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Studies on the influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of AZ63 magnesium alloy
Materials & Design Materials and Design 25 (2004) 637–643 www.elsevier.com/locate/matdes
Short Communication
Studies on the influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of AZ63 magnesium alloy Hikmet Altun *, Sadri Sen Department of Mechanical Engineering, Ataturk University, 25240, Erzurum, Turkey Received 28 August 2003; accepted 2 February 2004
Abstract Magnesium-based light alloys belong to a class of structural materials with increasing industrial attention. Magnesium alloys show the lowest density among the engineering metallic materials, low cost and large availability. As a consequence, this light alloys have a promising future. The paper presented reveals the influence of chloride ion concentration and pH on the corrosion and electrochemical behavior of AZ63 magnesium alloy in NaCl solution. The experimental techniques used include immersion studies and potentiodynamic polarization tests. The corrosion rate was very high in highly acidic solutions (pH 2) as compared to that in other solutions. The corrosion rate usually increased with the decrease in pH and the increase in chloride ion concentration. But the quantity of the increase in corrosion rate was different at separate pH and concentration regions. The corrosion potential usually shifted to more negative values with the increase in concentration of chloride ions and the decrease in pH of solution. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: AZ63 Alloy; Corrosion; Chloride ion; Concentration; pH
1. Introduction Magnesium-based light alloys belong to a class of structural materials with increasing industrial attention. Magnesium alloys show the lowest density among the engineering metallic materials, low cost and large availability [1]. As a consequence, this light alloys have a promising future [2]. Because of their low density, high specific strength and stiffness, magnesium alloys have become candidate materials for many applications in microelectronics and in automobile and aerospace industries. The relative density of magnesium is 1.74 g/cm3 , which is 35% lower than that of aluminum, and typical magnesium alloys weigh 35% lower than their aluminum counterparts at equal stiffness [3]. Magnesium and magnesium alloys are non-magnetic, have relatively
*
Corresponding author. Tel.: +90-442-2314839; fax: +90-4422360957. E-mail address: [email protected] (H. Altun). 0261-3069/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2004.02.002
high thermal and electrical conductivity, and good vibration and shock absorption ability [4]. A serious limitation for the potential use of several magnesium alloys is their susceptibility to corrosion. Magnesium alloys, especially those with high purity, have good resistance to atmospheric corrosion. However, the susceptibility to corrosion in chloride containing environments is a serious concern [3]. Alloying magnesium with aluminum in general improves the corrosion resistance [5]. Most of the magnesium–aluminum alloys contain 8–9% aluminum with small amounts of zinc [6]. AZ63 alloy, magnesium and the principal alloying element aluminum, is one of the most widely used magnesium alloys. Recently, the general corrosion and electrochemical corrosion of various magnesium alloys have been studied by various researchers [6–16]. Ambat et al. [6] studied the influence of chloride ion concentration and pH on the corrosion and electrochemical behavior of die-cast and ingot-cast AZ91D alloy. Song et al. [8] investigated the electrochemical behaviour of magnesium in representative
638
H. Altun, S. Sen / Materials and Design 25 (2004) 637–643
chloride and sulfate solutions including NaCl, Na2 SO4 , NaOH and their mixed solutions. Song et al. [9] studied the corrosion behaviour of AZ21, AZ501 and AZ91 magnesium alloys in 1N NaCl at pH 11. Baril and Pebere [12] studied the corrosion behaviour of pure magnesium in aerated and deaerated Na2 SO4 solutions. Daloz et al. [13] studied the corrosion behaviour of rapidly solidified magnesium–aluminum–zinc alloys (8%, 15% and 20% Al, and 1% and 3% Zn). The majority of the researches on magnesium alloys has focused on the effect of processing route and microstructure. To date, there has been no detailed study on the effect of pH and chloride ion concentration of cast AZ63 magnesium alloy. Because AZ63 magnesium alloy may behave differently under varying pH conditions and chloride concentrations, it is important to understand the effect of these parameters in detail on the corrosion behavior of the alloy. Therefore, the present study was aimed at investigating these aspects for AZ63 magnesium alloy.
2. Experimental details 2.1. Material
sured after each experiment and the corrosion rate was calculated in millimeters per year. The specimens were weighed on an analytical balance to an accuracy of 0.1mg. The specimens were taken photos after removing their corrosion products. 2.4. Electrochemical testing Electrochemical polarization experiments were carried out using a potentiostat, Potentioscan Wenking POS 73. The electrodes for this purpose were prepared by connecting a wire to one side of the sample that was covered with cold setting resin. One side of the specimen, whose area is 1 cm2 , was exposed to the solution. The specimens were treated with metallographic polishing prior to each experiment, followed by washing distilled water and acetone, and finally dried in warm air. The polarization measurements were carried out in corrosion cell containing 500 ml solution. The electrochemical cell consists of AZ63 alloy as the working electrode, a saturated calomel reference electrode, and a platinum counter electrode. The specimens were immersed in the test solution, and a polarization scan was carried out towards more noble values at a rate of 1 mV/s, after allowing a steady state potential to develop.
In this study, AZ63 magnesium alloy was used. The chemical composition of the alloy is given in Table 1. 3. Results 2.2. Corrosive media The effect of chloride ion concentration and pH on the corrosion and electrochemical behavior of AZ63 alloy was studied in NaCl solutions at different concentrations (0.01, 0.2, 0.6, 1 and 2 M) and pH values (2, 3, 8, 11 and 11.5). All the experiments were conducted at the same temperature of 28 0.5 °C. The solution was prepared using AR grade NaCl and distilled water. The pH value of the solution was adjusted to the desired value with HCl and NaOH.
Fig. 1 shows the corrosion rates obtained from immersion tests as a function of chloride ion concentration for the alloy at five pH values (2, 3, 8, 11, and 11.5). At all pH values, the materials usually exhibited a rise in corrosion rate with increase in chloride ion concentration. But the quantity of this rise was different. In such a way that, the change at chloride ion concentration at lower concentrations affected the corrosion rate much more as compared to that of higher concentrations.
2.3. Constant immersion testing
50 Na Cl pH =2
Corrosion Rate, mm/y
For constant immersion testing, the specimens were polished successively on finer grades of emery papers up to 800 level. The polished and preweight specimens have been exposed to the solution (500 ml) for 72 h. At the end of the experiment, cleaning of the specimens was carried out by dipping in a solution of 15% CrO3 + 1% Ag2 CrO4 in 100 ml water under boiling conditions. An acetone wash followed this. The weight loss was mea-
40 pH =3 pH =8 30
pH =11 pH=11. 5
20
10
0
Table 1 The chemical composition of AZ63 magnesium alloy
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
NaCl Concentration, M
Al
Zn
Cu
Ni
Mn
Si
5.65
2.7
0.08
0.01
0.11
0.12
Fig. 1. Corrosion rates obtained from immersion tests as a function of chloride ion concentration.
H. Altun, S. Sen / Materials and Design 25 (2004) 637–643
Namely, with the increase in chloride ion concentration, the rising rate at corrosion rate decreased. That is, the influence of chloride ion concentration was much lower at higher concentrations. In addition, it was seen that, when the chloride ion concentration was increased excessively, the rise at corrosion rate can hesitate, and even the corrosion rate may decrease a little. Fig. 2 shows the corrosion rates obtained from immersion tests against pH in NaCl solutions at five different concentrations (0.01, 0.2, 0.6, 1, and 2 M). At all concentrations, the materials usually exhibited a decrease in corrosion rate with increase in pH. The highest corrosion rate was observed at pH 2. In the neutral pH region, the corrosion rate remained constant approximately, and a comparatively low corrosion rate was observed in alkaline solutions. In addition, in Fig. 2, it was seen that the influence of pH was more at higher concentrations as compared to lower concentrations in neutral and alkaline solutions. Figs. 3 and 4 show typical surface features of the corroded surfaces exposed to the 0.01 M and 2 M NaCl solutions, respectively. When Fig. 3 was investigated, it 50 Na Cl 0.01M
Corrosion Rate, mm/y
40 0.2M 0.6M 30
1M 2M
20
10
0 0
2
4
6
8
10
12
14
pH
Fig. 2. Corrosion rates obtained from immersion tests against pH in NaCl solutions.
639
Fig. 4. Specimens after constant immersion test in 2 M NaCl solution. (a) before corrosion, (b) pH 2, (c) pH 3, (d) pH 8, (e) pH 11, (f) pH 11.5.
was observed that, at lower concentrations, the surface of the specimen was relatively slightly corroded in neutral and alkaline solutions while severely corroded in acidic solutions. In addition, it was seen in Fig. 4 that the specimen surfaces were severely corroded at all pH values at higher chloride concentrations. At pH 2 at all concentrations, the entire surface was attacked. The surface pictures after constant immersion tests are in agreement with corrosion rate measurements. Figs. 5–9 show the effect of pH on the potentiodynamic polarization curves for AZ63 alloy in 0.01, 0.2, 0.6, 1, and 2 M NaCl solutions, respectively. It is seen in Figs. 5–9 that, with the decrease at pH value of the solutions, the anodic curve for the materials usually revealed a shift to higher current density values. The effect of pH in the acidic region was more especially at lower concentrations. The polarization curves of AZ63 alloy are similar to each other, which indicates a similar electrochemical behaviour over pH range investigated. Figs. 10–14 show the effect of chloride ion concentration on the potentiodynamic polarization curves for
-800 0.01M NaCl pH =2
Potential, V (mV)
-1000
pH =3 -1200
pH =8 pH =11
-1400
pH=11. 5
-1600
-1800
-2000 1E -3
1E -2
1E-1
1E +0
1E +1
1E+2
1E +3
Current Density, log i (mA cm -2 )
Fig. 3. Specimens after constant immersion test in 0.01M NaCl solution. (a) before corrosion, (b) pH 2, (c) pH 3, (d) pH 8, (e) pH 11, (f) pH 11.5.
Fig. 5. Potentiodynamic polarization curves for AZ63 alloy in 0.01 M NaCl solution at various pH conditions.
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H. Altun, S. Sen / Materials and Design 25 (2004) 637–643 -800
-800
0.2M NaCl
pH =3 -1200
pH =8 pH =11
-1400
pH=11. 5
-1600
pH =2
-1000
Potential, V (mV)
-1000
Potential, V (mV)
2M NaCl
pH =2
pH =3 -1200
pH =8 pH =11
-1400
pH=11. 5
-1600
-1800
-1800
-2000
-2000 1E -3
1E -2
1E -1
1E +0
1E +1
1E+2
1E -3
1E +3
1E-2
1E -1
1E +0
1E +1
1E +2
1E+3
Current Density, log i (mA cm -2 )
Current Density, log i (mA cm -2 )
Fig. 6. Potentiodynamic polarization curves for AZ63 alloy in 0.2 M NaCl solution at various pH conditions.
Fig. 9. Potentiodynamic polarization curves for AZ63 alloy in 2 M NaCl solution at various pH conditions. -800
-800
pH=2 NaCl
0.6M NaCl
-1200
pH =8 pH =11
-1400
pH=11.5
Potential, V (mV)
Potential, V (mV)
pH =3
0.01M
-1000
pH =2
-1000
-1600
0.2M -1200
0.6M 1M
-1400
2M
-1600
-1800
-1800
-2000
-2000 1E -3
1E -2
1E-1
1E +0
1E +1
1E+2
1E -3
1E +3
1E -2
1E-1
1E +0
1E +1
1E+2
1E +3
Current Density, log i (mA cm -2 )
Current Density, log i (mA cm -2 )
Fig. 7. Potentiodynamic polarization curves for AZ63 alloy in 0.6 M NaCl solution at various pH conditions.
Fig. 10. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 2 at different concentrations. -800
-800
pH =3 NaCl
1M NaCl
pH =8 pH =11
-1400
pH=11. 5
Potential, V (mV)
Potential, V (mV)
pH =3 -1200
0.01M
-1000
pH =2
-1000
0.2M -1200
0.6M 1M
-1400
-1600
-1600
-1800
-1800
2M
-2000
-2000 1E -3
1E -2
1E-1
1E +0
1E +1
1E+2
1E +3
Current Density, log i (mA cm -2 )
1E -3
1E -2
1E-1
1E +0
1E +1
1E+2
1E +3
Current Density, log i (mA cm -2 )
Fig. 8. Potentiodynamic polarization curves for AZ63 alloy in 1 M NaCl solution at various pH conditions.
Fig. 11. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 3 at different concentrations.
AZ63 alloy in NaCl solutions of pH 2, 3, 8, 11, and 11.5, respectively. It is seen in Figs. 10–14 that the anodic curve for the materials usually revealed a shift to higher current density values with the increase in chloride ion concentration of the solutions. Especially the effect of chloride ion concentration was more at lower concen-
trations. The polarization curves of AZ63 alloy are similar to each other, which indicates a similar electrochemical behaviour over concentration range investigated. AZ63 alloy showed no passivity in NaCl solutions over pH and chloride ion concentration studied, which can be observed in the polarization curves.
-800 pH =8 NaCl 0.01M
Potential, V (mV)
-1000
0.2M -1200
0.6M 1M
-1400
2M
-1600
-1800
-2000 1E -3
1E -2
1E-1
1E+0
1E +1
1E+2
Corrosion Current Density, icorr (mA cm -2 )
H. Altun, S. Sen / Materials and Design 25 (2004) 637–643
641
3.50 Na Cl 3.00
0.01M 0.2M
2.50 0.6M 2.00
1M 2M
1.50 1.00 0.50 0.00
1E +3
0
Current Density, log i (mA cm-2 )
2
4
6
8
10
12
14
pH
Fig. 12. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 8 at different concentrations.
Fig. 15. Dependence on pH of icorr measured from the polarization curves. -1250
-800
Na Cl
pH =11 NaCl -1300
Potential, V (mV)
0.2M -1200
0.6M 1M
-1400
2M
-1600
-1800
0.01M
Corrosion Potential, E corr (mV)
0.01M
-1000
-1350
0.2M
-1400
0.6M
-1450
1M 2M
-1500 -1550 -1600 -1650 -1700
-2000 1E -3
1E -2
1E-1
1E+0
1E +1
1E +2
0
1E +3
2
4
6
-800 pH=11. 5 NaCl -1000
0.01M
Potential, V (mV)
0.2M -1200
0.6M 1M
-1400
2M
-1600
-1800
-2000 1E -2
1E-1
1E +0
1E +1
1E+2
10
12
14
1E +3
Current Density, log i (mA cm-2 )
Fig. 16. Dependence on pH of Ecorr measured from the polarization curves.
Corrosion Current Density, i corr (mA cm-2 )
Fig. 13. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 11 at different concentrations.
1E -3
8
pH
Current Density, log i (mA cm-2 )
3.50 Na Cl 3.00
pH =2 pH =3
2.50 pH =8 2.00
pH =11 pH=11. 5
1.50 1.00 0.50 0.00 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
NaCl Concentration (M)
Fig. 14. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 11.5 at different concentrations.
Fig. 17. Dependence on NaCl concentration of icorr measured from the polarization curves.
The cathodic and anodic current density was the highest at pH 2. Corrosion current density (icorr ), which is the measurement of corrosion rate, and corrosion potential (Ecorr ) values were also derived from the polarization
curves. Figs. 15 and 16 show the effect of pH on icorr and Ecorr , respectively, for AZ63 alloy in NaCl solutions of different concentrations. Figs. 17 and 18 show the effect of chloride ion concentration on icorr and Ecorr , respectively, for AZ63 alloy in NaCl solutions of different pH
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H. Altun, S. Sen / Materials and Design 25 (2004) 637–643 -1250 Na Cl
Corrosion Potential, E corr (mV)
-1300 pH =2 -1350
pH =3
-1400
pH =8
-1450
pH =11 pH=11. 5
-1500 -1550 -1600 -1650 -1700 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
NaCl Concentration (M)
Fig. 18. Dependence on NaCl concentration of Ecorr measured from the polarization curves.
values. When Figs. 1 and 2 are compared with Figs. 15 and 17, it is seen that the graphics showing the effect of pH and chloride ion concentration on the corrosion rate obtained from immersion and potentiodynamic polarization tests are compatible with each other. It is seen in Fig. 16 that, with the decrease at pH of the solution, corrosion potential shifted to more negative (more active) values. Fig. 18 shows that, with the increase in chloride ion concentration, corrosion potential usually shifted to more negative (more active) values (except for pH 2). At pH 2, the change in chloride ion concentration did not considerably affect the corrosion potential.
4. Discussion Corrosion and electrochemical behavior of AZ63 alloy were significantly influenced by pH and chloride ions. It was observed that the corrosion rate usually increased with the decrease in pH value. The effect of pH on the corrosion behavior is in agreement with Eh –pH diagram (Pourbaix diagram) of magnesium [2]. The dissolution of magnesium in aqueous solutions proceeds by the reduction of water to produce magnesium hydroxide (Mg(OH)2 ) and hydrogen gas (H2 ). The reduction process was primarily water reduction. These reactions are reported to be insensitive to oxygen concentration [3]. Mg ! Mg2þ þ 2e 2H2 O þ 2e ! H2 þ 2ðOHÞ
It was observed that the corrosion rate usually increased with the increase in concentration of NaCl solution. However, it was observed that, with the increase in chloride ion concentration, the rising rate at corrosion rate decreased. The increase in corrosion rate with increasing chloride ion concentration may be attributed to the participation of chloride ions in the dissolution reaction. Chloride ions are aggressive for both magnesium and aluminum. The adsorption of chloride ions to oxide covered magnesium surface transforms Mg(OH)2 to easily soluble MgCl2 . Corrosion potential shifted to more negative (more active) values with the increase in chloride ion concentration, which may be explained by the adsorption of these ions on the alloy surface at weak parts of the oxide film [6]. Corrosion potential shifted to more negative (more active) values with the decrease in pH value of solution. Higher pH values favour the formation of Mg(OH)2 which protects the alloy from corrosion.
5. Conclusions 1. The decrease in pH of NaCl solution increased the corrosion rate for AZ63 alloy. The highest corrosion rate was observed at pH 2, and the lowest corrosion rate was observed at pH 11.5. The influence of pH was more at higher concentrations as compared to lower concentrations in neutral and alkaline solutions. 2. The increase in chloride ion concentration increased the corrosion rate for AZ63 alloy. However, with the increase in chloride ion concentration, the rising rate at corrosion rate decreased. The influence of chloride ion concentration was lower at higher concentrations. 3. The corrosion potential shifted more negative (more active) values with the decrease in pH. 4. The corrosion potential usually shifted more negative (more active) values with the increase in chloride concentration (except for pH 2). At pH 2, the change in chloride ion concentration did no t considerably affect the corrosion potential.
References
ð1Þ
Mg2þ þ 2ðOHÞ ! MgðOHÞ2
ð2Þ ð3Þ
The equilibrium pH value required for the precipitation of Mg(OH)2 is around 11. Highly acidic solutions are aggressive to both magnesium and aluminum, hence a very high corrosion rate. In magnesium–aluminum alloys, a pH above 9 favour the formation of Mg(OH)2 (depending on the concentration of magnesium) [6].
[1] Hollstein F, Wiedemann R, Scholz J. Surf Coat Tech 2003;162: 261–8. [2] Song G, Atrens A. Adv Eng Mater 1999;1(1):11–33. [3] Makar GL, Kruger J. Int Mater Rev 1993;38(3):138–53. [4] Murray RW, Hillis J. Magnesium finishing: chemical treatment and coating practices. SAE Technical Paper Series #900791, Detroid; 1990. [5] Makar GL, Kruger J. J Electrochem Soc 1990;137(2):414–21. [6] Ambat R, Aung NN, Zhou W. J Appl Electron 2000;30:865–74. [7] Udhayan R, Bhatt DP. J Power Sources 1996;63:103–7. [8] Song G, Atrens A, John DS, Wu X, Nairn J. Corr Sci 1997;39: 1981–2004.
H. Altun, S. Sen / Materials and Design 25 (2004) 637–643 [9] Song G, Atrens A, Wu X, Bo Z, Zhang B. Corr Sci 1998;40:1769– 91. [10] Song G, Atrens A, John DS, Nairn J, Li Y. Corr Sci 1997;39:855– 75. [11] Ambat R, Aung NN, Zhou W. Corr Sci 2000;42:1433–55. [12] Baril G, Pebere N. Corr Sci 2001;43:471–84.
643
[13] Daloz D, Steinmetz P, Michot G. Corrosion 1997;53:944– 54. [14] Khaselev O, Weiss D, Yahalom J. Corr Sci 2001;43:1295–307. [15] Daloz D, Rapin C, Steinmetz P, Michot G. Corrosion 1998;54: 444–50. [16] Khaselev O, Yahalom J. Corr Sci 1998;40:1149–60.
Short Communication
Studies on the influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of AZ63 magnesium alloy Hikmet Altun *, Sadri Sen Department of Mechanical Engineering, Ataturk University, 25240, Erzurum, Turkey Received 28 August 2003; accepted 2 February 2004
Abstract Magnesium-based light alloys belong to a class of structural materials with increasing industrial attention. Magnesium alloys show the lowest density among the engineering metallic materials, low cost and large availability. As a consequence, this light alloys have a promising future. The paper presented reveals the influence of chloride ion concentration and pH on the corrosion and electrochemical behavior of AZ63 magnesium alloy in NaCl solution. The experimental techniques used include immersion studies and potentiodynamic polarization tests. The corrosion rate was very high in highly acidic solutions (pH 2) as compared to that in other solutions. The corrosion rate usually increased with the decrease in pH and the increase in chloride ion concentration. But the quantity of the increase in corrosion rate was different at separate pH and concentration regions. The corrosion potential usually shifted to more negative values with the increase in concentration of chloride ions and the decrease in pH of solution. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: AZ63 Alloy; Corrosion; Chloride ion; Concentration; pH
1. Introduction Magnesium-based light alloys belong to a class of structural materials with increasing industrial attention. Magnesium alloys show the lowest density among the engineering metallic materials, low cost and large availability [1]. As a consequence, this light alloys have a promising future [2]. Because of their low density, high specific strength and stiffness, magnesium alloys have become candidate materials for many applications in microelectronics and in automobile and aerospace industries. The relative density of magnesium is 1.74 g/cm3 , which is 35% lower than that of aluminum, and typical magnesium alloys weigh 35% lower than their aluminum counterparts at equal stiffness [3]. Magnesium and magnesium alloys are non-magnetic, have relatively
*
Corresponding author. Tel.: +90-442-2314839; fax: +90-4422360957. E-mail address: [email protected] (H. Altun). 0261-3069/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2004.02.002
high thermal and electrical conductivity, and good vibration and shock absorption ability [4]. A serious limitation for the potential use of several magnesium alloys is their susceptibility to corrosion. Magnesium alloys, especially those with high purity, have good resistance to atmospheric corrosion. However, the susceptibility to corrosion in chloride containing environments is a serious concern [3]. Alloying magnesium with aluminum in general improves the corrosion resistance [5]. Most of the magnesium–aluminum alloys contain 8–9% aluminum with small amounts of zinc [6]. AZ63 alloy, magnesium and the principal alloying element aluminum, is one of the most widely used magnesium alloys. Recently, the general corrosion and electrochemical corrosion of various magnesium alloys have been studied by various researchers [6–16]. Ambat et al. [6] studied the influence of chloride ion concentration and pH on the corrosion and electrochemical behavior of die-cast and ingot-cast AZ91D alloy. Song et al. [8] investigated the electrochemical behaviour of magnesium in representative
638
H. Altun, S. Sen / Materials and Design 25 (2004) 637–643
chloride and sulfate solutions including NaCl, Na2 SO4 , NaOH and their mixed solutions. Song et al. [9] studied the corrosion behaviour of AZ21, AZ501 and AZ91 magnesium alloys in 1N NaCl at pH 11. Baril and Pebere [12] studied the corrosion behaviour of pure magnesium in aerated and deaerated Na2 SO4 solutions. Daloz et al. [13] studied the corrosion behaviour of rapidly solidified magnesium–aluminum–zinc alloys (8%, 15% and 20% Al, and 1% and 3% Zn). The majority of the researches on magnesium alloys has focused on the effect of processing route and microstructure. To date, there has been no detailed study on the effect of pH and chloride ion concentration of cast AZ63 magnesium alloy. Because AZ63 magnesium alloy may behave differently under varying pH conditions and chloride concentrations, it is important to understand the effect of these parameters in detail on the corrosion behavior of the alloy. Therefore, the present study was aimed at investigating these aspects for AZ63 magnesium alloy.
2. Experimental details 2.1. Material
sured after each experiment and the corrosion rate was calculated in millimeters per year. The specimens were weighed on an analytical balance to an accuracy of 0.1mg. The specimens were taken photos after removing their corrosion products. 2.4. Electrochemical testing Electrochemical polarization experiments were carried out using a potentiostat, Potentioscan Wenking POS 73. The electrodes for this purpose were prepared by connecting a wire to one side of the sample that was covered with cold setting resin. One side of the specimen, whose area is 1 cm2 , was exposed to the solution. The specimens were treated with metallographic polishing prior to each experiment, followed by washing distilled water and acetone, and finally dried in warm air. The polarization measurements were carried out in corrosion cell containing 500 ml solution. The electrochemical cell consists of AZ63 alloy as the working electrode, a saturated calomel reference electrode, and a platinum counter electrode. The specimens were immersed in the test solution, and a polarization scan was carried out towards more noble values at a rate of 1 mV/s, after allowing a steady state potential to develop.
In this study, AZ63 magnesium alloy was used. The chemical composition of the alloy is given in Table 1. 3. Results 2.2. Corrosive media The effect of chloride ion concentration and pH on the corrosion and electrochemical behavior of AZ63 alloy was studied in NaCl solutions at different concentrations (0.01, 0.2, 0.6, 1 and 2 M) and pH values (2, 3, 8, 11 and 11.5). All the experiments were conducted at the same temperature of 28 0.5 °C. The solution was prepared using AR grade NaCl and distilled water. The pH value of the solution was adjusted to the desired value with HCl and NaOH.
Fig. 1 shows the corrosion rates obtained from immersion tests as a function of chloride ion concentration for the alloy at five pH values (2, 3, 8, 11, and 11.5). At all pH values, the materials usually exhibited a rise in corrosion rate with increase in chloride ion concentration. But the quantity of this rise was different. In such a way that, the change at chloride ion concentration at lower concentrations affected the corrosion rate much more as compared to that of higher concentrations.
2.3. Constant immersion testing
50 Na Cl pH =2
Corrosion Rate, mm/y
For constant immersion testing, the specimens were polished successively on finer grades of emery papers up to 800 level. The polished and preweight specimens have been exposed to the solution (500 ml) for 72 h. At the end of the experiment, cleaning of the specimens was carried out by dipping in a solution of 15% CrO3 + 1% Ag2 CrO4 in 100 ml water under boiling conditions. An acetone wash followed this. The weight loss was mea-
40 pH =3 pH =8 30
pH =11 pH=11. 5
20
10
0
Table 1 The chemical composition of AZ63 magnesium alloy
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
NaCl Concentration, M
Al
Zn
Cu
Ni
Mn
Si
5.65
2.7
0.08
0.01
0.11
0.12
Fig. 1. Corrosion rates obtained from immersion tests as a function of chloride ion concentration.
H. Altun, S. Sen / Materials and Design 25 (2004) 637–643
Namely, with the increase in chloride ion concentration, the rising rate at corrosion rate decreased. That is, the influence of chloride ion concentration was much lower at higher concentrations. In addition, it was seen that, when the chloride ion concentration was increased excessively, the rise at corrosion rate can hesitate, and even the corrosion rate may decrease a little. Fig. 2 shows the corrosion rates obtained from immersion tests against pH in NaCl solutions at five different concentrations (0.01, 0.2, 0.6, 1, and 2 M). At all concentrations, the materials usually exhibited a decrease in corrosion rate with increase in pH. The highest corrosion rate was observed at pH 2. In the neutral pH region, the corrosion rate remained constant approximately, and a comparatively low corrosion rate was observed in alkaline solutions. In addition, in Fig. 2, it was seen that the influence of pH was more at higher concentrations as compared to lower concentrations in neutral and alkaline solutions. Figs. 3 and 4 show typical surface features of the corroded surfaces exposed to the 0.01 M and 2 M NaCl solutions, respectively. When Fig. 3 was investigated, it 50 Na Cl 0.01M
Corrosion Rate, mm/y
40 0.2M 0.6M 30
1M 2M
20
10
0 0
2
4
6
8
10
12
14
pH
Fig. 2. Corrosion rates obtained from immersion tests against pH in NaCl solutions.
639
Fig. 4. Specimens after constant immersion test in 2 M NaCl solution. (a) before corrosion, (b) pH 2, (c) pH 3, (d) pH 8, (e) pH 11, (f) pH 11.5.
was observed that, at lower concentrations, the surface of the specimen was relatively slightly corroded in neutral and alkaline solutions while severely corroded in acidic solutions. In addition, it was seen in Fig. 4 that the specimen surfaces were severely corroded at all pH values at higher chloride concentrations. At pH 2 at all concentrations, the entire surface was attacked. The surface pictures after constant immersion tests are in agreement with corrosion rate measurements. Figs. 5–9 show the effect of pH on the potentiodynamic polarization curves for AZ63 alloy in 0.01, 0.2, 0.6, 1, and 2 M NaCl solutions, respectively. It is seen in Figs. 5–9 that, with the decrease at pH value of the solutions, the anodic curve for the materials usually revealed a shift to higher current density values. The effect of pH in the acidic region was more especially at lower concentrations. The polarization curves of AZ63 alloy are similar to each other, which indicates a similar electrochemical behaviour over pH range investigated. Figs. 10–14 show the effect of chloride ion concentration on the potentiodynamic polarization curves for
-800 0.01M NaCl pH =2
Potential, V (mV)
-1000
pH =3 -1200
pH =8 pH =11
-1400
pH=11. 5
-1600
-1800
-2000 1E -3
1E -2
1E-1
1E +0
1E +1
1E+2
1E +3
Current Density, log i (mA cm -2 )
Fig. 3. Specimens after constant immersion test in 0.01M NaCl solution. (a) before corrosion, (b) pH 2, (c) pH 3, (d) pH 8, (e) pH 11, (f) pH 11.5.
Fig. 5. Potentiodynamic polarization curves for AZ63 alloy in 0.01 M NaCl solution at various pH conditions.
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H. Altun, S. Sen / Materials and Design 25 (2004) 637–643 -800
-800
0.2M NaCl
pH =3 -1200
pH =8 pH =11
-1400
pH=11. 5
-1600
pH =2
-1000
Potential, V (mV)
-1000
Potential, V (mV)
2M NaCl
pH =2
pH =3 -1200
pH =8 pH =11
-1400
pH=11. 5
-1600
-1800
-1800
-2000
-2000 1E -3
1E -2
1E -1
1E +0
1E +1
1E+2
1E -3
1E +3
1E-2
1E -1
1E +0
1E +1
1E +2
1E+3
Current Density, log i (mA cm -2 )
Current Density, log i (mA cm -2 )
Fig. 6. Potentiodynamic polarization curves for AZ63 alloy in 0.2 M NaCl solution at various pH conditions.
Fig. 9. Potentiodynamic polarization curves for AZ63 alloy in 2 M NaCl solution at various pH conditions. -800
-800
pH=2 NaCl
0.6M NaCl
-1200
pH =8 pH =11
-1400
pH=11.5
Potential, V (mV)
Potential, V (mV)
pH =3
0.01M
-1000
pH =2
-1000
-1600
0.2M -1200
0.6M 1M
-1400
2M
-1600
-1800
-1800
-2000
-2000 1E -3
1E -2
1E-1
1E +0
1E +1
1E+2
1E -3
1E +3
1E -2
1E-1
1E +0
1E +1
1E+2
1E +3
Current Density, log i (mA cm -2 )
Current Density, log i (mA cm -2 )
Fig. 7. Potentiodynamic polarization curves for AZ63 alloy in 0.6 M NaCl solution at various pH conditions.
Fig. 10. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 2 at different concentrations. -800
-800
pH =3 NaCl
1M NaCl
pH =8 pH =11
-1400
pH=11. 5
Potential, V (mV)
Potential, V (mV)
pH =3 -1200
0.01M
-1000
pH =2
-1000
0.2M -1200
0.6M 1M
-1400
-1600
-1600
-1800
-1800
2M
-2000
-2000 1E -3
1E -2
1E-1
1E +0
1E +1
1E+2
1E +3
Current Density, log i (mA cm -2 )
1E -3
1E -2
1E-1
1E +0
1E +1
1E+2
1E +3
Current Density, log i (mA cm -2 )
Fig. 8. Potentiodynamic polarization curves for AZ63 alloy in 1 M NaCl solution at various pH conditions.
Fig. 11. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 3 at different concentrations.
AZ63 alloy in NaCl solutions of pH 2, 3, 8, 11, and 11.5, respectively. It is seen in Figs. 10–14 that the anodic curve for the materials usually revealed a shift to higher current density values with the increase in chloride ion concentration of the solutions. Especially the effect of chloride ion concentration was more at lower concen-
trations. The polarization curves of AZ63 alloy are similar to each other, which indicates a similar electrochemical behaviour over concentration range investigated. AZ63 alloy showed no passivity in NaCl solutions over pH and chloride ion concentration studied, which can be observed in the polarization curves.
-800 pH =8 NaCl 0.01M
Potential, V (mV)
-1000
0.2M -1200
0.6M 1M
-1400
2M
-1600
-1800
-2000 1E -3
1E -2
1E-1
1E+0
1E +1
1E+2
Corrosion Current Density, icorr (mA cm -2 )
H. Altun, S. Sen / Materials and Design 25 (2004) 637–643
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3.50 Na Cl 3.00
0.01M 0.2M
2.50 0.6M 2.00
1M 2M
1.50 1.00 0.50 0.00
1E +3
0
Current Density, log i (mA cm-2 )
2
4
6
8
10
12
14
pH
Fig. 12. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 8 at different concentrations.
Fig. 15. Dependence on pH of icorr measured from the polarization curves. -1250
-800
Na Cl
pH =11 NaCl -1300
Potential, V (mV)
0.2M -1200
0.6M 1M
-1400
2M
-1600
-1800
0.01M
Corrosion Potential, E corr (mV)
0.01M
-1000
-1350
0.2M
-1400
0.6M
-1450
1M 2M
-1500 -1550 -1600 -1650 -1700
-2000 1E -3
1E -2
1E-1
1E+0
1E +1
1E +2
0
1E +3
2
4
6
-800 pH=11. 5 NaCl -1000
0.01M
Potential, V (mV)
0.2M -1200
0.6M 1M
-1400
2M
-1600
-1800
-2000 1E -2
1E-1
1E +0
1E +1
1E+2
10
12
14
1E +3
Current Density, log i (mA cm-2 )
Fig. 16. Dependence on pH of Ecorr measured from the polarization curves.
Corrosion Current Density, i corr (mA cm-2 )
Fig. 13. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 11 at different concentrations.
1E -3
8
pH
Current Density, log i (mA cm-2 )
3.50 Na Cl 3.00
pH =2 pH =3
2.50 pH =8 2.00
pH =11 pH=11. 5
1.50 1.00 0.50 0.00 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
NaCl Concentration (M)
Fig. 14. Potentiodynamic polarization curves for AZ63 alloy in NaCl solutions of pH 11.5 at different concentrations.
Fig. 17. Dependence on NaCl concentration of icorr measured from the polarization curves.
The cathodic and anodic current density was the highest at pH 2. Corrosion current density (icorr ), which is the measurement of corrosion rate, and corrosion potential (Ecorr ) values were also derived from the polarization
curves. Figs. 15 and 16 show the effect of pH on icorr and Ecorr , respectively, for AZ63 alloy in NaCl solutions of different concentrations. Figs. 17 and 18 show the effect of chloride ion concentration on icorr and Ecorr , respectively, for AZ63 alloy in NaCl solutions of different pH
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H. Altun, S. Sen / Materials and Design 25 (2004) 637–643 -1250 Na Cl
Corrosion Potential, E corr (mV)
-1300 pH =2 -1350
pH =3
-1400
pH =8
-1450
pH =11 pH=11. 5
-1500 -1550 -1600 -1650 -1700 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
NaCl Concentration (M)
Fig. 18. Dependence on NaCl concentration of Ecorr measured from the polarization curves.
values. When Figs. 1 and 2 are compared with Figs. 15 and 17, it is seen that the graphics showing the effect of pH and chloride ion concentration on the corrosion rate obtained from immersion and potentiodynamic polarization tests are compatible with each other. It is seen in Fig. 16 that, with the decrease at pH of the solution, corrosion potential shifted to more negative (more active) values. Fig. 18 shows that, with the increase in chloride ion concentration, corrosion potential usually shifted to more negative (more active) values (except for pH 2). At pH 2, the change in chloride ion concentration did not considerably affect the corrosion potential.
4. Discussion Corrosion and electrochemical behavior of AZ63 alloy were significantly influenced by pH and chloride ions. It was observed that the corrosion rate usually increased with the decrease in pH value. The effect of pH on the corrosion behavior is in agreement with Eh –pH diagram (Pourbaix diagram) of magnesium [2]. The dissolution of magnesium in aqueous solutions proceeds by the reduction of water to produce magnesium hydroxide (Mg(OH)2 ) and hydrogen gas (H2 ). The reduction process was primarily water reduction. These reactions are reported to be insensitive to oxygen concentration [3]. Mg ! Mg2þ þ 2e 2H2 O þ 2e ! H2 þ 2ðOHÞ
It was observed that the corrosion rate usually increased with the increase in concentration of NaCl solution. However, it was observed that, with the increase in chloride ion concentration, the rising rate at corrosion rate decreased. The increase in corrosion rate with increasing chloride ion concentration may be attributed to the participation of chloride ions in the dissolution reaction. Chloride ions are aggressive for both magnesium and aluminum. The adsorption of chloride ions to oxide covered magnesium surface transforms Mg(OH)2 to easily soluble MgCl2 . Corrosion potential shifted to more negative (more active) values with the increase in chloride ion concentration, which may be explained by the adsorption of these ions on the alloy surface at weak parts of the oxide film [6]. Corrosion potential shifted to more negative (more active) values with the decrease in pH value of solution. Higher pH values favour the formation of Mg(OH)2 which protects the alloy from corrosion.
5. Conclusions 1. The decrease in pH of NaCl solution increased the corrosion rate for AZ63 alloy. The highest corrosion rate was observed at pH 2, and the lowest corrosion rate was observed at pH 11.5. The influence of pH was more at higher concentrations as compared to lower concentrations in neutral and alkaline solutions. 2. The increase in chloride ion concentration increased the corrosion rate for AZ63 alloy. However, with the increase in chloride ion concentration, the rising rate at corrosion rate decreased. The influence of chloride ion concentration was lower at higher concentrations. 3. The corrosion potential shifted more negative (more active) values with the decrease in pH. 4. The corrosion potential usually shifted more negative (more active) values with the increase in chloride concentration (except for pH 2). At pH 2, the change in chloride ion concentration did no t considerably affect the corrosion potential.
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ð1Þ
Mg2þ þ 2ðOHÞ ! MgðOHÞ2
ð2Þ ð3Þ
The equilibrium pH value required for the precipitation of Mg(OH)2 is around 11. Highly acidic solutions are aggressive to both magnesium and aluminum, hence a very high corrosion rate. In magnesium–aluminum alloys, a pH above 9 favour the formation of Mg(OH)2 (depending on the concentration of magnesium) [6].
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