Effect of boronizing on the oxidation of niobium

• Nb has poor resistance to air oxidation above 300 °C.
• Nb is boronized to improve its oxidation resistance.
• Scales of the as-received samples spalled after oxidation
• Boronized samples gained less weight and had adherent scales at 1000 °C.
• A liquid phase containing boron decreased scale growth and increased its strength.

Niobium (Nb) and its alloys are used in various industrial applications including metal processing, nuclear power generation and manufacturing of metallic superconductors and rocket nozzles. Although it is resistant to corrosion in several liquid media, Nb has a high oxidation rate in environments such as air at temperatures above 400 °C. In this study, the effect of the boronizing surface treatment on the morphology and structure of the oxide scales developed over Nb at 800–1000 °C, in air, was investigated. As-received Nb samples were observed to lose weight at and above 800 °C by the cracking, spallation and decohesion of thick Nb2O5 scales. However, oxide scales developed over the boronized samples at the same temperatures were comparatively protective for the metal. Based on the oxidation test results and characterization of the products, it is concluded that the positive effect of the boronizing surface treatment is due to the development of oxidation products including a liquid phase containing boron compounds.