*2.1.1.2. Effect of Mo content*

*2.1.1.1. Effect of Cr content*

114 Metallic Glasses - Properties and Processing

One of the pioneer corrosion resistance studies on Fe-based amorphous alloys with P and C contents in acid chloride-based solutions was performed by Naka et al. [9]. Both uniform and localized corrosion resistances of these glassy alloys were mainly attributed to the growth of protective oxide films when appropriate additions of C and P are used [10]. A comparison was made between the corrosion rates of crystalline Fe-Cr alloys and amorphous Fe-Cr-P-C alloys as a function of chromium concentration as shown in **Figure 1**. At low chromium atomic content levels (<4 at.%), the amorphous alloy corrodes at a higher rate than the crystalline material. However, at even higher chromium levels (4 at.%), there is a significant decrease in the corrosion rate of the MG alloy, whereas the crystalline material remains unchanged. At more elevated and intermediate content Cr level (8 at.%), no corrosion was detected on the MG alloy by weight loss experiments after immersion for 168 h. Surprisingly, it was found that the concentration of HCl electrolyte, which has a significant effect on the corrosion behavior of the crystalline alloys, had no effect on the corrosion of the Fe-Cr-P-C or Fe-Ni-Cr-P-C base MG alloy systems, and which exhibited no weight loss after exposure for a period of 168 h [11].

Chromium was shown to be very effective to confer pitting resistance, such as for the metallic glass alloys Fe-CrxB13C7 and Fe-CrxB13Si7 in 3% NaCl [8]. With chromium level contents of 2 and 5 at.%, both MG alloy types suffered pitting at potentials more anodic than the free corrosion potential of approximately 0.6 V/SCE. The addition of 8 at.% Cr extended the pitting resistance nearly to 1 V/SCE, which represents an extreme condition of competition for alloys containing such a low level of chromium. By contrast, type 304 stainless steel contains

**Figure 1.** Comparison of the corrosion rates of glassy FeCrXP13C7 alloys and crystalline iron-chromium alloys in 1 N

NaCl solution at 30°C. Reproduced from [11] with permission from Elsevier Science.

Molybdenum benefits the pitting resistance of metallic glass alloys and crystalline steels. The addition of Mo to glassy Fe-MoxP13C7 alloys suppressed pitting, decreased the critical current density for passivation and the passive current density [15]. As little as 4 at.% Mo prevented pitting corrosion in 1 N HCl; likewise, low additions of Mo were shown to be more effective in decreasing the corrosion rates than Cr. Molybdenum has been shown to facilitate the formation of a passive film of hydrated chromium or iron oxyhydroxide through its enrichment in the corrosion product layer during active dissolution [16]. The enrichment helps for the accumulation of passivating species in the film by lowering the dissolution rate of the species; the Mo-rich product subsequently dissolves, leaving little Mo behind in the film. However, in another case study [17], the increase of Mo content level up to 22.5 at.% was shown to be detrimental to the corrosion resistance of Fe-based BMG, more likely by increasing the risk of formation of secondary phases in the ferritic matrix zone.
