US3814673A - Process for tantalliding and niobiding base metal compositions - Google Patents

Abstract

A TANTALLIDE OR NIOBIDE COATING IS FORMED ON SPECIFIED BASE METAL COMPOSITIONS BY MAKING THE BASE METAL THE CATHODE JOINED THROUGH AM EXTERNAL ELECTRICAL CIRCUIT TO A TANTAUM OR NIOBIUM ANODE IN AN ELECTRIC CELL HAVING A SPECIFIED FUSED SALT ELECTROLYTE AT A TEMPERATURE OF AT LEAST 900*C., BUT BELOW THE MLETING POINT OF THE METAL COMPOSITION. SUCH A COMBINATION IS A SELF-GENERATING CELL PRODUCING ELECTRIC BUT AN EXTERNAL E.M.F. MAY BE IMPRESSED PROVIDING THE CURRENT DENSITY DOES NOT EXCEED 10 EMPERS/CM.2. THE PROCESS IS USEFUL IN MAKING TIGHT ADHERENT COATINGS COMPOSED OF TANTALUM OR NIOBIUM AND THE BASE METAL ON THE SURFACE OF THE SUBSTRATE.

Description

United States Patent No. 838,636, July 2, 1969. This application Oct. 15,

1971, Ser. No. 189,763 The portion of the term of the patent subsequent to May 26, 1987, has been disclalmed Int. Cl. C23b 5/30, 5/48 US. Cl. 204-39 11 Claims ABSTRACT OF THE DISCLOSURE A tantallide or niobide coating is formed on specified base metal compositions by making the base metal the cathode joined through an external electrical circuit to a tantalum or niobium anode in an electric cell having a specified fused salt electrolyte at a temperature of at least 900 C., but below the melting point of the metal composition. Such a combination is a self-generating cell producing electricity, but an external E.MF. may be impressed providing the current density does not exceed amperes/cm. The process is useful in making tight adherent coatings composed of tantalum or niobium and the base metal on the surface of the substrate.

This is a continuation of application Ser. No. 838,636, filed July 2, 1969, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method for metalliding a based metal composition. More particularly, this invention is concerned with a process for tantalliding and niobiding a base metal composition in a fused salt bath.

It is knovm that tantalum and niobium can be electrodeposited at 650 C. to 850 C. on certain metal compositions to form a firmly adherent layer of tantalum or niobium joined to the metal composition by a metal-tometal bond by electrodeposition in a fused salt bath. This method also requires that the molten fluorides must contain at least one of the fluorides of the group potassium, rubidium, or cesium.

It has been found, however, that these prior art proccesses have a number of shortcomings which substantially limit their usefulness. For example, it has been found that the rate of diffusion and alloying is a function of the bath temperature. In fact, at lower temperatures, 850 C. and below, the process is principally one in which the tantalum and niobium is plated onto the substrate metal with very little or no alloying taking place. Above 850 C., on the other hand, the displacement and volatilization of the potassium, rubidium and cesium in the molten fluorides introduces considerable diificulties. Potassium, perhaps, raises the most difficult problems as it volatilizes at 850 C. Any attempts to operate the bath above 850 C. results in the presence of potassium vapors which combine either with the substrate metals or the metals of other fluorides in the bath. Hence, these prior art processes could not be operated at temperatures at which substantial diffusion coatings of tantalum and niobium, as opposed to electrodeposited layers of these metals, are formed.

We have now discovered that uniform, tough, adherent diffusion coatings of tantalum or niobium can be formed on certain metal compositions employing certain alkali and alkaline earth fluoride molten salts in the complete or substantial absence of the fluorides of potassium, rubidium, or cesium, at temperatures in excess of 850 C. This electrodeposition of tantalum and niobium and sub- 3,814,673. Patented June 4, 1974 sequent formation of diffusion coatings is possible if certain critical steps are taken to insure the substantial absence of oxygen and oxide salts in the fused salt bath.

In accordance with the process of this invention, the tantalum or niobium metal is employed as the anode and is immersed in a fused salt bath composed essentially of a member of the class consisting of the alkali metal fluorides of lithium and sodium and mixtures thereof and mixtures of the alkali metal fluorides with magnesium, calcium, strontium or barium fluoride and containing from 0.01-5 mole percent of tantalum or niobium fluoride.

The cathode employed is the base metal upon which the diffusion coating is to be made. We have found that such a combination is an electric cell in which an electric current will be generated when an electrical connection, which is external to the fused bath, is made between the base metal cathode and the anode. Under such conditions, the metal of the anode dissolves in the fused salt bath and the metal ions are discharged at the surface of the base metal cathode where they form a deposit of tantalum or niobium which immediately diffuses into and reacts with the base metal to form a metallide coating.

The alkali metal fluorides which can be used in accordance with the process of this invention includes the fluorides of lithium and sodium, the mixtures thereof.

Lithium fluoride is preferred, however, because of its lower reactivity at temperatures above 850 C. Eutectic mixtures of lithium and sodium fluoride can often be used, however, especially at the lower operating temperatures of the process.

Mixtures of the all lithium and sodium alkali metal fluorides with magnesium, calcium, strontium, or barium fluoride can also be employed as components of the molten salts in the process of this invention. Magnesium fluoride does not always function as an inert component, however, since it sometimes permits the incorporation of small amounts of magnesium in the diflfusion coating, and this is not always desirable.

The chemical composition of the fused salt bath is critical for optimum metalliding results. The starting salt should be as anhydrous and free of all impurities as is possible or should be easily dried or purified by simply heating during the fusion step. The process must be carried out in the substantial absence of oxygen since oxygen interferes with the process by forming tantalum or niobium oxides and thereby preventing a coherent dififusiou coating of tantalum or niobium from being formed on the base metal cathode. Thus, for example, the process can be carried out in an inert gas atmosphere. By the term substantial absence of oxygen it is meant that neither atmospheric oxygen nor oxides of metals are present in the fused salt bath. The best results are obtained by starting With reagent grade salts and by carrying out the process in an inert gas atmosphere, for example, in an atmosphere of argon, helium, neon, krypton or xenon.

We have sometimes found that even commercially available reagent grade salts must be purified further in order to operate satisfactorily in our process. This purification can be readily done by utilizing scrap metal articles as the cathodes and carrying out the initial metalliding runs with or without additional applied voltage, thereby plating out and removing from the bath those impurities which interfere with the formation of high quality metallide coatings.

We have found that in order for the electrolysis cell of this process to work properly and to form metallide coatings which are bright and smooth rather than dull and rough due to the formation of oxidized surfaces it is necessary to reduce the oxide content of the salt to an extremely low level, i.e., about a few parts-per-million,

and to maintain an inert atmosphere over the salt at all times to prevent re-contamination of the salt with oxygen. T antallided and niobided diffusion coatings can be made on some metal surfacessuch as nickel and iron into which tantalum and niobium readily difiusein the presence of considerable oxygen content in the salt, i.e., a few parts-per-thousand, but the surfaces are usually dull and microscopically rough due to surface oxidation. Such coatings usually need to be slightly thicker than the bright smooth coatings to give comparable corrosion resistances. In the diffusion of tantalum into metals such as chromium and complex alloys where diffusion is slow, it is much more desirable and often critical that the oxide content of the baths be extremely low. The oxygen can be removed from the fused salt bath by employing a carbon anode and running the bath as an electrolytic cell to remove the oxides and oxygen by means of the carbon anode. We have also found that the last traces of oxygen and oxides can be removed from the fused salt bath by maintaining the fused salt bath under an inert atmosphere and placing in the bath, strips or chips of tantalum or niobium for a period of time until the strips or chips upon removal from the bath showed no evidence of pitting or other deterioration of the glossy, shiny surface of the metal due to the reaction of the tantalum or niobium with oxygen. Such strips of metal can also be used as electrodes and will usually speedup the scavenging of the oxygen from the salt melt.

We have also found that when metals are to be tantallided or niobided, it is necessary to conduct the process in the absence of carbon and carbon compounds because carbon forms very stable and refractory carbides of tantalum or niobium on the surface of the base metals thereby rendering it very difficult to penetrate these diffusion barriers and form tantallided and niobided coating on the substrates. We have found that carbon can be removed from the fused salt bath by operating it as a cell employing as a cathode, the-base metals such as nickel or iron, until the carbide coating is no longer formed on the surface of the metal.

The base metals which can be tantallided or niobided, in accordance with the process of this invention includes the metals having atomic numbers of 23-29, 41-46 and 73-79 inclusive. These metals are, for example, vanadium, chromium, manganese, iron, cobalt, nickel, copper, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, tantalum, tungsten, rhenium, iridium, platinum and gold. Alloys of these metals with each other or alloys containing these metals as the major constituent, that is, over 50 mole percent, alloyed with other metals as a minor constituent, that is less than 50 mole percent, can also be metallided in accordance with our process, providing the melting point of the resulting alloy is not lower than the temperature at which the used salt bath is being operated.

In order to produce a reasonably fast diffusion rate and to insure the fusion of the metal into the base metal to form a tantallide or niobide coating, we have found it desirable to operate our process at a temperature no lower than about 850 C. It is usually preferred to operate at temperatures of from 900 C. to 1100 C.

When an electrical circuit is formed external to the fused salt bath by joining the anode to most of the previously listed base metal cathodes by means of a conductor, an electric current will flow through the circuit without any applied electromotive force. The anode acts by dissolving in the fused salt bath to produce electrons and the metal ions. The electrons flow through the external circuit formed by the conductor, and the metal ions migrate through the fused salt bath to the base metal cathode to be metallided, where the electrons discharge the metal ions, forming a metallide coating. The amount of current can be measured with an ammeter which enables one to readily calculate the amount of metal being deposited on the base metal cathode and being converted to the metallided layer. Knowing the area of the article being plated, it is possible to calculate the thickness of the metallide coating formed, thereby permitting accurate control of the process to obtain any desired thickness of the metallide layer.

Although the process operates on most of the substrates listed above very satisfactorily without impressing any additional electromotive force on the electrical circuit, we have found it possible to apply a small voltage when it is desired to obtain constant current densities during the reaction and to increase the deposition rate of the metal being deposited without exceeding the diffusion rate of the metal into the base metal cathode. The additional should not exceed 1.0 volt and preferably should fall between 0.1 to 0.5 volt.

When tantalum is being diffused into niobium, or vanadium it is always necessary to apply a small external cathodic potential to these metals since they are slightly more reactive than tantalum. The same is true for the niobiding of vanadium.

When it is desirable to apply additional voltage to the circuit in order to shorten the time of operation, the total current density should not exceed 10 amperes/cm. At current densities above 10 amperes/cm, the tantalum or niobium deposition rate exceeds the diffusion rate and the base metal cathode becomes coated with a plate of tantalum or niobium.

Since the diffusion rate of tantalum and niobium into the cathode article varies from one material to another, with temperature, and with the thickness of the coating being formed, there is always a variation in the upper limits of the current densities that may be employed. T herefore, the deposition rate of the iding agent must always be adjusted so as not to exceed the diffusion rate of the iding agent into the substrate material if high efliciency and high quality diffusion coatings are to be obtained. The maximum current density for good tantalliding or niobiding is 10 amperes/cmfi, when operating within the preferred temperature ranges of this disclosure. Higher current densities can sometimes be used to form coatings with tantalum and niobium but in addition to the formation of a metallide coating, plating of the iding agent occurs over the diffusion layer.

Very low current densities (0.01-0.l amp/cm?) are often employed when diffusion rates are correspondingly low, and when very dilute surface solutions or very thin coatings are desired. Often the composition of the diffusion coating can be changed by varying the current density, producing under one condition a composition suitable for one application and under another condition a composition suitable for another application. Generally, however, current densities to form good quality tantallide or niobide coatings fall between 0.2 and 4.0 amperes per cm. for the preferred temperature ranges of this disclosure.

If an applied E.M.F. is used, the source, for example, a

battery or other source of direct current, should be connected in series with the external circuit so that the negative terminal is connected to the external circuit, terminating at the metal being metallided and the positive terminal is connected to the external circuit terminating at the metal anode. In this way, the voltages of both sources are algebraically additive.

As will be readily apparent to those skilled in the art, measuring instruments such as voltmeters, ammeters, resistances, timers, etc., may be included in the external circuit to aid in the control of the process.

Because the tough, adherent, corrosion resistant properties of the tantallide or niobide coatings are uniform over the entire treated area, the coated metal compositions prepared by our process have a wide variety of uses. They can be used to protect reaction vessels and apparatus from chemical attack, electro-chemical corrosion, and anodic oxidation, to make gears, bearings, and other articles reqiuring hard, wear-resistant surfaces, and to prevent corrosion at high temperatures on gas turbine material, heating elements etc. Other uses will be readily apparent to those skilled in the art as well as other modifications and variations of the present invention in light 'of the above teachings.

In the specification and claims we use the term tantalide and niobide to designate any solid solutions or alloys of tantalum and nobium and the base regardless of whether the base metal does or does not form an intermetallic compound with tantalum and niobium in definite stoichiometric proportions which can be represented by a chemical formula.

The following examples serve to further illustrate our invention. All parts are by weight unless otherwise stated.

EXAMPLE I Into a Monel vessel (3%" in diameter x 12" deep) was placed lithium fluoride (1720 grams). The Monel vessel was placed in an electric furnace. After, a nickel plated steel cover was attached which contained a water channel for cooling, 2 ports for electrodes and another 2 ports for a thermocouple well and an argon sparge tube. Vacuum connections were made to one of the electrode ports and the salt was then alternately evacuated and repressurized with argon three times at room temperature, and then heated to 300 C. under argon at which temperature evacuation and repressurizing with argon was repeated three more times; the salt was then heated under argon to melting (M.P.'=842 C.). The salt temperature was raised to 900 C. at which point 78 grams of potassium fluorotantalate salt (K TaF- was added through an electrode port.

A A" diameter tantalum rod (anode) was immersed into the salt and clean-up of salt impurities was accomplished by immersing nickel screens'(cathodes -50 cm. each) in the salt at 900 C. and electrolyzing at 4 amps for 15 minutes. After 10 amp-hours of electrolysis coulombic etficiencies (based on reduction of Ta+ to Ta) approaching 100% were realized.

EXAMPLE II Strips of nickel (40 cm. in area) were then tantallided at 1050 C. and at voltages ranging from +0.3 to +0.9 volt (anode polarity) with the following results:

TABLE I Current Wt. Coulombic Time density gain efficiency (mins (amps/cm!) (grams) (percent) EXAMPLE III A sample of expanded nickel screen was next tantallided at 1050 C. with the following results:

TABLE II Volts Current (anode density polarity) (amps/cm!) 6 EXAMPLE IV Four other expanded nickel screens at 1050 C. with the following results:

were tantallided The voltage (anode polarity) remained negative at all times indicating that diffusion of Ta was exceeding the deposition rate.

The expanded nickel screen containing 6.5 mg. Ta/cm. was incorporated as an air cathode current collector in a phosphoric acid matrix fuel cell operating at 15 0 C. The tantallided screen exhibited excellent electrochemical corrosion resistance for the 111 hour test period as a current collector material, i.e. the screen performed as well as a gold screen current collector in the same type fuel cell.

The three remaining tantallided screens containing 7.1, 12.0 and 15.0 mg. Ta/cm. were incorporated as air cathode current collectors in a sulfonic acid solid polymer electrolyte fuel cell operating at 60 C. The tantallided screens exhibited excellent electrochemical corrosion resistance for 450 hours as current collector materials, i.e., the screens performed as well as a gold screen current collector int he same type fuel cell.

EXAMPLE V Strips of 1020 mild steel, 4340 tool steel and Carpenter 20 Cb-3 (50 cm. in area) were tantillided at 1080" C. and -.050 to +0.1 volts (anode polarity), with the following results.

TABLE IV Current Coulombie Time density mg. efiiciency Material (mins.) (amps/cm?) Ta/cm. (percent) The tantallide diffusion coatings formed with the 1020 mild steel and 4340 tool steel were shown to be predominately a tantalum carbide; the tantallide diffusion coatings formed with the Carpenter 20 Cb-3 was checked for Ta/ Fe alloy formation which was found to be present, along with the other components of the Carpenter 20 Cb-3.

Samples of tantallided 1020 mild steel, 4340 tool steel, and Carpenter 20 Cb-3 stainless steel were subjected to anodic electrochemical corrosion in 1.5 N H 80 at 80 C., the tantalided 1020 and 4340 steels had marginal corrosion resistance, but the tantallided Carpenter 20 Cb-3 exhibited excellent corrosion resistance comparable to the cororsion resistance of tantallided expanded nickel screens from Example III.

I EXAMPLE VI Strips of cupron (45% Ni/55% Cu alloy) and molybdenum (30 cm. in area) were tantallided at 1050 C. and -0.070 to +0.600 volt (anode polarity) with the following results:

I 7 EXAMPLE vn A niobium anode can be substituted for the tantalum anode and KzNbFq for the KgTaFq in the lithium fluoride bath and the cell operated as given in the above ex amples to give niobium diffusion'coatings on the various base metal cathodes discussed above.

It will, of course, be apparent'to those skilled in the art that conditions other than those set forth in the above examples can be employed in the process of this invention without departing from the scope thereof.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A method of forming a tantallide or niobide difiusion coating on a metal composition having a melting point greater than 900 C., at least 50 mole percent of said metal composition being at least one of the metals selected from the class consisting of metals whose atomic numbers are 23-29, 41-46, and 73-79, said method comprising,

(1) forming an electric cell containing said metal composition as the cathode, joined through an external electrical circuit to a tantalum or niobium anode and a fused salt electrolyte composed essentially of a member of the class consisting of lithium fluoride or sodium fluoride, mixtures thereof, mixtures of any of magnesium calcium, strontium and barium fluo rides of the alkaline earth fluoride group, or mixtures of the said lithium or sodium fluoride with any of the above named alkaline earth fluorides, and from 0.01- mole percent of tantalum or niobium fluorides, said electrolyte being maintained at a temperature of at least 900 C. but below the melting point of said metal composition in the substantial absence of oxygen,

(2) controlling the current flowing in said electric cell so that the current density of the cathode does not exceed amperes/cm. during the formation of the tantallide or niobium diffusion coating, and

(3) interrupting the flow of electrical current after the desired thickness of the tantallide or niobium difiusion coating is formed on the metal composition.

2. The process of claim 1 wherein the absence of oxygen is obtained by use of a vacuum.

3. The process as claimed in claim 1 which is also conducted in the substantial absence of carbon.

4. The process as claimed in claim 1 wherein the absence of oxygen is obtained by allowing tantalum or niobium metal to be in contact with the fused electrolyte prior to carrying out the process until the oxygen has been depleted from the electrolyte bath.

5. The method of claim 1 wherein the metal composition is nickel. v

6. The method of claim 1 wherein the metal composition is cobalt.

7. The method of claim 1 wherein the metal composition is an alloy of nickel and cobalt.

8. The method of claim 1 wherein the metal composition is an alloy of nickel and copper.

9. The method of claim 1 wherein the metal composition is iron.

10. The method of claim 1 wherein the metal composi tion is any stainless steel alloy.

11. The method of claim 1 wherein the metal composition is molybdenum.

References Cited UNITED STATES PATENTS 3,514,272 5/1970 Cook 29194 3,489,540 1/1970 Cook 29194 3,489,539 1/1970 Cook 29-194 3,479,159 11/1969 Cook 29194 3,479,158 11/1964 Cook 29-194 OTHER REFERENCES Table of Periodic Properties of the Elements, E. H. Sargent & Co.

TA-HSUNG TUNG, Primary Examiner R. L. ANDREWS, Assistant Examiner