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Corrosion behaviour of copper under chloride-containing thin electrolyte layer

Xiaoning Liao a,b, Fahe Cao a,⇑, Liyun Zheng a, Wenjuan Liu a, Anna Chen a, Jianqing Zhang a,c, Chunan Cao a,c

a Department of Chemistry, Zhejiang University, Hangzhou 310027, PR Chinab College of Science, Jiangxi Agricultural University, Nanchang 330045, PR Chinac State Key Laboratory for Corrosion and Protection, Institute of Metal Research, The Chinese Academy of Sciences, Shenyang 110016, PR China

a r t i c l e i n f o

Article history:Received 30 January 2011Accepted 5 June 2011Available online 13 June 2011

Keywords:A. CopperB. EISB. PolarizationC. Atmospheric corrosion

a b s t r a c t

The corrosion behaviour of copper under chloride-containing thin electrolyte layers (TEL) was investi-gated using electrochemical impedance spectroscopy (EIS), cathodic polarization, linear polarization,SEM/EDS and XRD. The results indicate that the copper corrosion rate increases as TEL thicknessdecreases during the initial stages. After 192 h of immersion, the corrosion rate of copper under TEL inthis order: 300 > 402 > 199 > bulk solution > 101 lm. The corrosion behaviour is uniform under TEL,and pitting is the primary corrosion type in the bulk solution. A corrosion model of the behaviour ofcopper under chloride-containing TEL is proposed.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Copper has long been employed as an outdoor building materialfor constructing statues, sculptures, and monuments because of itsgood mechanical properties and corrosion resistance. It is alsowidely used in the fabrication of metallic artefacts of artistic andcultural nature because of its ability to combine with otherchemical species to form a coloured layer on its surface [1]. Thiscoloured layer is of vital importance from the archaeological oraesthetic point of view, and its identification can help in under-standing the degradation process of Cu-based art and may providean appropriate restoration intervention method to conservators[2,3]. Moreover, this layer serves as a barrier against low aggressivemedia; this phenomenon is called the self-protective effect [4] andis the reason some copper-based cultural relics can last for severalhundred years.

Unfortunately, in spite of having this self-protective function,copper still suffers from serious damage in some highly aggressiveenvironments [5–7]. For instance, in an aerated chloride medium,the corrosion of copper occurs at a noticeable rate [6,8,9]. Overthe last several decades, the corrosion behaviour of copper inchloride bulk solutions has been extensively studied by a numberof researchers [10–13] and reviewed by Kear et al. [14]. Generally,in the presence of chloride ions, the electrochemical dissolution ofcopper undergoes a single electron process in accordance with thefollowing reactions [5,15]:

ðIÞ Cuþ 2Cl� $ CuCl�2 þ e� ð1Þ

ðIIÞ Cu$ Cuþ þ e� ð2ÞCuþ þ 2Cl� $ 2CuCl�2 ð3Þ

ðIIIÞ Cuþ Cl� $ CuCl� þ e� ð4ÞCuCl� þ Cl� $ CuCl�2 ð5Þ

Cases I and III present the direct formation of cuprous chloride(CuCl�2 ) from copper, whereas case II first involves the oxidation ofcopper to cuprous, followed by the reaction with the chloride ionsto form CuCl�2 . Although CuCl2�

3 and CuCl3�4 can be formed when

the concentration of the chloride ions is higher than 0.7 mol L�1,CuCl�2 is believed to be the main cuprous complex in seawaterand NaCl electrolytes containing Cl� concentrations of approxi-mately 0.55 mol L�1. The anodic dissolution of copper is dominatedby the movement rate of CuCl�2 away from the electrode surface tothe bulk electrolyte [16]. Through a precipitation reaction, CuCl�2 isconverted into cuprous oxide (Cu2O), which is then generally oxi-dized to cupric hydroxide (CuO) and atacamite [Cu2(OH)3Cl] inthe presence of chloride ions [14]. Although the corrosion mecha-nisms of copper in a bulk solution containing chloride ions are wellestablished, the corrosion behaviour of copper in an atmosphericenvironment containing chloride is still unclear for some technicalreasons [17,18].

Metal corrosion behaviour in atmospheric environments issignificantly different from that in bulk solutions. Atmosphericcorrosion is an electrochemical process occurring on a metal sur-face covered with a thin electrolyte layer (TEL) [19,20]. Over thelast decades, several laboratory investigations on metals or alloysunder TELs have been conducted to determine the significantfactors initiating metal corrosion in atmospheric environments

0010-938X/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.corsci.2011.06.004

⇑ Corresponding author. Tel.: +86 571 87952318; fax: +86 571 87951895.E-mail address: [email protected] (F. Cao).

Corrosion Science 53 (2011) 3289–3298

Contents lists available at ScienceDirect

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[21–34]. For copper, a classic laboratory atmospheric corrosionstudy was performed by Leygraf et al. [35]. In the current study,the effect of humidity on the corrosion behaviour of copper wasinvestigated. The results show that increasing the relative humid-ity enhances the nucleation rate of oxide grains, thereby increasingthe formation rate of Cu2O. Huang et al. [36] also investigated theeffect of the relative humidity on the corrosion behaviour ofPrinted Circuit Board-Cu on an adsorbed TEL and found thatincreasing the relative humidity increases the cathodic currentdensity. However, in both studies the exact electrolyte layer thick-ness is unknown. The thickness of TEL always plays an importantrole in the corrosion behaviour of a metal. A change in the thick-ness of the electrolyte layer affects a number of processes, suchas the mass transport of dissolved oxygen, the accumulation ofcorrosion products, the hydration of dissolved metal ions, and soon. As reported by Frankel et al. [37], the solution layer thicknesshas a strong correlation with the oxygen-limiting current densityfor the Type 304L stainless steel. In our previous work [30,38],the oxygen reduction current is inversely proportional to the layerthickness in the range of 200–100 lm for aluminium alloys2024-T3, whereas for AM60 magnesium alloys cathodic reductionis inhibited with decreasing TEL thickness.

Over the last fifty years, the amounts of aggressive agents suchas Cl� and NOx in air gradually increased as the global atmosphericenvironment got worse [39,40]. The present work was undertakenin view of the limited knowledge on the atmospheric corrosionbehaviour of copper in aggressive media. The effect of the thick-ness of chloride-containing TELs on the corrosion behaviour ofcopper was investigated using cathodic polarization, electrochem-ical impedance spectra (EIS), and linear polarization. The corrosionmorphologies were investigated by scanning electron microscopy(SEM) and the corrosion products were determined using X-raydiffraction (XRD) and energy dispersive spectroscopy (EDS). Theresults of the current study may provide an insight on the corro-sion behaviour of copper in an atmospheric environment seriouslypolluted with chloride ions.

2. Experimental

2.1. Materials

A high purity Cu rod (>99.99 wt%) was used in the currentinvestigation. Each working electrode was mechanically cut fromthe high purity Cu rod and embedded into nylon, leaving a0.5 cm2 exposed surface area. Prior to each experiment, the surfaceof the working electrode was gradually ground down to 1000 gritusing sand paper, followed by fine polishing with a 2.5 lmdiamond paste. Subsequently, the surface was rinsed with distilledwater, degreased with acetone, and dried in a cool air flow.

The electrolyte used in the experiment was a 3.5 wt% NaClsolution (pH 6.70), which was prepared from deionized waterand analytical grade reagent NaCl (Sinopharm Chemical ReagentCo., Ltd., Shanghai, China).

2.2. TEL set-up

The configuration of the TEL electrochemical cell used in thecurrent study is presented in Fig. 1. In the design of the experimen-tal setup, the key point was to ensure that the electrolyte layerformed on the working electrode is even and stable. Thus, the cellwas fabricated and installed with high precision to meet therequirements. The working electrode was firmly fixed in the cell,leaving only the upper surface exposed. A platinum wire (0.5 mmdiameter) was fixed around the working electrode and positionedbelow the exposed surface and served as the counter electrode. Asaturated calomel electrode (SCE) was inserted into the bulk solu-tion and used as the reference electrode. The electrochemical cell,which was placed on a horizontal stage in a vacuum desiccator,was adjusted to the horizontal level using a water level, so thateven if the electrolyte layer on the working electrode was ultrathin, the counter and reference electrodes would still be immersedin the bulk electrolyte. This adjustment can minimize the ohmicdrop between the reference and working electrodes.

The TEL thickness was determined using equipment consistingof a sharp Pt needle, an iron support with a micrometer, and anohmmeter, as shown in Fig. 1a. The first value on the micrometerwas recorded when the Pt needle touched the electrode surface,whereas the second one was recorded according to the suddenchange in the radian on the electrolyte surface to improve theaccuracy of the thickness measurement. The thickness of the layerwas determined from the two values on the micrometer as de-scribed in our previous work [38]. This technique enabled the mea-surement of the TEL thickness with an accuracy of 5 lm.

A vacuum desiccator with an electrochemical cell was com-pletely covered with a lid (Fig. 1b) and a NaCl solution of the sameconcentration as the test solution was placed at the bottom of thevacuum desiccator to maintain the stability of the TEL thickness forlong immersion times during the electrochemical measurements.The inside of the vacuum desiccator was connected to the outsideby a capillary filled with 3.5 wt% NaCl solution to keep the oxygenor carbon dioxide concentrations constant in the vacuum desicca-tor during the electrochemical measurements.

2.3. Electrochemical measurements

Electrochemical measurements were conducted on a VMP2multichannel potentiostat (PARC, USA) and all potentials werereported with respect to SCE. Cathodic polarization curve tests un-der various TEL thicknesses were conducted starting from the open

Vacuum drier

Teflon

Couter electrode

Horizontal stage

Working electrode

Reference electrode

(b)

A

Micrometer

(a)Capillary

Fig. 1. Schematic diagram of the apparatus for (a) the determination of TEL thickness and (b) the electrochemical measurement in the TEL corrosion study.

3290 X. Liao et al. / Corrosion Science 53 (2011) 3289–3298


circuit potential (OCP) to the direction of the cathode, with a sweeprate of 0.5 mV/s. In this study, only the cathodic polarization wasconducted to avoid the experimental errors caused by anodicpolarization under TEL conditions; this concern was discussed ina previous work [30]. EIS measurements were conducted in the100 kHz to 10 mHz frequency range at the OCP with a potentialperturbation of ±5 mV. For the linear polarization measurements,a sweep range of �10 to +10 mV versus OCP at a sweep rate of0.5 mV/s was used, and the polarization resistance (Rp) was deter-mined from the slope of the E versus i curve in the vicinity of thecorrosion potential. All electrochemical measurements wereperformed at room temperature (20 ± 1 �C).

2.4. Characterization

X-ray diffraction (XRD) analysis was used to determine thecrystalline corrosion products formed on the Cu electrode surface.An ARL X’TRA X-ray diffractometer with CuKa radiation (k =0.15406 nm) and a 10�–80� scanning range was used. Identificationof the presented phases was carried out using the Joint Committeeon Powder Diffraction Standards (JCPDS) database. Examinations ofthe surface and cross-sectional morphologies of the samples wereconducted using an FEI SIRION-100 scanning electron microscope(SEM). The samples for the cross-sectional analysis were embed-ded in an epoxy resin and gradually ground down to 1000 grit withsand paper, and then fine polished using a 2.5 lm diamond paste.Subsequently, the samples were rinsed with distilled water anddried in a cool air flow. The chemical composition of the corrosionproducts on the surface or in-depth of the working electrode wasdetermined using a GENESIS 4000 energy dispersive spectroscopy(EDS) attached to the SEM.

3. Results and discussion

3.1. Electrochemical measurements

3.1.1. Cathodic polarization behaviourFig. 2a shows the cathodic polarization curves of copper covered

with various thicknesses of chloride-containing electrolyte layersafter immersion for 15 min. Based on a previous study on thecathodic reduction of oxygen on Cu in a NaCl solution [41], thecathodic polarization curves are divided into three regions: RegionI corresponds to the polarization region in the vicinity of the OCP;Region II, at the more negative potentials, is ascribed to oxygenreduction under a mass-transfer control; and Region III, rising atthe negative potentials between �1.08 and �1.21 V vs. SCE, isunambiguously attributed to the hydrogen evolution reactiongiven in Eq. (6):

2H2Oþ 2e! H2 þ 2OH� ð6Þ

In Region I, the cathodic polarization behaviour under TEL wasvery different from that in the bulk solution. In all TEL cases, a verypronounced peak superimposed upon the linear region for theoxygen reduction was detected at a somewhat irreproducible poten-tial of�0.28 ± �0.09 V vs. SCE. However, in the bulk solution, only alinear region for oxygen reduction was observed. Extension of theimmersion time from 15 to 35 min caused a peak at �0.26 V vs.SCE, as shown in Fig. 2b. According to Deslouis et al. [10], the peakin the range of �0.28 ± �0.09 V vs. SCE can be attributed to thereduction of CuCl formed during the electrode equilibrium period.These results indicate that the formation rate of CuCl or the coppercorrosion rate under the TELs is faster than that in the bulk solutionduring the initial stages. In Region II, the limiting current density fordissolved oxygen reduction (ilim;O2 ) increased with decreasingelectrolyte layer thickness, which is the same as that for copper in

Na2SO4 solutions [42]. As previously reported for the metals underTEL [30,43,44], the diffuse limiting current is the major parameteraffected by the electrolyte thickness in the cathodic polarization.The theoretical ilim;O2 for the one-dimensional diffusion can becalculated according to the following Nernst–Fick equation:

ilim;O2 ¼ nFDO2 O2½ �=d ð7Þ

where n is the number of the electrons involved in the oxygenreduction reaction, F is the Faraday constant, d is the diffusion layerthickness, and DO2 and [O2] are the diffusion coefficient and theconcentration of the dissolved oxygen in the electrolyte layer,respectively.

Eq. (7) suggests that the increase in ilim;O2 is caused by thedecrease in the d value resulting from the thinning of the electro-lyte layer.

In Region III, the potential at which the cathodic evolution ofhydrogen starts is designated as E2Hþ=H2

, which varies with theTEL thickness. The cathodic Tafel slope, b2Hþ=H2

, for hydrogenevolution also varies with the TEL thickness.

The ilim;O2 values measured at �0.8 V vs. SCE, as well as theE2Hþ=H2

and �b2Hþ=H2values calculated by fitting, are listed in

Table 1 to characterize further the polarization behaviour of cooperunder various TEL thicknesses. The ilim;O2 value increased withdecreasing thicknesses of the covered layers, indicating that thecathodic reaction rate was controlled by the oxygen diffusion tothe copper surface [42]. The E2Hþ=H2

value shifted and became morenegative and the �b2Hþ=H2

value increased with decreasing TELthickness, suggesting that the hydrogen evolution on the coppersurface is inhibited with decreasing TEL thickness. According to

-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

10-6

10-5

10-4

10-3

10-2

μm

+22H /H

1/b

+22H /H

E

Potential (V/SCE)

98 202 301 398 bulk

Cur

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den

sity

(A·c

m-2

)

III

II I

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.010-4

10-3

10-2

10-1

100

IIIII

Cur

rent

den

sity

(A

·cm

-2)

Potential (V/SCE)

15 min 35 min

I

Fig. 2. Cathodic polarization curves for copper (a) covered with various thicknessesof electrolyte layers after a 15 min immersion and (b) in the bulk solution atdifferent immersion times.

X. Liao et al. / Corrosion Science 53 (2011) 3289–3298 3291


King et al. [12], oxygen reduction is the primary cathodic reactionon the electrode in Regions I and II:

O2 þ 2H2Oþ 4e! 4OH� ð8Þ

Furthermore, ilim;O2 increased with decreasing TEL thickness,which resulted in a thinner electrolyte layer and a higher hydrox-ide ion (OH�) concentration on the electrode surface. When thecovered electrolyte layers were relatively thin, the OH� diffusionfrom the surface to the bulk solution was inhibited, further increas-ing its concentration on the copper surface. The higher OH�

concentration thus resulted in a lower hydrogen evolution forthe thinner TEL.

From the results of the cathodic polarization tests, the OH� con-centration on the copper surface increased with decreasing TELthickness and the cathodic reaction was controlled by the oxygendiffusion during the initial stages.

3.1.2. EIS behaviourEIS is a powerful tool in the investigation of the corrosion

behaviour that not only provides a non-destructive assessment ofthe corrosion rate, but also enables the determination of the corro-sion mechanism [45]. The EIS results for copper during immersionfor 288 h under various TEL thicknesses are presented in Fig. 3. Theimpedance measured in the initial stages for all TEL thicknessesexhibited a tail corresponding to the Warburg impedance in thelow frequency region of the Nyquist diagrams, indicating that adiffusion-controlled corrosion process occurred on the copper sur-face [46]. This diffusion process can be attributed to the transportof oxygen through the copper-solution interface based on thecombined results of EIS and the above-mentioned cathodic polari-zation. The Warburg impedance disappeared and most of the lowfrequency limits of impedance progressively increased with theincrease in the immersion time. The changes in the impedancespectra in both size and shape with the extension of the immersiontime, including the disappearance of the Warburg impedance andthe increase in the capacitive loop in size, show that the corrosionproduct layer was thickened and the protection afforded by thecorrosion layer was improved. Some of the low frequency limitsof the impedance under 199, 300, and 402 lm TELs did not exhibita progressive increase during the immersion times, indicating abreakdown of the protective layer [1]. From the Bode plots inFig. 3, most of the phase angles exceeded 45�. According toNishikata et al. [47], the model of the current distribution can beestimated from EIS. The current distribution is considered uniform,at least in the lower frequency region, if the phase angle went fur-ther than 45� when the frequency was scanned from high to low.Therefore, the uneven current distribution on the copper electrodein these EIS tests is negligible and the TEL thickness on the elec-trode can be considered uniform and contiguous [27].

In addition, the Bode plots in Fig. 3 show that three relaxationsat the initial stages occurred, and the number of relaxations degen-erated into two as the immersion time progressed. Two equivalentelectrical circuits are given in Fig. 4 and were employed to fit theEIS of the metal-TEL to account for the Cu corrosion behaviourunder TEL quantitatively. The first equivalent circuit (Fig. 4a) wasused to fit the EIS data displaying a Warburg impedance, whereas

the second one (Fig. 4b) was used for the EIS data only displayingtwo capacitive loops. In both circuits, Rs is the solution resistanceand the high-frequency circuit Rf–Cf corresponds to the capacitanceand resistance of the surface film mainly of the oxide layer; themedium–low frequency circuit Rct–Cdl is the double-layer capaci-tance and charge transfer resistance, and ZW represents theWarburg diffusion impedance appearing in the low-frequencyregion. For the capacitive loops, the coefficients nf and ndl representa depressed feature in the Nyquist diagram.

The reciprocals of Rct as a function of the immersion time andderived from the curve fitting are shown in Fig. 5. In the currentstudy, the reciprocal of Rct (but not that of Rp) was used to charac-terize the corrosion rate because of its close correlation with thecorrosion rate [45,48]. Apparently, the corrosion rates in all casesdecreased with the increase in the immersion time, suggesting thata protective layer was gradually formed on the copper surfaces. Bycomparing the corrosion rates at the same immersion time undervarious TEL thicknesses in the first 2 h (inset of Fig. 5), the corro-sion rates increased as TEL thickness decreased, which indicatesthat the copper corrosion at this stage was controlled by the oxy-gen diffusion through the water layer to the electrode surface. Asimilar phenomenon was also observed by Nishikata et al. [27].After long immersion times, the corrosion rates of copper underdifferent electrolyte layers did not increase with decreasingthickness of the covered electrolyte layers. When the immersiontime was longer than 192 h, the order of the corrosion rates was300 lm > 402 lm > 199 lm > bulk solution > 101 lm. This phe-nomenon can be explained as follows: after long immersion times,more corrosion products are formed on the electrode because thediffusion of the corrosion products and the metal ions from theelectrode (anodic process) is relatively easy for the relatively thickelectrolyte layers. Thus, the corrosion process is still controlled bythe cathodic reaction but the rate-determining step of the cathodicreaction is transformed from an oxygen diffusion to an electro-chemical reaction, as evidenced by the disappearance of theWarburg diffusion in Fig. 3. As a result, the corrosion rates in thebulk solution and under ca. 400 lm TEL are lower than that onca. 300 lm TEL. The anodic process was inhibited when the TELthickness was below 300 lm because the diffusion of the corrosionproducts or the chloride ions becomes more difficult in thinnerTELs, resulting in the inhibition of corrosion. According toTomashov’s model [49], the maximum corrosion rate is at the pointof the transition from cathodic to anodic control, which corre-sponds to the 300 lm critical thickness in the current study. Thecopper corrosion covered with a TEL less than 300 lm thick wasunder anodic control and the corrosion rate decreased withdecreasing TEL thickness. For the 101 lm thick TEL, the corrosionrate was even lower than that in the bulk solution, which isattributed to the strong inhibition of the anodic process. For sucha thin layer, the diffusion of the corrosion products and thechloride ions is extremely difficult, as indicated in the later EDSresults.

3.1.3. Linear polarization measurementsLinear polarization was also used to evaluate the corrosion pro-

cess of copper under chloride-containing TELs. The reciprocal ofthe polarization resistance (Rp), regarded as the corrosion rate,was plotted against the immersion time and is presented inFig. 6. The corrosion rates in all cases during the first 2 h decreasedwith immersion time, and the corrosion rates at the same immer-sion time increased with decreasing TEL thickness. This trend isconsistent with that of EIS. However, a discrepancy between linearpolarization and EIS measurements was observed and increasedwith time at 2–6 h immersion times, as shown in the inset of Figs. 5and 6. According to Mansfeld and Epelboin et al. [45,48], the exis-tence of an inverse proportionality relationship between Rp and the

Table 1Electrochemical parameters for the cathodic polarization curves of pure Cu coveredwith electrolyte layers of various thicknesses.

Thickness (lm) Bulk 398 301 202 98

ilim;O2(lA cm�2) 33.5 54.2 76.6 79.6 179.0

E2Hþ=H2(V/SCE) 1.08 1.10 1.12 1.14 1.21

�b2Hþ=H2(mV/dec) 217 311 349 309 451



10-2 10-1 100 101 102 103 104 1050

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/deg

ree

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Bulk

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-Z''/

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m2

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0 2000 4000 6000 8000 100000

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Ω· c

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10-2 10-1 100 101 102 103 104 1050

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101 μm

0 2000 4000 6000 8000 10000 12000 140000

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14000

Z'/Ω·cm2

-Z''/

Ω·c

m2

2h 36h 72h 144h 192h 288h

101μm

Fig. 3. Nyquist diagrams and Bode plots of copper covered with chloride-containing TEL of various thicknesses during 288 h of immersion.



corrosion rate of a metal is related to highly restrictive conditionssuch as the electrochemical process, the concentration of the com-pounds implied in the reactions, the distribution of the reactionson the surface, and so on. Meanwhile, this relationship betweenRct and the corrosion rate exists even in the presence of coverageand a concentration gradient because Rct is correlated with theelectrochemical reaction.

In the model reported in [14], the oxygen reduction on theCu2O-filmed copper occurred at both the oxide-electrolyte inter-face and the metal-electrolyte interface, indicating that the reac-tion distribution on a freshly polished copper surface changesafter long immersion times in a chloride medium. Consequently,the use of Rct derived from EIS rather than the Rp obtained from lin-ear polarization may provide a more accurate determination of thecopper corrosion rate under chloride-containing TELs for longexposure times.

3.2. Characterization

XRD, SEM, and EDS characterizations were performed toinvestigate further the corrosion behaviour of copper underchloride-containing TELs.

Fig. 7 shows the XRD spectra of the copper surface covered withelectrolyte layers of different thicknesses after 80 and 288 himmersion. Only crystalline cuprite (Cu2O) and nantokite (CuCl)were detected on the copper surfaces for all cases after immersionfor 80 h. Atacamite [Cu2(OH)3Cl] and malachite [Cu2CO3(OH)2],together with Cu2O and CuCl, appeared at ca. 200, 300, and400 lm, respectively, after immersion for 288 h, which is in agree-ment with that of Leygraf’s work performed in an outdoor marineenvironment [50]. This result indicates that the protective Cu2Olayer was partly destroyed in these cases. However, no significantdifference in the surface composition between these two immer-sion times in the bulk solution or under a ca. 100 lm TEL wasfound, implying that no significant change in the corrosionproducts occurred with time. SEM/EDS characterization was thusperformed to investigate the corrosion process further.

Table 2 shows the EDS results of the chemical composition ofthe corrosion products on the copper surface covered withchloride-containing TEL of various thicknesses after 2 h of immer-sion. Only oxides and copper were detected, suggesting that Cu2Ois the primary corrosion product during the first 2 h of immersion.The oxide concentration increased with decreasing the electrolytelayer thickness, indicating that the copper corrosion is acceleratedby reducing the electrolyte layer thickness. The result is in goodagreement with that of EIS, as shown in Fig. 5 and further supportsthe assumption that oxygen gas diffusion through the solutionlayer is the rate determining step for copper corrosion under TELsduring the first 2 h (as discussed in Section 3.2).

The visual appearances of all samples were similar after 80 himmersion, and a homogeneous yellowish-red layer developedon all surfaces. A significant difference was found using SEM. Asshown in Fig. 8a, a pitting corrosion occurred on the surface ofcopper immersed in the bulk solution. For the copper under theTELs (Fig. 8b–e), no pitting was observed as the numerous corro-sion products were deposited on the copper surfaces. Evidently,the amounts of corrosion products increased with decreasing TELthickness. EDS results in Table 3 reveal that the corrosion productsmainly consist of Cu, O, and very small amounts of Cl. Therefore,the corrosion products are primarily Cu2O and CuCl after 80 hexposure to NaCl solutions. Moreover, compared to their respective2 h counterparts, the oxygen content of the corrosion productsincreased, suggesting that the thicknesses of the Cu2O protectivelayer were increased after 80 h of immersion. Therefore, the coppercorrosion rate was decreased with time, as shown in Fig. 5.

A number of marked changes in the visual appearance ofsurfaces occurred when the immersion time was extended to228 h. For the copper surfaces covered with 402, 300, and199 lm TEL, the colour changed from yellowish-red to greenishwith various degrees compared to their 80 h counterparts, but nosignificant change occurred for the copper in the bulk solution un-der the 101 lm TEL. However, several big holes developed on thecopper surface in the bulk solution (Fig. 9a), which are caused by

Rct

Rf

Rs Cf, nf

Cdl, ndl

ZW

Cf, nf Rs

Rf

Rct

Cdl, ndl

(a)

(b)

Fig. 4. Equivalent circuit models used to fit the experimental impedance data forcopper covered with chloride-containing TELs (a): for fitting the data displaying theWarburg impedance, (b): for fitting the data without displaying the Warburgimpedance.

0 50 100 150 200 250 300

10-4

10-3

10-2

0 2 4 6

0.0004

0.0008

0.0012

0.0016

t (h)

1/R

ct(Ω

- 1 ·cm-2

)

1/R

ct(Ω

-1·c

m-2

)

t (h)

101 199 300 402 bulk

Fig. 5. Plot of the corrosion rate (1/Rct) as a function of the immersion time ofcopper covered with chloride-containing TELs of different thicknesses.

0 1 2 3 4 5 6

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0.0008

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p (

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t (h)

98 200 295 398 bulk

Fig. 6. Plot of the reciprocal of the polarization resistance as a function of time forcopper covered chloride-containing TELs of various thicknesses.



the merging of multiple pits. The corrosion products on the coppersurfaces covered with approximately 400, 300, and 200 lm TEL atthis immersion time were thicker than their respective counter-parts at 80 h. EDS results in Table 4 reveal that the C and Clamounts of the three samples increased compared with their80 h counterparts (See Tables 3 and 4), especially for the 300 lmTEL, indicating the appearance of corrosion products Cu2(OH)3Cland Cu2(OH)2CO3, which is in accordance with the XRD results.Cu2(OH)3Cl and Cu2(OH)2CO3 cannot provide good protection tothe substrate because of their loose microstructure [51], which ac-counts for the maximum value of the copper corrosion rate underthe 300 lm TEL, as determined by EIS measurement after a longimmersion time. In addition, the total amounts of C, O, and Clcalculated from Table 4 in the bulk solution under the 402, 300,199, and 101 lm TELs are 29.85, 33.87, 40.95, 31.16, and22.33 wt%, respectively. If the total amount of C, O, and Cl areindicative of the corrosion degree, the order of corrosion degreeis 300 lm > 402 lm > 199 lm > bulk solution > 101 lm. This result

is in good agreement with that of EIS, confirming that the corrosionrate obtained by EIS is accurate.

The corrosion products on the copper surfaces were removedwith a brush and a typical SEM image for the 302 lm TEL is pre-sented in Fig. 9f to verify further the corrosion behaviour of copperunder on TELs. Evidently, no pit was found after the removal of thecorrosion products, which indicates that copper corrosion under aTEL is a uniform corrosion process. A corresponding EDS analysisconfirms that a Cu2O layer was formed in the inner surface.

Fig. 10 presents the cross-section SEM images, along with thecorresponding in-depth linear scan profiles of copper covered withdifferent thicknesses of chloride-containing TEL after 288 h ofimmersion. A linear scan was performed from the direction ofthe epoxy to the substrate, as indicated by the arrows. Fourelements, namely O, Cu, Cl, and C, are present in all the corrosionlayers. The in-depth linear scan line profiles (Fig. 10b), togetherwith the aforementioned SEM surface analysis, indicate that a

Fig. 7. XRD spectra of the copper surfaces covered with TELs of different thicknesses after (a) 80 and (b) 288 h of immersion.

Table 2EDS results of the chemical composition of the corrosion products on the coppersurface after 2 h immersion covered with chloride-containing TELs of variousthicknesses.

Element (Wt%) Bulk 401 302 201 99

Cu 97.88 97.29 97.29 96.42 95.76O 2.12 2.71 2.71 3.58 4.24

Fig. 8. SEM images of the copper surface after 80 h immersion time (a) in the bulk solution, and on (b) 401, (c) 298, (d) 201, and (e) 99 lm chloride-containing TELs.

Table 3EDS results of the chemical compositions of the corrosion products on the coppersurface after 80 h immersion covered with chloride-containing TELs of variousthicknesses.

Element (Wt%) Bulk 401 298 201 99

Cu 95.54 89.19 88.93 86.13 91.12O 3.88 9.92 8.61 10.42 8.11Cl 0.58 0.89 2.46 3.45 0.77



duplex-layer structure was formed on the copper surface for eachsample: the inner layer, in contact with the copper substrate,

consists of Cu2O, and the outer layer, which was in contact withthe environment, is a mixture of Cu2(OH)3Cl and Cu2 (OH)2CO3.These results are in agreement with previous studies in a marineenvironment [52]. This duplex-layer structure was especially welldefined under the 302 lm TEL, as indicated in Fig. 10b. In addition,the thicknesses of the corrosion layers for copper in the bulk solu-tion on 302 and 99 lm TEL are approximately 5, 30, and 6 lm,respectively. Previous studies have shown that the runoff rate inthe atmospheric environment is significantly lower than the corro-sion rate [18,53]. The runoff amounts of the products can thus beneglected under TEL compared with the deposited ones. Therefore,the copper surface covered with the 302 lm TEL suffered theheaviest corrosion attack, which is in agreement with the EISresults.

4. Mechanisms of copper corrosion

The evolution of copper corrosion under TELs containingchloride ions is schematically presented in Fig. 11 based on theelectrochemical behaviour and the morphologies of the surfaceand cross sections. The Cu evolution model in the bulk solutionis also shown for comparison.

During the initial stages of immersion (2 h), the electrochemicalcopper dissolution in all cases mainly occurred at the grainboundary of the freshly polished copper [54] in the form of Cu+.Cu+ was transformed into the slightly soluble CuCl and solubleCuCl�2 in the presence of chloride ions, and most of the CuCl�2 werefurther converted to Cu2O via precipitation as follows [15]:

2CuCl�2 þ 2OH� $ Cu2OþH2Oþ 4Cl� ð9Þ

The Cu2O layer, which is a p-type semiconductor and thus haslow electronic conductivity, was mainly responsible for the highresistance [55]. Therefore, a decreasing corrosion rate (Figs. 5 and6) with immersion time is observed in all cases because of theincreased thickness of the Cu2O layer. The amount of Cu2O layerincreased with decreasing TEL thickness, because the equilibriumin Eq. (9) was shifted to the right as the local concentration ofOH� increased and Cu2O was deposited in response. However,during the initial stages, only a small amount of corrosion productwas deposited on the copper surface; thus, the anodic reaction wasnot sufficiently inhibited. Therefore, copper corrosion was mainlyinfluenced by oxygen diffusion during the initial stages.

Fig. 9. SEM images of the copper surfaces after 288 h immersion time. (a) in thebulk solution and on (b) 402, (c) 300, (d) 199, (e) 101, and (f) 300 lm chloride-containing TEL after the removal of the outer-surface corrosion products.

Table 4EDS results of the chemical compositions of the corrosion products on the coppersurface after 288 h immersion time covered with chloride-containing TEL of variousthicknesses.

Element (Wt%) Bulk 402 300 199 101

Cu 70.15 66.13 59.05 68.84 77.67O 13.69 31.23 14.52 18.29 9.49Cl 7.26 8.98 9.48 3.77 0.84C 8.90 9.65 16.95 9.30 12.00

(a) (b) (c)resin resin resin

atina

substrate substrate substrate

atinaatina

20μm 20μm 20μm

0 5 10 15 20 25 30 35 40 45 50

0

50

100

150

200

250

Inte

nsit

y (a

.u.)

Distance (μm)

C

O

Cl

Cu

0 5 10 15 20 25 30 35

0

50

100

150

200

Inte

nsit

y (a

.u.)

Distance (μm)

C

O

Cl

Cu

0 5 10 15 200

50

100

150

200

250

Inte

nsit

y (a

.u.)

Distance (μm)

C

O

Cl

Cu

Fig. 10. Cross-sectional BEI-SEM images of Cu corrosion and the corresponding in-depth elemental distribution (see the respective bottom right image) (a) in bulk solution,and on (b) 302 and (c) 99 lm chloride-containing TELs.



Pitting occurred in the bulk solution whereas no pitting wasobserved under the TELs when the immersion time was increasedup to 80 h. Pitting corrosion is a very complex phenomenon thatpotentially involves several initiating factors, including pH,temperature, water chemistry, and other variables. Fully under-standing this process is still a challenge despite the numerousstudies that have been conducted [56]. Using cyclic voltammetry,Cong et al. [57] investigated the effect of water chemistry onpassivity, uniform corrosion, and pitting. The results revealed thathigh OH� concentration improves passivity and raises the pittingpotential. Therefore, OH� also plays a key role in copper corrosionunder TELs. In the bulk solution, the OH� concentration producedby the cathodic reaction was relatively low, as discussed in Section3.1.1, and the OH� easily diffused from the copper surface com-pared to the cases under TEL. As a consequence, the OH� concen-tration on the copper surface was relatively lower, resulting in alower deposited Cu2O amount in the bulk solution; the sites cor-roded by the chloride ions could not be quickly healed. Therefore,pitting increased with immersion time in the bulk solution. On theother hand, the defective sites were quickly healed under the TELswith the rapid precipitation of Cu2O because of the high OH�

concentration. Consequently, pitting was inhibited and thecorrosion proceeded uniformly.

Cu2(OH)3Cl and Cu2(OH)2CO3 appeared in all samples after along immersion time (288 h). The chloride ions caused the break-down of the passive Cu2O film on the copper surface because ofthe formation of soluble species, such as CuCl or CuCl�2 [9,58].

Cu2Oþ 4Cl� þ 2Hþ ! 2CuCl�2 þH2O ð10Þ

The CuCl�2 produced was further oxidized into Cu2(OH)3Cl. Theformation of Cu2(OH)3Cl resulted in a net release of OH� on thecopper surface according to Eq. (11).

2CuCl�2 þ O2 þ 2H2O! 2Cu2ðOHÞ3ClðsÞ þ OH� ð11Þ

Increasing the pH caused increased the dissolution of CO2 onthe electrolyte surface [58].

CO2ðgÞ þH2O! 2Hþ þ CO23 ð12Þ

Thus, Cu2ðOHÞ3Cl was transformed into the more stableCu2(OH)2CO3 [9].

5. Conclusions

The corrosion behaviour of copper under chloride-containingTELs of various thicknesses was investigated using cathodic polar-ization, EIS, and linear polarization. The results show that duringthe initial immersion stage (2 h), the copper corrosion rate underthe TELs increased with decreasing TEL thickness. When theimmersion time was longer than 192 h, the order of the corrosionrate was 300 lm > 402 lm > 199 lm > bulk solution > 101 lm.These results were confirmed by XRD and EDS. A morphologicalstudy using SEM revealed that copper corrosion under thechloride-containing TELs is uniform and is significantly differentfrom that in the bulk solution. At the initial immersion time, thecorrosion product on the copper surface was primarily Cu2O, andits amount increased with decreasing TEL thickness. After a longimmersion time, the protective Cu2O layer suffered various degreesof attack (depending on the TEL thickness) and was converted intoCu2(OH)3Cl and Cu2(OH)2CO3 in the presence of chloride ions. Thecorrosion layer formed is a duplex structure, consisting of an innerlayer corresponding to Cu2O, which adheres to the substrate,and an outer layer containing a mixture of Cu2(OH)3Cl andCu2(OH)2CO3. The corrosion system employed in the currentresearch is useful for the rapid evaluation of the corrosion processof copper-based outdoor structures in a marine atmospheric envi-ronment and for mechanistic studies.

Acknowledgements

The authors wish to acknowledge the financial support of theNational Natural Science Foundation of China (No. 50801056),Zhejiang Provincial Natural Science Foundation of China (No.Y4110074) and Doctoral Fund of Ministry of Education of China(No. 200803351092).

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2O2O

-2 2O 2H O 4e 4OH+ + →

2O

-2CuCl

-OH

2Cu O

Cl-

Substrate

Patina

2O2O

TEL

23C O −

2O-2C u C l

2C u O

bulk Substrate

-2CuCl

Cl-Cl-

Uniform corrosion

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Substrate

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