Micro- and Nano-Scale Corrosion in Iron-Based Bulk Metallic Glass Sam 1651 and Silver-cored MP35N Lt Composite

The corrosion of two engineering materials, an Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ (at.%) bulk metallic glass (SAM 1651) and a silver-cored 35%Co-35%Ni-20%Cr-10%Mo (wt.%) composite (silver-cored MP35N LT), was studied. The structural and compositional heterogeneities in the Fe-based metallic glass of nanometer scales and the dimension of the silver-cored composite of micrometer sizes enabled the formation of corrosion cells on the micro- and nano-scales. Selective dissolutions occurred at the Y-Mo-rich constituents (ca. 10 to 100 nm) in both fully amorphous and partially devitrified SAM 1651, at the nanometer Cr-depleted zones (ca. 10 nm) in the partially devitrified SAM 1651 and at the silver core (ca. 30 um) of the composite.

Heat treatment of SAM 1651 at 600, 700 and 800 °C caused devitrification of the amorphous structure with the precipitation of nanocrystalline (Cr, Fe)₂₃C₆ and (Cr, Fe)₇C₃ carbides following primary transformation. The formation of nanometer Cr-depleted zones surrounding the carbides is proposed to be the reason for the degradation in the corrosion resistance of SAM 1651 after heat treatment observed at both macroscopic and nano-scale. Under diffusion controlled growth, the sizes of the carbide particles and of the Cr-depleted zones increased with the increase of the heat treatment temperature. The increase in the size of the Cr-depleted zones resulted in the decrease of the corrosion resistance of SAM 1651 as observed when temperature increased from 600 to 800 °C. However, the heat treated material still exhibited good corrosion resistance in 6M HCl with the corrosion rate of less than 5 µm/year as measured in weight loss test.

The corrosion of 30 um diameter silver cores of the composite in vitro was studied. The exchange current density of the Ag/Ag⁺ redox reaction in NaCl solution was found to be on the order of 10^-4 A/cm². The deposition of AgCl corrosion product layer at the silver microelectrode slowed the kinetics of the anodic dissolution. Ionic transport via micro-channels running through AgCl layer was the rate controlling process. As the layer was thin, i.e. on the order of micrometers, the silver dissolution process was under mixed activation-ohmic controlled regime. As the layer was thick, i.e. on the order of tens micrometers or thicker, the silver dissolution process was under ohmic controlled regime.

The geometry and the apparent conductivity of the AgCl layer played an important role in determining the corrosion kinetics. The anodic dissolution current decreased with increase of the size and the decrease of the apparent conductivity of the AgCl layer. When the AgCl layer was in cylindrical or hemispherical shape, the corrosion kinetics was found to increase linearly with the square root of the AgCl layer apparent conductivity, to increase linearly with the square root of the potential difference between the cathode and the anode, and to decay linearly with the square root of time.