EFFECTS OF ALLOYING ON THE KINETICS OF OXIDATION OF NIOBIUM

Technical Report · Fri Oct 31 04:00:00 EST 1958

OSTI ID:4306520

Klopp, W D; Sims, C T; Jaffee, R I

Kinetics of the air oxidation of pure niobium and of niobium binary alloys containing up to 25 at.% vanadium, 10 at,% molybdenum, 24 at.% chromium, 10 at.% tungsten, 35 at.% titanium, and 45 at.% zirconium have been studied at elevated temperatures. Results of these studies have suggested several principles for the oxidation of niobium: (a) ionic diffusion is ratecontrolling under certain conditions when apparent linear oxidation is observed, and (b) alloying elements can effect the oxidation behavior by expanding or contracting the partially protective Nb/sub 2/O/sub 5/ scale, by valence effects on diffusion rates through the scale, and by formation of new scales. Pure niobium oxidizes linearly at 600 to 1200 deg C. Moisture increased the oxidation rate at 600 deg C. The scales consisted of an inner, adherent, oriented layer of Nb/sub 2/O/sub 5/ , containing traces of NbO, and an outer, porous layer of Nb/sub 2/O/sub 5/. The oxide initially forms as adherent, protective Nb/sub 2/O/sub 5/, but high compressive stresses developed as a result of the high oxide/ metal volume ratio (2.69) cause the scale to crumble at a relatively low thickness. A steady state reaction ensures, with oxidation following the linear time law. On the basis of preliminary experiments it is believed that ionic diffusion, probably of oxygen ions, through the inner layer is the rate-controllying reaction. Alloying can affect oxidation behavior by three mechanisms: (1) Alloying additions with smaller crystal radii than that of niobium contract the Nb/sub 2/O/sub 5/, promoting protective scales and parabolic oxidation, while larger ions expand the scale and increase the linear oxidation rates. (2) Valence of the alloying addition has a relatively minor effect on diffusion rates through the scale because of the large number of defects in Nb/sub 2/O/sub 5/. (3) At alloying levels beyond the solubility limit of Nb/sub 2/O/sub 5/ for the alloying ion, the oxidation rates are determined by the properties of new scales. (auth)

• Research Organization: Battelle Memorial Inst., Columbus, Ohio
• NSA Number: NSA-12-014822
• OSTI ID: 4306520
• Report Number(s): A/CONF.15/P/712
• Country of Publication: Country unknown/Code not available
• Language: English

Related Subjects

AIR-- CHEMICAL REACTIONS-- CHROMIUM ALLOYS-- CONTROL-- DIFFUSION-- HIGH TEMPERATURE-- HUMIDITY-- IONS-- LAYERS-- MOLYBDENUM ALLOYS-- NIOBIUM-- NIOBIUM ALLOYS-- NIOBIUM OXIDES-- OXIDATION-- REACTION KINETICS-- STRESSES-- THICKNESS-- TITANIUM ALLOYS-- TUNGSTEN ALLOYS-- VANADIUM ALLOYS-- ZIRCONIUM ALLOYS
CRYSTALS-- DEFECTS-- OXYGEN-- SOLUBILITY
MINERALOGY, METALLURGY, AND CERAMICS