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The effect of the local microstructure of MRI 201S magnesium alloy on its corrosion rate
Corrosion Science 93 (2015) 167–171
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
The effect of the local microstructure of MRI 201S magnesium alloy on its corrosion rate Ohad Gaon, Guy Dror, Omer Davidi, Alex Lugovskoy ⇑ Chemical Engineering Department, Ariel University, Ariel 40700, Israel
a r t i c l e
i n f o
Article history: Received 27 October 2014 Accepted 7 January 2015 Available online 13 January 2015 Keywords: A. Magnesium B. Weight loss
a b s t r a c t The corrosion rates of High Density Die Cast MRI 201S and three other magnesium alloys in 3% NaCl were measured. The side surfaces of some specimens were insulated by a polymer, while the other specimens had all the surfaces open to the corrosive media. No difference between the two types of specimens for was observed for AZ91D, MRI 153M and MRI 202S alloys, while for MRI 201S alloy the corrosion of the samples of the 1st type was significantly slower than of the 2nd type. For all the alloys studied, the coarser the grains the higher the corrosion rate. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The production of magnesium alloys having predictable and controlled corrosion behavior requires comprehensive understanding of the detailed mechanism of the corrosion processes occurring in these alloys. A serious obstacle presents the highly localized character of corrosion of magnesium alloys in many, in particular, chloride containing media. Due to this factor the reproducibility of experimental measurements of corrosion rates even for virtually identical specimens of a magnesium alloy is very difficult [1]. There are some plausible reasons for the large discrepancies in the measured corrosion rates. First, even minor impurities can drastically change the corrosion behavior of a magnesium alloy. For example, the presence of less than 0.2% of iron may cause 10–100 fold acceleration of corrosion of a magnesium alloy in salt water [2]. Minor contaminations are stochastically distributed in an alloy, so that two specimens cut from the same ingot may corrode at different rates. Second, the rate of corrosion is strongly influenced by the microstructure of an alloy. For example, the corrosion of Mg–Al alloys depends on the ratio and distribution between the alpha-phase (based on the magnesium lattice) and the beta-phase (intermetallic Mg17Al12 cathodic relative to the alpha-phase [3]). It was shown that small amounts of beta-phase accelerate the corrosion rate of an alloy, while larger amounts of beta-phase suppress the corrosion process [2]. Another source of the discrepancies in the measured corrosion rates is not intrinsic, but caused by the experimental setup ⇑ Corresponding author. E-mail address: [email protected] (A. Lugovskoy). http://dx.doi.org/10.1016/j.corsci.2015.01.018 0010-938X/Ó 2015 Elsevier Ltd. All rights reserved.
difficulties: a specimen of a magnesium alloy has to be connected to a measuring instrument (potentiostat) by a conductive wire and be placed firmly in a measuring vessel. As it was shown, very serious impacts on the observed corrosion rates are often caused by (a) the galvanic corrosion occurring between the specimen and the metal of a conductive wire, (b) crevices in the sites of a joint between a conductive wire and the specimen, and (c) uneven access of the electrolyte to different sites of the specimen resulted from the placement of the specimen in the vessel [1,4]. These factors are not always reported in the literature so that the values of corrosion rates measured, for example, for AZ91 alloy in 3–5% salt water by several authors vary from 0.14 to 16.8 mm/year [4–8]. MRI (the abbreviation of the ‘Magnesium Research Institute’ in Beer-Sheba, Israel, which is the inventor and the patent holder of the alloys) series magnesium alloys [5,9] are novel alloys having improved mechanical properties, especially at elevated temperatures (up to 350 °C). The corrosion behavior of these alloys has been only scarcely studied [5,10,11] so that a more detailed research is being performed in our group. The least studied alloys of the MRI series are MRI 201S and MRI 202S, for which there is only one report [5] indicating their corrosion rate measured by the salt spray technique. In our studies we have observed a phenomenon, when the corrosion damage to a plate-shaped specimen of MRI 201S occurred preferably at the middle of the sides of the specimen (Fig. 1) while the larger (top and bottom) surfaces remained almost intact. When the side surfaces of an identical specimen were insulated with a polymer coating tape, the visual damage to the sample was considerably less than for the completely open specimen (Fig. 1). A
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O. Gaon et al. / Corrosion Science 93 (2015) 167–171
Table 1 Chemical composition (weight%) of the magnesium alloys. Alloy
Al
Zr
Zn
Mn
Ca
Sr
Nd
Y
Mg
AZ91D MRI 153 MRI 201S MRI 202S
9.0 8.0 – –
– – 0.41 0.40
0.68 0.07 0.38 0.41
0.24 0.16 – –
– 1.0 0.07 0.06
– 0.28 – –
– – 3.06 3.08
– – 2.00 0.24
Bal. Bal. Bal. Bal.
similar ‘skin effect’ was reported for other die-cast alloys [12–14]. We decided to study the ‘side surface effect’ on the corrosion rate of the High Density Die-Cast MRI 201S magnesium alloy as compared to other MRI series alloys in more detail and report here the first results of this study. Fig. 1. Specimens of MRI 201S magnesium alloy after 14 day immersion in 3% NaCl. Left: the side surfaces were isolated with a polymer. Right: all the surfaces were open to the corrosive media. See the deep crack that has developed along the side surface.
2. Experimental High Density Die Cast rectangular 3 mm thick sheets of commercial MRI series and AZ91D alloys were received from the Dead Sea Magnesium Ltd. [9]. MRI 153M (beryllium-free Mg–Al–Ca–Sr based alloy), MRI 201S and MRI 202S (both are Mg–Zr–Nd–Y based alloys, the former contains 2% Y and the latter contains 0.2% Y) were chosen for the comparison. The chemical compositions of the alloys were determined by ICP after the dissolution of a sample of each alloy in the nitric acid and are given in Table 1. Small rectangular 35 50 3 mm plates were cut from the received sheets, ground with #2000 diamond paper, degreased with acetone and immersed in 3% NaCl solutions at room temperature. Gas evolution in the course of corrosion was monitored by funnel-and-burette systems similarly to those described by Atrens et al. [1]. Microstructure observations were performed with metallographic optical microscope and SEM JEOL JSM6510LV equipped with an NSS7 EDS analyzer (Correction Method Proza – Phi-Pho-Z was used for the quantitative analysis). X-ray Diffractometer (XRD) Panalytical X’Pert Pro with Cu Ka1 radiation (k = 0.154 nm) was used with the full pattern identification made by X’Pert HighScore Plus software package, version 2.2e (2.2.5) by PANalytical B.V. Materials identification and analysis made by the PDF-2 Release 2009 (Powder Diffraction File). Phase analysis identification made by XRD, 40 kV 40 mA. The XRD patterns were recorded in the GIXD geometry at 1° and 5° in the range of 20–80° (step size 0.05° and time per step 2 s). For each alloy 6–8 specimens were cut from the 3 mm thick die-cast plates. One half of the specimens of each alloy had all the sides open to the electrolyte while the side surfaces of the other half of the specimens were sealed with a polymer tape so that only their top and bottom faces remained open. Each specimen was immersed into 3% NaCl at room temperature for 14–18 days. The pH was not kept constant, rather it changed from 6.2–6.8 to 9.5–10.2 in the course of an experiment. Gas evolution was monitored in the course of the experiment. After that, a specimen was taken out of the salt solution and the layer of the corrosion products was dissolved in the solution of 200 g/L CrO3 and 10 g/L AgNO3 [4,7] until the achievement of a constant mass and cessation of bubbles evolution (5–10 min of immersion).
Fig. 2. The time dependencies of the gas evolution for two specimens of MRI 201S alloy: the specimen no. 2 had all the sides open to the corrosion process while the side surfaces of the specimen no. 3 were sealed.
3. Results and discussions For each specimen the mass loss after 14–18 days of the full immersion in 3% NaCl at room temperature was measured and the corrosion rate was calculated for the mass loss (Pw) and for the gas evolution (PH). The averaged for 3–4 samples results of the measurement are summarized in Table 2. As is seen from Table 2, the sealing of the side surfaces does not affect the corrosion rate of AZ91D, has only a small effect on MRI 153M and MRI 202S and reduces almost by 50% the corrosion rates of MRI 201S. The time dependency of the gas evolution for MRI 201S (Fig. 2) clearly demonstrates the significant effect of the side surfaces on the corrosion rate. The microstructures of all the studied alloys are presented in Fig. 3a–d. As is seen from Fig. 3, while for AZ91D, MRI 153M and MRI 202S there is no microstructural difference between the
Table 2 Averaged for 3–4 samples corrosion rates measured by the mass loss (Pw) and gas evolution (PH) for plates of magnesium alloys having (a) all sides open and (b) side surfaces sealed. Alloy
Pw, mm/year (all sides open)
Pw, mm/year (sides sealed)
PH, mm/year (all sides open)
PH, mm/year (sides sealed)
AZ91D MRI 153M MRI 201S MRI 202S
0.21 ± 0.03 0.73 ± 0.06 1.46 ± 0.04 6.21 ± 0.05
0.20 ± 0.04 0.62 ± 0.03 0.79 ± 0.02 5.90 ± 0.06
0.20 ± 0.06 0.84 ± 0.04 1.29 ± 0.06 7.73 ± 0.05
0.19 ± 0.04 0.54 ± 0.07 0.69 ± 0.04 6.55 ± 0.08
O. Gaon et al. / Corrosion Science 93 (2015) 167–171
169
(a) AZ91D
(b) MRI 153D
(c) MRI 201S
(d) MRI 202S Fig. 3. SEM BSE images of the microstructures of the studied magnesium alloys.
surfaces and the cross-sections, this is clearly not the case for MRI 201S. The surface of MRI 201S (Fig. 3c) is formed by 10 lm grains of primary magnesium a-phase separated one from another with
fine ‘filaments’ of b-phase. The cross-section structure is formed with considerably larger and less regularly sized a-phase grains and much larger inclusions of b-phase. Some additional
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Fig. 4. Eutectic crystallization in the middle of the cross-section structure of MRI 201S. The numbered frames are (1) a-phase, (2) b-phase and (3) the eutectics.
Table 3 Element composition in weight% of the main constituents of MRI 201S measured by EDS point analysis. Phase
Mg
Zn
Y
Zr
Nd
a-Phase
97 91 67
0 0.5 3
0.6 3 8
0.8 0.2 0
0.3 5 22
b-Phase Eutectics
perfectly met. As the corrosion process has started, b-phase cannot surround larger grains of a-phase and thus cannot protect them. The macroscopic result (i.e. the fast degradation of the alloy) is well seen in Fig. 1. This process proceeds with the positive feedback so that deep corrosion pits and cracks are formed. The microstructures of AZ91D and MRI 153M are composed of fine grains on the surfaces and in the center of the cast sheets. In accordance with this, no difference is observed between the corrosion behaviors of the surface and the center, and the total corrosion rate is relatively low. On the contrary, the structure of MRI 202S is coarse both in the center and in the periphery. For that reason, no difference is observed between the corrosion of different parts of a sheet, although the total corrosion rates are quite high. It deserves to be noted that other authors observed considerable ‘skin effect’ in the corrosion of AZ91D in NaCl. So, Atrens et al. [13] reported that for 6 mm thick specimens of AZ91D the corrosion rate of the skin was by the factor of 10 higher than of the interior. Aghion and Lulu [12] measured the corrosion rates of AZ91D for different thicknesses of a specimen and found that the corrosion rate of 1.5 mm thick samples was always higher than for 3–9 mm thick samples, that is some ‘inverse skin effect’ was observed. This is not consistent with our observations, in which for all the alloys we saw that coarser grain structure resulted in higher corrosion rates and vice versa.
4. Conclusions
Fig. 5. XRD pattern of MRI 201S alloy.
information can be gained by somewhat larger magnification of the central part of the cross-section of an MRI 201S specimen (Fig. 4). The composition of the main phases observed in Fig. 4 is given in Table 3 and the XRD pattern of the alloy is given in Fig. 5. We believe that the strong effect of the side surfaces on the corrosion process of MRI 201S alloy can be explained by the difference of the microstructures between the flat surfaces of the die-cast sheets and the inner part of the sheets exposed when the large sheet are cut into smaller plates. In fact, this is the ‘skin-effect’ described previously in the literature for other High Density Die Cast magnesium alloys [12–14]. The periphery of a sheet is formed by fine grains of a-phase containing >97% Mg and only very small amount of b-phase enriched with yttrium and neodymium. Such a structure favors better corrosion protection [2,15], particularly for the Nd-containing magnesium alloys [16], because the ‘filaments’ of the b-phase are too small to form cathodes and cannot launch the process of micro-galvanic corrosion. The inner part of the cast sheet is composed of larger a-phase grains and also large sites of the primary and eutectic b-phases, both enriched with rare-earth elements. For that case, the conditions for the micro-galvanic corrosion are
The corrosion rates of High Density Die Cast AZ91D and three MRI series magnesium alloys were measured by the mass loss and gas evolution in the course of the complete immersion of samples in 3% NaCl at the room temperature. One half of the specimens had all the surfaces open to the corrosive media, while the side surfaces of the other half were insulated by a polymer. It was demonstrated that there is essentially no difference between the two types of specimens for AZ91D, MRI 153M and MRI 202S alloys. Unlike those, MRI 201S alloy demonstrated pronounced ‘skineffect’: the corrosion of the samples whose side surfaces (i.e. the interior of the die-cast plates) were insulated was significantly slower than of the samples for which all the sides were open. For all the alloys studied, there is a correlation between the coarseness of the grain structure of an alloy and the corrosion rate: the coarser the grains the higher the corrosion rate. The pronounced ‘skin effect’ of MRI 201S may be caused by the higher content of the Mg3Nd containing eutectic b-phase in the interior of the casts.
References [1] A. Atrens, G.-L. Song, F. Cao, Z. Shi, P.K. Bowen, Advances in Mg corrosion and research suggestions, J. Magnesium Alloys 1 (2013) 177–200. [2] G. Song, A. Atrens, Understanding magnesium corrosion—a framework for improved alloy performance, Adv. Eng. Mater. 5 (2003) 837–858. [3] J.A. Lyndon, R.K. Gupta, M.A. Gibson, N. Birbilis, Electrochemical behaviour of the b-phase intermetallic (Mg2Al3) as a function of pH as relevant to corrosion of aluminium–magnesium alloys, Corros. Sci. 70 (2013) 290–293. [4] Z. Shi, A. Atrens, An innovative specimen configuration for the study of Mg corrosion, Corros. Sci. 53 (2011) 226–246. [5] E. Aghion, B. Bronfi, F. Von Buch, S. Schumann, H. Friedrich, Newly developed magnesium alloys for powertrain applications, JOM (2003) 30–33. [6] R. Ambat, N.N. Aung, W. Zhou, Evaluation of microstructural effects on corrosion behaviour of AZ91D magnesium alloy, Corros. Sci. 42 (2000) 1433– 1455. [7] R. Arrabal, A. Pardo, M.C. Merino, M. Mohedano, P. Casajús, K. Paucar, et al., Effect of Nd on the corrosion behaviour of AM50 and AZ91D magnesium alloys in 3.5 wt.% NaCl solution, Corros. Sci. 55 (2012) 301–312. [8] M.-C. Zhao, M. Liu, G. Song, A. Atrens, Influence of the b-phase morphology on the corrosion of the Mg alloy AZ91, Corros. Sci. 50 (2008) 1939–1953. [9] MRI alloys (Accessed on 30.12.14).
O. Gaon et al. / Corrosion Science 93 (2015) 167–171 [10] E. Aghion, N. Lulu, The corrosion performance of die-cast magnesium alloy MRI230D in 3.5% NaCl solution saturated with Mg(OH)2, Mater. Charact. 61 (2010) 1221–1226. [11] G.-L. Song, D. Radovic, Lab evaluation and comparison of corrosion performance of Mg alloys (2010). [12] E. Aghion, N. Lulu, The effect of skin characteristics on the environmental behavior of die cast AZ91 magnesium alloy, J. Mater. Sci. 44 (2009) 4279–4285. [13] G. Song, A. Atrens, M. Dargusch, Influence of microstructure on the corrosion of diecast AZ80D, Corros. Sci. 41 (1999) 249–273. [14] Z. Shan, A.M. Gokhale, Utility of micro-indentation technique for characterization of the constitutive behavior of skin and interior
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microstructures of die-cast magnesium alloys, Mater. Sci. Eng. A 361 (2003) 267–274. [15] H.S. Kim, W.J. Kim, Enhanced corrosion resistance of ultrafine-grained AZ61 alloy containing very fine particles of Mg17Al12 phase, Corros. Sci. 75 (2013) 228–238. [16] G.R. Argade, S.K. Panigrahi, R.S. Mishra, Effects of grain size on the corrosion resistance of wrought magnesium alloys containing neodymium, Corros. Sci. 58 (2012) 145–151.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
The effect of the local microstructure of MRI 201S magnesium alloy on its corrosion rate Ohad Gaon, Guy Dror, Omer Davidi, Alex Lugovskoy ⇑ Chemical Engineering Department, Ariel University, Ariel 40700, Israel
a r t i c l e
i n f o
Article history: Received 27 October 2014 Accepted 7 January 2015 Available online 13 January 2015 Keywords: A. Magnesium B. Weight loss
a b s t r a c t The corrosion rates of High Density Die Cast MRI 201S and three other magnesium alloys in 3% NaCl were measured. The side surfaces of some specimens were insulated by a polymer, while the other specimens had all the surfaces open to the corrosive media. No difference between the two types of specimens for was observed for AZ91D, MRI 153M and MRI 202S alloys, while for MRI 201S alloy the corrosion of the samples of the 1st type was significantly slower than of the 2nd type. For all the alloys studied, the coarser the grains the higher the corrosion rate. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The production of magnesium alloys having predictable and controlled corrosion behavior requires comprehensive understanding of the detailed mechanism of the corrosion processes occurring in these alloys. A serious obstacle presents the highly localized character of corrosion of magnesium alloys in many, in particular, chloride containing media. Due to this factor the reproducibility of experimental measurements of corrosion rates even for virtually identical specimens of a magnesium alloy is very difficult [1]. There are some plausible reasons for the large discrepancies in the measured corrosion rates. First, even minor impurities can drastically change the corrosion behavior of a magnesium alloy. For example, the presence of less than 0.2% of iron may cause 10–100 fold acceleration of corrosion of a magnesium alloy in salt water [2]. Minor contaminations are stochastically distributed in an alloy, so that two specimens cut from the same ingot may corrode at different rates. Second, the rate of corrosion is strongly influenced by the microstructure of an alloy. For example, the corrosion of Mg–Al alloys depends on the ratio and distribution between the alpha-phase (based on the magnesium lattice) and the beta-phase (intermetallic Mg17Al12 cathodic relative to the alpha-phase [3]). It was shown that small amounts of beta-phase accelerate the corrosion rate of an alloy, while larger amounts of beta-phase suppress the corrosion process [2]. Another source of the discrepancies in the measured corrosion rates is not intrinsic, but caused by the experimental setup ⇑ Corresponding author. E-mail address: [email protected] (A. Lugovskoy). http://dx.doi.org/10.1016/j.corsci.2015.01.018 0010-938X/Ó 2015 Elsevier Ltd. All rights reserved.
difficulties: a specimen of a magnesium alloy has to be connected to a measuring instrument (potentiostat) by a conductive wire and be placed firmly in a measuring vessel. As it was shown, very serious impacts on the observed corrosion rates are often caused by (a) the galvanic corrosion occurring between the specimen and the metal of a conductive wire, (b) crevices in the sites of a joint between a conductive wire and the specimen, and (c) uneven access of the electrolyte to different sites of the specimen resulted from the placement of the specimen in the vessel [1,4]. These factors are not always reported in the literature so that the values of corrosion rates measured, for example, for AZ91 alloy in 3–5% salt water by several authors vary from 0.14 to 16.8 mm/year [4–8]. MRI (the abbreviation of the ‘Magnesium Research Institute’ in Beer-Sheba, Israel, which is the inventor and the patent holder of the alloys) series magnesium alloys [5,9] are novel alloys having improved mechanical properties, especially at elevated temperatures (up to 350 °C). The corrosion behavior of these alloys has been only scarcely studied [5,10,11] so that a more detailed research is being performed in our group. The least studied alloys of the MRI series are MRI 201S and MRI 202S, for which there is only one report [5] indicating their corrosion rate measured by the salt spray technique. In our studies we have observed a phenomenon, when the corrosion damage to a plate-shaped specimen of MRI 201S occurred preferably at the middle of the sides of the specimen (Fig. 1) while the larger (top and bottom) surfaces remained almost intact. When the side surfaces of an identical specimen were insulated with a polymer coating tape, the visual damage to the sample was considerably less than for the completely open specimen (Fig. 1). A
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Table 1 Chemical composition (weight%) of the magnesium alloys. Alloy
Al
Zr
Zn
Mn
Ca
Sr
Nd
Y
Mg
AZ91D MRI 153 MRI 201S MRI 202S
9.0 8.0 – –
– – 0.41 0.40
0.68 0.07 0.38 0.41
0.24 0.16 – –
– 1.0 0.07 0.06
– 0.28 – –
– – 3.06 3.08
– – 2.00 0.24
Bal. Bal. Bal. Bal.
similar ‘skin effect’ was reported for other die-cast alloys [12–14]. We decided to study the ‘side surface effect’ on the corrosion rate of the High Density Die-Cast MRI 201S magnesium alloy as compared to other MRI series alloys in more detail and report here the first results of this study. Fig. 1. Specimens of MRI 201S magnesium alloy after 14 day immersion in 3% NaCl. Left: the side surfaces were isolated with a polymer. Right: all the surfaces were open to the corrosive media. See the deep crack that has developed along the side surface.
2. Experimental High Density Die Cast rectangular 3 mm thick sheets of commercial MRI series and AZ91D alloys were received from the Dead Sea Magnesium Ltd. [9]. MRI 153M (beryllium-free Mg–Al–Ca–Sr based alloy), MRI 201S and MRI 202S (both are Mg–Zr–Nd–Y based alloys, the former contains 2% Y and the latter contains 0.2% Y) were chosen for the comparison. The chemical compositions of the alloys were determined by ICP after the dissolution of a sample of each alloy in the nitric acid and are given in Table 1. Small rectangular 35 50 3 mm plates were cut from the received sheets, ground with #2000 diamond paper, degreased with acetone and immersed in 3% NaCl solutions at room temperature. Gas evolution in the course of corrosion was monitored by funnel-and-burette systems similarly to those described by Atrens et al. [1]. Microstructure observations were performed with metallographic optical microscope and SEM JEOL JSM6510LV equipped with an NSS7 EDS analyzer (Correction Method Proza – Phi-Pho-Z was used for the quantitative analysis). X-ray Diffractometer (XRD) Panalytical X’Pert Pro with Cu Ka1 radiation (k = 0.154 nm) was used with the full pattern identification made by X’Pert HighScore Plus software package, version 2.2e (2.2.5) by PANalytical B.V. Materials identification and analysis made by the PDF-2 Release 2009 (Powder Diffraction File). Phase analysis identification made by XRD, 40 kV 40 mA. The XRD patterns were recorded in the GIXD geometry at 1° and 5° in the range of 20–80° (step size 0.05° and time per step 2 s). For each alloy 6–8 specimens were cut from the 3 mm thick die-cast plates. One half of the specimens of each alloy had all the sides open to the electrolyte while the side surfaces of the other half of the specimens were sealed with a polymer tape so that only their top and bottom faces remained open. Each specimen was immersed into 3% NaCl at room temperature for 14–18 days. The pH was not kept constant, rather it changed from 6.2–6.8 to 9.5–10.2 in the course of an experiment. Gas evolution was monitored in the course of the experiment. After that, a specimen was taken out of the salt solution and the layer of the corrosion products was dissolved in the solution of 200 g/L CrO3 and 10 g/L AgNO3 [4,7] until the achievement of a constant mass and cessation of bubbles evolution (5–10 min of immersion).
Fig. 2. The time dependencies of the gas evolution for two specimens of MRI 201S alloy: the specimen no. 2 had all the sides open to the corrosion process while the side surfaces of the specimen no. 3 were sealed.
3. Results and discussions For each specimen the mass loss after 14–18 days of the full immersion in 3% NaCl at room temperature was measured and the corrosion rate was calculated for the mass loss (Pw) and for the gas evolution (PH). The averaged for 3–4 samples results of the measurement are summarized in Table 2. As is seen from Table 2, the sealing of the side surfaces does not affect the corrosion rate of AZ91D, has only a small effect on MRI 153M and MRI 202S and reduces almost by 50% the corrosion rates of MRI 201S. The time dependency of the gas evolution for MRI 201S (Fig. 2) clearly demonstrates the significant effect of the side surfaces on the corrosion rate. The microstructures of all the studied alloys are presented in Fig. 3a–d. As is seen from Fig. 3, while for AZ91D, MRI 153M and MRI 202S there is no microstructural difference between the
Table 2 Averaged for 3–4 samples corrosion rates measured by the mass loss (Pw) and gas evolution (PH) for plates of magnesium alloys having (a) all sides open and (b) side surfaces sealed. Alloy
Pw, mm/year (all sides open)
Pw, mm/year (sides sealed)
PH, mm/year (all sides open)
PH, mm/year (sides sealed)
AZ91D MRI 153M MRI 201S MRI 202S
0.21 ± 0.03 0.73 ± 0.06 1.46 ± 0.04 6.21 ± 0.05
0.20 ± 0.04 0.62 ± 0.03 0.79 ± 0.02 5.90 ± 0.06
0.20 ± 0.06 0.84 ± 0.04 1.29 ± 0.06 7.73 ± 0.05
0.19 ± 0.04 0.54 ± 0.07 0.69 ± 0.04 6.55 ± 0.08
O. Gaon et al. / Corrosion Science 93 (2015) 167–171
169
(a) AZ91D
(b) MRI 153D
(c) MRI 201S
(d) MRI 202S Fig. 3. SEM BSE images of the microstructures of the studied magnesium alloys.
surfaces and the cross-sections, this is clearly not the case for MRI 201S. The surface of MRI 201S (Fig. 3c) is formed by 10 lm grains of primary magnesium a-phase separated one from another with
fine ‘filaments’ of b-phase. The cross-section structure is formed with considerably larger and less regularly sized a-phase grains and much larger inclusions of b-phase. Some additional
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Fig. 4. Eutectic crystallization in the middle of the cross-section structure of MRI 201S. The numbered frames are (1) a-phase, (2) b-phase and (3) the eutectics.
Table 3 Element composition in weight% of the main constituents of MRI 201S measured by EDS point analysis. Phase
Mg
Zn
Y
Zr
Nd
a-Phase
97 91 67
0 0.5 3
0.6 3 8
0.8 0.2 0
0.3 5 22
b-Phase Eutectics
perfectly met. As the corrosion process has started, b-phase cannot surround larger grains of a-phase and thus cannot protect them. The macroscopic result (i.e. the fast degradation of the alloy) is well seen in Fig. 1. This process proceeds with the positive feedback so that deep corrosion pits and cracks are formed. The microstructures of AZ91D and MRI 153M are composed of fine grains on the surfaces and in the center of the cast sheets. In accordance with this, no difference is observed between the corrosion behaviors of the surface and the center, and the total corrosion rate is relatively low. On the contrary, the structure of MRI 202S is coarse both in the center and in the periphery. For that reason, no difference is observed between the corrosion of different parts of a sheet, although the total corrosion rates are quite high. It deserves to be noted that other authors observed considerable ‘skin effect’ in the corrosion of AZ91D in NaCl. So, Atrens et al. [13] reported that for 6 mm thick specimens of AZ91D the corrosion rate of the skin was by the factor of 10 higher than of the interior. Aghion and Lulu [12] measured the corrosion rates of AZ91D for different thicknesses of a specimen and found that the corrosion rate of 1.5 mm thick samples was always higher than for 3–9 mm thick samples, that is some ‘inverse skin effect’ was observed. This is not consistent with our observations, in which for all the alloys we saw that coarser grain structure resulted in higher corrosion rates and vice versa.
4. Conclusions
Fig. 5. XRD pattern of MRI 201S alloy.
information can be gained by somewhat larger magnification of the central part of the cross-section of an MRI 201S specimen (Fig. 4). The composition of the main phases observed in Fig. 4 is given in Table 3 and the XRD pattern of the alloy is given in Fig. 5. We believe that the strong effect of the side surfaces on the corrosion process of MRI 201S alloy can be explained by the difference of the microstructures between the flat surfaces of the die-cast sheets and the inner part of the sheets exposed when the large sheet are cut into smaller plates. In fact, this is the ‘skin-effect’ described previously in the literature for other High Density Die Cast magnesium alloys [12–14]. The periphery of a sheet is formed by fine grains of a-phase containing >97% Mg and only very small amount of b-phase enriched with yttrium and neodymium. Such a structure favors better corrosion protection [2,15], particularly for the Nd-containing magnesium alloys [16], because the ‘filaments’ of the b-phase are too small to form cathodes and cannot launch the process of micro-galvanic corrosion. The inner part of the cast sheet is composed of larger a-phase grains and also large sites of the primary and eutectic b-phases, both enriched with rare-earth elements. For that case, the conditions for the micro-galvanic corrosion are
The corrosion rates of High Density Die Cast AZ91D and three MRI series magnesium alloys were measured by the mass loss and gas evolution in the course of the complete immersion of samples in 3% NaCl at the room temperature. One half of the specimens had all the surfaces open to the corrosive media, while the side surfaces of the other half were insulated by a polymer. It was demonstrated that there is essentially no difference between the two types of specimens for AZ91D, MRI 153M and MRI 202S alloys. Unlike those, MRI 201S alloy demonstrated pronounced ‘skineffect’: the corrosion of the samples whose side surfaces (i.e. the interior of the die-cast plates) were insulated was significantly slower than of the samples for which all the sides were open. For all the alloys studied, there is a correlation between the coarseness of the grain structure of an alloy and the corrosion rate: the coarser the grains the higher the corrosion rate. The pronounced ‘skin effect’ of MRI 201S may be caused by the higher content of the Mg3Nd containing eutectic b-phase in the interior of the casts.
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