Materials and Corrosion 2007, 58, No. 12
DOI: 10.1002/maco.200704091
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Corrosion and corrosion testing of magnesium alloys S. Bender*, J. Goellner, A. Heyn and E. Boese
The corrosion behaviour of magnesium alloys is not substantially comparable to other metals, such as iron, nickel and copper. It is always accompanied by hydrogen evolution. More hydrogen is evolved at a more positive potential or a higher anodic current density. The ‘strange’ hydrogen evolution behaviour is a common phenomenon for magnesium alloys and it is called negative difference effect (NDE). The NDE continues to receive considerable discussion. Furthermore, the corrosion behaviour of magnesium alloys depends mainly on the pH value of the surrounding electrolyte. Voluminous reaction products, formed in neutral electrolytes, lead to a diffusion-controlled dissolution on the surface of the underlying magnesium alloy. Therefore, influences from structure and alloying are suppressed very strongly. In alkaline environments, passivation
occurs as a result of the formation of a hydroxide layer on the magnesium surface. Therefore, differences in the corrosion behaviour between the alloys are hardly detectable. Measurable effects can only be detected using very ‘aggressive’ corrosion conditions. Present methods do not adequately take into account the specific character of the corrosion of magnesium alloys. It can be better characterized using a rotating disc electrode for electrochemical measurements, which enables model defined flow conditions on the surface. Furthermore, the application of electrochemical noise offers the possibility of a simple and sensitive assessment of the corrosion susceptibility of magnesium alloys. Due to the high sensitivity of this measurement procedure, it is also possible to carry out examinations under more practical conditions.
1 Unusual features of magnesium corrosion
electron- consuming process in the hydrogen evolution. The ‘strange’ hydrogen evolution behaviour is a common phenomenon for magnesium alloys and is called the ‘negative difference effect’ (NDE). Several models for the explanation of this phenomenon are given in the literature [7]. However, there are still a lot of questions. Resulting from own examinations and considerations, the following explanation of the NDE arises for us [8]. Magnesium is produced with a great effort of energy. For instance, 23 520 kJ/kg are necessary for the extraction of magnesium from magnesium oxide. Since matter always tends to a low energetically condition, the dwell time of the metallic state is restricted. If the metal is in contact with water it immediately sends ions into the solution, as shown below
The corrosion behaviour of magnesium alloys depends on a variety of factors, mainly on the pH values (pH of the surrounding medium and pH near the metal surface) and on the surface film (in dependence of the alloy composition). The type and distribution of the intermetallic phases are also important. The publications of Song and Atrens [1], Kainer [2], Hansen [3], Makar and Kruger [4], Haferkamp et al. [5] and Virtanen et al. [6] describe the essential aspects of the corrosion of magnesium alloys such as the type of corrosion, the corrosion mechanism and influences of the surrounding medium. If the pH is above 12, a stable and self-healing passive layer develops, which is responsible for the high corrosion resistance. The layer varies between a stable and an unstable state for a pH in a range of 10–11. Because of the active dissolution, the hydrogen development increases with decreasing pH values. Relative thick layers of loosely adhering corrosion products are developed, which appear like passivity. The corrosion rate is controlled by diffusion and depends mainly on the layer thickness. During the dissolution and hydrogen development, the pH rises in the area near the surface. The result is a change in the formation of the layer and a higher stability. On magnesium and magnesium alloys, an unusual feature appears which does not have to be noticed on other metals during corrosion examinations with external polarization. Magnesium alloys exhibit a ‘strange’ hydrogen evolution behaviour because more hydrogen is evolved at a more positive potential or higher anodic current density. The weight loss calculated with Faraday’s law is lower than the gravimetric examined weight loss. The reason is the
S. Bender, J. Goellner, A. Heyn, E. Boese Otto-von-Guericke-Universita¨t Magdeburg, IWF, P.O. Box 4120, D-39016 Magdeburg (Germany) E-mail:
[email protected]
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Mg ! Mgþþ þ 2e
(1)
The electrons remain free in the metal. The reaction will stop as the electrostatic force (positive ions in the medium, negative charge carrier in the metal) prevents a charge separation. An electron-consuming process is necessary for the further solution of the metal. The electron consumption on magnesium happens by the unloading of hydrogen ions (acidic corrosion), as can be seen in 2Hþ þ 2e ! H2
(2)
The source of the hydrogen ions (more exactly the hydronium ion H3Oþ) is the dissociated water, as shown below H2 O ! OH þ Hþ
(3)
The concentration of the hydrogen ions in neutral water (pH ¼ 7) is 107 mol. Because of secondary reactions, magnesium ions react with the hydroxide ions as seen in Mgþþ þ 2OH ! MgðOHÞ2
ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
(4)
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Materials and Corrosion 2007, 58, No. 12
Now, the equilibrium between water and its ions is disturbed and the reaction becomes stronger in one direction (principle of Le Chatelier) as seen in H2 O ! ðOH ÞMg þ Hþ
(5)
An acid is produced which discharges the free electrons and the metal dissolution is continued. That means for the process, the more magnesium ions go into the solution, the stronger is the water dissociation and the more electrons are consumed by hydrogen ions. The dissolution of magnesium increases rapidly. In addition, electrons are consumed while the recording of current density–potential curves obtained by electrochemical techniques and the magnesium dissolution increases. This further disturbs the non-equilibrium of the dissociation and an increasing hydrogen evolution can be noticed. The corrosion of magnesium alloys depends on two cathodic processes. One can be measured as a flowing current on the ammeter and the other can be seen by the formation of gas bubbles. The use of electrochemical techniques makes it possible to distinguish and compare different magnesium alloys from each other [9]. The NDE should be regarded as common in the corrosion of magnesium. Furthermore, the NDE decreases if the corrosion resistance of magnesium alloys increases. Magnesium alloys with a high corrosion resistance send less magnesium ions into the solution and therefore, the hydrogen evolution rate decreases. Particularly, corrosion rates estimated by Tafel extrapolation are different from that estimated by the weight-loss measurement. But electrochemical techniques also show other characteristic data (potential, current density and potential noise or current noise), which allow statements about the influence of alloying elements. The advantage of these methods is its ease of obtaining an instantaneous corrosion rate. So, it can be used to monitor the change of corrosion rate in real time rather than an average rate. Results of the corrosion investigations of magnesium alloys using polarization measurements in combination with the rotating disc electrode (RDE) and electrochemical noise measurements are presented as follows.
2.2 Polarization measurements The polarization measurements were carried out with a standard three-electrode set-up in the NaCl solution (0.01 M, 23 8C). The working electrode was built as an RDE. A saturated silver/silver chloride (EH ¼ 197 mV) electrode was used as a reference and platinum as a counter electrode. The polarization curves were scanned at a rate of 60 mV/min within a scanning range from 2000 mV to about 0 mV. Once the current density reached a predefined threshold of 50 A/m2, the measurement was stopped. It is important that the freshly ground working electrode is immediately dipped in the electrolyte to minimise the formation of surface layers. 2.3 Electrochemical noise measurements Electrochemical noise investigations were performed to determine the potential, potential noise, the current and current noise. A specific experimental set-up was used for the electrochemical noise measurements [10]. These measurements were carried out in a three-electrode cell consisting of two working electrodes, which are macroscopically identical specimens (same size and preparation), and a reference electrode (silver/silver chloride). Due to sensitivity of the measurements, a faraday cage was used to suppress external influences. The experiments were carried out with a zeroresistance ammeter (ZRA) and a high-impedance potential measuring device. The direct parts of the current and potential were filtered by a low-pass filter (cut-off frequency of 1 Hz). The alternating parts of the current and potential were filtered by a band-pass filter resulting in an effective frequency range of 0.1–10 Hz. The sampling rate of the 14-bit data acquisition was 20 samples per second due to the cut-off frequency of the filter. All experiments were carried out in a 0.05 M sodium sulphate solution to avoid the effect of chloride ions at different pH values. One common method for the quantification of noise data is the calculation of the standard deviation (S) over fixed periods and the further calculation of a noise resistance (RN ¼ SE/Si). Another examination method is the charge (Q), which can be calculated by integration of the rectified current noise over fixed time intervals [11].
2 Materials and experimental methods
3 Results
2.1 Specimen
3.1 Polarization measurements
For the investigations of the corrosion behaviour, the following magnesium alloys were available as shown in Table 1. It can be seen that the concentration of iron (Fe ¼ max. 0.004%) and copper (Cu ¼ max. 0.025%) in AM20 and AM20 þ Li is higher than the maximal permissible value.
For the development of a magnesium-specific corrosion testing method, the commonly used polarization measurement method in combination with RDE had to be modified. To achieve this, different test parameters were varied to determine their influence on the corrosion behaviour of
Table 1. Chemical composition of investigated magnesium alloys (manufactures data) Alloy type Diecast Diecast Diecast Diecast Wrought alloy Diecast Pig casting
Alloy
% Al
% Zn
% Mn
% Si
% Fe
% Cu
AM20 AM20Li59 AM20Li110 AM20Li123 AZ31 AZ91 D AZ91 PC
2.37 0.8 0.64 0.70 2.36 9.3 9.1
0.12 0.37 0.28 0.27 0.89 0.79 0.81
0.365 0.165 0.172 0.184 0.210 0.120 0.125
0.006 0.015 0.015 0.017 0.024 0.020 0.049
0.005 0.034 0.018 0.016 0.004 0.004 0.002
0.022 0.041 0.028 0.027 0.005 0.007 0,009
% Li 9.83 12.40 11.52
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Materials and Corrosion 2007, 58, No. 12
Corrosion testing of magnesium alloys
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Table 2. Results of Mg-Al-Zn alloys investigated with optimized parameters (2000 rpm, pH ¼ 9, Ar, 0.01 N NaCl)
Ecorr (mVAg/AgCl) icorr (A/m2)
AZ91 DC
AZ91 PC
AZ31
1580 5 0.008 0.002
1570 15 0.009 0.001
1470 15 0.014 0.001
Table 3. Results of Mg-Al-Mn alloys investigated with optimized parameters (2000 rpm, pH ¼ 9, Ar, 0.01 N NaCl)
Ecorr (mVAg/AgCl) icorr (A/m2)
AM20
AM20 þ Li
1540 20 0.010 0.001
1620 10 0.018 0.002
magnesium alloys. After preliminary investigations, the following test parameters for polarization measurements were found to be most suitable. A rotation speed of 2000 rounds per minute was chosen because at this speed a stable flow field develops which removes the reaction products and therefore causes more reproducible corrosion data. Aqueous electrolytes with pH ¼ 9 were chosen to slow the dissolution rate (slower than at pH ¼ 7) and prevent the formation of dense layers on the surface of the magnesium alloys (which occurs at higher pH values). During the polarization measurements, argon was bubbled into the electrolyte to avoid an adsorption of carbon dioxide from ambient air and, consequently, a decrease in the pH value. Subsequently, the corrosion behaviour of different magnesium alloys was tested under those optimized conditions.
In Table 2, the positions of free corrosion potential Ecorr and current density icorr determined from Tafel plots from different Mg-Al-Zn alloys are illustrated. AZ91 DC has the lowest free corrosion potential (1580 mVAg/AgCl), followed by AZ91 PC (1570 mVAg/AgCl) and AZ31 (1470 mVAg/AgCl). It can be seen that the values of icorr are not significantly different for AZ91 D (0.008 A/m2) and AZ91 PC (0.009 A/m2), but AZ31 shows the highest current density with approximately 0.014 A/m2. In Table 3, the positions of free corrosion potential Ecorr and current density icorr determined from Tafel plots from different Mg-Al-Zn alloys are illustrated. AM20 modified with Li shows a higher current density of approximately 0.018 A/m2 as compared to AM20 with, approximately 0.010 A/m2. The values of the current density indicate that AM20 has a better corrosion behaviour compared to AM20 with Li. 3.2 Electrochemical noise measurements The materials tested in Section 3.1 were also examined by electrochemical noise measurements. The electrochemical current noise (ECN) and electrochemical potential noise
Fig. 1. Electrochemical potential and the current noise of three Mg-Al-Zn alloys under open circuit conditions over 30 min in sodium sulphate (0.05 M, pH ¼ 9); (a) AZ91 D: Qi ¼ 2.89 mAs, RN ¼ 2390 V, (b) AZ91 PC: Qi ¼ 6.92 mAs, RN ¼ 1971 V, (c) AZ31 wrought alloy: Qi ¼ 2,94 mAs, RN ¼ 1923 V www.wiley-vch.de/home/wuk
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Materials and Corrosion 2007, 58, No. 12
Fig. 2. Electrochemical potential and the current noise of three Mg-Al-Mn alloys under open circuit conditions in sodium sulphate (0.05 M, pH ¼ 12); (a) AM20Li123: Qi ¼ 11.8 mAs, RN ¼ 1764 V, (b) AM20Li110: Qi ¼ 26.1 mAs, RN ¼ 1481 V, (c) AM20Li59: Qi ¼ 74.4 mAs, RN ¼ 2513 V; the Fe concentration at the AM20 and AM20 þ Li alloys increases from (a) to (c)
(EPN) were measured under open circuit conditions in a 0.05 M Sodium sulphate solution without chlorides. In Fig. 1, EPN and ECN of the three Mg-Al-Zn alloys are illustrated. AZ91 D (Fig. 1(a)) has the finest-grained microstructure and the highest Al concentration of the three tested Mg-Al-Zn alloys. Therefore, the current noise is less intensive and the respective charge Qi of 2.89 mAs is the lowest value. AZ91 PC (Fig. 1(b)) has nominally the same elemental composition as AZ91 D but the grain size is larger. It can be seen that the potential noise and current noise has higher intensities and the resulting charge increases to 6.92 mAs. AZ31 (Fig. 1(c)) is a wrought alloy and contains only 3% Al. The current noise is slightly higher compared to
AZ91 D and lower than that for AZ91 PC. The resulting charge of AZ31 (2.94 mAs) is comparable to the charge of AZ91 D (2.89 mAs). In Fig. 2, the EPN and ECN of Mg-AlMn alloys with different lithium contents are illustrated. As can be seen in Figs. 2(a)–(c), the potential noise and the current noise obviously increase with increasing Fe concentration. Specimens of AZ31 were covered with defined conversion layers (Alodine1 layers) and were investigated at different pH values to obtain information about their corrosion behaviour under these conditions. The results are shown in Figs. 3 and 4. The dependence of the charge on the pH value of AZ31 without coating and AZ31 þ Alodine is shown in
Fig. 3. Dependence of the charge on the pH value of AZ31 and Alodine coated AZ31 in 0.05 M Na2SO4
Fig. 4. Dependence of the noise resistance on the pH value at AZ31 and Alodine coated AZ31 in 0.05 M Na2SO4 www.wiley-vch.de/home/wuk
Materials and Corrosion 2007, 58, No. 12
Fig. 3. The charge of AZ31 þ Alodine has the highest value at pH ¼ 7 (2.9 mAs). In weak alkaline solutions, the charge decreases to a lower value (0.8 mAs) and in strong alkaline solutions, the charge (0.7 mAs) has approximately the same value. Regarding the charge values of the uncoated AZ31, the Alodine coating shows lower charge values. In Fig. 4, the dependence of noise resistance on the pH value at AZ31 and AZ31 þ Alodine is shown in a semi-logarithmic scale. In a neutral solution (pH ¼ 7), AZ31 with Alodine coating has a noise resistance of about 65 kV. With increasing pH value of the electrolyte, the noise resistance increases to higher values. In weak alkaline solutions, the noise resistance has a value of about 239 kV. However, if the pH increases to 12, the noise resistance increases to over 1 GV. In comparison to the magnesium alloy AZ31 without coating, the AZ31 þ Alodine layer reaches at all pH values higher noises resistances.
4 Discussion It has been reported that magnesium has an abnormal polarization behaviour because of the ‘strange’ hydrogen evolution. The basic feature of the ‘strange’ hydrogen phenomenon is that hydrogen evolution increases as the polarization current density or the potential becomes more positive. This paper shows a current idea about the magnesium NDE and describes the phenomenology of the NDE. The more magnesium ions go into solution, the stronger the water dissociation and the more electrons are consumed by hydrogen ions. The presented polarization measurements with the RDE show reproducible results. The advantage of polarization measurements with the RDE compared to the typically used salt spray test is its ease and the possibility of obtaining an instantaneous corrosion rate. Regarding the current density, polarization measurements with the RDE are suited to characterize different magnesium alloys. The corrosion behaviour of the available alloys in 0.01 M NaCl can be distinguished under the optimized testing conditions (pH ¼ 9, Argon and 2000 rounds per minute). The cast alloy AZ91 D (with a current density of 0.008 A/m2) shows the lowest current density followed by AZ91 PC (0.009 A/m2) and AZ31 wrought alloy (0.012 A/m2). The cast alloy AM20 has a current density of approximately 0.010 A/m2. The Mg-Al-Mn alloys with lithium show higher values of the current density (approximately 0.018 A/m2) than the pure AM20. The high Fe concentration of AM20 þ Li can be seen as a major influence factor. All tested Mg-Al-Mn-Li alloys are not high purity alloys. This is the reason that no possible influence of lithium concentration on the corrosion behaviour of Mg-Al-Mn-Li alloys could be determined. However, the anodic current density increases most for the alloy AM20 modified with lithium, which has the lowest aluminium concentration (2%) and the highest Fe concentration (
