Use of Molten Salt Fluxes and Cathodic Protection for Preventing the Oxidation of Titanium at Elevated Temperatures
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INTRODUCTION
CATHODIC protection, where a modest potential is applied to a metallic structure, is a well-known method of preventing oxidation and corrosion in aqueous solutions.[1] However, cathodic protection has rarely been applied at more elevated temperatures, especially those above the boiling point of water. The only example thus far has been in the nuclear industry, where reactor materials were protected from the attack of fluoride salts.[2] Titanium (Ti) and its alloys, as well as the other group IV metals and alloys, are frequently exposed to oxidizing gases during processing or application. These metals have the unusual ability to dissolve vast quantities of oxygen and other metalloids before they form oxide or other compound phases. This is in contrast to the majority of metals, where oxide films form rather readily, which in some cases can prevent further oxidation and attack. For Ti, however, oxygen solubility can be as high as 15 pct by mass in the solid state and even higher in the liquid state. This is visualized on the phase diagram of Ti and O in Figure 1.[3] The presence of oxygen in the surface region of a solid Ti component gives rise to the so-called a-case. It has inferior mechanical and fatigue properties to pure Ti and therefore has to be removed. Conventionally, this is done either by grinding away the surface layer or by dissolving it in hydrofluoric/nitric acids. Both these treatments cause problems with the disposal of the waste CARSTEN SCHWANDT, formerly Senior Research Fellow with the Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, U.K., is now National Chair Professor of Materials Science and Metallurgy of the Sultanate of Oman with the Department of Materials Science and Metallurgy, University of Nizwa, Birkat Al Mouz Initial Campus, Nizwa, Sultanate of Oman, and also Distinguished Visiting Fellow with the Department of Materials Science and Metallurgy, University of Cambridge. Contact e-mail: [email protected]; carsten@unizwa. edu.om DEREK J. FRAY, Professor Emeritus, Director of Research, is with the Department of Materials Science and Metallurgy, University of Cambridge. Manuscript submitted January 28, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
and invariably alter the dimensions of the component. It would thus be advantageous if oxygen ingress and a-case formation could be prevented straight from the outset and, beyond that, if catastrophic oxidation or corrosion could be avoided. In 2000, it was shown that it was possible to remove the a-case from a Ti metal or alloy component by placing it in a bath of molten calcium chloride (CaCl2) and making it the cathode vs a graphite anode.[4,5] Under these conditions, the oxygen in the surface scale is expelled into the molten salt in the form of oxide ions (O2) such that the scale is converted back into pure metal. Critical requirements for this process are that the molten salt employed is thermally and chemically stable, and possesses a wide electrochemical stabi
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