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Effects of grain size on the corrosion resistance of wrought magnesium alloys containing neodymium
Corrosion Science 58 (2012) 145–151
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Effects of grain size on the corrosion resistance of wrought magnesium alloys containing neodymium G.R. Argade a, S.K. Panigrahi a, R.S. Mishra a,b,⇑ a b
Center for Friction Stir Processing and Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA
a r t i c l e
i n f o
Article history: Received 17 October 2011 Accepted 24 January 2012 Available online 2 February 2012 Keywords: A. Magnesium B. Weight loss B. SEM C. Intergranular corrosion
a b s t r a c t A range of grain size from 70 lm to 0.7 lm was studied for corrosion resistance of Mg–Y–RE magnesium alloy using electrochemical and constant immersion testing in 3.5 wt.% NaCl solution. The linear polarization resistance (Rp) showed a clear trend of increasing Rp value with grain refinement. The ultrafine grained sample showed the most positive pitting potential as compared to coarse grained samples. One order of magnitude decrease in corrosion rate was observed between coarsest and ultrafine grained microstructure. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In the current state of art, the precise understanding of the effect of grain size on the corrosion response of materials, unlike, that on the mechanical properties, is lacking. Grain refinement leading to enhancement in strength of polycrystalline materials can be predicted using classical Hall–Petch relationship [1,2]. Recent efforts have been made towards development of a relationship correlating the grain size of materials and corresponding corrosion response [3,4]. Effects of grain size on corrosion behavior have been investigated for various metallic systems and enhancement as well as deterioration in corrosion resistance with grain refinement has been reported [5–24]. Many of these studies have been carried out for high purity metals and very few of them have been carried out for engineering materials [6–19]. The increase in repassivation tendencies of fine grain microstructure and break-up of second phase particles with more uniform distribution achieved through severe plastic deformation (SPD) leads to increase in corrosion resistance [6–9,11–16]. However, the increased chemical activity at grain boundary regions increases the dissolution rate of fine grained materials [17,23,24]. For light weight structural applications, magnesium alloys with their attractive properties can compete with existing materials. However, poor corrosion resistance of magnesium alloys is one of the impeding factors for their full potential use as structural materials [25]. High strength wrought magnesium alloys developed using alloy design principles can be an ⇑ Corresponding author at: Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA. Tel.: +1 940 565 2316; fax: +1 940 565 4824. E-mail address: [email protected] (R.S. Mishra). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2012.01.021
alternative [26]. Microstructural refinement of magnesium can lead to significant improvement in the corrosion response due to the alteration of passive layer characteristics [4]. However, Song et al. [17] reported decrease in corrosion resistance of pure magnesium with grain refinement in aggressive 3.5 wt.% NaCl solution. Song et al. [23] observed similar behavior in equal-channel angular pressed (ECAP) bulk ultra-fine grained AZ91D. The high strain induced mass crystalline defects and refinement of net like b-phase into isolated particles in matrix increased the dissolution rate in 3.5 wt.% NaCl [17,23]. Hoog et al. [14] observed no improvement in corrosion resistance of pure magnesium processed with surface mechanical attrition treatment (SMAT), whereas the ECAP material showed increased corrosion resistance. Therefore, it is likely that the improvement in corrosion resistance of materials with grain refinement is also a function of employed processing route. Present work is solely focused on the effect of grain size on corrosion resistance of Mg–Y–RE wrought magnesium alloy. A spectrum of grain size with two orders of magnitude (70 lm to 0.7 lm) difference was considered. Friction stir processing (FSP) was used as a SPD tool to achieve fine grain and ultrafine grain microstructures. Electrochemical testing and constant immersion testing were carried out to compare the effect of grain refinement on the corrosion resistance of the alloy.
2. Experimental 2.1. Material and conditions Samples for corrosion experiments were extracted from a hot-rolled Mg–Y–RE (nominal composition: Mg–4Y–3Nd) alloy
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Fig. 1. OIM micrographs of (a) CG 1, (b) CG 2, (c) FG, and (d) UFG. (e) Grain boundary misorientation plot for CG 1, CG 2, FG, and UFG.
(as-received condition). To achieve coarser grain size than asreceived condition, small pieces were cut from as-received material and were subjected to thermal treatment of 520 °C for 4 h. Friction stir processing (FSP) was employed to achieve finer grain microstructures. The FSP tool dimensions were: shoulder diameter12 mm, pin height-2.2 mm, pin root diameter-6 mm, and pin tip diameter-3.75 mm. One pass FSP of as-received plate was carried out with tool rotation rate of 600 RPM and tool traverse speed of 101.6 mm/min to achieve grain size refinement by one order. To attain ultrafine grained microstructure, multi-pass FSP was employed. As-received plate was subjected to first pass with tool rotation rate of 600 RPM and tool traverse speed of 101.6 mm/min. Thereafter, overlapping second pass was carried out with tool rotation rate of 300 RPM and tool traverse speed of 101.6 mm/min. All the FSP runs were conducted with tool tilt angle of 2.5°. Here onwards the specimens are designated as: as-received + thermal
treatment CG 1, as-received CG 2, as-received + 1 pass FSP FG, and as-received + 2 pass FSP UFG. Orientation imaging microscopy (OIM) was used to estimate the grain size. Scanning electron microscopy (SEM) was conducted to understand the corrosion morphology of tested samples. 2.2. Electrochemical corrosion testing Linear polarization resistance and cyclic potentiodynamic polarization were carried out for all the conditions. All the samples were polished to 1 lm surface finish using conventional metallographic steps. Thereafter, sample surface was cleaned with ethanol and subjected to electrochemical testing. Before starting the potential scan, the samples were given incubation period of 2 h to stabilize initial conditions. During the incubation period potential measurements with respect to time at a sampling rate of 1 s1
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Fig. 2. Linear polarization resistance plots of different conditions of Mg–Y–RE alloy in 3.5 wt.% NaCl. Fig. 4. Weight loss normalized with area for different conditions of Mg–Y–RE alloy after constant immersion testing in 3.5 wt.% NaCl for 3 weeks.
UFG, samples were extracted from the processed region only. All the samples were ground to 600 grit emery paper to make them flat on all the surfaces and were subsequently cleaned with ethanol. A small hole was drilled to suspend the samples in the corrosive medium through Teflon thread. Initial mass (up to 0.001 mg accuracy) and surface area was measured for all the samples. Three samples from each microstructural condition, i.e. CG 1, CG 2, FG, and UFG, were then subjected to constant immersion in corrosive medium for 3 weeks. At the end of experiment, final cleaning of each sample was done by immersing them in boiling 15% chromic acid solution to dissolve the corrosion products from the surface, immediately followed by cleaning with acetone. Weight loss was measured and corrosion rate (mg cm2 day1) was calculated. Fig. 3. Cyclic potentiodynamic polarization plots of different conditions of Mg–Y– RE alloy in 3.5 wt.% NaCl.
Table 1 Electrochemical parameters from cyclic potentiodynamic polarization test. Alloy condition
Breakdown potential, Eb (mV vs. SCE)
Repassivation potential, Ep (mV vs. SCE)
Eb–Ecorr (mV)
Eb–Ep (mV)
CG 1 CG 2 FG UFG
1589 ± 22 1591 ± 30 1617 ± 18 1597 ± 25
– 1754 ± 28 1710 ± 24 1658 ± 30
138 ± 32 192 ± 35 177 ± 29 160 ± 38
– 173 ± 25 83 ± 19 61 ± 28
2.4. Corrosive medium All the electrochemical corrosion experiments were carried out in 3.5 wt.% NaCl solution as electrolyte using saturated calomel electrode (SCE) as reference electrode, and platinum mesh as counter electrode in a standard flat cell with three electrode configuration. For immersion testing, the samples were suspended in 3.5 wt.% NaCl solution. Due care was taken to maintain sufficiently higher ratio of volume of electrolyte to surface area exposed of the samples. 3. Results and discussion
were made till a stable potential value was achieved. The potential scan range for linear polarization resistance was selected as ±30 mV from the stabilized value of potential specific to each sample at a scan rate of 0.166 mV/s. For cyclic polarization the potential was scanned from 250 mV to +500 mV, back to 250 mV from the stabilized value of potential specific to each sample at a scan rate of 0.5 mV/s. To check for the repeatability of the polarization resistance and cyclic potentiodynamic polarization, each sample condition was scanned three times.
2.3. Immersion testing The corrosion rate was estimated using weight loss method under constant immersion in the aqueous solution. For FG and
3.1. Microstructural examination The microstructural characterization of spectrum of grain size for Mg–Y–RE alloy was carried out using orientation imaging microscopy (OIM). The OIM micrographs for CG 1, CG 2, FG and UFG samples are shown in Fig. 1. Note that in Fig. 1(a) & (b) and (C) & (d) were taken at same magnification. The average grain size estimated from OIM micrographs for CG 1, CG 2, FG, and UFG are 70 ± 58 lm, 20 ± 12 lm, 2.4 ± 2 lm, and 0.65 ± 0.44 lm, respectively. OIM micrographs suggested a random texture for different conditions of Mg–Y–RE alloy. The grain boundary misorientation plot is shown in Fig. 1e. It can be noted that in all the conditions the fraction of high angle grain boundaries (HAGBs) is >85%.
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Fig. 5. Grain size effect on corrosion properties for Mg–Y–RE alloy, (a) open circuit potential (OCP) and polarization resistance as a function of grain size, (b) corrosion rate estimated from constant immersion testing as a function of grain size, and (c) and (d) comparison of corrosion rates in magnesium as a function of grain size [16,17].
3.2. Electrochemical corrosion studies 3.2.1. Linear polarization resistance Linear polarization resistance plots for different grain size Mg– Y–RE alloy samples tested in 3.5 wt.% NaCl solution are shown in Fig. 2. The slope of E–I plot gives polarization resistance (Rp). The Rp value is inversely proportional to the corrosion rate exhibited by the material. Thus, higher Rp value corresponds to higher corrosion resistance of the material. For the present work, higher Rp values were observed for UFG and FG samples as compared to CG 1 and CG 2 samples.
3.2.2. Cyclic potentiodynamic polarization Cyclic potentiodynamic polarization plots for different grain size Mg–Y–RE alloy samples in 3.5 wt.% NaCl solution are shown in Fig. 3. The electrochemical parameters obtained from cyclic polarization study are summarized in Table 1. It can be seen that the breakdown potential (Eb) observed for all the samples was in the similar range. The range of passive region indicated by Eb–Ecorr was also observed in the similar range for all the grain sizes. However, increase in repassivation potential (Ep) was observed with grain refinement. CG 1 sample showed no intersection between reverse scan and anodic region of forward scan indicating no repassivation. Gradual decrease in difference between Eb and Ep was observed for samples CG 2, FG and UFG with grain refinement. The Ecorr in reverse scan for FG and UFG samples was higher than that in the forward scan which was not observed in CG 1 and CG 2 samples. The growth of the pits formed in the transpassive region continues to grow in CG 1 and CG 2 samples, whereas for FG and UFG samples the higher Ecorr in reverse scan is an indication of no further growth of pits formed in transpassive region. Thus, FG
and UFG samples showed better repassivation characteristics with arresting the growth of pits as compared to CG 1 and CG 2 samples. 3.3. Weight loss measurements Constant immersion is a direct method to estimate the corrosion resistance of the materials. The weight loss experienced by the materials during constant immersion is proportional to the corrosion resistance. For the present work, the weight loss measured from constant immersion of different conditions of Mg–Y–RE alloy in 3.5 wt.% NaCl solution for 3 weeks are shown in Fig. 4. The CG 1 samples showed the highest weight loss and UFG samples showed the lowest weight loss. 3.4. Grain size effect Fig. 5a shows the polarization resistance (Rp) and the open circuit potentials (OCP) observed during electrochemical testing plotted as a function of grain size. For the present work, the grain refinement achieved through FSP of Mg–Y–RE magnesium alloy results in shift of OCP towards positive potentials. This indicates better thermodynamic ability of refined microstructure to resist corrosion. Similar behavior is also evident from the polarization resistance data. The slope of E–I plot (Fig. 2) increased with grain refinement suggesting higher corrosion resistance for fine grained microstructure as compared to coarse grained microstructure. The constant immersion testing results also indicated identical trends. The weight loss of the samples from the immersion testing was normalized with respect to the area of the samples. Fig. 5b shows area normalized weight loss as a function of d1/2, where d is the average grain size. During immersion testing, a distinctive trend
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Fig. 6. Backscattered SEM micrographs depicting constituent particles in (a) CG 1, (b) CG 2, and (c) FG. (d) Particle size distribution observed in each condition.
was observed between corrosion rate and grain size. Lower corrosion rates were observed in FG samples as compared to CG 1 and CG 2 samples. The lowest corrosion rates were observed in UFG samples. Birbilis et al. [16] observed a similar relationship between Icorr, a measure of corrosion rate, and grain size for pure magnesium. However, Song et al. [17] reported an inverse behavior in a similar study. Recent literature suggests that corrosion behavior of magnesium and its alloys can be significantly altered by grain refinement and researchers have reported improvement in corrosion resistance of commercially pure magnesium [14–16]. Pilling–Bedworth ratio (PBR), a measure of stability of passive oxide layer, for magnesium is 0.81. Moreover, in case of magnesium, unlike aluminum (PBR 1.2) and stainless steels (PBR 2), the passive layer is crystalline in nature [28]. The structural discontinuity between the crystal structure of oxide layer and the HCP lattice of magnesium results into development of high compressive stress within the oxide layer [16]. As suggested by Birbilis et al. [16], one of the ways of reducing the mismatch and have lower disorder between oxide layer and metal surface is to introduce large volume fraction of grain boundaries per unit area. The OIM micrograph analysis of FG and UFG samples indicated that FSP of Mg–Y–RE alloy resulted in >85% fraction of HAGBs in the microstructure. The HAGBs fraction in CG 1 and CG 2 microstructure was also >85% (Fig. 1e). Therefore, the higher fraction of HAGB per unit volume in UFG microstructure resulted in formation of more tenacious and adherent passive layer and thus lowest corrosion rate. The microstructural condition in UFG is altering the characteristics of formation of passive layer. This is clearly evident in the cyclic polarization scans with UFG sample showing repassivation in the reverse scan. There have been some studies on the influence of texture on the corrosion behavior of hexagonal crystalline materials [8,30]. Hoseini et al. [8] observed higher corrosion resistance in commercially pure ECAP titanium surface with basal texture. In a similar study, Song et al. [30] estimated better electrochemical stability and higher corrosion resistance in rolled
AZ31 in the surfaces parallel to basal plane direction. In the present work, the OIM micrographs (see Fig. 1) indicate a random texture in all the samples. The possible reason may be due to the presence of rare earth elements in the present magnesium alloy which tend to reduce the anisotropy in the microstructure. 3.5. Processing route impact The corrosion behavior of magnesium is highly sensitive to the processing route employed to achieve the microstructural refinement [14–17]. The corrosion rate obtained from the weight loss experiments was plotted on log–log scale as a function of d1/2 and was compared with the Icorr, a parameter of corrosion rate from the work of Birbilis et al. [16] and Song et al. [17] on pureMg. The difference in the slopes and intercepts in Fig. 5c are attributed to the difference in material, the processing route employed to achieve grain refinement and electrolyte concentration [3]. This difference in slopes can be explained as follows. The nature of FSP leads to formation of large fraction of high angle grain boundaries in comparison to other SPD techniques [27]. This is a major distinction as the microstructure evolution during FSP involves dynamic recrystallization and grain growth. This results in lower dislocation density in the microstructure. Comparing the plots in Fig. 5c and d, the magnitude of slope of corrosion rate-grain size plot for the present work is steeper than the slope observed by Birbilis et al. [16]. This suggests a significant decrease in the magnitude of corrosion rate with grain refinement of FSP Mg–Y–RE alloy. The microstructural evolution in ECAP is dislocation based sub-structure growth which results into higher fraction of low angle grain boundaries (LAGB) in ECAP microstructure as compared to FSP microstructure [29]. Also, ECAP introduces higher dislocation density in the microstructure as compared to FSP. Further, rare earth (RE) addition to magnesium in general improves the corrosion response of the alloy. This is attributed to the contribution of RE, in solid solution with magnesium matrix, to
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Fig. 7. SEM micrographs of surface morphology of samples tested after constant immersion in 3.5 wt.% NaCl for 3 weeks, (a) CG 1, (b) CG 2, (c) FG, and (d) UFG.
form a protective layer to resist corrosion attack. Moreover, FSP is expected to break-up and uniformly distribute second phase particles in the matrix [31]. This can lead to reduced propensity of localized corrosion attack in FG and UFG samples. Fig. 6 shows the SEM backscatter images of CG 1, CG 2, and FG conditions depicting the constituent particles. The particle size distribution observed in each case is also shown in Fig. 6. It can be clearly observed that the particle size distribution shifted towards lower size with 1 pass FSP. Also, note that the clustering of particles observed in CG 1 and CG 2 was eliminated in FG because of FSP. The fragmentation of constituent particles due to FSP could be a possible reason of reduced density of particles resolved under backscattered SEM for the FG condition. For the UFG condition, the 2 pass FSP is expected to further reduce the size of constituent particles and this was the reason why the constituent particles were not observed under backscattered SEM. The 2 pass FSP will reduce the particle size to the nanometer scale (<100 nm) and significantly lower the intensity of the corrosion attack due to the galvanic coupling with the matrix. The improvement in corrosion behavior due to dynamically recrystallized microstructure and fine homogenous distribution of second phase obtained via ECAP of ZE41A rare earth containing magnesium alloy has been reported [32]. Thus, the alloy chemistry together with nature of processing resulted in one order lower magnitude of corrosion rate in UFG microstructure as compared to CG 1. The inverse relationship observed by Song et al. [17] was attributed to high dislocation density and more energetic crystalline defects imparted during ECAP of pure magnesium. Moreover, electrolyte concentration in the work of Song et al. [17] was 0.6 M NaCl which can produce severe attack as reported. The work on AZ91D by Song et al. [23] also indicated similar trends in
corrosion rate estimated from weight loss experiments. The mass loss in 3.5 wt.% NaCl solution in as-cast AZ91D was reported to be lowest and increased with increase in number of ECAP passes. The stacked dislocations developed in the microstructures resulted in higher mass loss for ECAP specimens. From the present work, the corrosion rate (CR) dependence on grain size (Fig. 5c and d) is expressed as: 1
logðC R Þ ¼ A þ B logðd 2 Þ 0:3
C R ¼ Cd
ð1Þ ð2Þ
where d is the average grain size and A, B, and C are constants. The present results exhibit a power law relationship between grain size and corrosion rate. This power law relationship has lower grain size dependence as compared to Birbilis et al. [16], indicating higher dominance of alloy chemistry on the corrosion rate over grain size. This raises an open question about the grain size dependence of corrosion rate of materials. Birbilis et al. [3] proposed a relationship similar to classical Hall–Petch type dependence. From the present study, it is evident that the corrosion mechanism of magnesium can be significantly altered by introducing large fractions of high angle grain boundaries. The single crystal magnesium with infinitesimally small d1/2 value without any grain boundaries is expected to have lowest corrosion rate. The introduction of grain boundaries, the high energy areas in the microstructure, can lead to increase in corrosion rate by formation of galvanic couple between relatively anodic grain boundary and cathodic grain interior. This behavior is expected to change again when the grain refinement is very high although the transition grain size for this is not established yet. In the ultrafine grained regime, the very high fraction of grain
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boundaries in the microstructure is likely to reduce corrosion rates in two ways: (1) accelerating the passivation kinetics, and (2) reduce the intensity of galvanic couple between grain interior and grain boundary. Therefore, the UFG microstructure will lead to formation of closely spaced electrochemical batteries of anodic–cathodic regions. Thus the difference between the rates of anodic and cathodic reactions are expected to alter significantly, which leads to more uniform attack and lower corrosion rates. In the present work, the FSP refined microstructure with lower gradient of electrochemical potential within the matrix led to a more homogeneous microstructure. The corrosion surface morphology of CG 1 and UFG samples after constant immersion testing was observed using scanning electron microscopy (SEM). Fig. 7 shows SEM micrographs of CG 1, CG 2, FG, and UFG samples after 3 weeks of exposure to chloride environment. The pit morphology as observed on the surface of CG 1 and UFG clearly indicates higher corrosion attack with larger and deeper pits in CG 1 as compared to UFG showing shallower pits (see inset). This is in agreement with the weight loss observed during constant immersion testing. The larger and deeper pits in CG 1 samples and shallower pits in UFG samples lead to highest and lowest weight loss, respectively. The pit growth arrest in case of UFG microstructure was found to be better in cyclic polarization study as compared to coarse grained microstructure. The SEM observation of immersion tested sample surface indicates similar behavior. 4. Conclusions (1) Grain refinement of Mg–Y–RE achieved through FSP lead to increase in corrosion resistance of the alloy. (2) UFG microstructure showed highest polarization resistance and the most positive pitting potentials and repassivation potentials as compared to coarse grain microstructure. (3) During constant immersion testing a clear relationship between grain size and corrosion rate was observed with lowest corrosion rates in UFG samples and highest corrosion rates in coarse grain samples, respectively.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Effects of grain size on the corrosion resistance of wrought magnesium alloys containing neodymium G.R. Argade a, S.K. Panigrahi a, R.S. Mishra a,b,⇑ a b
Center for Friction Stir Processing and Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA
a r t i c l e
i n f o
Article history: Received 17 October 2011 Accepted 24 January 2012 Available online 2 February 2012 Keywords: A. Magnesium B. Weight loss B. SEM C. Intergranular corrosion
a b s t r a c t A range of grain size from 70 lm to 0.7 lm was studied for corrosion resistance of Mg–Y–RE magnesium alloy using electrochemical and constant immersion testing in 3.5 wt.% NaCl solution. The linear polarization resistance (Rp) showed a clear trend of increasing Rp value with grain refinement. The ultrafine grained sample showed the most positive pitting potential as compared to coarse grained samples. One order of magnitude decrease in corrosion rate was observed between coarsest and ultrafine grained microstructure. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In the current state of art, the precise understanding of the effect of grain size on the corrosion response of materials, unlike, that on the mechanical properties, is lacking. Grain refinement leading to enhancement in strength of polycrystalline materials can be predicted using classical Hall–Petch relationship [1,2]. Recent efforts have been made towards development of a relationship correlating the grain size of materials and corresponding corrosion response [3,4]. Effects of grain size on corrosion behavior have been investigated for various metallic systems and enhancement as well as deterioration in corrosion resistance with grain refinement has been reported [5–24]. Many of these studies have been carried out for high purity metals and very few of them have been carried out for engineering materials [6–19]. The increase in repassivation tendencies of fine grain microstructure and break-up of second phase particles with more uniform distribution achieved through severe plastic deformation (SPD) leads to increase in corrosion resistance [6–9,11–16]. However, the increased chemical activity at grain boundary regions increases the dissolution rate of fine grained materials [17,23,24]. For light weight structural applications, magnesium alloys with their attractive properties can compete with existing materials. However, poor corrosion resistance of magnesium alloys is one of the impeding factors for their full potential use as structural materials [25]. High strength wrought magnesium alloys developed using alloy design principles can be an ⇑ Corresponding author at: Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203, USA. Tel.: +1 940 565 2316; fax: +1 940 565 4824. E-mail address: [email protected] (R.S. Mishra). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2012.01.021
alternative [26]. Microstructural refinement of magnesium can lead to significant improvement in the corrosion response due to the alteration of passive layer characteristics [4]. However, Song et al. [17] reported decrease in corrosion resistance of pure magnesium with grain refinement in aggressive 3.5 wt.% NaCl solution. Song et al. [23] observed similar behavior in equal-channel angular pressed (ECAP) bulk ultra-fine grained AZ91D. The high strain induced mass crystalline defects and refinement of net like b-phase into isolated particles in matrix increased the dissolution rate in 3.5 wt.% NaCl [17,23]. Hoog et al. [14] observed no improvement in corrosion resistance of pure magnesium processed with surface mechanical attrition treatment (SMAT), whereas the ECAP material showed increased corrosion resistance. Therefore, it is likely that the improvement in corrosion resistance of materials with grain refinement is also a function of employed processing route. Present work is solely focused on the effect of grain size on corrosion resistance of Mg–Y–RE wrought magnesium alloy. A spectrum of grain size with two orders of magnitude (70 lm to 0.7 lm) difference was considered. Friction stir processing (FSP) was used as a SPD tool to achieve fine grain and ultrafine grain microstructures. Electrochemical testing and constant immersion testing were carried out to compare the effect of grain refinement on the corrosion resistance of the alloy.
2. Experimental 2.1. Material and conditions Samples for corrosion experiments were extracted from a hot-rolled Mg–Y–RE (nominal composition: Mg–4Y–3Nd) alloy
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Fig. 1. OIM micrographs of (a) CG 1, (b) CG 2, (c) FG, and (d) UFG. (e) Grain boundary misorientation plot for CG 1, CG 2, FG, and UFG.
(as-received condition). To achieve coarser grain size than asreceived condition, small pieces were cut from as-received material and were subjected to thermal treatment of 520 °C for 4 h. Friction stir processing (FSP) was employed to achieve finer grain microstructures. The FSP tool dimensions were: shoulder diameter12 mm, pin height-2.2 mm, pin root diameter-6 mm, and pin tip diameter-3.75 mm. One pass FSP of as-received plate was carried out with tool rotation rate of 600 RPM and tool traverse speed of 101.6 mm/min to achieve grain size refinement by one order. To attain ultrafine grained microstructure, multi-pass FSP was employed. As-received plate was subjected to first pass with tool rotation rate of 600 RPM and tool traverse speed of 101.6 mm/min. Thereafter, overlapping second pass was carried out with tool rotation rate of 300 RPM and tool traverse speed of 101.6 mm/min. All the FSP runs were conducted with tool tilt angle of 2.5°. Here onwards the specimens are designated as: as-received + thermal
treatment CG 1, as-received CG 2, as-received + 1 pass FSP FG, and as-received + 2 pass FSP UFG. Orientation imaging microscopy (OIM) was used to estimate the grain size. Scanning electron microscopy (SEM) was conducted to understand the corrosion morphology of tested samples. 2.2. Electrochemical corrosion testing Linear polarization resistance and cyclic potentiodynamic polarization were carried out for all the conditions. All the samples were polished to 1 lm surface finish using conventional metallographic steps. Thereafter, sample surface was cleaned with ethanol and subjected to electrochemical testing. Before starting the potential scan, the samples were given incubation period of 2 h to stabilize initial conditions. During the incubation period potential measurements with respect to time at a sampling rate of 1 s1
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Fig. 2. Linear polarization resistance plots of different conditions of Mg–Y–RE alloy in 3.5 wt.% NaCl. Fig. 4. Weight loss normalized with area for different conditions of Mg–Y–RE alloy after constant immersion testing in 3.5 wt.% NaCl for 3 weeks.
UFG, samples were extracted from the processed region only. All the samples were ground to 600 grit emery paper to make them flat on all the surfaces and were subsequently cleaned with ethanol. A small hole was drilled to suspend the samples in the corrosive medium through Teflon thread. Initial mass (up to 0.001 mg accuracy) and surface area was measured for all the samples. Three samples from each microstructural condition, i.e. CG 1, CG 2, FG, and UFG, were then subjected to constant immersion in corrosive medium for 3 weeks. At the end of experiment, final cleaning of each sample was done by immersing them in boiling 15% chromic acid solution to dissolve the corrosion products from the surface, immediately followed by cleaning with acetone. Weight loss was measured and corrosion rate (mg cm2 day1) was calculated. Fig. 3. Cyclic potentiodynamic polarization plots of different conditions of Mg–Y– RE alloy in 3.5 wt.% NaCl.
Table 1 Electrochemical parameters from cyclic potentiodynamic polarization test. Alloy condition
Breakdown potential, Eb (mV vs. SCE)
Repassivation potential, Ep (mV vs. SCE)
Eb–Ecorr (mV)
Eb–Ep (mV)
CG 1 CG 2 FG UFG
1589 ± 22 1591 ± 30 1617 ± 18 1597 ± 25
– 1754 ± 28 1710 ± 24 1658 ± 30
138 ± 32 192 ± 35 177 ± 29 160 ± 38
– 173 ± 25 83 ± 19 61 ± 28
2.4. Corrosive medium All the electrochemical corrosion experiments were carried out in 3.5 wt.% NaCl solution as electrolyte using saturated calomel electrode (SCE) as reference electrode, and platinum mesh as counter electrode in a standard flat cell with three electrode configuration. For immersion testing, the samples were suspended in 3.5 wt.% NaCl solution. Due care was taken to maintain sufficiently higher ratio of volume of electrolyte to surface area exposed of the samples. 3. Results and discussion
were made till a stable potential value was achieved. The potential scan range for linear polarization resistance was selected as ±30 mV from the stabilized value of potential specific to each sample at a scan rate of 0.166 mV/s. For cyclic polarization the potential was scanned from 250 mV to +500 mV, back to 250 mV from the stabilized value of potential specific to each sample at a scan rate of 0.5 mV/s. To check for the repeatability of the polarization resistance and cyclic potentiodynamic polarization, each sample condition was scanned three times.
2.3. Immersion testing The corrosion rate was estimated using weight loss method under constant immersion in the aqueous solution. For FG and
3.1. Microstructural examination The microstructural characterization of spectrum of grain size for Mg–Y–RE alloy was carried out using orientation imaging microscopy (OIM). The OIM micrographs for CG 1, CG 2, FG and UFG samples are shown in Fig. 1. Note that in Fig. 1(a) & (b) and (C) & (d) were taken at same magnification. The average grain size estimated from OIM micrographs for CG 1, CG 2, FG, and UFG are 70 ± 58 lm, 20 ± 12 lm, 2.4 ± 2 lm, and 0.65 ± 0.44 lm, respectively. OIM micrographs suggested a random texture for different conditions of Mg–Y–RE alloy. The grain boundary misorientation plot is shown in Fig. 1e. It can be noted that in all the conditions the fraction of high angle grain boundaries (HAGBs) is >85%.
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Fig. 5. Grain size effect on corrosion properties for Mg–Y–RE alloy, (a) open circuit potential (OCP) and polarization resistance as a function of grain size, (b) corrosion rate estimated from constant immersion testing as a function of grain size, and (c) and (d) comparison of corrosion rates in magnesium as a function of grain size [16,17].
3.2. Electrochemical corrosion studies 3.2.1. Linear polarization resistance Linear polarization resistance plots for different grain size Mg– Y–RE alloy samples tested in 3.5 wt.% NaCl solution are shown in Fig. 2. The slope of E–I plot gives polarization resistance (Rp). The Rp value is inversely proportional to the corrosion rate exhibited by the material. Thus, higher Rp value corresponds to higher corrosion resistance of the material. For the present work, higher Rp values were observed for UFG and FG samples as compared to CG 1 and CG 2 samples.
3.2.2. Cyclic potentiodynamic polarization Cyclic potentiodynamic polarization plots for different grain size Mg–Y–RE alloy samples in 3.5 wt.% NaCl solution are shown in Fig. 3. The electrochemical parameters obtained from cyclic polarization study are summarized in Table 1. It can be seen that the breakdown potential (Eb) observed for all the samples was in the similar range. The range of passive region indicated by Eb–Ecorr was also observed in the similar range for all the grain sizes. However, increase in repassivation potential (Ep) was observed with grain refinement. CG 1 sample showed no intersection between reverse scan and anodic region of forward scan indicating no repassivation. Gradual decrease in difference between Eb and Ep was observed for samples CG 2, FG and UFG with grain refinement. The Ecorr in reverse scan for FG and UFG samples was higher than that in the forward scan which was not observed in CG 1 and CG 2 samples. The growth of the pits formed in the transpassive region continues to grow in CG 1 and CG 2 samples, whereas for FG and UFG samples the higher Ecorr in reverse scan is an indication of no further growth of pits formed in transpassive region. Thus, FG
and UFG samples showed better repassivation characteristics with arresting the growth of pits as compared to CG 1 and CG 2 samples. 3.3. Weight loss measurements Constant immersion is a direct method to estimate the corrosion resistance of the materials. The weight loss experienced by the materials during constant immersion is proportional to the corrosion resistance. For the present work, the weight loss measured from constant immersion of different conditions of Mg–Y–RE alloy in 3.5 wt.% NaCl solution for 3 weeks are shown in Fig. 4. The CG 1 samples showed the highest weight loss and UFG samples showed the lowest weight loss. 3.4. Grain size effect Fig. 5a shows the polarization resistance (Rp) and the open circuit potentials (OCP) observed during electrochemical testing plotted as a function of grain size. For the present work, the grain refinement achieved through FSP of Mg–Y–RE magnesium alloy results in shift of OCP towards positive potentials. This indicates better thermodynamic ability of refined microstructure to resist corrosion. Similar behavior is also evident from the polarization resistance data. The slope of E–I plot (Fig. 2) increased with grain refinement suggesting higher corrosion resistance for fine grained microstructure as compared to coarse grained microstructure. The constant immersion testing results also indicated identical trends. The weight loss of the samples from the immersion testing was normalized with respect to the area of the samples. Fig. 5b shows area normalized weight loss as a function of d1/2, where d is the average grain size. During immersion testing, a distinctive trend
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Fig. 6. Backscattered SEM micrographs depicting constituent particles in (a) CG 1, (b) CG 2, and (c) FG. (d) Particle size distribution observed in each condition.
was observed between corrosion rate and grain size. Lower corrosion rates were observed in FG samples as compared to CG 1 and CG 2 samples. The lowest corrosion rates were observed in UFG samples. Birbilis et al. [16] observed a similar relationship between Icorr, a measure of corrosion rate, and grain size for pure magnesium. However, Song et al. [17] reported an inverse behavior in a similar study. Recent literature suggests that corrosion behavior of magnesium and its alloys can be significantly altered by grain refinement and researchers have reported improvement in corrosion resistance of commercially pure magnesium [14–16]. Pilling–Bedworth ratio (PBR), a measure of stability of passive oxide layer, for magnesium is 0.81. Moreover, in case of magnesium, unlike aluminum (PBR 1.2) and stainless steels (PBR 2), the passive layer is crystalline in nature [28]. The structural discontinuity between the crystal structure of oxide layer and the HCP lattice of magnesium results into development of high compressive stress within the oxide layer [16]. As suggested by Birbilis et al. [16], one of the ways of reducing the mismatch and have lower disorder between oxide layer and metal surface is to introduce large volume fraction of grain boundaries per unit area. The OIM micrograph analysis of FG and UFG samples indicated that FSP of Mg–Y–RE alloy resulted in >85% fraction of HAGBs in the microstructure. The HAGBs fraction in CG 1 and CG 2 microstructure was also >85% (Fig. 1e). Therefore, the higher fraction of HAGB per unit volume in UFG microstructure resulted in formation of more tenacious and adherent passive layer and thus lowest corrosion rate. The microstructural condition in UFG is altering the characteristics of formation of passive layer. This is clearly evident in the cyclic polarization scans with UFG sample showing repassivation in the reverse scan. There have been some studies on the influence of texture on the corrosion behavior of hexagonal crystalline materials [8,30]. Hoseini et al. [8] observed higher corrosion resistance in commercially pure ECAP titanium surface with basal texture. In a similar study, Song et al. [30] estimated better electrochemical stability and higher corrosion resistance in rolled
AZ31 in the surfaces parallel to basal plane direction. In the present work, the OIM micrographs (see Fig. 1) indicate a random texture in all the samples. The possible reason may be due to the presence of rare earth elements in the present magnesium alloy which tend to reduce the anisotropy in the microstructure. 3.5. Processing route impact The corrosion behavior of magnesium is highly sensitive to the processing route employed to achieve the microstructural refinement [14–17]. The corrosion rate obtained from the weight loss experiments was plotted on log–log scale as a function of d1/2 and was compared with the Icorr, a parameter of corrosion rate from the work of Birbilis et al. [16] and Song et al. [17] on pureMg. The difference in the slopes and intercepts in Fig. 5c are attributed to the difference in material, the processing route employed to achieve grain refinement and electrolyte concentration [3]. This difference in slopes can be explained as follows. The nature of FSP leads to formation of large fraction of high angle grain boundaries in comparison to other SPD techniques [27]. This is a major distinction as the microstructure evolution during FSP involves dynamic recrystallization and grain growth. This results in lower dislocation density in the microstructure. Comparing the plots in Fig. 5c and d, the magnitude of slope of corrosion rate-grain size plot for the present work is steeper than the slope observed by Birbilis et al. [16]. This suggests a significant decrease in the magnitude of corrosion rate with grain refinement of FSP Mg–Y–RE alloy. The microstructural evolution in ECAP is dislocation based sub-structure growth which results into higher fraction of low angle grain boundaries (LAGB) in ECAP microstructure as compared to FSP microstructure [29]. Also, ECAP introduces higher dislocation density in the microstructure as compared to FSP. Further, rare earth (RE) addition to magnesium in general improves the corrosion response of the alloy. This is attributed to the contribution of RE, in solid solution with magnesium matrix, to
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Fig. 7. SEM micrographs of surface morphology of samples tested after constant immersion in 3.5 wt.% NaCl for 3 weeks, (a) CG 1, (b) CG 2, (c) FG, and (d) UFG.
form a protective layer to resist corrosion attack. Moreover, FSP is expected to break-up and uniformly distribute second phase particles in the matrix [31]. This can lead to reduced propensity of localized corrosion attack in FG and UFG samples. Fig. 6 shows the SEM backscatter images of CG 1, CG 2, and FG conditions depicting the constituent particles. The particle size distribution observed in each case is also shown in Fig. 6. It can be clearly observed that the particle size distribution shifted towards lower size with 1 pass FSP. Also, note that the clustering of particles observed in CG 1 and CG 2 was eliminated in FG because of FSP. The fragmentation of constituent particles due to FSP could be a possible reason of reduced density of particles resolved under backscattered SEM for the FG condition. For the UFG condition, the 2 pass FSP is expected to further reduce the size of constituent particles and this was the reason why the constituent particles were not observed under backscattered SEM. The 2 pass FSP will reduce the particle size to the nanometer scale (<100 nm) and significantly lower the intensity of the corrosion attack due to the galvanic coupling with the matrix. The improvement in corrosion behavior due to dynamically recrystallized microstructure and fine homogenous distribution of second phase obtained via ECAP of ZE41A rare earth containing magnesium alloy has been reported [32]. Thus, the alloy chemistry together with nature of processing resulted in one order lower magnitude of corrosion rate in UFG microstructure as compared to CG 1. The inverse relationship observed by Song et al. [17] was attributed to high dislocation density and more energetic crystalline defects imparted during ECAP of pure magnesium. Moreover, electrolyte concentration in the work of Song et al. [17] was 0.6 M NaCl which can produce severe attack as reported. The work on AZ91D by Song et al. [23] also indicated similar trends in
corrosion rate estimated from weight loss experiments. The mass loss in 3.5 wt.% NaCl solution in as-cast AZ91D was reported to be lowest and increased with increase in number of ECAP passes. The stacked dislocations developed in the microstructures resulted in higher mass loss for ECAP specimens. From the present work, the corrosion rate (CR) dependence on grain size (Fig. 5c and d) is expressed as: 1
logðC R Þ ¼ A þ B logðd 2 Þ 0:3
C R ¼ Cd
ð1Þ ð2Þ
where d is the average grain size and A, B, and C are constants. The present results exhibit a power law relationship between grain size and corrosion rate. This power law relationship has lower grain size dependence as compared to Birbilis et al. [16], indicating higher dominance of alloy chemistry on the corrosion rate over grain size. This raises an open question about the grain size dependence of corrosion rate of materials. Birbilis et al. [3] proposed a relationship similar to classical Hall–Petch type dependence. From the present study, it is evident that the corrosion mechanism of magnesium can be significantly altered by introducing large fractions of high angle grain boundaries. The single crystal magnesium with infinitesimally small d1/2 value without any grain boundaries is expected to have lowest corrosion rate. The introduction of grain boundaries, the high energy areas in the microstructure, can lead to increase in corrosion rate by formation of galvanic couple between relatively anodic grain boundary and cathodic grain interior. This behavior is expected to change again when the grain refinement is very high although the transition grain size for this is not established yet. In the ultrafine grained regime, the very high fraction of grain
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boundaries in the microstructure is likely to reduce corrosion rates in two ways: (1) accelerating the passivation kinetics, and (2) reduce the intensity of galvanic couple between grain interior and grain boundary. Therefore, the UFG microstructure will lead to formation of closely spaced electrochemical batteries of anodic–cathodic regions. Thus the difference between the rates of anodic and cathodic reactions are expected to alter significantly, which leads to more uniform attack and lower corrosion rates. In the present work, the FSP refined microstructure with lower gradient of electrochemical potential within the matrix led to a more homogeneous microstructure. The corrosion surface morphology of CG 1 and UFG samples after constant immersion testing was observed using scanning electron microscopy (SEM). Fig. 7 shows SEM micrographs of CG 1, CG 2, FG, and UFG samples after 3 weeks of exposure to chloride environment. The pit morphology as observed on the surface of CG 1 and UFG clearly indicates higher corrosion attack with larger and deeper pits in CG 1 as compared to UFG showing shallower pits (see inset). This is in agreement with the weight loss observed during constant immersion testing. The larger and deeper pits in CG 1 samples and shallower pits in UFG samples lead to highest and lowest weight loss, respectively. The pit growth arrest in case of UFG microstructure was found to be better in cyclic polarization study as compared to coarse grained microstructure. The SEM observation of immersion tested sample surface indicates similar behavior. 4. Conclusions (1) Grain refinement of Mg–Y–RE achieved through FSP lead to increase in corrosion resistance of the alloy. (2) UFG microstructure showed highest polarization resistance and the most positive pitting potentials and repassivation potentials as compared to coarse grain microstructure. (3) During constant immersion testing a clear relationship between grain size and corrosion rate was observed with lowest corrosion rates in UFG samples and highest corrosion rates in coarse grain samples, respectively.
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