Oxidation behavior of Mo–Si–B alloys in wet air

The present work investigates the effect of water vapor on the oxidation behavior of Mo–Si–B phase assemblages. Three alloys were studied: Alloy 1: Mo5Si3Bx (T1)–MoSi2–MoB; Alloy 2: T1–Mo5SiB2 (T2)–Mo3Si and Alloy 3: Mo–T2–Mo3Si. Tests were conducted at 1000 and 1100 °C in controlled atmospheres of dry air and wet air nominally containing up to 2.0×104 Pa H2O. The effect of moisture on the oxidation behavior of Mo–Si–B alloys has been reported for the first time. The initial mass loss of each alloy was approximately independent of the test temperature and moisture content of the atmosphere and varied according to the Mo content of the alloys. All alloys formed a continuous external glassy scale that protected against further mass change after volatilization of the initially formed MoO3. Each alloy underwent a small steady state mass change following the initial volatilization of MoO3. All alloys formed varying amounts of subscale Mo and MoO2, implying that the external glassy scale significantly reduced inward transport of oxygen. For all alloys compared to dry air, water vapor accelerated the growth of the multiphase Mo and MoO2 interlayer at the scale/alloy interface. The role of water vapor on the initial formation of the scale and its stabilization has been qualitatively discussed and analyzed.