Corrosion Study of Aluminum 7075 Alloy in Cationic Aqueous Medium by Surface Analysis

Abstract

The results of the corrosion nature of aluminum 7075 alloy in a cation-containing aqueous medium are reported by using gravimetric tests and surface analysis. The investigation showed a substantial increase in the mass of the specimens after the immersion experiments, while a minor mass change was seen in the Zn²⁺ containing solution. Energy dispersive spectroscopic mapping and cross-sectional pictures showed that the specimen submerged in the Zn²⁺ containing solution developed a zinc-linked layer. The results of X-ray photoelectron spectroscopy showed that aluminum hydroxide was deposited on the surface, and a tiny amount was deposited on the specimen that was sunk in the solution containing Zn²⁺. The present study suggests that the zinc-related layer prevents the alloy from corroding.

1. Introduction

Aluminum and its alloys have extensive usage in different areas like electric module packaging, automotive structure, solar energy management, and airframe production.¹⁾ The most wonderful and lucrative properties of aluminum alloys are their lightweight, high specific strength, processability, anti-corrosion behavior, and recoverability.^1,2) Aluminum 7075 alloy is commonly used in mold tool manufacturing due to its high strength, low density, and thermal characteristics, as well as its ability to be highly polished.²⁾ Aluminum 7075 alloy has good machinability, owing to its high mechanical strength and specific stiffness,^2–4) making it appropriate for the manufacture of structural components for the aerospace and automotive sectors.⁵⁾

Even though aluminum alloys are resistant to corrosion in the presence of water, chloride ions dissolve the oxide films^6–8) and accelerate the oxidation reaction of aluminum. Because freshwater contains both cations and hostile chloride ions.⁹⁾ Chloride ions and other negative ions tend to destroy the surface oxide film through different mechanisms. A number of research on the corrosion of aluminum,^10–13) aluminum 6061 alloy,^14–16) aluminum 6063 alloy,¹⁷⁾ aluminum 5XXX series alloy,¹⁸⁾ and other aluminum alloys in water^19–21) have been conducted.

Metal cations have a strong impact on the aqueous corrosion of mild steel,^22,23) stainless steel,^24,25) aluminum 2024-T3 alloy,²⁶⁾ and aluminum 6061 alloy²⁷⁾ at room temperature. Few studies^28,29) have investigated the effect of metal cations on the corrosion behavior of aluminum alloys in aqueous solution. A few studies have been conducted on the corrosion of aluminum 1100 alloy,³⁰⁾ aluminum A3003 alloy, and other aluminum alloys in model tap water,^31,32) and it has been found that metal cations have a considerable influence on the corrosion of the alloy. According to certain research, metal cations change the corrosion behavior in aqueous media^33–36) at room temperature.

Some studies have reported that the metal is first ionized from the alloy and then returns to the metal surface to form an oxide or hydroxide layer that plays an important role in preventing subsequent corrosion of that alloy. According to Sun et al.,³⁷⁾ corrosion products operate as a protective barrier against zinc-coated steel corrosion in aqueous media. According to Tsuru,³⁸⁾ zinc is dissolved from corrosion products and reabsorbed onto the steel surface, preventing further corrosion of that steel substrate. He also stated that the external metal ion present in the aqueous media is more effective than the internal ionized metal ion from the alloy in avoiding corrosion processes. The same results have been obtained when mild steel is subjected to an aqueous solution containing Zn²⁺.²²⁾ Mg and Zn are among the metals found in aluminum 7075 alloy (Table 1).

Table 1 Standard chemical composition of A7075 aluminum alloy (mass%).

Since the alloy contains zinc and magnesium as its allying element, it is unknown how this alloy will behave in an aqueous medium rich in magnesium and zinc ions. The interaction between the internal and external ions in aqueous medium is also unknown. Similarly, the effect of metal cations on the long-term immersion corrosion behavior of aluminum 7075 alloy in freshwater is not well understood.

As a result, the current study’s goal is to clarify the corrosion natures of aluminum 7075 alloy under the specified conditions based on the experimental results. To achieve the aforementioned goal, relevant experimental settings were used to investigate the corrosion conduct of this alloy. The effect of metal cations on the corrosion of aluminum 7075 alloy in a simulated freshwater environment was investigated using immersion tests, surface investigations, and analysis with a scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDS), and X-ray photoelectron spectroscope (XPS).

2. Experimental Procedure

2.1 Test sample

A sheet of 7075-aluminum alloy (1.5 mm thick) was used. Table 1 shows the sample’s standard chemical components. The alloy sheet was sliced into 1 cm² pieces for testing purposes. All of the samples were molded in epoxy resin, with one side surface left open. The open surface of each molded sample was ground using silicon carbide abrasive sheets ranging in grit size from #1000 to #4000 (Marumoto Struers S5629, LaboForce-3). The resin samples were removed and ultrasonically cleaned (SIBATA ultrasonic cleaner, SU-2T) in ethanol and highly filtered water. The samples were then placed in a desiccator until the immersion testing.

2.2 Testing solutions

As testing solutions, three distinct metal cation-containing solutions were prepared: 10⁻³ M NaCl (Na_sol), 5 × 10⁻⁴ M MgCl₂ (Mg_sol), and 5 × 10⁻⁴ M ZnCl₂ (Zn_sol). All solutions had a Cl⁻ content of 10⁻³ M, which is comparable to freshwater.^9,39) During both the preparation and testing phases, the prepared solutions were clear and transparent. The pH of the solutions was kept around neutral. The pH measurement of the solutions before and after immersion tests were carried out using the pH meter (Eutech Instruments Pte. Ltd., Cyber-Scan 6000). Special analytical grade chemicals were used in this experiment which were obtained from Kanto Chemical Co. Ltd., Japan.

2.3 Immersion corrosion tests

At 25°C, samples were submerged in the testing solutions for 30 and 60 d. The immersion experiments were carried out in glass vials (25 mL), which were left open to the air during the tests to allow for proper oxygen circulation. A microbalance was used to measure the mass of the samples before and after the immersion tests. A digital pH meter was used to record the pH of the solutions at each phase of immersion. The immersion tests began individually, with a different sample for each category (e.g., 30 d, 60 d). Each test was run three times to ensure repeatable results. Following the immersion tests, the samples were cleaned with ethanol and highly filtered water in an ultrasonic bath. The cleaned samples were maintained in a desiccator until the surface investigation to avoid humidity and contamination.

2.4 Surface analysis

The surface morphology of an alloy is a significant characteristic that represents its corrosion behavior in respect to the medium. The surface examination is necessary to determine the variation of the alloy’s overall corrosion behavior under the effect of metal cations. A digital camera and a scanning electron microscope (SEM, JEOL Ltd., JSL6510-LA) were used to examine the surface of the samples. EDS was used to examine the surface composition of the samples after immersion studies.

2.5 Cross-sectional analysis

For the cross-sectional observation and analysis, after the immersion test, the ultrasonically cleaned specimen was coated with resin very carefully. A cross-section polisher (CP, JEOL Ltd., SM-09010) was used to prepare cross-sections of the specimens. The cross-sectional structures of the immersed specimens were also analyzed by the SEM and EDS.

2.6 Surface XPS analysis

Surface analysis was carried out to clarify the situation of specimen surface after immersion in the solutions with different metal cations. The center area of the specimens after immersion was analyzed by X-ray Photoelectron Spectroscope (XPS, JEOL Ltd., JPS-9200) using a monochrome Al Kα X-ray source. The diameter of the specimen analyzed by the XPS was 3 mm. Before the analysis, the immersed specimens were cleaned ultrasonically by ethanol and then in highly purified water. After cleaning, specimens were kept in a desiccator to avoid humidity and any type of contamination.

3. Results and Discussions

3.1 Immersion corrosion tests

Figure 1 depicts a photographic view of the specimens and solutions after 30 and 60 d of immersion at 25°C. After both immersion periods, a very small amount of white cloudy form is observed in the solutions. Besides, there was no other discernible effect in the test solutions. However, after varying times of immersion, some white appearances on the specimens are noticeable except for the specimens immersed in the Zn_sol. Only a few changes are noticeable on the surface in the case of specimens immersed in the Zn_sol regarding shininess.

Fig. 1

Appearance of solutions and specimen surface after the immersion test for 30 d and 60 d at 25°C.

Figure 2 shows the mass change with the submergence period at 25°C. In all of the cases, the mass has been increased after the immersion. However, the largest mass change is calculated in the case of a specimen submerged in Na_sol and significantly the lowest mass change is calculated in the case of a specimen submerged in Zn_sol. Generally, steel specimens lose their mass after submersion in the aqueous medium.^9,22,23) This occurs due to the corrosion reactions between the alloy specimen and the aqueous solutions. In this experiment, the specimen’s mass became greater after immersion in the test solutions. After the destruction of the oxide film, severe oxidation occurred of the aluminum alloy in the case of Na⁺ and Mg²⁺ containing solution, and the Al³⁺ further formed aluminum hydroxide, and deposited on the specimen’s surface by physical or chemical absorption.^9,27) That’s why the mass of the specimen immersed in Na_sol and Mg_sol was increased. On the other hand, in Zn²⁺ containing solution, zinc hydroxide forms a layer with the oxide film that may reduce the oxidation of the aluminum alloy. Therefore, the lowest mass increase was observed in Zn_sol.

Fig. 2

Mass changes of specimen after the immersion for different periods of time at 25°C.

The test solution’s pH before and after 30 and 60 days of immersion is shown in Fig. 3. The pH of the solutions is near neutral before the testing, and it increases slightly with the submersion period for all of the cases (Fig. 3). Though the pH increase is not large, however, it happened due to the formation of hydroxide in the solution.^9,36,39)

Fig. 3

Changes of pH with immersion time at 25°C.

3.2 Surface analysis

Surface morphological characters are an essential scenario for corrosion assessment. SEM photographs of the specimen can be examined for a more comprehensive examination of the corrosion issue. Figure 4 shows a surface scanning electron microscopic view of specimens after varying times of immersion in solutions at 25°C. Some corrosion products that are dispersed across the surface of the specimens submerged in Na⁺ and Mg²⁺ containing solution are noticed. With increasing immersion time, the quantity of distributed products on the surface expands. After 30 d of immersion in Zn_sol, some products are detected on the specimen, and the quantity of products increases after 60 d. However, the appearance of the products differs from that of Na_sol and Mg_sol. After 60 d of immersion, the products are sprayed as a layer on the surface. To determine the surface elemental composition, EDS analysis was performed. Figure 5 displays the surface SEM image and corresponding EDS mapping of elements after 30 d of immersion at 25°C. The illustration displays the mapping of the elements Al, O, Na, Mg, and Zn. The graphic illustrates the Aluminum and Oxygen mapping on each specimen submerged in each solution. Zn is visible in a mapping image of a specimen immersed in Zn_sol. In the mapping photos of specimens submerged in sodium solution and magnesium solution, however, Na and Mg are not apparent. Data on EDS characterization was also gathered. After 30 d of submersion in the solution at 25°C, the EDS characterization results of the specimen surface (% of elements) are shown in Table 2. Table 2 further demonstrates that Zn is widespread on the surface, whereas Na and Mg are absent from the same solution. Figure 6 depicts the EDS spectra of the specimen surface of elements after 60 d of immersion in solutions at 25°C. All of the cases show Al peaks. In the case of Na_sol and Mg_sol, Zn peaks appeared with very low intensity due to the specimen containing zinc as a composition (Table 1). On the other hand, in the case of Zn_sol, a high-intensity peak of Zn appeared. Therefore, it could be recognized that in Na_sol and Mg_sol, only material-derived Zn appears as peaks, while in Zn_sol, solution-derived Zn appears as peaks.

Fig. 4

SEM images of specimens’ surface before and after immersion in the solutions for different periods of time at 25°C.

Fig. 5

Surface SEM image and corresponding EDS mapping of elements after immersion in the solutions for 30 d at 25°C.

Table 2 EDS quantification data of specimen’s surface (mass% of elements) after immersion in the solutions for 30 d at 25°C.

Fig. 6

Surface EDS spectra of elements after immersion in the solutions for 60 d at 25°C.

3.3 Cross-sectional analysis

Cross-sectional photographs of the specimen were examined to determine the extent of corrosion and corrosion pits with depth. Figure 7 shows an SEM cross-sectional picture of specimens after 60 d of immersion in solutions at 25°C. Pits of almost comparable depth are seen in all of the samples. On the other hand, the specimen immersed in Zn_sol, has fewer pits as compared to the other two solutions. Figure 7 also shows the cross-sectional mapping images corresponding to the SEM images. A mapping image of Zn_sol shows a Zn-related compound deposited on the surface that makes a layer-type appearance. On the other hand, in the case of Na_sol and Mg_sol, there is no such layer-type deposition on the surface.

Fig. 7

SEM cross sectional image and corresponding EDS mapping of Zn of specimens immersed in the different solutions for 60 d at 25°C.

SEM analysis revealed that certain products were deposited on the specimen surface following the immersion experiments. Zn is also present in the material (Table 1). However, there is no Zn-related layer on the surface of the specimens immersed in Na_sol and Mg_sol (Fig. 8). Figure 8 shows the SEM cross-sectional picture and associated EDS mapping of Zn of specimens immersed in various solutions for 60 d at 25°C. In the case of Na_sol and Mg_sol, this figure reveals pits and certain products deposited on the surface. On the other hand, a Zn-related layer has been detected on the surface, resulting in a smaller number of pits on the specimen immersed in Zn_sol.

Fig. 8

SEM cross sectional image and corresponding EDS mapping of Al, O, Na, Mg and Zn of specimens immersed in the different solutions for 60 d at 25°C.

3.4 Surface XPS analysis

XPS analysis of the specimen surface was performed to clarify the deposited materials. Figure 9 depicts the XPS wide scan spectra of the surface of the specimens after 60 days of immersion in the solutions at 25°C. All of the instances had C1s, O1s, FeLMM, Al2s, Cr2p3/2, and Al2p3/2 peaks. However, no Cl 1s or Cu2p3/2 peaks are seen. Na1s and Mg1s peaks are not found in Na_sol and Mg_sol, respectively, however, Zn2p1/2 and Zn2p3/2 peaks are observed in Zn_sol. XPS narrow scan spectrums were used to gain a better understanding of the Al and Zn peaks. The XPS narrow scan spectra of Al2p3/2 of specimen submerged in Na_sol, Mg_sol, and Zn_sol and Zn2p3/2 of specimen immersed in Na_sol, Mg_sol, and Zn_sol for 30 d at 25°C are shown in Fig. 10. The Al2p3/2 peaks are seen at binding energy 74.0–74.2 eV (Fig. 10(a), (b), and (c)) in all cases associated with the peak of hydroxide production.^9,30) However, only in the case of Zn_sol is the Zn2p3/2 peak detected at binding energy 1022.5 eV (Fig. 10(f)). This peak is similarly related to the peak of hydroxide formation.^9,22,23)

Fig. 9

XPS wide scan spectra of specimens’ surface after immersion in the solutions for 60 d at 25°C.

Fig. 10

XPS narrow scan spectra of Al2p3/2 and Zn2p3/2 of specimen immersed in Na_sol, Mg_sol, and Zn_sol for 30 d at 25°C.

4. Corrosion Mechanism

According to the experimental results, the external Zn²⁺ forms hydroxides and it further forms a layer on the specimen’s surface with the oxide film by physical or chemical absorption.^9,27) The zinc layer has the ability to protect the oxidation of alloy from the hostile chloride ions while also inhibiting the alloy corrosion. At 25°C, Fig. 11 depicts the corrosion mechanism of aluminum 7075 alloy in aqueous medium with a) Na⁺, Mg²⁺, and b) Zn²⁺. At the experimental pH, it is clear that Na⁺ and Mg²⁺ do not have the ability to produce hydroxide with the oxide film.^22,23,40) The alloy specimen contains Mg as a composition (Table 1) and may ionized a bit to the solution. These internal Mg²⁺ also may not have the ability to produce hydroxide with the oxide film. As a result, chloride ions quickly break down the oxide coating, accelerating alloy corrosion, and some aluminum hydroxide is produced in solution as well as on the steel surface (Fig. 11(a)). As a result, the mass of the specimen is raised following the immersion tests. Even if aluminum hydroxide is generated, it may be unstable or incapable of forming a connection with the surface oxide deposit. As a result, oxidation is not retarded and the alloy is corroded. On the other hand, Zn²⁺ forms hydroxides and creates a link with the surface oxide film and protects it from hostile chloride ions while also inhibiting corrosion reactions (Fig. 11(b)). The alloy specimen contains Zn as a composition (Table 1) and may ionize a bit to the solution. However, the external zinc ions are more effective to form hydroxides as well as for creating a protective layer with the oxide film. Sun et al. reported that ZnO and Zn(OH)₂ are produced as corrosion products when zinc coated steel is exposed in aqueous environment and corrosion protective ability is attributed by these corrosion products.³⁷⁾ Tsuru explained that Zn²⁺ is produced from the corrosion products and reabsorbs into the steel and prevents further corrosion of the steel substrate.³⁸⁾ The XPS investigation revealed a zinc-related layer on the alloy surface. The zinc-related layer on the specimen surface was also shown by SEM cross sectional mapping pictures. This layer serves as a barrier for the oxidation of aluminum alloy. As a result, when compared to the two solutions (Na⁺ and Mg²⁺), the specimen immersed in the Zn²⁺ containing solution showed the lowest mass change.

Fig. 11

Corrosion mechanism of aluminum 7075 alloy in aqueous medium with (a) Na⁺, Mg²⁺, and (b) Zn²⁺ at 25°C.

5. Conclusions

Metal cations in an aqueous solution had a substantial effect on aluminum 7075 alloy. The following are the conclusions:

1. (1)    After the immersion experiments, the mass of the specimen increased, with a lesser mass change recorded in the Zn²⁺ containing solution.
2. (2)    SEM examinations revealed deposited product on the specimen submerged in the Zn²⁺ containing solution, as well as a rather smooth surface.
3. (3)    The results of XPS and EDS validated the composition of the products on the surface as Al hydroxide.
4. (4)    XPS results showed the presence of Zn on the Al alloy surface as hydroxides upon immersion in the Zn²⁺ containing solution.
5. (5)    The Zn²⁺ produced a layer with the oxide film on the alloy surface which slowed Al corrosion processes.

Acknowledgements

A part of this work was conducted at the Laboratory of XPS analysis, Joint-use facilities, Hokkaido University, supported by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT).

REFERENCES

• 1)   D.  Varshney and  K.  Kumar: Ain Shams Eng. J. 12 (2021) 1143–1152. doi:10.1016/j.asej.2020.05.013
• 2)   D.N.  Zhang,  Q.Q.  Shangguan,  C.J.  Xie and  F.  Liu: J. Alloy. Compd. 619 (2015) 186–194. doi:10.1016/j.jallcom.2014.09.002
• 3)   S.  Rajakumar,  C.  Muralidharan and  V.  Balasubramanian: Mater. Des. 32 (2011) 535–549. doi:10.1016/j.matdes.2010.08.025
• 4)   S.M.  Kumar,  R.  Pramod,  M.E.S.  Kumar and  H.K.  Govindaraju: Procedia Eng. 97 (2014) 178–185. doi:10.1016/j.proeng.2014.12.240
• 5)   I.  Peter and  M.  Rosso: Materials and Mechanics 15 (2017) 7–11.
• 6)   K.  Hyun and  S.J.  Kim: Surf. Rev. Lett. 24 (2017) 1850030. doi:10.1142/S0218625X18500300
• 7)   R.T.  Foley: Corrosion 26 (1970) 58–70. doi:10.5006/0010-9312-26.2.58
• 8)  E. McCafferty: Introduction to Corrosion Science, (Springer, New York, 2010) pp. 283–286.
• 9)   K.  Otani and  M.  Sakairi: Corros. Sci. 111 (2016) 302–312. doi:10.1016/j.corsci.2016.05.020
• 10)   Y.M.  Panchenko,  A.I.  Marshakov,  L.A.  Nikolaeva and  V.V.  Kovtanyuk: AIMS Mater. Sci. 5 (2018) 624–649. doi:10.3934/matersci.2018.4.624
• 11)   M.S.  Kaiser and  S.  Dutta: Int. J. Mat. Sci. Eng. 1 (2014) 9–16.
• 12)   S.L.  Coleman,  V.D.  Scott and  B.  Mcenaney: J. Mater. Sci. 29 (1994) 2826–2834. doi:10.1007/BF01117589
• 13)   M.C.  Reboul: Corrosion 35 (1979) 423–428. doi:10.5006/0010-9312-35.9.423
• 14)  M.M. EL-Sayed El-Bedawy: Doctoral thesis, Cairo University, Giza, Egypt (2010).
• 15)   M.  Saxena,  A.K.  Jha and  G.S.  Upadhyaya: J. Mater. Sci. 28 (1993) 4053–4058. doi:10.1007/BF00351231
• 16)   M.K.  Abbass,  K.S.  Hassan and  A.S.  Alwan: Int. J. Mater. Mech. Manuf. 3 (2015) 31–35. doi:10.7763/IJMMM.2015.V3.161
• 17)   D.  Prabhu and  P.  Rao: Arabian J. Chem. 10 (2017) S2234–S2244. doi:10.1016/j.arabjc.2013.07.059
• 18)   I.K.  Choi,  S.H.  Cho,  S.J.  Kim,  Y.S.  Jo and  S.H.  Kim: Coatings 8 (2018) 39. doi:10.3390/coatings8010039
• 19)   E.  Canepa,  R.  Stifanese,  L.  Merotto and  P.  Traverso: Mar. Structures 59 (2018) 271–284. doi:10.1016/j.marstruc.2018.02.006
• 20)   W.B.  Wan Nik,  S.  Syahrullail,  R.  Rosliza,  M.M.  Rahman and  M.F.R.  Zulkifli: J. Teknologi (Sci. and Eng.) 58 (2012) 73–76. doi:10.11113/jt.v58.1552
• 21)   U.  Trdan and  J.  Grum: Adv. Mater. Sci. Eng. 2015 (2015) 705306. doi:10.1155/2015/705306
• 22)   Md.S.  Islam,  K.  Otani and  M.  Sakairi: Corros. Sci. 131 (2018) 17–27. doi:10.1016/j.corsci.2017.11.015
• 23)   Md.S.  Islam,  K.  Otani and  M.  Sakairi: ISIJ Int. 58 (2018) 1616–1622. doi:10.2355/isijinternational.ISIJINT-2018-071
• 24)   Md.S.  Islam,  K.  Otani and  M.  Sakairi: Corros. Sci. 140 (2018) 8–17. doi:10.1016/j.corsci.2018.06.028
• 25)   S.  Zhang,  T.  Shibata and  T.  Haruna: Corros. Sci. 47 (2005) 1049–1061. doi:10.1016/j.corsci.2004.06.014
• 26)   L.  Li,  M.  Sakairi,  Md.  Saiful Islam and  A.  Kaneko: Mater. Trans. 64 (2023) 540–547. doi:10.2320/matertrans.MT-C2022006
• 27)   Md.S.  Islam and  M.  Sakairi: Corros. Comm. 5 (2022) 39–48.
• 28)   K.  Otani,  M.  Sakairi,  R.  Sasaki,  A.  Kaneko,  Y.  Seki and  D.  Nagasawa: J. Solid State Electrochem. 18 (2014) 325–332. doi:10.1007/s10008-013-2260-7
• 29)   M.  Sakairi,  R.  Sasaki,  A.  Kaneko,  Y.  Seki and  D.  Nagasawa: Electrochim. Acta 131 (2014) 123–129. doi:10.1016/j.electacta.2014.02.010
• 30)   M.  Sakairi,  Y.  Shimoyama and  D.  Nagasawa: Corros. Sci. Tech. 7 (2008) 168–172.
• 31)   M.  Sakairi,  A.  Kaneko,  Y.  Seki and  D.  Nagasawa: Proc. Eurocorro. (2008).
• 32)  M. Sakairi, A. Kaneko, T. Kikuchi, Y. Seki and D. Nagasawa: Proceedings of Eurocorr. (2009) NO. SS 17-0-7947.
• 33)   K.  Otani,  M.  Sakairi,  T.  Kikuchi and  A.  Kaneko: Zairyo-to-Kankyo 59 (2010) 330–331. doi:10.3323/jcorr.59.330
• 34)   G.K.  Gomma: Mater. Chem. Phys. 55 (1998) 131–138. doi:10.1016/S0254-0584(98)00084-4
• 35)   J.  Telegdi,  M.M.  Shaglouf,  A.  Shaban,  F.H.  Kármán,  I.  Betróti,  M.  Mohai and  E.  Kálmán: Electrochim. Acta 46 (2001) 3791–3799. doi:10.1016/S0013-4686(01)00666-1
• 36)   T.  Prosek,  D.  Thierry,  C.  Taxén and  J.  Maixner: Corros. Sci. 49 (2007) 2676–2693. doi:10.1016/j.corsci.2006.11.004
• 37)   H.  Sun,  S.  Liu and  L.  Sun: Int. J. Electrochem. Sci. 8 (2013) 3494–3509. doi:10.1016/S1452-3981(23)14407-5
• 38)   T.  Tsuru: Zairyo-to-Kankyo 59 (2010) 404–409. doi:10.3323/jcorr.59.404
• 39)   Md.S.  Islam and  M.  Sakairi: Corros. Sci. 153 (2019) 100–108. doi:10.1016/j.corsci.2019.03.040
• 40)  M. Pourbaix: Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd English ed., (National Association of Corrosion Engineers, (NACE), Houston, Texas, 1974).