Effect of boronizing on the oxidation of niobium E. Dokumaci a, ⁎, I. Özkan b , M.B. Özyigit a , B. Önay a a Metallurgical and Materials Engineering Department, Faculty of Engineering, Dokuz Eylül University, Izmir, Turkey b Torbali Vocational School, Dokuz Eylül University, Izmir, Turkey abstract article info Article history: Received 8 January 2013 Accepted 25 April 2013 Keywords: Niobium Boronizing High temperature oxidation 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 Nb 2 O 5 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 con- cluded that the positive effect of the boronizing surface treatment is due to the development of oxidation products including a liquid phase containing boron compounds. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Niobium (Nb) belongs to a group of elements known as refractory metals. In steel manufacturing, Nb is used to improve mechanical prop- erties of carbon steels and to stabilize the phase structure of austenitic stainless steels. Because of their superconducting properties, some Nb alloys and compounds are utilized in electronic and diagnostic equip- ments such as NMR. High corrosion resistance of Nb–Zr alloys against organic acids and most liquid metals has enabled their use in heat treat- ment processes and some nuclear reactors which require reactive liquid media for heat transfer [1]. Because of its high melting temperature and low density (8.57 g/cm 3 ), Nb has attracted attention as a base-metal for the de- velopment of next-generation high temperature structural materials since the current high temperature materials such as Ni-based superal- loys are getting closer to their usable temperature limits. Although Nb al- loys have already been used in the manufacturing of rocket nozzles and thrusters, coatings are required when such metallic parts are exposed to air because Nb has poor resistance to air oxidation at temperatures above about 300 °C [2]. Even though the oxidation behavior of refractory metals has been investigated since the late 1950s, there is always an interest in the de- velopment of oxidation-resistant refractory metal-based materials which can be employed at temperatures much higher than those reached by the current alloys. The earlier studies conducted on the oxidation behavior of Nb and the effect of alloying on oxidation were reported by Stringer [3]. Most of those studies concluded that Nb and its alloys need oxidation- resistant coatings for long-time use, in air, at temperatures above 1000 °C. In a later study, Perkins et al. showed that oxidation-resistant alumina layers were able to form over the Al and Cr-containing Nb al- loys [4]. However, melting temperatures of these alloys were much lower than expected because of the large amount of alloying elements required to stabilize the alumina phase. Studies conducted in the last decade, on the other hand, showed that addition of Si to Nb promoted the formation of intermetallic phases such as Nb 5 Si 3 which, besides im- proving the alloy mechanical properties, increased the oxidation resis- tance of the alloy by promoting the formation of the SiO 2 phase in the oxide scale [5–8]. More recently, boron-containing Nb–Si–B ternary al- loys were observed to have better oxidation resistance at high temper- atures [9,10]. The present authors have been studying the production and charac- terization of Nb-based and Mo-based refractory alloys. They have also been working on the oxidation behavior of borides and the effect of the boronizing surface treatment on metal oxidation [11,12]. Although boronizing is usually employed to form hard and wear-resistant surface layers over metallic substrates such as steels [13,14], in this work, the authors have studied the effect of boronizing on the structure and mor- phology of the oxide scales formed over the Nb metal. In order to make more use of the refractory metals in high temperature applications, in- vestigation of surface treatment methods are as important as the alloy development. 2. Materials and methods For the boronizing and oxidation tests conducted in this work, commercially-pure Nb samples were used. Before the tests, samples Int. Journal of Refractory Metals and Hard Materials 41 (2013) 276–281 ⁎ Corresponding author. Tel.: +90 232 3017468; fax: +90 232 3017452. E-mail address: esra.dokumaci@deu.edu.tr (E. Dokumaci). 0263-4368/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2013.04.014 Contents lists available at ScienceDirect Int. 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