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Influence of pH and chloride ion concentration on the corrosion of Mg alloy ZE41
Corrosion Science 50 (2008) 3168–3178
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Influence of pH and chloride ion concentration on the corrosion of Mg alloy ZE41 Ming-Chun Zhao a,b,c, Ming Liu a,b, Guang-Ling Song a, Andrej Atrens a,b,* a b c
Division of Materials, The University of Queensland, Brisbane, Qld 4072, Australia Swiss Federal Laboratories for Materials Science and Technology, EMPA, Department 136, Überlandstrasse 129, CH-8600 Dubendorf, Switzerland School of Material Science and Engineering, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 4 June 2008 Accepted 14 August 2008 Available online 26 August 2008 Keywords: Mg alloy ZE41 Corrosion pH Chloride ion concentration Surface film
a b s t r a c t The influence of pH and chloride ion concentration on the corrosion behaviour of ZE41 was studied using immersion tests and electrochemical measurements. A shorter incubation period to the onset of corrosion; a more negative corrosion potential; and a higher corrosion rate correlated with a higher chloride ion concentration at each pH value and correlated with a lower pH value for each chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film, with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown and the known instability of Mg(OH)2 in solutions with pH less than 10.5. The electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Mg alloys are of great interest in transport applications, especially for automobiles, due to their low density and adequate strength/weight ratio. However, a critical limitation for the service application of Mg alloys is their susceptibility to corrosion [1–3] and hence there continues to be much research to understand and document the various aspects of the corrosion of Mg alloys [4–33]. Stress corrosion cracking (SCC) of common Mg alloys occurs for frequently encountered environments [34–43] so SCC may occur for stressed auto components subjected to road splash; the fact that SCC of common Mg alloys occurs in distilled water [34–39] indicates that water itself is the key environment factor causing SCC in aqueous solutions. The key points of the generally accepted corrosion mechanism for Mg alloys [1–3] in common environments like 3% NaCl solution are as follows. A partially protective film covers the surface and corrosion occurs at the breaks in this film (i.e. at film free areas). The cathodic reaction is hydrogen liberation. A step in the anodic reaction is the production of the uni-positive Mg ion, Mg+, and a fraction undergoes chemical reaction with water to liberate hydrogen [1–3]; thus, hydrogen is produced both chemically and as the cathodic partial reaction. The surface film on Mg is not particularly * Corresponding author. Address: Division of Materials, The University of Queensland, Brisbane, Qld 4072, Australia. Tel.: +61 7 3365 3748; fax: +61 7 3365 3888. E-mail address: [email protected] (A. Atrens). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.08.023
protective; as a result the corrosion rate is typically more than 1 mm/y in common environments like 3% NaCl solution; no alloying element has been found that produces a solid solution Mg alloy with a corrosion rate less than that of pure Mg in 3% NaCl solution (which typically has a corrosion rate of 1 mm/y). Consequently, it is useful to include pure Mg as a standard in any comparative study of corrosion performance of Mg alloys (see e.g. [2,7,8,45]). Moreover, it is indeed extremely important to use pure Mg as the standard for comparison; commercial purity Mg can have a corrosion rate more than 50 times that of pure Mg [2] and so ‘‘commercial purity Mg” should be designated as ‘‘low purity Mg” for clarity. Corrosion of common commercial Mg alloys typically involves microgalvanic acceleration of the corrosion of the alpha matrix by the second phase(s) [2,7,8,44,45] and corrosion rates are equal to or typically greater than that of pure Mg. The corrosion rate of a two phase Mg alloy can, however, be lower than that of pure Mg for an appropriate distribution of the second phase in the alloy (or at least at the alloy surface) [2,25,44]: namely, if the second phase is finely divided, essentially continuous and the second phase itself has a lower corrosion rate than that of pure Mg, then the second phase can act as a corrosion barrier. The corrosion form of magnesium is typically designated as localised corrosion [1,2,48,49] to distinguish it from the auto-catalytic pitting in stainless steels. Localised corrosion in magnesium typically initiates as irregular localised corrosion, which spread laterally and cover the whole surface. There does not seem to be much tendency for deep pitting. The most probable reason is that
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the cathodic reaction is the liberation of hydrogen. A by product of that cathodic reaction is the production of OH ions (or equivalently the consumption of H+ ions) with a concomitant increase of the pH and stabilisation of the local magnesium hydroxide film and a decrease in corrosion tendency. Thus localised corrosion in magnesium has an inherent tendency to be self-limiting. This is in marked contrast to the situation for stainless steels, where the occluded pit cell becomes more aggressive and accelerates the localised corrosion. Corrosion of magnesium has a number of seemingly strange phenomena. The best known may be the negative difference effect, commonly referred to as NDE [2–5]: the amount of hydrogen liberated increases with increasing applied anodic current (that decreases the cathodic reaction rate and the amount of associated ‘‘cathodic” hydrogen) and the amount of magnesium dissolved is greater than expected from application of the Faraday Law; both effects are explained by the generally accepted Mg corrosion mechanism involving the uni-positive Mg ion, Mg+. Another well know effect is that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, do not agree with direct measurements using weight loss or evaluated from the evolved hydrogen [2,10,46,47]. Much of the mechanistic understanding of the magnesium corrosion mechanism [1–3] has been gained in studies [44,48–50] using pure Mg and Mg–Al alloys. Are the same issues involved in the corrosion of other Mg alloy systems? To answer this question, we picked ZE41, a commercial alloy, and we are carrying out a systematic investigation of the corrosion behaviour of ZE41. In a recent paper, Aghion et al. [51] indicates that ZE41 is still preferred for certain applications although it exhibits poor corrosion resistance. ZE41 was used for the gear case of the French Freron super-helicopter [52] and for the wheels of the champion racing car in the years of 1990 and 1991 [53]. ZE41 has the nominal composition Mg-4 wt% Zn-1 wt% RE. Its microstructure, as-cast, typically consists of an a-phase matrix and b-phase distributed along the a-phase grain boundaries [8,54]. There has been a significant amount of research over the last few years exploring the corrosion behaviour of Mg–Zn and Mg–RE alloys [55–62], which give valuable insights into the corrosion behaviour of ZE41. Our prior paper [8] reported on the influence of microstructure on the corrosion of as-cast ZE41 in 1 M NaCl. The corrosion rate of ZE41 was approximately double that of AZ91 and about thirteen times that of pure Mg. The second phase in ZE41 did not act as a corrosion barrier and did not stop the advance of corrosion. As a consequence, the alpha Mg matrix corroded over the whole surface with little or no corrosion of the second phase. The mechanism of microgalvanic acceleration of corrosion for ZE41 was the same as for the Mg–Al alloys. Mg alloys in auto service may encounter environments, particularly aqueous solutions from road splash, with different pH values and chloride ion concentrations. Are there any qualitative differences in their corrosion behaviour in these different environments? The influence of pH, chlorides and sulphates on the corrosion and electrochemical behaviour of pure magnesium was studied by Song et al. [48,49]; the corrosion and electrochemical behaviour of pure magnesium, AZ21, AZ501 and AZ91 in sodium chloride solutions was studied by Song et al. [49,50]; the influence of pH and chloride ion concentration on the corrosion and electrochemical behaviour of AZ91D was then further studied by Ambat
et al. [63] and of cast AZ63 by Altun et al. [64]. To date, however, there has been no detailed study on the effect of pH value and chloride ion concentration on the corrosion behaviour of ZE41, nor in the Mg–Zn and Mg–RE alloys [55–62], which give valuable insights into the corrosion behaviour of ZE41. From a practical view point, understanding on the influence of pH and chloride ion concentration on the corrosion behaviour of ZE41 is of significance and is essential for the understanding of environmental factors controlling ZE41 corrosion. This work investigated these aspects for ZE41 using immersion corrosion tests and by evaluating the electrochemistry of ZE41. The aim was to determine if there were any qualitative differences in the corrosion behaviour of ZE41 in these different environments or if the corrosion behaviour is consistent with the current understanding of the corrosion mechanism of magnesium alloys [1–3]. The electrochemical measurements were included to explore the relationship, if any, between that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, and direct measurements using weight loss or evaluated from the evolved hydrogen, because of the known [2,10,46,47] issue that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, do not agree with direct measurements using weight loss or evaluated from the evolved hydrogen. 2. Experimental procedure ZE41 specimens were cut from an as-cast ingot; the chemical composition is presented in Table 1. The microstructure was examined in the un-etched condition by optical microscopy and scanning electron microscopy (SEM) after metallographic preparation (mechanical grinding successively to 1200 grit SiC paper, polishing to 0.5lm diamond, washing with distilled water and drying with warm flowing air). The corrosion behaviour was characterized by immersion tests at room temperature in 0 M, 0.1 M and 1 M NaCl solutions; the pH of the solutions was adjusted to the desired value (3, 7 and 11) with HCl and NaOH. Solutions were made with analytical grade reagents and distilled water. The solutions designated as 0 M NaCl consisted of distilled water + zero NaCl, adjusted to the desired pH value with HCl and NaOH. The aim was to investigate the corrosion behaviour of ZE41 in acid, neutral and alkaline solutions that might be encountered in auto service. The samples for the corrosion evaluation (including hydrogen gas collection and potentiodynamic polarisation curves) were mechanically ground to 1200 grit SiC paper, washed with distilled water and dried with warm flowing air. Corrosion macro-morphology was examined using optical microscopy after immersion. Samples for hydrogen gas collection, to characterise the corrosion rate during solution immersion, were cut into coupons and encapsulated into epoxy resin so that a surface with the dimension of 18 mm 27 mm was horizontally immersed in 1500 ml of quiescent test solution, in a beaker open to laboratory air. The hydrogen evolved during the corrosion experiment was collected in a burette above the corroding sample. The overall magnesium corrosion reaction,
Mg þ Hþ þ H2 O ¼ Mg2þ þ OH þ H2
ð1Þ
indicates that one molecule of hydrogen is evolved for each atom of corroded magnesium.
Table 1 Chemical composition of ZE41 (wt%) Zn
Ce
La
Pr
Mn
Fe
Al
Be
Sn
Pb
Ni
Cr
Cu
Zr
Sr
Nd
Mg
4.59
1.05
0.48
0.12
0.02
0.006
0.004
<0.001
<0.002
<0.002
<0.001
<0.001
<0.002
<0.002
<0.001
<0.01
Balance
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Potentiodynamic polarisation curves were measured immediately after immersion of the specimen in the solution in a glass cell using a PAR-2263 potentiostat system at a scan rate of 0.2 mV/s. In addition some potentiodynamic polarisation curves were measured, using a procedure similar to that described above, after the sample had been immersed for 48 h in the solution (0.1 M NaCl, pH 3 and 11 and 1 M NaCl,pH 3 and 7) and steady state corrosion conditions had become established. The polarisation curves were started by stepping the potential several hundred mV negative to the steady state potential and then polarising in an anodic direction at a scan rate of 0.2 mV/s. Samples for the potentiodynamic polarisation curves were cut into coupons and encapsulated into epoxy resin so that only a surface with the dimension of 10 mm 10 mm was exposed to the 500 ml of the solution. A platinum gauze (25 mm 25 mm, 52 mesh) was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. All potentials were referred to the SCE. The polarisation curves were used to explore the relationship, if any, between that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, and direct measurements using weight loss or evaluated from the evolved hydrogen. The corrosion current at the free corrosion potential was evaluated by Tafel extrapolation of the cathodic branch of the polarisation curve. The cathodic curves were all linear except for the curve measured in the 0 M NaCl pH 7 solution; a slope was used similar to those of the other plots. 3. Results 3.1. Microstructure Fig. 1 presents optical and SEM micrographs of ZE41. The microstructure consisted of the primary a-Mg grains plus a second phase (the b-phase) distributed along and/or adjacent to the boundaries of the a -Mg grains; the b-phase distribution was non-continuous. In addition, some b-phase particles appeared to be inside the a -Mg grains as isolated particles. Details of EDS and XRD analyses of the b-phase were described previously [8], these analyses were consistent with Mg7Zn3(RE) and Mg12RE.
Fig. 1. ZE41 microstructure:(a) low magnification optical micrograph and (b) higher magnification SEM micrograph.
gen evolution volume after 48 h immersion, are summarized in Tables 2 and 3.
3.2. Hydrogen evolution 3.3. Corrosion rate Fig. 2 presents the hydrogen evolution volume (ml/cm2), as a function of immersion time, for ZE41 immersed in 0 M, 0.1 M and 1 M NaCl solutions with pH 3, 7 and 11. The evolved hydrogen volume was essentially zero for the 48 h immersion in 0 M NaCl solutions with pH 7 and 11, which indicated that there was a low corrosion rate for ZE41 in neutral or alkaline solutions without chloride ions. For all other solutions, after an incubation period during which there was a low hydrogen evolution rate, there was an increase in hydrogen evolution volume with increasing immersion time. The incubation period and rate of hydrogen evolution depended on the solution. The incubation period decreased and the hydrogen evolution rate increased with increasing chloride ion concentration at each pH (Fig. 2a) and with decreasing pH for each chloride ion concentration (Fig. 2b). For example, in the pH 3 solution with 0 M NaCl (Fig. 2a) the hydrogen evolution started an obvious increase after 20 h and reached 2.9 ml/cm2 by an immersion time of 48 h, whereas in the pH 3 solution with 1 M NaCl, the hydrogen evolution started an obvious increase after 1 h and reached 17.5 ml/cm2 by 48 h immersion. In the 1 M NaCl pH 11 solution (Fig. 2b) the hydrogen evolution started an obvious increase after 5 h and reached 7.0 ml/cm2 by 48 h, whereas in the 1 M NaCl pH 3 solution, an obvious increase started after 1 h and reached 17.5 ml/cm2 by 48 h. The incubation time, and the hydro-
Fig. 3 presents the average hydrogen evolution rate (ml/cm2/ day) for ZE41 immersed in 0 M, 0.1 M and 1 M NaCl solutions of pH 3, 7 and 11. The hydrogen evolution rate was essentially zero for immersion in the 0 N NaCl solutions with pH 7 and 11. For the other cases, the hydrogen evolution rate increased with decreasing pH at each chloride ion concentration (Fig. 3a) and with increasing chloride ion concentration at each pH (Fig. 3b). This indicated that the corrosion rate of ZE41 depended on the combined influence of pH and chloride ion concentration. The influence of pH was the largest in the 1 M NaCl solution. The influence of chloride ion concentration was largest at low chloride ion concentrations, for which the rate of increase was higher than at higher chloride ion concentrations, although there was a much higher hydrogen evolution rate at higher chloride ion concentrations for each pH, Fig. 3b. The hydrogen evolution volume rate, VH (ml cm2 d1) can be related to the corrosion rate, PH (mm y1) using the following conversion equation [7,8,65,66]:
PH ¼ 2:279 V H
ð2Þ
Table 4 presents the corrosion rate, evaluated from the average hydrogen evolution rate, for immersion in the various solutions. The highest corrosion rate was in the 1 M NaCl pH 3 solution.
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a
b
20
20
-
pH3 pH7 pH11
0 M Cl 0.1 M Cl 1 M Cl
15
15
pH=3
10
-
5
5
0
0 10
20
30
40
0
10
20
30
40
H2 evolution volume, ml/cm
2
2
0
H2 evolution volume, ml/cm
0 M Cl
10
15
pH=7
10
5
0 0
10
20
30
15
10
0.1 M Cl
-
5
0
40
0
15
15
10
10
10
20
30
40
1 M Cl
pH=11 5
-
5
0
0 0
10
20
30
40
50
0
10
immersion time, h
20
30
40
50
immersion time, h
Fig. 2. Hydrogen evolution volume as a function of immersion time.
Table 2 Incubation time [h] for ZE41 immersed in the following chloride solutions pH
0 M NaCl
0.1 M NaCl
1 M NaCl
3 7 11
20 >48 >48
5 7 7
1 3 5
3.4. Macroscopic corrosion morphology Fig. 4 presents the macroscopic surface appearance of the corroded samples at the end of the 48 h immersion period. There was no obvious corrosion for the samples exposed to 0 M NaCl pH 7 and 11 solutions indicating that the air formed film provided
Table 3 Hydrogen evolution volume (ml/cm2) for ZE41 immersed for 48 h in the following chloride solutions pH
0 M NaCl
0.1 M NaCl
1 M NaCl
3 7 11
2.9 0 0
8.4 2.1 1.4
17.5 11.9 7.0
resistance to corrosion initiation in the 0 M NaCl neutral or alkaline solutions for at least 48 h. For all other samples, at the end of the 48 h immersion period, the whole surface was homogenously covered by a layer of corrosion products, indicating that the corrosion was uniformly distributed over the whole surface (at least at the macro-level). The amount of visible corrosion qualitatively
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Table 4 Corrosion rate, pH (mm/y), evaluated from the average hydrogen evolution rate, for ZE41 immersed in various NaCl solutions pH
0 M NaCl
0.1 M NaCl
1 M NaCl
3 7 11
3.2 0 0
9.7 2.3 1.5
20 14 8.0
a
9.0
2
H2 evolution volume rate, ml/cm day
-
0 M Cl 0.1 M Cl 1 M Cl
7.5 6.0 4.5 3.0 1.5 0.0 0
2
4
6
8
10
12
14
pH value
2
H2 evolution volume rate, ml/cm day
b
9.0 pH3 pH7 pH11
7.5
pH 7 and 11 solutions; the incubation time was much longer than the experimental times of Fig. 5. Group B contained the other curves. For group B, the open circuit potential curve had a maximum after several minutes, and subsequently the potential decreased gradually to less noble values. The initial potential increase to a maximum was consistent with an incubation period for the initiation of corrosion. However, the incubation period evaluated from the open circuit potential curve was much shorter than that evaluated from the hydrogen gas evolution volume, Table 2 and Fig. 2, indicating that the electrochemical technique was more sensitive in detecting the onset of corrosion than the technique based on hydrogen gas evolution. The subsequent decrease of the potential correlated with the occurrence of corrosion. The potential stabilised to a steady state value reflecting the dynamic balance between the advance of the corrosion and the deposit of the corrosion products; at steady state all the sample surface was corroding. The steady state open circuit potential decreased to less noble values with increasing chloride ion concentration at each pH (Fig. 5a) and with decreasing pH at each chloride ion concentration (Fig. 5b). In these cases, there was a correlation between the open circuit potential and the corrosion rate as determined from the hydrogen evolution volumes given in Tables 3 and 4, i.e. a more negative open circuit potential correlated with higher hydrogen evolution volumes and higher corrosion rates, consistent with the prior reports [67]. However, the decrease in the open circuit potential was small for a decrease in pH from 11 to pH 7 in the 0.1 M and 1 M solutions, which implied that the chloride ion has a small influence.
6.0
3.6. Potentiodynamic polarisation curves 4.5 3.0 1.5 0.0 0.0
0.2
0.4
0.6
0.8
1.0
Cl- concentration, M Fig. 3. Hydrogen evolution rate as a function of (a) pH and (b) Cl concentration.
increased with chloride ion concentration at each pH and with decreasing pH at each chloride ion concentration, in qualitative agreement with the hydrogen evolution results as presented in Fig. 3. As reported previously [8], and as observed in the present study, the corrosion initiated as localised corrosion. The corrosion initiated at some sites on the surface, subsequently expanded over the whole surface and finally a layer of corrosion products covered the whole surface. 3.5. Open circuit potential Fig. 5 shows the evolution of the open circuit potential for ZE41 immersed in 0 M, 0.1 M and 1 M NaCl solutions with pH 3, 7 and 11. Each measurement series started immediately after immersion of the ZE41 specimen in the test solution and reflected the initiation and propagation of corrosion. The curves were in two clusters. Group A contained the open circuit potential curves in 0 M NaCl pH 3, pH 7 and 11 solutions; the potential increased smoothly and continuously. This correlated with the relatively long incubation period for the initiation of corrosion in these solutions; visible corrosion did not occur in the experimental period. The open circuit potential data, Fig. 5 was consistent with the incubation time of the hydrogen gas evolution volume data, Table 2; the incubation period was long for the samples exposed to the 0 M NaCl pH 3,
Fig. 6 presents the potentiodynamic polarisation curves for freshly prepared ZE41 samples immediately after immersion in 0 M, 0.1 M and 1 M NaCl solutions with pH 3, 7 and 11. There was a significant deviation from Tafel behaviour for the cathodic curve in the pH 7, 0 M NaCl solution; this deviation was attributed to measurement errors, particularly iR drop. All the other cathodic curves showed Tafel behaviour, Fig. 6a. The extent of the cathodic Tafel region related to the onset of pitting at just prior to the corrosion potential, as was also noted by Song et al. [48–50]. There was a rapid increase in the anodic curve in all the pH 11 solutions and in the pH 7 solutions with 0.1 M and 1 M NaCl. In the pH 3 solutions there was also a rapid increase in the anodic curve and there was an inflection point in each of these curves. This probably indicates some sort of kinetic barrier effect, maybe the deposition of a corrosion product film and then its removal by intensively evolved hydrogen bubbles, which was more apparent at the higher currents in the pH 3 solutions. ZE41 showed no classical passivity type behaviour for any of the conditions studied. The polarisation curves were used to estimate the corrosion current density, icorr, at Ecorr, by Tafel extrapolation of the cathodic branch. The corrosion current density, icorr (mA cm2), can be related to the corrosion rate, Pi (mm.y1), using the following conversion equation [7,8,65,66].
Pi ¼ 22:85 icorr
ð3Þ
Table 5 presents the corrosion rate, evaluated from the icorr data for the freshly prepared ZE41 samples, for the various solutions. The highest corrosion rate was for the 1 M NaCl pH 3 solution. A comparison of the values in Table 4 with those in Table 5 indicated that (i) the corrosion rates were much higher when estimated from the hydrogen evolution rate and (ii) the corrosion rate estimated from the Tafel extrapolation showed the same trends in influence of pH and chloride ion concentration. The reason for the difference may be that different types of corrosion were measured. The corrosion rate from the Tafel extrapolation may relate to the onset of
M.-C. Zhao et al. / Corrosion Science 50 (2008) 3168–3178
3173
Fig. 4. Surface of the corroded samples after 48 h immersion.
corrosion, whereas the corrosion rate from the hydrogen evolution measurements relates to corrosion averaged over a considerable time period and includes corrosion some considerable time after corrosion onset, when the corrosion is well established. Fig. 7 presents the polarisation curves measured after 48 h immersion in the solution and compares these curves with the polarisation curves measured immediately after immersion in the solution. There were large differences for each solution. For each solution, the cathodic current (i.e. the rate of hydrogen evolution) was significantly higher and the corrosion potential was significantly more positive for the curves measured after 48 h immersion in the solution compared with the curves measured with a fresh sample. It was hoped that Tafel extrapolation would yield corrosion rates comparable to or related to those evaluated from the hydrogen evolution rate, Table 4. Table 6 presents the corrosion rate, Piss (estimated from icorr from polarisation curves measured for ZE41 after reaching steady state corrosion conditions) compared with the corrosion rate, Pio (estimated from icorr measured from polarisation curves for freshly prepared ZE41) and the corrosion rate, PH (evaluated from the average hydrogen evolution rate) for ZE41 immersed in various NaCl solutions. The corrosion rate, Piss, was always greater than the corrosion rate, Pio, but the difference was small for the 0.1 M NaCl solutions whereas the difference was larger in the 1 M NaCl solution. The trends for the corrosion
rate, Piss (related to pH and chloride concentration) were similar to those for the corrosion rate, PH; but any similarity of the numerical value of Piss and PH appears fortuitous. Fig. 8 presents the corrosion potential, Ecorr, determined from the polarisation curves for freshly prepared ZE41 samples, Fig. 6, as a function of chloride ion concentration (Fig. 8a) and pH (Fig. 8b), for immersed in 0 M, 0.1 M and 1 M NaCl solutions with pH 3, 7 and 11. Ecorr shifted to more negative (or more active) values with increasing chloride ion concentration at each pH (Fig. 8a) and with decreasing pH at each chloride ion concentration (Fig. 8b), showing similar tendencies as the open circuit potential. Fig. 9 plots the measured value of steady state open circuit potential (from Fig. 5) or the corrosion potential (from Fig. 6) (both plotted as Ecorr) against the corrosion rate as determined from (i) the average hydrogen evolution volume rate from Table 4 and from (ii) the Tafel extrapolated value of icorr, Table 5; a more negative Ecorr value correlated with a higher corrosion rate. 4. Discussion 4.1. Hydrogen evolution Many of the hydrogen evolution curves showed significant increases in evolution rate over the 48 h immersion time. This
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a
b
0N 0.1 N 1N
-1.2
-1.3
-1.3 -
-1.4
Cl = 0M
-1.4
pH=3 -1.5
-1.5
-1.6
-1.6
-1.7
-1.7
200
400
600
800
1000 1200 1400 1600 180
-1.2
-1.3
-1.4
pH=7
-1.5
-1.6
0
Open circuit potentials, VSCE
0
Open circuit potentials, VSCE
pH 3 pH 7 pH 11
-1.2
200
400
600
800
1000 1200 1400 1600
-1.2
-1.3
-
Cl = 0.1 M
-1.4
-1.5
-1.6
-1.7
-1.7 0
200
400
600
800
1000 1200 1400 1600 1800
-1.2
-1.2 0
-1.3
-1.3
pH=11
-1.4
-1.5
-1.6
-1.6
-1.7
-1.7
200
400
600
800
1000 1200 1400 1600 1800
400
600
800
1000 1200 1400 1600
-
Cl =1 M
-1.4
-1.5
0
200
0
200
Time, second
400
600
800
1000 1200 1400 1600 1800
Time, second Fig. 5. Evolution of the open circuit potential.
indicates that steady state corrosion conditions take a significant time to become established and that short-term measurements can give rise to optimistically low values of corrosion rate. 4.2. Polarisation curves There were significant differences between the polarisation curves measured for freshly prepared samples and those measured after the establishment of steady state corrosion conditions, Fig. 7. This indicates that there are significant differences in the details of the various electrochemical reactions under these two conditions. For the freshly prepared samples, the cathodic reaction of hydrogen evolution occurs on the air formed film on the surface of the
freshly prepared surface. As shown by a number of studies [47– 50,68], the corrosion potential in chloride containing solutions relates to the breakdown of the air formed film and the initiation of pitting corrosion. For the sample that had attained steady state corrosion conditions, the cathodic reaction, also assumed to be hydrogen evolution, occurs on the corroded surface, possibly on the surface of corrosion products, possibly Mg(OH)2. Clearly hydrogen evolution is much faster on the surface after steady state corrosion has been attained. In all cases, the steady state corroding surface had a free corrosion potential more positive than that of the freshly prepared surface and moreover the steady state corroding surface had a higher rate of hydrogen evolution. This means that, if a corroding area is
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a
-0.6
b
0M 0.1 M 1M
-0.8
pH=3 pH=7 pH=11
-0.8
-1.0
-1.0
pH=3
-1.2
-1.2
-1.4
-1.4
-1.6
-1.6
-1.8
0 M Cl
-
-1.8 -7
-6
-5
-4
-3
-2
-7
-0.8
-0.8
-1.0
-1.0
Potential, VSCE
Potential, VSCE
-0.6
pH=7
-1.2
-1.4
-6
-5
-4
-1.2
0.1 M Cl
-3
-2
-3
-2
-
-1.4
-1.6
-1.6
-1.8
-1.8 -7
-6
-5
-4
-3
-7
-2
-0.8
-0.8
-1.0
-1.0
-1.2
-6
-5
-1.2
pH=11
-1.4
-1.4
-1.6
-1.6
-4
1 M Cl
-
-1.8
-1.8 -7
-6
-5
-4
-3
-2
log (current density) or log (I), A cm
- 2
-7
-6
-5
-4
-3
log (current density) or log (I), A cm
-2 - 2
Fig. 6. Potentiodynamic polarisation curves measured immediately after immersion in the solution.
Table 5 Corrosion rate, Pio (mm/y), estimated from the icorr, measured for freshly prepared ZE41 exposed to various NaCl solutions pH
0 M NaCl
0.1 M NaCl
1 M NaCl
3 7 11
0.93 0.16 0.09
3.7 0.63 0.22
5.0 1.6 0.60
adjacent to a non-corroded area, there will be a galvanic cell causing the galvanic acceleration of the corrosion rate of the non-corroded area. Thus, once corrosion starts, there is an electrochemical driving force for the spread of the corrosion across
the surface. This is indeed what is observed experimentally. The galvanic acceleration of the corrosion of the non-corroding areas (i.e. the spread of corrosion across the non-corroding areas), is balanced by the galvanic protection of the corroded areas, so that the corrosion tends to be rather shallow in the corroded areas. This is also consistent with the observations. This spread of corrosion across the surface for ZE41 in the present study was similar to the observation for MEZ by Song [47] and in contrast to the tendency for pitting for AZ91 observed by Song [47]. The critical aspect is that the corrosion potential of the corroded areas is more positive than that of the fresh surface for ZE41 and MEZ, whereas for AZ91 the corrosion potential of the corroded areas is more negative than that of the fresh surface.
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a
-1.0 0.1 M pH3 with 48 h 0.1 M pH3 with 0 min 0.1 M pH11 with 48 h 0.1 M pH11 with 0 min
-1.1
pH 3 pH 7 pH 11
-1.2
-1.2
-1.3
Ecorr, VSCE
Potential, VSCE
-1.1
-1.3 -1.4
-1.4 -1.5
-1.5 -1.6
-1.6
-1.7
-1.7 0.0
0.2
0.4
-1.8
0.6
0.8
1.0
-
Cl concentration, M -7
-6
-5
-4
-3
log (current density) or log (I), A cm
b
- 2
-1.1 0M 0.1 M 1M
-1.2
b
1 M pH3 with 48 h 1 M pH3 with 0 min 1 M pH7 with 48 h 1 M pH7 with 0 min
-1.3
Ecorr, VSCE
-1.3
Potential, VSCE
-2
-1.4
-1.4 -1.5
-1.5
-1.6 -1.6
-1.7 0
2
4
6
-1.7
8
10
12
14
pH value Fig. 8. Corrosion potential as a function of (a) Cl concentration and (b) pH.
-1.8 -7
-6
-5
-4
-3
log (current density) or log (I), A cm
-2 - 2
-1.1
Fig. 7. Polarisation curves measured after 48 h immersion in the solution compared with curves measured immediately after immersion in the solution.
data from hydrogen evolution rate data from icorr
-1.2
Table 6 The corrosion rate, Piss (estimated from icorr from polarisation curves measured for ZE41 after reaching steady state corrosion conditions) compared with the corrosion rate, Pio (estimated from icorr measured from polarisation curves for freshly prepared ZE41) and the corrosion rate, PH (evaluated from the average hydrogen evolution rate) for ZE41 immersed in various NaCl solutions pH
3 7 11
0.1 M NaCl
Ecorr, VSCE
-1.3 -1.4 -1.5 -1.6
1 M NaCl
PH
Pio
Piss
PH
Pio
Piss
9.7 2.3 1.5
3.7 0.63 0.22
4.5 – 0.3
20 14 8.0
5.0 1.6 0.6
17.0 3.2 –
The units for the corrosion rate were mm/y.
4.3. Electrochemical measurement of corrosion rate The electrochemical measurements were included to explore the relationship, if any, between that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, and direct measurements using weight loss or evaluated from the evolved hydrogen, because of the known [2,10,46,47] issue that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, do not agree with direct measurements using weight loss or evaluated from the evolved hydrogen. Table 6 showed that the corrosion rate determined from the current at the free corrosion
-1.7 0
5
10
15
20
Corrosion rate, mm/y Fig. 9. Corrosion potential plotted against the corrosion rate.
potential, did not agree with direct measurements evaluated from the evolved hydrogen. Of most concern may be that there did not appear to be any relation between Piss and PH. PH is the corrosion rate evaluated from the average hydrogen evolution rate. There is substantial evidence [2,7,48] that PH is a valid measure of the corrosion rate for Mg alloys, equivalent to that of weight loss rate. Piss is the electrochemical measurement of corrosion rate (by Tafel extrapolation) after steady state corrosion conditions had been attained. However, any similarity of the numerical value of Piss and PH appears fortuitous; the ratio PH/ Piss varied seemingly randomly between 5.0 and 1.2 (the values are given in Table 7). The ratio PH/ Piss would be expected to be a constant if the only factor was the negative
M.-C. Zhao et al. / Corrosion Science 50 (2008) 3168–3178 Table 7 Ratios PH/Pio and PH/Piss based on the data in Table 7 pH
3 7 11
0.1 M NaCl
1 M NaCl
PH/Pio
PH/Piss
PH/Pio
PH/Piss
2.6 3.7 6.8
2.2 – 5.0
4.0 8.8 13.3
1.2 2.8 –
difference effect and if k was a constant, where k is the fraction of the uni-positive Mg ion, Mg+, that undergoes chemical reaction with water to liberate hydrogen. However, it is worth remembering the work of Petty et al. [69] indicated that k need not be a constant. In fact, due to the ‘‘anodic hydrogen evolution” phenomenon, the corrosion rate of a magnesium alloy estimated from its polarisation curve is in theory not reliable [47]. The comparison of corrosion rates obtained by hydrogen evolution and polarisation curve measurements in this study further confirmed the ‘‘anodic hydrogen evolution” effect [47]. 4.4. Ecorr versus Corrosion rate In this study more negative values of Ecorr correlated with a higher corrosion rate, Fig. 9. Is this a general phenomenon for Mg alloys? Maybe not. A plot of Ecorr versus corrosion rate from the work of Song et al. [44] generated a seemingly-random scatter plot. Südholz et al. [33] did carry out extensive measurements of Ecorr and icorr but did not publish any measurements relating directly to corrosion rate. These results are hard to use in their present version because of the fact that there may be no relationship between icorr and the corrosion rate for Mg alloys. 4.5. Corrosion mechanism The corrosion behaviour and electrochemical behaviour of ZE41 is significantly influenced by pH and chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by the characteristics of its surface film [1–3,48–50]. The surface film on magnesium alloys in aqueous solutions is thought to be mainly Mg(OH)2 [1,2,6]. The corrosion reactions are presented by the following equations.
Mg ) Mg2þ þ 2e
ð4Þ
2H2 O þ 2e ) H2 þ 2ðOHÞ
ð5Þ
Mg2þ þ 2ðOHÞ ) MgðOHÞ2
ð6Þ
Eq. (6) describes the surface film formation, this occurs because Mg2+ has a low solubility. The influence of pH on corrosion needs to take into account the magnesium E-pH diagram (Pourbaix diagram) [70]. Thermodynamics and the E-pH diagram predict that there should no film on a magnesium surface in a solution with a pH lower than 10.5 because Mg(OH)2 is not stable under such conditions (the pH value required for the precipitation of Mg(OH)2 is around 10.5). However, even though the surface film is not thermodynamically stable at low pH values, the dissolution kinetics may be slow and a surface film may be formed provided the dissolution kinetics are slower than the formation kinetics. Furthermore, a pH value over 10 was theoretically calculated and experimentally measured in the electrolyte layer near a corroding magnesium electrode, even though the bulk solution had a pH value as low as 4 [71], i.e. there is significant alkalization near the magnesium surface in an acidic medium and this alkalization is associated with the formation of surface film. This alkalization is consistent with Eq. (5).
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The corrosion rate of ZE41 increased with decreasing pH and increasing chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film [1–3,48–50], with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown, and the known instability of Mg(OH)2 in solutions with pH less than 10.5. 5. Conclusions
1. The corrosion of ZE41 in NaCl chloride solutions depended on the pH and the chloride ion concentration. A shorter incubation period to the onset of corrosion; a more negative open circuit potential and corrosion potential; and a higher corrosion rate correlated with a higher chloride ion concentration at each pH and correlated with a lower pH for each chloride ion concentration. 2. The most active corrosion potential and the highest corrosion rate occurred for ZE41 in the 1 M NaCl pH 3 solution. The corrosion rate was small in the 0 M NaCl pH 7 and 11 solutions. 3. The influence of chloride ion concentration was largest at low chloride concentrations, for which there was a higher rate of increase than at higher chloride concentrations. 4. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film, with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown, and the known instability of Mg(OH)2 in solutions with pH less than 10.5. 5. The steady state corroding surface had a free corrosion potential more positive than that of the freshly prepared surface and moreover the steady state corroding surface had a higher rate of hydrogen evolution. Galvanic interaction between the corroding and non-corroding surface provides a driving force for the spread of the corrosion over the whole surface and moreover tends to decrease the corrosion rate of the corroding area causing the corrosion penetration to be relatively shallow. 6. The trends for the corrosion rates, Piss and Pio (related to pH and chloride concentration) were similar to those for the corrosion rate, PH; but any similarity of the numerical value of Piss and PH appears fortuitous. Piss was estimated from icorr from polarisation curves measured for ZE41 after reaching steady state corrosion conditions, Pio was estimated from icorr measured from polarisation curves for freshly prepared ZE41 and PH was evaluated from the average hydrogen evolution rate. 7. The electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.
Acknowledgements This work was supported by the ARC Center of Excellence, Design of Light Alloys. Ming-Chun Zhao, Ming Liu and Andrej Atrens would like to thank EMPA for their support that allowed them to
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Influence of pH and chloride ion concentration on the corrosion of Mg alloy ZE41 Ming-Chun Zhao a,b,c, Ming Liu a,b, Guang-Ling Song a, Andrej Atrens a,b,* a b c
Division of Materials, The University of Queensland, Brisbane, Qld 4072, Australia Swiss Federal Laboratories for Materials Science and Technology, EMPA, Department 136, Überlandstrasse 129, CH-8600 Dubendorf, Switzerland School of Material Science and Engineering, Central South University, Changsha 410083, PR China
a r t i c l e
i n f o
Article history: Received 4 June 2008 Accepted 14 August 2008 Available online 26 August 2008 Keywords: Mg alloy ZE41 Corrosion pH Chloride ion concentration Surface film
a b s t r a c t The influence of pH and chloride ion concentration on the corrosion behaviour of ZE41 was studied using immersion tests and electrochemical measurements. A shorter incubation period to the onset of corrosion; a more negative corrosion potential; and a higher corrosion rate correlated with a higher chloride ion concentration at each pH value and correlated with a lower pH value for each chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film, with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown and the known instability of Mg(OH)2 in solutions with pH less than 10.5. The electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Mg alloys are of great interest in transport applications, especially for automobiles, due to their low density and adequate strength/weight ratio. However, a critical limitation for the service application of Mg alloys is their susceptibility to corrosion [1–3] and hence there continues to be much research to understand and document the various aspects of the corrosion of Mg alloys [4–33]. Stress corrosion cracking (SCC) of common Mg alloys occurs for frequently encountered environments [34–43] so SCC may occur for stressed auto components subjected to road splash; the fact that SCC of common Mg alloys occurs in distilled water [34–39] indicates that water itself is the key environment factor causing SCC in aqueous solutions. The key points of the generally accepted corrosion mechanism for Mg alloys [1–3] in common environments like 3% NaCl solution are as follows. A partially protective film covers the surface and corrosion occurs at the breaks in this film (i.e. at film free areas). The cathodic reaction is hydrogen liberation. A step in the anodic reaction is the production of the uni-positive Mg ion, Mg+, and a fraction undergoes chemical reaction with water to liberate hydrogen [1–3]; thus, hydrogen is produced both chemically and as the cathodic partial reaction. The surface film on Mg is not particularly * Corresponding author. Address: Division of Materials, The University of Queensland, Brisbane, Qld 4072, Australia. Tel.: +61 7 3365 3748; fax: +61 7 3365 3888. E-mail address: [email protected] (A. Atrens). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.08.023
protective; as a result the corrosion rate is typically more than 1 mm/y in common environments like 3% NaCl solution; no alloying element has been found that produces a solid solution Mg alloy with a corrosion rate less than that of pure Mg in 3% NaCl solution (which typically has a corrosion rate of 1 mm/y). Consequently, it is useful to include pure Mg as a standard in any comparative study of corrosion performance of Mg alloys (see e.g. [2,7,8,45]). Moreover, it is indeed extremely important to use pure Mg as the standard for comparison; commercial purity Mg can have a corrosion rate more than 50 times that of pure Mg [2] and so ‘‘commercial purity Mg” should be designated as ‘‘low purity Mg” for clarity. Corrosion of common commercial Mg alloys typically involves microgalvanic acceleration of the corrosion of the alpha matrix by the second phase(s) [2,7,8,44,45] and corrosion rates are equal to or typically greater than that of pure Mg. The corrosion rate of a two phase Mg alloy can, however, be lower than that of pure Mg for an appropriate distribution of the second phase in the alloy (or at least at the alloy surface) [2,25,44]: namely, if the second phase is finely divided, essentially continuous and the second phase itself has a lower corrosion rate than that of pure Mg, then the second phase can act as a corrosion barrier. The corrosion form of magnesium is typically designated as localised corrosion [1,2,48,49] to distinguish it from the auto-catalytic pitting in stainless steels. Localised corrosion in magnesium typically initiates as irregular localised corrosion, which spread laterally and cover the whole surface. There does not seem to be much tendency for deep pitting. The most probable reason is that
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the cathodic reaction is the liberation of hydrogen. A by product of that cathodic reaction is the production of OH ions (or equivalently the consumption of H+ ions) with a concomitant increase of the pH and stabilisation of the local magnesium hydroxide film and a decrease in corrosion tendency. Thus localised corrosion in magnesium has an inherent tendency to be self-limiting. This is in marked contrast to the situation for stainless steels, where the occluded pit cell becomes more aggressive and accelerates the localised corrosion. Corrosion of magnesium has a number of seemingly strange phenomena. The best known may be the negative difference effect, commonly referred to as NDE [2–5]: the amount of hydrogen liberated increases with increasing applied anodic current (that decreases the cathodic reaction rate and the amount of associated ‘‘cathodic” hydrogen) and the amount of magnesium dissolved is greater than expected from application of the Faraday Law; both effects are explained by the generally accepted Mg corrosion mechanism involving the uni-positive Mg ion, Mg+. Another well know effect is that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, do not agree with direct measurements using weight loss or evaluated from the evolved hydrogen [2,10,46,47]. Much of the mechanistic understanding of the magnesium corrosion mechanism [1–3] has been gained in studies [44,48–50] using pure Mg and Mg–Al alloys. Are the same issues involved in the corrosion of other Mg alloy systems? To answer this question, we picked ZE41, a commercial alloy, and we are carrying out a systematic investigation of the corrosion behaviour of ZE41. In a recent paper, Aghion et al. [51] indicates that ZE41 is still preferred for certain applications although it exhibits poor corrosion resistance. ZE41 was used for the gear case of the French Freron super-helicopter [52] and for the wheels of the champion racing car in the years of 1990 and 1991 [53]. ZE41 has the nominal composition Mg-4 wt% Zn-1 wt% RE. Its microstructure, as-cast, typically consists of an a-phase matrix and b-phase distributed along the a-phase grain boundaries [8,54]. There has been a significant amount of research over the last few years exploring the corrosion behaviour of Mg–Zn and Mg–RE alloys [55–62], which give valuable insights into the corrosion behaviour of ZE41. Our prior paper [8] reported on the influence of microstructure on the corrosion of as-cast ZE41 in 1 M NaCl. The corrosion rate of ZE41 was approximately double that of AZ91 and about thirteen times that of pure Mg. The second phase in ZE41 did not act as a corrosion barrier and did not stop the advance of corrosion. As a consequence, the alpha Mg matrix corroded over the whole surface with little or no corrosion of the second phase. The mechanism of microgalvanic acceleration of corrosion for ZE41 was the same as for the Mg–Al alloys. Mg alloys in auto service may encounter environments, particularly aqueous solutions from road splash, with different pH values and chloride ion concentrations. Are there any qualitative differences in their corrosion behaviour in these different environments? The influence of pH, chlorides and sulphates on the corrosion and electrochemical behaviour of pure magnesium was studied by Song et al. [48,49]; the corrosion and electrochemical behaviour of pure magnesium, AZ21, AZ501 and AZ91 in sodium chloride solutions was studied by Song et al. [49,50]; the influence of pH and chloride ion concentration on the corrosion and electrochemical behaviour of AZ91D was then further studied by Ambat
et al. [63] and of cast AZ63 by Altun et al. [64]. To date, however, there has been no detailed study on the effect of pH value and chloride ion concentration on the corrosion behaviour of ZE41, nor in the Mg–Zn and Mg–RE alloys [55–62], which give valuable insights into the corrosion behaviour of ZE41. From a practical view point, understanding on the influence of pH and chloride ion concentration on the corrosion behaviour of ZE41 is of significance and is essential for the understanding of environmental factors controlling ZE41 corrosion. This work investigated these aspects for ZE41 using immersion corrosion tests and by evaluating the electrochemistry of ZE41. The aim was to determine if there were any qualitative differences in the corrosion behaviour of ZE41 in these different environments or if the corrosion behaviour is consistent with the current understanding of the corrosion mechanism of magnesium alloys [1–3]. The electrochemical measurements were included to explore the relationship, if any, between that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, and direct measurements using weight loss or evaluated from the evolved hydrogen, because of the known [2,10,46,47] issue that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, do not agree with direct measurements using weight loss or evaluated from the evolved hydrogen. 2. Experimental procedure ZE41 specimens were cut from an as-cast ingot; the chemical composition is presented in Table 1. The microstructure was examined in the un-etched condition by optical microscopy and scanning electron microscopy (SEM) after metallographic preparation (mechanical grinding successively to 1200 grit SiC paper, polishing to 0.5lm diamond, washing with distilled water and drying with warm flowing air). The corrosion behaviour was characterized by immersion tests at room temperature in 0 M, 0.1 M and 1 M NaCl solutions; the pH of the solutions was adjusted to the desired value (3, 7 and 11) with HCl and NaOH. Solutions were made with analytical grade reagents and distilled water. The solutions designated as 0 M NaCl consisted of distilled water + zero NaCl, adjusted to the desired pH value with HCl and NaOH. The aim was to investigate the corrosion behaviour of ZE41 in acid, neutral and alkaline solutions that might be encountered in auto service. The samples for the corrosion evaluation (including hydrogen gas collection and potentiodynamic polarisation curves) were mechanically ground to 1200 grit SiC paper, washed with distilled water and dried with warm flowing air. Corrosion macro-morphology was examined using optical microscopy after immersion. Samples for hydrogen gas collection, to characterise the corrosion rate during solution immersion, were cut into coupons and encapsulated into epoxy resin so that a surface with the dimension of 18 mm 27 mm was horizontally immersed in 1500 ml of quiescent test solution, in a beaker open to laboratory air. The hydrogen evolved during the corrosion experiment was collected in a burette above the corroding sample. The overall magnesium corrosion reaction,
Mg þ Hþ þ H2 O ¼ Mg2þ þ OH þ H2
ð1Þ
indicates that one molecule of hydrogen is evolved for each atom of corroded magnesium.
Table 1 Chemical composition of ZE41 (wt%) Zn
Ce
La
Pr
Mn
Fe
Al
Be
Sn
Pb
Ni
Cr
Cu
Zr
Sr
Nd
Mg
4.59
1.05
0.48
0.12
0.02
0.006
0.004
<0.001
<0.002
<0.002
<0.001
<0.001
<0.002
<0.002
<0.001
<0.01
Balance
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Potentiodynamic polarisation curves were measured immediately after immersion of the specimen in the solution in a glass cell using a PAR-2263 potentiostat system at a scan rate of 0.2 mV/s. In addition some potentiodynamic polarisation curves were measured, using a procedure similar to that described above, after the sample had been immersed for 48 h in the solution (0.1 M NaCl, pH 3 and 11 and 1 M NaCl,pH 3 and 7) and steady state corrosion conditions had become established. The polarisation curves were started by stepping the potential several hundred mV negative to the steady state potential and then polarising in an anodic direction at a scan rate of 0.2 mV/s. Samples for the potentiodynamic polarisation curves were cut into coupons and encapsulated into epoxy resin so that only a surface with the dimension of 10 mm 10 mm was exposed to the 500 ml of the solution. A platinum gauze (25 mm 25 mm, 52 mesh) was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. All potentials were referred to the SCE. The polarisation curves were used to explore the relationship, if any, between that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, and direct measurements using weight loss or evaluated from the evolved hydrogen. The corrosion current at the free corrosion potential was evaluated by Tafel extrapolation of the cathodic branch of the polarisation curve. The cathodic curves were all linear except for the curve measured in the 0 M NaCl pH 7 solution; a slope was used similar to those of the other plots. 3. Results 3.1. Microstructure Fig. 1 presents optical and SEM micrographs of ZE41. The microstructure consisted of the primary a-Mg grains plus a second phase (the b-phase) distributed along and/or adjacent to the boundaries of the a -Mg grains; the b-phase distribution was non-continuous. In addition, some b-phase particles appeared to be inside the a -Mg grains as isolated particles. Details of EDS and XRD analyses of the b-phase were described previously [8], these analyses were consistent with Mg7Zn3(RE) and Mg12RE.
Fig. 1. ZE41 microstructure:(a) low magnification optical micrograph and (b) higher magnification SEM micrograph.
gen evolution volume after 48 h immersion, are summarized in Tables 2 and 3.
3.2. Hydrogen evolution 3.3. Corrosion rate Fig. 2 presents the hydrogen evolution volume (ml/cm2), as a function of immersion time, for ZE41 immersed in 0 M, 0.1 M and 1 M NaCl solutions with pH 3, 7 and 11. The evolved hydrogen volume was essentially zero for the 48 h immersion in 0 M NaCl solutions with pH 7 and 11, which indicated that there was a low corrosion rate for ZE41 in neutral or alkaline solutions without chloride ions. For all other solutions, after an incubation period during which there was a low hydrogen evolution rate, there was an increase in hydrogen evolution volume with increasing immersion time. The incubation period and rate of hydrogen evolution depended on the solution. The incubation period decreased and the hydrogen evolution rate increased with increasing chloride ion concentration at each pH (Fig. 2a) and with decreasing pH for each chloride ion concentration (Fig. 2b). For example, in the pH 3 solution with 0 M NaCl (Fig. 2a) the hydrogen evolution started an obvious increase after 20 h and reached 2.9 ml/cm2 by an immersion time of 48 h, whereas in the pH 3 solution with 1 M NaCl, the hydrogen evolution started an obvious increase after 1 h and reached 17.5 ml/cm2 by 48 h immersion. In the 1 M NaCl pH 11 solution (Fig. 2b) the hydrogen evolution started an obvious increase after 5 h and reached 7.0 ml/cm2 by 48 h, whereas in the 1 M NaCl pH 3 solution, an obvious increase started after 1 h and reached 17.5 ml/cm2 by 48 h. The incubation time, and the hydro-
Fig. 3 presents the average hydrogen evolution rate (ml/cm2/ day) for ZE41 immersed in 0 M, 0.1 M and 1 M NaCl solutions of pH 3, 7 and 11. The hydrogen evolution rate was essentially zero for immersion in the 0 N NaCl solutions with pH 7 and 11. For the other cases, the hydrogen evolution rate increased with decreasing pH at each chloride ion concentration (Fig. 3a) and with increasing chloride ion concentration at each pH (Fig. 3b). This indicated that the corrosion rate of ZE41 depended on the combined influence of pH and chloride ion concentration. The influence of pH was the largest in the 1 M NaCl solution. The influence of chloride ion concentration was largest at low chloride ion concentrations, for which the rate of increase was higher than at higher chloride ion concentrations, although there was a much higher hydrogen evolution rate at higher chloride ion concentrations for each pH, Fig. 3b. The hydrogen evolution volume rate, VH (ml cm2 d1) can be related to the corrosion rate, PH (mm y1) using the following conversion equation [7,8,65,66]:
PH ¼ 2:279 V H
ð2Þ
Table 4 presents the corrosion rate, evaluated from the average hydrogen evolution rate, for immersion in the various solutions. The highest corrosion rate was in the 1 M NaCl pH 3 solution.
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a
b
20
20
-
pH3 pH7 pH11
0 M Cl 0.1 M Cl 1 M Cl
15
15
pH=3
10
-
5
5
0
0 10
20
30
40
0
10
20
30
40
H2 evolution volume, ml/cm
2
2
0
H2 evolution volume, ml/cm
0 M Cl
10
15
pH=7
10
5
0 0
10
20
30
15
10
0.1 M Cl
-
5
0
40
0
15
15
10
10
10
20
30
40
1 M Cl
pH=11 5
-
5
0
0 0
10
20
30
40
50
0
10
immersion time, h
20
30
40
50
immersion time, h
Fig. 2. Hydrogen evolution volume as a function of immersion time.
Table 2 Incubation time [h] for ZE41 immersed in the following chloride solutions pH
0 M NaCl
0.1 M NaCl
1 M NaCl
3 7 11
20 >48 >48
5 7 7
1 3 5
3.4. Macroscopic corrosion morphology Fig. 4 presents the macroscopic surface appearance of the corroded samples at the end of the 48 h immersion period. There was no obvious corrosion for the samples exposed to 0 M NaCl pH 7 and 11 solutions indicating that the air formed film provided
Table 3 Hydrogen evolution volume (ml/cm2) for ZE41 immersed for 48 h in the following chloride solutions pH
0 M NaCl
0.1 M NaCl
1 M NaCl
3 7 11
2.9 0 0
8.4 2.1 1.4
17.5 11.9 7.0
resistance to corrosion initiation in the 0 M NaCl neutral or alkaline solutions for at least 48 h. For all other samples, at the end of the 48 h immersion period, the whole surface was homogenously covered by a layer of corrosion products, indicating that the corrosion was uniformly distributed over the whole surface (at least at the macro-level). The amount of visible corrosion qualitatively
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Table 4 Corrosion rate, pH (mm/y), evaluated from the average hydrogen evolution rate, for ZE41 immersed in various NaCl solutions pH
0 M NaCl
0.1 M NaCl
1 M NaCl
3 7 11
3.2 0 0
9.7 2.3 1.5
20 14 8.0
a
9.0
2
H2 evolution volume rate, ml/cm day
-
0 M Cl 0.1 M Cl 1 M Cl
7.5 6.0 4.5 3.0 1.5 0.0 0
2
4
6
8
10
12
14
pH value
2
H2 evolution volume rate, ml/cm day
b
9.0 pH3 pH7 pH11
7.5
pH 7 and 11 solutions; the incubation time was much longer than the experimental times of Fig. 5. Group B contained the other curves. For group B, the open circuit potential curve had a maximum after several minutes, and subsequently the potential decreased gradually to less noble values. The initial potential increase to a maximum was consistent with an incubation period for the initiation of corrosion. However, the incubation period evaluated from the open circuit potential curve was much shorter than that evaluated from the hydrogen gas evolution volume, Table 2 and Fig. 2, indicating that the electrochemical technique was more sensitive in detecting the onset of corrosion than the technique based on hydrogen gas evolution. The subsequent decrease of the potential correlated with the occurrence of corrosion. The potential stabilised to a steady state value reflecting the dynamic balance between the advance of the corrosion and the deposit of the corrosion products; at steady state all the sample surface was corroding. The steady state open circuit potential decreased to less noble values with increasing chloride ion concentration at each pH (Fig. 5a) and with decreasing pH at each chloride ion concentration (Fig. 5b). In these cases, there was a correlation between the open circuit potential and the corrosion rate as determined from the hydrogen evolution volumes given in Tables 3 and 4, i.e. a more negative open circuit potential correlated with higher hydrogen evolution volumes and higher corrosion rates, consistent with the prior reports [67]. However, the decrease in the open circuit potential was small for a decrease in pH from 11 to pH 7 in the 0.1 M and 1 M solutions, which implied that the chloride ion has a small influence.
6.0
3.6. Potentiodynamic polarisation curves 4.5 3.0 1.5 0.0 0.0
0.2
0.4
0.6
0.8
1.0
Cl- concentration, M Fig. 3. Hydrogen evolution rate as a function of (a) pH and (b) Cl concentration.
increased with chloride ion concentration at each pH and with decreasing pH at each chloride ion concentration, in qualitative agreement with the hydrogen evolution results as presented in Fig. 3. As reported previously [8], and as observed in the present study, the corrosion initiated as localised corrosion. The corrosion initiated at some sites on the surface, subsequently expanded over the whole surface and finally a layer of corrosion products covered the whole surface. 3.5. Open circuit potential Fig. 5 shows the evolution of the open circuit potential for ZE41 immersed in 0 M, 0.1 M and 1 M NaCl solutions with pH 3, 7 and 11. Each measurement series started immediately after immersion of the ZE41 specimen in the test solution and reflected the initiation and propagation of corrosion. The curves were in two clusters. Group A contained the open circuit potential curves in 0 M NaCl pH 3, pH 7 and 11 solutions; the potential increased smoothly and continuously. This correlated with the relatively long incubation period for the initiation of corrosion in these solutions; visible corrosion did not occur in the experimental period. The open circuit potential data, Fig. 5 was consistent with the incubation time of the hydrogen gas evolution volume data, Table 2; the incubation period was long for the samples exposed to the 0 M NaCl pH 3,
Fig. 6 presents the potentiodynamic polarisation curves for freshly prepared ZE41 samples immediately after immersion in 0 M, 0.1 M and 1 M NaCl solutions with pH 3, 7 and 11. There was a significant deviation from Tafel behaviour for the cathodic curve in the pH 7, 0 M NaCl solution; this deviation was attributed to measurement errors, particularly iR drop. All the other cathodic curves showed Tafel behaviour, Fig. 6a. The extent of the cathodic Tafel region related to the onset of pitting at just prior to the corrosion potential, as was also noted by Song et al. [48–50]. There was a rapid increase in the anodic curve in all the pH 11 solutions and in the pH 7 solutions with 0.1 M and 1 M NaCl. In the pH 3 solutions there was also a rapid increase in the anodic curve and there was an inflection point in each of these curves. This probably indicates some sort of kinetic barrier effect, maybe the deposition of a corrosion product film and then its removal by intensively evolved hydrogen bubbles, which was more apparent at the higher currents in the pH 3 solutions. ZE41 showed no classical passivity type behaviour for any of the conditions studied. The polarisation curves were used to estimate the corrosion current density, icorr, at Ecorr, by Tafel extrapolation of the cathodic branch. The corrosion current density, icorr (mA cm2), can be related to the corrosion rate, Pi (mm.y1), using the following conversion equation [7,8,65,66].
Pi ¼ 22:85 icorr
ð3Þ
Table 5 presents the corrosion rate, evaluated from the icorr data for the freshly prepared ZE41 samples, for the various solutions. The highest corrosion rate was for the 1 M NaCl pH 3 solution. A comparison of the values in Table 4 with those in Table 5 indicated that (i) the corrosion rates were much higher when estimated from the hydrogen evolution rate and (ii) the corrosion rate estimated from the Tafel extrapolation showed the same trends in influence of pH and chloride ion concentration. The reason for the difference may be that different types of corrosion were measured. The corrosion rate from the Tafel extrapolation may relate to the onset of
M.-C. Zhao et al. / Corrosion Science 50 (2008) 3168–3178
3173
Fig. 4. Surface of the corroded samples after 48 h immersion.
corrosion, whereas the corrosion rate from the hydrogen evolution measurements relates to corrosion averaged over a considerable time period and includes corrosion some considerable time after corrosion onset, when the corrosion is well established. Fig. 7 presents the polarisation curves measured after 48 h immersion in the solution and compares these curves with the polarisation curves measured immediately after immersion in the solution. There were large differences for each solution. For each solution, the cathodic current (i.e. the rate of hydrogen evolution) was significantly higher and the corrosion potential was significantly more positive for the curves measured after 48 h immersion in the solution compared with the curves measured with a fresh sample. It was hoped that Tafel extrapolation would yield corrosion rates comparable to or related to those evaluated from the hydrogen evolution rate, Table 4. Table 6 presents the corrosion rate, Piss (estimated from icorr from polarisation curves measured for ZE41 after reaching steady state corrosion conditions) compared with the corrosion rate, Pio (estimated from icorr measured from polarisation curves for freshly prepared ZE41) and the corrosion rate, PH (evaluated from the average hydrogen evolution rate) for ZE41 immersed in various NaCl solutions. The corrosion rate, Piss, was always greater than the corrosion rate, Pio, but the difference was small for the 0.1 M NaCl solutions whereas the difference was larger in the 1 M NaCl solution. The trends for the corrosion
rate, Piss (related to pH and chloride concentration) were similar to those for the corrosion rate, PH; but any similarity of the numerical value of Piss and PH appears fortuitous. Fig. 8 presents the corrosion potential, Ecorr, determined from the polarisation curves for freshly prepared ZE41 samples, Fig. 6, as a function of chloride ion concentration (Fig. 8a) and pH (Fig. 8b), for immersed in 0 M, 0.1 M and 1 M NaCl solutions with pH 3, 7 and 11. Ecorr shifted to more negative (or more active) values with increasing chloride ion concentration at each pH (Fig. 8a) and with decreasing pH at each chloride ion concentration (Fig. 8b), showing similar tendencies as the open circuit potential. Fig. 9 plots the measured value of steady state open circuit potential (from Fig. 5) or the corrosion potential (from Fig. 6) (both plotted as Ecorr) against the corrosion rate as determined from (i) the average hydrogen evolution volume rate from Table 4 and from (ii) the Tafel extrapolated value of icorr, Table 5; a more negative Ecorr value correlated with a higher corrosion rate. 4. Discussion 4.1. Hydrogen evolution Many of the hydrogen evolution curves showed significant increases in evolution rate over the 48 h immersion time. This
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a
b
0N 0.1 N 1N
-1.2
-1.3
-1.3 -
-1.4
Cl = 0M
-1.4
pH=3 -1.5
-1.5
-1.6
-1.6
-1.7
-1.7
200
400
600
800
1000 1200 1400 1600 180
-1.2
-1.3
-1.4
pH=7
-1.5
-1.6
0
Open circuit potentials, VSCE
0
Open circuit potentials, VSCE
pH 3 pH 7 pH 11
-1.2
200
400
600
800
1000 1200 1400 1600
-1.2
-1.3
-
Cl = 0.1 M
-1.4
-1.5
-1.6
-1.7
-1.7 0
200
400
600
800
1000 1200 1400 1600 1800
-1.2
-1.2 0
-1.3
-1.3
pH=11
-1.4
-1.5
-1.6
-1.6
-1.7
-1.7
200
400
600
800
1000 1200 1400 1600 1800
400
600
800
1000 1200 1400 1600
-
Cl =1 M
-1.4
-1.5
0
200
0
200
Time, second
400
600
800
1000 1200 1400 1600 1800
Time, second Fig. 5. Evolution of the open circuit potential.
indicates that steady state corrosion conditions take a significant time to become established and that short-term measurements can give rise to optimistically low values of corrosion rate. 4.2. Polarisation curves There were significant differences between the polarisation curves measured for freshly prepared samples and those measured after the establishment of steady state corrosion conditions, Fig. 7. This indicates that there are significant differences in the details of the various electrochemical reactions under these two conditions. For the freshly prepared samples, the cathodic reaction of hydrogen evolution occurs on the air formed film on the surface of the
freshly prepared surface. As shown by a number of studies [47– 50,68], the corrosion potential in chloride containing solutions relates to the breakdown of the air formed film and the initiation of pitting corrosion. For the sample that had attained steady state corrosion conditions, the cathodic reaction, also assumed to be hydrogen evolution, occurs on the corroded surface, possibly on the surface of corrosion products, possibly Mg(OH)2. Clearly hydrogen evolution is much faster on the surface after steady state corrosion has been attained. In all cases, the steady state corroding surface had a free corrosion potential more positive than that of the freshly prepared surface and moreover the steady state corroding surface had a higher rate of hydrogen evolution. This means that, if a corroding area is
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a
-0.6
b
0M 0.1 M 1M
-0.8
pH=3 pH=7 pH=11
-0.8
-1.0
-1.0
pH=3
-1.2
-1.2
-1.4
-1.4
-1.6
-1.6
-1.8
0 M Cl
-
-1.8 -7
-6
-5
-4
-3
-2
-7
-0.8
-0.8
-1.0
-1.0
Potential, VSCE
Potential, VSCE
-0.6
pH=7
-1.2
-1.4
-6
-5
-4
-1.2
0.1 M Cl
-3
-2
-3
-2
-
-1.4
-1.6
-1.6
-1.8
-1.8 -7
-6
-5
-4
-3
-7
-2
-0.8
-0.8
-1.0
-1.0
-1.2
-6
-5
-1.2
pH=11
-1.4
-1.4
-1.6
-1.6
-4
1 M Cl
-
-1.8
-1.8 -7
-6
-5
-4
-3
-2
log (current density) or log (I), A cm
- 2
-7
-6
-5
-4
-3
log (current density) or log (I), A cm
-2 - 2
Fig. 6. Potentiodynamic polarisation curves measured immediately after immersion in the solution.
Table 5 Corrosion rate, Pio (mm/y), estimated from the icorr, measured for freshly prepared ZE41 exposed to various NaCl solutions pH
0 M NaCl
0.1 M NaCl
1 M NaCl
3 7 11
0.93 0.16 0.09
3.7 0.63 0.22
5.0 1.6 0.60
adjacent to a non-corroded area, there will be a galvanic cell causing the galvanic acceleration of the corrosion rate of the non-corroded area. Thus, once corrosion starts, there is an electrochemical driving force for the spread of the corrosion across
the surface. This is indeed what is observed experimentally. The galvanic acceleration of the corrosion of the non-corroding areas (i.e. the spread of corrosion across the non-corroding areas), is balanced by the galvanic protection of the corroded areas, so that the corrosion tends to be rather shallow in the corroded areas. This is also consistent with the observations. This spread of corrosion across the surface for ZE41 in the present study was similar to the observation for MEZ by Song [47] and in contrast to the tendency for pitting for AZ91 observed by Song [47]. The critical aspect is that the corrosion potential of the corroded areas is more positive than that of the fresh surface for ZE41 and MEZ, whereas for AZ91 the corrosion potential of the corroded areas is more negative than that of the fresh surface.
3176
a
M.-C. Zhao et al. / Corrosion Science 50 (2008) 3168–3178
a
-1.0 0.1 M pH3 with 48 h 0.1 M pH3 with 0 min 0.1 M pH11 with 48 h 0.1 M pH11 with 0 min
-1.1
pH 3 pH 7 pH 11
-1.2
-1.2
-1.3
Ecorr, VSCE
Potential, VSCE
-1.1
-1.3 -1.4
-1.4 -1.5
-1.5 -1.6
-1.6
-1.7
-1.7 0.0
0.2
0.4
-1.8
0.6
0.8
1.0
-
Cl concentration, M -7
-6
-5
-4
-3
log (current density) or log (I), A cm
b
- 2
-1.1 0M 0.1 M 1M
-1.2
b
1 M pH3 with 48 h 1 M pH3 with 0 min 1 M pH7 with 48 h 1 M pH7 with 0 min
-1.3
Ecorr, VSCE
-1.3
Potential, VSCE
-2
-1.4
-1.4 -1.5
-1.5
-1.6 -1.6
-1.7 0
2
4
6
-1.7
8
10
12
14
pH value Fig. 8. Corrosion potential as a function of (a) Cl concentration and (b) pH.
-1.8 -7
-6
-5
-4
-3
log (current density) or log (I), A cm
-2 - 2
-1.1
Fig. 7. Polarisation curves measured after 48 h immersion in the solution compared with curves measured immediately after immersion in the solution.
data from hydrogen evolution rate data from icorr
-1.2
Table 6 The corrosion rate, Piss (estimated from icorr from polarisation curves measured for ZE41 after reaching steady state corrosion conditions) compared with the corrosion rate, Pio (estimated from icorr measured from polarisation curves for freshly prepared ZE41) and the corrosion rate, PH (evaluated from the average hydrogen evolution rate) for ZE41 immersed in various NaCl solutions pH
3 7 11
0.1 M NaCl
Ecorr, VSCE
-1.3 -1.4 -1.5 -1.6
1 M NaCl
PH
Pio
Piss
PH
Pio
Piss
9.7 2.3 1.5
3.7 0.63 0.22
4.5 – 0.3
20 14 8.0
5.0 1.6 0.6
17.0 3.2 –
The units for the corrosion rate were mm/y.
4.3. Electrochemical measurement of corrosion rate The electrochemical measurements were included to explore the relationship, if any, between that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, and direct measurements using weight loss or evaluated from the evolved hydrogen, because of the known [2,10,46,47] issue that electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, do not agree with direct measurements using weight loss or evaluated from the evolved hydrogen. Table 6 showed that the corrosion rate determined from the current at the free corrosion
-1.7 0
5
10
15
20
Corrosion rate, mm/y Fig. 9. Corrosion potential plotted against the corrosion rate.
potential, did not agree with direct measurements evaluated from the evolved hydrogen. Of most concern may be that there did not appear to be any relation between Piss and PH. PH is the corrosion rate evaluated from the average hydrogen evolution rate. There is substantial evidence [2,7,48] that PH is a valid measure of the corrosion rate for Mg alloys, equivalent to that of weight loss rate. Piss is the electrochemical measurement of corrosion rate (by Tafel extrapolation) after steady state corrosion conditions had been attained. However, any similarity of the numerical value of Piss and PH appears fortuitous; the ratio PH/ Piss varied seemingly randomly between 5.0 and 1.2 (the values are given in Table 7). The ratio PH/ Piss would be expected to be a constant if the only factor was the negative
M.-C. Zhao et al. / Corrosion Science 50 (2008) 3168–3178 Table 7 Ratios PH/Pio and PH/Piss based on the data in Table 7 pH
3 7 11
0.1 M NaCl
1 M NaCl
PH/Pio
PH/Piss
PH/Pio
PH/Piss
2.6 3.7 6.8
2.2 – 5.0
4.0 8.8 13.3
1.2 2.8 –
difference effect and if k was a constant, where k is the fraction of the uni-positive Mg ion, Mg+, that undergoes chemical reaction with water to liberate hydrogen. However, it is worth remembering the work of Petty et al. [69] indicated that k need not be a constant. In fact, due to the ‘‘anodic hydrogen evolution” phenomenon, the corrosion rate of a magnesium alloy estimated from its polarisation curve is in theory not reliable [47]. The comparison of corrosion rates obtained by hydrogen evolution and polarisation curve measurements in this study further confirmed the ‘‘anodic hydrogen evolution” effect [47]. 4.4. Ecorr versus Corrosion rate In this study more negative values of Ecorr correlated with a higher corrosion rate, Fig. 9. Is this a general phenomenon for Mg alloys? Maybe not. A plot of Ecorr versus corrosion rate from the work of Song et al. [44] generated a seemingly-random scatter plot. Südholz et al. [33] did carry out extensive measurements of Ecorr and icorr but did not publish any measurements relating directly to corrosion rate. These results are hard to use in their present version because of the fact that there may be no relationship between icorr and the corrosion rate for Mg alloys. 4.5. Corrosion mechanism The corrosion behaviour and electrochemical behaviour of ZE41 is significantly influenced by pH and chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by the characteristics of its surface film [1–3,48–50]. The surface film on magnesium alloys in aqueous solutions is thought to be mainly Mg(OH)2 [1,2,6]. The corrosion reactions are presented by the following equations.
Mg ) Mg2þ þ 2e
ð4Þ
2H2 O þ 2e ) H2 þ 2ðOHÞ
ð5Þ
Mg2þ þ 2ðOHÞ ) MgðOHÞ2
ð6Þ
Eq. (6) describes the surface film formation, this occurs because Mg2+ has a low solubility. The influence of pH on corrosion needs to take into account the magnesium E-pH diagram (Pourbaix diagram) [70]. Thermodynamics and the E-pH diagram predict that there should no film on a magnesium surface in a solution with a pH lower than 10.5 because Mg(OH)2 is not stable under such conditions (the pH value required for the precipitation of Mg(OH)2 is around 10.5). However, even though the surface film is not thermodynamically stable at low pH values, the dissolution kinetics may be slow and a surface film may be formed provided the dissolution kinetics are slower than the formation kinetics. Furthermore, a pH value over 10 was theoretically calculated and experimentally measured in the electrolyte layer near a corroding magnesium electrode, even though the bulk solution had a pH value as low as 4 [71], i.e. there is significant alkalization near the magnesium surface in an acidic medium and this alkalization is associated with the formation of surface film. This alkalization is consistent with Eq. (5).
3177
The corrosion rate of ZE41 increased with decreasing pH and increasing chloride ion concentration. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film [1–3,48–50], with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown, and the known instability of Mg(OH)2 in solutions with pH less than 10.5. 5. Conclusions
1. The corrosion of ZE41 in NaCl chloride solutions depended on the pH and the chloride ion concentration. A shorter incubation period to the onset of corrosion; a more negative open circuit potential and corrosion potential; and a higher corrosion rate correlated with a higher chloride ion concentration at each pH and correlated with a lower pH for each chloride ion concentration. 2. The most active corrosion potential and the highest corrosion rate occurred for ZE41 in the 1 M NaCl pH 3 solution. The corrosion rate was small in the 0 M NaCl pH 7 and 11 solutions. 3. The influence of chloride ion concentration was largest at low chloride concentrations, for which there was a higher rate of increase than at higher chloride concentrations. 4. This corrosion behaviour is consistent with the current understanding that the corrosion behaviour of magnesium alloys is governed by a partially protective surface film, with the corrosion reactions occurring predominantly at the breaks or imperfections of the partially protective film. The implication is that the fraction of film free surface increases with decreasing bulk pH and with increasing chloride ion concentration. This is consistent with the known tendency of chloride ions to cause film breakdown, and the known instability of Mg(OH)2 in solutions with pH less than 10.5. 5. The steady state corroding surface had a free corrosion potential more positive than that of the freshly prepared surface and moreover the steady state corroding surface had a higher rate of hydrogen evolution. Galvanic interaction between the corroding and non-corroding surface provides a driving force for the spread of the corrosion over the whole surface and moreover tends to decrease the corrosion rate of the corroding area causing the corrosion penetration to be relatively shallow. 6. The trends for the corrosion rates, Piss and Pio (related to pH and chloride concentration) were similar to those for the corrosion rate, PH; but any similarity of the numerical value of Piss and PH appears fortuitous. Piss was estimated from icorr from polarisation curves measured for ZE41 after reaching steady state corrosion conditions, Pio was estimated from icorr measured from polarisation curves for freshly prepared ZE41 and PH was evaluated from the average hydrogen evolution rate. 7. The electrochemical measurements of the corrosion rate, based on the corrosion current at the free corrosion potential, did not agree with direct measurements evaluated from the evolved hydrogen, in agreement with other observations for Mg.
Acknowledgements This work was supported by the ARC Center of Excellence, Design of Light Alloys. Ming-Chun Zhao, Ming Liu and Andrej Atrens would like to thank EMPA for their support that allowed them to
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