosion attack. Such behavior would result in the high current efficiency values at 500 em/s as seen in Fig. 1. In contrast, the current efficiency for the removal of pure Ni is highly dependent on both the flow rate and the current density. The current etficieney, in general, increases with current density but decreases with flow rate up to 2000 em/s. At 3000 em/s the current efficiency rises once more. The dotted lines in Fig. 1 show the behavior at low current densities as reported in the l i t e r a t u r e ) , " DISCUSSION Although the chief constituent of the conducting film formed on Ni anodes x4 is [3-NiOOH (hydrated Ni~Os), the film may be considered, depending on the potential, to be a mixture of Ni(OH)~ and I~-NiOOH~5 or a mixture of [~-NiOOH and NiO~. =6
438
JAMESP. HOAREand CHARLESR. WIESE
Since the Ni(III) and Ni(IV) oxides are formed anodically from the poorly conducting Ni(OH)s according to equations (I) and (2), Ni(OH)2 ~ 13-NiOOH + H + + e
(1)
{3-NiOOH -+ NiO2 + H + + e,
(2)
and
the composition of the film should be sensitive to the pH of the environment in which the anode finds itself. Consider, now, the data in Fig. 1 for Ni. At low current densities, a major portion of the current is consumed in the evolution of 02, 2H20 "+ 02 + 4H + + 4e,
(3)
because the potential is high enough so that the anodic film is highly conducting (containing only Ni(IIl) and Ni(IV) ions in the lattice). In tiffs region even at the lowest flow rate studied (500 cm/s), the flow rate is large enough to remove the H+ ions generated according to equation (3) so that significant changes in pH are not realized in the vicinity of the anode. Consequently, the current efficiency is virtually independent of the electrolyte flow rate between 500 and 2000 cm/s in the current density range from 5 to 20 A/cm 2. In the region of high current densities ( ~ 40 A/cm2), the vicinity of the anode becomes very acid due to the reaction described by equation (3). Either equations (1 and 2) are shifted to the left by this acidic environment causing a reduction in film conductivity because of an increase in the Ni(OH)z concentration in the film, or the film is removed chemically, such as, 2[3NiOOH -4- Ni + 6H + --* 3Ni +2 + 4HzO
(4)
exposing the underlying Ni to attack. In either case, the current efficiency for metal removal is increased with increasing current density in agreement with the data of Fig. 1. However, in this region, the H + ions are more efficiently removed from the neighborhood of the anode with increasing flow of electrolyte. As a result, the conducting film present at low current densities is less modified in the high current density range at the higher flow rate. According to this viewpoint, the current efficiency for metal removal should fall with increasing rate of solution flow in accord with the data of Fig. 1 in the range of solution flow between 500 and 200 cm/s. This behavior of the nickel oxides is responsible for the sigmoid curves found on Ni anode, Fig. 1, at flow rates between 500 and 2000 cm/s. Landolt9 also observed this behavior. At the highest flow rate studied, 3000 cm/s, a new mechanism has taken over which manifests itself as a change from the sigmoid behavior to a linear increase of the current efficiency for metal removal in the current density range from 5 to 40 A/cm 2. In general, the current efficiency is higher at 3000 cm/s than at lower flow rates. The curve at 2000 cm/s (inverted triangles in Fig. 1) between 40 and
Current efficiency during the electrochemical machining of iron and nickel
439
50 A/cm 2 appears to display a transition from the sigmoid to the linear behavior. At 3000 cm/s, such extreme hydrodynamic conditions cause chunks of the anodic film to be mechanically removed from the anode surface, thus exposing the underlying metal to attack. Undercutting of the film at these exposed sites will cause more chunks of film to flake off. With increasing current density, this undercutting effect would worsen causing an increase in the current efficiency for metal removal. The complex combination of events occurring at the anode surface produces, fortuitously, a linear increase in current efficiency with current density for this cell geometry. In the case of iron, the thick anodic film is removed by an ion exchange mechanism 17 at low current densities before any p H effects can become important. Since the thin uniformporous oxide film formed on iron in NaCIOa solutions at high current densities is extremely tenacious, little or no effect of current efficiency with flow rate is found. Consideration of the cell geometry in an ECM fixture turns out to be an important factor in studies designed to compare machining rates and current efficiencies of various metals or alloys in various electrolytes. When the ECM properties of steel were compared with those of Ni in various electrolytes during the machining in the flow cell described in Ref. 22, in the tube-end ECM test rig described in Ref. 13, and in the plunge-cut machine described in Ref. 18, somewhat different results were obtained. It is, therefore, important to describe in some detail the type of test rig on which the ECM measurements were carried out and only those data obtained from the same machining operation can be compared rigorously. These data, we believe, demonstrate undoubtedly that the ECM behavior (e.g. metal removal rate, current efficiency for metal removal) of a given metal in a given electrolyte may be directly related to the nature and properties of the anodic film formed on the metal surface in that electrolyte. Since, apparently, a universal electrolyte for the machining of all metals does not exist, it is necessary to understand the properties of the anodic films formed on a given metal so that a good E C M electrolyte can be tailor-made for the machining of that metal. Powerful tools for obtaining this understanding include the data recorded from potentiostatic polarization curves, constant current film stripping studies and mass balance or current efficiency determinations.
Acknowledgements--The authors are pleased to express their gratitude to Prof. Ernest Yeager of Case-Western Reserve University for his informative discussion and to Dr. Roger L. Saur of the Electrochemistry Department of General Motors Research Laboratories for making the electrode area measurements. They are also indebted to S. M. Dobrash of the Electrochemistry Department of General Motors Research Laboratories for making the electroformed ingots. REFERENCES J. P. HOARE, Nature, Loml. 219, 1034 (1968). J. P. HOARE, J. electrochem. Soc. 117, 142 (1970). K. CHIKAMORI and S. ITo, Denki Kagaku Oyobi Butsuri Kagaku 37, 602 (1969). D. LANDOLT, R. M. MULLER and C. W. ToaxAS, J. electrochem. Soc. U 8 , 36 (1971). A. D. DAVYDOV, R. A. MIRZOEV, V. D. KASHCrlEEVand B. N. KABANOV, Elektrok#nya 8, 1468 (1972). 6. ]~. DATTA and D. LANDOLT, Corros. Sei. 13, 187 (1973). 7. J. P. HOARE, M. A. LABODA, M. L. McMILLAN and A. J. WALLACE, J. electrochem. Soc. 116, 199 0969).
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