The effect of niobium and water on the high‐temperature oxidation behaviour of pure titanium and Ti4Nb (at.%) was studied. An oxidation apparatus was designed that allowed not only the performance of two‐stage oxidation experiments using synthetic air isotopically enriched in 18O2 and 15N2 but also the control of humidity using either H2 16O or H2 18O. Oxidation experiments were performed in dry and humid atmospheres at 800 °C. The oxide scales formed were analysed by secondary neutral mass spectrometry. The results obtained show that niobium significantly reduced the oxidation rate of titanium and also suppressed the influence of water. Moreover, the presence of niobium stabilizes the formation of a nitride layer formed at the scale/metal interface. In humid atmospheres water is the main oxygen source for the oxidation process. This phenomenon has to be taken into consideration when interpreting two‐stage oxidation experiments and discussing oxidation mechanisms. Copyright © 2000 John Wiley & Sons, Ltd.

The effect of niobium and water on the high-temperature oxidation behaviour of pure titanium and Ti4Nb (at.%) was studied. An oxidation apparatus was designed that allowed not only the performance of twostage oxidation experiments using synthetic air isotopically enriched in 18 O 2 and 15 N 2 but also the control of humidity using either H 2 16 O or H 2 18 O. Oxidation experiments were performed in dry and humid atmospheres at 800°C. The oxide scales formed were analysed by secondary neutral mass spectrometry. The results obtained show that niobium significantly reduced the oxidation rate of titanium and also suppressed the influence of water. Moreover, the presence of niobium stabilizes the formation of a nitride layer formed at the scale/metal interface. In humid atmospheres water is the main oxygen source for the oxidation process. This phenomenon has to be taken into consideration when interpreting two-stage oxidation experiments and discussing oxidation mechanisms.