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Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy
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Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy Hamed Seifiyan a, Mahmoud Heydarzadeh Sohi a,∗, Mohammad Ansari b, Donya Ahmadkhaniha c, Mohsen Saremi a a School
of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Iran of Mechanical and Mechatronics Engineering, University of Waterloo, Canada c Department of Materials and Manufacturing, Jonkoping University, Sweden
b Department
Received 13 May 2019; received in revised form 4 November 2019; accepted 6 November 2019 Available online xxx
Abstract The present study aims to investigate the effect of friction stir processing (FSP) conditions on the corrosion characteristic of AZ31B magnesium alloy. Specimens made of AZ31B alloy were friction stir processed under various processing conditions, and their microstructure and corrosion behavior were studied. The corrosion behavior was studied by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and immersion test in 3.5% sodium chloride (NaCl) solution. The results showed a substantial improvement in the corrosion resistance of the friction stir processed AZ31B alloy. The improvement is likely a result of more stable corrosion products and microstructure refinement formed after friction sir processing. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: Magnesium; Friction stir processing; Corrosion; Polarization; EIS.
1. Introduction Magnesium and its alloys have attracted considerable interest in a number of industries such as aerospace, automobile and electronics, and the demand for these materials has increased in recent years [1]. The rising demand for these alloys is due to their many desirable properties which range from being light weight to having a reasonably high strength-toweight ratio [2–4]. Among Mg alloys, AZ-series, especially AZ31 alloy, based on Mg-Al-Zn ternary system is the most widely used Mg alloy and they are considered as a promising choice to use in structural applications [5,6]. Despite the great properties of Mg and its alloys, these materials are chemically active and their applications are hindered by their poor corrosion performance in both atmospheric and aqueous media. The low corrosion resistance of ∗
Corresponding author. E-mail address: [email protected] (M. Heydarzadeh Sohi).
Mg and its alloys is directly related to mismatch between Mg(OH)2 lattice and bulk material on the surface region which leads to the lack of formation of protective passive oxide film [7,8]. Microstructural refinement is believed to be a promising approach to overcome the limitation of Mg and its alloys for their poor corrosion resistance [9,10]. Various surface refinement techniques have been investigated and studied on magnesium alloys by several researchers. Laser surface melting (LSM) [11–16], and electron beam melting (EBM) [17] of Mg alloys among such processes. Friction stir processing (FSP), which has its basis on friction stir welding (FSW), is another surface modification approach for refining the microstructure of metallic materials including magnesium since the beginning of this century [18]. In FSP, tool rotational (ɷ, rpm) and traverse speed (ν, mm/min) play key roles. While the rotation of the pin stirs the material, its traverse shifts the stirred substrate from the front side to the back of the rotating tool. The process is shown schematically in Fig. 1. This solid-state process has
https://doi.org/10.1016/j.jma.2019.11.004 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Please cite this article as: H. Seifiyan, M. Heydarzadeh Sohi and M. Ansari et al., Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.004
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Fig. 1. Schematic drawing of friction stir processing (FSP).
an advantage over the surface modification techniques based on surface melting processing such as LSP and EBM. This advantage is that FSP avoids the bulk melting of the base material that makes it capable of producing porosity and crack-free modified layers [18,19]. Up to now, FSP has been mostly used by researchers for improving the mechanical properties of materials [20–25] and there has been limited research studies dealing with the corrosion behavior of Mg alloys after FSP/FSW [26–34]. The corrosion behaviors of magnesium alloys are mainly related to a number of parameters such as microstructure, alloy composition, working conditions, and properties of the film developed during the exposure to the working atmosphere. Also, the uniformity of microstructural refinement and residual stresses induced by FSP is governed by processing conditions. Depending on FSP parameters, the corrosion resistance may be deteriorated. Although some researchers have briefly studied the corrosion resistance of AZ31 Mg alloy after FSP/FSW [35–42], it is still not well-understood how FSP conditions affect the corrosion behavior. To the knowledge of the authors, a thorough investigation on the influence of FSP conditions such as traverse and rotational speeds of the tool, and the number of FSP passes has not been performed yet and such an investigation is requisite to shed light on the corrosion behavior of AZ31 Mg alloy after FSP. Concerning this, the main purpose of the present work is to know how modifications in the microstructure cause changes in the corrosion behavior of AZ31B when different FSP conditions applied. 2. Material and methods 2.1. Materials and FSP conditions Commercially available wrought AZ31B-O Mg alloy plates (150 × 100 × 8 mm3 ) were used as base material. Prior to FSP, the surface oxides were removed and then the surface was cleaned using ethanol. The composition of AZ31B was
Table 1 Chemical composition of AZ31B Mg alloy. Element
Mg
Al
Zn
Mn
Si
Cu
Fe
Ni
Content (wt. %)
Bal
2.96
0.94
0.48
0.032
0.027
0.0025
0.003
analyzed using optical emission spectroscopy (OES); the results are given in Table 1. FSP was conducted using an H13 steel tool consisted of a square pin of 6 mm in diameter, 5.6 mm in height, and a shoulder of 18 mm in diameter. The detailed dimensions of the tool are given in Fig. 2. Several samples were friction stir processed (FSPed) at different processing conditions as shown in Table 2. Tilt angle of tools in direction of travel was set in 3° and penetration depth was 0.4 mm. After performing single-pass FSP, the parameters which led to the samples with the lowest average grain size were chosen as best single-pass processing conditions and 3-pass FSP was performed in those processing conditions. 2.2. Corrosion tests The specimens for corrosion tests were extracted from only the stir zone at the middle of the FSPed samples using wire electrical discharge machining. Thereafter, the specimens were ground using emery papers (3000 grit). To study the corrosion behavior, three techniques were employed which include potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and immersion test. A system of three electrodes including FSPed specimens as the working electrode, platinum as counter electrode, and Ag/AgCl as reference electrode for electrochemical studies. To prepare the electrodes, a wire covered with cold setting resin was connected to one side of the specimens, while their other side were faced to the corrosive solution. EIS tests were conducted using an EG&G Potentiostat/Galvanostat Model 273 with a Model 1260 frequency response analyzer (FRA), after exposing the specimens in a 3.5% NaCl solution for 1 h, at ambient temperature. The exposure area of each specimen
Please cite this article as: H. Seifiyan, M. Heydarzadeh Sohi and M. Ansari et al., Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.004
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Table 2 Processing conditions used for FSP of AZ31B alloy. Sample code
SP1
SP2
SP3
SP4
SP5
SP6
SP7
SP8
MP1
MP2
Tool rotational speed ɷ (rpm) Tool traverse speed ν (mm/min) Number of FSP passes
800 50 1
800 80 1
1000 50 1
1000 80 1
1250 50 1
1250 80 1
1600 50 1
1600 80 1
1000 50 3
1000 80 3
Single-pass and multi-pass FSPed samples denoted as SP and MP, respectively.
surface area of all specimens was 1 cm2 . After immersion, the corrosion products formed on the samples were removed by means of a reagent containing 200 g CrO3 and 10 g AgNO3 in 250 mL distilled water [44]. They were then washed with distilled water, dried and weighed to get the weight loss. 2.3. Microstructural and phase characterization Metallographic specimens were extracted from the middle of the FSPed samples and their cross sections were manually ground, polished and then etched in a solution of 4.2 g picric acid, 10 ml acetic acid, 70 ml ethanol and 10 ml H2 O [45]. While the grain size was measured using Digimizer Image Analysis Software [46], microscopic images were taken by an optical microscope (Gippon-Japan). In accordance with ASTM: E112-96 [47], the average grain size was measured. A Zeiss SIGMA VP scanning electron microscope (SEM) fitted with an Energy Dispersive Spectroscopy (EDS) microanalyzer was used to analyze the microstructure of the samples, surface morphology, and composition of the corroded surfaces. A Phillips X’Pert Pro diffractometer was used to identify the phases in the corrosion products after immersion tests. A CuKα radiation was used at operating parameters of 40 kV and 30 mA over a 2θ range of 10–110°. 3. Results 3.1. Microstructure
Fig. 2. Schematic drawing of FSP tool and its dimensions.
that was immersed in the solution was 1 cm2 . The frequency ranged from 100 kHz to 100 mHz using perturbative voltage amplitude of 10 mV was used for EIS measurements. ZView® software [43] was used for fitting the equivalent circuit with the impedance data obtained in these tests. The corroded layers after EIS tests were then characterized by SEM and XRD. Potentiodynamic polarization scans were performed in a cell containing 150 ml of 3.5% NaCl. The specimens were immersed in 3.5% NaCl solution and the test were carried out with a scan rate of 1 mV/s in a range of -250 to +250 mV with regard to the open circuit potential (OCP), after letting development of a steady state potential. For constant immersion tests, all the specimens were initially polished and then immersed in 3.5% NaCl solution for 24 h. The exposure
Fig. 3 is an optical micrograph of a typical cross-sectional view of a FSPed AZ31B Mg alloy showing friction stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and the base magnesium alloy. The right and left sides of the centered area are retreating (RS) and advancing sides (AS) of the rotating tool, respectively. All samples showed a typical view as Fig. 3 with no defects, except for the samples FSPed with tool rotational speed of 800 rpm that showed tunneling defects. Since these imperfect samples did not pass preliminary examinations, they were rejected and did not proceed further. SZ experiences high heat input and strain that induced by rotating FSP tool. The microstructure in this area has fine grains which result from dynamic recrystallization. Optical micrographs of as-received alloy and SZ of the FSPed samples at different FSP conditions are shown in Fig. 4. In addition, the average grain size values achieved at varying processing conditions are given in Table 3. It can be clearly substantiated that FSP refined the microstructure from an
Please cite this article as: H. Seifiyan, M. Heydarzadeh Sohi and M. Ansari et al., Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.004
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Fig. 3. Typical cross-sectional view of FSPed sample at ɷ =1000 rpm, ν = 50 mm/min.
Fig. 4. Optical micrographs of as-received alloy and SZ of FSPed samples at different processing conditions; (a) as-received alloy, (b) single-pass FSP at ɷ =1000 rpm, ν = 50 mm/min (SP3), (c) single-pass FSP at ɷ =1000 rpm, ν = 80 mm/min (SP4), (d) single-pass FSP at ɷ =1250 rpm, ν = 50 mm/min (SP5), (e) single-pass FSP at ɷ =1250 rpm, ν = 80 mm/min (SP6), (f) single-pass FSP at ɷ =1600 rpm, ν = 50 mm/min (SP7), (g) single-pass FSP at ɷ =1600 rpm, ν = 80 mm/min (SP8), (h) 3-pass FSP at ɷ =1000 rpm, ν = 50 mm/min (MP1), and (i) 3-pass FSP at ɷ =1000 rpm, ν = 80 mm/min (MP2). Table 3 Average grain size value for as-received alloy and SZ of FSPed samples at different processing conditions. Sample code
SP3
SP4
SP5
SP6
SP7
SP8
MP1
MP2
As-received
Average grain size (μm)
16.8
15.9
20.1
19.4
24.5
23.6
11.2
10.2
28.5
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Table 4 Fitting data obtained from EIS measurements for all samples immersed in 3.5% NaCl solution after 1 h.
Fig. 5. High-magnification SEM micrograph of FSPed sample at ɷ =1000 rpm, ν = 50 mm/min, indicating the presence of second phase Mn-Al particles at grain boundaries.
average grain size of about 29 ± 9 μm for the as-received alloy to an average grain size of 15.9± 5.6 μm for best single-pass FSP conditions and 10.2 ± 2.4 μm for best multipass FSP conditions. The microstructure of the base metal exhibits a combination of coarse and fine grains, while the grains of the FSPed samples are more or less uniform. It is seen that the grain size is slightly decreased after single-pass FSP; however, after 3-pass FSP, the grain size considerably reduced to about one-third of that of the as-received alloy. As it can be seen in Fig. 4, a few second phase particles are randomly distributed in the matrix of the as-received alloy and FSPed sample, which are believed to be Mn-Al intermetallics (confirmed by EDS results). A higher magnification SEM micrograph of these particles in a typical FSPed sample is shown in Fig. 5. The presence of these particles after FSP indicates that second phase Mn-Al particles in the as-received alloy were not dissolved during FSP. A number of other researchers have also come across similar results [48–50]. Nevertheless, the intermetallics were broken during FSP and their size reduced from about 3-7 μm in as-received AZ31B to about 1–3 μm in the FSPed samples. 3.2. Corrosion studies 3.2.1. Electrochemical impedance measurements Fig. 6a-c show the Nyquist and Bode plots obtained from the as-received material and single-pass FSPed samples at different conditions immersed in 3.5% NaCl solution for 1 h. The Nyquist diagrams for the as-received alloy and singlepass FSPed samples (Fig. 6a) show two-time constants corresponding to the presence of a capacitive loop at high frequency ranges and an inductive loop at low-frequency ranges. The Bode plots for the as-received alloy and single-pass FSPed samples appears to confirm the Nyquist diagrams. Charge transfer resistance of the electrode is the reason of the capacitive loop at high to medium frequencies. It should
Sample code
Rs (Ω.cm2 )
Cdl (μF/cm2 )
Rct (Ω.cm2 )
RL (Ω.cm2 )
SP3 SP4 SP5 SP6 SP7 SP8 MP1 MP2 As-received
28.5 34.56 20.85 19.95 26.24 27 22.57 23.3 22.2
7.6185E-6 4.5134E-6 2.8383E-6 1.1571E-5 8.6041E-6 4.656E-6 1.0284E-5 1.3014E-5 1.4606E-5
1081 790 805.7 564.7 545.7 452.9 1629 1206 313.3
549.6 486.9 1611 400 400.3 140 262.5
L (H.cm2 ) 245.8 95.02 304.7 351 186.1 97.55 135.7
be noted that the diameter of the capacitive loop directly relates to charge transfer resistance. The reason for the presence of inductive loop can be pitting corrosion or adsorbed surface of electrically active species such as Mg(OH)ads + or Mg(OH)2 [51,52]. The larger diameter of the capacitive loops for the FSPed samples in comparison to the as-received material shows higher charge transfer resistance or in other word better corrosion resistance of the FSPed samples. By increasing tool rotational and traverse speed, the diameter of the capacitive loops for the single-pass FSPed samples decreased, indicating low corrosion resistance. The Nyquist diagrams for the as-received alloy and singlepass and 3-pass FSPed samples at optimum conditions are shown in Fig. 6d, together with the Bode plots (Fig. 6e and f). Although the presence of an inductive loop and a capacitive loop are evident in the as-received alloy and single-pass FSPed samples, the 3-pass FSPed samples only show a capacitive loop and there is no inductive loop. It has been reported that the presence of an inductive loop in Nyquist diagrams is a sign of corrosion on the surface (localized corrosion especially pitting). Therefore, the absence of an inductive loop may be evidence for better corrosion protection [53]. In addition, by increasing the number of FSP passes the diameter of the capacitive loops for the 3-pass FSPed samples increased, indicating better corrosion resistance. The electrical equivalent circuit (EEC) was developed according to the overall corrosion reaction of Mg in aqueous environments, which generally contains the electrochemical reaction of Mg and H2 O to produce Mg(OH)2 and H2 , as previously reported by others [54–56]. Fig. 7a shows the EEC for the as-received alloy and single-pass FSPed samples and Fig. 7b shows the EEC for 3-pass FSPed samples. In these EECs, Rct is the charge transfer resistance, Rs is the electrolyte resistance, and Cdl represents a two-layer capacitance at the substrate/film interface. To take the inductive behavior into account, an inductor L and a resistance RL were introduced into the model for the as-received alloy and single-pass FSPed samples. The impedance data were fitted with the suggested EECs by ZView® software and the results are listed in Table 4. The Rs values are almost close and relatively small, which is due to the good electrical conductivity of NaCl solution.
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Fig. 6. Nyquist and Bode plots after 1 h immersion in 3.5% NaCl solution for (a-c) the as-received alloy and single-pass FSPed samples, and (d–f) the as-received alloy, and single-pass and 3-pass FSPed samples only at optimum conditions.
The Rct values for the FSPed samples are remarkably higher than that of the as-received alloy. According to Table 4, the highest corrosion resistance for the single- and multi-pass FSPed samples achieved at processing conditions of 1000 rpm and 50 mm/min. The Rct values for the single-pass FSPed samples decreased by increasing tool rotational and traverse speed. Although the grain size decreased by increasing traverse speed, the FSPed samples at processing condition of 50 mm/min showed a higher Rct value. The
reason is probably that of high traverse speed which can induce more residual stresses and lessen the positive effect of grain refinement. It has been reported that the presence of residual stresses and a large number of surface defects and high dislocation/grain boundary density have a detrimental effect on the corrosion behavior [57–59]. In addition, by increasing the number of FSP passes from 1 pass to 3, the Rct value significantly increased from 313.3 Ω.cm2 for the as-received alloy to 1081 and 1629 Ω.cm2 for the 1-pass and
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Table 5 Summary of electrochemical data calculated by Tafel extrapolation method for all samples immersed in 3.5% NaCl solution. Sample code
β a (mV/decade)
β c (mV/decade)
Ecorr vs. Ag/AgCl (V)
jcorr (A/cm2 )
SP3 SP4 SP5 SP6 SP7 SP8 MP1 MP2 As-received
–0.174 –0.1663 –0.2132 –0.1742 –0.1556 –0.1737 –0.1952 –0.1928 –0.2189
0.0119 0.0076 0.0179 0.0146 0.0371 0.0405 0.063 0.063 0.126
–1.204 –1.303 –1.293 –1.322 –1.388 –1.382 –1.320 –1.317 –1.483
5.87E-6 1.06E-5 3.94E-5 4.22E-5 4.75E-6 3.74E-6 5.53E-5 5.66E-5 4.23E-4
Fig. 7. EEC used for fitting EIS data of (a) as-received alloy and single-pass FSPed samples, and (b) 3-pass FSPed samples.
3-pass FSPed sample with tool rotational speed of 1000 rpm and tool traverse speed of 50 mm/min.
3.2.2. Potentiodynamic polarization measurements The polarization curves for the as-received and FSPed samples are shown in Fig. 8 and the electrochemical data calculated by Tafel extrapolation method are listed in Table 5. It is evident from Fig. 8 that the anodic polarization curve for all the FSPed samples shifts towards more noble potential, indicating low anodic dissolution which may be accredited to the fine-grained structure of the FSPed samples. FSP caused a decrease in anodic current density of samples due to facilitate passive film formation on the surface. In addition to the predominant effect of FSP on the acceleration of passive film formation for all samples, the decrease in cathodic kinetics was another reason for decreasing the corrosion current density of FSPed samples. From Table 5, it can be seen that the sample FSPed with rotational speed of 1000 rpm and tool traverse speed of 50 mm/min (SP3) has the lowest polarization current density among all the FSPed samples. Increasing of tool’s rotational speed and its traverse speed, decreases the corrosion potential (Ecorr ); although, the corrosion current density (jcorr ) does not have a meaningful trend. The anodic Tafel slope (β a ) for the FSPed samples is higher than that of the as-received alloy; however, the cathodic Tafel slope (β c ) for the FSPed samples is lower than that of the as-received alloy and by decreasing tool rotational speed from 1600 to
Fig. 8. Polarization curves after immersion in 3.5% NaCl solution for (a) as-received alloy and single-pass FSPed samples, and (b) as-received alloy, and single- and 3-pass FSPed samples only at optimum conditions.
1000 rpm, the value of cathodic slope decreases. The cathodic activity is attributed to the presence of second phase particles. The potentiodynamic polarization investigations revealed that the polarization curves, especially anodic polarization curves, had considerable deviations from linearity. Since the
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Table 6 Summary of corrosion rates obtained from constant immersion tests for all samples immersed in 3.5% NaCl solution for 24 h.
Table 7 EDS results of corrosion products on the surface of the corroded samples (atom %).
Sample code
Corrosion rate (mm/year)
Element Sample
Mg
O
Cl
Al
SP3 SP4 SP5 SP6 SP7 SP8 MP1 MP2 As-received
0.0801 0.1140 0.1339 0.2409 0.2731 0.2784 0.0598 0.0709 0.3293
MP1 SP3 As-received
38.7 37.6 38.5
58.2 57.3 59.4
2.3 2.4 1.5
0.8 0.6 0.5
anodic branch of the polarization curves deviates from linear behavior, the determination of anodic slope is not accurate. These observations suggest that Tafel extrapolation is not an accurate method for determining the electrochemical data in the case of Mg alloys. The reason why the determination of the anodic Tafel slope is not accurate has been elucidated by Curioni et al. [60,61]. In the case of Mg alloys, the measurement of the anodic slope is difficult due to the fact that magnesium oxidizes under anodic polarization that is leading to the domination of ohmic drop effects [60]. The measurement is not reliable owing to the presence of bubbles produced by substantial hydrogen evolution and local alkalinization in the proximity of the electrode surface [61]. The above-mentioned considerations are in agreement with the observations in this study, disclosing a complex polarization behavior that cannot be approximated by Tafel extrapolation method. 3.2.3. Immersion test The samples were immersed in 3.5 wt.% NaCl solution for 24 h and the corrosion rate was calculated by the following equation in accordance with NACE/ASTM TM0169 G0031 12A Standard [62]: k.(w0 − w1 ) CR = A.t.ρ In this equation CR is corrosion rate (mm/year), k = 8.76 × 104 , (w0 - w1 ) is weight change (g), A is specimen area (cm2 ), and t is exposure time (h), and ρ is density (g/cm3 ). The results of the immersion tests are given in Table 6. According to Table 6, the highest corrosion rate for the single- and multi-pass FSPed samples achieved at processing conditions of 1000 rpm and 50 mm/min. The corrosion rate value for the single-pass FSPed samples increased by increasing rotational and traverse speeds of the tool. In addition, by increasing the number of FSP passes from 1 pass to 3, the corrosion rate value significantly decreased from 0.3293 mm/year for the as-received alloy to 0.0801 and 0.0598 mm/year for the 1-pass and 3-pass FSPed samples with tool rotational speed of 1000 rpm and tool traverse speed of 50 mm/min. 3.2.4. Characterization of corrosion layer Fig. 9 compares the surface morphology of the as-received and FSPed samples after immersion in 3.5 wt.% NaCl solu-
tion for 24 h. The surface of the as-received sample was completely corroded, and the film was thick and loose with large cracks and flake-like structure (Fig. 9a and b). These cracks could act as routs for the penetration of the corrosive solution, that might intensify the corrosion [63]. Unlike the as received material, the surface of the corroded FSPed samples (Fig. 9c, d, e, f) showed a denser and more uniform corrosion layer with fewer cracks and discontinuities, which is more likely to be protective than that formed on the as-received sample. Table 7 gives the atomic percentage of the elements obtained by EDS microanalysis of the corrosion products formed on the surface of the as-received and FSPed samples after 24 h immersion in 3.5 wt.% NaCl solution. From the analyses carried out, it can be pointed out that the corrosion products mainly contain Mg, O, and Cl with a trifling Al enrichment on the corrosion layer. XRD patterns of the corrosion products after being immersed for 24 h in 3.5 wt.% NaCl solution can be seen in Fig. 10. The main corrosion product is revealed to be Mg(OH)2 which makes the XRD analysis to be consistent with the EDS results. The formation of Mg(OH)2 as the main corrosion product has been reported before [56,64]. Moreover, the diffraction intensities of the metallic Mg from the substrate decreased for the FSPed samples, which agrees with the presence of more amount of Mg(OH)2 on the FSPed samples and confirms the formation of a protective corrosion layer. 4. Discussion 4.1. Influence of FSP conditions on microstructure Recrystallization and grain growth are two metallurgical phenomena that compete with each other to control the grain size after FSP. Some factors which influence these two phenomena include maximum temperature (dominated by tool rotational speed), remaining time at high temperature (dominated by tool traverse speed), strain, and strain rate experienced during FSP (dominated by the number of FSP passes). These factors are highly affected by rotational and traverse speed of FSP tool as well as the number of FSP passes. As for the effect of tool rotational and traverse speed on the grain size, grain growth derived from heat input during the course of FSP is the dominant phenomenon. Heat input is directly proportional to the ratio of tool rotational speed and traverse speed (ɷ/ν) [65]. By increasing tool rotational speed and reducing its traverse speed (i.e. increasing ɷ/ν ratio), the heat input in SZ increases. Therefore, by increasing the tool
Please cite this article as: H. Seifiyan, M. Heydarzadeh Sohi and M. Ansari et al., Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.004
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Fig. 9. Low and high magnification SEM images showing the surface morphology of corroded samples after 24 h immersion in 3.5 wt.% NaCl solution; (a and b) as-received alloy, (c and d) single-pass FSPed sample at ɷ = 1000 rpm, ν = 50 mm/min (SP3), and (e and f) 3-pass FSPed sample at ɷ = 1000 rpm, ν = 50 mm/min (MP1).
rotational speed at constant tool traverse speed, maximum temperature increases, and grain growth occurs and coarse grains are formed in SZ. However, when the tool traverse speed is increased at constant tool rotational speed, the remaining time at high-temperature decreases which will provide less time for grains to grow and finer grains are formed in SZ. Regarding the effect of the number of FSP passes on the grain size, the average grain size showed the lowest values after multi-pass FSP. This is a result of acute plastic deformation and frictional heat triggered by multi-pass FSP. Multi-pass FSP induces more dislocation sources, resulting in more recrystallization and dynamic recovery. Recrystallization phenomenon is dominant and it is expected more by increasing the number of FSP passes. More strain induces and grain size decreases.
4.2. Influence of FSP conditions on corrosion behavior Since magnesium is an intrinsically passive element, its exposure to chloride ions in a non-oxidizing media leads to its pitting. In this case, the anode and cathodes are grain interior and grain boundary, respectively. It is also known that the corrosion behavior of Mg alloys is influenced by the distribution of microconstituents, which are nobler than the adjacent Mg matrix. Consequently, it has been well documented that the Mn-Al intermetallics and the β-phase present in Mg-Al alloys behave as potential cathodes, generating a micro-galvanic coupling and preferentially localized dissolution of the Mg matrix [66]. Whereas the role of grain size is dominant, that of the microconstituents is inconsequential in AZ31 Mg alloy. On the other hand, when Mg is
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Fig. 10. XRD patterns for corrosion layer after 24 h immersion in 3.5 wt.% NaCl solution; (a) as-received alloy, (b) single-pass FSPed sample at ɷ = 1000 rpm, ν = 50 mm/min (SP3), and (c) 3-pass FSPed sample at ɷ = 1000 rpm, ν = 50 mm/min (MP1).
electrochemically dissolved, there will be a production of OH− ions from the cathodic reaction (2). This results in an increase in pH at the interface, thereby giving rise to the formation and precipitation of MgO and/or Mg(OH)2 as corrosion products. Mg → Mg2+ + 2e−
Anodic reaction
2H2 O + 2e− → H2 + 2OH−
Cathodic reaction
Mg + 2H2 O → Mg(OH )2 + H2
Overall equation
(1)
(2)
(3)
In general, the higher corrosion rate of AZ31B alloy is attributed to the presence of large size microconstituents as well as coarse Mg grains. Fine-grained Mg in corrosive media shows different behaviors [67–69]. Recent research studies have found that the grain refinement can improve the corrosion resistance of Mg both in alkaline and neutral NaCl solutions; however, some of them reported that residual stresses in the material after grain refinement could increase the cathodic kinetics [58,59,70]. In a research study by Song et al. [57], the corrosion behavior of equal-channel-angular-pressed (ECAP) pure Mg was investigated. They reported that fine-grained Mg material produced by ECAP showed inferior corrosion resistance in 3.5% NaCl solution. Although the fine-grained Mg material has a greater tendency to form an oxide film on the surface, the magnesium oxide film is unstable and easily dissolves in NaCl solution owing to the presence of a large number of surface defects and high dislocation/grain boundary density induced by ECAP. In another research study by Jang et al. [35] on FSWed AZ31 alloy, it was shown that grain size was more effective
than residual stresses, especially in the case of 3.5% NaCl solution as corrosive media. Grain boundaries are more chemically active and have a higher energy in comparison with the bulk metal [67]. Therefore, high grain boundary density in the fine-grained material can increase the surface activity and leads to rapid formation of the passive protective film. High grain boundary density in fine-grained Mg alloys compensates the oxide/metal mismatch and encourages the formation of a denser and uniform passive film on the surface, resulting in an improved corrosion resistance. According to the literatures, the intensity of micro-galvanic couple is decreased and a quick semi-protective anodic layer is formed when the grain size is reduced [27,29]. Formation of Mg (OH)2 films on the surface is initially started on defects like grain boundaries. Reduction of the grain size, via processes like FSP, increases the nucleation site density for the fabrication of oxide films and intensifies the formation of passive layers. The adherence of the passive film to the surface may also improve due to the presence of fine grains, thereby decreasing the tendency for oxide cracking. Orlov et al. [71] also suggested that the degree of oxide cracking is reduced for a fine-grained material since it can relieve the tensile stress induced by the oxide/metal mismatch. Moreover, the micro-galvanic couple effect is minimized by the refinement of the microconstituents. Therefore, the improved corrosion resistance of FSPed AZ31 alloy is mainly attributed to the refinement of Mg grains and uniform redistribution of microconstituents. Grain refinement and compositional homogenization can reduce the volume fraction of the effective cathodes that in turn limits the cell action induced by the pile up of cathodic phases [2]. In the course of FSP, a considerable variation in the corrosion behavior may occur depending on the processing conditions. There are three factors that can affect the corrosion behavior and hence deteriorate or ameliorate the corrosion
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resistance. While large size microconstituents and residual stresses have adverse effects, grain refinement has a positive effect on the corrosion behavior. These three factors tend to compete with each other to determine corrosion behavior. This study shows that the role of grain size is predominant since all the FSPed samples revealed an improvement in the corrosion resistance; however, it may be possible that some electrochemical results can have no meaningful correlation with FSP conditions. FSP conditions can amplify or lessen the role of the three factors, although the mutual interaction of the three factors is complicated and it is still not wellunderstood. 5. Conclusions Friction stir processing was successfully employed for surface modification of AZ31B Mg alloy. The following conclusions can be drawn: • The FSP conditions including tool rotational and traverse speed as well as the number of FSP passes are found to be the most significant parameters, which affect the microstructure and corrosion behavior of FSPed AZ31B Mg alloy. • A set of FSP conditions consisting of tool rotational speed of 1000 rpm and tool traverse speed of 50 mm/min was found to result in maximum corrosion resistance. • Grain refinement was found to have a significant effect on the corrosion behavior. However, the presence of residual stress induced by FSP and large size microconstituents can lessen the positive effect of grain refinement on the improved corrosion resistance. • The lower corrosion current density and more noble potential observed in polarization tests as well as higher charge transfer resistance observed in EIS tests indicated a superior resistance against corrosion for the FSPed samples as compared to the AZ31B alloy. • The improved corrosion resistance is attributed to the formation of the passive protective film, supported by the microstructural refinement and the presence of fine microconstituents after FSP. Declaration of Competing Interest None. References [1] H. Friedrich, S. Schumann, J. Mater. Process. Technol. 117 (2001) 276– 281, doi:10.1016/S0924- 0136(01)00780- 4. [2] G.L. Makar, J. Kruger, Int. Mater. Rev. 38 (1993) 138–153, doi:10. 1179/imr.1993.38.3.138. [3] , Corrosion of Magnesium Alloys, in: G.-L. Song (Ed.), Corrosion of Magnesium Alloys, Woodhead Publishing, Cambridge, 2011. [4] G. Song, A. Atrens, Adv. Eng. Mater. 5 (12) (2003) 837–858, doi:10. 1002/adem.200310405. [5] S. Niknejad, L. Liu, M.-Y. Lee, S. Esmaeili, N.Y. Zhou, Mater. Sci. Eng. A. 618 (2014) 323–334, doi:10.1016/j.msea.2014.08.013.
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Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy Hamed Seifiyan a, Mahmoud Heydarzadeh Sohi a,∗, Mohammad Ansari b, Donya Ahmadkhaniha c, Mohsen Saremi a a School
of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Iran of Mechanical and Mechatronics Engineering, University of Waterloo, Canada c Department of Materials and Manufacturing, Jonkoping University, Sweden
b Department
Received 13 May 2019; received in revised form 4 November 2019; accepted 6 November 2019 Available online xxx
Abstract The present study aims to investigate the effect of friction stir processing (FSP) conditions on the corrosion characteristic of AZ31B magnesium alloy. Specimens made of AZ31B alloy were friction stir processed under various processing conditions, and their microstructure and corrosion behavior were studied. The corrosion behavior was studied by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and immersion test in 3.5% sodium chloride (NaCl) solution. The results showed a substantial improvement in the corrosion resistance of the friction stir processed AZ31B alloy. The improvement is likely a result of more stable corrosion products and microstructure refinement formed after friction sir processing. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: Magnesium; Friction stir processing; Corrosion; Polarization; EIS.
1. Introduction Magnesium and its alloys have attracted considerable interest in a number of industries such as aerospace, automobile and electronics, and the demand for these materials has increased in recent years [1]. The rising demand for these alloys is due to their many desirable properties which range from being light weight to having a reasonably high strength-toweight ratio [2–4]. Among Mg alloys, AZ-series, especially AZ31 alloy, based on Mg-Al-Zn ternary system is the most widely used Mg alloy and they are considered as a promising choice to use in structural applications [5,6]. Despite the great properties of Mg and its alloys, these materials are chemically active and their applications are hindered by their poor corrosion performance in both atmospheric and aqueous media. The low corrosion resistance of ∗
Corresponding author. E-mail address: [email protected] (M. Heydarzadeh Sohi).
Mg and its alloys is directly related to mismatch between Mg(OH)2 lattice and bulk material on the surface region which leads to the lack of formation of protective passive oxide film [7,8]. Microstructural refinement is believed to be a promising approach to overcome the limitation of Mg and its alloys for their poor corrosion resistance [9,10]. Various surface refinement techniques have been investigated and studied on magnesium alloys by several researchers. Laser surface melting (LSM) [11–16], and electron beam melting (EBM) [17] of Mg alloys among such processes. Friction stir processing (FSP), which has its basis on friction stir welding (FSW), is another surface modification approach for refining the microstructure of metallic materials including magnesium since the beginning of this century [18]. In FSP, tool rotational (ɷ, rpm) and traverse speed (ν, mm/min) play key roles. While the rotation of the pin stirs the material, its traverse shifts the stirred substrate from the front side to the back of the rotating tool. The process is shown schematically in Fig. 1. This solid-state process has
https://doi.org/10.1016/j.jma.2019.11.004 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Please cite this article as: H. Seifiyan, M. Heydarzadeh Sohi and M. Ansari et al., Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.004
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Fig. 1. Schematic drawing of friction stir processing (FSP).
an advantage over the surface modification techniques based on surface melting processing such as LSP and EBM. This advantage is that FSP avoids the bulk melting of the base material that makes it capable of producing porosity and crack-free modified layers [18,19]. Up to now, FSP has been mostly used by researchers for improving the mechanical properties of materials [20–25] and there has been limited research studies dealing with the corrosion behavior of Mg alloys after FSP/FSW [26–34]. The corrosion behaviors of magnesium alloys are mainly related to a number of parameters such as microstructure, alloy composition, working conditions, and properties of the film developed during the exposure to the working atmosphere. Also, the uniformity of microstructural refinement and residual stresses induced by FSP is governed by processing conditions. Depending on FSP parameters, the corrosion resistance may be deteriorated. Although some researchers have briefly studied the corrosion resistance of AZ31 Mg alloy after FSP/FSW [35–42], it is still not well-understood how FSP conditions affect the corrosion behavior. To the knowledge of the authors, a thorough investigation on the influence of FSP conditions such as traverse and rotational speeds of the tool, and the number of FSP passes has not been performed yet and such an investigation is requisite to shed light on the corrosion behavior of AZ31 Mg alloy after FSP. Concerning this, the main purpose of the present work is to know how modifications in the microstructure cause changes in the corrosion behavior of AZ31B when different FSP conditions applied. 2. Material and methods 2.1. Materials and FSP conditions Commercially available wrought AZ31B-O Mg alloy plates (150 × 100 × 8 mm3 ) were used as base material. Prior to FSP, the surface oxides were removed and then the surface was cleaned using ethanol. The composition of AZ31B was
Table 1 Chemical composition of AZ31B Mg alloy. Element
Mg
Al
Zn
Mn
Si
Cu
Fe
Ni
Content (wt. %)
Bal
2.96
0.94
0.48
0.032
0.027
0.0025
0.003
analyzed using optical emission spectroscopy (OES); the results are given in Table 1. FSP was conducted using an H13 steel tool consisted of a square pin of 6 mm in diameter, 5.6 mm in height, and a shoulder of 18 mm in diameter. The detailed dimensions of the tool are given in Fig. 2. Several samples were friction stir processed (FSPed) at different processing conditions as shown in Table 2. Tilt angle of tools in direction of travel was set in 3° and penetration depth was 0.4 mm. After performing single-pass FSP, the parameters which led to the samples with the lowest average grain size were chosen as best single-pass processing conditions and 3-pass FSP was performed in those processing conditions. 2.2. Corrosion tests The specimens for corrosion tests were extracted from only the stir zone at the middle of the FSPed samples using wire electrical discharge machining. Thereafter, the specimens were ground using emery papers (3000 grit). To study the corrosion behavior, three techniques were employed which include potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and immersion test. A system of three electrodes including FSPed specimens as the working electrode, platinum as counter electrode, and Ag/AgCl as reference electrode for electrochemical studies. To prepare the electrodes, a wire covered with cold setting resin was connected to one side of the specimens, while their other side were faced to the corrosive solution. EIS tests were conducted using an EG&G Potentiostat/Galvanostat Model 273 with a Model 1260 frequency response analyzer (FRA), after exposing the specimens in a 3.5% NaCl solution for 1 h, at ambient temperature. The exposure area of each specimen
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Table 2 Processing conditions used for FSP of AZ31B alloy. Sample code
SP1
SP2
SP3
SP4
SP5
SP6
SP7
SP8
MP1
MP2
Tool rotational speed ɷ (rpm) Tool traverse speed ν (mm/min) Number of FSP passes
800 50 1
800 80 1
1000 50 1
1000 80 1
1250 50 1
1250 80 1
1600 50 1
1600 80 1
1000 50 3
1000 80 3
Single-pass and multi-pass FSPed samples denoted as SP and MP, respectively.
surface area of all specimens was 1 cm2 . After immersion, the corrosion products formed on the samples were removed by means of a reagent containing 200 g CrO3 and 10 g AgNO3 in 250 mL distilled water [44]. They were then washed with distilled water, dried and weighed to get the weight loss. 2.3. Microstructural and phase characterization Metallographic specimens were extracted from the middle of the FSPed samples and their cross sections were manually ground, polished and then etched in a solution of 4.2 g picric acid, 10 ml acetic acid, 70 ml ethanol and 10 ml H2 O [45]. While the grain size was measured using Digimizer Image Analysis Software [46], microscopic images were taken by an optical microscope (Gippon-Japan). In accordance with ASTM: E112-96 [47], the average grain size was measured. A Zeiss SIGMA VP scanning electron microscope (SEM) fitted with an Energy Dispersive Spectroscopy (EDS) microanalyzer was used to analyze the microstructure of the samples, surface morphology, and composition of the corroded surfaces. A Phillips X’Pert Pro diffractometer was used to identify the phases in the corrosion products after immersion tests. A CuKα radiation was used at operating parameters of 40 kV and 30 mA over a 2θ range of 10–110°. 3. Results 3.1. Microstructure
Fig. 2. Schematic drawing of FSP tool and its dimensions.
that was immersed in the solution was 1 cm2 . The frequency ranged from 100 kHz to 100 mHz using perturbative voltage amplitude of 10 mV was used for EIS measurements. ZView® software [43] was used for fitting the equivalent circuit with the impedance data obtained in these tests. The corroded layers after EIS tests were then characterized by SEM and XRD. Potentiodynamic polarization scans were performed in a cell containing 150 ml of 3.5% NaCl. The specimens were immersed in 3.5% NaCl solution and the test were carried out with a scan rate of 1 mV/s in a range of -250 to +250 mV with regard to the open circuit potential (OCP), after letting development of a steady state potential. For constant immersion tests, all the specimens were initially polished and then immersed in 3.5% NaCl solution for 24 h. The exposure
Fig. 3 is an optical micrograph of a typical cross-sectional view of a FSPed AZ31B Mg alloy showing friction stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and the base magnesium alloy. The right and left sides of the centered area are retreating (RS) and advancing sides (AS) of the rotating tool, respectively. All samples showed a typical view as Fig. 3 with no defects, except for the samples FSPed with tool rotational speed of 800 rpm that showed tunneling defects. Since these imperfect samples did not pass preliminary examinations, they were rejected and did not proceed further. SZ experiences high heat input and strain that induced by rotating FSP tool. The microstructure in this area has fine grains which result from dynamic recrystallization. Optical micrographs of as-received alloy and SZ of the FSPed samples at different FSP conditions are shown in Fig. 4. In addition, the average grain size values achieved at varying processing conditions are given in Table 3. It can be clearly substantiated that FSP refined the microstructure from an
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Fig. 3. Typical cross-sectional view of FSPed sample at ɷ =1000 rpm, ν = 50 mm/min.
Fig. 4. Optical micrographs of as-received alloy and SZ of FSPed samples at different processing conditions; (a) as-received alloy, (b) single-pass FSP at ɷ =1000 rpm, ν = 50 mm/min (SP3), (c) single-pass FSP at ɷ =1000 rpm, ν = 80 mm/min (SP4), (d) single-pass FSP at ɷ =1250 rpm, ν = 50 mm/min (SP5), (e) single-pass FSP at ɷ =1250 rpm, ν = 80 mm/min (SP6), (f) single-pass FSP at ɷ =1600 rpm, ν = 50 mm/min (SP7), (g) single-pass FSP at ɷ =1600 rpm, ν = 80 mm/min (SP8), (h) 3-pass FSP at ɷ =1000 rpm, ν = 50 mm/min (MP1), and (i) 3-pass FSP at ɷ =1000 rpm, ν = 80 mm/min (MP2). Table 3 Average grain size value for as-received alloy and SZ of FSPed samples at different processing conditions. Sample code
SP3
SP4
SP5
SP6
SP7
SP8
MP1
MP2
As-received
Average grain size (μm)
16.8
15.9
20.1
19.4
24.5
23.6
11.2
10.2
28.5
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Table 4 Fitting data obtained from EIS measurements for all samples immersed in 3.5% NaCl solution after 1 h.
Fig. 5. High-magnification SEM micrograph of FSPed sample at ɷ =1000 rpm, ν = 50 mm/min, indicating the presence of second phase Mn-Al particles at grain boundaries.
average grain size of about 29 ± 9 μm for the as-received alloy to an average grain size of 15.9± 5.6 μm for best single-pass FSP conditions and 10.2 ± 2.4 μm for best multipass FSP conditions. The microstructure of the base metal exhibits a combination of coarse and fine grains, while the grains of the FSPed samples are more or less uniform. It is seen that the grain size is slightly decreased after single-pass FSP; however, after 3-pass FSP, the grain size considerably reduced to about one-third of that of the as-received alloy. As it can be seen in Fig. 4, a few second phase particles are randomly distributed in the matrix of the as-received alloy and FSPed sample, which are believed to be Mn-Al intermetallics (confirmed by EDS results). A higher magnification SEM micrograph of these particles in a typical FSPed sample is shown in Fig. 5. The presence of these particles after FSP indicates that second phase Mn-Al particles in the as-received alloy were not dissolved during FSP. A number of other researchers have also come across similar results [48–50]. Nevertheless, the intermetallics were broken during FSP and their size reduced from about 3-7 μm in as-received AZ31B to about 1–3 μm in the FSPed samples. 3.2. Corrosion studies 3.2.1. Electrochemical impedance measurements Fig. 6a-c show the Nyquist and Bode plots obtained from the as-received material and single-pass FSPed samples at different conditions immersed in 3.5% NaCl solution for 1 h. The Nyquist diagrams for the as-received alloy and singlepass FSPed samples (Fig. 6a) show two-time constants corresponding to the presence of a capacitive loop at high frequency ranges and an inductive loop at low-frequency ranges. The Bode plots for the as-received alloy and single-pass FSPed samples appears to confirm the Nyquist diagrams. Charge transfer resistance of the electrode is the reason of the capacitive loop at high to medium frequencies. It should
Sample code
Rs (Ω.cm2 )
Cdl (μF/cm2 )
Rct (Ω.cm2 )
RL (Ω.cm2 )
SP3 SP4 SP5 SP6 SP7 SP8 MP1 MP2 As-received
28.5 34.56 20.85 19.95 26.24 27 22.57 23.3 22.2
7.6185E-6 4.5134E-6 2.8383E-6 1.1571E-5 8.6041E-6 4.656E-6 1.0284E-5 1.3014E-5 1.4606E-5
1081 790 805.7 564.7 545.7 452.9 1629 1206 313.3
549.6 486.9 1611 400 400.3 140 262.5
L (H.cm2 ) 245.8 95.02 304.7 351 186.1 97.55 135.7
be noted that the diameter of the capacitive loop directly relates to charge transfer resistance. The reason for the presence of inductive loop can be pitting corrosion or adsorbed surface of electrically active species such as Mg(OH)ads + or Mg(OH)2 [51,52]. The larger diameter of the capacitive loops for the FSPed samples in comparison to the as-received material shows higher charge transfer resistance or in other word better corrosion resistance of the FSPed samples. By increasing tool rotational and traverse speed, the diameter of the capacitive loops for the single-pass FSPed samples decreased, indicating low corrosion resistance. The Nyquist diagrams for the as-received alloy and singlepass and 3-pass FSPed samples at optimum conditions are shown in Fig. 6d, together with the Bode plots (Fig. 6e and f). Although the presence of an inductive loop and a capacitive loop are evident in the as-received alloy and single-pass FSPed samples, the 3-pass FSPed samples only show a capacitive loop and there is no inductive loop. It has been reported that the presence of an inductive loop in Nyquist diagrams is a sign of corrosion on the surface (localized corrosion especially pitting). Therefore, the absence of an inductive loop may be evidence for better corrosion protection [53]. In addition, by increasing the number of FSP passes the diameter of the capacitive loops for the 3-pass FSPed samples increased, indicating better corrosion resistance. The electrical equivalent circuit (EEC) was developed according to the overall corrosion reaction of Mg in aqueous environments, which generally contains the electrochemical reaction of Mg and H2 O to produce Mg(OH)2 and H2 , as previously reported by others [54–56]. Fig. 7a shows the EEC for the as-received alloy and single-pass FSPed samples and Fig. 7b shows the EEC for 3-pass FSPed samples. In these EECs, Rct is the charge transfer resistance, Rs is the electrolyte resistance, and Cdl represents a two-layer capacitance at the substrate/film interface. To take the inductive behavior into account, an inductor L and a resistance RL were introduced into the model for the as-received alloy and single-pass FSPed samples. The impedance data were fitted with the suggested EECs by ZView® software and the results are listed in Table 4. The Rs values are almost close and relatively small, which is due to the good electrical conductivity of NaCl solution.
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Fig. 6. Nyquist and Bode plots after 1 h immersion in 3.5% NaCl solution for (a-c) the as-received alloy and single-pass FSPed samples, and (d–f) the as-received alloy, and single-pass and 3-pass FSPed samples only at optimum conditions.
The Rct values for the FSPed samples are remarkably higher than that of the as-received alloy. According to Table 4, the highest corrosion resistance for the single- and multi-pass FSPed samples achieved at processing conditions of 1000 rpm and 50 mm/min. The Rct values for the single-pass FSPed samples decreased by increasing tool rotational and traverse speed. Although the grain size decreased by increasing traverse speed, the FSPed samples at processing condition of 50 mm/min showed a higher Rct value. The
reason is probably that of high traverse speed which can induce more residual stresses and lessen the positive effect of grain refinement. It has been reported that the presence of residual stresses and a large number of surface defects and high dislocation/grain boundary density have a detrimental effect on the corrosion behavior [57–59]. In addition, by increasing the number of FSP passes from 1 pass to 3, the Rct value significantly increased from 313.3 Ω.cm2 for the as-received alloy to 1081 and 1629 Ω.cm2 for the 1-pass and
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Table 5 Summary of electrochemical data calculated by Tafel extrapolation method for all samples immersed in 3.5% NaCl solution. Sample code
β a (mV/decade)
β c (mV/decade)
Ecorr vs. Ag/AgCl (V)
jcorr (A/cm2 )
SP3 SP4 SP5 SP6 SP7 SP8 MP1 MP2 As-received
–0.174 –0.1663 –0.2132 –0.1742 –0.1556 –0.1737 –0.1952 –0.1928 –0.2189
0.0119 0.0076 0.0179 0.0146 0.0371 0.0405 0.063 0.063 0.126
–1.204 –1.303 –1.293 –1.322 –1.388 –1.382 –1.320 –1.317 –1.483
5.87E-6 1.06E-5 3.94E-5 4.22E-5 4.75E-6 3.74E-6 5.53E-5 5.66E-5 4.23E-4
Fig. 7. EEC used for fitting EIS data of (a) as-received alloy and single-pass FSPed samples, and (b) 3-pass FSPed samples.
3-pass FSPed sample with tool rotational speed of 1000 rpm and tool traverse speed of 50 mm/min.
3.2.2. Potentiodynamic polarization measurements The polarization curves for the as-received and FSPed samples are shown in Fig. 8 and the electrochemical data calculated by Tafel extrapolation method are listed in Table 5. It is evident from Fig. 8 that the anodic polarization curve for all the FSPed samples shifts towards more noble potential, indicating low anodic dissolution which may be accredited to the fine-grained structure of the FSPed samples. FSP caused a decrease in anodic current density of samples due to facilitate passive film formation on the surface. In addition to the predominant effect of FSP on the acceleration of passive film formation for all samples, the decrease in cathodic kinetics was another reason for decreasing the corrosion current density of FSPed samples. From Table 5, it can be seen that the sample FSPed with rotational speed of 1000 rpm and tool traverse speed of 50 mm/min (SP3) has the lowest polarization current density among all the FSPed samples. Increasing of tool’s rotational speed and its traverse speed, decreases the corrosion potential (Ecorr ); although, the corrosion current density (jcorr ) does not have a meaningful trend. The anodic Tafel slope (β a ) for the FSPed samples is higher than that of the as-received alloy; however, the cathodic Tafel slope (β c ) for the FSPed samples is lower than that of the as-received alloy and by decreasing tool rotational speed from 1600 to
Fig. 8. Polarization curves after immersion in 3.5% NaCl solution for (a) as-received alloy and single-pass FSPed samples, and (b) as-received alloy, and single- and 3-pass FSPed samples only at optimum conditions.
1000 rpm, the value of cathodic slope decreases. The cathodic activity is attributed to the presence of second phase particles. The potentiodynamic polarization investigations revealed that the polarization curves, especially anodic polarization curves, had considerable deviations from linearity. Since the
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Table 6 Summary of corrosion rates obtained from constant immersion tests for all samples immersed in 3.5% NaCl solution for 24 h.
Table 7 EDS results of corrosion products on the surface of the corroded samples (atom %).
Sample code
Corrosion rate (mm/year)
Element Sample
Mg
O
Cl
Al
SP3 SP4 SP5 SP6 SP7 SP8 MP1 MP2 As-received
0.0801 0.1140 0.1339 0.2409 0.2731 0.2784 0.0598 0.0709 0.3293
MP1 SP3 As-received
38.7 37.6 38.5
58.2 57.3 59.4
2.3 2.4 1.5
0.8 0.6 0.5
anodic branch of the polarization curves deviates from linear behavior, the determination of anodic slope is not accurate. These observations suggest that Tafel extrapolation is not an accurate method for determining the electrochemical data in the case of Mg alloys. The reason why the determination of the anodic Tafel slope is not accurate has been elucidated by Curioni et al. [60,61]. In the case of Mg alloys, the measurement of the anodic slope is difficult due to the fact that magnesium oxidizes under anodic polarization that is leading to the domination of ohmic drop effects [60]. The measurement is not reliable owing to the presence of bubbles produced by substantial hydrogen evolution and local alkalinization in the proximity of the electrode surface [61]. The above-mentioned considerations are in agreement with the observations in this study, disclosing a complex polarization behavior that cannot be approximated by Tafel extrapolation method. 3.2.3. Immersion test The samples were immersed in 3.5 wt.% NaCl solution for 24 h and the corrosion rate was calculated by the following equation in accordance with NACE/ASTM TM0169 G0031 12A Standard [62]: k.(w0 − w1 ) CR = A.t.ρ In this equation CR is corrosion rate (mm/year), k = 8.76 × 104 , (w0 - w1 ) is weight change (g), A is specimen area (cm2 ), and t is exposure time (h), and ρ is density (g/cm3 ). The results of the immersion tests are given in Table 6. According to Table 6, the highest corrosion rate for the single- and multi-pass FSPed samples achieved at processing conditions of 1000 rpm and 50 mm/min. The corrosion rate value for the single-pass FSPed samples increased by increasing rotational and traverse speeds of the tool. In addition, by increasing the number of FSP passes from 1 pass to 3, the corrosion rate value significantly decreased from 0.3293 mm/year for the as-received alloy to 0.0801 and 0.0598 mm/year for the 1-pass and 3-pass FSPed samples with tool rotational speed of 1000 rpm and tool traverse speed of 50 mm/min. 3.2.4. Characterization of corrosion layer Fig. 9 compares the surface morphology of the as-received and FSPed samples after immersion in 3.5 wt.% NaCl solu-
tion for 24 h. The surface of the as-received sample was completely corroded, and the film was thick and loose with large cracks and flake-like structure (Fig. 9a and b). These cracks could act as routs for the penetration of the corrosive solution, that might intensify the corrosion [63]. Unlike the as received material, the surface of the corroded FSPed samples (Fig. 9c, d, e, f) showed a denser and more uniform corrosion layer with fewer cracks and discontinuities, which is more likely to be protective than that formed on the as-received sample. Table 7 gives the atomic percentage of the elements obtained by EDS microanalysis of the corrosion products formed on the surface of the as-received and FSPed samples after 24 h immersion in 3.5 wt.% NaCl solution. From the analyses carried out, it can be pointed out that the corrosion products mainly contain Mg, O, and Cl with a trifling Al enrichment on the corrosion layer. XRD patterns of the corrosion products after being immersed for 24 h in 3.5 wt.% NaCl solution can be seen in Fig. 10. The main corrosion product is revealed to be Mg(OH)2 which makes the XRD analysis to be consistent with the EDS results. The formation of Mg(OH)2 as the main corrosion product has been reported before [56,64]. Moreover, the diffraction intensities of the metallic Mg from the substrate decreased for the FSPed samples, which agrees with the presence of more amount of Mg(OH)2 on the FSPed samples and confirms the formation of a protective corrosion layer. 4. Discussion 4.1. Influence of FSP conditions on microstructure Recrystallization and grain growth are two metallurgical phenomena that compete with each other to control the grain size after FSP. Some factors which influence these two phenomena include maximum temperature (dominated by tool rotational speed), remaining time at high temperature (dominated by tool traverse speed), strain, and strain rate experienced during FSP (dominated by the number of FSP passes). These factors are highly affected by rotational and traverse speed of FSP tool as well as the number of FSP passes. As for the effect of tool rotational and traverse speed on the grain size, grain growth derived from heat input during the course of FSP is the dominant phenomenon. Heat input is directly proportional to the ratio of tool rotational speed and traverse speed (ɷ/ν) [65]. By increasing tool rotational speed and reducing its traverse speed (i.e. increasing ɷ/ν ratio), the heat input in SZ increases. Therefore, by increasing the tool
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Fig. 9. Low and high magnification SEM images showing the surface morphology of corroded samples after 24 h immersion in 3.5 wt.% NaCl solution; (a and b) as-received alloy, (c and d) single-pass FSPed sample at ɷ = 1000 rpm, ν = 50 mm/min (SP3), and (e and f) 3-pass FSPed sample at ɷ = 1000 rpm, ν = 50 mm/min (MP1).
rotational speed at constant tool traverse speed, maximum temperature increases, and grain growth occurs and coarse grains are formed in SZ. However, when the tool traverse speed is increased at constant tool rotational speed, the remaining time at high-temperature decreases which will provide less time for grains to grow and finer grains are formed in SZ. Regarding the effect of the number of FSP passes on the grain size, the average grain size showed the lowest values after multi-pass FSP. This is a result of acute plastic deformation and frictional heat triggered by multi-pass FSP. Multi-pass FSP induces more dislocation sources, resulting in more recrystallization and dynamic recovery. Recrystallization phenomenon is dominant and it is expected more by increasing the number of FSP passes. More strain induces and grain size decreases.
4.2. Influence of FSP conditions on corrosion behavior Since magnesium is an intrinsically passive element, its exposure to chloride ions in a non-oxidizing media leads to its pitting. In this case, the anode and cathodes are grain interior and grain boundary, respectively. It is also known that the corrosion behavior of Mg alloys is influenced by the distribution of microconstituents, which are nobler than the adjacent Mg matrix. Consequently, it has been well documented that the Mn-Al intermetallics and the β-phase present in Mg-Al alloys behave as potential cathodes, generating a micro-galvanic coupling and preferentially localized dissolution of the Mg matrix [66]. Whereas the role of grain size is dominant, that of the microconstituents is inconsequential in AZ31 Mg alloy. On the other hand, when Mg is
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Fig. 10. XRD patterns for corrosion layer after 24 h immersion in 3.5 wt.% NaCl solution; (a) as-received alloy, (b) single-pass FSPed sample at ɷ = 1000 rpm, ν = 50 mm/min (SP3), and (c) 3-pass FSPed sample at ɷ = 1000 rpm, ν = 50 mm/min (MP1).
electrochemically dissolved, there will be a production of OH− ions from the cathodic reaction (2). This results in an increase in pH at the interface, thereby giving rise to the formation and precipitation of MgO and/or Mg(OH)2 as corrosion products. Mg → Mg2+ + 2e−
Anodic reaction
2H2 O + 2e− → H2 + 2OH−
Cathodic reaction
Mg + 2H2 O → Mg(OH )2 + H2
Overall equation
(1)
(2)
(3)
In general, the higher corrosion rate of AZ31B alloy is attributed to the presence of large size microconstituents as well as coarse Mg grains. Fine-grained Mg in corrosive media shows different behaviors [67–69]. Recent research studies have found that the grain refinement can improve the corrosion resistance of Mg both in alkaline and neutral NaCl solutions; however, some of them reported that residual stresses in the material after grain refinement could increase the cathodic kinetics [58,59,70]. In a research study by Song et al. [57], the corrosion behavior of equal-channel-angular-pressed (ECAP) pure Mg was investigated. They reported that fine-grained Mg material produced by ECAP showed inferior corrosion resistance in 3.5% NaCl solution. Although the fine-grained Mg material has a greater tendency to form an oxide film on the surface, the magnesium oxide film is unstable and easily dissolves in NaCl solution owing to the presence of a large number of surface defects and high dislocation/grain boundary density induced by ECAP. In another research study by Jang et al. [35] on FSWed AZ31 alloy, it was shown that grain size was more effective
than residual stresses, especially in the case of 3.5% NaCl solution as corrosive media. Grain boundaries are more chemically active and have a higher energy in comparison with the bulk metal [67]. Therefore, high grain boundary density in the fine-grained material can increase the surface activity and leads to rapid formation of the passive protective film. High grain boundary density in fine-grained Mg alloys compensates the oxide/metal mismatch and encourages the formation of a denser and uniform passive film on the surface, resulting in an improved corrosion resistance. According to the literatures, the intensity of micro-galvanic couple is decreased and a quick semi-protective anodic layer is formed when the grain size is reduced [27,29]. Formation of Mg (OH)2 films on the surface is initially started on defects like grain boundaries. Reduction of the grain size, via processes like FSP, increases the nucleation site density for the fabrication of oxide films and intensifies the formation of passive layers. The adherence of the passive film to the surface may also improve due to the presence of fine grains, thereby decreasing the tendency for oxide cracking. Orlov et al. [71] also suggested that the degree of oxide cracking is reduced for a fine-grained material since it can relieve the tensile stress induced by the oxide/metal mismatch. Moreover, the micro-galvanic couple effect is minimized by the refinement of the microconstituents. Therefore, the improved corrosion resistance of FSPed AZ31 alloy is mainly attributed to the refinement of Mg grains and uniform redistribution of microconstituents. Grain refinement and compositional homogenization can reduce the volume fraction of the effective cathodes that in turn limits the cell action induced by the pile up of cathodic phases [2]. In the course of FSP, a considerable variation in the corrosion behavior may occur depending on the processing conditions. There are three factors that can affect the corrosion behavior and hence deteriorate or ameliorate the corrosion
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H. Seifiyan, M. Heydarzadeh Sohi and M. Ansari et al. / Journal of Magnesium and Alloys xxx (xxxx) xxx
resistance. While large size microconstituents and residual stresses have adverse effects, grain refinement has a positive effect on the corrosion behavior. These three factors tend to compete with each other to determine corrosion behavior. This study shows that the role of grain size is predominant since all the FSPed samples revealed an improvement in the corrosion resistance; however, it may be possible that some electrochemical results can have no meaningful correlation with FSP conditions. FSP conditions can amplify or lessen the role of the three factors, although the mutual interaction of the three factors is complicated and it is still not wellunderstood. 5. Conclusions Friction stir processing was successfully employed for surface modification of AZ31B Mg alloy. The following conclusions can be drawn: • The FSP conditions including tool rotational and traverse speed as well as the number of FSP passes are found to be the most significant parameters, which affect the microstructure and corrosion behavior of FSPed AZ31B Mg alloy. • A set of FSP conditions consisting of tool rotational speed of 1000 rpm and tool traverse speed of 50 mm/min was found to result in maximum corrosion resistance. • Grain refinement was found to have a significant effect on the corrosion behavior. However, the presence of residual stress induced by FSP and large size microconstituents can lessen the positive effect of grain refinement on the improved corrosion resistance. • The lower corrosion current density and more noble potential observed in polarization tests as well as higher charge transfer resistance observed in EIS tests indicated a superior resistance against corrosion for the FSPed samples as compared to the AZ31B alloy. • The improved corrosion resistance is attributed to the formation of the passive protective film, supported by the microstructural refinement and the presence of fine microconstituents after FSP. Declaration of Competing Interest None. References [1] H. Friedrich, S. Schumann, J. Mater. Process. Technol. 117 (2001) 276– 281, doi:10.1016/S0924- 0136(01)00780- 4. [2] G.L. Makar, J. Kruger, Int. Mater. Rev. 38 (1993) 138–153, doi:10. 1179/imr.1993.38.3.138. [3] , Corrosion of Magnesium Alloys, in: G.-L. Song (Ed.), Corrosion of Magnesium Alloys, Woodhead Publishing, Cambridge, 2011. [4] G. Song, A. Atrens, Adv. Eng. Mater. 5 (12) (2003) 837–858, doi:10. 1002/adem.200310405. [5] S. Niknejad, L. Liu, M.-Y. Lee, S. Esmaeili, N.Y. Zhou, Mater. Sci. Eng. A. 618 (2014) 323–334, doi:10.1016/j.msea.2014.08.013.
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Please cite this article as: H. Seifiyan, M. Heydarzadeh Sohi and M. Ansari et al., Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.004
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Please cite this article as: H. Seifiyan, M. Heydarzadeh Sohi and M. Ansari et al., Influence of friction stir processing conditions on corrosion behavior of AZ31B magnesium alloy, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.11.004