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Applied Surface Science 258 (2011) 1235–12 41

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j ou rna l ho me p a g e : www.e l sev i e r. com/ loca t e / apsusc

Initial oxidation of brass induced by humidied air

Ping Qiu ∗, Christofer Leygraf Division of Surface and Corrosion Science, Royal Institute of Technology (KTH),Drottning Kristinas väg 51, SE-100 44 Stockholm, Sweden

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Article history:Received 9 March 2011Received in revised form 25 August 2011Accepted 20 September 2011Available online 24 September 2011

Keywords:Atmospheric corrosionBrass (Cu–20Zn)Humid airSurface analysisCu2 OZnO

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Complementary surface and near-surface analytical techniques have been used to explore a brass(Cu–20Zn) surface before, during, and after exposure in air at 90% relative humidity. Volta potentialvariations along the unexposed surface are attributed to variations in surface composition and resulted

in

an

accelerated

localized

growth

of

ZnO

and

a retarded

more

uniform

growth

of

an

amorphous

Cu2 O-likeoxide. After 3 days the duplex oxide has a total mass of 1.3 g/cm 2 , with improved corrosion protectiveproperties compared to the oxides grown on pure Cu or Zn. A schematic model for the duplex oxidegrowth on brass is presented.

© 2011 Elsevier B.V.

1. Introduction

The corrosion properties of brass, herein referred to as an alloyof copper and zinc, have attracted great attention over the yearsowing to the importance of brass in various technological andartistic applications [1–4] . Most corrosion-related studies havebeen performed in bulk solution, and the results largely explainedon the basis of selective dissolution and dezincication mecha-nismscaused by the difference in electrochemicalnobility betweenthe main alloy constituents [5–12] . Some studies have been con-cerned with thepassive behavior of brass in various solutions usingelectrochemical methods in combination with surface analyticaltechniques. The results suggest the passivation to be attributed totheformation of a complex passive layer on brass consisting of mix-tures ofZnO and Cu xO y, andwith a composition strongly inuencedby theanions of thesolution [13–16] . Other studies have focused ontheoxidation behavior of brass in eitherpure oxygenor airat ambi-ent or elevated temperature, and interpreted the results mainlythrough the semiconducting and electrochemical properties of thepassive layer formed [17–20] .

So far, the fundamental aspects of the atmospheric corrosion of Cu–Zn alloys have received relatively little attention, although adetailed understanding exists for their pure elements Cu and Zn[21–26] . Hence, this study is a rst effort to explore the funda-mental processes governing the atmospheric corrosion of a brass(Cu–20Zn) alloy through a multi-analytical in situ and ex situ

∗ Corresponding author. Tel.: +46 08 7909925; fax: +46 08 208284.E-mail address: pingq@kth.se (P. Qiu).

approach. Results from the oxidation of brass will be presentedbased on exposure in air at 90% relative humidity. This paper willbe followed by a second, in which the inuence of carboxylic acidson the atmospheric corrosion of brass will be explored in somedetail.

2. Materials and methods

2.1. Sample preparation

Commercially available brass sheets (Cu–Zn, with 20% Zn byweight, size 20mm × 20mm × 1mm) were abraded with SiC paperand then polished with diamond paste down to 0.25 m until aneven mirror-like surface was obtained. After sonication in 99.5%ethanol each sample was immediately ushed in dry nitrogen gas.

In order to revealthe grain boundaries of thebrasssurface,someof the polished samples were etched in a solution containing 2M

of hydrochloric acid (HCl) and 0.2 M of iron chloride (FeCl 3 ).

2.2. Exposure conditions

The experimental setup for preparation of humid air has beenpresented elsewhere [27] . The corrosive air was generated by mix-ing streams of synthetic dry air, puried from gases and particlesthrough lters, and humidied air. The resulting laminar air owvelocity was 3.5 cm/s along the sample surface. The desired expo-sure conditions throughout all exposures were a relative humidity(RH) of 90 ± 3% and a temperature of 19.5 ± 0.5 ◦ C. This relativehumidity represents oxidation conditions which are fast enough

0169-4332 © 2011 Elsevier B.V.

doi: 10.1016/j.apsusc.2011.09.080

Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

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to follow the oxide growth under current in situ measurementconditions.

2.3. In situ infrared reection absorption spectroscopy (IRAS)

The IRAS setup has been describedin more detail in earlier pub-lications [26] . IRAS spectra were recorded by using a commercialDigilab 4.0 Pro FTIR spectrometer with 1024 scans at a resolution

of 4 cm−

1 . P-polarized light impinged the sample with a grazingangle of around 78 ◦ from surface normal and passedthrough CdTe-windows when entering and leaving the exposure cell. The IRASspectra were recorded in absorbance units ( − log( R/R0 )), where R isthe reectance of the exposed sample and R0 is the reectance of the sample before exposure (background spectrum).

The sample was kept in humidied air for 30 min to col-lect a background spectrum. This was subtracted from all laterin situ spectra to eliminate overlap from physisorbed water [28] .Thereafter in situ IRAS spectra were collected continuously. Postspectra analysis was performed by using the Digilab ResolutionsPro4.0software.Undercurrent controlledexposureconditions, thetemperature ( ± 0.5 ◦ C) and relative humidity ( ± 3%) variations cor-respond to variations in the determination of cuprite thickness of

around 0.1nm,

which is below the detection limit of the IRAS sys-tem (one monolayer of cuprite). Hence, such variations have only aminor inuence on the quantication procedure presented in thisstudy.

2.4. Confocal Raman microspectroscopy (CRM)

The confocal Raman measurements were performed with aWITec alpha 300 system. The scan range was 25 m × 25 manda532 nm laser was used as laser source. The measurement was per-formed ex situ and the integration time per Raman spectrum wasin the order of 50ms. A Nikon objective, Nikon NA0.9 NGC, wasused for the measurements together with a pinhole with 50 mdiameter. The resulting stack Raman spectra were produced in the

scanning area with a lateral resolution of around 300 nm and a ver-ticalresolution of around 2 m. WITecProjectsoftwarewas appliedfor spectra imaging.

2.5. Scanning Kelvin probe force microscopy (SKPFM)

A Nanoscope IV with facilities for Volta potentialmeasurementsof unexposed brass was used. The principals and details of theSKPFM technique have been described elsewhere [29] . The datawere acquired in a two-pass mode: one for topography and theother for Volta potential mapping of the same surface area [30] .The probe, antimony-doped Si, supplied by Veeco, was lifted up toa constant distance from the surface (80 nm in this work) to collectVolta potentialdata.The lateral resolutionunder current conditions

isof the order of 100nm [29] .

2.6. Atomic force microscopy (AFM)

The topography of the exposed sample surface was investigatedby an AFM from Quesant Instrument Ltd. The measurements wereperformed in contact mode with a commercially obtained SiN can-tilever.

2.7. Scanning electron microscopy with energy dispersive X-rayanalysis (SEM/EDS)

A Philips XL 30 SEM/EDS instrument was used for surfacemicrostructure characterization of both polished and etched brass.

2.8. Electrochemical measurements

To aid in the identication of the oxides formed upon expo-sure and to deduce their average mass and overall polarizationresistance,cathodic reduction(CR) and electrochemical impedancespectroscopy (EIS) measurements were performed using a stan-dard three-electrode cell with a KCl saturated Ag/AgCl electrodeas reference and a platinum mesh as counter electrode. MinorCl− leakage from the reference electrode into the cell cannot beexcluded. For comparison, a saturated calomel reference electrodein a 25 mL electrochemical cell increased the Cl − concentrationfrom zero to about 1 M after a ten times longer exposure timethan was used here [31] . Exposed brass (Cu–20Zn), Cu and Znsheets with 1 cm 2 surface area acted as working electrodes. A 0.1Msodium perchlorate (NaClO 4 ) solution was used as electrolyte. TheKCl saturated Ag/AgCl reference electrode in contact with the per-chlorate solution may induce minor precipitation of KClO 4 , whichhas very low solubility in water. The solution was purged withN2 for 30min before starting each reduction and throughout itsduration.A potentiostatSolartron1287 wasapplied for the CR mea-surements. The scan rate was 1.0mV/s in the range from − 0.4 to− 1.3V Ag/AgCl . The equivalentmass ofZnO and Cu 2 O wasthen deter-mined from Faraday’s law [32] by integrating the correspondingreduction peak using a Corrview software. Combining the poten-tiostat with a frequency response analyzer Solartron 1255, EIS wasperformed at the open circuit potential with a sinusoidal voltagesignal of 10mV applied over the frequency range 10 4 –10 − 2 Hz. AZview (Scribner Associates Inc.) software was used for analysis of EIS spectra.

3. Results and discussion

3.1. Surface characterization of unexposed brass

3.1.1. SEM/EDS Fig. 1(a ) isa SEMimage obtainedfroma newlydiamondpolished

brass surface, whereas Fig. 1 (b) exhibits diamond polished brassafter slight etching to reveal the microstructure, including grainboundaries. A common observation is that complementary EDSmeasurements show slightly higher Zn-content in some microme-ter sized grains.

3.1.2. SKPFM Fig. 2 shows topography (a) and Volta potential (b) images

obtained by using the AFM-based SKPFM to map a newly diamondpolished brass surface. The images reveal the variations in sur-face morphology and corresponding relative nobility variations of the very same surface area. The Volta potential map displayed inFig. 2 (b) clearly shows numerous darker rounded features, a fewmicrometers in size, of lower Volta potential. Their size is compa-

rable tothe sizeof someof the grainsseenin Fig.1 (b ). Supportedbyseparate EDS-analysis that reveals variations in Zn-content in dif-ferent grains of the brass alloy (Section 3.1.1 ), the areas with lowerVoltapotential may be associated with grains of slightly higher Zn-content. Brass,in which Zn substitutes Cu in thealloy, consists of an

-phase with FCC crystal structure up to 35wt% Zn [33] . The varia-tions inZn-contentat differentgrains couldbe dueto differentgrainorientations in combination with thermal treatments, abrasion orpolishing procedures.

3.2. Surface characterization of brass exposed to humidied air

3.2.1. IRAS Fig. 3 displays the in situ IRAS spectrum collected during expo-

sure of polished brass in synthetic air at 90% RH up to 72h. For

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Fig. 4. Absorbance of the Cu 2 O and ZnO band at 660cm − 1 and 570cm − 1 versusexposure time during exposure of brass to synthetic air at 90% RH. The error barsarebased on triplicates.

previously reported for pure Zn and Cu [35,39] . Fig. 5 showsa representative curve recorded during cathodic reduction from− 0.4 to − 1.3V Ag/AgCl of the brass surface after exposure to air at90% RH for 72h. Two main reduction peaks are seen. The reduc-tion peak around − 0.62 V Ag/AgCl originates most likely from Cu 2 O,whereas the reduction peak around − 1.2V Ag/AgCl originates fromZnO. The estimated masses of Cu 2 O and ZnO on brass are 0.7 and0.6 g/cm 2 respectively, higher than what was obtained by IRAS(Fig. 4 ). A possible reason for this difference in results may be alater formed oxide during transport in ambient air or during partsof the cathodic reductionprocedure,despite careful handling of thesamples between exposure and cathodic reduction, and N 2 purg-ing during cathodic reduction. Consistent with the results based onIRAS (Fig. 4 ), the absolute amount for ZnO is smaller than for Cu 2 O.

The lack of any reduction peak around − 0.8V Ag/AgCl suggests thatno CuO has been formed.

The quantication procedure from a pure metal and an alloymay be different, in the same way as the properties of oxidesformed on brass most likely differ from those formed on the puremetals. Earlier studies in our group [38] have shown that when

Fig. 5. Cathodic reduction curve obtained in 0.1M NaClO 4 solution of the oxides

formed

onbrass exposed tohumidied air at 90%RH for 72h.

Fig. 6. Bode plots measured in 0.1M NaClO 4 solution for brass, pure Zn, and pureCuafter exposurein humidied air at 90% RH for 72h.

independently quantifying the amount of cuprite on pure copperby QCM, IRAS and cathodic reduction, there was an agreementbetween the results from all methods with a relative accuracy of 12% or better. Looking at the absoluteamount of cuprite on brass inthe present study obtained by cathodic reduction (0.7 g/cm 2 ) andby IRAS (0.6 g/cm 2 ), the difference seems to be of the same order,in particularly when considering the circumstance that cathodicreduction may result in slightly higher mass than IRAS due to theshort exposure in air. This forms evidence that the quanticationprocedures for cuprite on brass and on Cu are the same within arelative accuracy of approximately 15%.

In all, cathodic reduction supports the conclusions from IRASthat two main oxides have been formed on brass during exposurein humidied air: Cu 2 O and ZnO,with a total mass of1.3 g/cm 2 .

3.2.3. EIS EIS measurements were also performed, in order to explore the

overall corrosion protective properties of the oxides formed onbrass. Fig. 6 displays the Bode plots obtained in 0.1 M NaClO 4 forbrass exposed tohumidied air at90% RHfor 72h, and alsofor pureCu andpure Zn after thesame exposures. Thespectra allreveal onlyone time constant,in which thehigherfrequency regionrepresentsthe solution resistance ( Rs ), the middle frequency region’s slopeexhibits the capacity between the passive layer and the substrate,while the lower frequency region is a measure of the protective

ability and the electrolyte property by means of the polarization

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Fig. 9. Schematic illustration of important steps identied during initial oxidationof brass in humidied air. (a) Before exposure in humid synthetic air; (b) duringexposure in humid synthetic air; (c) formationof corrosionproducts.

The Cu 2 O band detected with IRAS ( Fig. 3 ) is broader on brassthan on pure Cu under similar exposure conditions [34] , whichsuggests a more amorphous structure. The band also undergoesa shift indicating a compositional and/or structural change of theCu2 O-like oxide during growth on brass. The ZnO band on brass,on the other hand, is similar to ZnO on pure Zn [35] , and suggestsno major differences in nature of the two oxides formed. The factthatmore amorphousstructures in oxides result in highercorrosionresistance is well-known from the literature [45,46] . Zn seems toenhance the stability of the Cu 2 O layer, a phenomenon also shownin previous studies [47,48] .

The measured mass of Cu 2 O and of ZnO can be transformedinto an average oxide thickness. Assuming bulk density of Cu 2 O(6.0 g/cm 3 ) and of ZnO (5.2 g/cm 3 ), respectively, the resulting

average oxide thickness is around 4 nm in all. This is signicantly

thinner than the corresponding average oxide thicknesses on pureCu (14 nm) [34] and on pure Zn (47.5 nm) [35] .

A previously reported XPS-studyof brass oxidized in air at roomtemperature has shown that preferential oxidation of Zn to ZnOtakes place initially with a concomitant zinc depletion layer in thesurface region of brass [18] . This promotes an overlayer of Cu 2 Oto form which has been reported to be thinner than on pure Cu,in agreement with the present work. Possible reasons for the thin-ner oxide on brass may be due to changes in the semiconductingand transport properties of the Cu 2 O-layer caused by the adjacentZnO-layer. The defect structures of Cu 2 O and of ZnO have a differ-ent nature, with Cu 2 O predominantly a p-type semiconductor andZnO an n-type semiconductor [2,49,50] . Due to the importance of Cu2 O/ZnO heterojunctions as candidates in solar cell devices [51] ,there are considerable interests in Zn-doping of Cu 2 O, which turnsout to have a profound inuence on, e.g., the electrical and struc-tural properties of Cu 2 O [52] . Evidence of different properties of the Cu 2 O-layer formedon brass compared to pure Cu in thepresentstudyare theshiftto higherwavenumber andthe broadening of theCu2 O-band in the IRAS spectra ( Fig. 3 ), also the higher polarizationresistance of the oxidized brass surface compared to pure Cu or Zn(Fig. 6 ).

4. Conclusions

The duplex oxide growth on a diamond polished brass(Cu–20Zn) surface exposed to humidied air at 90% RH has beenfollowed by means of a multi-analytical approach. The coherentresults provide new details on the initial oxidation of brass inhumidied air.

Prior to exposure the brass surface consists of slightly Zn-enriched areas, a few micrometers in size, characterized by lowerVolta potential than the surrounding matrix.

The variations in nobility along the brass surface create condi-tions forgalvanic effects which resultin an acceleratedlocalgrowthof ZnO and a retarded and more uniform growth of a Cu 2 O-likeoxide.

The total mass of the duplex oxide after 72h in humidied airat 90% RH is around 1.3 g/cm 2 . This is signicantly less than thecorresponding oxide mass on the pure metals after the same expo-sures. The improved corrosion protective properties of the duplexoxides formed on brass may be attributed to a more amorphousCu2 O-oxide layerwith altered semiconducting properties, possiblydue to Zn-intermixing.

Acknowledgments

Financial support from the Chinese Scholarship Council (CSC)and from the Swedish Research Council (VR) is gratefully acknowl-edged. We are grateful to Fan Zhang and Eleonora Bettini (KTH)for performing SKPFM measurements, to Majid Sababi for metal-lography assistance (KTH), and to Prof. Inger Odnevall Wallinder(KTH), Prof. Jinshan Pan (KTH), and Dr. Harveth Gil (University of Antioquia, Colombia) for valuable suggestions.

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