Assessment of microstructural and electrochemical behavior of severely deformed pure copper through equal channel angular pressing

Journal of Alloys and Compounds 723 (2017) 856e865

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Assessment of microstructural and electrochemical behavior of severely deformed pure copper through equal channel angular pressing D. Gholami a, *, O. Imantalab b, M. Naseri c, S. Vafaeian b, A. Fattah-alhosseini b a b c

School of Metallurgy and Materials Engineering, Iran University of Science and Technology, Tehran, Iran Department of Materials Engineering, Bu-Ali Sina University, Hamedan, 65178-38695, Iran Department of Materials Science and Engineering, Faculty of Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 March 2017 Received in revised form 22 June 2017 Accepted 27 June 2017 Available online 29 June 2017

In the present research, the passive and electrochemical responses of pure copper, heavily deformed by equal channel angular pressing (ECAP), in a phosphate buffer solution (pH ¼ 10.69) were investigated. For this purpose, various electrochemical tests such as potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and MotteSchottky (MeS) analysis were carried out. Also, the influences of ECAP on distribution of the plastic strain and maximum principal stress were analyzed using a three-dimensional (3D) simulation by finite element methods (FEM). According to the field emission scanning electron microscope (FESEM) micrograph, by increasing the ECAP passes, finer microstructure was obtained. Considering all the electrochemical tests results, it can be concluded that the passive and electrochemical responses of pure copper are improved under influence of ECAP process, mainly due to the formation of thicker and less defective passive film. © 2017 Elsevier B.V. All rights reserved.

Keywords: Equal channel angular pressing Grain refinement Electrochemical impedance spectroscopy MotteSchottky analysis Point defect model

1. Introduction Over the last decade, severe plastic deformation (SPD) techniques have drawn wide attention to ultrafine grained (UFG) bulk metals production. A unique type of SPD process involves imposing the ultrahigh strain by adding the process passes, without any significant change in the overall dimensions of the work piece. To have considerable advantages, SPD methods have been developed rapidly in recent years [1e3]. The microstructure of UFG bulk materials is fairly uniform and homogeneous, containing equiaxed grains with the majority of grain boundaries having high angles of misorientation [4,5]. Conventional techniques of heavy deformation such as drawing and cold rolling, are also accompanied by significant refinement in the microstructure. In this way, generally the resulted substructure is a cellular type with cells elongated in the direction of drawing or rolling, and containing high fraction of low angle grain boundaries (LAGBs) with misorientation between 2 and 15 . On the contrary, materials processed by SPD techniques have ultrafine granular structure, and containing mainly high angle

* Corresponding author. E-mail address: [email protected] (D. Gholami). http://dx.doi.org/10.1016/j.jallcom.2017.06.302 0925-8388/© 2017 Elsevier B.V. All rights reserved.

grain boundaries (HAGBs) with misorientation greater than 15 [6e9]. The main problems with SPD procedures are the specific required equipment, limits to the sample size, and a trade-off between strength and ductility to form desirable nano-grains with less lattice defects at reduced strain levels [10,11]. The most successful method, which was developed in late 1970's, is equal channel angular extrusion/pressing (ECAE/ECAP) [12,13]. This process has great potential for developing ultrafine grain structure consisting of homogeneous and equiaxed grains, and grain boundaries dominated by high angle disorientations [14,15]. Since the cross-section area of the sample is constant after each pass, a very high level of strain can be achieved by repeating the deformation. The effects of ECAP process parameters, such as the number of passes, die angle, deformation route and the back pressure on grain refinement have been extensively investigated [16e20]. Of the premier qualities in UFG materials fabricated through ECAP process, supreme strength and the possible superplasticity at lower degrees of temperature and increased strain rates can be named. Another significant aspect of deformation behavior about UFG materials produced by ECAP is the lack of strain hardening. Furthermore, hardness and yield stress reduction have been reported by increasing the number of passes during ECAP process [21e24].

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During the recent decade, bulk metals and UFG structures have attracted considerable interest owing to their unique physical and mechanical properties. Therefore, of great scientific interest is to understand their electrochemical behavior specifically to define their applications. There have been a number of studies on effects of the grain refinement on the alloys corrosion performance. However, there is still a serious lack of knowledge about the grain refinement impact on the alloys electrochemical behavior [25e29]. Indeed, the microstructure, which is influenced by the forming process, can affect the passive film formation and its behavior. For example, Akiyama et al. [30] reported that ECAP process resulted in deterioration of corrosion resistance of aluminum alloy in a neutral buffer solution containing 0.002 mol L1 chloride ion, while other researchers [31,32] observed that grain refinement by ECAP improved corrosion resistance of aluminum alloys in chloridecontaining solutions. Hoseini et al. [33] evaluated the effect of grain size and texture on the corrosion behavior of pure titanium deformed through ECAP in NaCl solutions. They concluded that fine-grained specimens with the (0002) planes parallel to the surface possess the highest corrosion resistance, regardless of their grain size. However, Eizadjou et al. [34] demonstrated that accumulative roll bonding (ARB) processing results in a significant reduction in corrosion resistance of pure aluminum in NaCl solution. The controversies are possibly due to the influence of grain refinement as the surface reactivity inducement, resulting in various corrosion behaviors depending on the exact combination of exposed environment, material and SPD procedure [35,36]. According to our careful review of the available literature, there is no systematic study on the influence of ECAP on the passivity and electrochemical behavior of pure copper. Accordingly, the multiplepass step ECAP technique was conducted on pure copper at room temperature in the current evaluation. This research is concentrated on the pure copper passive resistance enhancement, resulted from the grain refinement treatment in phosphate buffer solution. Secondly, the grain refinement influence on the semiconductivity of passive layers formed on pure copper was investigated. To reach these objectives the MotteSchottky plots for all specimens in phosphate buffer electrolyte, along with the point defect model (PDM) were implemented, and the consequence of grain refinement on the defects concentrations was assessed. To realize the pure copper passive film semiconductivity, the relation between defects density and the grain size was evaluated.

2.2. Finite element modeling (FEM) FEM simulation of ECAP process was carried out considering different involved parameters using DEFORM-3D V6.0. The specifications of dies, billet and punch were selected according to the experimental conditions. The diameter and length of the work piece were 20 mm and 120 mm, respectively. The total number of elements after solid mesh generation was 20,000 and automatic remeshing was used to accommodate large deformation during all the analyses (Fig. 1). The die and punch were assumed to be rigid, i.e.no deformation happens in these pieces. Deformation was modelled at room temperature assuming no heat generation, dissipation and transfer between the sample and die walls or the environment happen. Since the model is symmetrical about the middle plane of the ECAP, half of the sample, die and punch were analyzed and the symmetrical boundary conditions were applied. The boundary conditions applied to the model were as following: (i) Die was fixed by assigning zero degree of freedom to displacement and rotation in all three directions. (ii) The billet was permitted to move freely in Z direction during pressing operation in the input channel of die. (iii) The punch was set to move 120 mm in downward direction to press the material completely in the die to complete the process.

2.3. Microstructural characterizations The microstructures of all the samples were examined by MIRA3 TESCAN field emission scanning electron microscope (FESEM). Prior to FESEM, the longitudinal sections of the samples were mechanically ground and polished to 5000-grit using SiC paper, and then electro-polished in a chemical solution of 25% orthophosphoric acid, 25% ethanol and 50% distilled water at 10 V for 30 s at room temperature. 2.4. Electrochemical measurements All the electrochemical experiments were done in classicalthree-electrode glass cell at room temperature by using the mAutolab set (Type III/FRA2) conducted with a PC. An Ag/AgCl electrode

2. Experimental procedure 2.1. ECAP process In the current study, commercially pure copper (99.9 in wt.%) was received in the form of extruded rods with a diameter of 30 mm. The rods were machined into cylindrical samples with a length of 120 mm and a diameter of 20 mm. The samples were annealed at 773 K for 1200 s to remove the effects of deformation induced during the previous processing. Then, ECAP was performed using a die consisted of two round channels, having an inside diameter of 20 mm and equal cross section, intersecting at an angle of 90 , and with an outer curved corner of 22.5 . In the present research on the pure copper, ECAP was carried out at room temperature and at a ram speed of 1 mm/s for up to six passes using the route BC . Route BC refers to 90 rotation of the sample following consecutive passes. Also, molybdenum disulfide (MoS2) was used as solid lubricant to reduce the friction during ECAP process. A 20 mm long section at each end of the sample was discarded in order to remove any part with possible inhomogeneity in the microstructure, and then, the remaining part was cut into pieces in equal length of 10 mm.

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Fig. 1. Model for the billet with initial finite elements.

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saturated in 3 M KCl solution, a Pt plate and ECAPed pure copper samples were used as the reference, the counter and the working electrodes, respectively. Prior to the electrochemical test, the surface of the working electrode was ground with wet emery papers (up to 5000 grit). After grinding, the working electrodes were immersed in the specified phosphate buffer solution (pH ¼ 10.69) for 30 min to form stable passive films on their surfaces. The PDP, EIS and MeS measurements were started after the stability of open-circuit potential (OCP). The PDP plots were obtained using a scan rate of 1 mVs1 starting from 0.25 V (vs. OCP) to above transpassive potential. The frequency range of 100 kHz to 10 mHz was used to record EIS curves. The potential of MeS test was swept from 0.7 VAg/AgCl down to 0.1 VAg/AgCl (passive domain). The AC perturbation amplitude of 10 mV and 5 mV were selected for EIS and MeS tests, respectively. To confirm reproducibility of the experimental results, each test was repeated 5 times. Fig. 3. Distribution of maximum principal stress in conventional die with 4 ¼ 90 and j ¼ 22.5 pressed at room temperature after one ECAP pass.

3. Results and discussion 3.1. FEM simulation The accumulated strain was calculated by FEM as a function of the starting radius position in one ECAP pass; the result is presented in Fig. 2. As can be seen, the redundant shear is making significant contribution to the total strain near the external radius. Therefore, the average value of the strain distribution εav y1:1 is considered as the average strain value imposed to the sample in each ECAP pass in this study (with the die geometry parameters). Eq. (1) presents the equivalent strain imposed on the billet after N passes of ECAP process [37]:

     N 4þj 4þj þ j cos ec εN ¼ pffiffiffi 2 cot 2 2 3

(1)

where 4, j and N are die angle, outer curved corner angle, and the number of passes, respectively. Because of the friction influence, plastic flow inhomogeneity occurs; top regions of the sample are deformed in a relatively short time, while the bottom layers are deformed with a delay [16,18]. Fig. 3 shows that the central regions of the samples are deformed in other way than the surface areas. The top sample surface in the ECAP die is not influenced by friction, so that the deformation is more likely to be influenced by the center

Fig. 2. Plastic strain distribution in conventional die with 4 ¼ 90 and j ¼ 22.5 pressed at room temperature after one ECAP pass.

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region deformation behavior than the bottom surface contiguous to the die. As it had been suggested in several experiments [20,23], the corner gap occurrence is influenced by the strain hardening coefficient n; friction coefficient is of an indispensable influence on the dead zone creation and its size as well. Despite the fact that friction increases strain inhomogeneity throughout the sample, better filling of the die's corner zone with material is supported by higher friction factor [17,19]. ECAP is conducted not only at a room temperature, but also at elevated temperatures. Although the former is usual because it can fully utilize the grain refining effect and strain

859

hardening results, the latter may be important for difficult-to work alloys [12,13]. The temperature change, due to a plastic work release, die chill and frictional heat between the die and the specimen, is important under both cold and hot working conditions, because a temperature rise can induce phase transformation and an alteration in grain structure, especially in the grain boundary structure. These effects, in turn, can modify the flow stress of the workpiece material as well as other mechanical properties. According to a simple lumped heat transfer analysis, the temperature rise DT of the workpiece during ECAP can be

Fig. 4. SEM micrographs of (a) as-annealed, (b) 1 pass, (c) 2 pass, (d) 4 pass and (e) 6 pass of ECAP process.

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Fig. 5. OCP curves of ECAPed pure copper samples in the phosphate buffer solution.

1.0 1 pass 2 pass

0.8

4 pass 6 pass

E / VAg/AgCl

0.6 0.4 0.2 0.0 -0.2 -0.4 1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

Fig. 7. (a) Nyquist and (b) bode diagrams of ECAPed pure copper samples in the phosphate buffer solution.

log i /A cm-2

Fig. 6. PDP curves recorded in the phosphate buffer solution for pure copper samples following 1, 2, 4 and 6 passes of ECAP process.

expressed as follows [37]:

h

 .pffiffiffi  i 0:9sε þ 0:5m s 3 m VA Dt  DT ¼  rC þ VA hDt

(2)

where ε, s, m, m, r, C, V, A, Dt, and h denote the effective strain and the effective stress, the friction factor, the relative velocity between the die and the workpiece, the density of the material, its heat capacity, the volume of the calculation domain (i.e. main deforming zone), the outer surface area of the domain contacting the die, the dwell time of the domain within the deforming zone, and the heat transfer coefficient between the workpiece and the die, respectively. According to related investigation [38], it is apparent that smaller ultrafine grains with higher angles of misorientation are obtained when the material is subjected to a larger plastic strain. This is fulfilled when the pressing happens preferably in dies with lower channel angles as the die with angle of 90 induces more plastic strain. During ECAP of bulk metallic materials, three general parameters control the microstructural evolution and mechanical behavior of the severely deformed billets [16,18e20]: (i) Factors related to the experimental features of ECAP like die angle (4) and outer arc angle (j).

(ii) Processing factors such as pressing speed, working ature, pass number, back pressure, pressing route design that can be set by operator or designer. (iii) Factors with impact on grain refinement of the microstructure including the crystallographic stacking fault energy (SFE) and crystal structure.

temperand die ECAPed texture,

3.2. Microstructural observations The microstructure of the as-annealed and ECAP processed specimens are shown in Fig. 4. As can be seen, the application of one ECAP pass significantly evolved the microstructure, and the processed microstructure is substantially different from the initial microstructure of the annealed copper. It should be noted that processing by six passes of ECAP resulted in the formation of submicrometer equiaxed grains. While plastic deformation happens, large SFE effectively restricts partial dislocations separation simplifying the cross-slip movement, so that cells or subgrains are created as dislocation substructures in three dimensions. In addition, a low SFE creating a large space occupied by the partial dislocations, limits the cross-slip and organizes dislocations to be set in planar arrays. During the ECAP process using materials like copper and aluminum, with the SFE in the range of medium to high, dislocation activity significantly impacts deformation and grain refinement at ambient temperature. With the application of ECAP,

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Fig. 8. KeK transformations of the EIS data of pure copper: (a) 1 pass, (b) 2 pass, (c) 4 pass, and (d) 6 pass of ECAP process.

nucleation, HAGBs with misorientation greater than 15 pass through long distance [41]. This mechanism can also lead to grain refinement of copper during ECAP process. Despite the fact that the copper microstructural evolution during ECAP is not the aim of current investigation, in order to specify the final mechanism of grain refinement, further microstructural investigations must be conducted utilizing high resolution transmission electron microscopy (HRTEM). 3.3. OCP and PDP measurements Fig. 9. Best equivalent electrical circuit of the analysis of experimental EIS data.

as a result of dislocations propagation and interaction, they become twisted together and finally form different cells and cell blocks as spatially sub-structures [38]. In addition to the first mechanism based on slip, it is generally accepted that during low strain deformation, discontinuous dynamic recrystallization (DDRX) is another mechanism of grain structure evolution in materials with medium SFE like copper [15,22,39]. While DDRX, grains reform in two steps. First, bulge of grain boundary starts nucleation and then high angle grain boundaries migrate to build a new microstructure. To clarify, when dislocation density rises over a period, discontinuities appear, that is, local discrepancies in dislocation density, and as a result new grains nucleate [40,41]. Accordingly, with the mentioned

The OCP of the ECAPed samples in phosphate buffer solution is depicted in Fig. 5. These results reveal two main points. Firstly, the OCP of samples becomes stable within about 900 s and after that it remains almost constant. Secondly, all the curves are directed towards the positive values of potential. Such a behavior is indicative of the formation and growth of stable passive layer with increasing the immersion time. In other passive metals and alloys, these trends of potential variations have been also reported by other researchers [42e44]. Some important characters concerned with the passivation behavior of metals and alloys in certain media, are obtained by the PDP curves. Fig. 6 represents the PDP curves of the ECAPed specimens in phosphate buffer solution. As seen, all the curves have the same behavior with a distinct active-passive-transpassive regions. In addition, with increasing the ECAP passes, the passive region of polarization curves moves to the left-hand side, namely to the lower current density. This denotes an enhancement in protectivity

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Fig. 12. Calculated acceptor density of the passive films formed on pure copper samples in the phosphate buffer solution vs. pass number of ECAP process.

Fig. 10. Variations of (a) the passive film and charge transfer resistance and (b) the passive film and double layer capacitance of pure copper sample vs. number of ECAP passes at OCP.

2.0E+10 1 pass 2 pass 4 pass

1.6E+10

6 pass

C-2/ F-2 cm4

1.2E+10

8.0E+09

4.0E+09

0.0E+00 -0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

E/ VAg/AgCl Fig. 11. MeS plots of ECAPed pure copper samples in the phosphate buffer solution.

of passive film as a result of increase in the number of ECAP passes. The same behavior of PDP curves for pure copper in alkaline solution has been reported [45e49]. 3.4. EIS measurements In this section, the passivation behavior of ECAPed specimens will be compared by EIS method as a nondestructive, rapid and convenient technique. In Fig. 7, the EIS spectra at OCP in phosphate buffer solution are presented in Nyquist and bode formats. Comparing the general plot shapes of the ECAPed samples, it can be concluded that the passivation mechanism of pure copper

specimen is not affected by the number of ECAP passes [50]. As seen, distinct capacitive arcs are presented in all cases, and the overall impedance of pure copper increased by increasing the number of ECAP passes. This shows that protective behavior of passive film on the surface of pure copper has become stronger in phosphate buffer solution. KramerseKronig (KeK) transforms are often a prelude to validation of experimental EIS data. Fig. 8 reflects that KeK transforms overlap the experimental EIS data. Considering the requirements of the linear system theory (stability, causality, and linearity) [51], the experimental EIS data are valid (reliable). The details of the KeK transforms have been reported elsewhere [51e53]. Although different equivalent circuit models have been proposed to interpret the EIS data for pure copper in alkaline solutions [54,55], in this work, the equivalent circuit model consisting of two-time constant (as the best proposed model by NOVA software) was used to simulate the EIS data (Fig. 9). Based on the presented model in Fig. 9, passive film can be assumed as a defective and inhomogeneous layer [56]. The elements of this equivalent circuit are defined as follows: The two elements Qp and Rp are used for description of passive film and are respectively the constant phase element and the passive film resistance. On the other hand, Qdl and Rct are used to illustrate the constant phase element and charge transfer resistance of the double layer, respectively. Rs is also an element that exhibits the solution resistance. According to Fig. 9, it can be noted that two processes occurred during the EIS analysis. In the first detected process, Qp indicates the capacitive behavior of the passive film on the surface of pure copper. This is coupled with the resistance of passive film Rp because of (resulting from) the ionic paths within the passive layer at middle frequencies. In the second process, at low frequencies, Qdl indicates capacitive behavior of the double layer at the interfaces related to resistance of the charge-transfer, Rct [56]. The constant phase element (Q) is widely used for simulating behavior of frequency dispersion due to the formation of porous oxide layer, existence active sites (such as grain boundaries and impurities) and surface heterogeneity [57]. The above-mentioned explanations can be summarized as follows: the high frequency domain of impedance is related to the resistance of solution, the middle frequency zone describes the passive layer, and the low frequency indicates the double layer behavior. It should be noted that for the equivalent circuit shown in Fig. 9 different interpretations were also expressed in other alloy systems and other environments such as works done by Lin et al. [58,59]. Fig. 10 is the illustration of the values extracted according to the equivalent circuit of Fig. 9. Both values of passive film resistance and charge transfer resistance increased by increasing the ECAP

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 ¼ copper vacancy on the copper sublattice of the Fig. 13. Modified PDM for passivation of (a) 1-pass, and (b) 6-pass ECAPed pure copper samples in a phosphate buffer solution. VCu 2þ barrier layer, Cuþ ¼ interstitial copper, Cu ¼ copper cation on the copper sublattice of the barrier layer, V ¼ oxygen vacancy on the oxygen sublattice of the barrier layer, Cu O i OO ¼ oxygen anion on the oxygen sublattice of the barrier layer, Cuþ ðaqÞ ¼ copper cation in solution.

passes. Thus, the polarization resistance (Rpol ¼ Rp þ Rct) increased with increment of ECAP passes, reflecting the better capability of corrosion prevention [45]. In addition, the higher number of ECAP passes led to reduction in the capacitance values of the constant phase element of the passive film (Cp) and the double layer (Cdl). Reduction in the value of Cp can be related to the compactness of the passive film increased with the number of ECAP passes [57]. The obtained results are consistent with the PDP measurements. Sometimes the value of Cp is utilized as the thickness scale of the passive film. Although this approach is not a precise method to measure the passive film quality [60], but it is useful to obtain a trend for thickness variation. Considering the general inverse relation between the value of Cp and the thickness of the passive film [60], Fig. 10 shows improvement in the passive film thickness on the surface of pure copper sample in phosphate buffer solution as the number of ECAP passes increases. 3.5. MeS analysis The passive behavior of pure copper in alkaline or neutral electrolytes is related to the semiconducting behavior of its oxide layer (passive film), thus semiconducting response of the passive films is an important subject to be investigated. The MotteSchottky method is widely applied for studying the mentioned semiconductivity. The MeS curves of the worked copper in terms of ECAP passes after immersion for 30 min (to form stable passive film) in phosphate buffer solution are presented in Fig. 11. As it is shown, with the increase in ECAP passes, C2 rises. Accordingly, the passive film capacitance tends to decline with increasing the passes

of ECAP technique. The other point inferred from Fig. 7, is that the semiconductive nature of the passive film formed on copper is the p-type, because of the negative slope seen in the linear part of the curves. This behavior has been previously reported for worked copper in other solutions [45e47]. Based on equation (1) [61e63] and the values of negative slope in Fig. 11, the acceptor density of the passive layer of each sample was calculated and displayed in Fig. 12. The elements of equation (3) are defined as follows:

  1 2 kT For p  type semiconductor E  E ¼   FB εε0 eNA e C2

(3)

where K is Boltzmann constant, T is the absolute temperature, e stands for the electron charge, EFB represents the flat band potential, NA denotes the acceptor density, ε illustrates the dielectric constant of the passive layer and for copper is usually taken as 12 [62], and ε0 is the term considered as vacuum permittivity. MeS plots (Fig. 11) of ECAPed pure copper samples in the phosphate buffer solution only depicted the p-type behavior. Thus, the flux of cation vacancies through the passive film is essential to the film growth process (based on the PDM). According to a newly modified PDM for 1-pass and 6-pass ECAPed pure copper samples (Fig. 13) which propounds incorporation of hydroxyl vacancies alongside other defects, the flux of oxygen vacancies and/or hydroxyl vacancies and/or cation interstitials through the passive film is essential to the film growth process. The more details of the formation of point defects for pure copper is published elsewhere [48,49].

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Observing Fig. 12, it is well found that the acceptor density of the passive film decreased with the increase of ECAP passes. It is evident that stability of passive film enhances due to the restriction of electrochemical reaction inside the passive film [64]. According to Fig. 12, the values of acceptor density are in the order of 1020 cm3, which is in accordance with the reported values for the pure copper in alkaline solutions [45e49]. Fattah-alhosseini et al. [46] reported lower acceptor density for pure copper sample after eight cycles of ARB compared to the annealed specimen. They claimed that grain refinement, high dislocation density and residual stress are responsible for the improved passive behavior of ARBed pure copper in alkaline solution. Similar results have been also reported through investigation on the role of grain refinement on passive behavior of AA1050 [42], pure titanium [65] and aluminum-based hybrid composite [66] in passive media. Finally, take all results obtained from the electrochemical tests into consideration, it can be concluded that passive and electrochemical responses of pure copper are improved under influence of ECAP process, mainly due to the formation of thicker and less defective passive film.

[12] [13] [14]

[15]

[16]

[17]

[18]

[19] [20]

[21]

4. Conclusion In the present paper, the passive and electrochemical response of pure copper, heavily deformed by ECAP, in a phosphate buffer solution (pH ¼ 10.69) was studied. The PDP plot revealed that with increasing the ECAP passes, the passive region of polarization curves moves to the left-hand side. This denotes an enhancement in protectivity of passive film by increase in the number of ECAP passes. Also, EIS results showed that the polarization resistance increased with the increment of ECAP passes, reflecting the better capability of corrosion prevention. MeS plots depicted that with the increase in ECAP passes, C2 rises. Moreover, it is well found that the acceptor density of the passive film decreased with the increase of ECAP passes. Finally, it is concluded that passive and electrochemical response of pure copper is improved under influence of ECAP process, mainly due to the formation of thicker and less defective passive film.

[22]

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