- Email: [email protected]
Mechanoelectrochemical behavior and creep corrosion of magnesium alloys
Materials Science and Engineering A302 (2001) 63 – 67 www.elsevier.com/locate/msea
Mechanoelectrochemical behavior and creep corrosion of magnesium alloys E.M. Gutman *, A. Eliezer, Ya Unigovski, E. Abramov Department of Material Engineering, Ben-Gurion Uni6ersity of the Nege6, PO Box 653, Beer-She6a 84105, Israel
Abstract Both creep and corrosion resistances are significant problems in the application of magnesium alloys. The synergistic effect of corrosion and stress on the viscoelasticity of magnesium alloys named corrosion creep has been studied in die-cast AZ91D (Mg–9%Al–1%Zn) and AM50 (Mg–5% Al–0.4% Mn) alloys in air and in the borate buffer solution. The highest sensitivity to creep in the corrosive environment is observed in the alloy with the highest Al content. An electrochemical study under a tensile strain demonstrates a good correlation with corrosion creep tests. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnesium alloys; Mechanochemistry; Electrochemical corrosion; Corrosion creep; Borate buffer solution
1. Introduction Attractive mechanical properties of light materials such as magnesium alloys have increased the scope of their use in the transportation industry. Some magnesium applications (for example, for seat frames) require a good creep resistance. Some cast alloys were tested for stress corrosion and demonstrated a high sensitivity to this kind of degradation [1,2]. While all magnesium alloys stress corrodes to some extent, the most susceptible are those containing aluminum as an alloying element. For example, the failure of die-cast AZ91 alloy exposed to a rural atmosphere while loaded under constant tension (102 MPa) occurred within 10–100 days [3]. However, creep behavior of Mg alloys in electrolytes is still not studied despite the fact that these alloys do show creep even at room temperature. In general, it was reported that any environmental media, which removed or altered the oxide film, e.g. acids or salt solution, for this reason alone increased the plasticity and accelerated the creep rate of some metals [4,5]. But such effects probably resulted from both the effects of metal dissolution and oxide removal. Indeed, there exists a theoretical and experimental background of * Corresponding author. Tel.: +972-7-6461478; fax: + 972-76519247. E-mail address: [email protected] (E.M. Gutman).
chemomechanical effect, which consists of the environmentally assisted plastization of metal in an active state without any passive film [6]. This effect can promote an additional creep in the absence of surface barriers, e.g. oxide films or ‘debris layers’. Some magnesium alloys have been developed in the past several years to meet the needs of structural applications. The most common die-cast alloy is AZ91D with 9% aluminum content. However, many structural applications require an appreciable amount of energy absorption during their operation. Therefore, magnesium alloys having lower aluminum content, for example, AM50 (5% Al) were found to be more ductile, especially under impact. Thus, it was essential to study corrosion creep of magnesium die-cast alloys and to investigate the correlation of corrosion creep resistance with the mechanochemical behavior of these promising alloys.
2. Experimental Various types of die-cast test specimens were used to determine standard mechanical properties, mechanoelectrochemical behavior and creep resistance of the alloys. The specimens of Mg alloys AM50 and AZ91D (Table 1) with gauge length of 75 mm and diameter of 5.9 mm used for this study were die cast in a 200-t cold chamber machine. Samples for mechanoelectrochemical
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 3 5 5 - 1
E.M. Gutman et al. / Materials Science and Engineering A302 (2001) 63–67
64
Table 1 Chemical composition (mass%) of alloys, Mg–the rest Alloy
Al
Mn
Zn
Si
Ni
Cu
Fe
Be
AZ91D AM50
8.4 5.1
0.17 0.57
0.85 0.15
0.010 0.013
0.0007 0.0006
0.0008 0.0007
0.0013 0.0040
0.0003 0.0013
investigations were employed with as-cast surfaces after polishing, rinsing with distilled water and wiping with acetone. The compositions of electrolytes was 0.1 N Na2B4O7 aqueous buffer solution (pH 9.3) for corrosion creep test and 0.1 N Na2B4O7 +Mg(OH)2 saturated (pH 10.5) for mechanoelectrochemical tests. All solutions were prepared from analytical grade chemicals and distilled water. The ultimate tensile strength (UTS), tensile yield strength (TYS) and elongation to fracture were equal to 224.7, 180.0 MPa and 2.8% for AZ91D alloy and 229.0, 136.0 MPa and 11.7% for AM50 alloy, respectively. All polarization regimes for mechanoelectrochemical tests were realized using corrosion measurement system consisting of versastat – potentiostat/galvanostat with floating option [7,8]. Clamping an electrochemical cell to a specimen of a tensile machine makes it possible to conduct electrochemical investigations under static and dynamic conditions of loading. The cell includes a platinum counter electrode, a Luggin capillary, a reference (calomel) electrode and a working electrode (the test specimen). The latter is clamped between the plastic wall of the cell (having a circle aperture of 5 mm in diameter) and clamping plate with a screw. The diameter of the aperture may be altered to change the area of polarization. Thus, the opened area of the work electrode was constant during the deformation. The potentiodynamic polarization measurements can be conducted with or without an external load. Tensile deformation of the specimen was step-wise and on every level of plastic strain the machine was stopped and the potentiodynamic curve was measured. [7,8]. The analysis of polarization curves was carried out by the linear polarization method and by cross-sections of curves at different selected potential levels to find corresponding current densities. Corrosion rates were derived from polarization data by a common method [9]. Corrosion creep tests with a duration of 20–450 h were carried out on Model 3 Satec machine at room temperature 25°C in air and in the buffer solution. Stress values in the creep tests were equal to 0.85 and 0.89 of TYS for each alloy. A creep specimen 1 in a glass electrochemical cell 3 was placed into an averaging high-temperature extensometer 2 (Fig. 1). The volume of the buffer solution was about 50 ml (the solution height in the cell being 40 mm, and the inner cell diameter was 40 mm). The pH value change during long-term tests was less than 20%.
The elemental composition of the magnesium alloys at the surface and in-depth concentration distributions (‘sputter depth-profiling’) were estimated using surfacesensitive AES combined with controlled Ar ion bombardment. A PHI 549 SAM/AES/XPS apparatus with standard pressure of 2× 10 − 9 Torr in the chamber and cylindrical mirror analyzer were used for AES measurements. Quantitative analysis was based on atomic sensitivity factors [10] and was carried out proceeding from general survey spectra. The sputtering procedure was accomplished by a differential pumped small-spot ion gun (IQE 12/38) under Ar pressure of 2.5×10 − 7 Torr and a power supply voltage of 3 kV with the sputtering yield of 1292 nm min − 1 at the maximum sputtering duration of 240 min. Microstructure studies were carried out using an optical microscope ‘Nicon’ with a computer program ‘Omnimet’ for quantitative phase analysis and a model Jeol JSM-35CF scanning electron microscope with a ‘Link system’ AN 10000 energy dispersive spectroscope.
3. Results and discussion Mechanoelectrochemical measurements demonstrate a higher corrosion rate of AZ91D in a deformed state than that of AM50, while in the undeformed state (strain o= 0) AZ91D has a lower corrosion rate than AM50 (Fig. 2). These corrosion rate C–strain o dependencies for AZ91D and AM50 alloys are well described by linear equations — C= 16.19o +9.65 and C= 2.58o +15.78 with correlation coefficients of 0.92 and 0.84, respectively. Namely, the least effect is observed in AM50 alloy, and the greatest in AZ91D alloy. This
Fig. 1. Creep specimen 1 within an electrochemical cell 3 placed in the averaging high temperature extensometer 2.
E.M. Gutman et al. / Materials Science and Engineering A302 (2001) 63–67
65
Fig. 2. Corrosion rate of Mg alloys in the 0.1 N Na2B4O7 + Mg(OH)2 saturated solution vs. tensile strain. Fig. 3. Typical creep curves for AZ91D alloy in air (solid line) and in 0.1 N Na2B4O7 buffer solution (dotted line).
corresponds exactly to the arrangement of AM20, AM50, AM40 and AZ81 alloys in the order of increasing strain-hardening coefficient [7,8]. As it can be observed in Table 2 and Figs. 3 and 4, the creep rate of AZ91D in the buffer solution is significantly higher than in air at relative stresses s/ TYS of 0.85 and 0.89. However, AM50 alloy containing lower percentage of aluminum is more stable in the buffer solution during the first creep stage. The analysis of the initial portions of creep curves (test time B 0.5 h) shows that the time to a specified creep strain o (for example, o= 0.03%), was 1.4 – 5 times shorter for AZ91D in the corrosive environment than in air. However, for AM50 alloy in the corrosive environment this time was 2–10 times longer than in air (Table 2). The creep life of the AZ91D alloy in the corrosive environment was equal to 1309 20 and 45 9 15 h at s/TYS of 0.85 and 0.89, respectively (Table 2, Fig. 3). However, in air the creep rupture was not observed at the test duration of 450 and 150 h at s/TYS values of 0.85 and 0.89, respectively. The AM50 alloy was stressed up to 350 h without rupture at s/TYS of 0.85 both in air and in the buffer solution. An increase in the relative stress from 0.85 to 0.89 leads to a rupture of AM50 in the buffer solution, however, in air the creep rupture was not observed during the test duration of 450 h (Table 2, Fig. 4). An estimation of the elemental composition of the alloys at their surface and sputter depth-profiling shows that in the die-cast surface layer there are such compo-
Fig. 4. Typical creep curves for AM50 alloy in air (solid line) and in 0.1 N Na2B4O7 buffer solution (dotted line).
nents as Mg2 + , Mg, Al3 + , Al, O2 − , B4O27 − , and carbon, apparently, in CO23 − form. The contents of borate anions in the oxide film of specimens stressed in the buffer solution amounted to 17–25 and 20–45% for AZ91D and AM50 alloys, respectively. The content of the metal portion (Mg+Al)/(Mg + Al+MgO+ Al2O3) in the surface layer depending on the sputtering time is presented in Fig. 5 for two alloys stressed in air and in the buffer solution. The thickness of the oxide film is estimated at the sputtering time corresponding to
Table 2 Effect of the environment and stress on creep behavior of Mg alloys Alloy
AZ91D AM50
a b
s/TYS
0.85 0.89 0.85 0.89
No rupture up to 450 h. No rupture up to 150 h.
Time to strain, 0.03% (h)
oc for 100 h (%)
Time to rupture tr (h)
Air
Solution
Air
Solution
Air
0.18 0.10 0.04 0.02
0.13 0.02 0.40 0.04
0.12 0.18 0.16 0.19
0.12 (trB100 h) 0.12 0.23
No No No No
Solution rupturea ruptureb rupturea rupturea
130 9 20 459 15 No rupturea 340 9 30
66
E.M. Gutman et al. / Materials Science and Engineering A302 (2001) 63–67
the metal portion below 0.1. In AZ91D and AM50 it amounted to 40 and 100 nm in air, 1300 and 1900 nm in the borate solution. These results are related to the films formed on as-cast surfaces. Earlier data [11] for die-cast AM80 (Mg – 8%Al – 0.3%Mn) and AM50 (Mg–5%Al – 0.3%Mn) alloys relating to the oxide films formed on fresh surfaces of TEM foils show that the thickness of these films was equal to 7.59 2 and 109 5 nm in the dry air, and 200 and 300 nm in distilled water, respectively. We can explain the obtained results with the account for surface phenomena. Creep deformation of a brittle AZ91D alloy in a corrosive environment leads to the surface film break-up. This process leads to an increase in the metal dissolution and corrosion rate on fresh surfaces. Apparently, a thicker film on the surface of a ductile AM50 alloy containing a higher content of the borate anion B4O27 − decreases the metal dissolution and corrosion rate. Here the borate anion acts as a corrosion inhibitor at the first stage of creep. It is known, for example, that these ions suppress the intergranular stress corrosion cracking of type 303 stainless steel by delaying the crack initiation time and reducing the crack initiation frequency [12]. The influence of Al content on the sensitivity to creep in active environment can be understood on the basis of studying the cast alloy microstructure. The grain size of Mg–Al solid solution in the surface layer of about 0.5-mm thick and in the bulk of die-castings amounts to 1–10 and 10– 15 mm, respectively. Since pressure die-casting involves rapid cooling rates even alloys with relatively low Al content contain a certain volume fraction of the divorced eutectic b-phase (Mg17Al12). In addition, in the as-cast microstructure, hard intermetallic Al–Mn–Fe particles are found. As the Al content is increased in the alloy there is progressive increase in the amount of these intermetallics. As it was established by
quantitative phase analysis, the Mg17Al12 content in the form of coarse discontinuous precipitates varied in the ‘as-cast’ AZ91D and AM50 alloys within the limits of 25–32 and 10–18%, respectively. The hard intermetallics promote strain hardening and, thus, increase chemical potential of metal atoms (shifting the standard potential towards negative potential values due to this reason), i.e. increase mechanochemical dissolution [6]. Thus, similar to the data obtained in the study of mechanoelectrochemical behavior and corrosion fatigue of Mg alloys [7,8] the highest sensitivity to creep in a corrosive environment is observed in the alloy with the highest Al content.
4. Conclusions A synergistic effect of corrosion and stress on viscoelasticity of magnesium alloys named corrosion creep was studied using die-cast AZ91D and AM50 alloys. The highest sensitivity to creep in a corrosive environment is observed in the alloy with the highest Al content. Thus, the creep rate of AZ91D alloy in the buffer solution is significantly higher than in air. However, it is observed in AM50 alloy at the first stage of creep that the creep rate of the alloy in the buffer solution is lower than in air. The thickness of the oxide films formed on as-cast surfaces of AZ91D and AM50 alloys amounted to 40 and 100 nm in air, and 1300 and 1900 nm in the borate solution, respectively. The content of borate anions in the oxide film of specimens stressed in the buffer solution amounted to 17–25% for AZ91D and 20–45% for AM50 alloys. Apparently, a thicker film on the surface of a ductile AM50 alloy with a higher content of the borate anion B4O27 − decreases metal dissolution and corrosion rate. Here the borate anion acts as a corrosion inhibitor at the first stage of creep.
Acknowledgements The present work was sponsored by the Consortium of Magnesium Technologies Development (Israel’s Ministry of Industry and Commerce). We would like to thank Dr N. Frumin and O. Nabutovski (Ben-Gurion University of the Negev) for kind assistance in AES and SEM. We thank Z. Koren and H. Rosenson (Israel Institute of Metals, Technion. Haifa) for supplying us with die-cast specimens.
Fig. 5. AES (Al + Mg)/(Al+ Mg+ MgO+ Al2O3) mole ratios on the surface of die-cast AZ91D (1 and 3) and AM50 (2 and 4) magnesium alloys in the process of sputtering of as-cast specimens stressed in air (1 and 2) and in the buffer solution (3 and 4).
References [1] R.I. Stephens, C.D. Schrader, K.B. Lease, J. Eng. Mater. Technol. 117 (1995) 293 – 298.
E.M. Gutman et al. / Materials Science and Engineering A302 (2001) 63–67 [2] H.R. Mayer, H. Lipovsky, M. Papakyriacou, R. Rosch, A. Stich, S. Stanzl-Tschegg, Fatigue Fract. Eng. Mater. Struct. 22 (7) (1999) 591 – 599. [3] R.S. Busk, Magnesium Products Design, Marcel Dekker, New York, 1987, pp. 248, 279, 282, 448. [4] E. Andrade, Nature 164 (1949) 536. [5] R.W. Revie, H. Uhlig, Acta Metall. 22 (1974) 619–627. [6] E.M. Gutman, Mechanochemistry of Solid Surfaces, World Scientific, Singapore, 1994, p. 332. [7] A. Eliezer, E.M. Gutman, E. Abramov, E. Aghion, Corrosion Rev. 16 (1 – 2) (1997) 1–26.
.
67
[8] A. Eliezer, E. Abramov, E.M. Gutman, J. Mater. Sci. Lett. 17 (1998) 801 – 803. [9] H.H. Uhlig, Corrosion and Corrosion Control, Wiley, New York, 1963, p. 53. [10] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach, R.E. Weber, Handbook of Auger Electron Spectroscopy, Perkin Elmer, Eden Prairie, USA, 1976. [11] J.H. Nordlien, K. Nisancioglu, S. Ono, N. Masuko, J. Electrochem. Soc. 143 (8) (1996) 2564 – 2572. [12] S. Zhang, T. Shibata, T. Haruna, Corrosion 54 (6) (1998) 428 – 434.
Mechanoelectrochemical behavior and creep corrosion of magnesium alloys E.M. Gutman *, A. Eliezer, Ya Unigovski, E. Abramov Department of Material Engineering, Ben-Gurion Uni6ersity of the Nege6, PO Box 653, Beer-She6a 84105, Israel
Abstract Both creep and corrosion resistances are significant problems in the application of magnesium alloys. The synergistic effect of corrosion and stress on the viscoelasticity of magnesium alloys named corrosion creep has been studied in die-cast AZ91D (Mg–9%Al–1%Zn) and AM50 (Mg–5% Al–0.4% Mn) alloys in air and in the borate buffer solution. The highest sensitivity to creep in the corrosive environment is observed in the alloy with the highest Al content. An electrochemical study under a tensile strain demonstrates a good correlation with corrosion creep tests. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnesium alloys; Mechanochemistry; Electrochemical corrosion; Corrosion creep; Borate buffer solution
1. Introduction Attractive mechanical properties of light materials such as magnesium alloys have increased the scope of their use in the transportation industry. Some magnesium applications (for example, for seat frames) require a good creep resistance. Some cast alloys were tested for stress corrosion and demonstrated a high sensitivity to this kind of degradation [1,2]. While all magnesium alloys stress corrodes to some extent, the most susceptible are those containing aluminum as an alloying element. For example, the failure of die-cast AZ91 alloy exposed to a rural atmosphere while loaded under constant tension (102 MPa) occurred within 10–100 days [3]. However, creep behavior of Mg alloys in electrolytes is still not studied despite the fact that these alloys do show creep even at room temperature. In general, it was reported that any environmental media, which removed or altered the oxide film, e.g. acids or salt solution, for this reason alone increased the plasticity and accelerated the creep rate of some metals [4,5]. But such effects probably resulted from both the effects of metal dissolution and oxide removal. Indeed, there exists a theoretical and experimental background of * Corresponding author. Tel.: +972-7-6461478; fax: + 972-76519247. E-mail address: [email protected] (E.M. Gutman).
chemomechanical effect, which consists of the environmentally assisted plastization of metal in an active state without any passive film [6]. This effect can promote an additional creep in the absence of surface barriers, e.g. oxide films or ‘debris layers’. Some magnesium alloys have been developed in the past several years to meet the needs of structural applications. The most common die-cast alloy is AZ91D with 9% aluminum content. However, many structural applications require an appreciable amount of energy absorption during their operation. Therefore, magnesium alloys having lower aluminum content, for example, AM50 (5% Al) were found to be more ductile, especially under impact. Thus, it was essential to study corrosion creep of magnesium die-cast alloys and to investigate the correlation of corrosion creep resistance with the mechanochemical behavior of these promising alloys.
2. Experimental Various types of die-cast test specimens were used to determine standard mechanical properties, mechanoelectrochemical behavior and creep resistance of the alloys. The specimens of Mg alloys AM50 and AZ91D (Table 1) with gauge length of 75 mm and diameter of 5.9 mm used for this study were die cast in a 200-t cold chamber machine. Samples for mechanoelectrochemical
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 3 5 5 - 1
E.M. Gutman et al. / Materials Science and Engineering A302 (2001) 63–67
64
Table 1 Chemical composition (mass%) of alloys, Mg–the rest Alloy
Al
Mn
Zn
Si
Ni
Cu
Fe
Be
AZ91D AM50
8.4 5.1
0.17 0.57
0.85 0.15
0.010 0.013
0.0007 0.0006
0.0008 0.0007
0.0013 0.0040
0.0003 0.0013
investigations were employed with as-cast surfaces after polishing, rinsing with distilled water and wiping with acetone. The compositions of electrolytes was 0.1 N Na2B4O7 aqueous buffer solution (pH 9.3) for corrosion creep test and 0.1 N Na2B4O7 +Mg(OH)2 saturated (pH 10.5) for mechanoelectrochemical tests. All solutions were prepared from analytical grade chemicals and distilled water. The ultimate tensile strength (UTS), tensile yield strength (TYS) and elongation to fracture were equal to 224.7, 180.0 MPa and 2.8% for AZ91D alloy and 229.0, 136.0 MPa and 11.7% for AM50 alloy, respectively. All polarization regimes for mechanoelectrochemical tests were realized using corrosion measurement system consisting of versastat – potentiostat/galvanostat with floating option [7,8]. Clamping an electrochemical cell to a specimen of a tensile machine makes it possible to conduct electrochemical investigations under static and dynamic conditions of loading. The cell includes a platinum counter electrode, a Luggin capillary, a reference (calomel) electrode and a working electrode (the test specimen). The latter is clamped between the plastic wall of the cell (having a circle aperture of 5 mm in diameter) and clamping plate with a screw. The diameter of the aperture may be altered to change the area of polarization. Thus, the opened area of the work electrode was constant during the deformation. The potentiodynamic polarization measurements can be conducted with or without an external load. Tensile deformation of the specimen was step-wise and on every level of plastic strain the machine was stopped and the potentiodynamic curve was measured. [7,8]. The analysis of polarization curves was carried out by the linear polarization method and by cross-sections of curves at different selected potential levels to find corresponding current densities. Corrosion rates were derived from polarization data by a common method [9]. Corrosion creep tests with a duration of 20–450 h were carried out on Model 3 Satec machine at room temperature 25°C in air and in the buffer solution. Stress values in the creep tests were equal to 0.85 and 0.89 of TYS for each alloy. A creep specimen 1 in a glass electrochemical cell 3 was placed into an averaging high-temperature extensometer 2 (Fig. 1). The volume of the buffer solution was about 50 ml (the solution height in the cell being 40 mm, and the inner cell diameter was 40 mm). The pH value change during long-term tests was less than 20%.
The elemental composition of the magnesium alloys at the surface and in-depth concentration distributions (‘sputter depth-profiling’) were estimated using surfacesensitive AES combined with controlled Ar ion bombardment. A PHI 549 SAM/AES/XPS apparatus with standard pressure of 2× 10 − 9 Torr in the chamber and cylindrical mirror analyzer were used for AES measurements. Quantitative analysis was based on atomic sensitivity factors [10] and was carried out proceeding from general survey spectra. The sputtering procedure was accomplished by a differential pumped small-spot ion gun (IQE 12/38) under Ar pressure of 2.5×10 − 7 Torr and a power supply voltage of 3 kV with the sputtering yield of 1292 nm min − 1 at the maximum sputtering duration of 240 min. Microstructure studies were carried out using an optical microscope ‘Nicon’ with a computer program ‘Omnimet’ for quantitative phase analysis and a model Jeol JSM-35CF scanning electron microscope with a ‘Link system’ AN 10000 energy dispersive spectroscope.
3. Results and discussion Mechanoelectrochemical measurements demonstrate a higher corrosion rate of AZ91D in a deformed state than that of AM50, while in the undeformed state (strain o= 0) AZ91D has a lower corrosion rate than AM50 (Fig. 2). These corrosion rate C–strain o dependencies for AZ91D and AM50 alloys are well described by linear equations — C= 16.19o +9.65 and C= 2.58o +15.78 with correlation coefficients of 0.92 and 0.84, respectively. Namely, the least effect is observed in AM50 alloy, and the greatest in AZ91D alloy. This
Fig. 1. Creep specimen 1 within an electrochemical cell 3 placed in the averaging high temperature extensometer 2.
E.M. Gutman et al. / Materials Science and Engineering A302 (2001) 63–67
65
Fig. 2. Corrosion rate of Mg alloys in the 0.1 N Na2B4O7 + Mg(OH)2 saturated solution vs. tensile strain. Fig. 3. Typical creep curves for AZ91D alloy in air (solid line) and in 0.1 N Na2B4O7 buffer solution (dotted line).
corresponds exactly to the arrangement of AM20, AM50, AM40 and AZ81 alloys in the order of increasing strain-hardening coefficient [7,8]. As it can be observed in Table 2 and Figs. 3 and 4, the creep rate of AZ91D in the buffer solution is significantly higher than in air at relative stresses s/ TYS of 0.85 and 0.89. However, AM50 alloy containing lower percentage of aluminum is more stable in the buffer solution during the first creep stage. The analysis of the initial portions of creep curves (test time B 0.5 h) shows that the time to a specified creep strain o (for example, o= 0.03%), was 1.4 – 5 times shorter for AZ91D in the corrosive environment than in air. However, for AM50 alloy in the corrosive environment this time was 2–10 times longer than in air (Table 2). The creep life of the AZ91D alloy in the corrosive environment was equal to 1309 20 and 45 9 15 h at s/TYS of 0.85 and 0.89, respectively (Table 2, Fig. 3). However, in air the creep rupture was not observed at the test duration of 450 and 150 h at s/TYS values of 0.85 and 0.89, respectively. The AM50 alloy was stressed up to 350 h without rupture at s/TYS of 0.85 both in air and in the buffer solution. An increase in the relative stress from 0.85 to 0.89 leads to a rupture of AM50 in the buffer solution, however, in air the creep rupture was not observed during the test duration of 450 h (Table 2, Fig. 4). An estimation of the elemental composition of the alloys at their surface and sputter depth-profiling shows that in the die-cast surface layer there are such compo-
Fig. 4. Typical creep curves for AM50 alloy in air (solid line) and in 0.1 N Na2B4O7 buffer solution (dotted line).
nents as Mg2 + , Mg, Al3 + , Al, O2 − , B4O27 − , and carbon, apparently, in CO23 − form. The contents of borate anions in the oxide film of specimens stressed in the buffer solution amounted to 17–25 and 20–45% for AZ91D and AM50 alloys, respectively. The content of the metal portion (Mg+Al)/(Mg + Al+MgO+ Al2O3) in the surface layer depending on the sputtering time is presented in Fig. 5 for two alloys stressed in air and in the buffer solution. The thickness of the oxide film is estimated at the sputtering time corresponding to
Table 2 Effect of the environment and stress on creep behavior of Mg alloys Alloy
AZ91D AM50
a b
s/TYS
0.85 0.89 0.85 0.89
No rupture up to 450 h. No rupture up to 150 h.
Time to strain, 0.03% (h)
oc for 100 h (%)
Time to rupture tr (h)
Air
Solution
Air
Solution
Air
0.18 0.10 0.04 0.02
0.13 0.02 0.40 0.04
0.12 0.18 0.16 0.19
0.12 (trB100 h) 0.12 0.23
No No No No
Solution rupturea ruptureb rupturea rupturea
130 9 20 459 15 No rupturea 340 9 30
66
E.M. Gutman et al. / Materials Science and Engineering A302 (2001) 63–67
the metal portion below 0.1. In AZ91D and AM50 it amounted to 40 and 100 nm in air, 1300 and 1900 nm in the borate solution. These results are related to the films formed on as-cast surfaces. Earlier data [11] for die-cast AM80 (Mg – 8%Al – 0.3%Mn) and AM50 (Mg–5%Al – 0.3%Mn) alloys relating to the oxide films formed on fresh surfaces of TEM foils show that the thickness of these films was equal to 7.59 2 and 109 5 nm in the dry air, and 200 and 300 nm in distilled water, respectively. We can explain the obtained results with the account for surface phenomena. Creep deformation of a brittle AZ91D alloy in a corrosive environment leads to the surface film break-up. This process leads to an increase in the metal dissolution and corrosion rate on fresh surfaces. Apparently, a thicker film on the surface of a ductile AM50 alloy containing a higher content of the borate anion B4O27 − decreases the metal dissolution and corrosion rate. Here the borate anion acts as a corrosion inhibitor at the first stage of creep. It is known, for example, that these ions suppress the intergranular stress corrosion cracking of type 303 stainless steel by delaying the crack initiation time and reducing the crack initiation frequency [12]. The influence of Al content on the sensitivity to creep in active environment can be understood on the basis of studying the cast alloy microstructure. The grain size of Mg–Al solid solution in the surface layer of about 0.5-mm thick and in the bulk of die-castings amounts to 1–10 and 10– 15 mm, respectively. Since pressure die-casting involves rapid cooling rates even alloys with relatively low Al content contain a certain volume fraction of the divorced eutectic b-phase (Mg17Al12). In addition, in the as-cast microstructure, hard intermetallic Al–Mn–Fe particles are found. As the Al content is increased in the alloy there is progressive increase in the amount of these intermetallics. As it was established by
quantitative phase analysis, the Mg17Al12 content in the form of coarse discontinuous precipitates varied in the ‘as-cast’ AZ91D and AM50 alloys within the limits of 25–32 and 10–18%, respectively. The hard intermetallics promote strain hardening and, thus, increase chemical potential of metal atoms (shifting the standard potential towards negative potential values due to this reason), i.e. increase mechanochemical dissolution [6]. Thus, similar to the data obtained in the study of mechanoelectrochemical behavior and corrosion fatigue of Mg alloys [7,8] the highest sensitivity to creep in a corrosive environment is observed in the alloy with the highest Al content.
4. Conclusions A synergistic effect of corrosion and stress on viscoelasticity of magnesium alloys named corrosion creep was studied using die-cast AZ91D and AM50 alloys. The highest sensitivity to creep in a corrosive environment is observed in the alloy with the highest Al content. Thus, the creep rate of AZ91D alloy in the buffer solution is significantly higher than in air. However, it is observed in AM50 alloy at the first stage of creep that the creep rate of the alloy in the buffer solution is lower than in air. The thickness of the oxide films formed on as-cast surfaces of AZ91D and AM50 alloys amounted to 40 and 100 nm in air, and 1300 and 1900 nm in the borate solution, respectively. The content of borate anions in the oxide film of specimens stressed in the buffer solution amounted to 17–25% for AZ91D and 20–45% for AM50 alloys. Apparently, a thicker film on the surface of a ductile AM50 alloy with a higher content of the borate anion B4O27 − decreases metal dissolution and corrosion rate. Here the borate anion acts as a corrosion inhibitor at the first stage of creep.
Acknowledgements The present work was sponsored by the Consortium of Magnesium Technologies Development (Israel’s Ministry of Industry and Commerce). We would like to thank Dr N. Frumin and O. Nabutovski (Ben-Gurion University of the Negev) for kind assistance in AES and SEM. We thank Z. Koren and H. Rosenson (Israel Institute of Metals, Technion. Haifa) for supplying us with die-cast specimens.
Fig. 5. AES (Al + Mg)/(Al+ Mg+ MgO+ Al2O3) mole ratios on the surface of die-cast AZ91D (1 and 3) and AM50 (2 and 4) magnesium alloys in the process of sputtering of as-cast specimens stressed in air (1 and 2) and in the buffer solution (3 and 4).
References [1] R.I. Stephens, C.D. Schrader, K.B. Lease, J. Eng. Mater. Technol. 117 (1995) 293 – 298.
E.M. Gutman et al. / Materials Science and Engineering A302 (2001) 63–67 [2] H.R. Mayer, H. Lipovsky, M. Papakyriacou, R. Rosch, A. Stich, S. Stanzl-Tschegg, Fatigue Fract. Eng. Mater. Struct. 22 (7) (1999) 591 – 599. [3] R.S. Busk, Magnesium Products Design, Marcel Dekker, New York, 1987, pp. 248, 279, 282, 448. [4] E. Andrade, Nature 164 (1949) 536. [5] R.W. Revie, H. Uhlig, Acta Metall. 22 (1974) 619–627. [6] E.M. Gutman, Mechanochemistry of Solid Surfaces, World Scientific, Singapore, 1994, p. 332. [7] A. Eliezer, E.M. Gutman, E. Abramov, E. Aghion, Corrosion Rev. 16 (1 – 2) (1997) 1–26.
.
67
[8] A. Eliezer, E. Abramov, E.M. Gutman, J. Mater. Sci. Lett. 17 (1998) 801 – 803. [9] H.H. Uhlig, Corrosion and Corrosion Control, Wiley, New York, 1963, p. 53. [10] L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach, R.E. Weber, Handbook of Auger Electron Spectroscopy, Perkin Elmer, Eden Prairie, USA, 1976. [11] J.H. Nordlien, K. Nisancioglu, S. Ono, N. Masuko, J. Electrochem. Soc. 143 (8) (1996) 2564 – 2572. [12] S. Zhang, T. Shibata, T. Haruna, Corrosion 54 (6) (1998) 428 – 434.