*2.2.1. Other BMG-based system materials*

showed that the Zr55Cu30Al10Ni5 exhibited an immune response to localized corrosion either

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

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

> SO4 , Na<sup>2</sup> SO4

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

that an increase in the concentration of NaCl of 0.01–0.2 M, added to a 0.5 M H<sup>2</sup>

**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>

or in a 0.1 M NaOH solution over the entire potential scanning range (−1000 up

SO4

SO4

, and NaOH solutions.

electrolyte,

solution (pH 8).

in a 0.1 M Na<sup>2</sup>

SO4

122 Metallic Glasses - Properties and Processing

amorphous metal systems.

while their best performance was satisfied in H<sup>2</sup>

Reproduced from [45] with permission from Elsevier Science.

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 under investigation.

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 high strength with lightweight materials [54].

The electrochemical properties of Ca-based BMGs (Ca65Mg15Zn20, Ca55Mg18Zn11Cu16, and Ca50Mg20Cu30) were investigated in a 0.05 M Na<sup>2</sup> SO4 electrolyte [51]. The Ca65Mg15Zn20 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 stainless steel when identical test conditions were prevailed.

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 exhibited superior corrosion resistance among the other three alloys.
