Grain refinement of bronze alloy by equal-channel angular ... · Grain refinement of bronze alloy...

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Grain refinement of bronze alloy by equal-channel angular pressing (ECAP) and its effect on corrosion behaviour M.M. SADAWY a, *, M. GHANEM b a Mining and Pet. Dept., Faculty of Engineering,Al-Azhar University, Nasr City 11371, Cairo, Egypt b Mechanical Dept., Faculty of Industrial Education, Suez University, Egypt Received 17 December 2015; revised 31 January 2016; accepted 31 January 2016 Available online 2 March 2016 Abstract The corrosion behaviour of bronze alloy prepared by equal channel angular pressing (ECAP) was investigated in 3.5 wt. % NaCl solution. Immersion corrosion tests and different electrochemical techniques were carried out. The results showed that ECAPed bronze samples exhibited higher corrosion resistance compared with the as-cast alloy and the passive current density decreased with increasing number of passes. Moreover, the morphology of alloys indicated that the corrosion damage on the surface of ECAPed bronze was smooth and uniform while the as-cast alloy suffered from selective corrosion. © 2016 China Ordnance Society. Production and hosting by Elsevier B.V.All rights reserved. Keywords: Bronze; ECAP; Corrosion resistance; Passive film; Pitting 1. Introduction Great efforts have been devoted for producing ultrafine grained and nanostructured materials due to their enhanced physical and mechanical properties [1–5]. Several severe plastic deformation (SPD) techniques have been developed and now are available to fabricate bulk ultrafine-grained (UFG) and nano-structured materials, including high-pressure torsion, fric- tion stir processing (FSP), accumulative roll bonding (ARB), and equal channel angular pressing (ECAP) [6]. ECAP is one of the most effective procedures among all SPD techniques to improve the quality of the products [7–11]. In ECAP, the sample is simply pressed through the channel and a shear strain is introduced to the sample when it passes through the bending point of the channel [12]. To produce UFG, the samples must subject to multiple passes through the die as the cross-section remains unchanged. It was proved that, ECAP is a good refining technique to improve the material properties of Al [13], Mg [14], Ti [15], Fe [10] and Cu [16] alloys. The corrosion resistance of materials processed using SPD could be affected by the applied manufacturing process. However, the literature reports wide scattering results. Some researchers have reported that grain refinement can improve corrosion resistance of different materials [17,18] while others stated that the strain-induced grain refinement with more crys- talline defects weakens corrosion resistance [19]. A detailed literature review showed that the corrosion behaviour of the copper and its alloys produced by the SPD methods gained only limited attention. Xu et al. [20] revealed that the corrosion current of UFG copper fabricated by ECAP is higher than that of the coarse-grained copper in Hanks solution. Miyamoto et al. [21] showed that UFG copper exhibited a lower corrosion current in comparison with that in its recrystallized coarse grain (CG). Nikfahm et al. [22] found that ultra-fine grain copper fabricated by accumulative roll bonding process (ARB) showed better corrosion resistance compared to as-cast alloy. On the other hand, the literature showed that there is a little investigation on the corrosion behaviour of bronze alloy fabri- cated by the SPD techniques. Therefore, the present work focused on the effect of ECAP number of passes on the corro- sion and electrochemical behaviour of the bronze alloy. 2. Experimental work 2.1. Material The present study was carried out using the bronze alloy. Its chemical composition is shown in Table 1. The ECAP process was carried out up to 5 passes. The ECAP process and the die design were performed and discussed in details by Elshafey Peer review under responsibility of China Ordnance Society. * Corresponding author. Tel.: +0201113302663. E-mail address: [email protected] (M.M. SADAWY). http://dx.doi.org/10.1016/j.dt.2016.01.013 2214-9147/© 2016 China Ordnance Society. Production and hosting by Elsevier B.V.All rights reserved. Available online at www.sciencedirect.com Defence Technology 12 (2016) 316–323 www.elsevier.com/locate/dt ScienceDirect

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Page 1: Grain refinement of bronze alloy by equal-channel angular ... · Grain refinement of bronze alloy by equal-channel angular pressing (ECAP) and its effect on corrosion behaviour M.M.

Grain refinement of bronze alloy by equal-channel angular pressing(ECAP) and its effect on corrosion behaviour

M.M. SADAWY a,*, M. GHANEM b

a Mining and Pet. Dept., Faculty of Engineering, Al-Azhar University, Nasr City 11371, Cairo, Egyptb Mechanical Dept., Faculty of Industrial Education, Suez University, Egypt

Received 17 December 2015; revised 31 January 2016; accepted 31 January 2016

Available online 2 March 2016

Abstract

The corrosion behaviour of bronze alloy prepared by equal channel angular pressing (ECAP) was investigated in 3.5 wt. % NaCl solution.Immersion corrosion tests and different electrochemical techniques were carried out. The results showed that ECAPed bronze samples exhibitedhigher corrosion resistance compared with the as-cast alloy and the passive current density decreased with increasing number of passes. Moreover,the morphology of alloys indicated that the corrosion damage on the surface of ECAPed bronze was smooth and uniform while the as-cast alloysuffered from selective corrosion.© 2016 China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.

Keywords: Bronze; ECAP; Corrosion resistance; Passive film; Pitting

1. Introduction

Great efforts have been devoted for producing ultrafinegrained and nanostructured materials due to their enhancedphysical and mechanical properties [1–5]. Several severeplastic deformation (SPD) techniques have been developed andnow are available to fabricate bulk ultrafine-grained (UFG) andnano-structured materials, including high-pressure torsion, fric-tion stir processing (FSP), accumulative roll bonding (ARB),and equal channel angular pressing (ECAP) [6].

ECAP is one of the most effective procedures among allSPD techniques to improve the quality of the products [7–11].In ECAP, the sample is simply pressed through the channel anda shear strain is introduced to the sample when it passes throughthe bending point of the channel [12]. To produce UFG, thesamples must subject to multiple passes through the die as thecross-section remains unchanged. It was proved that, ECAP is agood refining technique to improve the material properties ofAl[13], Mg [14], Ti [15], Fe [10] and Cu [16] alloys.

The corrosion resistance of materials processed using SPDcould be affected by the applied manufacturing process.However, the literature reports wide scattering results. Some

researchers have reported that grain refinement can improvecorrosion resistance of different materials [17,18] while othersstated that the strain-induced grain refinement with more crys-talline defects weakens corrosion resistance [19].

A detailed literature review showed that the corrosionbehaviour of the copper and its alloys produced by the SPDmethods gained only limited attention. Xu et al. [20] revealed thatthe corrosion current of UFG copper fabricated by ECAP ishigher than that of the coarse-grained copper in Hanks solution.Miyamoto et al. [21] showed that UFG copper exhibited a lowercorrosion current in comparison with that in its recrystallizedcoarse grain (CG). Nikfahm et al. [22] found that ultra-fine graincopper fabricated by accumulative roll bonding process (ARB)showed better corrosion resistance compared to as-cast alloy.

On the other hand, the literature showed that there is a littleinvestigation on the corrosion behaviour of bronze alloy fabri-cated by the SPD techniques. Therefore, the present workfocused on the effect of ECAP number of passes on the corro-sion and electrochemical behaviour of the bronze alloy.

2. Experimental work

2.1. Material

The present study was carried out using the bronze alloy. Itschemical composition is shown in Table 1. The ECAP processwas carried out up to 5 passes. The ECAP process and the diedesign were performed and discussed in details by Elshafey

Peer review under responsibility of China Ordnance Society.* Corresponding author. Tel.: +0201113302663.E-mail address: [email protected] (M.M. SADAWY).

http://dx.doi.org/10.1016/j.dt.2016.01.0132214-9147/© 2016 China Ordnance Society. Production and hosting by Elsevier B.V. All rights reserved.

Available online at www.sciencedirect.com

Defence Technology 12 (2016) 316–323www.elsevier.com/locate/dt

ScienceDirect

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et al. [23]. Discs of 10 mm in diameter and 5 mm thick were cutfrom bars using disc cutter. The discs were mechanically pol-ished using SiC emery paper of grades 180–1200 mesh andalumina emulsion. Specimens were etched using etchant pre-pared by 2 g K2Cr2O7, 8 mL H2SO4, 4 mL saturated NaCl solu-tion, and 100 mL H2O. The metallographic study was carriedout using Olympus optical microscope.

2.2. XRD

For determining the different phases and grain sizes of theas-cast and ECAPed bronze, a computer-controlled X-raydiffractometer (XRD, Philips Analytical X-ray B.V. machine)with monochromatic Cu-K radiation (l = 0.154 nm) was used. Tocover all the possible diffraction lines of bronze, the scanningrange was 5–100 (2θ) with a step size of 0.05o (2θ) and countingtime of 5 seconds per step. This measurable range was chosenbecause all probable phases of Cu and Sn lie in that range. Peakfit and data analysis (peak position, integrated intensity and grainsize) were accomplished by the WinFit computer program.

2.3. Immersion tests

Immersion corrosion tests were carried out at room tempera-ture according to ASTM G1 and G31 [24]. Initially, bronzesamples were cleaned in HCl acid solution for 2 minutes,degreased with acetone and washed with distilled water anddried. The initial weight of the bronze was taken using an ana-lytical balance for the original weight.After immersion in 3.5 wt.%NaCl solution for 28 days, the specimens removed and cleanedwith HCl solution for 3 min and dried. After cleaning, all speci-mens were reweighed and the loss in weight was calculated.

2.4. Electrochemical techniques

All electrochemical experiments except the electrochemicalimpedance spectra were carried out using Potentiostat/Galvanostat (EG&G model 273). M352 corrosion softwarefrom EG&G Princeton Applied Research was used. A three-electrode cell composed of bronze as a working electrode, Ptcounter electrode, and Ag/AgCl reference electrode were usedfor the tests.

The open-circuit potential (OCP) was recorded after immer-sion of samples in the test solution for 15 min vs. Ag/AgClreference electrode. Tafel polarization tests were carried out ata scan rate of 0.5 mV/s. The PAR Calc Tafel Analysis routinestatistically fits the experimental data to the Stern–Geary modelfor a corroding system. The cyclic potentiodynamic curveswere obtained by scanning the potential in the forward directionfrom −250 mVAg/AgCl towards the anodic direction at a scanrate of 1.0 mV/s. The potential scan was reversed in the back-ward direction when the current density reached a value of0.10 A/cm2. At least three separate experiments were carried

out for each run to ensure reproducibility of results. After cyclicpolarization measurements, all specimens were taken out anddried for microscopic analysis.

Potentiostatic measurements were carried out at +500 mVvs. Ag/AgCl for 5 minutes where the anodic current wasrecorded as a function of time.

An electrochemical impedance spectrum (EIS) was per-formed using Gamry PCI300/4 Potentiostat/Galvanostat/Zraanalyzer. The Echem Analyst software (version 5.21) was usedfor impedance data analysis. Measurements of EIS were per-formed at the OCP. The amplitude of applied potential was10 mV and the frequency ranged from 0.01 Hz to 100 kHz.

All corrosion experiments were carried out in 3.5 wt. % NaClsolution as electrolyte. The solutions were prepared from analyti-cal grade and chemically pure reagents using distilled water.

2.5. Surface examination study after corrosion tests

The as-cast and ECAPed bronze specimens were immersed in3.5 wt. % NaCl solution for a period of 28 days. After that, thespecimens were taken out and dried. The morphology of thesurface was investigated using X-ray and scanning electronmicroscope (SEM), model JEOL JSM-6330F at voltage of20 keV.

3. Results and discussions

3.1. Microstructure and XRD

The optical micrographs of the as-cast bronze and ECAPedsamples of 1, 3 and 5 passes are shown in Fig. 1. The micro-structure of the as-cast bronze (Fig. 1(a)) consists mainly ofequiaxed grains with average grain size of 400 μm. After onepass of ECAP, the grains were elongated with a big width alongextrusion direction as shown in Fig. 1(b). After 3 passes, thegrains were elongated with a small width (Fig. 1(c)). On theother hand Fig. 1(d) indicates that the grains of the bronze alloyafter 5 passes became so refined that grains could not be seen byoptical microscope.

X-ray diffraction profiles of the as-cast and ECAPed bronzeis shown in Fig. 2. It shows that the as-cast bronze containsα-Cu solid solution with CuSn and Cu6Sn5. After 1st pass thebronze contains α-Cu solid solution with minor CuSn andCu6Sn5. After the 2nd, 3rd 4th and 5th passes, the second phasecould not be detected. The crystallite size calculated from X-raydiffraction profiles is shown in Fig. 3. It can be seen that thecrystal size decreased from 604 to 232 Angstrom with increas-ing number of passes to 5 passes. This is attributed to accumu-lative plastic strain introduced by ECAP.

3.2. Immersion tests

Fig. 4 shows the time dependence of the corrosion rate of theas-cast and ECAPed bronze samples calculated from the massloss tests. It can be seen that the corrosion rates of all samplesincreased as the immersion time increased to 16 days andattained stable values after 20 days. This attributed to the patinafilm which formed on the surface of the bronze alloy. On theother hand, Fig. 4 indicates that ECAPed samples have lowerdissolution rate in aqueous NaCl solution than as-cast alloy, and

Table 1Chemical composition (wt. %) of bronze alloy.

Sn Zn Pb Fe Mn Cu

2.019 0.306 0.112 0.02 0.02 Bal.

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the dissolution rate decreases with the increase of ECAPpasses. The dissolution rate of the 5-passes sample is nearly twotimes lower than the as-cast bronze during immersion for 28days. This may be due to more homogenized microstructure anddissolving intermetallic compounds of CuSn and Cu6Sn5 byincreasing ECAP number of passes to 5 passes.

3.3. Open circuit potential measurements (OCP)

The variation of the free corrosion potential with time for theas-cast bronze and ECAPed samples in 3.5%wt. NaCl solution

is shown in Fig. 5. The results reveal that corrosion potential forall samples tends from the moment of immersion towards morenegative values. This behaviour represents the breakdown of thepre-immersion air formed an oxide film and dissolution ofbronze. After passing a long time of immersion, the potentialstabilizes. This represents the formation of the passive film onthe surface, which will enhance the free corrosion potential ofsamples. Furthermore, Fig. 5 indicates that the ECAPedsamples need less time to reach the stable OCP values while theas-cast sample seems to need more time to reach the stable OCPvalue. On the other hand, the results indicate that the corrosionpotential shifts to more positive potential with increasing ECAP

Fig. 1. Effect of equal-channel angular pressing on microstructure of bronze alloy. (a) 0- pass, (b) 1-pass, (c) 3-passes and (d) 5-passes.

Fig. 2. X-ray diffraction of the as-cast bronze and deformed by ECA. Fig. 3. Effect of ECAP number of passes on crystallite size of bronze.

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number of passes. This means that increasing number of passesimproves the patina film on the surface and consequently theelectrochemical reaction decreases.

3.4. Tafel polarization

Tafel polarization curves of the as-cast bronze and deformedby ECAP are shown in Fig. 6. All samples were immersed in3.5 wt. % NaCl solution for about 15 min before polarizationtests to achieve their stable OCP values. It is clear that thecathodic and anodic curves of ECAPed samples display closepolarization behaviour similar to that of as-cast bronze.However, the anodic polarization curves show that whenincreasing ECAP number of passes up to 5 passes, the corro-sion potential (Ecorr) shifts to more noble potential and thecorrosion rate decreased from 7.13 to 3.81 mpy. This behaviourmay be attributed to two reasons: first, no second phases exist inthe homogenized specimen and the uniform electrochemicalproperty leads to small potential difference as well as smallcorrosion driving force. Secondly, the decrease in the grain sizeleads to lower amount of impurities segregated at grain

boundaries. It is well-known that dilution of segregated impu-rities at grain boundaries increases the corrosion resistance[21,25]. It was found that when grain size is decreased from 10to 0.3 μm, segregated impurities is estimated to be diluted byabout 1/30 [21]. Such drastic decreasing of impurities couldincrease the corrosion resistance at the grain boundaries.

On the other hand, the electrochemical parameters includingcorrosion potential (Ecorr), corrosion current density (icorr), cor-rosion rate (mpy), anodic and cathodic slopes (βa and βc) werecalculated from Tafel plots and summarized in Table 2.

Inspection the data in Table 2 reveals that the corrosion rateobtained from Tafel polarization curves is in a good agreementwith the results obtained from weight loss method. Moreover,Table 2 indicates that the anodic and cathodic Tafel slopeschanges with increasing ECAP number of passes. This meansthat ECAP number of passes has an obvious effect on anodicand cathodic reactions.

3.5. Effect of ECAP on the pitting corrosion resistanceof bronze

Cyclic polarization technique was performed to evaluate theeffect of ECAP number of passes on the passive stability andpitting of as-cast and ECAPed bronze alloys in 3.5 wt. % NaClsolution. Fig. 7 shows the cyclic polarization curves of investi-gated samples. It can be seen that all anodic curves are charac-terized by a broad passive region for about 2.3 V from +0.5 V to

Fig. 4. Weight loss-time curves for the corrosion of the as-cast and ECAPedbronze alloys in 3.5 wt. % NaCl solution.

Fig. 5. Potential-time curves of the as-cast bronze and deformed by ECAP in3.5 wt. % NaCl solution.

Fig. 6. Tafel polarization curves of the as-cast bronze and deformed by ECAPin 3.5 wt. % NaCl solution.

Table 2Electrochemical parameters obtained from potentiodynamic polarization mea-surements for the as-cast and ECAPed bronze alloy in 3.5 wt. % NaCl solution.

Sample Ecor/mV icor/(A·cm−2) mpy βc /(mV·dec−1) βa /(mV·dec−1)

0-pass −229.6 13.86 × 10−6 7.132 188.9 84.401-pass −220.3 11.47 × 10−6 5.905 239.6 70.992-passes −228.1 10.92 × 10−6 5.619 201.8 68.393-passes −218.4 8.530 × 10−6 4.390 145.4 51.854-passes −218.0 7.673 × 10−6 3.950 147.1 58.495-passes −212.7 7.411 × 10−6 3.814 126.1 70.31

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+2.8 V over which the current density is constant. This plateau isattributed to the formation of protective oxide film on surfaces ofsamples. The extent of this passivity did not change with decreas-ing grain size. However, the current densities in the passive rangedecrease with increasing number of passes, indicating more pro-tective patina film on the surface of 5-passes sample. Theimproved passivity of the ECAP-processed bronze is due to thesmall grain size promoting the uniform distribution of the ele-ments and facilitating the rapid formation of the passive film[21,26]. Moreover, Fig. 7 indicates that the reverse anodic curvesare shifted to lower currents for as-cast and ECAPed bronzealloys. Therefore, no pitting is expected for all samples in 3.5 wt.% NaCl solution. This behaviour is due to patina which acts as aprotective coating against further corrosion. Also, it can be seenthat the protective potentials for all samples are similar.

3.6. Potentiostatic current–time measurements

The time-dependence of the anodic current density of theas-cast and ECAPed bronze alloys is shown in Fig. 8.All sampleswere held at +0.5 V vs. (Ag/AgCl) to be in the passive regions. Itcan be seen from Fig. 8 that the overall process can be divided

into two stages. In the first stage, the current density rapidlydecreases in few seconds and this represents the formation of thepatina film on the surface. The second stage represents the for-mation and stabilizing of the passive film on the surface. On theother hand, Fig. 8 indicates that the passive current densitydecreases with the increase of ECAP the number of passes. Thiscan be ascribed to more compact passive oxide film obtainedwith decreasing the grain size and dispersion of impurities. It wasfound that the passive current density is related to the structure ofthe passive film. A large number of crystalline defects providedabundant nucleation sites for the passive film, which promotedthe formation of the passive film [26].

3.7. Electrochemical impedance spectroscopymeasurements (EIS)

EIS measurements were performed in order to get furtherinformation on the protective properties of the patina layerformed on the as-cast and ECAPed bronze alloys in 3.5 wt. %NaCl solution. Fig. 9 indicates Nyquist plots of the as-cast andECAPed bronze alloys in 3.5 wt. % NaCl solution. It can beseen that all samples show a capacitive loop in high frequency

Fig. 7. (a,b). Cyclic polarization curves of the as-cast bronze and deformed byECAP in 3.5 wt. % NaCl solution.

Fig. 8. Potentiostatic current–time curves for the as-cast and ECAPed bronzealloys after cyclic polarization tests in 3.5 wt. % NaCl solution.

Fig. 9. Nyquist plots for the as-cast and ECAPed bronze in 3.5 wt. % NaClsolution.

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and a straight line region in low frequency. The capacitive loopin the high-frequency region can be related to the combinationof charge transfer resistance and the double-layer capacitance.The straight line region reflects the anodic diffusion process ofcopper from the surfaces of bronze alloys to the bulk solutionand the cathodic diffusion process of dissolved oxygen from thebulk solution to the surfaces of bronze. Moreover, Fig. 9 showsthat the diameter of the semi-circle increases with increasingECAP number of passes, indicating that the corrosion reactionat bronze/solution interface decreases with the decrease ofgrain size. The Bode phase diagrams of the EIS data (Fig. 10)for the as-cast and ECAPed bronze alloys in 3.5 wt. % NaClsolution show two phase maxima, which reveals the presence oftwo-time constants of the corrosion process.

The equivalent circuit model used to fit the impedancespectra is shown in Fig. 11. This model is based on the follow-ing equation

Z R

R WY

= +

++ ( )

s

p d

j

11

0 w a(1)

The model includes four elements: (Rs) resistance of 3.5 wt.% NaCl solution, (Rp) electric charge transfer resistance forphase boundary of bronze/solution, (CPE) constant phaseelement characterizing the electrical properties of the doublelayer at the interface, and (Wd) Warbrug diffusion impedance,

which characterizes the control of corrosion processes by dif-fusion of mass in the area of the electrolyte at the metal surface.

The calculated equivalent circuit parameters for the as-castand ECAPed bronze alloys are listed in Table 3. The values ofα reveal that alloy surface show some degree of heterogeneity,where α < 1 (α ≈ 0.7). Moreover, Table 3 shows that values ofthe polarization resistance increases with increasing number ofpasses which is in agreement with the results of immersion testsand polarization method.

3.8. Surface examination study after corrosion tests

In order to understand the nature of patina layer formed onthe surface of the bronze, XRD patterns (Fig. 12) were obtainedfrom the surface of as-cast and ECAPed bronze samples afterimmersion in 3.5 wt. % NaCl solution for 28 days. The XRDanalysis indicates the presence of diffraction peaks for CuCl2and Cu2O on the surface of the as-cast and ECAPed bronzesamples. The absence of Sn product peaks in the XRD patternindicates that the concentration of Sn may be less in the outersurface layer. According to this data, the formation of patinafilm in oxygenated chloride solution on the surface of investi-gated alloy may involve the following electrochemical steps[27–29]

Cu Cl CuClads+ =− − (2)

CuCl Cl CuClads− − −+ = 2 (3)

Cu Cl CuCl e+ = +− − −2 2 (4)

Fig. 10. Bode plots for the as-cast and ECAPed bronze in 3.5 wt. % NaClsolution.

Fig. 11. Equivalent circuit model used to fit the impedance spectra for theas-cast and ECAPed bronze in 3.5 wt. % NaCl solution.

Table 3Equivalent circuit parameters for the as-cast and ECAPed bronze alloys in3.5 wt. % NaCl solution at 25 °C.

Sample Rs/(Ωcm2) Rp/(Ωcm2) Y0/μF α Wd/(Ss1/2)

0-pass 5.6 647.3 463.4 0.68 0.0123-passes 3.5 1203 412.0 0.68 0.015-passes 20.3 1670.2 74.4 0.68 0.003

Fig. 12. X-ray diffraction of patina formed on bronze alloys after cyclic polar-ization tests.

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The presence of high concentration of CuCl2 at the metalsurface leads to hydrolysis of CuCl2 and the production of CuO2

according to

CuCl H O Cu O Cl H2 2 2 4 2− − ++ = + + (5)

Fig. 13 shows the surface morphology of the as-cast andECAPed bronze alloys after immersion in 3.5 wt. % NaCl solu-tion for 28 days. Fig. 13(a) indicates that the patina formed on thesurface of the as-cast bronze suffers from selective dissolution.This can be ascribed to the precipitation of (Cu-Sn) phase whichacts as cathode or due to the difference of surfaces crystallo-graphic orientation [30]. On the other hand, Fig. 13(b–f) shows

that ECAPed bronze alloys are corroded uniformly when theaggressive ions attack their surfaces. This is due to the potentialdifference along the surface is very low. Furthermore, Fig. 13(f)indicates that patina formed on ECAPed bronze alloys exhibitshomogeneously compact and smooth passive layer free of theporous film with increasing number of passes to 5-passes.

4. Conclusions

The effect of reducing the grain size by ECAP on the corrosionresistance of bronze alloy was investigated in 3.5 wt. % NaClsolution. The following main observations are made from thisstudy:

Fig. 13. Surface morphology of the as-cast and ECAPed bronze alloys after cyclic polarization tests in 3.5 wt. % NaCl solution. (a) 0-pass, (b) 1-pass, (c) 2-passes,(d) 3-passes (e) 4- passes and (f) 5-passes.

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1) The increase of ECAP number of passes increases thecorrosion resistance of the bronze alloy.

2) The corrosion potential of bronze shifts to more noblepotential with increasing ECAP number of passes.

3) The time and current density of the passive film decreasewith increasing ECAP number of passes.

4) The increase of ECAP number of passes does not haveany effect on the passive zone or the protective potentialsof bronze in 3.5 wt. % NaCl solution.

5) Corrosion behaviour of the as-cast and ECAPed bronzealloys is diffusion controlled mechanism.

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