Original Research Article

An investigation into the effect of alloying elementson corrosion behavior of severely deformed Cu-Snalloys by equal channel angular pressing

M. Ebrahimi a,*, Sh. Attarilar b, M.H. Shaeri c,C. Gode d, H. Armoon c, F. Djavanroodi e

aDepartment of Mechanical Engineering, Faculty of Engineering, University of Maragheh, P.O. Box 55136-533,Maragheh, IranbSchool of Metallurgy and Materials Engineering, Iran University of Science and Technology, Tehran, IrancDepartment of Materials Science and Engineering, Imam Khomeini International University, Qazvin, IrandSchool of Denizli Vocational Technology, Program of Machine, Pamukkale University, Denizli, TurkeyeDepartment of Mechanical Engineering, Prince Mohammad Bin Fahd University, Al Khobar 31952, Saudi Arabia

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a r t i c l e i n f o

Article history:

Received 16 December 2018

Accepted 28 March 2019

Available online

Keywords:

Copper-tin dilute alloys

Severe plastic deformation

Electrochemical measurement

Corroded surface morphology

Microstructure analysis

a b s t r a c t

To overcome some possible deficiencies of pure copper, dilute alloying and employment of

equal channel angular pressing seem the cost-effective solutions. In this work, dilute copper

alloys with the tin amount of 0.18, 0.3, and 0.5 wt.% were obtained with continuous casting

and subsequently, they were subjected to ECAP process up to four passes. It was shown that

integrated treatment by dilute alloying and ECAP lead to 182% improvement of the corrosion

resistance as compared to the as-received condition due to the grain refinement. Meanwhile,

the alloying impact on current density is decreased with the ECAP process which may result

from the changes at the distribution of Sn atoms in Cu. The difference in measured corrosion

current density of unprocessed and ECAPed samples for the alloys Cu-0.3%Sn and Cu-0.5%Sn

are 15% and 2%, respectively. The corrosion improvement by means of current density

reduction due to the alloying before the ECAP process is about 45% while this value after the

ECAP diminishes to 35%. Microstructure analysis showed that four passes of ECAP process

cause the average grain size of the pure copper to less than 700 nm and the Cu-0.5%Sn to

about 550 nm. Also, the HAGBs fraction of the ECAPed pure Cu is 74%, while the correspond-

ing magnitude for the Cu-0.5%Sn is 78%.

© 2019 Politechnika Wroclawska. Published by Elsevier B.V. All rights reserved.

Available online at www.sciencedirect.com

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journal homepage: http://www.elsevier.com/locate/acme

1. Introduction

As known, large structural refinements can be achievedthrough imposing deformation by the simple shear. Severe

* Corresponding author.E-mail address: ebrahimi@maragheh.ac.ir (M. Ebrahimi).

https://doi.org/10.1016/j.acme.2019.03.0091644-9665/© 2019 Politechnika Wroclawska. Published by Elsevier B.V

plastic deformation (SPD) methods are one of the pronouncedmetal-forming processes which utilize this phenomenon toproduce ultrafine-grained (UFG) or even nanostructured (NS)metals and alloys [1]. Usually, SPD methods are described as aprocess in which high hydrostatic pressure is applied in bulk

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solid materials which impose considerable amounts of plasticstrains without any dimensional changes in the sample'sshape [2,3]. So far, number of versions of SPD techniques havebeen introduced and examined including equal channelangular pressing (ECAP) which is one of the most popularprocesses due to its simplicity and low cost [4], high pressuretorsion (HPT) [5], accumulative roll bonding (ARB) [6], con-strained groove pressing (CGP) [7,8], equal channel forwardextrusion (ECFE) [9], multi directional forging (MDF) [10,11],planar twist extrusion (PTE) [12], etc. It is generally acceptedthat ECAP is a well-established SPD process in which thesample is pressed through a die with two intersectingchannels by means of a plunger. Therefore, the simple shearoccurs as the sample passes through the die [13]. Byconsidering that dimensional change is minimal, the processcan be performed repetitively; consequently, the excessiveplastic strains generated would lead to the intense grainrefinement of the materials [14].

Copper is one of the most widely used industrial materialsespecially because of the high electrical and thermal conduc-tivity, easy to deform feature, and high corrosion resistance. Inthe meantime, copper-tin (Cu-Sn) alloys are of particularimportance especially in soldering due to the absence of toxicelements such as lead [15]. Considering wide applications ofthe copper in the automotive and electrical industries, cost-effective solutions are essential; so, methods for improvingstrength, mechanical properties, and corrosion behavior canbe of interest. Therefore, to enhance the mechanical propertiesof the copper, various copper alloys have been studied. One ofthese alloys is the dilute alloys of tin [16]. As known, poormechanical properties are among the copper limitationswhich is specifically necessary for the magnetic applications[17] and one of the solutions to enhance mechanical propertyof copper is to utilize SPD processing. Over the past sixty years,many Cu-X alloys have been developed mainly with additiveelements such as Ag, Nb, and Cr which have been subjected tothe cold rolling and cold processing techniques [18–20]. It isobvious that the chemical composition, thermal treatment,and deformation process play an important role in theproperties of these alloys, resulting in high mechanicalstrength, favorable electrochemical response and also main-tenance of the electrical conductivity after subjecting them tothe high levels of deformation. As demonstrated, conventionalhardening mechanisms include hardening through solidsolutions, precipitate hardening, and cold work; but virtually,all strengthening mechanisms weaken the electrical conduc-tivity. In this regard, dilute alloys are among the exceptions[21,22]; consequently, such alloying systems become particu-larly significant. The presence of smaller atoms such as tin inthe copper crystalline structure leads to the lattice contractionand the increment of deformation resistance; thus, thematerial becomes harder [21]. For instance, tin can solveabout 15.8 wt.% in copper. Additionally, the microstructure ofmaterials plays a major role in strength, ductility, corrosionresistance, and electrical conductivity [23,24].

In one study on the Cu-Sn alloying system, the continuouscasting was used to improve mechanical properties, corrosionresistance, and electrical conductivity [25]. Accordingly, thestructure of Cu-Sn alloy consists of columnar grains whichsurrounded with the smaller non-columnar grains of the same

phase. The tensile strength and ductility of this alloy weregreatly improved, the corrosion resistance was about 15 timeshigher, and the electrical conductivity was increased by 12.2%.The high strength and corrosion resistance could be due to thisfact that the dislocations movement are effectively blocked bysmall isolated grains which increase the strength and preventthe onset of the corrosion. Small isolated grains can be foundin the structures including both columnar and equiaxed grains[25]. Irfan et al. [26] reported the effect of ECAP processing onthe erosion–corrosion behavior of pure copper in the simulat-ed sea-water medium. It was found that the microhardnessvalues increase by 200% after the four passes as compared tothe annealed condition. It was also concluded that ECAPprocessing promotes the erosion–corrosion resistance ofcopper and increasing the pass numbers leads to the bettererosion–corrosion behavior of the samples. Jandaghi andPouraliakbar studied the combined effect of constrainedgroove pressing and rolling with different reductions alongdirect and cross orientations as employed dual-straining routeon Al-Mn-Si alloy. They found that apart from processingroute, more lattice swelling facilitates the electrons' travelingbetween the planes which improves the sample's conductivity[27]. Also, dynamic recovery during the deformation reducesthe corrosion rate significantly. By dislocations annihilationthrough the annealing, icorr decreases and reaches its mini-mum magnitude for the sample experienced static recovery[28]. Microstructural transformation and electrochemicalbehavior of the ECAPed pure copper were studied by Hadzimaet al. [29]. The results showed that the corrosion product ismore homogeneous in the UFG copper than that of the coarse-grained (CG) counterpart which indicates unfavorable local-ized intergranular corrosion form, but the electrochemicalbehavior including the corrosion resistance and the dissolu-tion rate was not considerably different from each other. Thecorrosion behavior of the UFG copper produced by the ECAPprocess was also investigated by Miyamoto et al. [30]. It wasrevealed that UFG copper displays notable lower corrosioncurrent in comparison with the CG counterpart regardless ofthe higher dislocations density and the total fraction of grainboundaries according to Ralston's rule [31]. Additionally, thecorrosion damage of the deformed sample is more uniform,whereas there is a clear existence of the selective corrosion ofsome grain interiors and evidence of preferential grainboundary degradation in the CG Cu. Stress corrosion cracking(SCC) susceptibility of the UFG copper produced by ECAPtechnique was studied by Yamasaki et al. [32]. According to thefindings, UFG copper has a considerably improved SCCbehavior than its CG counterpart, because SCC occursintergranularly in the UFGs while the fracture of CGs wasalmost transgranularly with a cleavage pattern. The promotedresistance to SCC damage in the UFG state and its intergranu-lar nature have been attributed to the microstructuralevolution due to the ECAP processing. The effect of micro-structure on the corrosion properties of copper was exploredby Janecek et al. [33]. It is reported that ECAP processing leadsto the grain size reductions of about 100 times; but regardlessof this huge amount of structural refinement [34,35], thegeneral corrosion characteristics are almost unchanged. Also,similar to the other researches, the corrosion damage is moreuniform in the UFG materials. Based on the above literature

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survey, grain refinement through SPD methods and especiallyECAP process does not degrade the corrosion properties andeven enhances it; but for the copper case, it seems that it hasnot a sizeable effect [36]. It was claimed that the inhomoge-neous distribution of the solute atoms in the lattice leads to theproduction of galvanic cells between the regions of high andlow concentrations; hence, the overall corrosion performanceis decreased [36]. This also may depend on the solute atomtype, for example, it is a passivation form or not, and it shouldbe clarified carefully [37].

In this study, it is attempted to investigate both alloyingand ECAP processing influence on the corrosion behavior ofCu-Sn alloy in comparison with the pure copper condition,because it seems that there are very limited investigations onthe combined effect of alloying and ECAP processing,particularly in the case of the Cu-Sn alloys and the resultsmay be beneficial from both industrial and theoretical aspects.In addition, a lot of contradictory findings on the corrosionresistance of UFG copper have been reported in the literature.

2. Materials and methods

In this study, two hardening procedures were utilized. At first,a mechanism of precipitate hardening was applied by additionof Sn alloying element into the pure copper and then, theobtained dilute Cu-Sn alloys were subjected to the SPDprocessing via ECAP technique up to the four passes atambient temperature. To achieve this goal, dilute Cu-Sn alloyswere manufactured by the continuous casting technique at atemperature range of 1200 in 20 mm diameter. It is worthmentioning that the obtained samples by dilute alloying wereprepared in a continuous casting furnace with a specialgraphite pot for non-ferrous materials. The Cu-Sn alloys wereprepared with three various types including rods with Sncontent of 0.18, 0.30, and 0.50 in weight percent (wt.%). Table 1lists the chemical composition of each alloyed and the as-received pure copper samples. For ECAP processing, billetswith the length and diameter of 120 mm and 12 mm were cutand machined, respectively.

In order to apply the ECAP process on the prepared pure anddilute samples, a split-die with the channel angle of 908 and outercorner angle of 208 was designed and manufactured. Thesamples were processed up to four passes by route BC whichinvolves respective sample rotation of 908 between eachconsecutive pass. For this aim, the hydraulic press was usedwith the plunger speed of about 0.5 mm/s. Subsequent to theECAP processing, various examinations were performed in orderto investigate the effect of ECAP pass numbers and alloyingtreatment on the corrosion behavior of the attained samples.

Table 1 – The chemical composition of utilized Cu-Snalloys and pure copper sample.

Material Cu Sn P Mg Zn Pb Se

Pure Cu Bal. <0.001 <0.020 <0.002 <0.002 <0.001 <0.001Cu-0.18%Sn Bal. 0.18 <0.020 <0.002 <0.002 <0.001 <0.001Cu-0.30%Sn Bal. 0.30 <0.020 <0.002 <0.002 <0.001 <0.001Cu-0.50%Sn Bal. 0.50 <0.020 <0.002 <0.002 <0.001 <0.001

For electrochemical measurements, disk shaped-speci-mens with 1 mm thickness and 12 mm diameter weresectioned perpendicular to the pressing axis by the electricaldischarge machine. Then, the specimens were mechanicallypolished and subsequently electropolished in the solution of70 ml H3PO4 and 30 ml CH3COOH to attain mirror-like surfacewithout any damaged layers. In order to ensure the10mm � 10 mm working electrode area, the residual sur-rounding area and the connecting copper wire were coveredcarefully. Afterwards, the corrosion tests were conducted witha conventional three-electrode cell setup containing a satu-rated calomel reference electrode (SCE), lugging capillary, Ptcounter electrode, and the prepared specimen as a workingelectrode in 3.5 wt.% NaCl aqueous solution. All experimentswere carried out at room temperature and repeated threetimes to ensure the validity of the results, then the averagemagnitudes were reported. Previous to each measurement,the specimens were kept in the testing solution for 15 min toreach the steady-state open-circuit potential. Each potentio-dynamic curve was plotted by a potential sweep rate of10 mV s�1 starting from �450 mV SCE and terminating at apotential of 100 mV SCE. Eventually, corrosion potential (Ecorr)and corrosion current density (icorr) were calculated from eachpotentiodynamic plots employing the well-known Tafelextrapolation procedure [38]. Calculation of corrosion rateswas performed through the electrochemical measurements inaccordance with the ASTM G102. The corroded surfacemorphology was examined after the potentiodynamic polari-zation tests using scanning electron microscopy (SeronAlS2300C SEM microscope) operated at 30 kV. For microstruc-tural examinations and grain size measurements of the initialand ECAPed samples, Quanta 3D FEI field emission scanningelectron microscopy (FE-SEM) equipped with electron back-scatter diffraction (EBSD) at an accelerating voltage of 15 kVand a beam current of 10 nA was utilized. The EBSD imageswere obtained using the step sizes of 500 nm and 50 nm for theunECAPed and ECAPed samples, respectively. The raw datawas analyzed using the OIM TSL7 software. Prior to EBSDcharacterization, the surface of samples' cross-section wasmanually polished down to 1 mm via diamond polishingfollowed by electropolishing in a 30% nitric acid and 70%methanol solution with a DC voltage of 15 V for 10 s at �25.

3. Results and discussion

Fig. 1 shows the potentiodynamic polarization curves of theCu-Sn alloys and pure Cu samples before and after processingby ECAP, respectively. The detailed information about theelectrochemical reactions taken place on copper in NaCl andother saline solutions as well as the anodic polarizationbehavior of copper can be found elsewhere [27,28]. Also,polarization data and corrosion parameters have beencalculated and represented in Fig. 2. It is evident that theshape of all curves is almost similar and there is not any visiblesubstantial change. It means that similar electrochemicalbehavior is observed regardless of the ECAP processing oralloying by Sn. Similar behavior for pure copper processed byARB has been previously reported [39]. The best corrosionperformance belongs to the Cu-0.5%Sn alloy after the ECAP

Fig. 1 – Polarization curve of the pure Cu and Cu-Sn alloys: (a) before the ECAP and (b) after the ECAP processing.

Fig. 2 – Corrosion parameters of pure Cu and Cu-Sn alloys before and after the ECAP processing.

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processing. It shows approximately 181% improvement of thecorrosion rate as compared to the pure Cu before the ECAPprocessing with respective magnitudes of 11.6 and 32.6 mm/year. This result reveals that simultaneous treatment by ECAPand alloying by addition of Sn in the Cu matrix is a suitablescheme for improvement of electrochemical and corrosionbehavior. It was reported earlier by El-Sherif et al. [40] thatslight additions of Al or Sn (1–2%) can improve the corrosionresistance of the Cu-Zn alloy. This enhancement may berelated to the microstructural evolution, defects magnitude,segregation of alloying elements, and its properties as well asthe composition of the produced passive film. Tin can be easilycontributed to the chemical reactions of both acidic andalkaline solutions even if its valence +2 state is considerablystable. It is worth mentioning that the Sn (II) transforms to Sn(IV) in the aqueous solutions by means of the oxidationprocess. As claimed, Sn can combine with the other elementssuch as Cu and Sb which leads to enhancement of thecorrosion behavior [41]. Moreover, Pardo et al. [42,43] indicatedthat high Cu-Sn synergy effect can reduce the corrosion rate ofstainless steel. The beneficial effect of Sn content for corrosionbehavior and passive film formation has been previouslypublished in various studies [44,45]. As a result, Sn has beenwidely considered as an effective alloying element to improvethe corrosion resistance of steels. However, it was found thatonly the slight amount of Sn would be beneficial for corrosionperformance of the materials and consequently, the largeamount of Sn leads to high acidification and occurrence of thepitting damage [43]. The same beneficial electrochemicaleffect of tin in pure Cu is observed in this study. By analysesof given data in Fig. 2, it can be easily seen that the corrosioncurrent density is decreased in all samples by the addition ofSn content regardless of the ECAP processing. Also, the effectof alloying on the reduction of the current density is less for theECAPed samples as compared to the unECAPed conditions. Inother words, no considerable effect on the current density wasobserved by further addition of Sn after the ECAP processing.For example, the difference of the corrosion current densitybetween the Cu-0.3%Sn and Cu-0.5%Sn samples are 15% forthe unECAPed condition, on the other hand, for similar alloysafter ECAP processing, only 2% change in corrosion currentdensity has been observed. This is probably due to the ECAP-induced structural evolution which will be discussed later.

According to Fig. 2, it can be claimed that the effect ofalloying is diminished through the ECAP processing whichmay result from the alterations at the distribution of Sn atomsin the Cu matrix. For instance, corrosion improvement(reduction of current density) due to the alloying before theECAP process is about 45% compared to the initial conditionwhile this value after the ECAP diminishes to 35%. Overall, thevariation of corrosion behavior between different alloyedsamples after the ECAP process is minor except for the puresample. Accordingly, the addition of just 0.18% Sn decreasesthe corrosion rate up to 32% for the unECAPed condition whilethis amount is about 19% after the ECAP processing.

Another interesting finding is about the corrosion behaviorof Cu-0.18%Sn sample because this alloy is the most affectedsample by the ECAP processing, the icorr magnitude isdecreased up to 54% in the ECAP processed samples. Thismay arise from the distribution change of the Sn atoms within

the ECAP process. In other words, the distribution of Sn atomsmay change the copper matrix and its grain boundaries [30].Better corrosion resistance of SPD processed materials hasbeen reported in several investigations. For example, Balyanovet al. studied the UFG pure titanium and suggested that highpassivation rate and low impurities segregation at the grainboundaries are responsible for the corrosion enhancement[46]. SPD techniques and especially ECAP process induce adegree of microstructural heterogeneities such as grainboundaries and dislocations density. These variations maybe the reason for the observed different electrochemicalproperties [47,48].

Fig. 3 displays scanning electron microscopy of corrosionproducts on the surface of pure Cu and Cu-0.5%Sn samplesbefore and after the ECAP processing conditions. As can beobserved, these images show relatively good accordance withthe obtained electrochemical results. Some semi-localizedcorrosion products can be observed in the initial pure Cusample. Also, it seems that most of the corrosion products arelocated around the copper grains. Moreover, some black areasare seen in this micrograph possibly showing the occurrence ofsome pitting-type corrosion in the pure copper which has beenpreviously reported for Cu in chloride media [47]. After ECAPprocessing (Fig. 3(c)) and alloying (Fig. 3(b)), the current densityand corrosion rate of the sample is decreased from 2.8 mA/cm2

and 0.0325 mm/year at the initial Cu sample (Fig. 3(a)), to about1.55 mA/cm2 and 0.0180 mm/year at the Cu-0.5%Sn sample(Fig. 3(b)) and 1.54 mA/cm2 and 0.0179 mm/year at the ECAPedpure Cu (Fig. 3(c)). By considering Fig. 3(a) and (c) and thecorrosion products of the processed pure copper, the corrosionrate is decreased and the occurrence of localized corrosionproducts is completely obvious in Fig. 3(c). So, some regions areprotected and are free of any corrosion, but some regions arecorroded rapidly. However, after the four-pass ECAP proces-sing (Fig. 3(c)), the localized corrosion behavior is increasedand led to the formation of square-shaped regions. Addition-ally, alloying has a great impact on the corrosion behavior as itis observed in Fig. 3(b) and 3(d), it shows less corrosionproducts as compared to the initial pure Cu sample. Fig. 3(b)and (d) confirms that the dilute alloying leads to the reductionof corrosion localization and less square-shape regions(marked by red arrows in Fig. 3(c)). Also, the degree of localgrain boundaries' attack in the alloyed sample seems to belower which indicates the beneficial effect of the Sn element inthe Cu structure. ECAP process improves the corrosionperformance of the sample, but it may lead to a higheramount of localization in limited square-shape regions. In fact,the interior of these square-shaped regions is protected whilethe surrounding areas are severely attacked. According to thedetailed discussion of Miyamoto et al. [36], SPD can have a lotof effects regarding the corrosion performance, such asredistribution of impurity and solute elements, the morphol-ogy change of precipitation by partial dissolution and physicalfragmentation, residual dislocation density, texture, etc. Thesesquare-shaped regions (marked by red arrows in Fig. 3(c)) maybe the direct result of these complex interactions especially byconsidering that ECAP has a great effect in occurrence oftexture [49] and distortion of lattice [50] which may lead todifferent accumulation of alloying elements [51] and inclu-sions that ultimately changes the electrochemical response of

Fig. 3 – SEM micrographs of the corroded surface morphology after the polarization test: (a) pure copper and (b) Cu-0.5%Snbefore the ECAP condition, (c) pure copper and (d) Cu-0.5%Sn after the ECAP processing.

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materials. According to Fig. 3(b) and (d), the square-shapedcorrosion products are diminished by dilute alloying. It can beobserved that there is just a slight amount of corrosionproducts in Fig. 3(d) which is also confirmed by Fig. 4(b). Thisconfirms that impurities have an essential effect in which theaddition of Sn content possibly affects the behavior ofimpurities due to the good synergy between the Cu and Snatoms.

Fig. 5 depicts some microstructural aspects of the initialand ECAPed processed pure copper and Cu-0.5%Sn alloyincluding the grain orientation maps and related grainboundaries. The standard unit stereographic triangle deter-mined by the crystallographic orientations of the grains is also

Fig. 4 – Energy dispersive spectroscopy of the corrosion productsof the corresponded elements: (a) ECAPed pure copper and (b) E

included in Fig. 5. It is worth mentioning that the area fractionmethod has been utilized to calculate the average grain size byTSL OIM software. Also, the boundaries belonged to theneighbor grains with the misorientation angles larger than 158(high angle grain boundaries) have been considered as theeffective grain boundaries. Misorientation is the difference incrystallographic orientation between two grains/crystallites inpolycrystalline materials. The EBSD analyses show that theaverage grain size of the pure copper is reduced from about20 mm to less than 700 nm after being processed by four passesof the ECAP, while the average grain size of about 550 nm isachieved after four passes of the ECAP process in the Cu-0.5%Sn. It is also observed that most of the grains have relatively

of Fig. 3 marked by red positive sign and the weight percentCAPed Cu-0.5%Sn.

Fig. 5 – EBSD grains orientation and related grains boundaries maps of (a) initial pure copper, (b) four-pass ECAP processedpure copper, and (c) four-pass ECAP processed Cu-0.5%Sn alloy.

Table 2 – The results of microstructure analyses regarding the length of grain boundaries and fraction of LAGBs and HAGBs.

Misorientaionangle

Initial pure Cu Four-passECAPed pure Cu

Four-pass ECAPedCu-0.5%Sn

Fraction (%) Boundarylength in

100 mm2 (mm)

Fraction (%) Boundarylength in

100 mm2 (mm)

Fraction (%) Boundarylength in

100 mm2 (mm)

2–58 8.9 3.3 12.7 63.4 12.8 66.55–158 5.3 1.9 12.9 64.5 9.6 49.9>158 85.8 36.6 74.4 372.5 77.6 403.3

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spherical and equiaxed morphology after ECAP processing ofboth pure Cu and Cu-0.5%Sn. The main reason for theremarkable grain refinement is the increment of dislocationsdensity and consequently, the formation of low angle grainboundaries (LAGBs) at the initial stages of the ECAP process. Bythe addition of plastic strain at increased ECAP pass numbers,the aforementioned LAGBs are transformed to high angle grainboundaries (HAGBs) and subsequently, ultrafine grain struc-ture can be formed [2,52]. In addition, EBSD maps indicate thatthe effect of ECAP processing on the grain refinementincreases gradually by the addition of Sn alloying elementto the pure copper. This may be due to the reduction ofstacking fault energy as well as the increase of twiningcapacity, reported in Refs. [53,54].

The corresponding grain boundaries maps of Fig. 5 indicatethe formation of LAGBs and HAGBs in the ECAP processedsamples where the boundaries with misorientation anglelarger than 158 have been shown by blue lines and theboundaries with misorientation angle between 5–158 and 2–58have been displayed by green and red lines, respectively.Table 2 lists information regarding the length of grainboundaries and fraction of LAGBs and HAGBs. As can beobserved, the surface density of LAGBs and HAGBs is increasedextremely after processing by four-pass ECAP as a result ofimposing a high amount of plastic strain. The HAGBs fractionof the four-pass ECAPed pure Cu is more than 74%, while thecorresponding magnitude for the Cu-0.5%Sn is about 78%.Therefore, it can be concluded that the fraction of HAGBs isincreased gradually by the addition of Sn alloying element. It isalso found that the microstructure of both samples consistsmainly of HAGBs after four passes of the ECAP process.

4. Conclusions

Dilute alloying of pure copper by tin element with the weightpercent of 0.18, 0.3, and 0.5 wt.% was performed by continuouscasting and subsequently, ECAP method was applied up to fourpasses. The effect of dilute alloying and ECAP was investigatedon the corrosion behavior and microstructure properties of theobtained samples. The main conclusions may be drawn asfollows:

� Combined treatment by dilute alloying and ECAP seems tobe a suitable scheme for improvement of corrosion behavior.In this regard, the best corrosion performance belongs to theCu-0.5%Sn alloy after the ECAP process with approximately182% improvement of corrosion rate as compared to the as-received pure Cu.

� Effect of dilute alloying on the reduction of current density isless for the ECAPed samples in comparison with theunECAPed counterparts. For instance, about 15% variationis found at the corrosion current density of processed Cu-0.3%Sn and Cu-0.5%Sn, on the other hand, for the samealloys, there is only 2% difference in the current density afterthe ECAP processing.

� The influence of alloying is lower in the ECAP processedsamples which possibly is due to the alterations of Snatoms distribution in the Cu matrix. For example, thereduction of current density in the dilute alloyedsamples is about 45% higher than the initial conditionwhile this magnitude is decreased to 35% for the ECAPedsamples.

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� The corrosion rate of initial pure Cu is higher than theECAPed Cu-0.5%Sn. It was confirmed that the corrosionproducts may consist of CuCl, CuO, and Cu2O, according tothe energy dispersive spectroscopy analysis.

� The density of grain boundaries and dislocations is anindicative characteristic of the UFGed materials. According-ly, the average grain size of four-pass ECAPed pure Cu andCu-0.5%Sn are respectively less than 700 nm and 550 nm.Also, most of the grains have relatively spherical andequiaxed morphology.

� The impact of ECAP processing on grain refinementincreases gradually by alloying due to the reduction ofstacking fault energy as well as the increment of twiningcapacity. The analyses indicated that the fraction of HAGBsis 74% for the final pass of ECAPed pure Cu, while thecorresponding magnitude for the Cu-0.5%Sn reaches ap-proximately 78%. Therefore, the fraction of HAGBs isgradually increased by the addition of Sn alloying element.

Data availability

All data generated or analyzed during this study are includedin this published article.

Acknowledgments

This research was funded by Iran National Science Foundation(INSF) with grant number of 94810544.

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