- Email: [email protected]
Combining equal-channel angular pressing and heat treatment to obtain enhanced corrosion resistance in 6061 aluminum alloy
Journal of Alloys and Compounds 648 (2015) 912e918
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Combining equal-channel angular pressing and heat treatment to obtain enhanced corrosion resistance in 6061 aluminum alloy Omid Nejadseyfi a, *, Ali Shokuhfar a, Amirreza Dabiri b, Amin Azimi a a
Advanced Materials and Nanotechnology Research Center, Department of Mechanical Engineering, K.N. Toosi University of Technology, Tehran 19395-1999, Iran b Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2015 Received in revised form 22 May 2015 Accepted 27 May 2015 Available online 10 June 2015
The corrosion behavior of 6061 aluminum alloy processed by heat treatment and equal-channel angular pressing (ECAP) is investigated. Current transient, potentiodynamic polarization and electrochemical impedance spectroscopy are used to assess the corrosion resistance of 6061 aluminum alloy in different processing conditions. The results confirm that the peak-aged sample after two ECAP passes and annealed sample after four ECAP passes are the most resistant samples against corrosion. Using ECAP leads to substantial increase in the dislocation density, re-distribution of precipitates, crystallite refinement, alteration of the surface area of the cathodic sites, and increase in the volume fraction of grain boundaries that affects corrosion resistance. Assessment of the corroded surfaces of the materials shows that some coarse-grains are corroded selectively in the annealed sample leading to formation of deep and large pits on the sample surface. However, after ECAP, shallow and uniform corroded surface is observed that is in agreement with the corrosion diagrams. © 2015 Elsevier B.V. All rights reserved.
Keywords: Corrosion Electrochemical impedance spectroscopy Metals and alloys Potentiodynamic polarization Scanning electron microscopy Transmission electron microscopy
1. Introduction One of the main characteristics of aluminum alloys is excellent corrosion resistance that makes them suitable to be widely used in automotive, aerospace, and marine industry. A wide range of heat treatments are also available for aluminum alloys that make it possible to tailor their properties to fit specific applications. In addition, development of severe plastic deformation (SPD) methods to produce nanostructured (NS) and ultrafine-grained (UFG) materials makes it possible to control the microstructure and properties of heat treatable aluminum alloys. ECAP is one of the most used SPD processes that induces simple shear deformation to the samples without any cross-sectional change and due to the constancy of the cross-sectional area of the billet, the process is repeatable in further passes [1,2]. Recently, researchers have widely investigated the strength, toughness, wear, and fatigue properties of the samples processed by SPD methods [3e9]. More recently, the corrosion resistance of
* Corresponding author. E-mail addresses: o.nejadseyfi@sina.kntu.ac.ir, (O. Nejadseyfi), [email protected] (A. Shokuhfar). http://dx.doi.org/10.1016/j.jallcom.2015.05.177 0925-8388/© 2015 Elsevier B.V. All rights reserved.
o.nejadseyfi@gmail.com
different materials after SPD is also investigated. Darmiani et al. [10] studied the corrosion behavior of AleSiC nano-composite, produced by accumulative roll bonding (ARB). They showed that increasing the ARB cycles led to a decrease in the number of pits and an increase in the pitting corrosion resistance. Nie et al. [11] investigated the corrosion behavior of commercial purity titanium processed by high-pressure torsion (HPT). They stated that there is a complicated relation between grain refinement and corrosion resistance, due to contradictory effects of inhomogeneous microstructure and grain size reduction. Zheng et al. [12] investigated the effects of microstructural changes during ECAP on the corrosion behavior of 304 stainless steel. They reported that the corrosion resistance of the samples improved after ECAP due to increase in the stability of the passive film on 304 stainless steel though the thickness and composition of the passive films on the as-received and 8-pass samples were very similar. Miyamoto et al. [13] studied the corrosion behavior of copper with and without processing by ECAP in a modified Livingstone etchant. Their study revealed that UFG copper exhibited remarkably lower corrosion current in comparison with that in the coarse-grained (CG) counterpart. Jiang et al. [14] reported the enhanced corrosion resistance of hypereutectic AleSi alloy subjected to ECAP. They found that in UFG structure, the easier formation of an oxide layer with an
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
improved adhesion force and protection efficacy improved the corrosion behavior. Several other investigations have been performed to investigate the influence of SPD on the corrosion behavior of different materials. Akiyama et al. [15] reported improvement of corrosion resistance of the AleNi alloy by ECAP, Hosseini et al. [16] investigated the effects of grain size and texture on the corrosion behavior tna et al. of commercial purity titanium processed by ECAP, and Vra [17] reported enhanced corrosion resistance in AZ31 magnesium alloy processed by ECAP. In addition, Shi et al. [18] studied the effect of Sc and Zr addition on the corrosion behavior of AleZneMgeCu alloys, Canakci et al. [19] assessed the influence of milling time in mechanical alloying on the formation of a protective coating layer, and Hockauf et al. [20] stated that the solid-solution heat treated 6082 aluminum alloy combined with post-ECAP aging showed excellent corrosion resistance. In this study, corrosion behavior of 6061 aluminum alloy processed by a combination of heat treatment and ECAP is investigated to reveal the simultaneous effects of grain refinement and heat treatment on corrosion resistance. In order to compare the results, the tests were performed in four processing conditions and the influence of heat treatment, severe plastic deformation, and a combination of heat treatment and severe plastic deformation are discussed. The results of different corrosion tests on the annealed material were used as the reference case. The aim is to address the individual and combined effects of heat treatment and ECAP on corrosion resistance of 6061 aluminum alloy. The information obtained from these analyses can be used to address failure reasons due to corrosion and propose an appropriate method for materials processing to promote corrosion resistance. 2. Experimental procedure Four groups of cylindrical billets with a diameter of 10 mm and a length of 60 mm were machined from an ingot of aluminum alloy 6061. The composition of this alloy is presented in Table 1. All billets were initially annealed in 686 K for five hours. The billets of first group, named as group A, were kept in the annealed condition (O temper). The billets of the second group, named as group B, were solution heat treated and artificially aged to achieve the T6 temper. Solution heat treatment was performed by heating the billets to the super-saturated condition at around 800 K for 30 min followed by rapid quenching in cold water. Artificial aging was also performed at 433 K for 18 h. Third group contained the billets of O temper, which were pressed using an ECAP die by route C up to four passes. (Group C). The last group contained the billets in T6 condition that were ECA pressed up to two passes using route C. A split die with an internal angle of 90 and an arc of curvature of 20 was implemented to conduct ECAP at room temperature. MOS2 was used as lubricant in order to reduce friction [21]. Punch speed was chosen to be 5 mms-1. Prior to corrosion tests, small disks were prepared from the center of each billet and were polished using SiC papers and polishing powders. Electrochemical measurement was performed at room temperature for each specimen in a solution containing 3.5% NaCl. A reference electrode of Ag/AgCl was used for electrochemical impedance spectroscopy (EIS) using an auxiliary electrode of platinum. The frequency range was from 105 to 102 Hz using
sinusoidal AC voltage of 10 mV amplitude. Subsequently, potentiodynamic polarization was carried out in the range of 1.2 to 0.2 V at a scan rate of 0.25 mVs1. The analysis of current transients was another method used to assess the mechanisms of localized corrosion at a constant voltage, the variation of current density was recorded during 1200s. This test was used to analyze pit initiation and re-passivation processes in different samples. All tests were repeated at least three times in order to maintain a high statistical accuracy. XRD analysis was performed using Cu Ka radiation (l ¼ 0.15406 nm) at a voltage and electrical current of 40 kV and 30 mA, respectively. The XRD patterns were recorded in the 2q range of 0e100 . Corroded surfaces were investigated using scanning electron microscopy (SEM) to observe the microscopic features of corrosion in each case. Optical microscopy (OM) and highresolution transmission electron microscopy (HR-TEM) were also used in order to obtain the microstructure of the processed materials before and after ECAP. 3. Results 3.1. Current transient test results Fig. 1 shows the graphs of the current density versus immersion time for the samples of four groups where the magnitude of the voltage remained constant. The variation of electrode current with immersion time is depicted to investigate the pit initiation and repassivation processes. In all samples, the current magnitude is increased during the initial stages of immersion. However, after about 200s, the current values remain almost stable with very little fluctuation. It is shown that the highest current is obtained for annealed sample showing the lowest pitting corrosion resistance for this sample. After processing the annealed sample by ECAP, the current values are decreased showing improved pitting corrosion resistance. The heat-treated sample also shows an improved pitting corrosion resistance compared to the annealed sample. However, one ECAP pass on the annealed specimen seems to be more effective in improving the pitting corrosion resistance than peak aging (T6 heat treatment). It is also shown that the pitting corrosion resistance for both annealed and peak-aged samples after two ECAP passes is very similar and the annealed specimen after four ECAP passes is the most resistant sample against pitting corrosion. Nevertheless, the formation of the protective layer in the peak-aged sample after two ECAP passes is immediate after immersion, while the oxide layer in the annealed sample after four ECAP passes needs more time to be formed. Careful inspection of these curves confirms that after 200s, there is gradual increase in the current value for the annealed and peak-
Table 1 Composition of the Al6061 Alloy in weight percentage. Al
Mg
Si
Fe
Cu
Mn
Base
0.807
0.669
0.627
0.23
0.12
913
Fig. 1. Current density versus immersion time at a constant voltage.
914
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
aged samples. However, for the ECA pressed samples the current becomes more stable during higher immersion times. This phenomenon suggests that the annealed and heat-treated samples need more immersing time to form a protective passive film, which indicates that pitting occurs gradually. This observation can be related to the quick dissolution of the oxide film in the aggressive solution. However, the current density remains almost constant in the samples processed by ECAP. Although practical information can be obtained from these curves, other electrochemical tests are still needed to accurately judge the corrosion resistance of different samples. 3.2. Potentiodynamic polarization Determining corrosion rate and evaluation of corrosion properties is usually performed by potentiodynamic polarization test. Fig. 2 illustrates the polarization curves for the samples of each group. Corrosion current and potential extracted from potentiodynamic polarization diagrams is also illustrated in Fig. 3. It is shown that the corrosion current of the annealed specimen after ECAP is more than that of the unprocessed specimen. A reverse trend is observed for the peak-aged sample before and after ECAP. Complex passive corrosion behavior in NaCl solution is observed in both annealed and peak-aged samples. The corrosion current density and corrosion potential are evaluated using the Tafel slope fitting from the polarization curves and the corrosion rates are included in Table 2 and Fig. 3. For the annealed sample, the corrosion rate is increased after processing by ECAP. In addition, heat treatment leads to increase in the corrosion rate. However, these modifications are very narrow in the polarization curves and actually preceding cathodic reaction on the sample surface may compromise the accuracy of the obtained results [11]. Therefore, it is suggested to use the EIS technique, which is more sensitive in characterizing the corrosion behavior of the materials of different groups accurately. 3.3. Electrochemical impedance spectroscopy Although polarization curves have been used widely for corrosion analysis, preceding cathodic reaction on the sample surface may compromise the accuracy of the obtained results. Therefore, it is better to use EIS technique to characterize the electrochemical interface between electrodes and electrolytes, which is more sensitive. Fig. 4 shows representative Nyquist plots obtained for different samples in 3.5% NaCl solution. The results show that, increasing the number ECAP passes leads to increase in the corrosion resistance. The annealed sample has the lowest resistance
Fig. 3. Corrosion current and potential extracted from potentiodynamic polarization diagrams.
against corrosion. Peak aging leads to a gradual improvement in the corrosion resistance of aluminum alloy. However, one ECAP pass is more effective in improvement of the corrosion resistance than peak aging. These observations accompany the obtained results in the current transients. It is notable that the corrosion resistance of both annealed and peak-aged specimens increases when the number of passes are increased. However, the peak-aged sample obtains exceptional corrosion resistance after two ECAP passes. This observation could be directly related to the microstructural evolution, distribution, size and volume fraction of the precipitates [20]. Fig. 5 shows X-Ray diffraction (XRD) patterns for the samples of four groups. The constituting phases and the relative amount of these phases could be distinguished by comparing the ratio of the intensities. Therefore, it is notable that the relative amount of AlFeSi phase increases after heat treatment. This observation confirms the increase in corrosion resistance after heat treatment compared with that of the annealed sample. As expected, processing by ECAP reduces the size of precipitations and leads to a uniform distribution of the second phase particles [22,23]. The induced strain, grain refinement mechanism and induction of high amount of dislocation density are almost identical in both annealed and peak-aged samples after ECAP. The peak broadening in all cases shows that the average crystallite size of the soft and hard phases decreases after two ECAP passes. However, significant difference between the corrosion behavior of annealed and peak-aged samples after two ECAP passes may arise from the presence of the fine second phase particles with a uniform distribution in the aluminum matrix. It is also notable the influence of the presence of fine second phase particles and their uniform distribution is more effective than repeating ECAP and gradual conversion of low angle grain boundaries to high angle grain boundaries. In other words, the corrosion resistance of the peak-aged sample after two ECAP passes is even higher than that of the annealed sample after four ECAP passes.
3.4. Microstructure and corroded surface
Fig. 2. Potentiodynamic polarization curves for different samples.
In the previous sections, the results of different corrosion tests were presented. This section aims to clearly represent the underlying mechanisms in the corrosion. Fig. 6 illustrates the microstructure of the annealed specimen before and after ECAP. As shown in this figure, the average grain size is reduced from 20 to 30 mm to below 500 nm after only two ECAP passes. In addition to that, a high number of dislocations and sub-grain boundaries that appear as dark lines in Fig. 6b are easily detectable. A high dislocation density together with refined grains is the main reason that improves corrosion resistance of the sample. In addition, Fig. 5 showed the constitutive phases and their relative amount before
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
915
Table 2 Data extracted from potentiodynamic polarization diagrams. Sample
Ecorr/V
Icorr/A cm2
Anneal Anneal þ 2Passes Anneal þ 4Passes Peak age Peak age þ 2Passes
0.623 0.696 0.680 0.707 0.728
8.588 3.133 6.613 3.446 3.723
107 106 106 106 107
Fig. 4. Electrochemical impedance spectroscopy diagrams for different samples.
ba/V dec1
bc/V dec1
Rp/ohm
0.688 0.278 0.778 0.428 0.166
0.014 0.014 0.015 0.021 0.018
3.284 3.433 4.989 7.312 2.264
Vcorr/mm y1 103 102 102 102 103
1.009 3.680 7.767 4.048 4.384
102 102 102 102 103
and after precipitation hardening and ECAP. Fig. 7 depicts the corroded surfaces of the samples of each group to show how the microstructural features affect the pitting on the samples during the test. For the annealed sample (Fig. 7a), there are a lot of local environmental attacks. Large and deep pits are easily detectable in this case. After peak aging, these local environmental attacks reduce, but the size and depth of the pits remain relatively large (Fig. 7b). Most of the sample surface remained intact, while there are selective corrosion areas on the surface. Both annealed and peak-aged sample after two ECAP passes show relatively small pits as shown in Fig. 7c and d. Nevertheless, they are well-distributed in the peakaged sample. This might have originated from the uniform distribution of the second phase particles. Finally, the annealed sample after four ECAP passes represents shallow and uniformly corroded surface (Fig. 7e). Selective corrosion of grains can be attributed to the differences in the crystallographic orientations of specific grains. When some coarse-grains are corroded selectively, deep and large pits are formed on the sample surface. That is why in the annealed sample deeper and larger pits are detectable than the other samples. A closer inspection of the pits in Fig. 8 confirms the presence of abundant and deeper corrosion pits on the corroded surface of the annealed sample in which the grain size is more than 40 times bigger than that of the annealed sample after two ECAP passes. Fig. 9 shows the EDS analysis of point A and B shown in Fig. 7. As shown in this figure, the concentration of Na and Cl around the pit is detectable. The Mg element is totally solved on point B. However, the formation of oxide layer on point B protects the surface and the major alloying elements are retained. 4. Discussion
Fig. 5. X-ray diffraction (XRD) patterns.
The graphs of the current densities versus immersion time for the samples of four groups in a constant voltage are shown in Fig. 1. These curves showed that there was gradual increase in the current value for the annealed and peak-aged sample. Aluminum alloys exhibit relatively good corrosion resistance due to spontaneous formation of an oxide film in an atmospheric environment.
Fig. 6. The microstructure of (a) annealed specimen and (b) annealed specimen after two ECAP passes using route C.
916
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
Fig. 7. Corroded surfaces of (a) annealed specimen, (b) peak-aged sample, (c) annealed specimen after two ECAP passes using route C, (d) peak-aged sample after two ECAP passes using route C, and (e) annealed specimen after four ECAP passes using route C.
Fig. 8. Three dimensional images from the corroded surfaces of (a) annealed specimen, 1500, and (b) annealed specimen after two ECAP passes, 1500X (To view this image correctly, the reader is recommended to refer to the web version of the article and to wear a passive 3-D red-cyan glasses.).
However, they are prone to pitting corrosion mainly in chloriderich solutions in which the oxide film is attacked by chloride ions. For both annealed and peak-aged samples some immersing time is needed to form a protective passive film once pitting occurs gradually. Quick dissolution of the oxide film in the aggressive solution
leads to gradual increase in current value. However, for the ECA pressed samples the current becomes more stable in the initial immersion times as it was observed previously [12]. The relative dissolution or passivation of a surface can be linked to the total length of the grain boundary. As shown by Orlov et al. [24], a change
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
Fig. 9. EDS analysis of points A and B shown in Fig. 7.
in grain boundary length can either increase or decrease the corrosion rate, which depends on specific material/environment combinations. In their potentiodynamic polarization study on Mg alloy ZK60, icorr appeared to decrease as the grain size was reduced. Additionally, corrosion behavior could not be exclusively attributed to the grain size while ignoring solute population throughout the microstructure [24]. Nevertheless, in the absence of precipitates in high-purity materials, the grain boundaries will play the key role in controlling the corrosion behavior. Such an investigation was one of the main challenges addressed in this study. X-ray diffraction patterns shown in Fig. 5, showed that the relative amount of second phase particles in the annealed sample was very low and ECAP led to decreasing corrosion rate. However, the peak-aged sample, in which the relative amount of second phase particles was higher than that of the annealed sample, showed improved corrosion resistance after ECAP. Similar observation was reported for the corrosion resistance of two Mg alloys [25]. The corrosion resistance of AE21 alloy decreased after ECAP while the corrosion resistance of AE42 alloy increased. As the grain size of both alloys after ECAP was nearly the same, the variation in the corrosion resistance of these alloys originated from variation of the alloying elements and the uniform spatial distribution of the alloying elements in AE42 led to enhanced corrosion resistance. As indicated, peak-aged sample after ECAP with fine dispersion of second phase particles showed improved corrosion resistance. Jiang et al. [14] observed such a phenomenon for cast Al-26 wt% Si alloy. The improved corrosion resistance was related to grain refinement that provided more nucleation sites to form denser and thicker natural oxide film resulting in higher impermeability to aggressive media [14]. Macroscopic assessments also showed that the samples were generally eroded with coincident pitting corrosion due to the presence of large secondary phases that created
917
local galvanic cells. Additionally, abundant and deeper corrosion pits on the corroded surface of the as-cast sample were replaced by shallow corrosion grooves. The main reason was the uniform distribution of fine secondary-phase Si particles on the UFG Al matrix that weakened the susceptibility to pitting corrosion, while it inhibited general microgalvanic reactions as observed in this study. Fig. 7 clearly showed that how the precipitation hardening as well as the size and distribution of precipitates can alter the corrosion mechanisms. Such an observation was also reported by Akiyama et al. [15] who performed ECAP on Al-5 wt% Cu alloy and stated that the size and the total area of Al2Cu particles reduced with the repetition of ECAP passes. The refinement of Al2Cu particles, which acted as cathode and the Cu-depleted zone surrounding Al2Cu particles where pitting preferentially took place had been diminished. These changes in microstructure of the processed samples seemed to be responsible for improvement in the pitting resistance after ECAP. The dissolution of Al8Fe2Si inclusions that were within grain boundaries together with the existing grain boundaries were also introduced as a main reason for changing the morphology of pitting corrosion from low-density deep pits to high-density shallow pits which led to transition from inter-granular corrosion to pitting corrosion [26]. In addition to that, comparing Fig. 7c, d and e indicated that the microstructural homogeneity achieved after repeating the ECAP process up to four passes was also beneficial in achieving enhanced corrosion resistance, because it can reduce the localized corrosion in some grains. In fact, the homogeneity of corrosive damages seems to be the main advantages of ultrafine-grained materials in comparison with conventional polycrystals, which are susceptible to highly localized corrosion [27]. Figs. 7 and 8 clearly showed the differences between the highly localized corrosion and shallow pits on the surface of annealed sample before and after ECAP, respectively. The results of potentiodynamic polarization test shown in Figs. 2 and 3, and specifically the corrosion volume calculated in Table 2 suggested that the predictions of this test in different processing conditions vary slightly. In other words, the variation of corrosion rate obtained for annealed, peak-aged, and annealed sample after ECAP is very narrow, which also can be affected by experimental errors and preceding cathodic reaction on the sample surface [16]. The only conclusion that can be made confidently is improvement of the corrosion resistance of peak-aged sample after two ECAP passes, because the corrosion rate obtained for this specimen is about one tenth of the corrosion rate obtained for the rest of the samples. In addition, it seems that ECAP deteriorates the corrosion resistance of annealed sample in this test. These differences are usually explained considering various and counteracting effects of ECAP on different samples. The processing by ECAP leads to substantial increase in the dislocation density, re-distribution of precipitates, crystallite refinement, alteration of the surface area of the cathodic sites, and increase in the volume fraction of grain boundaries. Each of these microstructural changes modifies the cathodic sites prone to corrosion, formation of passive film, and in general, the corrosion behavior of the samples [26]. Nevertheless, it is suggested that results of the EIS technique be used, being a sensitive method in the investigation of the corrosion behavior. The results of the EIS tests on the samples of four groups shown in Fig. 4 exhibit a little bit difference with the data obtained in polarization curves. While potentiodynamic polarization suggested that the corrosion resistance of the annealed sample after ECAP reduced, the EIS technique indicated that the corrosion resistance of that sample is improved similar to the current transient test results. As mentioned, preceding cathodic reaction on the sample surface may compromise the accuracy of the obtained results.
918
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
Therefore, the results of the EIS technique that is more sensitive could be reliable in terms of corrosion resistance. Nevertheless, all corrosion tests performed in this study imply that the most resistant sample against corrosion is the peak-aged sample after two passes of ECAP. It should be noted that the material on which ECAP is applied is also important to assess the corrosion behavior. For instance, the corrosion behavior of copper does not qualitatively change after ECAP [27]. However, the microstructural evolution during severe plastic deformation may lead to enhanced stability and homogeneity of coating [28,29]. The state of grain boundary, which depends on the pre and post ECAP heat treatment, is also another important factor. Therefore, all these complicated cases needs to be investigated in detail to clearly understand the corrosion behavior of the materials processed by a combination of severe plastic deformation and heat treatment. 5. Conclusions In this study, the corrosion behavior of 6061 aluminum alloy processed by a combination of heat treatment and ECAP was investigated. Current transient, potentiodynamic polarization, and EIS tests were used to reveal the effects of processing history and different microstructural backgrounds on the corrosion resistance of aluminum alloy. The following conclusions were made as the main results of this investigation: The steady state anodic current density that served as a measure of the average dissolution rate at a certain potential was reduced after ECAP for both annealed and peak-aged samples. In addition, the steady state anodic current density for peak-aged sample was lower than that of the annealed specimen. These observations were related to the increase in the total length of the grain boundaries by which the formation of passive layer with an improved adhesion force and protection efficacy led to improved corrosion resistance. Potentiodynamic polarization tests showed that for annealed samples, the corrosion rate increased after processing by ECAP. In addition, heat treatment led to an increase in the corrosion rate. At the same time, peak-aged samples after two ECAP passes showed enhanced corrosion resistance. As the differences were very narrow in this test, they could be affected by preceding cathodic reaction or experimental/setup errors. Electrochemical impedance spectroscopy (EIS) showed that the corrosion resistance of both annealed and peak-aged specimens increased when the number of ECAP passes increased. The results that were obtained confirmed that the peak-aged sample after two ECAP passes was the most resistant sample against corrosion. Using ECAP led to substantial increase in the dislocation density, re-distribution of precipitates, crystallite refinement, alteration of the surface area of the cathodic sites, and increase in the volume fraction of grain boundaries. Each of these microstructural changes modified the cathodic sites prone to corrosion, formation of passive film, and in general, the corrosion behavior of the samples. Acknowledgments The authors would like to thank Iran Nanotechnology Initiative Council (INIC) Tehran, Iran, for the financial support of this work under the grant numbers of 38288 and 59117. References [1] R.Z. Valiev, T.G. Langdon, Principles of equal-channel angular pressing as a processing tool for grain refinement, Prog. Mater. Sci. 51 (2006) 881e981.
[2] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, The process of grain refinement in equal-channel angular pressing, Acta Mater. 46 (1998) 3317e3331. [3] C.S. Chung, J.K. Kim, H.K. Kim, W.J. Kim, Improvement of high-cycle fatigue life in a 6061 Al alloy produced by equal channel angular pressing, Mater. Sci. Eng. A 337 (2002) 39e44. [4] A. Canakci, F. Arslan, Abrasive wear behaviour of B4C particle reinforced Al2024 MMCs, Int. J. Adv. Manuf. Technol. 63 (2012) 785e795. [5] L. Meyer, M. Hockauf, B. Zillmann, I. Schneider, Strength, ductility and impact toughness of the magnesium alloy az31b after equal-channel angular pressing, Int. J. Mater. Form. 2 (2009) 61e64. [6] A. Azimi, A. Shokuhfar, O. Nejadseyfi, Mechanically alloyed Al7075eTiC nanocomposite: Powder processing, consolidation and mechanical strength, Mater. Des. 66 (Part A) (2015) 137e141. [7] A. Vinogradov, T. Kawaguchi, Y. Kaneko, S. Hashimoto, Fatigue crack growth and related microstructure evolution in ultrafine grain copper processed by ECAP, Mater. Trans. 53 (2012) 101e108. [8] A. Canakci, Microstructure and abrasive wear behaviour of B4C particle reinforced 2014 Al matrix composites, J. Mater. Sci. 46 (2011) 2805e2813. [9] A. Azimi, H. Fallahdoost, O. Nejadseyfi, Microstructure, mechanical and tribological behavior of hot-pressed mechanically alloyed AleZneMgeCu powders, Mater. Des. 75 (2015) 1e8. [10] E. Darmiani, I. Danaee, M.A. Golozar, M.R. Toroghinejad, Corrosion investigation of AleSiC nano-composite fabricated by accumulative roll bonding (ARB) process, J. Alloys Compd. 552 (2013) 31e39. [11] M. Nie, C. Wang, M. Qu, N. Gao, J. Wharton, T. Langdon, The corrosion behaviour of commercial purity titanium processed by high-pressure torsion, J. Mater. Sci. 49 (2014) 2824e2831. [12] Z.J. Zheng, Y. Gao, Y. Gui, M. Zhu, Corrosion behaviour of nanocrystalline 304 stainless steel prepared by equal channel angular pressing, Corros. Sci. 54 (2012) 60e67. [13] H. Miyamoto, K. Harada, T. Mimaki, A. Vinogradov, S. Hashimoto, Corrosion of ultra-fine grained copper fabricated by equal-channel angular pressing, Corros. Sci. 50 (2008) 1215e1220. [14] J. Jiang, A. Ma, D. Song, D. Yang, J. Shi, K. Wang, L. Zhang, J. Chen, Anticorrosion behavior of ultrafine-grained Al-26 wt% Si alloy fabricated by ECAP, J. Mater. Sci. 47 (2012) 7744e7750. [15] E. Akiyama, Z. Zhang, Y. Watanabe, K. Tsuzaki, Effects of severe plastic deformation on the corrosion behavior of aluminum alloys, J. Solid State Electrochem 13 (2009) 277e282. [16] M. Hoseini, A. Shahryari, S. Omanovic, J.A. Szpunar, Comparative effect of grain size and texture on the corrosion behaviour of commercially pure titanium processed by equal channel angular pressing, Corros. Sci. 51 (2009) 3064e3067. [17] J. Vr atn a, B. Hadzima, M. Bukovina, M. Jane cek, Room temperature corrosion properties of AZ31 magnesium alloy processed by extrusion and equal channel angular pressing, J. Mater. Sci. 48 (2013) 4510e4516. [18] Y. Shi, Q. Pan, M. Li, X. Huang, B. Li, Effect of Sc and Zr additions on corrosion behaviour of AleZneMgeCu alloys, J. Alloys Compd. 612 (2014) 42e50. [19] A. Canakci, T. Varol, F. Erdemir, S. Ozkaya, H. Mindivan, Microstructure and properties of FeeAl intermetallic coatings on the low carbon steel synthesized by mechanical alloying, Int. J. Adv. Manuf. Technol. 73 (2014) 849e858. [20] M. Hockauf, L. Meyer, D. Nickel, G. Alisch, T. Lampke, B. Wielage, L. Krüger, Mechanical properties and corrosion behaviour of ultrafine-grained AA6082 produced by equal-channel angular pressing, J. Mater. Sci. 43 (2008) 7409e7417. [21] A. Shokuhfar, O. Nejadseyfi, The influence of friction on the processing of ultrafine-grained/nanostructured materials by equal-channel angular pressing, J. Mat. Eng. Perform. 23 (3) (2014) 1038e1048. [22] P. Li, X. Zhang, K.-m. Xue, X. Li, Effect of equal channel angular pressing and torsion on SiC-particle distribution of SiCpeAl composites, Trans. Nonferrous Metals Soc. China 22 (Suppl. 2) (2012) s402es407. [23] A. Shokuhfar, O. Nejadseyfi, A comparison of the effects of severe plastic deformation and heat treatment on the tensile properties and impact toughness of aluminum alloy 6061, Mater. Sci. Eng. A 594 (2014) 140e148. [24] D. Orlov, K.D. Ralston, N. Birbilis, Y. Estrin, Enhanced corrosion resistance of Mg alloy ZK60 after processing by integrated extrusion and equal channel angular pressing, Acta Mater. 59 (2011) 6176e6186. l, M. Jane [25] P. Min arik, R. Kra cek, Effect of ECAP processing on corrosion resistance of AE21 and AE42 magnesium alloys, Appl. Surf. Sci. 281 (2013) 44e48. [26] O. Jilani, N. Njah, P. Ponthiaux, Transition from intergranular to pitting corrosion in fine grained aluminum processed by equal channel angular pressing, Corros. Sci. 87 (2014) 259e264. [27] A. Vinogradov, T. Mimaki, S. Hashimoto, R. Valiev, On the corrosion behaviour of ultra-fine grain copper, Scr. Mater. 41 (1999) 319e326. [28] A. Canakci, F. Erdemir, T. Varol, S. Ozkaya, Formation of FeeAl intermetallic coating on low-carbon steel by a novel mechanical alloying technique, Powder Technol. 247 (2013) 24e29. [29] A. Canakci, F. Erdemir, T. Varol, R. Dalmıs¸, S. Ozkaya, Effects of a new premilling coating process on the formation and properties of an FeeAl intermetallic coating, Powder Technol. 268 (2014) 110e117.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Combining equal-channel angular pressing and heat treatment to obtain enhanced corrosion resistance in 6061 aluminum alloy Omid Nejadseyfi a, *, Ali Shokuhfar a, Amirreza Dabiri b, Amin Azimi a a
Advanced Materials and Nanotechnology Research Center, Department of Mechanical Engineering, K.N. Toosi University of Technology, Tehran 19395-1999, Iran b Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 March 2015 Received in revised form 22 May 2015 Accepted 27 May 2015 Available online 10 June 2015
The corrosion behavior of 6061 aluminum alloy processed by heat treatment and equal-channel angular pressing (ECAP) is investigated. Current transient, potentiodynamic polarization and electrochemical impedance spectroscopy are used to assess the corrosion resistance of 6061 aluminum alloy in different processing conditions. The results confirm that the peak-aged sample after two ECAP passes and annealed sample after four ECAP passes are the most resistant samples against corrosion. Using ECAP leads to substantial increase in the dislocation density, re-distribution of precipitates, crystallite refinement, alteration of the surface area of the cathodic sites, and increase in the volume fraction of grain boundaries that affects corrosion resistance. Assessment of the corroded surfaces of the materials shows that some coarse-grains are corroded selectively in the annealed sample leading to formation of deep and large pits on the sample surface. However, after ECAP, shallow and uniform corroded surface is observed that is in agreement with the corrosion diagrams. © 2015 Elsevier B.V. All rights reserved.
Keywords: Corrosion Electrochemical impedance spectroscopy Metals and alloys Potentiodynamic polarization Scanning electron microscopy Transmission electron microscopy
1. Introduction One of the main characteristics of aluminum alloys is excellent corrosion resistance that makes them suitable to be widely used in automotive, aerospace, and marine industry. A wide range of heat treatments are also available for aluminum alloys that make it possible to tailor their properties to fit specific applications. In addition, development of severe plastic deformation (SPD) methods to produce nanostructured (NS) and ultrafine-grained (UFG) materials makes it possible to control the microstructure and properties of heat treatable aluminum alloys. ECAP is one of the most used SPD processes that induces simple shear deformation to the samples without any cross-sectional change and due to the constancy of the cross-sectional area of the billet, the process is repeatable in further passes [1,2]. Recently, researchers have widely investigated the strength, toughness, wear, and fatigue properties of the samples processed by SPD methods [3e9]. More recently, the corrosion resistance of
* Corresponding author. E-mail addresses: o.nejadseyfi@sina.kntu.ac.ir, (O. Nejadseyfi), [email protected] (A. Shokuhfar). http://dx.doi.org/10.1016/j.jallcom.2015.05.177 0925-8388/© 2015 Elsevier B.V. All rights reserved.
o.nejadseyfi@gmail.com
different materials after SPD is also investigated. Darmiani et al. [10] studied the corrosion behavior of AleSiC nano-composite, produced by accumulative roll bonding (ARB). They showed that increasing the ARB cycles led to a decrease in the number of pits and an increase in the pitting corrosion resistance. Nie et al. [11] investigated the corrosion behavior of commercial purity titanium processed by high-pressure torsion (HPT). They stated that there is a complicated relation between grain refinement and corrosion resistance, due to contradictory effects of inhomogeneous microstructure and grain size reduction. Zheng et al. [12] investigated the effects of microstructural changes during ECAP on the corrosion behavior of 304 stainless steel. They reported that the corrosion resistance of the samples improved after ECAP due to increase in the stability of the passive film on 304 stainless steel though the thickness and composition of the passive films on the as-received and 8-pass samples were very similar. Miyamoto et al. [13] studied the corrosion behavior of copper with and without processing by ECAP in a modified Livingstone etchant. Their study revealed that UFG copper exhibited remarkably lower corrosion current in comparison with that in the coarse-grained (CG) counterpart. Jiang et al. [14] reported the enhanced corrosion resistance of hypereutectic AleSi alloy subjected to ECAP. They found that in UFG structure, the easier formation of an oxide layer with an
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
improved adhesion force and protection efficacy improved the corrosion behavior. Several other investigations have been performed to investigate the influence of SPD on the corrosion behavior of different materials. Akiyama et al. [15] reported improvement of corrosion resistance of the AleNi alloy by ECAP, Hosseini et al. [16] investigated the effects of grain size and texture on the corrosion behavior tna et al. of commercial purity titanium processed by ECAP, and Vra [17] reported enhanced corrosion resistance in AZ31 magnesium alloy processed by ECAP. In addition, Shi et al. [18] studied the effect of Sc and Zr addition on the corrosion behavior of AleZneMgeCu alloys, Canakci et al. [19] assessed the influence of milling time in mechanical alloying on the formation of a protective coating layer, and Hockauf et al. [20] stated that the solid-solution heat treated 6082 aluminum alloy combined with post-ECAP aging showed excellent corrosion resistance. In this study, corrosion behavior of 6061 aluminum alloy processed by a combination of heat treatment and ECAP is investigated to reveal the simultaneous effects of grain refinement and heat treatment on corrosion resistance. In order to compare the results, the tests were performed in four processing conditions and the influence of heat treatment, severe plastic deformation, and a combination of heat treatment and severe plastic deformation are discussed. The results of different corrosion tests on the annealed material were used as the reference case. The aim is to address the individual and combined effects of heat treatment and ECAP on corrosion resistance of 6061 aluminum alloy. The information obtained from these analyses can be used to address failure reasons due to corrosion and propose an appropriate method for materials processing to promote corrosion resistance. 2. Experimental procedure Four groups of cylindrical billets with a diameter of 10 mm and a length of 60 mm were machined from an ingot of aluminum alloy 6061. The composition of this alloy is presented in Table 1. All billets were initially annealed in 686 K for five hours. The billets of first group, named as group A, were kept in the annealed condition (O temper). The billets of the second group, named as group B, were solution heat treated and artificially aged to achieve the T6 temper. Solution heat treatment was performed by heating the billets to the super-saturated condition at around 800 K for 30 min followed by rapid quenching in cold water. Artificial aging was also performed at 433 K for 18 h. Third group contained the billets of O temper, which were pressed using an ECAP die by route C up to four passes. (Group C). The last group contained the billets in T6 condition that were ECA pressed up to two passes using route C. A split die with an internal angle of 90 and an arc of curvature of 20 was implemented to conduct ECAP at room temperature. MOS2 was used as lubricant in order to reduce friction [21]. Punch speed was chosen to be 5 mms-1. Prior to corrosion tests, small disks were prepared from the center of each billet and were polished using SiC papers and polishing powders. Electrochemical measurement was performed at room temperature for each specimen in a solution containing 3.5% NaCl. A reference electrode of Ag/AgCl was used for electrochemical impedance spectroscopy (EIS) using an auxiliary electrode of platinum. The frequency range was from 105 to 102 Hz using
sinusoidal AC voltage of 10 mV amplitude. Subsequently, potentiodynamic polarization was carried out in the range of 1.2 to 0.2 V at a scan rate of 0.25 mVs1. The analysis of current transients was another method used to assess the mechanisms of localized corrosion at a constant voltage, the variation of current density was recorded during 1200s. This test was used to analyze pit initiation and re-passivation processes in different samples. All tests were repeated at least three times in order to maintain a high statistical accuracy. XRD analysis was performed using Cu Ka radiation (l ¼ 0.15406 nm) at a voltage and electrical current of 40 kV and 30 mA, respectively. The XRD patterns were recorded in the 2q range of 0e100 . Corroded surfaces were investigated using scanning electron microscopy (SEM) to observe the microscopic features of corrosion in each case. Optical microscopy (OM) and highresolution transmission electron microscopy (HR-TEM) were also used in order to obtain the microstructure of the processed materials before and after ECAP. 3. Results 3.1. Current transient test results Fig. 1 shows the graphs of the current density versus immersion time for the samples of four groups where the magnitude of the voltage remained constant. The variation of electrode current with immersion time is depicted to investigate the pit initiation and repassivation processes. In all samples, the current magnitude is increased during the initial stages of immersion. However, after about 200s, the current values remain almost stable with very little fluctuation. It is shown that the highest current is obtained for annealed sample showing the lowest pitting corrosion resistance for this sample. After processing the annealed sample by ECAP, the current values are decreased showing improved pitting corrosion resistance. The heat-treated sample also shows an improved pitting corrosion resistance compared to the annealed sample. However, one ECAP pass on the annealed specimen seems to be more effective in improving the pitting corrosion resistance than peak aging (T6 heat treatment). It is also shown that the pitting corrosion resistance for both annealed and peak-aged samples after two ECAP passes is very similar and the annealed specimen after four ECAP passes is the most resistant sample against pitting corrosion. Nevertheless, the formation of the protective layer in the peak-aged sample after two ECAP passes is immediate after immersion, while the oxide layer in the annealed sample after four ECAP passes needs more time to be formed. Careful inspection of these curves confirms that after 200s, there is gradual increase in the current value for the annealed and peak-
Table 1 Composition of the Al6061 Alloy in weight percentage. Al
Mg
Si
Fe
Cu
Mn
Base
0.807
0.669
0.627
0.23
0.12
913
Fig. 1. Current density versus immersion time at a constant voltage.
914
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
aged samples. However, for the ECA pressed samples the current becomes more stable during higher immersion times. This phenomenon suggests that the annealed and heat-treated samples need more immersing time to form a protective passive film, which indicates that pitting occurs gradually. This observation can be related to the quick dissolution of the oxide film in the aggressive solution. However, the current density remains almost constant in the samples processed by ECAP. Although practical information can be obtained from these curves, other electrochemical tests are still needed to accurately judge the corrosion resistance of different samples. 3.2. Potentiodynamic polarization Determining corrosion rate and evaluation of corrosion properties is usually performed by potentiodynamic polarization test. Fig. 2 illustrates the polarization curves for the samples of each group. Corrosion current and potential extracted from potentiodynamic polarization diagrams is also illustrated in Fig. 3. It is shown that the corrosion current of the annealed specimen after ECAP is more than that of the unprocessed specimen. A reverse trend is observed for the peak-aged sample before and after ECAP. Complex passive corrosion behavior in NaCl solution is observed in both annealed and peak-aged samples. The corrosion current density and corrosion potential are evaluated using the Tafel slope fitting from the polarization curves and the corrosion rates are included in Table 2 and Fig. 3. For the annealed sample, the corrosion rate is increased after processing by ECAP. In addition, heat treatment leads to increase in the corrosion rate. However, these modifications are very narrow in the polarization curves and actually preceding cathodic reaction on the sample surface may compromise the accuracy of the obtained results [11]. Therefore, it is suggested to use the EIS technique, which is more sensitive in characterizing the corrosion behavior of the materials of different groups accurately. 3.3. Electrochemical impedance spectroscopy Although polarization curves have been used widely for corrosion analysis, preceding cathodic reaction on the sample surface may compromise the accuracy of the obtained results. Therefore, it is better to use EIS technique to characterize the electrochemical interface between electrodes and electrolytes, which is more sensitive. Fig. 4 shows representative Nyquist plots obtained for different samples in 3.5% NaCl solution. The results show that, increasing the number ECAP passes leads to increase in the corrosion resistance. The annealed sample has the lowest resistance
Fig. 3. Corrosion current and potential extracted from potentiodynamic polarization diagrams.
against corrosion. Peak aging leads to a gradual improvement in the corrosion resistance of aluminum alloy. However, one ECAP pass is more effective in improvement of the corrosion resistance than peak aging. These observations accompany the obtained results in the current transients. It is notable that the corrosion resistance of both annealed and peak-aged specimens increases when the number of passes are increased. However, the peak-aged sample obtains exceptional corrosion resistance after two ECAP passes. This observation could be directly related to the microstructural evolution, distribution, size and volume fraction of the precipitates [20]. Fig. 5 shows X-Ray diffraction (XRD) patterns for the samples of four groups. The constituting phases and the relative amount of these phases could be distinguished by comparing the ratio of the intensities. Therefore, it is notable that the relative amount of AlFeSi phase increases after heat treatment. This observation confirms the increase in corrosion resistance after heat treatment compared with that of the annealed sample. As expected, processing by ECAP reduces the size of precipitations and leads to a uniform distribution of the second phase particles [22,23]. The induced strain, grain refinement mechanism and induction of high amount of dislocation density are almost identical in both annealed and peak-aged samples after ECAP. The peak broadening in all cases shows that the average crystallite size of the soft and hard phases decreases after two ECAP passes. However, significant difference between the corrosion behavior of annealed and peak-aged samples after two ECAP passes may arise from the presence of the fine second phase particles with a uniform distribution in the aluminum matrix. It is also notable the influence of the presence of fine second phase particles and their uniform distribution is more effective than repeating ECAP and gradual conversion of low angle grain boundaries to high angle grain boundaries. In other words, the corrosion resistance of the peak-aged sample after two ECAP passes is even higher than that of the annealed sample after four ECAP passes.
3.4. Microstructure and corroded surface
Fig. 2. Potentiodynamic polarization curves for different samples.
In the previous sections, the results of different corrosion tests were presented. This section aims to clearly represent the underlying mechanisms in the corrosion. Fig. 6 illustrates the microstructure of the annealed specimen before and after ECAP. As shown in this figure, the average grain size is reduced from 20 to 30 mm to below 500 nm after only two ECAP passes. In addition to that, a high number of dislocations and sub-grain boundaries that appear as dark lines in Fig. 6b are easily detectable. A high dislocation density together with refined grains is the main reason that improves corrosion resistance of the sample. In addition, Fig. 5 showed the constitutive phases and their relative amount before
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
915
Table 2 Data extracted from potentiodynamic polarization diagrams. Sample
Ecorr/V
Icorr/A cm2
Anneal Anneal þ 2Passes Anneal þ 4Passes Peak age Peak age þ 2Passes
0.623 0.696 0.680 0.707 0.728
8.588 3.133 6.613 3.446 3.723
107 106 106 106 107
Fig. 4. Electrochemical impedance spectroscopy diagrams for different samples.
ba/V dec1
bc/V dec1
Rp/ohm
0.688 0.278 0.778 0.428 0.166
0.014 0.014 0.015 0.021 0.018
3.284 3.433 4.989 7.312 2.264
Vcorr/mm y1 103 102 102 102 103
1.009 3.680 7.767 4.048 4.384
102 102 102 102 103
and after precipitation hardening and ECAP. Fig. 7 depicts the corroded surfaces of the samples of each group to show how the microstructural features affect the pitting on the samples during the test. For the annealed sample (Fig. 7a), there are a lot of local environmental attacks. Large and deep pits are easily detectable in this case. After peak aging, these local environmental attacks reduce, but the size and depth of the pits remain relatively large (Fig. 7b). Most of the sample surface remained intact, while there are selective corrosion areas on the surface. Both annealed and peak-aged sample after two ECAP passes show relatively small pits as shown in Fig. 7c and d. Nevertheless, they are well-distributed in the peakaged sample. This might have originated from the uniform distribution of the second phase particles. Finally, the annealed sample after four ECAP passes represents shallow and uniformly corroded surface (Fig. 7e). Selective corrosion of grains can be attributed to the differences in the crystallographic orientations of specific grains. When some coarse-grains are corroded selectively, deep and large pits are formed on the sample surface. That is why in the annealed sample deeper and larger pits are detectable than the other samples. A closer inspection of the pits in Fig. 8 confirms the presence of abundant and deeper corrosion pits on the corroded surface of the annealed sample in which the grain size is more than 40 times bigger than that of the annealed sample after two ECAP passes. Fig. 9 shows the EDS analysis of point A and B shown in Fig. 7. As shown in this figure, the concentration of Na and Cl around the pit is detectable. The Mg element is totally solved on point B. However, the formation of oxide layer on point B protects the surface and the major alloying elements are retained. 4. Discussion
Fig. 5. X-ray diffraction (XRD) patterns.
The graphs of the current densities versus immersion time for the samples of four groups in a constant voltage are shown in Fig. 1. These curves showed that there was gradual increase in the current value for the annealed and peak-aged sample. Aluminum alloys exhibit relatively good corrosion resistance due to spontaneous formation of an oxide film in an atmospheric environment.
Fig. 6. The microstructure of (a) annealed specimen and (b) annealed specimen after two ECAP passes using route C.
916
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
Fig. 7. Corroded surfaces of (a) annealed specimen, (b) peak-aged sample, (c) annealed specimen after two ECAP passes using route C, (d) peak-aged sample after two ECAP passes using route C, and (e) annealed specimen after four ECAP passes using route C.
Fig. 8. Three dimensional images from the corroded surfaces of (a) annealed specimen, 1500, and (b) annealed specimen after two ECAP passes, 1500X (To view this image correctly, the reader is recommended to refer to the web version of the article and to wear a passive 3-D red-cyan glasses.).
However, they are prone to pitting corrosion mainly in chloriderich solutions in which the oxide film is attacked by chloride ions. For both annealed and peak-aged samples some immersing time is needed to form a protective passive film once pitting occurs gradually. Quick dissolution of the oxide film in the aggressive solution
leads to gradual increase in current value. However, for the ECA pressed samples the current becomes more stable in the initial immersion times as it was observed previously [12]. The relative dissolution or passivation of a surface can be linked to the total length of the grain boundary. As shown by Orlov et al. [24], a change
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
Fig. 9. EDS analysis of points A and B shown in Fig. 7.
in grain boundary length can either increase or decrease the corrosion rate, which depends on specific material/environment combinations. In their potentiodynamic polarization study on Mg alloy ZK60, icorr appeared to decrease as the grain size was reduced. Additionally, corrosion behavior could not be exclusively attributed to the grain size while ignoring solute population throughout the microstructure [24]. Nevertheless, in the absence of precipitates in high-purity materials, the grain boundaries will play the key role in controlling the corrosion behavior. Such an investigation was one of the main challenges addressed in this study. X-ray diffraction patterns shown in Fig. 5, showed that the relative amount of second phase particles in the annealed sample was very low and ECAP led to decreasing corrosion rate. However, the peak-aged sample, in which the relative amount of second phase particles was higher than that of the annealed sample, showed improved corrosion resistance after ECAP. Similar observation was reported for the corrosion resistance of two Mg alloys [25]. The corrosion resistance of AE21 alloy decreased after ECAP while the corrosion resistance of AE42 alloy increased. As the grain size of both alloys after ECAP was nearly the same, the variation in the corrosion resistance of these alloys originated from variation of the alloying elements and the uniform spatial distribution of the alloying elements in AE42 led to enhanced corrosion resistance. As indicated, peak-aged sample after ECAP with fine dispersion of second phase particles showed improved corrosion resistance. Jiang et al. [14] observed such a phenomenon for cast Al-26 wt% Si alloy. The improved corrosion resistance was related to grain refinement that provided more nucleation sites to form denser and thicker natural oxide film resulting in higher impermeability to aggressive media [14]. Macroscopic assessments also showed that the samples were generally eroded with coincident pitting corrosion due to the presence of large secondary phases that created
917
local galvanic cells. Additionally, abundant and deeper corrosion pits on the corroded surface of the as-cast sample were replaced by shallow corrosion grooves. The main reason was the uniform distribution of fine secondary-phase Si particles on the UFG Al matrix that weakened the susceptibility to pitting corrosion, while it inhibited general microgalvanic reactions as observed in this study. Fig. 7 clearly showed that how the precipitation hardening as well as the size and distribution of precipitates can alter the corrosion mechanisms. Such an observation was also reported by Akiyama et al. [15] who performed ECAP on Al-5 wt% Cu alloy and stated that the size and the total area of Al2Cu particles reduced with the repetition of ECAP passes. The refinement of Al2Cu particles, which acted as cathode and the Cu-depleted zone surrounding Al2Cu particles where pitting preferentially took place had been diminished. These changes in microstructure of the processed samples seemed to be responsible for improvement in the pitting resistance after ECAP. The dissolution of Al8Fe2Si inclusions that were within grain boundaries together with the existing grain boundaries were also introduced as a main reason for changing the morphology of pitting corrosion from low-density deep pits to high-density shallow pits which led to transition from inter-granular corrosion to pitting corrosion [26]. In addition to that, comparing Fig. 7c, d and e indicated that the microstructural homogeneity achieved after repeating the ECAP process up to four passes was also beneficial in achieving enhanced corrosion resistance, because it can reduce the localized corrosion in some grains. In fact, the homogeneity of corrosive damages seems to be the main advantages of ultrafine-grained materials in comparison with conventional polycrystals, which are susceptible to highly localized corrosion [27]. Figs. 7 and 8 clearly showed the differences between the highly localized corrosion and shallow pits on the surface of annealed sample before and after ECAP, respectively. The results of potentiodynamic polarization test shown in Figs. 2 and 3, and specifically the corrosion volume calculated in Table 2 suggested that the predictions of this test in different processing conditions vary slightly. In other words, the variation of corrosion rate obtained for annealed, peak-aged, and annealed sample after ECAP is very narrow, which also can be affected by experimental errors and preceding cathodic reaction on the sample surface [16]. The only conclusion that can be made confidently is improvement of the corrosion resistance of peak-aged sample after two ECAP passes, because the corrosion rate obtained for this specimen is about one tenth of the corrosion rate obtained for the rest of the samples. In addition, it seems that ECAP deteriorates the corrosion resistance of annealed sample in this test. These differences are usually explained considering various and counteracting effects of ECAP on different samples. The processing by ECAP leads to substantial increase in the dislocation density, re-distribution of precipitates, crystallite refinement, alteration of the surface area of the cathodic sites, and increase in the volume fraction of grain boundaries. Each of these microstructural changes modifies the cathodic sites prone to corrosion, formation of passive film, and in general, the corrosion behavior of the samples [26]. Nevertheless, it is suggested that results of the EIS technique be used, being a sensitive method in the investigation of the corrosion behavior. The results of the EIS tests on the samples of four groups shown in Fig. 4 exhibit a little bit difference with the data obtained in polarization curves. While potentiodynamic polarization suggested that the corrosion resistance of the annealed sample after ECAP reduced, the EIS technique indicated that the corrosion resistance of that sample is improved similar to the current transient test results. As mentioned, preceding cathodic reaction on the sample surface may compromise the accuracy of the obtained results.
918
O. Nejadseyfi et al. / Journal of Alloys and Compounds 648 (2015) 912e918
Therefore, the results of the EIS technique that is more sensitive could be reliable in terms of corrosion resistance. Nevertheless, all corrosion tests performed in this study imply that the most resistant sample against corrosion is the peak-aged sample after two passes of ECAP. It should be noted that the material on which ECAP is applied is also important to assess the corrosion behavior. For instance, the corrosion behavior of copper does not qualitatively change after ECAP [27]. However, the microstructural evolution during severe plastic deformation may lead to enhanced stability and homogeneity of coating [28,29]. The state of grain boundary, which depends on the pre and post ECAP heat treatment, is also another important factor. Therefore, all these complicated cases needs to be investigated in detail to clearly understand the corrosion behavior of the materials processed by a combination of severe plastic deformation and heat treatment. 5. Conclusions In this study, the corrosion behavior of 6061 aluminum alloy processed by a combination of heat treatment and ECAP was investigated. Current transient, potentiodynamic polarization, and EIS tests were used to reveal the effects of processing history and different microstructural backgrounds on the corrosion resistance of aluminum alloy. The following conclusions were made as the main results of this investigation: The steady state anodic current density that served as a measure of the average dissolution rate at a certain potential was reduced after ECAP for both annealed and peak-aged samples. In addition, the steady state anodic current density for peak-aged sample was lower than that of the annealed specimen. These observations were related to the increase in the total length of the grain boundaries by which the formation of passive layer with an improved adhesion force and protection efficacy led to improved corrosion resistance. Potentiodynamic polarization tests showed that for annealed samples, the corrosion rate increased after processing by ECAP. In addition, heat treatment led to an increase in the corrosion rate. At the same time, peak-aged samples after two ECAP passes showed enhanced corrosion resistance. As the differences were very narrow in this test, they could be affected by preceding cathodic reaction or experimental/setup errors. Electrochemical impedance spectroscopy (EIS) showed that the corrosion resistance of both annealed and peak-aged specimens increased when the number of ECAP passes increased. The results that were obtained confirmed that the peak-aged sample after two ECAP passes was the most resistant sample against corrosion. Using ECAP led to substantial increase in the dislocation density, re-distribution of precipitates, crystallite refinement, alteration of the surface area of the cathodic sites, and increase in the volume fraction of grain boundaries. Each of these microstructural changes modified the cathodic sites prone to corrosion, formation of passive film, and in general, the corrosion behavior of the samples. Acknowledgments The authors would like to thank Iran Nanotechnology Initiative Council (INIC) Tehran, Iran, for the financial support of this work under the grant numbers of 38288 and 59117. References [1] R.Z. Valiev, T.G. Langdon, Principles of equal-channel angular pressing as a processing tool for grain refinement, Prog. Mater. Sci. 51 (2006) 881e981.
[2] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, The process of grain refinement in equal-channel angular pressing, Acta Mater. 46 (1998) 3317e3331. [3] C.S. Chung, J.K. Kim, H.K. Kim, W.J. Kim, Improvement of high-cycle fatigue life in a 6061 Al alloy produced by equal channel angular pressing, Mater. Sci. Eng. A 337 (2002) 39e44. [4] A. Canakci, F. Arslan, Abrasive wear behaviour of B4C particle reinforced Al2024 MMCs, Int. J. Adv. Manuf. Technol. 63 (2012) 785e795. [5] L. Meyer, M. Hockauf, B. Zillmann, I. Schneider, Strength, ductility and impact toughness of the magnesium alloy az31b after equal-channel angular pressing, Int. J. Mater. Form. 2 (2009) 61e64. [6] A. Azimi, A. Shokuhfar, O. Nejadseyfi, Mechanically alloyed Al7075eTiC nanocomposite: Powder processing, consolidation and mechanical strength, Mater. Des. 66 (Part A) (2015) 137e141. [7] A. Vinogradov, T. Kawaguchi, Y. Kaneko, S. Hashimoto, Fatigue crack growth and related microstructure evolution in ultrafine grain copper processed by ECAP, Mater. Trans. 53 (2012) 101e108. [8] A. Canakci, Microstructure and abrasive wear behaviour of B4C particle reinforced 2014 Al matrix composites, J. Mater. Sci. 46 (2011) 2805e2813. [9] A. Azimi, H. Fallahdoost, O. Nejadseyfi, Microstructure, mechanical and tribological behavior of hot-pressed mechanically alloyed AleZneMgeCu powders, Mater. Des. 75 (2015) 1e8. [10] E. Darmiani, I. Danaee, M.A. Golozar, M.R. Toroghinejad, Corrosion investigation of AleSiC nano-composite fabricated by accumulative roll bonding (ARB) process, J. Alloys Compd. 552 (2013) 31e39. [11] M. Nie, C. Wang, M. Qu, N. Gao, J. Wharton, T. Langdon, The corrosion behaviour of commercial purity titanium processed by high-pressure torsion, J. Mater. Sci. 49 (2014) 2824e2831. [12] Z.J. Zheng, Y. Gao, Y. Gui, M. Zhu, Corrosion behaviour of nanocrystalline 304 stainless steel prepared by equal channel angular pressing, Corros. Sci. 54 (2012) 60e67. [13] H. Miyamoto, K. Harada, T. Mimaki, A. Vinogradov, S. Hashimoto, Corrosion of ultra-fine grained copper fabricated by equal-channel angular pressing, Corros. Sci. 50 (2008) 1215e1220. [14] J. Jiang, A. Ma, D. Song, D. Yang, J. Shi, K. Wang, L. Zhang, J. Chen, Anticorrosion behavior of ultrafine-grained Al-26 wt% Si alloy fabricated by ECAP, J. Mater. Sci. 47 (2012) 7744e7750. [15] E. Akiyama, Z. Zhang, Y. Watanabe, K. Tsuzaki, Effects of severe plastic deformation on the corrosion behavior of aluminum alloys, J. Solid State Electrochem 13 (2009) 277e282. [16] M. Hoseini, A. Shahryari, S. Omanovic, J.A. Szpunar, Comparative effect of grain size and texture on the corrosion behaviour of commercially pure titanium processed by equal channel angular pressing, Corros. Sci. 51 (2009) 3064e3067. [17] J. Vr atn a, B. Hadzima, M. Bukovina, M. Jane cek, Room temperature corrosion properties of AZ31 magnesium alloy processed by extrusion and equal channel angular pressing, J. Mater. Sci. 48 (2013) 4510e4516. [18] Y. Shi, Q. Pan, M. Li, X. Huang, B. Li, Effect of Sc and Zr additions on corrosion behaviour of AleZneMgeCu alloys, J. Alloys Compd. 612 (2014) 42e50. [19] A. Canakci, T. Varol, F. Erdemir, S. Ozkaya, H. Mindivan, Microstructure and properties of FeeAl intermetallic coatings on the low carbon steel synthesized by mechanical alloying, Int. J. Adv. Manuf. Technol. 73 (2014) 849e858. [20] M. Hockauf, L. Meyer, D. Nickel, G. Alisch, T. Lampke, B. Wielage, L. Krüger, Mechanical properties and corrosion behaviour of ultrafine-grained AA6082 produced by equal-channel angular pressing, J. Mater. Sci. 43 (2008) 7409e7417. [21] A. Shokuhfar, O. Nejadseyfi, The influence of friction on the processing of ultrafine-grained/nanostructured materials by equal-channel angular pressing, J. Mat. Eng. Perform. 23 (3) (2014) 1038e1048. [22] P. Li, X. Zhang, K.-m. Xue, X. Li, Effect of equal channel angular pressing and torsion on SiC-particle distribution of SiCpeAl composites, Trans. Nonferrous Metals Soc. China 22 (Suppl. 2) (2012) s402es407. [23] A. Shokuhfar, O. Nejadseyfi, A comparison of the effects of severe plastic deformation and heat treatment on the tensile properties and impact toughness of aluminum alloy 6061, Mater. Sci. Eng. A 594 (2014) 140e148. [24] D. Orlov, K.D. Ralston, N. Birbilis, Y. Estrin, Enhanced corrosion resistance of Mg alloy ZK60 after processing by integrated extrusion and equal channel angular pressing, Acta Mater. 59 (2011) 6176e6186. l, M. Jane [25] P. Min arik, R. Kra cek, Effect of ECAP processing on corrosion resistance of AE21 and AE42 magnesium alloys, Appl. Surf. Sci. 281 (2013) 44e48. [26] O. Jilani, N. Njah, P. Ponthiaux, Transition from intergranular to pitting corrosion in fine grained aluminum processed by equal channel angular pressing, Corros. Sci. 87 (2014) 259e264. [27] A. Vinogradov, T. Mimaki, S. Hashimoto, R. Valiev, On the corrosion behaviour of ultra-fine grain copper, Scr. Mater. 41 (1999) 319e326. [28] A. Canakci, F. Erdemir, T. Varol, S. Ozkaya, Formation of FeeAl intermetallic coating on low-carbon steel by a novel mechanical alloying technique, Powder Technol. 247 (2013) 24e29. [29] A. Canakci, F. Erdemir, T. Varol, R. Dalmıs¸, S. Ozkaya, Effects of a new premilling coating process on the formation and properties of an FeeAl intermetallic coating, Powder Technol. 268 (2014) 110e117.