Tantalum or tantalum alloys comprising pure or substantially pure tantalum and at least one metallic element selected from Ru, Rh, Pd, Os, Ir,, Pt, Mo, W, and Re, forming tantalum alloys resistant to water corrosion. This invention also relates to methods for preparing tantalum alloys.
When the hydrogen concentration is greater than 100 ppm, the hydrogen embrittlement effect of pure tantalum and tantalum alloys begins to become significant. In the chemical processing industry (CPI), pure tantalum absorbs hydrogen and becomes brittle when exposed to hot HCl and hot H₂SO₄. Ta-3W exhibits better resistance to hydrogen absorption than pure tantalum. When tantalum and tantalum alloys are used in the chemical processing industry to contain very hot concentrated acids, the primary mechanism of failure is hydrogen embrittlement rather than wall thinning due to corrosion. Controlled addition and retention of nitrogen can improve the oxidation resistance of the alloy. In other words, it has been found that by adding nitrogen to form microalloys, the microstructure, particularly the grain size, of the type of alloy studied can be controlled, or a relatively stable structure can be achieved at elevated temperatures over extended periods of time. Furthermore, and most advantageously, to extend service life, the specific ratio of silicon to titanium should be observed, as shown in this document.
Methods for improving resistance to hydrogen embrittlement include forming a microalloy with at least one metallic element selected from Ru, Rh, Pd, Os, Ir, Pt, Mo, W, and Re, and pure or substantially pure tantalum or tantalum alloys. A preferred embodiment involves adding platinum to NRC76. The chemical processing industry is seeking new tantalum alloys that allow for higher operating temperatures in their processing equipment. One objective is to obtain improved tantalum alloys with better resistance to water corrosion and hydrogen embrittlement. A tantalum alloy comprising pure or substantially pure tantalum or tantalum alloys and at least one metallic element selected from Ru, Rh, Pd, Os, Ir, Pt, Mo, W, and Re is also described. The content of the metallic element can reach up to the solubility limit of the metal in tantalum.
Tantalum or tantalum-based alloys are preferably prepared using a vacuum melting method. Vacuum arc remelting (VAR), electron beam melting (EBM), or plasma arc melting (PAM) are also vacuum melting methods that can be used to form alloys. To formulate the actual alloy, at least one element selected from ruthenium, rhodium, palladium, osmium, Iridium, platinum, molybdenum, tungsten, and rhenium (Ru, Rh, Pd, Os, Ir, Pt, Mo, W, and Re) is added to pure tantalum material or substantially pure tantalum material or tantalum alloy using one of the above vacuum melting methods. The tantalum alloy preferably contains tungsten as well as platinum, molybdenum, rhenium, or mixtures thereof. Although it is stated above that VAR, EBM, or PAM can be used, VAR is the preferred technique.
Examples of tantalum alloys with a tantalum content of at least 89% include, but are not limited to, Ta-3W (tantalum-tungsten) containing at least about 3% tungsten, Ta-3W-Pt (tantalum-tungsten and platinum alloy) containing at least about 3% tungsten, Ta-3W-Mo (tantalum-tungsten and molybdenum alloy) containing at least about 3% tungsten, and Ta-3W-Re (tantalum-tungsten and rhenium alloy) containing at least about 3% tungsten. Ta-3W-Pt, Ta-3W-Mo, and Ta-3W-Re can be formulated and manufactured in a manner similar to that used in the preparation of Ta-3W. The alloys are preferably prepared by forming microalloys with other metals in the Ta-3W (tantalum-tungsten) alloy.
Adding platinum is the most preferred embodiment because platinum has the most free electrons, which theoretically can attract additional oxygen atoms to seal holes in the Ta2O5 oxide layer and/or provide sites for low hydrogen overvoltage, thereby stabilizing the Ta2O5 oxide layer. Another preferred embodiment involves the addition of ruthenium, rhodium, palladium, osmium, and iridium (also known as "platinum group metals," or PGMs), which also provide low hydrogen overvoltage sites, thereby stabilizing the Ta₂O₅ oxide layer.
Samples are prepared using either the Laser Additive Manufacturing (LAM) method or the conventional Vacuum Arc Remelting (VAR) technique. In the former technique, powders of tantalum, tungsten, and platinum are mixed according to the desired composition, and then melted and solidified under inert conditions using a laser. In these samples, the final tantalum alloy contains 2.8 wt% tungsten and 500 ppm platinum. In the latter technique, powders of tantalum and platinum are pressed into leech-shaped objects according to the desired composition and welded to the side of an NRC76 rod (this assembly is referred to herein as the "electrode"). The electrode is then melted using the conventional Vacuum Arc Remelting (VAR) technique. In these samples, the final tantalum alloy contains 2.8 wt% tungsten and 10,000 ppm platinum.
