Niobium oxidised in air during 16 hours at 1130 °C

Niobium coated with aluminium oxidised in air at 650 °C.

Oxidation of niobium and of niobium coated with aluminium in steam-air mixtures

W.M.M. Huijbregts and M.J. Brabers

(Technological University, Delft, the Netherlands)

Proceedings S.E.R.A.I. Journee internationales d'étude sur l'óxydation des metaux. Bruxelles, oct - 1965. (paper 01)

Paper available as pdf

SUMMARY.

The oxidation behaviour of pure niobium and of niobium coated with aluminium has been studie in the temperature range of 400 to 600 °C in different atmospheres: superheated steam of 1 atm., a mixture of steam and air, and in air.

Depending on temperature and on oxidation rate the following oxides have been found: NbO, NbOz needles, mixtures of NbO, and T Nb2O5 and a T Nb2O5 scale. The NbOz phase not only is formed below 500 °C and at low oxygen pressures, as reported in the literature, but also at high oxidation rates in air up to 600 °C.

The rate of oxidation is determined by the properties of the different oxides. Below 600 °C the Nb2O5 formed in air is a loose powder and formed in steam and steam+air is a rather dense oxide. The Nb2O5 scale flakes off at cooling down to room temperature. The NbOz needles furnish a better adherence of oxide to metal than to NbOz and Nb2O5. This may be due to a network formed by NbOz which is continued in the Nb2O5 phase.

The rate of oxidation of aluminium coated niobium in steam, steam+air and in air is alike when the temperature does not exceed 600 °C. A non-porous layer of NbAI3 gives a reasonable protection. At temperatures higher than 600 °C two types of failures have been observed :

• oxidation of aluminium between NbAl3 grains
• formation of cracks in the NbAl3 layer as a result of diffusion and thermal stresses. Once a crack has reached the metal substrate oxide is formed between metal and NbAI3 layer.

It has been observed that the oxidation resistance is high at temperatures of about 1100 °C in air. On cooling, however, cracks are formed which destroy the oxidation resistance of aluininium coated niobium.

on NbOz needles are also formed at temperatures higher than 500 °C for instance, at 575 °C. This is in contradiction with the results of Norman who assumed that NbOz only occurred at low oxygen pressures and at a temperature below 500 °C. The NbOz is probably formed here because the niobium is supersaturated with oxygen as a result of the powdery nature of the T Nb2O5 in this case.

Conclusions.

The results of the oxidation of niobium may be summarised as follows : Three oxide layers are present :

1. An outside layer consisting of T Nb2O5 which is a white, polarising oxide. The Nb2O5 is formed in layers except in air-oxidation, where it appears as a powdery oxide.
2. An in-between layer, consisting of T Nb2O5 + NbO2. This layer is non-polarising.
3. NbOz or NbO. NbOz is formed by oxidation in air below 575 °C as a result of rapid supersaturation of niobium with oxygen, whereas it occurs in steam oxidation below 500 °C on account of a low oxygen pressure. NbO is formed, when niobium is oxidised in air above 600 °C and in steam and steam-air above 550 °C.

The flaking of the Nb2O5 layer and the transition of the oxidation rate can be connected with the presence of the NbOz needles and the NbO-scale. The NbOz needles pin the Nb2O5. The break-away occurs sooner when NbOz needles are present because generally they are less protective than NbO.

The intermetallic compound NbAl3 affords some protection against oxidation. At the interface Nb /NbAl3 oxygen diffuses to the NbAl3 giving a niobate which acts as a diffusion barrier.

However, when a crack is present NbO is formed which grows between niobium and niobate.

Cracks may arise by oxidation of free alu minium between NbAl3 grains or by thermal stresses during cooling in a dense NbAl3 diffusion layer.

Some interesting Figures from the paper.

Photograph 1. By oxidising at 525 °C NbO. needles and the white uniform NbO-scale occur together. Non polarised light

Photograph 2. Polarising NbO. needles growing from the Nb.O. layer into niobium. Polarised light.

Photograph 3. The transition phase of NbOz and NbO. The NbOz needles become broader. Non polarised light.

Photograph 4. The NbOz needles pin the outside Nb2O5 scale; polarised light,

Photograph 5. Aluminium canals formed during dipping of niobium in molten aluminium.

Photograph 6. The NbAI, coating is pushed away by the formation of Nb205.

Photograph 7. Cracks ina dense NbAI3 coating.

Photograph 8. The crack runs into the niobium.

Photograph 9. NbO. needles are formed near the NbAI3.

Photograph 10. The wide NbO-5cale near the NbAl3.