The Properties of Oxidation-Resistant Scales Formed on Molybdenum-Base Alloys at Elevated Temperatures

Due to the limitations of externally applied claddings for protecting molybdenum-base alloys from oxidation at elevated temperatures, a program of developing protective self-regenerative scales on molybdenum-base alloys was conducted. Various molybdates and molybdenum-containing oxide complexes were tested for stability at elevated temperatures in order to determine if they would be stable at operating temperatures and thus possibly present a barrier to further oxidation. Nickel and cobalt molybdates appeared promising and were further studied in terms of their properties and their ability to form protective scales upon molybdenum alloys containing them. Results of oxidation tests show that molybdate scales will form on molybdenum-nickel and molybdenum-cobalt alloys and are protective. Both nickel and cobalt molybdates spall upon thermal cycling. Spalling can be suppressed by the addition of certain third components which stabilize the molybdate crystal structure. The nature and suppression of spalling are discussed.