Long-Term Corrosion Resistance of Iron-Based Amorphous Metal Coatings

Novel iron-based amorphous metals, including SAM2X5 (Fe_49.7Cr_17.7Mn_1.9Mo_7.4W_1.6B_15.2C_3.8Si_2.4), SAM1651 (Fe_48.0Cr_15.0Mo_14.0B_6.0C_15.0Y_2.0), and other compositions have been developed for use as corrosion-resistant coatings for spent nuclear fuel containers, as criticality control materials, and as ultra-hard corrosion-resistant material for ship applications. These amorphous alloys appear to have corrosion resistance comparable to (or better than) that of Ni-based Alloy C-22 (UNS # N06022), based on measurements of breakdown potential and corrosion rate in seawater. Chromium (Cr) and molybdenum (Mo) provide corrosion resistance, boron (B) enables glass formation, and rare earths such as yttrium (Y) lower critical cooling rate (CCR). SAM1651 has a nominal critical cooling rate of only 80 Kelvin per second due to the additions of yttrium, while SAM2X5 is characterized by relatively high critical cooling rates of approximately 600 Kelvin per second and has no yttrium. As previously reported, the hardness of Type 316L Stainless Steel is approximately 150 VHN, that of Alloy C-22 is approximately 250 VHN, and that of HVOF SAM2X5 ranges from 1100-1300 VHN. Such hardness makes these materials particularly attractive for applications where corrosion-erosion and wear are also issues. Since SAM2X5 has high boron content, it can absorb neutrons efficiently, and may therefore find useful applications as a criticality control material within the nuclear industry. In addition to using a variety of characterization techniques including electron microscopy with focused ion beam milling to determine the effects of processing on microstructure, we have used cyclic polarization, linear polarization, and electrochemical impedance spectroscopy to develop an understanding of passive film behavior for this material during 4-month exposures in natural seawater and a wide variety of concentrated brines at temperatures from 30 to 90 degrees Centigrade. A variety of impedance models have been explored for fitting the data, including transmission line models. Results on these important new alloys will be presented.