An XPS and AES study of the free corrosion of Cu-, Ni- and Zn-based alloys in synthetic sweat

\ PERGAMON

Corrosion Science 30 "0888# 0940Ð0954

An XPS and AES study of the free corrosion of Cu!\ Ni! and Zn!based alloys in synthetic sweat S[ Colina\ \ E[ Becheb\ R[ Berjoanc\ H[ Joliboisa\ A[ Chambaudeta a

Equipe d|Accueil de Microanalyse des Materiaux\ L[M[N[U[F[R[ des Sciences et Techniques\ La Bouloie\ Route de Gray\ 14929 Besancž on Cedex\ France b LERMPS\ IPSe\ 89909 Belfort Cedex\ France c IMP:C[N[R[S[\ BP 4\ Odeillo\ 55014 Font Romeu\ France Received 16 November 0885^ accepted 10 September 0887

Abstract This paper deals with the investigations into the behaviour during free corrosion of Cu:Zn:Ni\ Cu:Ni:Zn and Ni:Cu alloys in a synthetic sweat medium by means of surface analyses[ For the three alloys studied\ our experimental results indicate the presence of a corrosion layer\ the thickness of which increases with the increase of the copper content of the alloy[ The corrosion layers are composed of Cu! and Ni!based compounds[ For the alloys with a high Cu content\ the latter is mainly composed of copper "I# oxide in which chloride anions are included[ For the alloys with a high Ni content\ Cu1 "OH#2Cl and Ni!compounds like Ni"OH#1 and NiO are detected in the corrosion layer[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] copper^ nickel^ XPS^ AES

0[ Introduction Copper!\ nickel! and zinc!based alloys are frequently used in making metallic objects "earrings\ jewels\ watches\ etc[# that are liable to come into contact with the skin[ Following sweat corrosion\ nickel "II# ions are released and can provoke contact dermatitis ð0\ 1Ł[ In order to de_ne factors that limit the nickel release\ we must study the corrosion resistance of Cu!\ Ni! and Zn!based alloys in sweat "neutral aqueous solution of chloride sodium and lactic acid#[ Studies ð2Ð07Ł on the corrosion of Cu:Ni\ Ni:Cu and Cu:Zn alloys in a chlorinated

 Corresponding author[ Tel[] ¦9922!2!70!55!54!22^ Fax] ¦9922!2!70!55!54!11[ 9909!827X:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[ PII] S 9 9 0 9 ! 8 2 7 X " 8 7 # 9 9 0 3 0 ! 2

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medium have shown the formation of a passive layer whose composition and structure regulate the rate of corrosion ð2Ł[ These experiments\ using essentially electrochemistry ð3Ð7Ł and electron di}raction ð3\ 8\ 09Ł\ have demonstrated that the passive _lm depends on the composition of the alloy ð09Ł and the solution ð4Ł\ the temperature ð00Ł and the corrosion time ð3Ł[ The corrosion resistance of Cu! and Ni!based alloys is better in NaCl solution than in seawater because of the di}erent nature of the surface corrosion products] copper and nickel oxides in chloride solution\ cuprous chloride in seawater ð4Ł[ In seawater\ the corrosion resistance of the Cu:Ni 69:29 alloy strongly increases with an increase in the temperature because of the change in the passive layer] the nickel content of the passive layer is 19) at 14>C and 79) at 79>C ð00Ł[ At 79>C\ the aim components of the passive _lms formed on the Cu:Ni 69:29 surface are Ni!compounds "Ni"OH#1\ NiO# after a short exposure time and Cu! compounds "CuO\ Cu"OH#1# after a longer exposure time ð3Ł[ The electrochemical behaviour of a!brass in a NaCl solution "pH8# at 14>C has been interpreted in terms of its own passive layer _lm and dezinci_ed alloy surface[ The lower resistance of brasses\ as compared to Cu\ was assigned to the presence of a complex ZnO\x! H1O:Cu1OÐCuO layer\ which is less protective towards Cl! ion attack than a copper oxide _lm ð09Ł[ Nevertheless\ despite the wide number of references concerning the corrosion layer formed in a chlorinated medium\ very few data were found on corrosion products of Cu!\ Ni! and Zn!based alloys formed in a chlorinated medium in the presence of lactic acid[ Accordingly\ this paper deals with a detailed XPSÐAES study whose aim is to characterize the passive layer formed on copper!\ nickel! and zinc!based alloys in contact with synthetic sweat[ Experimental conditions are atuned to the EEC directive 65:658:EEC concerning the nickel allergenic power of Ni!alloys[ 1[ Experimental method 1[0[ Materials and corrosion conditions The parallelipedal samples "thickness] 0[4 mm\ width] 3!4 mm# are obtained from metal sheets or wires that have been reduced by hammering and pressing[ They are made from commercial Cu:Zn:Ni "Arf#\ Cu:Ni:Zn "Brf# and Ni:Cu "Crf# alloys and have varying nickel\ copper and zinc contents[ Their mass composition is checked by WDS "Table 0#[ The Arf alloy has a high Cu content "72) weight# and the Crf alloy

Table 0 Theoretical mass composition of alloys Arf\ Brf and Crf ")#[ Alloys

Nickel

Copper

Zinc

Tin

Iron

Alloy A Alloy B Alloy C

1[9 12[9 55[9

72[9 51[4 21[9

09[9 01[9 9[9

4[9 1[4 9[9

9[9 9[9 1[9

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has a high Ni content "55) weight#[ The Brf alloy has an intermediate composition between Arf and Crf[ The samples are immersed in freshly synthetic sweat with a pH of 5[49 "adjusted by adding ammonia "AldrichÐChimie\ solution in water\ 17Ð29) NH2\ ACS Reagent## ð0\ 1Ł[ This solution\ used to evaluate the nickel release\ is composed of] , 9[4) sodium chloride "AldrichÐChimie\ 88¦)\ ACS Reagent# , 9[0) lactic acid "AldrichÐChimie\ 74¦)\ solution in water\ ACS Reagent# , 9[0) urea "AldrichÐChimie\ 88¦)\ ACS Reagent#[ The percentages are expressed in mass[ A protective plastic _lm prevents sweat from evaporating and supplies oxygen from the outside[ The sweat solution is not shaken "in accordance with European Standardization project prEN0700#[ The immersion parameter\ pi "volume of sweat per unit of immersed surface#\ is _xed at 9[714×09!1 m ð08Ł[ The experiment is con! ducted at a temperature of 2921>C[ After one week of free corrosion\ the samples are rinsed in distilled water\ dried by ambiant air\ and the corrosion surface is analysed by XPS and AES[ 1[1[ XPS and AES analyses XPS and AES analyses are performed by using a SIA 199 RIBER!CAMECA UHV device[ The Auger transitions are stimulated by an electron beam with an incident energy _xed at about 2 keV[ Photo!electron emissions are obtained by using an AlÐ Ka X!ray source "hn0375[5 eV# striking the surface of the sample[ The kinetic energy of the Auger electrons and photo!electrons is measured by means of a RIBER CAMECA MAC 1 spectrometer two stage analyser[ The energy resolution of the analyser is _xed at 0 eV[ The depth pro_le analysis is performed on specimens sputtered repeatedly with a RIBER CAMECA C[I[ 49 ion gun[ The Ar¦ ions are accelerated under 1 keV and the ion current density is _xed at 09 mA:cm1[ In XPS analysis\ a defocalized Ar¦ ion beam\ accelerated under 1 keV\ is used to remove adsorbed contaminants on the surface "9[4 mA:cm1 current density#[ AES spectra are collected in the direct N"Ek# mode "Ek] Kinetic energy of the electron# at various sputtering times in order to measure depth pro_le concentrations and the nature of the elements through the passive _lm[ Next\ the N"Ek# spectra are mathematically converted into Ek[N"Ek# mode[ The peak upon background ratio values "P:B ratio# ð19Ł are measured on the Ek[N"Ek# spectra in order to evaluate the relative compositional changes with increasing sputtering times[ We prefer to analyse the P:B values rather than the peak height values\ because the P:B ones do not change with the variation of the background levels[ Auger spectra of uncorroded CuÐNi alloys are used as references[ XPS spectra are recorded in direct N"Ek# mode "Ek] Kinetic energy of the emitted photo!electron and Auger electron#\ either without surface sputtering or after sput! tering\ to evaluate the composition of the passive _lm as well as the nature of oxide species and the nature of the chemical bonding[ Surface and bulk atomic

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concentrations are determined from the peak areas using the atomic sensitivity factor reported by Sco_eld and corrected by the transmission function of the analyser ð10Ł[ Auger spectra energy calibrations are made using the C KVV ð11\ 12Ł and Ar LVV ð11\ 12Ł Auger transitions located at 152 and 100 eV\ respectively\ without any charging e}ects[ XPS spectra binding energy normalizations are referenced to the C 0s peak of carbon contamination reported currently at 174 eV ð13\ 14Ł[ We also measured the Auger modi_ed parameter\ a?\ introduced by Wagner ð15Ł[ This parameter is the sum of the kinetic energy of the Auger electron "Ek# and the binding energy "Eb# of the photo!electron\ which are all referenced to the Fermi level] a?Ek¦Eb[ The Auger modi_ed parameter\ a?\ can be used successfully\ especially to di}erentiate chemical environments of copper ð15Ð18Ł[

2[ Experimental results 2[0[ XPS analyses of the corrosion layers The passive _lm surface and bulk are examined by XPS in order to determine either the atomic concentration of Cl\ C\ O\ Ni\ Cu and Zn species\ or the chemical environment of Cu and Ni atoms[ 2[0[0[ Atomic concentration of Cl\ C\ O\ Ni\ Cu and Zn species Table 1 summarizes the atomic concentrations "atomic percentage] at[)# calculated from Cl 1p0:1\2:1\ C 0s\ O 0s\ Ni 1p2:1\ Cu 1p2:1 and Zn 1p2:1 photo!electron lines collected] , on the reference alloy "uncorroded alloys#\ after 3 minutes of 1 keV Ar¦ sputtering of the surface "labelled Arf\ Brf and Crf#

Table 1 Atomic concentrations "at[)# of Cl\ C\ O\ Ni\ Cu and Zn elements measured by XPS and EPMA on the Arf\ Brf and Crf samples measured by XPS on samples A0\ A1\ B0\ B1\ C0 and C1[ Atomic concentrations of elements "at[)# EPMA

XPS

Elements

Arf

Brf

Crf

Arf

Brf

Crf

A0

B0

C0

A1

B1

C1

Cl C O Ni Cu Zn

: : : 2 74 00

: : : 13 52 02

: : : 56 22 :

: : 1 5 73 7

: 2 1 11 51 00

: 1 1 55 29 :

3 45 11 : 07 :

3 43 13 : 07 :

2 34 29 09 01 :

5 5 15 : 51 :

6 2 16 1 59 0

: 2 2 53 29 :

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, on the corrosion _lms formed on the alloy surfaces *without ionic sputtering "labelled A0\ B0 and C0# *after 3 minutes of 1 keV Ar¦ sputtering "labelled A1\ B1 and C1#[ In Table 1\ we also indicate the atomic concentrations measured for the Arf\ Brf and Crf alloys by using an EPMAÐWDS spectrometer[ For the reference alloys\ we note that the atomic concentrations calculated from XPS measurements are consistent with the atomic concentrations of the alloys mea! sured by using the EPMAÐWDS technique[ Very high carbon contents were detected on the corroded samples A0\ B0 and C0 "45)\ 43) and 34)\ respectively#[ This result is attributed to the presence of hydrocarbon species deposited on the surfaces of the samples during air exposure ð29Ł[ After 3 minutes of 1 keV Ar¦ sputtering "samples A1\ B1 and C1#\ the atomic concentration of Cu and Ni has increased\ while the carbon concentration decreased[ Moreover\ we noted that there was no zinc in the analysed oxidation layer\ whatever the composition of the alloy studied[ "Detected zinc in the case of sample B1 was not signi_cant#[ As for the presence of Cl\ it was detected only on the surface of the A1 and B1 samples[ For the A and B alloys\ oxygen was still present in considerable amounts after ion sputtering[ Furthermore\ with this study\ we noted that the composition of alloy C after an Ar¦ sputtering "sample C1# was similar to that obtained for reference sample Crf[ The oxidation of alloy C was only super_cial[ Thus\ in order to characterize the thin passive _lm formed on the surface of alloy C\ we must examine only the results obtained for the corroded sample without sputtering "sample C0#[ We can conclude that the oxidation layer of alloy C is distinctly thinner than the one formed on alloys A and B[ The XPS and AES experimental results indicate that the oxidation layer thickness  for sample C1[ This latter  for samples A1 and B1\ and about 49 A is greater than 299 A value is in accordance with the result obtained by a chemical method\ which leads us [ to a very approximate oxidation layer thickness contained between 19 and 099 A This method consists of] the chemical dissolution of oxydation products in a de! aerated 19) sulfuric acid solution\ and the measurement of the dissolved elements by atomic emission spectrophotometry ð5Ł[ For samples A1 and B1\ the corrosion layer thickness can be estimated at 9[0!9[1 mm and 9[6!9[7 mm\ respectively[ These experimental results can be explained by decreasing oxidation of CuÐNi alloys with increasing nickel content ð2Ł[ Moreover\ these results indicate that the XPS analyses carried out after Ar¦ sputtering are performed on the corrosion _lm of alloys A and B[ The last two are basically composed of Cu\ O and a small amount of Cl\ with similar contents[ 2[0[1[ Chemical environment of Cu atoms Figure 0 shows the Cu 1p2:1 spectra collected from] the surfaces of the CuO and Cu1O reference pellets^ from the surface of the as!grown corrosion layers "A0#\ "B0# and "C0#^ and from the surface of the corrosion layers "A1# and "B1# after 3 minutes of 1 keV Ar¦ sputtering[

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Fig[ 0[ Cu 1p2:1 spectra collected from] the surfaces of CuO and Cu1O reference pellets^ from the surface of the as!grown corrosion layers "A0\ B0\ C0#^ and from the surface of the corrosion layer "A1\ B1# after 3 minutes of 1 keV Ar¦ sputtering[

This _gure shows that the Cu 1p2:1 photo!emisson peak collected for CuO\ in the binding energy range 829!836 eV\ presents two features[ The main one is located at about 822[5 eV\ the wider one is observed between 828 and 835 eV[ These two features are generally attributed to the emission of the Cu 1p2:1 electrons with two di}erent _nal states for the electronic levels of the ionized copper atom[ The main feature is 4 2d 09 L _nal states[ The _lling of the 2d 09 state is caused by an electron related to 1p2:1 transfer from the ligand labelled L "oxygen in this case# to the 2d electronic level of copper during the emission process of the 1p2:1 electron ð20Ł[ The presence of the 1p2:1 double feature is characteristic of the Cu1¦ state of copper ð20Ð22Ł[ Figure 0 shows 4 that the Cu 1p2:1 satellite related to the 1p2:1 2d 8 _nal state is clearly observed only for the CuO reference sample[ The absence of this Cu 1p2:1 satellite in the A0\ A1\ B0 and B1 spectra also indicates that Cu¦ is the major copper state present in the corrosion layers[ Accordingly\ Cu1¦ is either absent or present in a very small amount in the corrosion _lms of alloys A and B[ The low intensity feature observed in the C0 spectrum would indicate a slight contribution of the Cu1¦ state for this surface[

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Table 2 Binding energy "Eb# and full width at half maximum "F[W[H[M[# measured for the main 4 Cu 1p2:1 2d 09 peaks on samples CuO\ Cu1O\ A0\ A1\ B0\ B1 and C0[ Cu 1p photo!electron peak Samples

Eb "eV#

FHWM "eV#

CuO Cu1O A0 A1 B0 B1 C0

822[5 822[9 822[0 822[0 822[9 822[9 822[9

2[3 1[0 1[3 1[2 1[3 1[2 1[3

Table 2 contains the binding energy "Eb# and the full width at half maximum 4 2d 09 peaks presented in Fig[ 0[ The "F[W[H[M[# measured for all the main Cu 1p2:1 binding energies obtained for the peaks collected for the corrosion layers are consistent with those measured for Cu1O[ On the other hand\ we noted that the FWHM values measured for CuO and Cu1O are very distinct "2[3 eV and 1[0 eV\ respectively#[ 4 2d 09 peaks collected on the Moreover\ the FWHM values measured for the Cu 1p2:1 corrosion layers "between 1[2 and 1[3 eV# are higher than the FWHM value obtained for Cu1O "1[0 eV#[ Therefore\ we conclude that these results suggest the presence of various copper chemical environments in which Cu1O is the major compound in the corrosion layers[ Thus\ the Cu 1p2:1 lines are _tted in two Gaussian components\ which are attributed to various chemical Cu!bonds in the corrosion layers[ According to the Shirley method ð23Ł\ Fig[ 1 shows the Cu 1p2:1 line deconvolution performed for samples A0\ A1\ B0\ B1 and C0 after subtraction of the inelastic electron background[ Each _tted Cu 1p2:1 line is composed of two components "a and b#[ The binding energy positions and the FWHM values of these components are summarized in Table 3[ The main components "a# present in the A0\ A1\ B0\ B1 and C0 spectra are located at about Eb82229[0 eV[ This binding energy can only be attributed to the presence of the Cu¦ state ð3\ 13\ 16\ 21\ 22\ 24Ð27Ł[ However\ according to several authors ð20\ 22\ 28Ð31Ł\ the shift of the binding energy of Cu 1p2:1 obtained for Cu metal and Cu1O is often very slight "generally less than 9[1 eV#[ Accordingly\ we cannot completely di}erentiate them[ Therefore\ the components "a# may contain contributions from the Cu9 and Cu¦ states[ We also carefully examined the kinetic energy positions of the XAES Cu L12M34M34 Auger transition collected from the same surfaces[ For samples A0\ A1\ B0 and B1\ the Cu L12M34M34 peak is located at Ek805[4 eV ð01\ 16\ 21\ 26\ 32Ł[ Thus\ the modi_ed Auger parameter calculated from the Cu L12M34M34 kinetic energy and the Cu 1p2:1 binding energy has a value of

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Fig[ 1[ Cu 1p2:1 line deconvolution obtained from the samples labelled A0\ A1\ B0\ B1 and C0 after subtraction of the inelastic electron background[

Table 3 Binding energy "Eb# and full width at half maximum "F[W[H[M[# measured for the com! 4 ponents located in the Cu 1p2:1 2d 09 features of samples A0\ A1\ B0\ B1 and C0[ Cu 1p line decomposition "A0\ A1\ B0\ B1\ C0# Components

Eb "eV#

FHWM "eV#

"a# "b#

822[0 824[1

1[0 2[0

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a?"A0\A1\B0\B1#0738[5 eV[ This parameter is consistent with the values of other authors and is characteristic of Cu1O ð01\ 16\ 33\ 34Ł[ For the C0 sample\ the Cu L12M34M34 XAES transition is broadened on the high kinetic energy side when compared with the same transitions recorded for samples A0\ B0\ A1 and B1[ The latter Auger transition is located between 805[4 and 80729[4 eV[ Thus\ two values of the Auger a? parameter were calculated from the two kinetic energy values\ 0738[529[4 eV and 074029[4 eV\ respectively[ According to other authors ð3\ 21\ 22\ 35Ð37Ł\ these values are characteristic of Cu¦ and Cu9 states[ Therefore\ we conclude that the very thin oxidation layer on the C sample  contains Cu¦ oxidized copper[ However\ the thickess of this layer is lower than 49 A because the XPS analyses performed on the surface without Ar¦ sputtering detected the Cu9 state\ which is the copper state present in the bulk of the sample[ The  :min[ This result is in accordance determinated Ar¦ sputtering rate is close to 09 A with the fact that the bulk composition of sample C was reached after sputtering Argon for 3 mins[ The minor components "b# located at about 824[129[0 eV Fig[ 1 are characteristic of Cu1¦ ions surrounded by OH!\ Cl! and CO21− anions ð3\ 6\ 8\ 24\ 34\ 38Ð41Ł[ To our knowledge\ the copper!\ oxygene!\ chloride! and carbonate!based species\ which are liable to be formed "in a chlorinated medium# ð42\ 43Ł\ are] , copper "II# hydroxide\ , copper "II# hydroxychloride\ especially Cu1 "OH#2Cl ð3\ 6\ 8\ 40\ 41\ 44Ł\ , copper "II# hydroxycarbonate "verdigris#[ For alloys A and B\ the absence of the Cu 1p2:1 satellite peak\ observed in Fig[ 0\ can be attributed either to a very small and undetectable amount of Cu!based com! pounds or to the absence of these last species[ Within the framework of this second hypothesis\ the minor component "b# can be assigned to CuÐCl bonds in a Cu!\ O! and Cl!based compound] a defective copper "I# oxide in which chloride anions are included "Cu1O doped by chloride anions# eq[ "0#[ This experimental result is con! sistent with Beccaria|s data ð3Ł and those of other authors ð6\ 40\ 41Ł[ Substitution mechanism "with formation of a cationic gap# ð40Ł ð1Cu¦¦O1− Łoxide ¦ðxCl− Łsolution ¦ð1xH2 O ¦ Łsolution :ð"1−x#Cu¦¦"0−x#O 1− ¦xCl− Łoxide ¦ðxCu¦Łsolution ¦ð2xH1 OŁsolution

"I#

In conclusion\ these XPS results show that the corrosion layer formed on the surface of the alloys is mainly composed of Cu1O[ Cl is also detected in the corrosion layer[ Unfortunately\ the small amount of Cl!based compounds does not permit us to de_ne their exact nature[ However it seems that] , for alloys A and B\ Cl atoms are more!or!less included in a defective copper "I# oxide[ The very small quantity of Zn detected in the corrosion layer of sample B1 is insigni_cant "0)#[ , for alloy C\ the presence of Cl atoms may be explained by the formation of compounds\ such as Cu1 "OH#2Cl[

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2[0[2[ Chemical environment of Ni atoms The Ni 1p2:1 photo!electron peak of the corrosion layer formed on alloy C "with a high nickel content# is collected in the C0 spectrum "without sputtering# "Figure 2#[ It is also _tted into several components with the aim of distinguishing the Ni!based compounds present in the corrosion layer[ This Ni 1p2:1 peak decomposition is more complicated than the Cu 1p2:1 peak _tting because the main Ni 1p2:1 peak of the Ni metal ð3\ 01\ 45Ð52Ł or the main Ni 1p2:1 peak of NiO ð01\ 45\ 48Ð52Ł are associated with two or three satellites\ respectively[ Figure 2 illustrates the curve _tting of the Ni 1p2:1 spectrum for the C0 corrosion layer[ This shows two main peaks located at 742[029[0 eV and 744[729[0 eV[ The energy shift between these two photo!electron peaks "1[6 eV# is higher than the data from other articles on this subject[ The metal peak is located at 741[329[0 eV with a satellite at 747[829[0 eV ð3\ 01\ 46Ð52Ł[ The oxide peak is located at 743[129[0 eV with one satellite at 744[629[0 eV and another at 759[629[0 eV ð3\ 01\ 46Ð52Ł[ This di}erence observed for the two experimental main peaks can also be attributed to the presence of the Ni 1p2:1 peak of nickel hydroxide Ni"OH#1[ Indeed\ this has been noted at 745[029[0 eV with a satellite peak at 751[829[0 eV ð3\ 47Ð52Ł[ Therefore\ the experimental Ni 1p2:1 curve is _tted into three main components attributed to Ni metal\ NiO and Ni"OH#1[ In Fig[ 2\ the Gaussian component "a# and its satellite "a0# are attributed to Ni metal "or alloy#[ The Gaussian component "b# and its satellites "b0# and "b1# are characteristic of NiO[ The component "c# with its satellite "c0# are characteristic of Ni"OH#1[ The binding energy positions and the FWHM values of these three components are summarized in Table 4[ The small amount of Ni detected for sample B1 is insigni_cant[ However\ the small peak "Ni 1p2:1#\ located at about 743 eV\ could be attributed to NiO compounds[

Fig[ 2[ Curve _tting of the Ni 1p2:1 spectrum C0 for the corrosion layer formed at the surface of alloy C[

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Table 4 Binding energy "Eb# and full width at half maximum "F[W[H[M[# measured for the com! ponents located in the Ni 1p feature of sample C0[ Ni 1p line decomposition "C0# Components

Eb "eV#

FHWM "eV#

"a# "b# "c#

741[7 743[1 745[4

1[9 1[7 2[1

Figure 3 shows the curve _tting of the O 0s spectrum collected from the same corrosion layer "sample C0#[ The O 0s experimental curve is _tted in two Gaussian components located at 429[729[0 eV and 422[029[0 eV\ attributed to O 1! and OH! species ð24\ 34\ 53Ð55Ł\ respectively[ Our curve _ttings are very similar to those obtained by other authors ð24\ 34\ 53Ð55Ł for the Ni 1p2:1 and O 0s photo!electron peaks collected from oxidized nickel surfaces in de!oxygenated or oxygenated solu! tions of sodium chloride[ Finally\ the examinations of the experimental O 0s and Ni 1p2:1 spectra completed by the curve _tting procedures enable us to conclude that the interface layer formed on the surface of alloy C contains mainly Ni metal\ or Ni!Cu alloy and Ni"OH#1[ NiO is noted as a minor constituent[

Fig[ 3[ Curve _tting of the O 0s spectrum C0 collected from the corrosion layer formed at the surface of alloy C[

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2[1[ Au`er depth pro_le measurements] P:B ratios In Fig[ 4\ we have indicated the variations of the P:B values "oxidized samples A\ B and C# measured for Cl LVV\ C KVV\ O KLL\ Ni L2M34M34 and Cu L2M34M34 Auger transitions\ for di}erent sputtering times[ The P:B values "samples A0\ B0 and C0# collected before ionic sputtering are characteristic of contaminated surfaces with a high concentration of carbon species " for example\ P:BC9[4 for the sample A0#[ For the oxidized A and B alloys\ Fig[ 4"a\b# shows that the P:B values measured for the Cu L2M34M34 transition on the corrosion layer is stable "P:B9[05 and P:B9[02\ respectively# and lower than the P:B value measured for alloys\ Arf and Brf "P:B9[08 and P:B9[05\ respectively#[ Fixed P:B values for O KLL and Cl LVV were also measured for the corrosion layer on oxidized samples A "P:BO9[02 and P:BCl9[93# and B "P:BO9[09 and P:BCl9[92#[ These results indicate that the composition of the corrosion layer formed on samples A and B is homogeneous from the surface to the depth reached after 29 minutes of Ar¦ etching[ These experimental results are consistent with previous observations] the presence of a homogeneous passive _lm formed on alloys A and B[ This layer is a copper "I# oxide containing Cl species\ probably bonded to Cu atoms in a defective oxide[ For alloy C\ the P:B values for Ni L2M34M34 and Cu L2M34M34 Auger transitions rapidly reach the P:B values measured for the reference alloys ðFig[ 4"c#Ł[ A very fast elimination of oxygen was also noted during the sputtering of sample C[ This result indicates that the corrosion layer formed on alloy C "with a high Ni content# and

Fig[ 4[ Depth pro_le obtained by sputtering the surface of sample A "spectrum 4a[#\ sample B "spectrum 4b[# and sample C "spectrum 4c#[

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immersed in the synthetic sweat\ is much thinner than the corrosion _lm formed on alloys A and B "with a high Cu content#[ This experimental result is consistent with the data in other articles concerning a chlorinated medium ð56Ð58Ł[

3[ Discussion The Cu!based corrosion product formed on the three CuÐNi alloys studied in synthetic sweat is Cu1O with minor Cu!\ O! and Cl!based compounds[ This result is in accordance with experiments performed in seawater and in NaCl solution ð3\ 4\ 6\ 09\ 04Ł[ However\ the composition of minor compounds depends on the alloy|s[ Cu1 "OH#2Cl\ which is analysed in great quantity by Kato et al[ ð6Ł and EIselstein et al[ ð00Ł\ is only present in small amounts on the surface of the Ni!rich alloy[ For the Ni!poor alloys\ chloride ions seem to be a dopant in the Cu1O defect structure\ as demonstrated by North et al[ ð40Ł and Blundy et al[ ð02Ł[ The Ni!based corrosion compounds\ especially nickel hydroxide and nickel oxide\ were essentially identi_ed on the surface of the Ni!rich alloy[ The presence of these Ni!compounds are in accordance with the works of Beccaria et al[ ð3Ł\ Chauhan et al[ ð02Ł and Blundy et al[ ð01Ł[ However\ the presence of nickel ions as dopants in Cu1O ð04\ 42Ł cannot be ruled out and could explain the weak corrosion of the Ni!rich alloy[ Indeed\ Uhlig ð56\ 57Ł has suggested that nickel present in the solid solution improves its resistance to corrosion by making the passivation reaction easier[ The Zn!based compounds were not detected on the surface of the corroded alloys[ However\ Morales et al[ ð7\ 09Ł has demonstrated ZnO formation during the a!brass corrosion\ resulting from the dezinci_cation process\ i[e[\ the preferential dissolution of zinc that leaves a porous substrate rich in copper ð69Ð63Ł[ However\ this passivation layer breaks down in the presence of chloride ions ð7\ 09Ł[ Therefore\ our experimental result "no Zn!compound# can be explained by faster destruction of the layer than that of the ZnO formation[

4[ Conclusions In order to understand the free corrosion behaviour of Cu!\ Ni! and Zn!based alloys in sweat\ the surface of the corroded samples of Cu:Zn:Ni\ Cu:Ni:Zn and Ni:Cu alloys was characterized by XPS and AES[ The following conclusions were drawn] 0[ Ni!compounds "Ni"OH#1^ NiO# are essentially present in the corrosion layer of Ni!rich alloys[ 1[ For the three alloys studied\ the corroded _lm is also composed of Cu1O with a small amount of Cl!based compounds[ 2[ The composition of these minor compounds depends on the alloy|s composition]

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*For Cu!rich alloys "alloys A and B#] the chloride ions are present as a dopant in the Cu1O defect structure[ *For Ni!rich alloys "alloy C#] the minor compound seems to be Cu1 "OH#2Cl[ 3[ The thickness of the corroded _lm increases when the copper content is increased[

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