Influence of Al Concentration and Zn Addition on the Corrosion Resistance of Rolled Mg–Al–(Zn)–Ca Magnesium Alloys

Abstract

The effects of Al concentration and Zn addition on the corrosion resistance of Mg–Al–(Zn)–Ca-based magnesium alloy rolled sheets were investigated. AXM (Mg–Al–Ca) alloys containing 1 mass% Ca and AZX (Mg–Al–Zn–Ca) alloys containing 1 mass% Ca plus 1 mass% Zn with the Al concentration adjusted to 6, 7, 8, 9, and 11 mass% were prepared. The corrosion behaviors of the alloys in a 5 mass% NaCl solution were evaluated by weight loss and the penetration depth. Electrochemical methods based on corrosion potential measurements and impedance spectroscopy was applied to monitor the corrosion behavior. The hydroxide film formed on the alloy surface was characterized by GDOES and SEM/EDS. The AXM alloy with an Al content of approximately 8 mass% exhibited minimum corrosion rate, whereas the AZX alloy showed the opposite result in the same concentration range of Al. This phenomenon was analyzed in terms of the Al concentration in the matrix (α-phase), which was reduced by the precipitation of Mg₁₇Al₁₂ (β-phase) and the presence of Zn. The tendency of localized corrosion in high Al-containing AZX alloys was pointed out.

 

This Paper was Originally Published in Japanese in J. JILM 70 (2020) 56–62. The caption of Fig. 3 is slightly modified.

Weight loss corrosion rate of AXM and AZX alloys in 5% NaCl solution after 72 h.

1. Introduction

Mg–Al- and Mg–Al–Zn-based magnesium alloys, such as AM60 and AZ91, containing aluminum are general-purpose alloys having a good balance between strength and corrosion resistance. Recently, a flame-retardant Mg alloy containing 1–2 mass% (hereinafter, mass% is expressed as %) calcium has attracted considerable attention for application as structural members of high-speed trains and aircrafts.¹⁾

To optimize the mechanical properties of wrought flame-retardant alloys, the effects of alloy composition on the mechanical properties were investigated.^2,3) Saito et al.²⁾ evaluated the microstructure and mechanical properties of a rolled Mg–x%Al–1%Zn–1%Ca–0.2%Mn alloy (x = 6–11) and discovered that the alloy without zinc (hereinafter, abbreviated as “AXM alloy”) had the highest tensile strength and fracture elongation at x = 8%. This was explained by the balance between the precipitation strengthening of the fine Mg₁₇Al₁₂ particles (β-phase) during alloy formation and the decrease in ductility due to the coarsening of the second-phase Al₂Ca particles. The alloy containing 1% Zn (hereinafter, abbreviated as “AZX alloy”) was found to possess a higher strength than the AXM alloy due to the solid solution strengthening of Zn and enhanced precipitation of the β-phase. The strength and elongation of the extruded Mg–9%Al–0.7%Zn–2%Ca alloy were optimum.³⁾

The alloy composition also influences the corrosion resistance.⁴⁾ In the case of the Mg–Al alloys, the corrosion rate rapidly decreases when the Al concentration is increased from 0 to 4–5% and gradually decreases with a further increase in the Al concentration.^5,6) The initial decrease is due to the formation of a dense oxide/hydroxide film containing Al⁷⁾ on the alloy surface. Higher presence of Al contributes the formation of Al-rich α- and β-phases with high corrosion resistance.^5,6,8–10) Especially in die-cast alloys, the β-phase forms a network along the grain boundaries and protects the primary α-phase.^5,10) However, the behavior of the β-phase, which possesses more noble corrosion potential (E_corr) than the α-phase, remains a matter of discussion.^5,11) Song et al.^8,9,12) suggested that whether the β-phase acts as a galvanic cathode to accelerate corrosion or a barrier to the protect the α-phase depends on its proportion and distribution in the alloy. The correlation of the β-phase with the corrosion rate of the Mg–Al cast alloys was investigated using various heat treatments and Mg/Al ratios, and the Al concentration in the α-phase was found to be a dominant factor in the corrosion resistance rather than the β-phase proportion.¹¹⁾ Contrary to the case of cast alloys, the β-phase in wrought alloys does not create a network structure but is evenly distributed as nano-sized grains that are pulverized or dynamically reprecipitated during the process.^2,3) The corrosion resistance of extruded Mg–x%Al–0.7%Zn–2%Ca–0.2% Mn alloys (x = 3, 6, or 9) was evaluated, and it was found that the finely precipitated β-phase did not influence the corrosion rate.¹³⁾

Similarly, several studies have been reported on the effect of Zn on the corrosion of Mg alloys. The addition of more than 2–3% of Zn to a Mg–Zn binary alloy impairs the corrosion resistance.¹⁴⁾ Moreover, Zn addition to Mg–Al cast alloys reduces the anodic reactivity of the Mg–Al solid solution and the cathodic reactivity of the β-phase, which is beneficial to the corrosion resistance.⁶⁾ The corrosion rate of the Mg–5.3%Al–x%Zn cast alloy (x = 0–2.7) was investigated, and the area ratio of the β-phase plus Mg₃₂(Al, Zn)₄₉ to the α-phase was found to be critical for the corrosion rate.¹⁵⁾ The addition of Ca in small amounts contributes to corrosion resistance due to the refinement of the microstructure,¹⁶⁾ whereas the addition of more than 0.5–1% Ca reduces the corrosion resistance by promoting the precipitation of the secondary phase as Mg₂Ca, (Mg, Al)₂Ca, or Al₂Ca.^17,18)

In summary, the AZX alloys contain multiple precipitated phases that affect the corrosion behavior. Hence, it is reasonable to investigate the effect of alloy composition on not only mechanical strength but also corrosion properties. Herein, the corrosion of rolled AXM and AZX alloys with the Al concentration of 6–11% was investigated. The corrosion rate in the 5% NaCl solution was evaluated by weight loss and electrochemical methods, and its correlation with the alloy microstructure was discussed.

2. Experimental Procedure

Table 1 shows the designations and chemical compositions of the AXM and AZX alloy ingots. Most samples met the target composition, and the level of impurities, including iron, was less than the industrial standard (JIS H4201). After grinding the alloy surface, the alloys were rolled at the reduction rate of 77% under heating at 523 K to prepare 3-mm thick sheets, followed by annealing at 473 K for 1 h.²⁾ They were cut into the dimensions of 35 mm × 25 mm. The testing surface was polished with up to #1000 SiC paper, washed with pure water, and degreased with ethanol.

Table 1 Chemical composition of Mg–Al–Zn–Ca based magnesium alloys used in the present experiment.

Corrosion testing was performed under open-air conditions according to JIS H 0541. The test samples were immersed in a 1 L of 0.86 mol/L (5%) NaCl aqueous solution at 303 K for 72 h. The solution pH was adjusted to 10–11 using Mg(OH)₂. After the test, the corrosion products were removed with a 1 mol/L boiling CrO₃ solution to calculate the weight-loss corrosion rate (W_corr). The maximum penetration depth (P_corr) was measured using a 3D profiler (Keyence VR-5100). The corrosion potential (E_corr) and electrochemical impedance were monitored in the same environment. A silver–silver chloride reference electrode, a platinum counter electrode, and a test holder regulating the alloy sample area to 1 cm² were immersed in 400 mL of test solution. The electrochemical impedance at the E_corr was measured with the amplitude of 5 mV in the frequency range of 10 kHz–100 mHz using VersaSTAT 3. The spectrum was analyzed using curve fitting software (supplied by Scribner Associates, ZView ver. 3.5). The depth profiles of the elements in the corroded AXM801 and AZX811 alloy surfaces were analyzed using glow discharge optical emission spectroscopy (GDOES, Horiba Profiler 2). The microstructures of these alloys were analyzed using scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS, JSM-IT500).

3. Results and Discussions

3.1 W_corr and P_corr

Figure 1 shows the surface profile images and the maximum penetration depth of several alloys after the test. The corrosion morphology was different among the alloys. The corrosion damage of the AXM alloy was minor, leaving some gloss on the surface. Most of the AZX series alloys exhibited general corrosion along with several dimples. A trench of corrosion with a maximum depth of 0.7 mm was visible in the AZX811 cross section. A few streaks were also observed in the AXM901 cross section; however, they were significantly shallower, approximately 0.04 mm in depth. Figure 2 shows W_corr. At a 6% Al concentration, the difference between the W_corr of the AXM and AZX alloys was small. At higher Al concentrations, this difference was noticeable as the W_corr of the AXM alloy was minimum (8 g·m⁻²·d⁻¹ at 8%), whereas that of the AZX alloy was maximum (42 g·m⁻²·d⁻¹ at 7%). At further higher Al concentrations, this difference reduced, and the W_corr of approximately 20 g·m⁻²·d⁻¹ was achieved.

Fig. 1

Surface profile images and maximum penetration depth of several AXM and AZX alloys corroded in 5% NaCl solution after 72 h.

Fig. 2

Weight loss corrosion rate of AXM and AZX alloys in 5% NaCl solution after 72 h.

Figure 3 shows the maximum penetration depth P_corr. The AXM alloys showed a value of approximately 0.2 mm regardless of the Al concentration. In the case of AZX alloys, the Al concentration increased sharply from 7% and showed a maximum value of 0.6 mm at 8%. Although P_corr decreases slightly with increasing Al concentration, it is still higher than that of AXM alloys. There is also a large variation among the AZX alloys.

Fig. 3

Maximum penetration depth of AXM and AZX alloys in 5% NaCl solution after 72 h.

3.2 E_corr and electrochemical impedance

The variation of the E_corr with time is presented in Fig. 4. For the AXM alloys, E_corr was approximately between −1.58 and −1.55 V depending on the Al concentration. After 72 h, the E_corr varied in the following order: (base) AXM801 < AXM701 < AXM901 < AXM601 (noble). The AZX alloys presented a more noble E_corr (approximately 50 mV more than those of the AXM alloys) along with a fluctuation of approximately 10 mV. The dependence of E_corr on the Al concentration was unclear.

Fig. 4

Time evolution of corrosion potential in 5% NaCl solution: (a) AXM alloys, (b) AZX alloys.

Figure 5(a) shows an example of the Nyquist diagram of electrochemical impedance measured for AXM alloys. For the Mg alloys corroding in the NaCl solution, a capacitive semicircle along with an inductive loop is often obtained in the lower frequency region.^13,19–21) This trajectory is expressed using an electric equivalent circuit shown in Fig. 5(b).^19,20) As the center of the capacitive semicircle is located below the real axis Z′, one of the capacitive elements has been replaced by a constant phase element; this reflects the inhomogeneity of the electrode surface.²¹⁾ Curve fitting analysis was performed, and its result was superimposed on the experimental results in Fig. 5(a). The employed equivalent circuit adequately simulated the experimental system. The value of each element was listed in Table 2. Herein, the sum of R1 + R2 was selected as a resistance to corrosion rate, which is approximately equivalent to R_ct, the difference between the maximum and minimum of Z′ at −Z′′ = 0.¹⁹⁾ The reciprocal of R_ct was designated as the conductance Y_ct, and its time variation was analyzed, as shown in Fig. 6. The AZX alloys exhibited higher Y_ct than the AXM alloys. The AXM alloys, except for AXM601, initially exhibited a small Y_ct, which gradually increased with time. The higher the Al concentration, the greater the tendency of Y_ct to increase. The AXM601 and AZX611 alloys showed a similar behavior regardless of the existence of Zn. Figure 7 shows the average Y_ct and W_corr. A linear relationship was observed in both alloys, indicating that Y_ct reflects their W_corr.

Fig. 5

Result of electrochemical impedance spectroscopy: (a) Nyquist diagram of AXM801 alloy after 72 h, (b) electrical equivalent circuit of corroding magnesium.¹⁹⁾

Table 2 Result of fitting analysis of electrochemical impedance spectroscopy of corroding AXM801 alloy after 72 h.

Fig. 6

Time evolution of charge transfer conductance Y_ct in 5% NaCl solution: (a) AXM alloys, (b) AZX alloys.

Fig. 7

Relation between the average of charge transfer conductance and the weight loss corrosion rate of AXM and AZX alloys in 5% NaCl solution.

3.3 GD-OES of the corroded AXM and AZX alloy surfaces

Figure 8 shows the appearance of the AXM801 and AZX811 alloys after 3 h of immersion. The AZX811 alloy was slightly covered with filiform-type corrosion; contrarily, the lusty appearance was still recognizable in the AXM801 alloy. The depth profiles of the surface elements are shown in Fig. 9. Speculating that the profiles of H and O reflect the presence of the hydroxide film, its thickness was estimated to be approximately 0.8 µm and 3.2 µm for the AXM801 and AZX811 alloys, respectively. Figure 9(c) shows a magnification of a part of Fig. 9(b). The carbon content was maximum on the outermost surface and then rapidly decreased in both alloys. However, the presence of C was detected up to a 3 µm thickness for the AZX811 alloy. It is possible that a layered double hydroxide (LDH)²²⁾ was formed on this alloy. Furthermore, Zn was detected in the AZX811 alloy layer, suggesting that Zn was once dissolved and incorporated inside the hydroxide.

Fig. 8

Appearances of AXM801 and AZX811 alloy surface in 5%NaCl solution after 8 h.

Fig. 9

Depth profile of elements measured by GD-OES: (a) AXM801 alloy, (b) AZX811 alloy, (c) enlargement of dotted area of (b). Measurement areas are encircled in Fig. 8.

3.4 Microstructure observation by SEM/EDS

Figure 10 presents the SEM images of the AXM801 and AZX811 alloys. Their microstructures consist of coarse Al₂Ca precipitates along the rolling direction, a fine β-phase with a size of approximately 0.5 µm, and an α-phase.²⁾ The presence of the β-phase is not evident in the present magnification. Therefore, 10 points of the α-phase were randomly selected and the EDS analysis was carried out. The range of Al concentration was presented as box and data plots in Fig. 11. The average Al concentrations were found to be 4.36% and 3.83% for the AXM801 and AZX811 alloys, respectively. A one-sided testing of the difference of population means based on statistical t-distribution was conducted, and the probability P value was found to be 0.0022, which was judged to be “significant”.

Fig. 10

SEM image: (a) AXM801 alloy and (b) AZX811 alloy. Triangle symbols indicate the spots where EDS analysis was carried out.

Fig. 11

Box and data plots showing the distribution of the Al concentration of AXM801 alloy and AZX811 alloy measured by EDS.

3.5 Effects of Al concentration and Zn addition on the corrosion resistance of the Mg–Al alloy

For the AXM alloys, the effect of Al concentration on the W_corr was almost consistent with previous observations, whereas for the AZX alloys, the W_corr showed an unexpected peak in the Al concentration range of 7–8%. The initiation and progress of Mg corrosion is accompanied by an increase in the cathode area.²³⁾ More noble E_corr and larger Y_ct of the AZX alloys than those of the AXM alloy support this phenomenon. Generally, the addition of alloying elements more than the solid solubility limit induces the precipitation of secondary phase particles, which reduces the corrosion resistance of Mg alloys.⁴⁾ As the difference between the corrosion resistances of the AXM601 and AZX611 alloys is small, the presence of the β-phase, which starts to precipitate at an Al concentration of 6% or higher, seems important. Based on the previous results,^11,13) the corrosion phenomenon was interpreted as follows: the present alloy system contains 1% Ca, which causes the precipitation of the Al₂Ca phase in the microstructure. For example, in an 8%Al–1%Ca (7.22 mol%Al–0.61 mol%Ca) alloy, approximately 6 mol% Al (= 7.22–0.61 × 2) exists in the α- and β-phases. Zn in the AZX alloy generates a Zn-containing β-phase (4–19 mol%) and acts as the nuclei of the β-phase.²⁾ Consequently, the Al content in the α-phase of the AZX alloy is lower than that in the α-phase of the AMX alloy; this was verified by the SEM/EDS analysis. The insufficient Al content in the α-phase would cause a micro-galvanic corrosion with a larger amount of cathodic β-phase. The presence of further Al (>8%) leads to a high Al content in the α-phase, and the effect of the Zn concentration is gradually diminished.

The corrosion resistance of the Mg–Al alloy is improved by solution heat treatment.²⁴⁾ This process induces the dissolution of the β-phase and some Al₂Ca such that the Al concentration in the α-phase increases. The application of 7–8% Al in the AZX alloy to improve the corrosion resistance is expected if the decrease in the mechanical strength is acceptable.

The addition of 1% Zn was found to have an adverse effect on the corrosion resistance of the present alloy system. However, it may be premature to judge that the addition of Zn tends to decrease the corrosion resistance of Mg alloys. The W_corr of Mg–5.3%Al alloy attains the maximum value at 1% Zn concentration.¹⁵⁾ The electrochemical properties of the second phase appearing in the Mg–x%Al–(15–x)%Zn cast alloy differed according to the Zn content.²⁵⁾ Further studies on the variation of Zn content in a wider range and the effect of heat treatment would provide a more general perspective on the corrosion behavior of the AZX alloys.

3.6 Relationship between Al concentration in alloy and localized corrosion

It is generally known that high Al-containing magnesium diecast alloys as AZ91 possess good corrosion resistance.⁶⁾ The low corrosion rate W_corr of AZX911 alloy and AZX1011 alloy shown in Fig. 2 also supports this trend. However, P_corr of both alloys shown in Fig. 3 does not decrease as much as W_corr. Herein, pitting factor²⁶⁾ was introduced to compare the susceptibleness to localized corrosion. It is calculated by the ratio between P_corr and the average penetration depth obtained by dividing W_corr with the density (here the density was fixed to 1.8 Mg·m⁻³ for all alloys). The results are shown in Fig. 12. The tendency is different between AXM alloys and AZX alloys. The AXM alloys exhibit a slightly protruding value at the Al concentration of 8%, which is because the average penetration depth (corrosion rate) itself is small at this concentration. On the other hand, the pitting factor of AZX alloys increase linearly with Al concentration, suggesting that the increased tendency of localized corrosion. In fact, a deep profile observed in AZX811 alloy in Fig. 1 is different from the filiform corrosion found in AZ91 alloy.²⁷⁾ In addition, a layered corrosion similar to exfoliation corrosion in high strength aluminum alloy²⁸⁾ was observed on the cross section. Such viewpoint of localized corrosion would be required if the magnesium alloys are utilized under corrosive and loaded conditions.

Fig. 12

Dependence of pitting factor on the Al concentration in AXM and AZX alloys.

4. Conclusion

1. (1)    The W_corr of rolled AXM alloys decreased with the increasing Al concentration, showing a minimum at 7–8% Al in the 5% NaCl solution. The 1%Zn-containing AZX alloy exhibited the opposite result at the same Al concentration. P_corr of both alloys followed the same tendency.
2. (2)    E_corr of the AZX alloy was approximately 50 mV higher than that of the AXM alloy and showed a value of around −1500 mV. Its correlation with the Al concentration was low. The E_corr of the AXM alloy varied according to the Al concentration.
3. (3)    The electrochemical impedance indicated a trajectory consisting of a capacitive semicircle and an inductive loop for all the alloys. Y_ct was chosen to analyze the W_corr. Their time variation differed according to the Al concentration and the presence of Zn. The time average value of Y_ct showed a good correlation with the W_corr.
4. (4)    GDOES characterized the presence of a dense hydroxide film with a thickness of approximately 1 µm on the AXM801 alloy surface and more than 3 µm on the AZX811 alloy surface containing Zn and carbonate.
5. (5)    SEM/EDS detected a lower Al content in the α-phase of the AZX811 alloy than that of the AMX801 alloy; this triggered micro-galvanic corrosion with an enhanced volume of the β-phase.
6. (6)    The AZX alloy with high Al concentration was prone to exhibit localized corrosion.

Acknowledgements

This paper is based on the results obtained from a future pioneering program (innovative structural materials project) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

REFERENCES

• 1)   H.  Mori,  K.  Fujino,  T.  Kurita,  Y.  Chino,  N.  Saito,  H.  Komai and  H.  Ohara: Materia 52 (2013) 484–490.
• 2)   N.  Saito,  K.  Suzuki,  Y.  Fukuda,  T.  Ito,  M.  Noda,  Y.  Gonda and  Y.  Chino: J. JILM 66 (2016) 246–251.
• 3)  S. Huang, Y. Chino, H. Ueda, M. Inoue, T. Kido and T. Matsumoto: Collected Abstracts of the 2016 Autumn Meeting of the Japan Inst. Light Metals, (2016) pp. 71–72.
• 4)   A.D.  Südholz,  N.T.  Kirkland,  R.G.  Buchheit and  N.  Birbilis: Electrochem. Solid-State Lett. 14 (2011) C5–C7.
• 5)   O.  Lunder,  K.  Nisancioglu and  R.S.  Hansen: SAE Technical Paper (1993) 930755.
• 6)   K.  Nisancioglu and  O.  Lunder: J. Surf. Finish. Soc. Jpn 44 (1993) 883–889.
• 7)   J.H.  Nordlien,  K.  Nişancioglu,  S.  Ono and  N.  Masuko: J. Electrochem. Soc. 143 (1996) 2564–2572.
• 8)   G.  Song and  A.  Atrens: Adv. Eng. Mater. 5 (2003) 837–858.
• 9)   G.  Song,  A.  Atrens and  M.  Dargusch: Corros. Sci. 41 (1998) 249–273.
• 10)   D.  Zander and  C.  Schnatterer: Corros. Sci. 98 (2015) 291–303.
• 11)   M.  Grimm,  A.  Lohmüller,  R.F.  Singer and  S.  Virtanen: Corros. Sci. 155 (2019) 195–208.
• 12)   M.  Zhao,  M.  Liu,  G.  Song and  A.  Atrens: Corros. Sci. 50 (2008) 1939–1953.
• 13)  I. Nakatsugawa, X. Huang, N. Saito, K. Suzuki and Y. Chino: Magnesium 2018, ed. by Z. Fan and C. Mendis, (Brunel University Press, London, 2018) pp. 318–323.
• 14)   K.  Gusieva,  C.H.J.  Davies,  J.R.  Scully and  N.  Birbilis: Int. Mater. Rev. 60 (2015) 169–194.
• 15)   Y.  Asakawa,  F.  Sato,  T.  Nakayama and  H.  Sato: J. JILM 42 (1992) 759–764.
• 16)   C.  Yim,  Y.  Kim and  B.  You: Mater. Trans. 48 (2007) 1023–1028.
• 17)   P.  Wu,  F.  Xu,  K.  Deng,  F.  Han,  Z.  Zhang and  R.  Gao: Corros. Sci. 127 (2017) 280–290.
• 18)   S.  Baek,  H.  Kim,  H.  Jeong,  S.  Sohn,  H.  Shin,  K.  Choi,  K.  Lee,  J.  Lee,  C.  Yim,  B.  You,  H.  Ha and  S.  Park: Corros. Sci. 112 (2016) 44–53.
• 19)   M.  Curioni,  F.  Scenini,  T.  Monetta and  F.  Bellucci: Electrochim. Acta 166 (2015) 372–384.
• 20)   A.D.  King,  N.  Birbilis and  J.R.  Scully: Electrochim. Acta 121 (2014) 394–406.
• 21)  M.E. Orazem and B. Tribollet: Electrochemical Impedance Spectroscopy, 2nd Edition, (Wiley, Hoboken, 2017).
• 22)   R.  Lindstrom,  L.G.  Johansson and  J.E.  Svensson: Mater. Corros. 54 (2003) 587–594.
• 23)   G.  Williams,  N.  Birbilis and  H.N.  McMurray: Electrochem. Commun. 36 (2013) 1–5.
• 24)  Sumitomo Electric, M. Yamakawa, M. Numano, T. Sugihara, Y. Kubo, M. Iguchi and O. Mizuno: JP 2012-140656, 2012.7.26.
• 25)   H.  Feng,  S.  Liu,  Y.  Du,  T.  Lei,  R.  Zeng and  T.  Yuan: J. Alloy. Compd. 695 (2017) 2330–2338.
• 26)  N. Taniguchi, H. Suzuki and M. Naito: JAEA-Research, 2009-066.
• 27)   O.  Lunder,  J.E.  Lein,  S.M.  Hesjevik,  T.K.  Aune and  K.  Nisancioglu: Werkst. Korros. 45 (1994) 331–340.
• 28)   S.  Yamamoto,  M.  Kawamoto and  T.  Takenaka: J. Jpn Inst. Metals 60 (1996) 1187–1191.