Electropolishing of copper and copper-based alloys in ortho-phosphoric acid

Corrosion Science, 1972, Vol. 12, pp. 113 to 120. Pergamon Press. Printed in Great Britain

ELECTROPOLISHING OF COPPER AND COPPER-BASED ALLOYS IN ORTHO-PHOSPHORIC ACID* D. R. GABE Department of Metallurgy, The University, Sheffield Abstract--The behaviour of copper, a single-phase brass and a duplex aluminium bronze, during electropolishing in orthophosphoric acid has been examined in terms of polarization curves and the effects of temp. and acid concentration. A change in mechanism of electropolishing was apparent at about 37°C; below this temp. an activation energy of 5.9 kcal/mole was found while above it was 4.2 kcal/mole. The copper alloys were satisfactorily electropolished in similar solutions to those used for pure copper, but at high overpotentials the duplex bronze was subject to dealuminification of the second (y~) phase. R6sum& On a examin6 le comportement de cuivre, d'un laiton h une phase et d'un bronze ~t l'aluminium lors de polissage 61ectrochimique dans l'acide orthophosphorique. Les effets de la temp6rature et de la concentration en acide ont 6t6 examin6s au moyen de courbes de polarisation. Une modification du m6canisme de polissage apparait h 37°C; on a trouv6 une 6nergie d'activation de 5,9 kcal/mole en dessous de 37°C et de 4,2 kcal/mole au-dessus de 37°C. Les alliages cuivreux sont polis correctement darts des solutions semblables h celles utilis6es pour le cuivre pur, mais ~t des surtensions 61ev~s la seconde phase ('f~) du bronze d'aluminium s'est d6salumin6e. Zusammenfassung--DasVerhalten yon Kupfer, einem Einphasenmessing und einer Duplexaluminiumbronze wahrend des Elektropolierens in Orthophosphorsiiure wurde zwecks Gewinnung yon Polarisationskurven und der Feststellung der Einfliisse yon Temperatur und Siiurekonzentration untersucht. Bei etwa 37°C trat eine ~nderung des Mechanismus des Elektropolierens in Erscheinung; unterhalb dieser Temperatur wurde eine Aktivierungsenergie yon 5,9 kcal/Mol gefunden, wiihrend sie dartiber 4,2 kcal/Mol betrug. Die Kupferlegierungen liessen sich in fihnlichen L6sungen wie den fiir reines Kupfer verwendeten zufriedenstellend elektropolieren, aber bei hohen Obersparmungen war die Duplexbronze der Entaluminisierungder zweiten (Y2) Phase unterworfen. INTRODUCTION ORTHO-VHOSVHORICacid is widely used for the electropolishing of copper a n d c o p p e r based alloys 1,2 and since its original development 3,4 the polarization characteristics have received considerable attention. 5-10 A d d i t i o n a l factors which have been examined in recent years include the effects of solution flow n-13 a n d heat transfer 1. although little has been reported o n the effects of alloying elements. The m e c h a n i s m o f electropolishing has been of considerable interest, owing to the peculiar properties of phosphoric acid in n o t only p r o m o t i n g passivity a n d thereby film-controlled dissolution, b u t also p r o m o t i n g solution diffusion-control in view of its viscosity. These factors have all received consideration in the development of a theory for electropolishing.a, 16-21 Nevertheless, some discrepancies in the polarization data exist a n d a n attempt has been made to c o m p a r e these results a n d show how they relate to data for some c o p p e r - b a s e d alloys. The basis o f the investigation has been the r e c o m m e n d a t i o n that o p t i m u m metallographic electropolishing m a y be achieved in solutions c o n t a i n i n g 60-70% v/v ortho-phosphoric acid (8.7-10M) at temp. within the range 15-25°C 1,~ *Manuscript received 5 April 1971 ; in revised form 21 July 1971. 113

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and the assertion that brightening only occurs for concentrations above about 36% (5.3M). 5 EXPERIMENTAL

Samples of O.F.H.C. copper strip, cold-rolled and annealed, were degreased and pickled in 50% nitric acid prior to electropolishing in ortho-phosphoric acid solutions. The basic solution used was 67% v/v (10M) although the effect of concentration was examined at 24°C and the effect of temp. examined over the range 4-75°C. Potentiodynamic polarization measurements were made with an Amel potentiostat using a scanning speed of 133 mV/min and a copper reference electrode. The anode was held vertically such that convection currents could develop naturally without significant restriction and the cathode was a cylindrical sheet surrounding anode and again vertical. The effect of stirring was examined by using a small electromagnetic stirrer at the bottom of the cell which was rotated quite slowly (e.g. 30-60 rev/min). In addition to the "pure" copper mentioned above, two other alloys were examined - - a single-phase 71 Cu-29 Zn u-brass and a duplex 9 AI-91 Cu aluminium bronze --and their electropolishing characteristics examined. Both alloys had been rolled and annealed, the latter having a relatively large grain size with the second (72) phase present essentially as a massive grain boundary precipitate. RESULTS

Effects of temp. variation Potentiodynamic polarization curves were measured for a range of temps. (4-75°C under conditions of no specific agitation, although it was clear that at the higher temps, some thermal convection was developing. The critical electropolishing or bdghteuing potential was apparent at about 250 mV over-potential for the lower temps, rising to almost 400 mV at 75°C (Fig. 1). The polishing current, i,o~, increased i

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Electropolishing of copper and copper-based alloys

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Effects of acid concentration A decrease in concentration of ortho-phosphoric acid had little effect on the value of Ecrit but the polishing c.d. increased markedly (Fig. 3). If the values of ipoI are plotted against acid concentration (Fig. 4) it may be seen that a maximum occurs at about 20% (3M), polishing only being satisfactory on the high-concentration side of this peak. If other data is included it may be seen that the general shape and maximum position, which could be affected by the experimental procedure and materials, show some discrepancy. The line shift of Fig. 2 can now be clearly placed in context. Acid concentration, % V/V 5 I0 20 30 44950 70 /-% 180

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Effects of alloy composition Polarization curves in orthophosphoric at 23°C were obtained for the two copper alloys at three concentrations (Fig. 5) and are directly comparable with the data of Fig. 3. The curves are clearly very similar in the case of 70/30 brass although it may be noticed that the breakdown of polishing "passivity" takes place at about 150 mV lower-potential when compared with copper. In the case of the duplex aluminium bronze a two-stage polishing "plateau" is apparent, each stage clearly associated with one of the phases. With increasing temp. (Fig. 6), the secondary polishing region becomes less significant but may still be detected at 50°C. At high over-potentials (Ets > 1800 mV) the aluminium bronze is subject to dealuminification which can be associated with the second phase (72), being preferentially dissolved yielding, after a length of time, a porous copper-like appearance. Potentiodynamic cycling reveals that irreversibility occurs only when over-potentials above about 1500 mV are exceeded (Fig. 7). Good metallographic polishes are achieved for both alloys at over-potentials of 400-700 mV and temps, below about 40°C, and the black-film characteristics are similar to those observed for pure copper.

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FIG. 7. Anodic potentiodynamic curves for the 9 AI-91 Cu alloy in 67% orthophosphoric acid at 25°C. DISCUSSION Electropolishing is usually attributed to the formation of a surface film or a diffusion layer at the electrode surface such that dissolution is a "randomized" process and preferential etching is suppressed. Clearly the form of the film is important otherwise true passivation may take place--in Fig. 4 concentrations below 2M cause low c.ds. but this is due to passive phosphating films and not electropolishing films. Electropolishing continues to take place on alloys as well as pure metals in essentially the same manner, although if a second phase appears, having its own well-defined electrochemical characteristics, preferential dissolution may take place within an apparently polishing film condition. The difficulties of examining the role of either surface films or diffusion layers in the mechanism of electropolishing are obvious although impedance measurements s or ellipsometry22 have been used. However, if activation energies are determined for the process over a range of temp. some indications of mechanism may be obtained. Assuming an Arrhenius-type relation* the data of Fig. 2 has been replotted as log i against 1/T (Fig. 8) and it may be seen that two lines result with a transition at about 37°C. The data of Honeycombe and Hughan 5 is insufficient to establish a transition temp. although a change in slope may be perceived. However, data of Hoar and Rothwel112 can be compared; by taking relatively few values around the transition temp. they obtained a spurious activation energy value, although with hindsight, the change in slope may be discerned. From the data presented here values of the activation energy may be found to be 5.9 kcal/mole below 37°C and 4.2 kcal/mole above 37°C compared with the earlier values x2 of 3.4-3-6 kcal/mole. These two values, together with the appearance of a transition, seem to be distinctive, although it is not possible to be dogmatic about possible mechanisms in view of the similarity of the values. At low temps, the viscosity of the ortho-phosphoric acid solutions is readily apparent and electropolishing at lower over-potentials is *ipoj = A exp (-- Q/RT) where Q is the activation energy and T the absolute temp.

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accompanied by a black film (see Refs. 9, 12); at higher temps, the viscosity, is less apparent, thermal convection currents may be observed and the black film stage tends to be less important. Circumstantial evidence, therefore, suggests that at lower temps, electropolishing may be diffusion-layer controlled while at higher temps, it may be surface-film controlled. (It may be noted that data for the single-phase brass fits the same pattern of behaviour). When the behaviour of the aluminium-bronze is compared to that for copper it is clear that the breakdown of the "secondary plateau", which might be expected to be due to copper dissolution, is in fact due to preferential dissolution of the second phase 3'2 leading to dealuminification. The presence of about 8 % aluminium in the primary solid solution (~) therefore appears to have ennobled the matrix phase sufficiently to avoid dissolution at these potentials (En ~ 1850 mV). Rowlands 2a has shown that the Y, phase is markedly anodic with respect to et in sea-water (a difference in corrosion potential of about 300 mV) and practical experience indicates that corrosive attack of this type will occur in acid solutions. 24 An adequate metallographic electropolish is nevertheless achieved at conditions recommended for copper but clearly the low potential range, associated with black film polishing, is preferable otherwise selective attack of the second phase may take place. Acknowledgement--The author is indebted to Professor G. W. Greenwood for making laboratory

facilities available and to various colleagues for helpful discussion. REFERENCES 1. P. A. JACQUET,Met. Rev. 1, 157 (1956). 2. W. J. M. TEGART,Electrolytic Polishing of Metals and Alloys, 2nd Ed. Pergamon (1959). 3. P. A. JACQ~T, C.R. Acad. Sci. 201, 1473 (1935). 4. P. A. JACQUET,Trans. electrochem. Sac. 69, 629 (1936).

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R. W. K. HONEYCOMBEand R. R. HUGHAN, J. Counc. Sci. Ind. Res. Aust. 20, 297 (1947). A. I-I.tcKtaNG and J. K. H[GGINS, Trans. Inst. Met. Fin. 29, 274 0953). K. F. LORKrNG, J. electrochem. Soc. 102, 479 (1955). M. COLE and T. P. HOAR, Proc. 8th CITCE 1956. Butterworths 0958). K. F. LORKING, Electrochim. Acta 7, 101 0962). F. H. GILES and J. H. BARTLETT,J. electrochem. Soc. 108, 266 (1961). H. F. WALTON,J. electrochem. Soc. 97, 2,19 (1950). T. P. HOAR and G. P. ROTHWELL, Electrochim. Acta 9, 135 0964). E. S. VARENKO et al., Prof. Met. 6, 106 (1970). D. T. PORTER, M. DONIMIRSKA and R. WALL, Corros. Sci. 8, 833 (1968). T. P. HOAR and J. A. S. MOWAT, Nature 165, 64 (1950). T. P. HOAR and T. W. FARTHING, Nattire 169, 324 0952). I. EDWARDS, 3". electrochem. Soc. 100, 189C, 223C (1953). C. WAGNER,J. electrochem. Soc. 101,225 (1954). E. C. WILLIAMS and M. A. BARRETT, J. electrochem. Soc. 103, 363 (1956). T. P. HOAR, Modern Aspects t~f Electrochemistry (Ed. BOCKmS) 2, 262. Buttcrworths, London (1959). 21a. T. P. HoAR, D. C. MEARS and G. P. ROTHWELL, Corro~. Sci. 5, 279 (1965). 21b. T. P. HOAR, Corros. Sci. 7, 341 (1967). 22. M. NOVAK, A. K. N. REDDY and H. WROBLOWA,J. electrochem. Soc. 117, 733 (1970). 23. J. C. ROWLANDS, Corros. Sci. 2, 89 (1962). 24. "Aluminium Bronze", Copper Devel. Assoc. Pub. 31.