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Effect of lead and oxygen on the passivation ofcopper anodes
\
Canadian Metallur`ical Quarterly\ Vol[ 27\ No[ 0\ pp[ 12Ð21\ 0888 Þ 0888 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd Printed in Great Britain[ All rights reserved 9997Ð3322:88:, ! see front matter
Pergamon
PII ] S9997Ð3322"87#99918Ð8
EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES H[ GAUTHIER\ M[ MANZINI and E[ GHALI Department of Mining and Metallurgy\ Laval University\ Quebec\ G0K 6P3\ Canada "Received 15 May 0887# Abstract*The e}ect of lead and oxygen upon the passivation of copper anodes during electrore_ning was investigated in sulfuric acid at 54[929[1>C by electrochemical methods and by surface analysis[ CuO was present in the copper matrix[ It was generated through oxidation of Cu1O present during heat treatment[ It was found that CuO accelerates the dissolution of the anode[ The voltammetric technique showed that a higher quantity of lead increases the passivation of copper[ When both lead and oxygen were present\ less passivation was observed[ Voltammetric reduction of the passive layer showed the presence of CuO\ Cu1O and lead compounds[ Surface analysis con_rmed the presence of CuO and Cu1O in the passive layer[ Finally\ the impedance results have demonstrated that a three step mechanism with two adsorbed species can be employed to explain this passivation reaction[ Þ 0888 Canadian Institute of Mining and Metallurgy Published by Elsevier Science Ltd[ All rights reserved[ Resume*L|e}et du plomb et de l|oxygene sur la passivation des anodes de cuivre lors de l|electrora.nage en milieu d|acide sulfurique a une temperature de 5429[1>C a ete etudie a l|aide de methodes electro! chimiques et d|analyses de surface[ Le CuO etait present dans la matrice de cuivre[ Il a ete genere par l|oxidation du Cu1O present lors du traitement thermique[ On a observe que le CuO accelere la dissolution de l|anode[ Les techniques voltamperometriques ont montre que plus la quantite de plomb est elevee\ plus la passivation du cuivre augmente[ Lorsque le plomb et l|oxygene sont tous les deux presents\ la passivation observee est moins importante[ La reduction voltamperometrique de la couche passive a demontre la presence de CuO\ de Cu1O et de composes de plomb[ L|analyse de surface a con_rme la presence de CuO et de Cu1O dans la couche passive[ Finalement\ les resultats d|impedance ont demontre qu|un mecanisme a trois etapes ayant deux especes adsorbees peut e¼tre utilise pour expliquer la reaction de passivation[ Þ 0888 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd[ All rights reserved[
0[ INTRODUCTION
1[ EXPERIMENTAL CONDITIONS 1[0[ Solutions
The passivation of copper anodes is a problem often en! countered in the re_ning industry[ This phenomenon is a}ected by the electrolysis conditions and by the impurities present in the anodes[ The consequences of the copper passivation are the lower current densities used in the process and the lower quality of re_ned copper obtained[ Several studies regarding the mechanism of copper pas! sivation have been done in the past ð0Ð4Ł[ Some authors ð4Ł consider that the precipitation of non!conductive solid copper sulfate on the anode surface is the cause of passivation[ Others ð5Ł show that passivation is due to the formation of a cuprous oxide _lm[ Since the impurities present in the anode have a detrimental e}ect on the electrore_ning process\ studies have been done ð6Ł to determine a critical limit at which the pas! sivation is accelerated[ The purpose of the present study is to elucidate the behavior of lead and lead!oxygen copper alloys in the passivation[ Several electrochemical techniques and surface analyses have been used to get a better understanding of the anodic passivation of copper ð7Ł[
All the chemical reagents used during this investigation were Fischer ACS grade[ Deionized water was used to prepare the solutions[ The solution was composed of 0[55 M H1SO3¦9[55 M CuSO3=4H1O¦9[18 M NiSO3=5H1O\ very close to the elec! trolyte used in the industry[ This solution was used for most of the electrochemical tests and for the preparation of the anodes for the surface analysis[ For the passive layer electroreduction a solution of 0[16 M Na1SO3\ pH 2 "adjusted with H1SO3# was used[ This solution was employed to avoid the reduction of Cu1¦ ions present in the industrial solution[ 1[1[ Electrodes and cell The synthetic electrodes used in this work were prepared from pure electrolytic copper "Aldrich#[ Small pieces of copper were cut and placed in a graphite crucible which was sub! sequently placed in a vacuum stock induction furnace[ The amount of oxygen was controlled by a surperpressure of dry argon[ The temperature was increased to 0199>C and then lead wrapped in a pure copper foil was added[ The mixture of copper and lead was kept at 0199>C for a few min[ The molten metal
To whom all correspondence should be addressed[ Tel[] 990 307 545 6518^ fax] 990 307 545 4232^ e!mail] edward[ghaliÝgmn[ulaval[ca 12
13
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES Table 0[ Lead and oxygen quantities in the studied anodes
Alloys CuÐPb 3999 CuÐPb 1999 CuÐPb 499 CuÐPbÐO
Lead "ppm#
Oxygen "ppm#
2299 0499 499 2799
8 02 84 0999
collected in the normal direction to the sample[ The software PIXAS ð8Ł was used to analyze the spectra[
2[ RESULTS AND DISCUSSION 2[0[ First observations
was then poured into an iron mold[ In the anodes obtained\ the impurities are situated in the boundary grains[ In order to obtain the same structure\ the synthetic anodes were then heated to 899>C for 1 h and then swaged[ Table 0 gives the quantity of lead and oxygen for each alloy studied and the notation used in this work[ A three electrode cell was used during experiments[ The ref! erence electrode was a mercurous sulfate electrode "MSE#\ Hg\ Hg1SO3:sat[ K1SO3 "9[53 V vs NHE#^ all potentials quoted in this paper are given with reference to this electrode[ The auxili! ary electrode was a pure copper disk with a surface area of 1[72 cm1[ For the impedance experiments a foil of pure copper was used to encounter the electrode system[ Before each experiment\ the electrodes were polished using emery paper "from 139 to 0199 grit#\ cleaned with water and ethanol in order to remove polishing products[ A double!walled glass cell was used[ The solution temperature was kept constant at 54[929[1>C with a thermostat[
First of all\ the assumption is made that since lead is less noble than copper\ lead oxide will be formed _rst in the matrix and then\ if oxygen is present in large amounts\ copper oxide will appear ð09Ł[ Table 1 shows the compounds obtained for each alloy according to this assumption[ A Cu"NO2#1=2H1O 0 M solution was used to attack the anodes[ With this solution\ only metallic lead will be dissolved\ while the other compounds will stay in the matrix[ Figure 0 shows a CuÐPbÐO anode before and after etching[ Scanning electron microscopy "SEM# studies show that before etching the black spots are related to copper oxide and the white ones to lead compounds[ After etching\ the black particles are associated with dissolved lead\ the white ones with lead oxides and the gray ones with copper oxides[ These results con_rm that CuÐPb 3999 and CuÐPb 1999 anodes do not have any copper oxide in the matrix\ but they have metallic lead[ Lead oxide is present in all anodes[ These results can be used to partially con_rm the _rst assumption[ Copper oxide is de_nitely present in the CuÐPb 499 and CuÐPbÐO anodes which have an excess of oxygen[ The copper oxide present in these anodes is CuO since the anodes were heat treated at 899>C under an oxygen atmosphere ð00Ł[
1[2[ Instrumentation
2[1[ Voltammetric experiments
All electrochemical tests were carried out using an EG+G potentiostat:galvanostat model 162 coupled with an IBM com! puter[ The SOFTCORR software was used to acquire data[ For the impedance measurements a Schlumberger Solartron potentiostat model 0175 and a Schlumberger Solartron fre! quency analyzer model 0144 were used[ An IBM computer was coupled with these instruments and the ZPLOT software was used to obtain the data[ Scanning electron microscopy "SEM# analysis was performed using a JEOL model 739 equipped with an energy dispersive X! ray "EDX# analyzer[ Auger electron spectra were recorded on a VG Scienti_c Instrument model ESCALAB !MKII photo! electron spectrometer[ The non!monochromatized Mg in a dou! ble A0:Mg anode was used as the X!ray excitation source[ The anode voltage was 04 kV for an emission current of 19 mA[ The incidence angle was 34> and the Auger electrons were
Figure 1 shows the polarization curves recorded at a scan rate of 9[94 mVs−0 from the open circuit potential up to 0099 mV for pure copper and copper alloys in the acidic copper sulfate bath at 54[929[1>C without agitation[ These vol! tammograms exhibit four characteristic regions] the active dis! solution region\ the dissolution and precipitation region\ a current plateau passivation region and a transpassive region[ The results of voltammetric studies are shown in Table 2[ The anodes can be grouped two by two[ The _rst group is the anodes having metallic lead and lead oxide in the matrix "CuÐPb 3999 and CuÐPb 1999#[ The second group is the anodes having copper oxide in the matrix "CuÐPb 499 and CuÐPbÐO#[ Since the passivation current "ipass# is high\ it cannot be attri! buted to a real passivation[ In this case\ it is assigned to pseudo! passivation[ The ipass value can be related to the quality of the barrier layer which prevents the di}usion of Cu1¦ ions[
1
Area 9[510 cm [
Table 1[ Compounds present in the copper matrix ð09Ł Alloys
Lead "ppm#
Oxygen "ppm#
Metal[Pb "ppm#
O1 "Pb ox# "ppm#
O1 "Cu ox# "ppm#
PbO "ppm#
CuO "ppm#
CuÐPb 499 CuÐPb 1999 CuÐPb 3999 CuÐPbÐO
499 0499 2299 2799
84 02 8 0999
* 0221 2072 *
28 02 8 183
45 * * 695
428 070 015 3983
179 * * 2402
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
CuO¦H1SO3 : CuSO3¦H1O
14
"0#
In this case\ this layer facilitates the copper ions di}usion to the anode surface and the passivation decreases[ Epic values show that passivation is accelerated in presence of lead\ but not in presence of lead and oxygen in enough quantity to form CuO[ This behaviour is not observed for the pure copper[ The voltammetric reduction of the passive layer in Na1SO3 solution "pH 2# was done in order to get more information about the compounds present in this layer[ Figure 2 shows the experimental results obtained\ compared with the standard values of potential reduction[ Reduction of the surface anode without passivation was done to verify that the reduction peak obtained did not originate from the matrix[ For each alloy the _rst run showed a large peak[ The second run gave separated peaks and provides more information[ For pure copper and copper!lead alloys\ CuO and Cu1O reduction peaks were observed[ Part of the CuO peak can be attributed to CuSO3 reduction[ When reductions of the stan! dard compounds were carried out\ no peak was observed for the CuSO3 since it is a salt with a large resistance in this solution[ In this case\ this technique was worthless in distinguishing between these two compounds[ For the CuÐPbÐO anode only one reduction peak was observed and it can be attributed to CuO or CuSO3[ In the case of lead compounds peaks\ PbO and PbSO3 reduction peaks cannot be distinguished[ Two peaks were observed and can be attributed to the two kinds of lead oxide or a mixture of PbO and PbSO3[ In fact\ this behavior con_rms that the lead compounds are not stable ð01\ 02Ł[ All the metallic lead and lead oxide will be transformed to PbSO3 with time[ 2[2[ Surface analysis
Fig[ 0[ "a# CuÐPbÐO anode before etching "0999×#\ "b# CuÐPbÐO anode after etching "0999×#[ SEM photo[
Table 2[ Voltammetric parameters for copper anodes Alloys Pure Cu CuÐPb 3999 CuÐPb 1999 CuÐPb 499 CuÐPbÐO
Epic ðVŁ
ipass ðmA cm−1Ł
DE ðVŁ
−9[115 −9[175 −9[186 −9[146 −9[151
69 43 41 46 59
39[062 39[019 39[062 39[062 39[199
v 9[94 mVs−0\ standard conditions[
Comparison with pure copper shows that the presence of lead makes the barrier layer thicker due to the formation of PbSO3 ð01\ 02Ł[ When lead and oxygen are both present\ the formation of copper oxide makes the barrier layer more porous with the dissolution of CuO ð03Ł]
It was demonstrated that only the passive layer reduction of the anodes cannot be used to determine the compounds present after passivation[ Surface analysis has been carried out in order to elucidate this problem[ Figure 3 shows the SEM photo obtained for the passive layer in the CuÐPb 3999 and CuÐPbÐO anodes[ On both pictures\ the presence of a large amount of copper powder\ CuSO3 and PbSO3 can be seen[ The passive layer is mainly the same one for both anodes but some di}erences can be pointed out in the formation of PbSO3[ In Fig[ 3a the PbSO3 is situated in the boundary grain\ while in Fig[ 3b the PbSO3 seems to grow up from the matrix[ Figure 4 con_rms this behavior[ In this case\ the oxide layer which causes the passivation is not continuous at the anode surface[ There are other compounds present in this layer and lead oxide is probably dissociated from the matrix to form PbSO3 in the barrier layer[ Metallic lead will form PbSO3 in situ\ and the upward growth of PbSO3 can facilitate the passivation[ The same behavior was observed for CuSO3 ð04Ł[ The ratio of copper powder\ CuO and Cu1O present in the passive layer has been obtained by Auger peaks "CuL2M3\4M3\4#[ These results have been obtained by comparing the spectra of commercial powder standards[ Table 3 gives the results obtained[ The results shown in Table 3 were obtained by the addition of standard curves[ An example of this process is shown in the Fig[ 5 for the copper species "Fig[ 5a# and for lead species "Fig[ 5b#[ Copper sulfate was not considered[ The copper powder
15
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
Fig[ 1[ Anodic polarization of pure copper and copper alloy electrodes in an acidic sulfate bath 9[94 mV s−0 at 54[929[1>C without agitation[
Fig[ 2[ Experimental results obtained\ compared with the standards values of potential reduction for the studied compounds[
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
16
Fig[ 3[ SEM photo obtained for the passive layer in the "a# CuÐPb 3999\ a# PbSO3\ b# Cu matrix\ and "b# CuÐPbÐO\ a# PbSO3\ b# Cu crystals\ c# Cu powder[
Table 3[ Ratio of copper compounds obtained by Auger analysis Alloys
Cu powder ")#
Cu1O ")#
CuO ")#
CuÐPb 3999 CuÐPb 1999 CuÐPb 499 CuÐPbÐO
70 68 64 63
6 00 01 01
01 09 02 01
amount is slightly higher for CuÐPb 3999 and CuÐPb 1999 anodes which have only metallic lead present in the matrix[ No signi_cant change in the quantities of Cu1O and CuO were noticed[ This phenomenon indicates that the CuO present in the matrix does not in~uence the CuO present in the passive layer[ Cu1O and CuO were formed by electrochemical reactions[ The only lead compound detected by this technique was PbSO3[ All the spectra for lead were modi_ed to the energy value of
17
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
Fig[ 4[ SEM photo obtained for the passive layer in the "a# CuÐPb 3999 and "b# CuÐPbÐO[
aromatic carbon "174[9 and V# and the results were compared with the literature values ð05Ł[ Since this technique is an ex situ one\ the oxidation reaction can occur during manipulation[ This may explain the small di}erence observed in the CuO and Cu1O amounts[ 2[3[ ac Impedance The application of relaxation techniques and steady state sinusoidal modulation "ac impedance# methods can be used in
order to get more information on the copper passivation process[ This technique gives information about quantitative parameters like constant rate adsorption\ capacitances and con! ductivity in the interface electrode:solution[ An alternating potential is superimposed on a continuous potential at di}erent frequencies[ The double layer con! tributions are available at audio frequencies[ Faradaic e}ects are observed at lower frequencies[ In this study the range of frequencies is 094Ð09−0 Hz[
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
18
Fig[ 5[ "a# Auger spectra CuL2M3\4M3\4 "CuÐPbÐO# and "b# ESCA spectra for lead compounds "CuÐPbÐO#[
From the theory of copper dissolution and adsorption ð06Ł a model is proposed to illustrate the mechanism of copper passivation[ k0
Cu 0−u0−u1 Cu"I#ads u0
−0
k1
\ k
Cu"I#ads u0
Cu"II#ads
−1
Cu"II#ads u1
\ k
k2
\ k
−2
u1
¦e
¦e
Cu¦1 0−u0−u1
"1#
Only the reactions "1# and "2# depend on the current\ so] i F"v0¦v1#
"4#
where v0 and v1 are the rate reactions and F the Faraday constant[ The mechanism proposed above can be mathematically described as]
"2# y¼f A¦ "3#
In this model k means the constant rate\ u0\ u1 are the covering ratios of surface area[
jwB¦C jwD¦Ew1¦G
"5#
where y¼f is the faradaic admittance "0:Z# and A\ B\ C\ D\ E and G are the constants which describe the system[ j is the imaginary number and w the angular frequency[ This mechanism is a simple one since the _lm resistance is not considered and the contribution of the lead compounds is considered constant "uPb is constant#[
29
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
Fig[ 6[ Nyquist impedance diagrams of CuÐPb 3999 obtained at di}erent potentials] "a# 9 mV\ "b# 099 mV\ "c# 199 mV and "d# 299 mV[ Standard experimental conditions[
The experimental and approximated curves obtained using this model are shown in Figs[ 6Ð8[ The shapes of the curves indicate the presence of two adsorbed species ð07Ł[ For the copper passivation system some problems were found[ Each impedance measurement requires about 29 min and as mentioned above this system presents a high current passivation "pseudo!passivation#[ In this case\ this quantity of current may change the surface electrochemical properties\ even if an equilibrium state is obtained after 1 h[ Another problem was that at the lowest frequencies\ there is a lot of noise[ This phenomenon could not be avoided because it is probably caused by the _lm formation[ It can be seen in Figs[ 6Ð8 that the potentials of 9 and 099 mV correspond to the active!passive range\ while the passive range is observed for 199 and 299 mV[
3[ CONCLUSIONS It can be observed that thermal treatment accelerates the anode dissolution[ This phenomenon can be explained by the presence of CuO which comes from Cu1O oxidation reaction[ When Cu1O is in the matrix\ a copper powder layer is formed according to the following reaction] Cu1O¦1H¦:Cu1¦¦Cuo¦H1O
"6#
In this case the density of the barrier layer increases[ The voltametric results show that an increase of lead facili! tates the anode passivation\ but when lead and oxygen are both present in the anode the passivation decreases[ In the _rst case the barrier layer is thicker or more dense "presence of lead
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
20
Fig[ 7[ Nyquist impedance diagrams of CuÐPbÐO obtained at di}erent potentials] "a# 9 mV\ "b# 099 mV\ "c# 199 mV and "d# 299 mV[ Standard experimental conditions[
21
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
Impedance studies indicate the presence of two adsorbed species in the passive layer[ The mechanism proposed is a simple one since the _lm resistance is not considered and the con! tribution of the lead compounds is considered constant[ Acknowled`ements*The authors wish to express their thanks to Dr A[ Adnot and L[ Patry "Univ[ Laval# and Dr A[ Lasia "Univ[ de Sher! brooke# for helpful discussions[ The _nancial support of Natural Sci! ences and Engineering Research Council of Canada is gratefully acknowledged[
REFERENCES Fig[ 8[ Nyquist impedance diagram of CuÐPb 1999 obtained at 299 mV[ Standard experimental conditions[
sulfate# than in the second case[ It should be emphasized that CuO and not Cu1O was present in the matrix because of heat treatment which was performed in an oxygen atmosphere[ When the passive layer is reduced by voltammetric techniques the presence of CuO\ Cu1O and a mix of lead compounds were detected[ Lead oxide is not stable and PbSO3 is always found in the passive layer[ SEM results show that there is a di}erence between the PbSO3 formed when only metallic lead is present in the matrix and when lead oxide is present[ In the _rst case PbSO3 grows up in situ in the matrix and can form one part of the passive layer[ In the second case PbSO3 is formed in the barrier layer and is not detected in the passive layer[ ESCA:AUGER technique is the best way to identify the compounds which are present in the passive layer[ A large quantity of copper powder\ CuO and Cu1O was detected in all the studied anodes[ Of the lead species only PbSO3 was observed[
0[ Jin\ S[ and Ghali\ E[\ J[ Appl[ Electrochem[\ 0880\ 10\ 136[ 1[ Moreira\ A[ H[\ Benedetti\ A[V[\ Cabot\ P[L[ and Sumodjo\ P[T[A[\ Electrochim[ Acta\ 0882\ 27"6#\ 870[ 2[ Minotas\ J[ C[\ Djellab\ H[ and Ghali\ E[\ J[ Appl[ Electrochem[\ 0878\ 08\ 666[ 3[ Bangna\ Z[\ Shan`ai Metals\ 0870\ 1"2#\ 3[ 4[ Abe\ S[\ Burrows\ B[W[ and Ettel\ V[ A[\ Can Met Quart 0879\ 08\ 178[ 5[ Choku\ I[\ Min[ J[ Metall[ Inst[ Japan\ 0858\ 74"863#\ 312[ 6[ Abe\ S[ and Goto\ S[ J[\ Min[ Metall[ Inst[ Japan\ 0873\ 099\ 318[ 7[ Gauthier\ H[\ MSc[ thesis[ Universite Laval\ 0885[ 8[ Adnot\ A[\ Techniques d|analyse de surface[ Universite Laval\ 0883[ 09[ Gumowska\ W[ and Sedzimir\ J[\ Hydrometallur`y\ 0881\ 17\ 126[ 00[ Chen\ T[ T[ and Dutrizac\ J[ E[\ Can[ Met[ Quart[\ 0878\ 17\ 1[ 01[ Chen\ T[ T[ and Dutrizac\ J[ E[\ Scan[ Microscopy\ 0877\ 1\ 624[ 02[ Ho}mann\ J[ E[\ JOM\ 0889\ 27[ 03[ Abe\ S[ and Goto\ S[\ J[\ Min[ Metall[ Inst[ Japan\ First report\ 0879[ 04[ Shize\ J[\ Ph[D[ thesis[ Universite Laval\ 0883[ 05[ Hewitt\ R[W[ and Winograd\ N[ Surf[ Sci[\ 0867\ 67\ 0[ 06[ Epelboin\ I[\ Gabrielli\ C[\ Keddam\ M and Takenouti H[\ The study of passivation process by electrode impedance analysis[ Com! prehensive Treatise of Electrochemistry[ Plenum Press\ New York\ 0870[ 07[ Cao\ C[\ Electrochim[ Acta\ 0889\ 24"4#\ 726[
Canadian Metallur`ical Quarterly\ Vol[ 27\ No[ 0\ pp[ 12Ð21\ 0888 Þ 0888 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd Printed in Great Britain[ All rights reserved 9997Ð3322:88:, ! see front matter
Pergamon
PII ] S9997Ð3322"87#99918Ð8
EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES H[ GAUTHIER\ M[ MANZINI and E[ GHALI Department of Mining and Metallurgy\ Laval University\ Quebec\ G0K 6P3\ Canada "Received 15 May 0887# Abstract*The e}ect of lead and oxygen upon the passivation of copper anodes during electrore_ning was investigated in sulfuric acid at 54[929[1>C by electrochemical methods and by surface analysis[ CuO was present in the copper matrix[ It was generated through oxidation of Cu1O present during heat treatment[ It was found that CuO accelerates the dissolution of the anode[ The voltammetric technique showed that a higher quantity of lead increases the passivation of copper[ When both lead and oxygen were present\ less passivation was observed[ Voltammetric reduction of the passive layer showed the presence of CuO\ Cu1O and lead compounds[ Surface analysis con_rmed the presence of CuO and Cu1O in the passive layer[ Finally\ the impedance results have demonstrated that a three step mechanism with two adsorbed species can be employed to explain this passivation reaction[ Þ 0888 Canadian Institute of Mining and Metallurgy Published by Elsevier Science Ltd[ All rights reserved[ Resume*L|e}et du plomb et de l|oxygene sur la passivation des anodes de cuivre lors de l|electrora.nage en milieu d|acide sulfurique a une temperature de 5429[1>C a ete etudie a l|aide de methodes electro! chimiques et d|analyses de surface[ Le CuO etait present dans la matrice de cuivre[ Il a ete genere par l|oxidation du Cu1O present lors du traitement thermique[ On a observe que le CuO accelere la dissolution de l|anode[ Les techniques voltamperometriques ont montre que plus la quantite de plomb est elevee\ plus la passivation du cuivre augmente[ Lorsque le plomb et l|oxygene sont tous les deux presents\ la passivation observee est moins importante[ La reduction voltamperometrique de la couche passive a demontre la presence de CuO\ de Cu1O et de composes de plomb[ L|analyse de surface a con_rme la presence de CuO et de Cu1O dans la couche passive[ Finalement\ les resultats d|impedance ont demontre qu|un mecanisme a trois etapes ayant deux especes adsorbees peut e¼tre utilise pour expliquer la reaction de passivation[ Þ 0888 Canadian Institute of Mining and Metallurgy[ Published by Elsevier Science Ltd[ All rights reserved[
0[ INTRODUCTION
1[ EXPERIMENTAL CONDITIONS 1[0[ Solutions
The passivation of copper anodes is a problem often en! countered in the re_ning industry[ This phenomenon is a}ected by the electrolysis conditions and by the impurities present in the anodes[ The consequences of the copper passivation are the lower current densities used in the process and the lower quality of re_ned copper obtained[ Several studies regarding the mechanism of copper pas! sivation have been done in the past ð0Ð4Ł[ Some authors ð4Ł consider that the precipitation of non!conductive solid copper sulfate on the anode surface is the cause of passivation[ Others ð5Ł show that passivation is due to the formation of a cuprous oxide _lm[ Since the impurities present in the anode have a detrimental e}ect on the electrore_ning process\ studies have been done ð6Ł to determine a critical limit at which the pas! sivation is accelerated[ The purpose of the present study is to elucidate the behavior of lead and lead!oxygen copper alloys in the passivation[ Several electrochemical techniques and surface analyses have been used to get a better understanding of the anodic passivation of copper ð7Ł[
All the chemical reagents used during this investigation were Fischer ACS grade[ Deionized water was used to prepare the solutions[ The solution was composed of 0[55 M H1SO3¦9[55 M CuSO3=4H1O¦9[18 M NiSO3=5H1O\ very close to the elec! trolyte used in the industry[ This solution was used for most of the electrochemical tests and for the preparation of the anodes for the surface analysis[ For the passive layer electroreduction a solution of 0[16 M Na1SO3\ pH 2 "adjusted with H1SO3# was used[ This solution was employed to avoid the reduction of Cu1¦ ions present in the industrial solution[ 1[1[ Electrodes and cell The synthetic electrodes used in this work were prepared from pure electrolytic copper "Aldrich#[ Small pieces of copper were cut and placed in a graphite crucible which was sub! sequently placed in a vacuum stock induction furnace[ The amount of oxygen was controlled by a surperpressure of dry argon[ The temperature was increased to 0199>C and then lead wrapped in a pure copper foil was added[ The mixture of copper and lead was kept at 0199>C for a few min[ The molten metal
To whom all correspondence should be addressed[ Tel[] 990 307 545 6518^ fax] 990 307 545 4232^ e!mail] edward[ghaliÝgmn[ulaval[ca 12
13
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES Table 0[ Lead and oxygen quantities in the studied anodes
Alloys CuÐPb 3999 CuÐPb 1999 CuÐPb 499 CuÐPbÐO
Lead "ppm#
Oxygen "ppm#
2299 0499 499 2799
8 02 84 0999
collected in the normal direction to the sample[ The software PIXAS ð8Ł was used to analyze the spectra[
2[ RESULTS AND DISCUSSION 2[0[ First observations
was then poured into an iron mold[ In the anodes obtained\ the impurities are situated in the boundary grains[ In order to obtain the same structure\ the synthetic anodes were then heated to 899>C for 1 h and then swaged[ Table 0 gives the quantity of lead and oxygen for each alloy studied and the notation used in this work[ A three electrode cell was used during experiments[ The ref! erence electrode was a mercurous sulfate electrode "MSE#\ Hg\ Hg1SO3:sat[ K1SO3 "9[53 V vs NHE#^ all potentials quoted in this paper are given with reference to this electrode[ The auxili! ary electrode was a pure copper disk with a surface area of 1[72 cm1[ For the impedance experiments a foil of pure copper was used to encounter the electrode system[ Before each experiment\ the electrodes were polished using emery paper "from 139 to 0199 grit#\ cleaned with water and ethanol in order to remove polishing products[ A double!walled glass cell was used[ The solution temperature was kept constant at 54[929[1>C with a thermostat[
First of all\ the assumption is made that since lead is less noble than copper\ lead oxide will be formed _rst in the matrix and then\ if oxygen is present in large amounts\ copper oxide will appear ð09Ł[ Table 1 shows the compounds obtained for each alloy according to this assumption[ A Cu"NO2#1=2H1O 0 M solution was used to attack the anodes[ With this solution\ only metallic lead will be dissolved\ while the other compounds will stay in the matrix[ Figure 0 shows a CuÐPbÐO anode before and after etching[ Scanning electron microscopy "SEM# studies show that before etching the black spots are related to copper oxide and the white ones to lead compounds[ After etching\ the black particles are associated with dissolved lead\ the white ones with lead oxides and the gray ones with copper oxides[ These results con_rm that CuÐPb 3999 and CuÐPb 1999 anodes do not have any copper oxide in the matrix\ but they have metallic lead[ Lead oxide is present in all anodes[ These results can be used to partially con_rm the _rst assumption[ Copper oxide is de_nitely present in the CuÐPb 499 and CuÐPbÐO anodes which have an excess of oxygen[ The copper oxide present in these anodes is CuO since the anodes were heat treated at 899>C under an oxygen atmosphere ð00Ł[
1[2[ Instrumentation
2[1[ Voltammetric experiments
All electrochemical tests were carried out using an EG+G potentiostat:galvanostat model 162 coupled with an IBM com! puter[ The SOFTCORR software was used to acquire data[ For the impedance measurements a Schlumberger Solartron potentiostat model 0175 and a Schlumberger Solartron fre! quency analyzer model 0144 were used[ An IBM computer was coupled with these instruments and the ZPLOT software was used to obtain the data[ Scanning electron microscopy "SEM# analysis was performed using a JEOL model 739 equipped with an energy dispersive X! ray "EDX# analyzer[ Auger electron spectra were recorded on a VG Scienti_c Instrument model ESCALAB !MKII photo! electron spectrometer[ The non!monochromatized Mg in a dou! ble A0:Mg anode was used as the X!ray excitation source[ The anode voltage was 04 kV for an emission current of 19 mA[ The incidence angle was 34> and the Auger electrons were
Figure 1 shows the polarization curves recorded at a scan rate of 9[94 mVs−0 from the open circuit potential up to 0099 mV for pure copper and copper alloys in the acidic copper sulfate bath at 54[929[1>C without agitation[ These vol! tammograms exhibit four characteristic regions] the active dis! solution region\ the dissolution and precipitation region\ a current plateau passivation region and a transpassive region[ The results of voltammetric studies are shown in Table 2[ The anodes can be grouped two by two[ The _rst group is the anodes having metallic lead and lead oxide in the matrix "CuÐPb 3999 and CuÐPb 1999#[ The second group is the anodes having copper oxide in the matrix "CuÐPb 499 and CuÐPbÐO#[ Since the passivation current "ipass# is high\ it cannot be attri! buted to a real passivation[ In this case\ it is assigned to pseudo! passivation[ The ipass value can be related to the quality of the barrier layer which prevents the di}usion of Cu1¦ ions[
1
Area 9[510 cm [
Table 1[ Compounds present in the copper matrix ð09Ł Alloys
Lead "ppm#
Oxygen "ppm#
Metal[Pb "ppm#
O1 "Pb ox# "ppm#
O1 "Cu ox# "ppm#
PbO "ppm#
CuO "ppm#
CuÐPb 499 CuÐPb 1999 CuÐPb 3999 CuÐPbÐO
499 0499 2299 2799
84 02 8 0999
* 0221 2072 *
28 02 8 183
45 * * 695
428 070 015 3983
179 * * 2402
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
CuO¦H1SO3 : CuSO3¦H1O
14
"0#
In this case\ this layer facilitates the copper ions di}usion to the anode surface and the passivation decreases[ Epic values show that passivation is accelerated in presence of lead\ but not in presence of lead and oxygen in enough quantity to form CuO[ This behaviour is not observed for the pure copper[ The voltammetric reduction of the passive layer in Na1SO3 solution "pH 2# was done in order to get more information about the compounds present in this layer[ Figure 2 shows the experimental results obtained\ compared with the standard values of potential reduction[ Reduction of the surface anode without passivation was done to verify that the reduction peak obtained did not originate from the matrix[ For each alloy the _rst run showed a large peak[ The second run gave separated peaks and provides more information[ For pure copper and copper!lead alloys\ CuO and Cu1O reduction peaks were observed[ Part of the CuO peak can be attributed to CuSO3 reduction[ When reductions of the stan! dard compounds were carried out\ no peak was observed for the CuSO3 since it is a salt with a large resistance in this solution[ In this case\ this technique was worthless in distinguishing between these two compounds[ For the CuÐPbÐO anode only one reduction peak was observed and it can be attributed to CuO or CuSO3[ In the case of lead compounds peaks\ PbO and PbSO3 reduction peaks cannot be distinguished[ Two peaks were observed and can be attributed to the two kinds of lead oxide or a mixture of PbO and PbSO3[ In fact\ this behavior con_rms that the lead compounds are not stable ð01\ 02Ł[ All the metallic lead and lead oxide will be transformed to PbSO3 with time[ 2[2[ Surface analysis
Fig[ 0[ "a# CuÐPbÐO anode before etching "0999×#\ "b# CuÐPbÐO anode after etching "0999×#[ SEM photo[
Table 2[ Voltammetric parameters for copper anodes Alloys Pure Cu CuÐPb 3999 CuÐPb 1999 CuÐPb 499 CuÐPbÐO
Epic ðVŁ
ipass ðmA cm−1Ł
DE ðVŁ
−9[115 −9[175 −9[186 −9[146 −9[151
69 43 41 46 59
39[062 39[019 39[062 39[062 39[199
v 9[94 mVs−0\ standard conditions[
Comparison with pure copper shows that the presence of lead makes the barrier layer thicker due to the formation of PbSO3 ð01\ 02Ł[ When lead and oxygen are both present\ the formation of copper oxide makes the barrier layer more porous with the dissolution of CuO ð03Ł]
It was demonstrated that only the passive layer reduction of the anodes cannot be used to determine the compounds present after passivation[ Surface analysis has been carried out in order to elucidate this problem[ Figure 3 shows the SEM photo obtained for the passive layer in the CuÐPb 3999 and CuÐPbÐO anodes[ On both pictures\ the presence of a large amount of copper powder\ CuSO3 and PbSO3 can be seen[ The passive layer is mainly the same one for both anodes but some di}erences can be pointed out in the formation of PbSO3[ In Fig[ 3a the PbSO3 is situated in the boundary grain\ while in Fig[ 3b the PbSO3 seems to grow up from the matrix[ Figure 4 con_rms this behavior[ In this case\ the oxide layer which causes the passivation is not continuous at the anode surface[ There are other compounds present in this layer and lead oxide is probably dissociated from the matrix to form PbSO3 in the barrier layer[ Metallic lead will form PbSO3 in situ\ and the upward growth of PbSO3 can facilitate the passivation[ The same behavior was observed for CuSO3 ð04Ł[ The ratio of copper powder\ CuO and Cu1O present in the passive layer has been obtained by Auger peaks "CuL2M3\4M3\4#[ These results have been obtained by comparing the spectra of commercial powder standards[ Table 3 gives the results obtained[ The results shown in Table 3 were obtained by the addition of standard curves[ An example of this process is shown in the Fig[ 5 for the copper species "Fig[ 5a# and for lead species "Fig[ 5b#[ Copper sulfate was not considered[ The copper powder
15
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
Fig[ 1[ Anodic polarization of pure copper and copper alloy electrodes in an acidic sulfate bath 9[94 mV s−0 at 54[929[1>C without agitation[
Fig[ 2[ Experimental results obtained\ compared with the standards values of potential reduction for the studied compounds[
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
16
Fig[ 3[ SEM photo obtained for the passive layer in the "a# CuÐPb 3999\ a# PbSO3\ b# Cu matrix\ and "b# CuÐPbÐO\ a# PbSO3\ b# Cu crystals\ c# Cu powder[
Table 3[ Ratio of copper compounds obtained by Auger analysis Alloys
Cu powder ")#
Cu1O ")#
CuO ")#
CuÐPb 3999 CuÐPb 1999 CuÐPb 499 CuÐPbÐO
70 68 64 63
6 00 01 01
01 09 02 01
amount is slightly higher for CuÐPb 3999 and CuÐPb 1999 anodes which have only metallic lead present in the matrix[ No signi_cant change in the quantities of Cu1O and CuO were noticed[ This phenomenon indicates that the CuO present in the matrix does not in~uence the CuO present in the passive layer[ Cu1O and CuO were formed by electrochemical reactions[ The only lead compound detected by this technique was PbSO3[ All the spectra for lead were modi_ed to the energy value of
17
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
Fig[ 4[ SEM photo obtained for the passive layer in the "a# CuÐPb 3999 and "b# CuÐPbÐO[
aromatic carbon "174[9 and V# and the results were compared with the literature values ð05Ł[ Since this technique is an ex situ one\ the oxidation reaction can occur during manipulation[ This may explain the small di}erence observed in the CuO and Cu1O amounts[ 2[3[ ac Impedance The application of relaxation techniques and steady state sinusoidal modulation "ac impedance# methods can be used in
order to get more information on the copper passivation process[ This technique gives information about quantitative parameters like constant rate adsorption\ capacitances and con! ductivity in the interface electrode:solution[ An alternating potential is superimposed on a continuous potential at di}erent frequencies[ The double layer con! tributions are available at audio frequencies[ Faradaic e}ects are observed at lower frequencies[ In this study the range of frequencies is 094Ð09−0 Hz[
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
18
Fig[ 5[ "a# Auger spectra CuL2M3\4M3\4 "CuÐPbÐO# and "b# ESCA spectra for lead compounds "CuÐPbÐO#[
From the theory of copper dissolution and adsorption ð06Ł a model is proposed to illustrate the mechanism of copper passivation[ k0
Cu 0−u0−u1 Cu"I#ads u0
−0
k1
\ k
Cu"I#ads u0
Cu"II#ads
−1
Cu"II#ads u1
\ k
k2
\ k
−2
u1
¦e
¦e
Cu¦1 0−u0−u1
"1#
Only the reactions "1# and "2# depend on the current\ so] i F"v0¦v1#
"4#
where v0 and v1 are the rate reactions and F the Faraday constant[ The mechanism proposed above can be mathematically described as]
"2# y¼f A¦ "3#
In this model k means the constant rate\ u0\ u1 are the covering ratios of surface area[
jwB¦C jwD¦Ew1¦G
"5#
where y¼f is the faradaic admittance "0:Z# and A\ B\ C\ D\ E and G are the constants which describe the system[ j is the imaginary number and w the angular frequency[ This mechanism is a simple one since the _lm resistance is not considered and the contribution of the lead compounds is considered constant "uPb is constant#[
29
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
Fig[ 6[ Nyquist impedance diagrams of CuÐPb 3999 obtained at di}erent potentials] "a# 9 mV\ "b# 099 mV\ "c# 199 mV and "d# 299 mV[ Standard experimental conditions[
The experimental and approximated curves obtained using this model are shown in Figs[ 6Ð8[ The shapes of the curves indicate the presence of two adsorbed species ð07Ł[ For the copper passivation system some problems were found[ Each impedance measurement requires about 29 min and as mentioned above this system presents a high current passivation "pseudo!passivation#[ In this case\ this quantity of current may change the surface electrochemical properties\ even if an equilibrium state is obtained after 1 h[ Another problem was that at the lowest frequencies\ there is a lot of noise[ This phenomenon could not be avoided because it is probably caused by the _lm formation[ It can be seen in Figs[ 6Ð8 that the potentials of 9 and 099 mV correspond to the active!passive range\ while the passive range is observed for 199 and 299 mV[
3[ CONCLUSIONS It can be observed that thermal treatment accelerates the anode dissolution[ This phenomenon can be explained by the presence of CuO which comes from Cu1O oxidation reaction[ When Cu1O is in the matrix\ a copper powder layer is formed according to the following reaction] Cu1O¦1H¦:Cu1¦¦Cuo¦H1O
"6#
In this case the density of the barrier layer increases[ The voltametric results show that an increase of lead facili! tates the anode passivation\ but when lead and oxygen are both present in the anode the passivation decreases[ In the _rst case the barrier layer is thicker or more dense "presence of lead
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
20
Fig[ 7[ Nyquist impedance diagrams of CuÐPbÐO obtained at di}erent potentials] "a# 9 mV\ "b# 099 mV\ "c# 199 mV and "d# 299 mV[ Standard experimental conditions[
21
H[ GAUTHIER et al[] EFFECT OF LEAD AND OXYGEN ON THE PASSIVATION OF COPPER ANODES
Impedance studies indicate the presence of two adsorbed species in the passive layer[ The mechanism proposed is a simple one since the _lm resistance is not considered and the con! tribution of the lead compounds is considered constant[ Acknowled`ements*The authors wish to express their thanks to Dr A[ Adnot and L[ Patry "Univ[ Laval# and Dr A[ Lasia "Univ[ de Sher! brooke# for helpful discussions[ The _nancial support of Natural Sci! ences and Engineering Research Council of Canada is gratefully acknowledged[
REFERENCES Fig[ 8[ Nyquist impedance diagram of CuÐPb 1999 obtained at 299 mV[ Standard experimental conditions[
sulfate# than in the second case[ It should be emphasized that CuO and not Cu1O was present in the matrix because of heat treatment which was performed in an oxygen atmosphere[ When the passive layer is reduced by voltammetric techniques the presence of CuO\ Cu1O and a mix of lead compounds were detected[ Lead oxide is not stable and PbSO3 is always found in the passive layer[ SEM results show that there is a di}erence between the PbSO3 formed when only metallic lead is present in the matrix and when lead oxide is present[ In the _rst case PbSO3 grows up in situ in the matrix and can form one part of the passive layer[ In the second case PbSO3 is formed in the barrier layer and is not detected in the passive layer[ ESCA:AUGER technique is the best way to identify the compounds which are present in the passive layer[ A large quantity of copper powder\ CuO and Cu1O was detected in all the studied anodes[ Of the lead species only PbSO3 was observed[
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