XPS and UPS studies of gas‐phase oxidation, electrochemistry and corrosion behaviour of Ti and Ti5Ta

Owing to its high corrosion resistance, Ti is used as a containment material for processes in hot nitric acid. The corrosion stability of Ti can be improved still further by alloying with 5% Ta. The oxidation behaviour of Ti and Ti5Ta and the composition and stability of their oxide layers are compared by means of XPS, UPS, electrochemical measurements and corrosion test. Thin oxide layers of up to 2 nm have been formed by gas‐phase oxidation. Three regions can be observed due to chemisorption, oxide formation and oxide growth. The average composition of the oxide layer after an oxygen exposure of 1000 L is Ti 2 O 3 , with a layer thickness of 1.8 nm. The change of Ti oxidation states during oxidation shows that the metal/oxide interface is a mixed oxide of TiO, Ti 2 O 3 and TiO 2 , while TiO 2 is formed at the oxide/vacuum interface. The change of work function during oxidation is similar for Ti and Ti5Ta. Thicker oxide layers have been built electrochemically, with no differences in capacity measurements between Ti and Ti5Ta, indicating similar electronic properties. No significant Ta enrichment in 1 N H 2 SO 4 and 1 N HNO 3 is found up to 10 V. Preferential dissolution of Ti in corrosion tests in hot HNO 3 leads to Ta enrichments by factors of >10, which explains the improved corrosion stability of the alloy. The surface is covered by a barrier oxide of ∼10 nm and a porous outer layer with a thickness of >50 nm.