*2.1.2.1. Effect of elements content (Ti, Zr, Ta, Nb, Cr, P)*

NaCl solutions. Although the base alloy, Fe-P13C7, did not passivate; additions of any of the foregoing elements at levels from 0.5 up to 40 at.% enabled passivation to occur during

**Figure 2.** Potentiodynamic polarization curves of the cast glassy Fe-based BMGs (rods with a diameter of 1.2 mm) in 1

and titanium were very beneficial. No pitting was observed in 3% NaCl for passivated alloys. The alloys that did not passivate, such as Fe-Co-P13C7, did not pit, but rather they dissolved

Fe-W resisted to pitting corrosion up to 2.5 V/SCE in both acidic and neutral chloride solutions (pH 1 and 7, respectively) [19]. Addition of tungsten to Fe-WxP13C7 has the effect of increasing the critical pitting potential, *E*crit, to a level above 2 V/SCE for x = 6 at.%, but when x = 10 at.% of W was added it caused transpassive dissolution at 1 V/SCE of the MG alloy [19].

Generally, Ni-based metallic glass systems exhibit a good resistance to uniform and localized corrosion. A number of investigations on the electrochemical characteristics of Ni-based amorphous alloys have been performed on ribbons [20–22] due to the struggle in producing amorphous bulk samples (i.e., having thickness > 1.5 mm). The elemental constituents that have typically been used to ensure a good corrosion resistance were either additions of metal-

loids such as P [20, 22] or additions of metals such as Ta [20, 21] and Nb [20].

. Chromium was the most efficient, still, molybdenum,

SO4

and 6 M HCl open to air at 298 K. Reproduced from [13, 29] with permission from Elsevier Science.

anodic polarization in 0.1 N H<sup>2</sup>

116 Metallic Glasses - Properties and Processing

*2.1.2. Ni-based BMG materials*

uniformly.

There has been a consensus that additions of suitable metals to amorphous Ni-based alloys tend to increase their corrosion resistance. The electrochemical behavior of Ni-based amorphous alloys containing Ti, Ta, Zr, Nb, Cr, and/or P has been of a great concern for a number of investigators [20, 25–27]. One of the most significant studies was led by Shimamura et al. [20] who investigated the effect of P and other valve metals (e.g., Ta) on the corrosion properties of Ni-based amorphous ribbons immersed either in boiling 9 M HNO<sup>3</sup> solutions with and without any of Cr6+ ions content or in a boiling 6 M HCl electrolytes. Ta additions have been proven to be the most efficient in lowering the corrosion rates of Ni-based MG alloy. The addition of critical amounts of Ta resulted in undetectable corrosion rates (<10−3 mm year−1). For example, after being immersed in a boiling 9 N HNO<sup>3</sup> solution for 168 h, the corrosion rate of Ni60Ti40 was estimated to be close to 1 mm year−1. After adding of 30 at.% Ta, however, the Ni60Ti10Ta30 MG alloy exhibited an immune response to corrosion for the same period of exposure, i.e., 168 h. Immunity was attributed to the formation of a kind of a protective layer. Although, the authors Shimamura et al. [20] claimed that amorphous Ni-Ta alloys required more than 35 at.% Ta in a boiling 6 M HCl solution to form a tantalum oxyhydroxide (TaO<sup>2</sup> [OH]) shielding passive film.

Alternatively, the addition of a small amount of P to Ni-Ta glassy alloys has been proven to be effective in significantly reducing their corrosion rates. The corrosion rate of Ni70Ta30 in a boiling 6 M HCl solution was more than 104 times greater than that of Ni68Ta30P2 alloy when tested under similar conditions [20]. The authors believed that the addition of P promoted the growth of TaO<sup>2</sup> (OH) passive film by accelerating selective dissolution of elements unnecessary for the passive film formation [20]. However, when experiments were performed in solutions with a high oxidizing power, the authors found that the addition of P to Ni-Ta alloys was not necessary to promote the growth of the passive film. Interestingly, many research works have suggested that a Ta-enriched passive film would probably be one of the reasons for the high corrosion resistance of Ni-based amorphous alloys in aggressive solutions [20, 21].

Moreover, it has been proven that the addition of approximately of 7 at.% Cr was sufficient to prevent pitting corrosion of Ni-Cr-P-B alloy systems immersed in 10% FeCl<sup>3</sup> .H<sup>2</sup> O at 30°C [27]. In another electrochemical study on the Ni-based MG alloy, Habazaki et al. [25] carried out potentiodynamic polarization tests on Ni75-XCrXTa5P16B4 BMGs (X = 5, 10, and 15 at.%) in a 6 M HCl solution in open air at 303 K. The passive current density was shown to decrease as the Cr (at.%) content increased in Ni60Cr15Ta5P16B4.

In line with previous conclusions, and in order to better understand the origin of corrosion resistance in MT-MT systems, another supported corrosion study was carried out on alloys

except the alkaline one, that is, NaOH, both crystalline and metallic glass Cu-Ti alloys exhibited corrosion rates lower than those of pure Cu, and in all cases, the corrosion resistance of the metallic glass alloy was better than that of the crystalline alloy. The metallic glass alloys in such compositional systems are not unusually corrosion-resistant; in fact, neither the crystalline nor the glassy forms of the alloys were more corrosion resistant than pure Ti or pure Zr. This may suggest that the presence of passivation elements, such as Ti, or Zr promotes the corrosion resistance of the metallic glass alloys, and therefore it cannot barely be the result of the presence of the glassy or vitreous state. These results are consistent with previous findings in that the corrosion resistance could be determined by the behavior of the most corrosion-

The synthesis of new bulk metallic glass alloys has subsequently extended the field of corrosion study of many alloy systems, including BMGs. Thus, the development of Cu-based BMGs has been a relatively new occurrence. Lin and Johnson [30] have successfully synthesized Cu-Zr-Ti-Ni glassy alloys with thicknesses of at least 4 mm. Afterward, Inoue et al. [31] developed Cu-Zr-Ti amorphous alloys containing at least 50 at.% Cu with critical diameters between 4 and 5 mm. Among the Cu-Ti glassy alloys, the Cu-Zr-Ti [32] and Cu-Hf-Ti [31] BMGs earned more attention because of their excellent mechanical properties (*viz.* compressive fracture strengths of 2.06–2.15 GPa), which put forward the concept of their possible

The chemical resistance of Cu-BMG systems has become more and more interesting. Unfortunately, the corrosion resistance of most amorphous Cu-based bulk alloys has not been as impressive as their mechanical properties. Nevertheless, the influence of the content of the composition on the corrosion behavior of Cu-based BMGs with a variety of additions of elements is still in a continuous evaluation phase. Many of such additions have shown to improve the corrosion resistance of Cu-based BMGs [33–37, 39]. Additions of small amounts of Nb [33, 35, 36, 39] led to increase the corrosion resistance of Cu-based BMGs. The alloy with other elements such as Cr [34], Ta [33], or Mo [33, 34, 37] has also proved to be effective in improving the electrochemical properties of the Cu-base BMG systems (e.g., corrosion and pitting potentials, etc.). This was the case of one of the comparative studies conducted by Inoue, and Qin et al. [33, 35, 36], who evaluated the effect of low additions of Nb, Mo, and Ta (to a Cu-Zr-Ti-X (X = Nb, Mo, Ta)) on the corrosion behavior of the Cu60Zr30Ti10 (at.%) BMGs

The typical potentiodynamic polarization curves of these Cu-based BMGs are shown in **Figure 4**. The electrochemical behavior of Cu59.4Zr29.7Ti9.9Nb1, Cu59.4Zr29.7Ti9.9Mo1, and Cu59.4Zr29.7Ti9.9Ta1 demonstrated that the addition of Nb was the most effective element in lowering the corrosion rate of Cu-based systems in all the test solutions considered. An increase in the Nb content (up to 5 at.%) led to a decrease in the rate of corrosion in all the solutions tested. As can be seen in **Figure 4**, the additions of Nb to Cu-base alloy resulted into more positive values of the *E*cor, an indication of the improvement of its nobility, thereby suggesting a better behavior of corrosion resistance. This trend was supported by the lower

, 1 M NaOH, or 0.5 M (3%) NaCl.

, and NaOH solutions [15]. In all solutions

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119

A Tribo-Electrochemical Investigation of Degradation Processes in Metallic Glasses

, HCl, HNO<sup>3</sup>

SO4

resistant component of the glassy alloy in the transition metal-metal systems.

of Cu-Zr and Cu-Ti systems in H<sup>2</sup>

usage as engineering materials.

exposed to solutions of 1 M HCl, 1 M HNO<sup>3</sup>

Not all metal additions can improve the corrosion resistance of Ni-based alloys in aggressive solutions. This was the case for Cobalt. Pang et al. [26] studied the anodic polarization behavior of Ni60-XCoXNb20Ti10Zr10 (X = 0, 5, and 20 at.%) in a 6 M HCl solution. The Co additions did not significantly alter the polarization behavior; however, the three tested compositions exhibited spontaneous passivation in the 6 M HCl solution and no pitting was experienced during the anodic polarization. Passive current densities for the three tested alloys were almost identical, that is, about 10<sup>2</sup> mA m−2. The beneficial effects behind the modification of Ni-based amorphous alloys are summarized in **Figure 3**.

**Figure 3.** Anodic polarization curves of the bulk glassy Ni60-xCoxNb20Ti10Zr10 alloys with their critical diameters for glass formation and pure niobium, titanium and zirconium in 6 N HCl solution open to air at 298 K. Reproduced from [26] with permission from Elsevier science.

#### *2.1.3. Cu-based BMG materials*

Recently, the corrosion behavior of either Cu, Zr, or Cu-Zr glassy alloy systems has been investigated [28]. The potentiodynamic anodic polarization behavior of the alloy system exhibited the characteristics of two components, namely Cu, and Zr, while the corrosion resistance of the alloy was not greater than that of the more passive (and noble) metal of the alloy, *viz.* zirconium [28]. Thus, it can be deduced that the corrosion resistance in some transition metal-metal alloy systems could seemingly be due to the presence of a key passivation element in the solid substitute solution, and not to the glassy structure.

In line with previous conclusions, and in order to better understand the origin of corrosion resistance in MT-MT systems, another supported corrosion study was carried out on alloys of Cu-Zr and Cu-Ti systems in H<sup>2</sup> SO4 , HCl, HNO<sup>3</sup> , and NaOH solutions [15]. In all solutions except the alkaline one, that is, NaOH, both crystalline and metallic glass Cu-Ti alloys exhibited corrosion rates lower than those of pure Cu, and in all cases, the corrosion resistance of the metallic glass alloy was better than that of the crystalline alloy. The metallic glass alloys in such compositional systems are not unusually corrosion-resistant; in fact, neither the crystalline nor the glassy forms of the alloys were more corrosion resistant than pure Ti or pure Zr. This may suggest that the presence of passivation elements, such as Ti, or Zr promotes the corrosion resistance of the metallic glass alloys, and therefore it cannot barely be the result of the presence of the glassy or vitreous state. These results are consistent with previous findings in that the corrosion resistance could be determined by the behavior of the most corrosionresistant component of the glassy alloy in the transition metal-metal systems.

The synthesis of new bulk metallic glass alloys has subsequently extended the field of corrosion study of many alloy systems, including BMGs. Thus, the development of Cu-based BMGs has been a relatively new occurrence. Lin and Johnson [30] have successfully synthesized Cu-Zr-Ti-Ni glassy alloys with thicknesses of at least 4 mm. Afterward, Inoue et al. [31] developed Cu-Zr-Ti amorphous alloys containing at least 50 at.% Cu with critical diameters between 4 and 5 mm. Among the Cu-Ti glassy alloys, the Cu-Zr-Ti [32] and Cu-Hf-Ti [31] BMGs earned more attention because of their excellent mechanical properties (*viz.* compressive fracture strengths of 2.06–2.15 GPa), which put forward the concept of their possible usage as engineering materials.

The chemical resistance of Cu-BMG systems has become more and more interesting. Unfortunately, the corrosion resistance of most amorphous Cu-based bulk alloys has not been as impressive as their mechanical properties. Nevertheless, the influence of the content of the composition on the corrosion behavior of Cu-based BMGs with a variety of additions of elements is still in a continuous evaluation phase. Many of such additions have shown to improve the corrosion resistance of Cu-based BMGs [33–37, 39]. Additions of small amounts of Nb [33, 35, 36, 39] led to increase the corrosion resistance of Cu-based BMGs. The alloy with other elements such as Cr [34], Ta [33], or Mo [33, 34, 37] has also proved to be effective in improving the electrochemical properties of the Cu-base BMG systems (e.g., corrosion and pitting potentials, etc.). This was the case of one of the comparative studies conducted by Inoue, and Qin et al. [33, 35, 36], who evaluated the effect of low additions of Nb, Mo, and Ta (to a Cu-Zr-Ti-X (X = Nb, Mo, Ta)) on the corrosion behavior of the Cu60Zr30Ti10 (at.%) BMGs exposed to solutions of 1 M HCl, 1 M HNO<sup>3</sup> , 1 M NaOH, or 0.5 M (3%) NaCl.

**Figure 3.** Anodic polarization curves of the bulk glassy Ni60-xCoxNb20Ti10Zr10 alloys with their critical diameters for glass formation and pure niobium, titanium and zirconium in 6 N HCl solution open to air at 298 K. Reproduced from

Recently, the corrosion behavior of either Cu, Zr, or Cu-Zr glassy alloy systems has been investigated [28]. The potentiodynamic anodic polarization behavior of the alloy system exhibited the characteristics of two components, namely Cu, and Zr, while the corrosion resistance of the alloy was not greater than that of the more passive (and noble) metal of the alloy, *viz.* zirconium [28]. Thus, it can be deduced that the corrosion resistance in some transition metal-metal alloy systems could seemingly be due to the presence of a key passivation

element in the solid substitute solution, and not to the glassy structure.

In another electrochemical study on the Ni-based MG alloy, Habazaki et al. [25] carried out potentiodynamic polarization tests on Ni75-XCrXTa5P16B4 BMGs (X = 5, 10, and 15 at.%) in a 6 M HCl solution in open air at 303 K. The passive current density was shown to decrease as

Not all metal additions can improve the corrosion resistance of Ni-based alloys in aggressive solutions. This was the case for Cobalt. Pang et al. [26] studied the anodic polarization behavior of Ni60-XCoXNb20Ti10Zr10 (X = 0, 5, and 20 at.%) in a 6 M HCl solution. The Co additions did not significantly alter the polarization behavior; however, the three tested compositions exhibited spontaneous passivation in the 6 M HCl solution and no pitting was experienced during the anodic polarization. Passive current densities for the three tested alloys were almost identical, that is, about 10<sup>2</sup> mA m−2. The beneficial effects behind the modification of

the Cr (at.%) content increased in Ni60Cr15Ta5P16B4.

118 Metallic Glasses - Properties and Processing

Ni-based amorphous alloys are summarized in **Figure 3**.

[26] with permission from Elsevier science.

*2.1.3. Cu-based BMG materials*

The typical potentiodynamic polarization curves of these Cu-based BMGs are shown in **Figure 4**. The electrochemical behavior of Cu59.4Zr29.7Ti9.9Nb1, Cu59.4Zr29.7Ti9.9Mo1, and Cu59.4Zr29.7Ti9.9Ta1 demonstrated that the addition of Nb was the most effective element in lowering the corrosion rate of Cu-based systems in all the test solutions considered. An increase in the Nb content (up to 5 at.%) led to a decrease in the rate of corrosion in all the solutions tested. As can be seen in **Figure 4**, the additions of Nb to Cu-base alloy resulted into more positive values of the *E*cor, an indication of the improvement of its nobility, thereby suggesting a better behavior of corrosion resistance. This trend was supported by the lower

Attention was paid to the dependence of the corrosion behavior of Cu-based BMGs on the test environment [33, 35, 36, 38, 39]. Different electrolytes have been selected to study this dependency. In their corrosion study of Cu-Zr-Al-Nb BMGs in 1 M HCl, 0.5 M NaCl, or 0.5 M H<sup>2</sup>

and regardless of the composition, Tam et al. [38] showed that the corrosion rate of Cu-Zr-Al-Nb BMGs was higher in the case of more aggressive solutions (i.e., 1 M HCl and 0.5 M NaCl)

ions, the Cu-BMGs exhibited active behavior, demonstrating the deleterious effects of chloride ions. However, an active-passive behavior was found in the case where Cu-BMGs were

BMG exhibited an active behavior in a 0.5 M NaCl and escorted with high corrosion rates, and

So far, there have been only a few attempts to study the electrochemical properties of Zr-based BMGs in recent years. The majority of these studies have been devoted to systems such as Zr-Ni-Cu-Al [40], Zr-Ti-Ni-Cu-Be [41], and Zr-Ti-Ni-Cu [30] alloys. These Zr-based BMG families are attractive due to their good glass-forming ability and excellent mechanical properties.

Since then, the procedure of adding noble elements (e.g., Nb, Pd, Ti, Ta) has enabled it possible to develop new compositions of MG capable of improving both the capacity of glass-forming ability and the resistance to uniform and localized corrosion [42–45]. Raju et al. [45] investigated the corrosion behavior of Zr-Cu-Al-Ni-X (X = Nb or Ti up to 5 at.%) BMGs in low alkaline

resulted in a slight decrease in *E*cor and an increase in the passive current density in the sulfate solution. On the contrary, such an increase in the contents of Nb and Ti leads to an increase in the values of ηpit and ηp in the NaCl solution, thus revealing the improvement of the alloy *vis-a-vis* its corrosion resistance. A typical comparative graph of the anodic polarization curves

SO4

In a similar study, Asami et al. [42] investigated the electrochemical behavior of Zr60-

An increase in the concentration of Nb resulted in an increase in the *E*pit. The ηpit, however, did not always increase when the Nb content was increased. Substitution of 20 at.% of Nb for Zr resulted in a decrease in the rate of corrosion penetration (CPR) in the 1 M HCl solution from 100 down to 1 μm year−1. The effectiveness of Hf addition (at.%) in improving the passivation ability of the Zr-Cu-Ni-Al BMG has been proven by Liu et al. [46]. Nevertheless, it was pointed out that the addition of noble metals does not always guarantee a better corrosion resistance of Zr-base BMGs.

Qin et al. [43] investigated the corrosion resistance of Zr-base bulk amorphous alloys with three different compositions, namely Zr55Al10Cu30Ni5-XPdX (with X = 0, 1, 3, and 5 at.%) in 0.6 M NaCl. Their findings have shown that the additions of Pd to the Zr-based MG lead to a decrease in the value of ηpit. Therefore, a change in the composition of Zr-based BMG does not

The effect of the environment is as challenging as that of alloying. It has been shown to play a significant role in the corrosion behavior of Zr-based bulk amorphous alloys. Gebert et al. [47]

an active-passive response accompanied with low rates of corrosion in 0.5 M H<sup>2</sup>

SO4

. Similarly, Qin et al. [39] showed that either Cu-Zr-Al BMG or Cu-Zr-Al-Nb

A Tribo-Electrochemical Investigation of Degradation Processes in Metallic Glasses

), and chloride electrolytes (0.01 M NaCl). Increasing the Nb or Ti content

electrolytes (pH 8) is shown in **Figure 5**.

SO4

and lower in the less aggressive solution (0.5 M H<sup>2</sup>

exposed to H<sup>2</sup>

sulfate (0.1 M Na<sup>2</sup>

SO4

of these related Zr-based BMGs in 0.1 M Na<sup>2</sup>

always have a shielding effect for the MG alloy.

XNbXAl10Ni10Cu20 (X = 0, 5, 10, 15, and 20 at.%) BMGs in 0.5 M H<sup>2</sup>

SO4

*2.1.4. Zr-based BMG materials*

SO4 , 121

). In both solutions containing chloride

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SO4

and 1 M HCl solutions.

solution.

**Figure 4.** Potentiodynamic polarization curves of Cu-based BMGs (Cu0.6Zr0.3Ti0.1)100xNbx (x = 0, 2 and 6 at.%) alloys) in either 1 N HCl, 0.5 M (3%) NaCl, 1 N H<sup>2</sup> SO4 or 1 N HNO<sup>3</sup> open to air at 298 K. Reproduced from [35] with permission from Elsevier Science.

*i* cor, and the larger *E*pit values. Ta was not to, as effective as Nb, more likely due to its lower concentration (only 0.2 at.% Ta was added).

Liu and Liu [34] investigated the electrochemical behavior of Cu47Zr11Ti34Ni8 and (Cu47Zr11Ti34Ni8)99.5X0.5 (X = Cr, Mo, and W) BMGs in aqueous solutions of 0.5 M H<sup>2</sup> SO4 and 1 M NaOH. Additions of Cr, Mo, and W have led to the extension of the passive region, the lowering of passive current densities, and lowered the corrosion rates. The alloy with Mo addition, however, showed the most improved corrosion resistance in the two tested solutions, namely H<sup>2</sup> SO4 and NaOH. All Cu-based BMGs with Mo additions had passive films enriched in ZrO<sup>2</sup> and TiO<sup>2</sup> but depleted in Cu-oxides, which are less chemically stable and denser than ZrO<sup>2</sup> and TiO<sup>2</sup> [37]. It was believed that the addition of Mo was most efficient in improving the corrosion resistance of the alloy because its lower ionization energy compared to that of Cr and W, and leading to faster film formation [34, 37].

Attention was paid to the dependence of the corrosion behavior of Cu-based BMGs on the test environment [33, 35, 36, 38, 39]. Different electrolytes have been selected to study this dependency. In their corrosion study of Cu-Zr-Al-Nb BMGs in 1 M HCl, 0.5 M NaCl, or 0.5 M H<sup>2</sup> SO4 , and regardless of the composition, Tam et al. [38] showed that the corrosion rate of Cu-Zr-Al-Nb BMGs was higher in the case of more aggressive solutions (i.e., 1 M HCl and 0.5 M NaCl) and lower in the less aggressive solution (0.5 M H<sup>2</sup> SO4 ). In both solutions containing chloride ions, the Cu-BMGs exhibited active behavior, demonstrating the deleterious effects of chloride ions. However, an active-passive behavior was found in the case where Cu-BMGs were exposed to H<sup>2</sup> SO4 . Similarly, Qin et al. [39] showed that either Cu-Zr-Al BMG or Cu-Zr-Al-Nb BMG exhibited an active behavior in a 0.5 M NaCl and escorted with high corrosion rates, and an active-passive response accompanied with low rates of corrosion in 0.5 M H<sup>2</sup> SO4 solution.

## *2.1.4. Zr-based BMG materials*

*i*

tions, namely H<sup>2</sup>

from Elsevier Science.

enriched in ZrO<sup>2</sup>

denser than ZrO<sup>2</sup>

concentration (only 0.2 at.% Ta was added).

in either 1 N HCl, 0.5 M (3%) NaCl, 1 N H<sup>2</sup>

120 Metallic Glasses - Properties and Processing

SO4

and TiO<sup>2</sup>

and TiO<sup>2</sup>

to that of Cr and W, and leading to faster film formation [34, 37].

SO4

cor, and the larger *E*pit values. Ta was not to, as effective as Nb, more likely due to its lower

**Figure 4.** Potentiodynamic polarization curves of Cu-based BMGs (Cu0.6Zr0.3Ti0.1)100xNbx (x = 0, 2 and 6 at.%) alloys)

or 1 N HNO<sup>3</sup>

Liu and Liu [34] investigated the electrochemical behavior of Cu47Zr11Ti34Ni8 and (Cu47Zr11Ti34Ni8)99.5X0.5 (X = Cr, Mo, and W) BMGs in aqueous solutions of 0.5 M H<sup>2</sup>

and 1 M NaOH. Additions of Cr, Mo, and W have led to the extension of the passive region, the lowering of passive current densities, and lowered the corrosion rates. The alloy with Mo addition, however, showed the most improved corrosion resistance in the two tested solu-

improving the corrosion resistance of the alloy because its lower ionization energy compared

and NaOH. All Cu-based BMGs with Mo additions had passive films

but depleted in Cu-oxides, which are less chemically stable and

open to air at 298 K. Reproduced from [35] with permission

[37]. It was believed that the addition of Mo was most efficient in

SO4

So far, there have been only a few attempts to study the electrochemical properties of Zr-based BMGs in recent years. The majority of these studies have been devoted to systems such as Zr-Ni-Cu-Al [40], Zr-Ti-Ni-Cu-Be [41], and Zr-Ti-Ni-Cu [30] alloys. These Zr-based BMG families are attractive due to their good glass-forming ability and excellent mechanical properties.

Since then, the procedure of adding noble elements (e.g., Nb, Pd, Ti, Ta) has enabled it possible to develop new compositions of MG capable of improving both the capacity of glass-forming ability and the resistance to uniform and localized corrosion [42–45]. Raju et al. [45] investigated the corrosion behavior of Zr-Cu-Al-Ni-X (X = Nb or Ti up to 5 at.%) BMGs in low alkaline sulfate (0.1 M Na<sup>2</sup> SO4 ), and chloride electrolytes (0.01 M NaCl). Increasing the Nb or Ti content resulted in a slight decrease in *E*cor and an increase in the passive current density in the sulfate solution. On the contrary, such an increase in the contents of Nb and Ti leads to an increase in the values of ηpit and ηp in the NaCl solution, thus revealing the improvement of the alloy *vis-a-vis* its corrosion resistance. A typical comparative graph of the anodic polarization curves of these related Zr-based BMGs in 0.1 M Na<sup>2</sup> SO4 electrolytes (pH 8) is shown in **Figure 5**.

In a similar study, Asami et al. [42] investigated the electrochemical behavior of Zr60- XNbXAl10Ni10Cu20 (X = 0, 5, 10, 15, and 20 at.%) BMGs in 0.5 M H<sup>2</sup> SO4 and 1 M HCl solutions. An increase in the concentration of Nb resulted in an increase in the *E*pit. The ηpit, however, did not always increase when the Nb content was increased. Substitution of 20 at.% of Nb for Zr resulted in a decrease in the rate of corrosion penetration (CPR) in the 1 M HCl solution from 100 down to 1 μm year−1. The effectiveness of Hf addition (at.%) in improving the passivation ability of the Zr-Cu-Ni-Al BMG has been proven by Liu et al. [46]. Nevertheless, it was pointed out that the addition of noble metals does not always guarantee a better corrosion resistance of Zr-base BMGs.

Qin et al. [43] investigated the corrosion resistance of Zr-base bulk amorphous alloys with three different compositions, namely Zr55Al10Cu30Ni5-XPdX (with X = 0, 1, 3, and 5 at.%) in 0.6 M NaCl. Their findings have shown that the additions of Pd to the Zr-based MG lead to a decrease in the value of ηpit. Therefore, a change in the composition of Zr-based BMG does not always have a shielding effect for the MG alloy.

The effect of the environment is as challenging as that of alloying. It has been shown to play a significant role in the corrosion behavior of Zr-based bulk amorphous alloys. Gebert et al. [47]

**2.2. Corrosion and associated mechanisms in the case of the binary, ternary, or** 

The majority of previous corrosion studies involved BMG systems based on Cu, Fe, Ni, and Zr, but there are other BMG systems, such as Ca- [51] Mg- [52], and Ti-based MG [53] still

A Tribo-Electrochemical Investigation of Degradation Processes in Metallic Glasses

The selection of materials and design alloys are, inter alia, the major factors driving the global BMGs market. More new BMG systems will emerge and become commercially available in mass production in the near future as these amorphous alloys have many attractive properties for everyday life such as biomaterials, electronic devices, structures, and so on. This is the case for Ca- and Ti-based BMGs, which have shown great interest because of their potential applications as biomaterials. The Mg-based system is interesting for applications requiring

The electrochemical properties of Ca-based BMGs (Ca65Mg15Zn20, Ca55Mg18Zn11Cu16, and

BMG experienced pitting at free corrosion conditions and had a CPR of 5691 μm year−1. However, both Ca50Mg20Cu30 and Ca55Mg18Zn11Cu16 were slightly passivated at *E*cor conditions and exhibited CPR values in the order of 1503 and 311 μm year−1 respectively.

The electrochemical behavior of the Ti43.3Zr21.7Ni7.5Be27.5 BMG immersed in a phosphate buffered saline (PBS) solution at 310 K was examined by Morrison et al. [53]. The Ti-base BMG exhibited a passive behavior at *E*cor conditions but it showed a localized corrosion susceptibility at more increasing potentials. The ηpit value was about 589 mV/SCE. The Ti-base BMG alloy had a CPR value of about 2.9 μm year−1. The authors concluded that the alloy resistance to localized corrosion in the PBS solution was equivalent to or greater than that of the 316 L

Gebert et al. [52] has performed a comparative corrosion study of both Mg65Y10Cu15Ag10 and Mg65Y10Cu25 BMGs in a borate buffer solution (pH 8.4) with pure Mg and Mg65Y10Cu25 crystalline alloys. The electrochemical behavior of both amorphous and crystalline Mg65Y10Cu25 alloys was similar, but superior to that of pure Mg. Although, the Mg65Y10Cu15Ag10 BMG

There is still considerable interest in how the electrochemical properties of metallic glasses and BMGs compare to those of conventional crystalline alloys. Amorphous alloys are believed

• their compositions, which are not constrained by the solubility limits, and they can be

SO4

electrolyte [51]. The Ca65Mg15Zn20

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123

**quaternary amorphous alloy systems**

*2.2.1. Other BMG-based system materials*

high strength with lightweight materials [54].

Ca50Mg20Cu30) were investigated in a 0.05 M Na<sup>2</sup>

stainless steel when identical test conditions were prevailed.

exhibited superior corrosion resistance among the other three alloys.

*2.2.2. Corrosion resistance of BMGs in comparison to crystalline alloys*

to exhibit corrosion resistance due to:

allied to elements promoting passivation [47],

under investigation.

**Figure 5.** Anodic polarization curves of Zr-Cu-Al-Ni-x (x = Nb or Ti) BMG alloys in 0.1 M Na<sup>2</sup> SO4 solution (pH 8). Reproduced from [45] with permission from Elsevier Science.

showed that the Zr55Cu30Al10Ni5 exhibited an immune response to localized corrosion either in a 0.1 M Na<sup>2</sup> SO4 or in a 0.1 M NaOH solution over the entire potential scanning range (−1000 up to 2000 mV/SCE). However, the susceptibility to pitting corrosion was observed on the BMG surface during anodic polarization experiments for chloride concentrations as low as 10−3 M. The *E*pit was shown to decrease as the chloride concentrations increased. However, this trend has been tempered by the anodic pre-growth of a passive film. Similarly, Mudali et al. [48] have found that an increase in the concentration of NaCl of 0.01–0.2 M, added to a 0.5 M H<sup>2</sup> SO4 electrolyte, significantly decreased the *E*pit of the Zr-base BMG. Other studies [47, 49] further support this outcome. It has been shown that a decrease in localized corrosion resistance is appropriately associated with the exposure of the Zr-base BMG surfaces to solutions with increasing concentrations of chloride ions [47, 49]. In agreement with the foregoing conclusions, many researchers [42, 44, 48, 50, 56] have concluded that the majority of the degradation of a number of Zr-based BMG systems due to localized corrosion involved exposure to solutions containing chloride ions while their best performance was satisfied in H<sup>2</sup> SO4 , Na<sup>2</sup> SO4 , and NaOH solutions.

The effect of other factors, despite what has been mentioned above, such as the test temperature and passivation level, was found to affect the electrochemical properties of Zr-based BMGs. Gebert et al. [50] studied this effect on the corrosion behavior of Zr55Cu30Al10Ni5 BMG. The anodic polarization was carried out at 298, 423 and 523 K in a 0.001 M NaCl electrolyte on the pre-passivated Zr55Cu30Al10Ni5 samples and those without any specific treatment. For untreated and pre-passivated BMG samples, the *E*pit decreases with increasing temperature. It was concluded that a decrease in temperature and prior passivation treatment promoted the tendency of the Zr55Cu30Al10Ni5 BMG to resist pitting in the chloride solution, which was consistent with the observations of many crystalline and amorphous metal systems.
