To understand the potential degradation of metallic interconnects, tubes of alloy 446 were exposed to air/hydrogen dual atmosphere at elevated temperatures for 1030 h. The scale developed on the air side consisted of two layers. Because the two sublayers in the oxide scale were too thin for evaluation by regular X-ray diffraction, nanobeam X-ray diffraction was used to study the phase compositions in the scales that formed on both the air and hydrogen sides of alloy 446. The results indicate that Mn1-xCr2-xO4 spinel is the major phase in the oxide scale on alloy 446. The valence of manganese in the oxide scale was also evaluated by X-ray near-edge absorption. (C) 2007 The Electrochemical Society.