US2964399A - Tantalum-titanium corrosion resistant alloy - Google Patents

Description

limited States hatent Leland K. Lyons, Boulder City, Nevt, assignor to Titanium Metals Corporation of America, New York, N.Y., a

corporation of Delaware N Drawing. Filed June 25, 1959, Ser. No. 822,727 3 Claims. (Cl. 75-174) This invention relates to corrosion resistant alloys and more particularly to an alloy containing tantalum and titanium.

The corrosion resistant characteristics of tantalum metal are well known. It is employed, as a pure metal in various fabricated forms, under severe service conditions such as exposure to boiling acids for extended periods. To be useful in such applications, however, tantalum must be extremely pure inorder to provide ductile metal which can be readily worked into bars, sheets, rods, tubes and other fabricated forms. In addition, it has been found that many impurities, including metallic and nonmetallic elements andcornpounds, seriously reduce the corrosion resistant properties of tantalum, often when present in only small amounts.

Another factor affecting the usefulness of pure tantalum is its extremely high cost whichis of the order of $50.00 to $80.00 per pound in fabricated form. Thus; While tantalum has valuable corrosion resistant properties its general usefulness has been severely restricted due to its high cost. 7 7

It isa principal object of this invention to provide an improved corrosion resistant alloy. Another object of this invention is to provide an alloy having corrosion re-' sistant properties approaching that of pure tantalumbut at substantially less cost. Another object of this invention is to provide a. corrosion resistant and readily fabricated tantalum, alloy.. These and other objects of this invention will be apparent from the following detailed description thereof.

In its broadest aspects this invention contemplates an alloy of tantalum and titanium in which the tantalum is present in amount more than 40% of the alloy, with the balance essentially all titanium. Preferably the alloy consists essentially of more than 40% and up to about 65% tantalum, balance substantially all titanium. An alternative embodiment of this invention contemplates the same type of preferred alloy composition in which less than one-half of its. tantalum content is replaced with columbium without substantial impairment of its corrosion resistant properties.

I have discovered that a loys of tantalum and-titanium containing, more than 40% tantalum show remarkable corrosion resistant properties which approach those of pure tantalum. Table l which follows, shows the corrosion in mils per year of alloys containing 50% and 60% tantalum, balance titanium, when immersed in. various extremely corrosive acids at boiling temperature;

Table I Corrosion RateMi1s per year 1 Gain due to oxide coating.

Patented Dec. 13, 11960 Pure tantalum has been found to have a corrosion rate" corrosion art a corrosion rate of less than 5 mils per year is considered resistant, and materials showing such corrosion resistance may be and are, used in corresponding corrosive environments with anticipation in most applica-' tions of substantially unl'irnitedlife.

It is essential that the binary alloys of this invention contain more than 40% tantalum. When 40% or less tantalum is present in the alloy, an inexplicable but nevertheless extremely sharp and critical reduction in corrosion resistance results. Table 2 following, shows the corrosion inmils per year of a 40% tantalum 60% titanium alloy and these results should be compared with those shown in Table 1. It is evident that reductions of the tantalum content to 40% has. resulted in a fortyfold increase in the corrosion rate in 20% HCl, a substantial increase in the H SO; corrosion rate, and at least atenfold increase in the corrosion rate in oxalic acid.

Table 2 Corrosion Rate-Mi1s:per year The alloy of this invention may contain higher per- Eeritag'es of tantalum if desired, but the best combination of characteristics andcost areobtained when the tantalum content is within the preferred range of more than 40% and up to about. 65% of the alloy. Since the price of tantalum powder is about $50.00 per'pound and the price of titanium sponge is about $3.00 per pound, itwill be evident that the material cost, as an example, for a- 50% Ta, 50% Tialloy will be only about $2650 orlitt-le' morethan one-half that of pure tantalum; and such an alloy will have excellent corrosion resistant properties as shown above. It will also be' obviousthat as the tantalum content is increased the cost of the materials increases, approaching at the highest percentages that of pure tantalum. The upper limit of 65% tantalum in thepreferred' range, represents a percentage at which the excelent properties of the allows are obtained at Very reasonable cost. Binary alloys of titanium and tantalum containing more than 60% and up to 65 are more particularly described and claimed in my copending patent application Serial No. 40,561, filed July 5, 1960. In addition, tantalumtitanium alloy compositions containing m0re tl'ian40%' and up to about 65 tantalum are ductile and readily fabricated by forging, rolling, and other known working techniques, into bars, rods, sheets, plates, tubes. and other" fabricated forms for use in industry. At the same time, such alloys have relatively high tensile strength and mechanical properties which are more than adequate for use as structural materials in corrosion environment applications.

A unique feature of the alloys of this invention containing more than 40% and up to about 65% tantalum is that part of the tantalum content thereof may be replaced With columbium, provided the replacement is less than one-half of the aggregate of these metals, without appreciable impairment of their corrosion resistance and other properties. Table 3 following, shows the corrosion rates of a 50% tantalum, 50% titanium alloy in which various proportions of the tantalum have been replaced by columbium.

Table 3 Corrosion Rate-Mils per year The amount of columbium present in the alloys shown in Table 3 above may readily be calculated as more than about 5% (for the alloy containing 2.9% Ob) and up to about 32% (for the alloy containing 16% Ch) of the aggregate amount of tantalum and columbium.

It will be evident from Table 3 that excellent corrosion resistance is shown by the alloys in which columbium is substituted in part for tantalum and these alloys also show mechanical properties and ease of fabrication similar to those of alloys previously described.

Only less than one-half of the tantalum can be replaced with columbium since otherwise the corrosion properties will be adversely affected. Table 4 below shows the corrosion resistance of an alloy containing 25% tantalum, 25% columbium and 50% titanium exposed to the same conditions discussed hereinbefore. It is clear that the corrosion resistance of this alloy, in which one-half the tantalum has been replaced by columbium, is not nearly as good as the resistance shown by the alloys in Table 3.

Table 4 Corrosion Rate-Mils per year The tolerance for columbium in the alloys of these embodiments of this invention is of the utmost significance. Tantalum and columbium often occur together in nature as a combined tantalite-combumbite ore. Tantalum and columbium, however, are difficult to separate by chemical or metallurgical methods and one of the principal reasons for the high cost of pure tantalum is the difliculty and expensive processing required to completely separate it from impurity columbium. Such pure tantalum is not essential in the alloys of this invention, as previously explained, and a much cheaper and more impure (with respect to columbium content) grade may be employed. Provided the columbium content of the tantalum plus columbium is less than 50%, such impure tantalum may be employed in the alloys of this invention without significant deleterious effect thereon, and at a substantial cost saving even over the pure tantalumtitanium alloys.

A unique and most valuable property of alloys of this invention is their low specific gravity. Tantalum is heavy, having a specific gravity of 16.6 as metal and 14.4 as powder. Titanium has a specific gravity of 4.5. However, the specific gravity of the 50% Ta--50% Ti alloy, for example, has been determined to be only 7.1. Therefore, there is an anomaly in that the specific gravities of these alloys are significantly less than would be normally expected, considering the specific gravities of their constituent metals. Substitution of columbium, which has a specifice gravity of 8.55, even further reduces the specific gravity of the alloys of this invention, as an example, an alloy comprising 41.5% Ta8.5% Co-50% Ti, has a specific gravity of only 6.9. These low specific gravities are of material importance, since the amount or volume per pound is obviously much greater. Thus, for example, ten pounds of pure tantalum in sheet of inch thickness will amount to 0.92 square feet, while 10 pounds of the 41.5% Ta8.5% Cb50% Ti alloy in sheet of the same thickness will amount to 2.23 square feet.

Thus, this invention provides corrosion resistant alloys comparable in many respects to pure tantalum at a small fraction of the cost.

Commercial grades of pure tantalum, columbium, and titanium, which will contain incidental impurities in amount less than that which would cause deleterious embrittlement in the aloy or otherwise effect its usefulness, are suitable for use to produce the alloys of this invention.

The alloys may be produced according to methods well known in the art, such as by melting together the constituent metals; for example, tantalum (or tantalum and columbium) powder and titanium powder or subdivided sponge, may be admixed and compacted to form a consumable electrode which is melted under vacuum in an arc furnace into a so-called cold mold to form an ingot. The ingot may be heated and forged or otherwise wrought or cast into useful forms such as rods, tubes, sheets, plates, bars, and wire. Pure tantalum must be rolled or formed cold because of sensitivity to oxidation at elevated temperatures, but the alloys of this invention containing up to about tantalum are much a less sensitive to this effect, and may be rolled or formed hot if desirable or convenient although a finish rolling cold will generally produce a better surface condition. These may be further fabricated to produce, for example, tank and reactor linings, piping, and other chemical plant apparatus. And as has been explained hereinbefore, such products are useful in many applications where previously only pure tantalum or more noble metals were known to have adequate corrosion resistance.

I claim:

1. A corrosion resistant alloy consisting essentially of more than 40% and up to about 65 in the aggregate of tantalum and columbium, with the columbium present in amount more than about 5% and less than one-half the aggregate amount of tantalum and columbium, balance substantially all titanium.

2. A corrosion resistant alloy consisting essentially of more than 40% and up to about 65% in the aggregate of tantalum and columbium, with the columbium present in amount more than about 5% and up to about 32% of the aggregate amount of tantalum and columbium, balance substantially all titanium.

3. A corrosion resistant alloy consisting essentially of about 34% tantalum and about 16% columbium, balance substantially all titanium.

References Cited in the file of this patent UNITED STATES PATENTS 2,754.204 Jatfee et a1. July 10, 1956 2,819.960 V Bomberger Jan. 14, 1958 2,922,714 Benham Jan. 26, 1960 FOREIGN PATENTS 718,822 Germany Mar. 24, 1942 OTHER REFERENCES Journal of the Institute of Metals, voume 81, 1952-53, article by Summers-Smith on pages 73-76.

Claims (1)

1. A CORROSION RESISTANT ALLOY CONSISTING ESSENTIALLY OF MORE THAN 40% AND UP TO ABOUT 65% IN THE AGGREGATE OF TANTALUM AND COLUMBIUM, WITH THE COLUMBIUM PRESENT IN AMOUNT MORE THAN ABOUT 5% AND LESS THAN ONE-HALF THE AGGREGATE AMOUNT OF TANTALUM AND COLUMBIUM, BALANCE SUBSTANTIALLY ALL TITANIUM.