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

M.M.SADAWY*，M.GHANEM

aMining and Pet.Dept.，F(xiàn)aculty of Engineering，Al-Azhar University，Nasr City 11371，Cairo，Egypt

bMechanical Dept.，F(xiàn)aculty of Industrial Education，Suez University，Egypt

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

M.M.SADAWYa，*，M.GHANEMb

aMining and Pet.Dept.，F(xiàn)aculty of Engineering，Al-Azhar University，Nasr City 11371，Cairo，Egypt

bMechanical Dept.，F(xiàn)aculty of Industrial Education，Suez University，Egypt

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.

Bronze；ECAP；Corrosion resistance；Passive f i lm；Pitting

1.Introduction

Great efforts have been devoted for producing ultraf i ne 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 ultraf i ne-grained （UFG）and nano-structured materials，including high-pressure torsion，friction 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 ref i ning technique to improve the material properties ofAl［13］，Mg ［14］，Ti ［15］，F(xiàn)e ［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.Someresearchers have reported that grain ref i nement can improve corrosion resistance of different materials ［17，18］while others stated that the strain-induced grain ref i nement with more crystalline defects weakens corrosion resistance ［19］.

A detailed literature review showed that the corrosion behaviour of the copper and its alloys produced by the SPD methodsgainedonlylimitedattention.Xuetal.［20］revealedthat 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-f i ne 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 fabricated by the SPD techniques.Therefore，the present work focused on the effect of ECAP number of passes on the corrosion 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 Elshafeyet al.［23］.Discs of 10 mm in diameter and 5 mm thick were cut from bars using disc cutter.The discs were mechanically polished using SiC emery paper of grades 180-1200 mesh and alumina emulsion.Specimens were etched using etchant prepared by 2 g K2Cr2O7，8 mL H2SO4，4 mL saturated NaCl solution，and 100 mL H2O.The metallographic study was carried out using Olympus optical microscope.

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

2.2.XRD

For determining the different phases and grain sizes of the as-cast and ECAPed bronze，a computer-controlled X-ray diffractometer （XRD，Philips Analytical X-ray B.V.machine）with monochromatic Cu-K radiation （l=0.154 nm）was used.To cover all the possible diffraction lines of bronze，the scanning range was 5-100 （2θ）with a step size of 0.05o （2θ）and counting time of 5 seconds per step.This measurable range was chosen because all probable phases of Cu and Sn lie in that range.Peak f

i t and data analysis （peak position，integrated intensity and grain size）were accomplished by the WinFit computer program.

2.3.Immersion tests

Immersion corrosion tests were carried out at room temperature according to ASTM G1 and G31 ［24］.Initially，bronze samples were cleaned in HCl acid solution for 2 minutes，degreased with acetone and washed with distilled water and dried.The initial weight of the bronze was taken using an analytical balance for the original weight.After immersion in 3.5 wt. %NaClsolutionfor28days，thespecimensremovedandcleaned with HCl solution for 3 min and dried.After cleaning，all specimens were reweighed and the loss in weight was calculated.

2.4.Electrochemical techniques

All electrochemical experiments except the electrochemical impedance spectra werecarried outusing Potentiostat/ Galvanostat （EG&G model 273）.M352 corrosion software from EG&G Princeton Applied Research was used.A threeelectrode cell composed of bronze as a working electrode，Pt counter electrode，and Ag/AgCl reference electrode were used for the tests.

The open-circuit potential（OCP）was recorded after immersion of samples in the test solution for 15 min vs.Ag/AgCl reference electrode.Tafel polarization tests were carried out at a scan rate of 0.5 mV/s.The PAR Calc Tafel Analysis routine statistically f i ts the experimental data to the Stern-Geary model for a corroding system.The cyclic potentiodynamic curves were obtained by scanning the potential in the forward direction from-250 mV Ag/AgCl towards the anodic direction at a scan rate of 1.0 mV/s.The potential scan was reversed in the backward direction when the current density reached a value of 0.10 A/cm2.At least three separate experiments were carried out for each run to ensure reproducibility of results.After cyclic polarization measurements，all specimens were taken out and dried for microscopic analysis.

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

An electrochemical impedance spectrum （EIS）was performed using Gamry PCI300/4 Potentiostat/Galvanostat/Zra analyzer.The Echem Analyst software （version 5.21）was used for impedance data analysis.Measurements of EIS were performed at the OCP.The amplitude of applied potential was 10 mV and the frequency ranged from 0.01 Hz to 100 kHz.

All corrosion experiments were carried out in 3.5 wt.%NaCl solution as electrolyte.The solutions were prepared from analytical 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 in 3.5 wt.%NaCl solution for a period of 28 days.After that，the specimens were taken out and dried.The morphology of the surface was investigated using X-ray and scanning electron microscope （SEM），model JEOL JSM-6330F at voltage of 20 keV.

3.Results and discussions

3.1.Microstructure and XRD

The optical micrographs of the as-cast bronze and ECAPed samples of 1，3 and 5 passes are shown in Fig.1.The microstructure of the as-cast bronze （Fig.1（a））consists mainly of equiaxed grains with average grain size of 400 μm.After one pass of ECAP，the grains were elongated with a big width along extrusion direction as shown in Fig.1（b）.After 3 passes，the grains were elongated with a small width （Fig.1（c））.On the other hand Fig.1（d）indicates that the grains of the bronze alloy after 5 passes became so ref i ned that grains could not be seen by optical microscope.

X-ray diffraction prof i les of the as-cast and ECAPed bronze is shown in Fig.2.It shows that the as-cast bronze contains α-Cu solid solution with CuSn and Cu6Sn5.After 1st pass the bronze contains α-Cu solid solution with minor CuSn and Cu6Sn5.After the 2nd，3rd 4th and 5th passes，the second phase could not be detected.The crystallite size calculated from X-ray diffraction prof i les is shown in Fig.3.It can be seen that the crystal size decreased from 604 to 232 Angstrom with increasing number of passes to 5 passes.This is attributed to accumulative plastic strain introduced by ECAP.

3.2.Immersion tests

Fig.4 shows the time dependence of the corrosion rate of the as-cast and ECAPed bronze samples calculated from the mass loss tests.It can be seen that the corrosion rates of all samples increased as the immersion time increased to 16 days and attained stable values after 20 days.This attributed to the patina f i lm which formed on the surface of the bronze alloy.On the other hand，F(xiàn)ig.4 indicates that ECAPed samples have lower dissolution rate in aqueous NaCl solution than as-cast alloy，andthe dissolution rate decreases with the increase of ECAP passes.The dissolution rate of the 5-passes sample is nearly two times lower than the as-cast bronze during immersion for 28 days.This may be due to more homogenized microstructure and dissolving intermetallic compounds of CuSn and Cu6Sn5 by increasing ECAP number of passes to 5 passes.

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.

3.3.Open circuit potential measurements （OCP）

Fig.2.X-ray diffraction of the as-cast bronze and deformed by ECA.

The variation of the free corrosion potential with time for the as-cast bronze and ECAPed samples in 3.5%wt.NaCl solution is shown in Fig.5.The results reveal that corrosion potential for all samples tends from the moment of immersion towards more negative values.This behaviour represents the breakdown of the pre-immersion air formed an oxide f i lm and dissolution of bronze.After passing a long time of immersion，the potential stabilizes.This represents the formation of the passive f i lm on the surface，which will enhance the free corrosion potential of samples.Furthermore，F(xiàn)ig.5 indicates that the ECAPed samples need less time to reach the stable OCP values while the as-cast sample seems to need more time to reach the stable OCP value.On the other hand，the results indicate that the corrosion potential shifts to more positive potential with increasing ECAPnumber of passes.This means that increasing number of passes improves the patina f i lm on the surface and consequently the electrochemical reaction decreases.

Fig.3.Effect of ECAP number of passes on crystallite size of bronze.

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

Fig.5.Potential-time curves of the as-cast bronze and deformed by ECAP in 3.5 wt.%NaCl solution.

3.4.Tafel polarization

Tafel polarization curves of the as-cast bronze and deformed by ECAP are shown in Fig.6.All samples were immersed in 3.5 wt.%NaCl solution for about 15 min before polarization tests to achieve their stable OCP values.It is clear that the cathodic and anodic curves of ECAPed samples display close polarization behaviour similar to that of as-cast bronze. However，the anodic polarization curves show that when increasing ECAP number of passes up to 5 passes，the corrosion potential （Ecorr）shifts to more noble potential and the corrosion rate decreased from 7.13 to 3.81 mpy.This behaviour may be attributed to two reasons:f i rst，no second phases exist in the homogenized specimen and the uniform electrochemical property leads to small potential difference as well as small corrosion driving force.Secondly，the decrease in the grain size leads to lower amount of impurities segregated at grain boundaries.It is well-known that dilution of segregated impurities at grain boundaries increases the corrosion resistance［21，25］.It was found that when grain size is decreased from 10 to 0.3 μm，segregated impurities is estimated to be diluted by about 1/30 ［21］.Such drastic decreasing of impurities could increase the corrosion resistance at the grain boundaries.

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

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

On the other hand，the electrochemical parameters including corrosion potential （Ecorr），corrosion current density （icorr），corrosion rate （mpy），anodic and cathodic slopes （βaand βc）were calculated from Tafel plots and summarized in Table 2.

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

3.5.Effect of ECAP on the pitting corrosion resistance

of bronze

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

Cyclic polarization technique was performed to evaluate the effect of ECAP number of passes on the passive stability and pitting of as-cast and ECAPed bronze alloys in 3.5 wt.%NaCl solution.Fig.7 shows the cyclic polarization curves of investigated samples.It can be seen that all anodic curves are characterized by a broad passive region for about 2.3 V from+0.5 V to +2.8 V over which the current density is constant.This plateau is attributed to the formation of protective oxide f i lm on surfaces of samples.Theextentofthispassivitydidnotchangewithdecreasing grain size.However，the current densities in the passive range decrease with increasing number of passes，indicating more protective patina f i lm on the surface of 5-passes sample.The improved passivity of the ECAP-processed bronze is due to the small grain size promoting the uniform distribution of the elements and facilitating the rapid formation of the passive f i lm［21，26］.Moreover，F(xiàn)ig.7 indicates that the reverse anodic curves are shifted to lower currents for as-cast and ECAPed bronze alloys.Therefore，no pitting is expected for all samples in 3.5 wt. %NaCl solution.This behaviour is due to patina which acts as a protective coating against further corrosion.Also，it can be seen that the protective potentials for all samples are similar.

3.6.Potentiostatic current-time measurements

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

The time-dependence of the anodic current density of the as-castandECAPedbronzealloysisshowninFig.8.Allsamples were held at+0.5 V vs.（Ag/AgCl）to be in the passive regions.It can be seen from Fig.8 that the overall process can be divided into two stages.In the f i rst stage，the current density rapidly decreases in few seconds and this represents the formation of the patina f i lm on the surface.The second stage represents the formation and stabilizing of the passive f i lm on the surface.On the other hand，F(xiàn)ig.8 indicates that the passive current density decreases with the increase of ECAP the number of passes.This can be ascribed to more compact passive oxide f i lm obtained withdecreasingthegrainsizeanddispersionofimpurities.Itwas foundthatthepassivecurrentdensityisrelatedtothestructureof the passive f i lm.A large number of crystalline defects provided abundant nucleation sites for the passive f i lm，which promoted the formation of the passive f i lm ［26］.

3.7.Electrochemical impedance spectroscopy measurements （EIS）

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

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

EIS measurements were performed in order to get further information on the protective properties of the patina layer formed on the as-cast and ECAPed bronze alloys in 3.5 wt.% NaCl solution.Fig.9 indicates Nyquist plots of the as-cast and ECAPed bronze alloys in 3.5 wt.%NaCl solution.It can be seen that all samples show a capacitive loop in high frequency and a straight line region in low frequency.The capacitive loop in the high-frequency region can be related to the combination of charge transfer resistance and the double-layer capacitance. The straight line region ref l ects the anodic diffusion process of copper from the surfaces of bronze alloys to the bulk solution and the cathodic diffusion process of dissolved oxygen from the bulk solution to the surfaces of bronze.Moreover，F(xiàn)ig.9 shows that the diameter of the semi-circle increases with increasing ECAP number of passes，indicating that the corrosion reaction at bronze/solution interface decreases with the decrease of grain size.The Bode phase diagrams of the EIS data （Fig.10）for the as-cast and ECAPed bronze alloys in 3.5 wt.%NaCl solution show two phase maxima，which reveals the presence of two-time constants of the corrosion process.

The equivalent circuit model used to f i t the impedance spectra is shown in Fig.11.This model is based on the following equationwhich characterizes the control of corrosion processes by diffusion of mass in the area of the electrolyte at the metal surface.

Table 3Equivalent circuit parameters for the as-cast and ECAPed bronze alloys in 3.5 wt.%NaCl solution at 25 °C.

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

3.8.Surface examination study after corrosion tests

In order to understand the nature of patina layer formed on the surface of the bronze，XRD patterns （Fig.12）were obtained from the surface of as-cast and ECAPed bronze samples after immersion in 3.5 wt.%NaCl solution for 28 days.The XRD analysis indicates the presence of diffraction peaks for CuCl2and Cu2O on the surface of the as-cast and ECAPed bronze samples.The absence of Sn product peaks in the XRD pattern indicates that the concentration of Sn may be less in the outer surface layer.According to this data，the formation of patina f i lm in oxygenated chloride solution on the surface of investigated alloy may involve the following electrochemical steps［27-29］

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

Fig.11.Equivalent circuit model used to f i t the impedance spectra for the as-cast and ECAPed bronze in 3.5 wt.%NaCl solution.

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

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.

The presence of high concentration of CuCl2at the metal surface leads to hydrolysis of CuCl2and the production of CuO2according to

Fig.13 shows the surface morphology of the as-cast and ECAPed bronze alloys after immersion in 3.5 wt.%NaCl solutionfor28days.Fig.13（a）indicatesthatthepatinaformedonthe surface of the as-cast bronze suffers from selective dissolution. This can be ascribed to the precipitation of （Cu-Sn）phase which acts as cathode or due to the difference of surfaces crystallographic orientation ［30］.On the other hand，F(xiàn)ig.13（b-f）shows that ECAPed bronze alloys are corroded uniformly when the aggressive ions attack their surfaces.This is due to the potential difference along the surface is very low.Furthermore，F(xiàn)ig.13（f）indicates that patina formed on ECAPed bronze alloys exhibits homogeneously compact and smooth passive layer free of the porous f i lm with increasing number of passes to 5-passes.

4.Conclusions

TheeffectofreducingthegrainsizebyECAPonthecorrosion resistance of bronze alloy was investigated in 3.5 wt.%NaCl solution.The following main observations are made from this study:

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

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

3）The time and current density of the passive f i lm decrease with increasing ECAP number of passes.

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

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

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Received 17 December 2015；revised 31 January 2016；accepted 31 January 2016 Available online 2 March 2016

Peer review under responsibility of China Ordnance Society.

*Corresponding author.Tel.:+0201113302663.

E-mail address:mosaadsadawy@yahoo.com （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.

? 2016 China Ordnance Society.Production and hosting by Elsevier B.V.All rights reserved.