Effect of boronizing on the oxidation of niobium

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 degrees 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 degrees C, in air, was investigated. As-received Nb samples were observed to lose weight at and above 800 degrees 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. (C) 2013 Elsevier Ltd. All rights reserved.