Corrosion Science 74 (2013) 44–49

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Corrosion Science

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Ultrafine-grained copper produced by machining and its unusualelectrochemical corrosion resistance in acidic chloride pickling solutions

0010-938X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2013.04.007

⇑ Corresponding author. Tel.: +86 20 87114634; fax: +86 20 22236360.E-mail address: dengwj@scut.edu.cn (W. Deng).

Wenjun Deng a,⇑, Ping Lin a, Qing Li a, Guangquan Mo b

a School of Mechanical & Automotive Engineering, South China University of Technology, Guangzhou 510641, PR Chinab Department of Chemistry, Guangzhou Medical College, Guangzhou 510182, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 October 2012Accepted 6 April 2013Available online 22 April 2013

Keywords:A. CopperB. PolarizationB. EISC. Acid corrosion

Ultrafine-grained (UFG) copper was prepared by facile machining procedure. High resolution transmis-sion electron microscopy images revealed that, in UFG Cu, minimum grain size of 80 nm could be formedwhen a small machining rake angle was applied. The electrochemical corrosion behavior of UFG Cu in0.5 M HCl was investigated by potentiodynamic polarization and electrochemical impedance spectros-copy. Comparing with coarse-grained Cu, UFG Cu exhibited notably declined corrosion current density.Particularly, when the size of Cu grains were reduced from 500 lm to 80 nm, the charge transfer resis-tance of anodic dissolution step dramatically increased from 200 to 621 X cm2.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Copper has been widely used in industry and microelectronicapparatus including heat exchangers, wiring technology, electro-magnetic interference shielding and electrostatic dissipation tech-nology because of its excellent mechanical workability and goodelectrical and thermal conductivities [1]. Developments in portableelectronic devices have resulted in the increasing use of Cu in cor-rosive environments such as marine and urban applications withexposure to high levels of pollution [1]. Despite the relatively highstandard electrode potential of this metal, corrosion occurs at a sig-nificant rate under such strenuous conditions. Therefore, corrosioninhibition of Cu is an important field of research and there havebeen studies worldwide in this area to alleviate Cu corrosion [2,3].

Currently, one of the most effective measures to prevent Cucorrosion is the using of organic corrosion inhibitors [2]. N-hetero-cyclic azole derivatives such as benzotriazole, mercaptobenzothia-zole, benzimidazole and imidazole are typical and efficientcorrosion inhibitors for Cu [3,4]. The corrosion inhibiting actionof these organic compounds could be attributed to their interac-tions with the Cu surface via adsorption [2]. Surface adsorptionleads to the formation of polymeric complexes, which in turn forman adherent protective film on the metal and act as a barrier toaggressive ions such as chloride [2]. However, the adsorption ofthe inhibitor on the surface usually depends on the nature andthe surface charge of the metal, the adsorption mode, the inhibi-tor’s chemical structure and the type of the electrolyte solution

[3,4]. Besides, most organic inhibitors are harmful, which mightcause serious environment pollution [2,4]. Novel strategy for Cuprotection continues to be a big challenge in metal protection engi-neering [3].

Recently, transforming or further refining the microstructure ofmetals has been reported as another promising strategy to improvemetal corrosion resistance [5–15]. To reconstruct the originalcoarse-grained (CG) structure in raw materials, severe plasticdeformation (SPD) technique is used to obtain a nanocrystallineor sub-microcrystalline structure [5,7,10,12,14]. The significantgrain size reduction, in combination with higher dislocation den-sity after SPD, could result in some unique physical and mechanicalproperties [16]. Properties such as intergranular corrosion andstress corrosion cracking (SCC) of these ultrafine-grained (UFG)materials in elevated environment have elicited significant atten-tion in recent times [5,12,14]. The beneficial effect of grain refiningin reconstructing process was initially demonstrated for intergran-ular corrosion in electrodeposited nanocrystalline nickel in the pio-neering work of Rofagha et al. [17], and followed by Kim et al. [18].Both of them observed a smooth surface with a smaller penetrationrate at grain boundaries and thus notable improvement in inter-granular corrosion resistance [17,18]. Vinogradov et al. firstly re-ported the anodic polarization behavior of SPD-fabricated UFGCu, and found that the corroded surface of UFG Cu appearedsmooth with shallow corrosion grooves at grain boundarieswhereas deeper grooves were formed at the grain boundaries inCG Cu [5]. Susceptibility to localized corrosion, the most dangerousform of environmental degradation, was found to be lower in UFGCu [5]. Yamasaki et al. reported that, in 1 M NaNO2 aqueoussolution, UFG Cu materials produced by equal-channel angular

Fig. 1. Schematic representation of machining for preparing UFG Cu chip.

W. Deng et al. / Corrosion Science 74 (2013) 44–49 45

pressing (ECAP) technique showed notably better resistance to SCCthan their CG counterpart [6]. The corrosion behavior of ECAP-pro-duced UFG Cu in a modified Livingstone etchant was described byVinogradov et al., which showed that, comparing with its CG coun-terpart, UFG Cu exhibited a rather small anodic corrosion current[5,12].

With respect to the SPD technique, ECAP is a common andwidely used procedure. ECAP is often used to prepare UFG puremetals or alloys [16,19–21]. Despite of the widespread applicationsof ECAP, current approaches still have the following limitations: (i)multiple stages of deformation are needed to create large plasticstrains, (ii) high strength metals and alloys are difficult to deformdue to constraints imposed by the forming equipment and (iii)there are uncertainties in the knowledge and control of deforma-tion field parameters [19,22]. A potential attractive process involv-ing large strain and high-strain rates in a single stage ofdeformation, while simultaneously overcoming the aforemen-tioned limitations, is the process of chip formation by machining[22–25]. Machining could be considered as a promising SPD tech-nique for mass production of UFG materials [22,23]. However, re-cent investigations on machining-produced materials have onlyfocused on structural characterization, thermal stability, elasticand damping properties, microhardness, compression and tensilebehavior and fatigue [22,23]. To the best of our knowledge, the cor-rosion resistance of machining-produced materials has not beenstudied yet. The characterization of corrosion behavior of thesematerials is important both for prospective engineering applica-tions and for better understanding of their fundamental physicalproperties. Previously, we reported the formation of UFG pure met-als or alloys by facile machining and the characteristics of thedeformation fields [22,23]. A series of experiments were conductedwith pure Cu, mild carbon steel and Ti6Al4V alloy to study struc-ture refinement and deformation behavior in machining. Finite ele-ment models were developed to characterize the deformation fieldassociated with chip formation in plain orthogonal machining[22,23]. In this work, the effect of machining process on the micro-structure, particularly, the grain size of UFG Cu chip has been stud-ied in detail. The electrochemical corrosion behavior of machining-produced UFG Cu chip in acidic chloride pickling solutions (0.5 MHCl) has also been studied by potentiodynamic polarization andelectrochemical impedance spectroscopy (EIS).

2. Experimental

2.1. Materials and reagents

Commercial purity (99.96%) Cu rod with a diameter of 40.5 mmwas employed as the raw material and served as the workpiece.Metallographic emery paper and alumina slurries were purchasedfrom local agents. Transparent metallographic resin and all otherchemicals with analytical grade were from local chemical agents.Ultra-purified water (18.4 MX cm) was used throughout.

2.2. Copper chip and electrode preparation

To obtain UFG Cu chip, machining process was performed on Curod workpiece. Fig. 1 shows a representative machining process onCu workpiece. It can be seen that the cutting tool is fixed with cer-tain inclined rake angle, and Cu rod workpiece is moving forwardat a certain cutting velocity. The cutting velocity (V) was kept atvery low level (0.052 m s�1) to minimize any temperature influ-ences. During a machining process, with concentrated shearing,Cu chip formation occurred within a narrow severe plastic defor-mation zone (Fig. 1). The geometry of the primary deformationzone and shear strain depended on the shear angle and the rakeangle. Severe plastic deformation could refine the microstructure

of Cu workpiece. To investigate the effect of rake angle on the grainsize of Cu chip, rake angles of 20�, 10� and 0� were employed toprepare 20 D, 10 D and 0 D Cu chip samples. The thickness of eachsample was about 3 mm. These samples were randomly takenfrom a consecutive machining process, so that the observed resultcould represent the refined Cu materials produced by machining.

For the preparation of Cu chip electrode, the as-prepared samplewas connected to a Cu wire and embedded in a transparent metallo-graphic resin, dried at 60 �C for 1 h. The working interface of Cu chipelectrode was wet-abraded by successive emery paper treatments upto No. 2000 grade until a smooth and planar surface (10� 3 mm2)was obtained. After that, the resulting Cu chip electrode was polishedwith 0.3 and 0.05 lm alumina slurry repeatedly, and then sonicatedfor 5 min in water and acetone successively. After the natural evap-oration of solvent, the as-prepared 20 D, 10 D and 0 D working elec-trodes were immersed into acidic chloride pickling solutions (0.5 MHCl) for electrochemical measurements. For comparison, a smallsample was taken Cu rod directly. Then, pristine Cu electrode withoutmachining treatment was also prepared by the same procedure, anddenoted as P–Cu electrode.

2.3. Instruments and procedure

Optical microscopy photos were obtained by using LeicaDMI5000M (Leica, Germany). High resolution transmission elec-tron microscopy (HR-TEM) investigations were performed onCM-300 (Philips, Netherlands). All electrochemical tests were per-formed on CHI660D electrochemical work station (Shanghai, Chi-na) with typical three-electrode system consisted of Cu workingelectrode, Ag/AgCl (3 M KCl) reference electrode and Pt auxiliaryelectrode. The electrolyte was 0.5 M HCl and used without any pre-treatment. Potentiodynamic polarization curves were obtained bysweeping potential from �0.4 to 0.1 V vs. Ag/AgCl at a scan rateof 1 mV s�1. EIS measurements were performed at the open circuitpotential over a frequency range of 100 kHz–0.1 Hz, with an ACwave of ±5 mV peak-to-peak overlaid on a DC bias potential, andthe impedance data were collected at a rate of 12 points per decadechange in frequency. The open circuit potential was obtained by a30 min measurement on working electrode. Electrochemicalexperimental results were reproduced and each experiment wascarried out three times where a very good agreement was ob-tained. Stable and representative potentiodynamic polarizationTafel plots and EIS curves were recorded and presented finally.

46 W. Deng et al. / Corrosion Science 74 (2013) 44–49

All the measurements were performed at room temperature(25 ± 1 �C).

3. Results and discussion

3.1. Morphology characterizations

In order to understand the influence of machining treatment onthe microstructure and grain boundary distribution, machining-produced Cu was examined by optical microscopy and HR-TEM.Fig. 2A shows the optical microscopy image of untreated Cu rawmaterial. It can be seen that the grain boundary and individualcoarse grain are well-defined (Fig. 2A). The size of coarse grain inbulk Cu is widely distributed between 10 and 500 lm indicatingthat bulk Cu raw material is composed of large equiaxed grains.Microscopic image of typical sample from a machining processwas shown in Fig. 2B. It can be seen that the bottom of Cu work-piece without machining treatment is still with the characteristicof pristine CG Cu. However, SPD of Cu caused by the concentratedshearing of cutting tool at the primary deformation zone forces thecoarse grain to undergo a shape change. Cu coarse grain becomesmore elongated along the direction of strain gradient (Fig. 2B). Itis obvious that the grain size and shape in the shearing Cu chipis different from that in the bulk phase of Cu raw material. A lackof visible grain size in the Cu chip suggests that the grain size is ex-tremely small (Fig. 2B). The grain size and boundary are not clearbut the sub deformation band with different width can be observed

Fig. 2. (A) Optical micrograph prior to machining; (B

Fig. 3. TEM images of Cu chips produced b

as the ‘‘flow-line’’ type to some extent. These results indicate thatSPD of machining on Cu raw material could result in a severe grainsize reduction in Cu chip.

Fig. 3 shows the HR–TEM images of 20� and 0� Cu chips. One cansee that, comparing with CG Cu (Fig. 2A), the machining cuttingprocess can lead to a significant grain size reduction in combina-tion with higher dislocation density. Fig. 3A shows the stria grainsin UFG Cu produced by deforming and elongating along the direc-tion of chip flow when the tool rake angle is 20�. The grain width iswith a large distribution ranging from 80 to 200 nm. The elongateddislocation structure is the typical feature of the deformationmicrostructure when a large rake angle is applied [22,23]. A fairlyuniform microstructure is presented in 0� specimen, Fig. 3B. Thegrains with the size ranging from 80 to 200 nm are nearly equiaxialand separated by boundaries creating long-range strain fields(Fig. 3B). Minimum grain size in 0� specimen is 80 nm, which iseven smaller than those of the ECAP-produced samples [6]. Dislo-cations tend to accumulate in the vicinity of interfaces. Smallergrains are with a lower dislocation density. Chaotically distributeddislocations are visible in larger grains. This effect might be causedby the large shear strain at small tool rake angle [22,23]. When therake angle of cutting tool was decreased from 20� to 0�, strongshear strain compelled most of the elongated dislocation cells tosubdivide into smaller sub-grains [23]. The significant size reduc-tion and typical interface are often regarded as a non-equilibriumgrain boundary [9–11] and supposed to strongly affect the proper-ties of UFG materials [6].

) optical micrograph of the machined Cu sample.

y the rake angel of 20� (A) and 0� (B).

W. Deng et al. / Corrosion Science 74 (2013) 44–49 47

3.2. Potentiodynamic polarization behavior

The significant grain size reduction in combination with higherdislocation density in machining-produced UFG Cu can result insome unique properties [5]. The effect of grain size reduction onthe electrochemical corrosion behavior of machining-producedCu in acidic chloride pickling solutions (0.5 M HCl) was investi-gated through potentiodynamic polarization experiments. Fig. 4presents the Tafel plots recorded on P–Cu, 20 D, 10 D and 0 Dworking electrodes. For the untreated raw P–Cu electrode (Fig. 4,dotted line), it can be seen that the cathodic branch shows a dis-tinct Tafel region between �0.4 and �0.2 V vs. Ag/AgCl, a lower po-tential less than �t0.4 V vs. Ag/AgCl results in a current eruptioncausing by hydrogen evolution in acidic chloride solution [26].Notably, as for the anodic branch (Fig. 4, dotted line), the anodicdissolution current of Cu increases rapidly with the positive shift-ing of applied potential and shows three distinct regions. The ano-dic branch of P–Cu electrode is similar to the previous resultsreported by Sherif et al. [4,26]. According to their results [26], theseregions could be described as (i) the Tafel region at lower overpo-tentials extending to the peak current density at about 0.05 V vs.Ag/AgCl due to the fast dissolution of Cu into Cu+, then slow oxida-tion of Cu+ to Cu2+ increasing the applied potential, (ii) the regionof decreasing current until a minimum value is reached at about0.08 V vs. Ag/AgCl due to the formation of CuCl, because Cu+ reactsfaster with Cl� than Cu2+ and (iii) the region of sudden increase incurrent density due to the decrease in adhesion of the adsorbedCuCl layer because of the acidic solution, and resulting formationof soluble Cu complex, CuCl�2 . It has been reported that the forma-tion of CuCl�2 and its diffusion from the surface to bulk solution, orits further oxidation to cupric ions are the main reasons for Cu cor-rosion [4]. The corrosion current density (jcorr) and corrosion po-tential (Ecorr) parameters can be obtained from the extrapolationof anodic and cathodic Tafel lines located adjacent to the linearizedcurrent regions. jcorr and Ecorr for P–Cu electrode are 15.85 lA cm�2

and �0.16 V vs. Ag/AgCl, respectively. These values are also similarto results reported earlier by Sherif et al. [4,26], which indicates thenatural electrochemical corrosion behavior on Cu raw materialwith coarse grains.

In the case of all machining-prepared UFG Cu electrodes (20 D,10 D and 0 D), however, it can be seen from Fig. 4 that both anodic

Fig. 4. Potentiodynamic polarization Tafel plots in 0.5 M HCl recorded on P–Cu(dotted line), 20 D (dot-dash line), 10 D (dash line) and 0 D (solid line) workingelectrodes.

current density in three distinct regions and cathodic current den-sity are remarkably lower than that of P–Cu electrode. For thecathodic region of Tafel plot, there is little effect of different rakeangle on cathodic current density. Nevertheless, when the rake an-gle of cutting tool changes from 20� to 0� causing a significant grainsize reduction, the anodic branch of Tafel plot drops sharply. Com-paring with P–Cu, 20 D and 10 D electrodes, the 0 D electrode(Fig. 4, solid line) displays the lowest current density in the anodicTafel region when the potential is between �0.16 and 0 V vs. Ag/AgCl, which indicates a decreased dissolution of Cu into Cu+ [26].When the potential is above 0 V vs. Ag/AgCl (Fig. 4, solid line),the current density on 0 D electrode due to the oxidation of Cu+

to Cu2+ increases rapidly [4]. This result indicates that the formerproduced Cu+ could be immediately oxidized to Cu2+. The secondregion of decreasing current density due to the formation of CuCldisplays a minimum value at about 0.05 V vs. Ag/AgCl. A minimumjcorr of 5.89 lA cm�2 is seen for 0 D electrode (Fig. 4, solid line),which is almost the same as previous reports using 2-amino-5-ethyl-1,3,4-thiadiazole [28], 5-(3-aminophenyl)-tetrazole [4], N-phenyl-1,4-phenylenediamine [26] or 3-amino-1,2,4-triazole-5-thiol [27] as effective corrosion inhibitor. Besides, this result iseven comparable to recent works using other corrosion inhibitors,especially in such a severe ambient [29–31]. Reducing the grainsize of Cu could efficiently enhance the electrochemical corrosionresistance. Similarly, Yamasaki and co-workers reported thatECAP-produced UFG Cu materials displayed notably better resis-tance to SCC compared to their CG counterpart [6]. Miyamotoand co-workers found that UFG Cu exhibited a lower corrosion cur-rent compared with its recrystallized CG counterpart.

The corrosion mechanism in UFG materials is still not clear. Bal-yanov and co-workers suggested that the higher resistance to cor-rosion in the UFG state could be attributed to a higher passivationrate and lower amount of impurities segregated at grain bound-aries [14]. Intergranular corrosion is usually induced by impuritysegregation and precipitation at grain boundaries. Defects suchas dislocations and grain boundaries have also intrinsic susceptibil-ity to local attack, and reactivity of these defects increases withincreasing extra free energy associated with intrinsic structuraldisorder. In thermodynamic consideration, the extra free energylowers half-cell electrode potential, resulting in greater tendencyfor electrochemical dissolution in certain corrosive environments.Thus, UFG structures with larger number of larger grain boundariesshould be active compared with CG structures. This, however, ap-pears to be inconsistent with the present results. In fact, as for acorrosion process, two steps must be considered. The first one isthe initiation stage which is controlled largely by metallurgical fac-tors and inhomogeneity of structures. The other is the growth andpropagation stages which may be structure sensitive but also influ-enced by kinetic factors such as division reactions and polarization[12]. Since the degree of degradation in the present experiment ismainly determined by the second step, the discussion hereafter isconcentrated on the second step. Dilution of segregated impuritiesat grain boundaries is well-known beneficial effect of grain sizereduction for corrosion resistance [12]. If grain size is reduced from10 to 0.3 lm, the volume fraction of intercrystalline region in-creases from 0.03% to about 1% [12]. Thus, segregated impuritiesare estimated to be diluted by about 1/30. For example, bulk impu-rity content in Cu of commercial purity is assumed to be about0.04 at.%, therefore the segregated impurity at grain boundariesin CG Cu can be multiplied to almost 100 at.% because bulk impu-rity content is over the volume fraction of intercrystalline region.On the other hand, segregated impurity in UFG Cu is estimatedto be about 4 at.%. Such dramatic diminishing of impurities from100 to 4 at.% could efficiently reduce the corrosion rate at grainboundaries. Thus, refining the microstructure of metals could effi-ciently improve their corrosion resistance [5–15].

48 W. Deng et al. / Corrosion Science 74 (2013) 44–49

For all machining-prepared UFG Cu electrodes, it can be seenfrom Fig. 4 that a significant grain size reduction in Cu electrodecan shift the Ecorr to a slightly more negative value. The Ecorr for20 D (dot-dash line), 10 D (dash line) and 0 D (solid line) electrodesis �0.19, �0.18 and �0.17 V vs. Ag/AgCl, respectively. This negativeshifting effect of Ecorr on UFG Cu electrode is similar to that of add-ing organic inhibitors or refining microstructure of Cu throughECAP [4–15,26], and may be due to the formation of corrosionproducts partially protecting the Cu surface and reducing the chlo-ride ion attack [27]. These corrosion products have been reportedto be cuprous chloride or oxychloride complexes and/or an oxidefilm formed on the surface due to the hydrolysis of CuCl [26].

3.3. The impedance measurements

EIS is an effective method to probe the electrochemical corro-sion behavior of electrodes [4,26,27]. To further illustrate the effectof grain size reduction on the electrochemical corrosion behaviorof UFG Cu chip in acidic chloride pickling solutions (0.5 M HCl),impedance experiments were also performed. Fig. 5 displays theEIS result on P–Cu (circles), 20 D (black dots), 10 D (black squares)and 0 D (black diamonds) working electrodes. The simple Randle’sequivalent circuit is also presented (inset of Fig. 5, top). In Randle’scircuit, it is assumed that the resistance to charge transfer (Rct) andthe diffusion impedance (W) were both in parallel to the interfacialcapacity (Cdl). This parallel combination of Rct and Cdl gives rise to asemicircle in the Nyquist complex plots. The semicircle diameterequals the charge transfer resistance, Rct. This resistance exhibitsthe electron transfer kinetics of the Cu+ dissolution at the Cu elec-trode interface. Therefore, Rct can be used to characterize the elec-trochemical corrosion resistance of Cu [32]. As shown in Fig. 5, theRct increases from 200 to 621 X cm2 (Fig. 5, circles and black dia-mongs), indicating that the electron transfer resistance of Cu disso-lution at 0 D electrode is extremely high. This value is comparableto the result using Co porphyrin derivatives considering acidicchloride pickling solution is a strong corrosive mixture [32]. Be-sides, according to previous research [26], the immersion time ofCu electrode could affect the formation of passivation film at sur-face. Prolonging the time of immersion could facilitate the forma-tion of passivation film, which could result in a large semicircle inthe Nyquist complex plots [4]. Thus, the corrosion resistance ofUFG Cu is also comparable to those reports using high-perfor-

Fig. 5. EIS results of P–Cu (circles), 20 D (black dots), 10 D (black squares) and 0 D(black diamonds) working electrodes in 0.5 M HCl.

mance corrosion inhibitors since the immersion time of electrodeis only 30 min for measuring open circuit potential [4,26]. Refiningthe grain size of Cu could efficiently enhance the electrochemicalcorrosion resistance.

4. Conclusions

In this study, machining was developed as an efficient methodto refine the microstructure of Cu material. This technique couldresult in the preparation of UFG Cu chip with a minimum grain sizeof 80 nm when a rake angle of 0� was applied. The electrochemicalcorrosion behavior of the UFG Cu chip in acidic chloride picklingsolutions (0.5 M HCl) was studied in terms of potentiodynamicpolarization and EIS. Electrochemical potentiodynamic polariza-tion measurements showed that, in contrast to CG Cu, UFG Cu dis-played notably reduced anodic corrosion currents and decreasedcorrosion rates. EIS results further revealed that when the size ofCu grains decreased from 500 lm to about 80 nm, the chargetransfer resistance (Rct) of the anodic dissolution step dramaticallyincreased about 3 times (from 200 to 621 X cm2). UFG Cu pos-sesses better resistance to electrochemical corrosion compared toits CG counterpart. Machining is an efficient method to producewell-refined microstructure in Cu with ultrafine grains and en-hanced electrochemical corrosion resistance. Machining techniquetherefore holds considerable promise in industrial and microelec-tronic applications.

Acknowledgments

This research was supported by the National Natural ScienceFoundation of China (50605022, 51075154), Fundamental Re-search Funds for the Central Universities (2012ZZ0057), NaturalScience Foundation of Guangdong Province (06300160), and Zhuji-ang Science Technology New Stars Foundation (2011J2200066).

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