US3547709A - Corrosion-resistant uranium - Google Patents

Description

United States Patent 3,547,709 CORROSION-RESISTANT URANIUM George S. Petit and Ralph R. Wright, Oak Ridge, Tenn.,

assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed May 14, 1968, Ser. No. 728,910 Int. Cl. C23f 7/02 US. Cl. 148-63 10 Claims ABSTRACT OF THE DISCLOSURE Metallic uranium is provided with a surface film of uranium oxide which is highly protective against corrosion-producing agents such as dry or moist air and water. The corrosion-resistant uranium oxide film is provided by heating the uranium in an evacuated furnace to a temperature in a range of about 500 to 650 C. and then contacting the uranium with a selected quantity of dry oxygen.

The present invention relates generally to corrosionresistant uranium and more particularly to a method of providing metallic uranium with a corrosion-resistant film of uranium oxide. This invention was made in the course of, or under, a contract with the US. Atomic Energy Commission.

Metallic uranium is by nature a highly reactive metal which readily reacts with and is corroded by many gases and liquids including air and water. Extensive investigations have been previously conducted in an effort to minimize or obviate this corrosion problem. These efforts have resulted in several techniques for alleviating uranium corrosion and include such protective measures as alloying, plastic coatings, covering exposed surfaces with layers of nickel or aluminum by vapor deposition, electroplating with nickel, etc.

While previous techniques for protecting uranium metal from corrosion such as those mentioned above have met with some success, the practice of these techniques is often very difficult and cumbersome with the resulting corrosion barrier in certain instances failing to accomplish its intended purpose. For example, the tendency of uranium metal surfaces to become passive after preplating treatment renders uranium a highly difficult metal to successfully plate. Further, impervious protective platings are not easily provided, particularly when the platings are relatively thin, i.e., of a thickness in the order of about 1 mil (0.001 of an inch).

It is the aim of the present invention to provide exposed surfaces of metallic uranium with a protective layer or film of uranium oxide which is very adherent, hard, and sufliciently resistant to penetration by or reaction with air and water to prevent corrosion of the uranium even when exposed to severe corrosion-causing conditions such as prolonged exposure to heated air containing water vapor. The novel protective uranium oxide film afforded by the present invention is formed of uranium oxides which are not wettable by water or reactive with moist or dry air and water. These uranium oxides consist of a gradient of uranium dioxide and lesser oxides including uranium monoxide with the uranium dioxide being at the surface of the protective film and the uranium monoxide being at the interface between the metallic uranium and the film.

An object of the present invention is to provide metallic uranium with a surface film or coating which obviates or substantially minimizes corrosion of the uranium.

Another object of the present invention is to provide metallic uranium with a surface film of uranium oxide which is resistant to penetration or reaction with air and Water vapor.

A further object of the present invention is to provide a method of treating metallic uranium in order to render the latter resistant to corrosion by air and water. The method comprises the formation of a uranium oxide film on the surface of the metallic uranium by heating the uranium in an evacuated chamber and contacting the uranium with a selected quantity of dry oxygen.

Other and further objects of the invention will be obvious upon an understanding of the illustrative method for protecting uranium metal from corrosion about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.

Described generally, the present invention relates to the treatment of uranium metal for rendering it highly resistant to attacks by normally corrosive agents, particularly water and moist or dry air. The uranium metal is protected from these corrosive agents by providing the uranium metal with a surface film of uranium oxide which is composed of uranium dioxide (U0 and lesser oxides in a gradient ranging down to uranium monoxide (UO). This film is produced on. the metallic uranium by heating the uranium metal in vacuum after the surfaces thereof have been cleansed of excess naturally occurring oxides and other contaminants and thereafter contacting the heated uranium With a selected quantity of virtually moisture-free or dry oxygen.

The protective film of the present invention is believed to be formed by a mobile layer of U0. A thin film of U0 will form on the uranium metal in the furnace due to the residual oxygen in the furnace atmosphere and the oxygen on the surface of the uranium metal. With the uranium metalin the evacuated furnace a preselected quantity of oxygen is slowly admitted into the furnace and comes in contact with the metallic uranium. This contact between the oxygen and the thin UO film on the metallic uranium causes the U0 to consume the oxygen and form a thin layer of U0 which, in the presence of excess uranium metal, will permit the reaction for forming a new layer of U0. Thus, the interface between the uranium oxide film and the metal continually progresses into the uranium metal while at the same time maintaining the same chemical bonding as the initial film (U0) formed. There is probably some diifusion of oxygen through the uranium oxide film, permitting the formation of some UO due to a limited quantity of oxygen reacting directly with excess uranium metal at the interface. As the quantity of excess uranium diminishes with the increasing thickness of the protective film, the quantityof oxygen in the film increases. Thus, there is a gradually increasing gradient of oxygen content in the protective film from the interface surface outwardly. The structure of the film effected by this gradient ranges from U0 at the interface to U0 at the film surface, with the latter providing the predominant proportion of the protective film thickness. An analytical determination performed on the protective film indicated that the oxygento-uranium ratio of the film is about 1.9. Accordingly, in a film thickness of about 0.2 to 1.0 mil (0.001 of an inch), the surface of the uranium metal is protected by U0 rather than U0.

The use of excess quantities of dry oxygen for the formation of a uranium oxide film on metallic uranium can be deleterious in that the film resulting from the exposure of the heated uranium to excessive amounts of oxygen is a relatively thick, non-wetting layer of U0 which is not significantly protective and which often exhibits cracking and flaking. It is believed that this nonprotective film results when a sufficient quantity of dry oxygen is present to cause the direct formation of U by the reaction U+O; UO rather than the indirect formation indicative by the reaction described above.

In order to provide the corrosion-resistant uranium oxide film of the present invention, the uranium metal to be treated must be thoroughly cleansed prior to the for mation of the protective film thereon. The naturally occurring uranium oxide on the surface of the metallic uranium must be removed prior to heating the metallic uranium in the presence of dry oxygen since this oxide inhibits the formation of the protective film. The layer of natural uranium oxide may be readily removed by immersing the uranium in an acid bath, such as, for example, a solution of 8 N nitric acid, for approximately 20 minutes. Other contaminants may be removed prior to the acid bath by using conventional cleansing procedures such as scouring powders and distilled water rinses.

The best results provided by the protective uranium oxide film are obtained on metallic uranium articles having a surface finish of 80 root means square (RMS) or better. Pits and irregularities in the uranium metal produce the weakest areas in the film and corrosion will normally occur in these areas first, if at all. However, even when corrosion occurs in these relatively small areas, it does not spread over the entire piece as would normally occur with uranium metal which has been subjected to corrosion inhibiting techniques as previously practiced.

The heating of the metallic uranium for producing a protective film may be readily accomplished in an electrically heated vacuum furnace of conventional design that is capable of being fitted with oxygen leaks. This furnace should also be capable of being evacuated to a pressure in a range corresponding to about 1 to 10 microns and have a leak rate less than about one micron per hour.

After cleaning the metallic uranium article as above described, or in any other suitable manner, the uranium article is immediately placed in the furnace, which is then sealed and evacuated to a pressure within the desired range without delay. During this pumping down, it may be desirable to assure an oxygen-free atmosphere within the furnace by purging the latter with a suitable inert gas such as argon or the like. When the furnace is heated to a temperature sufficient to effect the formation of the protective film, a selected quantity of dry oxygen is introduced into the furnace. Normally, the temperatures sufiicient to provide the desired film are in the range of about 500 to about 650 C. It has been found that the better results occur at the higher temperatures, but care should be exercised to assure that the temperature does not get up to or above 665 C. since uranium changes from the alpha to the beta phase at about this temperature. It is believed that the conversion of the uranium metal to the beta phase should be avoided since the subsequent return to the alpha phase upon cooling would be accompanied by a crystal growth which might cause warpage and disruption of the protective film. The rate of heatup of the uranium article in the furnace is not particularly critical. However, the heatup rate should be sufficiently slow to permit outgassing of hydrogen and other entrapped gases from the uranium prior to the introduction of the dry oxygen into the furnace.

Upon obtaining a preferred furnace temperature of about 625 C., a suitable quantity of dry oxygen is introduced into the furnace in any suitable manner such as by providing the furnace with a number of standard leaks connected in parallel, with each leak being capable of providing the furnace volume with a particular quantity of oxygen. Satisfactory results have been achieved by using three leaks, each capable of providing the furnace interior with 900 microns of oxygen per cubic foot per hour when treating uranium articles having a surface area of about 7 square inches. It has been found that about 900 microns of oxygen per cubic foot per hour in the furnace at 625 C. will provide a protective film of about 0.25 mil thickness in one hour on a uranium article having about 7 square inches of surface area. With uranium articles of surface areas greater than about 7 square inches, correspondingly greater quanties of oxygen are required to provide protective films in the desired thickness range. In other words, with a uranium article having a surface area of about 14 square inches, the furnace interior should be provided with about 1800 microns of oxygen per cubic foot per hour to provide a protective film with a thickness of about 0.25 mil. The type of oxygen used in the furnace is highly critical in that it is necessarily free, or virtually free, of moisture since the protective film cannot be obtained when excessive moisture is present. The use of commercially available 99.9% pure oxygen has been found to be adequately dry for accomplishing the method of the present invention. The presence of an inert gas, e.g., argon, in the furnace does not appear to have any deleterious effects so long as it is virtually or entirely moisture-free and does not interfere with the contacting of the uranium article with the desired quantity of oxygen.

Successful preparation of the protective film of the present invention is highly dependent upon the use of dry oxygen together with a leak-proof vacuum furnace since inleakage of atmospheric gases, particularly moisture-laden gases, into the furnace during treatment considerably changes the character of the protective film and corrosion will occur in a matter of a few hours. For example, a defective gasket of the furnace assembly permitted trace amounts of atmospheric air to leak into the furnace, which resulted in the films being of the wet table type which failed in a short time upon exposure to heated air containing water vapor.

As briefly mentioned above, the quantity of dry oxygen in the furnace is critical in that the required reaction depends upon contacting the uranium metal with less than the quantity of oxygen which would cause the aforementioned undesirable reaction of U+Og UO The film thickness is varied by the quantity of dry oxygen in the furnace as well as the duration which the uranium metal remains in the heated furnace. Film thicknesses in the range of about 0.2 to 1.0 mil have been found to be highly satisfactory as corrosion inhibitors.

Upon completing the treatment of the uranium metal in the furnace, which is usually accomplished in a duration of about one hour at 625 C., the furnace is cooled down under vacuum to assure that the film is properly established prior to exposing it to corrosion-causing conditions.

The uranium oxide film produced by the method of the present invention provides the uranium metal with highly corrosion-resistant coatings or films in that uranium metal coupons so treated have been exposed to atmospheric air at relative humidity and heated to 200 F. for durations of over several hundred hours without any sign of corrosion or other adverse eifects. The average life of the protective film is expected to be somewhat greater than 600 hours under the severe test conditions just described. In fact, treated coupons have been exposed to these conditions for durations greater than 1700 hours without showing any corrosion. An electron microscope evaluation of coupons after being subjected to the corrosion test indicated no evidence of any destruction of the film from the exposure or any damage to the uranium. Other properties or characteristics of the protective film include a hardness of about 60-65 on the Rockwell C scale as compared to 2025 on the same scale for the uranium metal, a bluish color, a surface finish as good as the finish on the uranium metal prior to treatment in the furnace, and a reproducibility of film thickness better than :01 mil.

The corrosion resistance of the protective film produced by the present method is apparently achieved because, unlike naturally occurring uranium dioxide, it is not wetted by water. A quick check to determine if a uranium article is adequately protected is to dip the article in a container of distilled water and, if the article sheds water upon removal from the latter, it will be corrosion resistant. However, if the coated uranium article is wetted by the water, as was the case with the uranium articles where the furnace was inadequately sealed due to a gasket failure, the film will deteriorate in a relatively short time when subjected to the test conditions mentioned above.

While it is not entirely clear as to what particular mechanism provides the non-wettability of the film and hence the corrosion protection, it is believed that it may be due to a particular crystal orientation of the U in the film. X-ray diffraction patterns indicate a highly preferred 110 crystal orientation in a plane parallel to the surface of the uranium article. The U0, on the other hand, has a crystal orientation in the 100 plane.

In order to provide a more facile understanding of the invention, an example of a typical operation for providing a uranium metal coupon with the protective film of the present invention is set forth below. This example is merely illustrative of the subject invention and is not to be considered in a limiting sense since the scope of the invention is limited only by the scope of the appended claims.

EXAMPLE Two uranium metal coupons (1%" x 1" x /s) were vapor decreased, scrubbed with scouring powder, rinsed in distilled water, bathed in 8 N nitric acid for a period of 20 minutes, rinsed in distilled water, and then dried with acetone. Immediately after this drying, the coupons were placed in a vacuum furnace of the type described above. The furnace was immediately sealed and evacuated, purged three times with argon, and pumped down to a pressure corresponding to microns. The furnace heaters were then energized to heat the uranium coupons to a temperature of 625 C. at a rate of 250 C. per hour. During this heatup, outgasing, particularly hydrogen, occurred. When the furnace reached the temperature of 625 C., dry, 99.9 percent pure oxygen was admitted into the furnace interior through one of the standard leaks at a rate of 900 microns per cubic foot per hour. The dry oxygen entering the furnace volume through this leak flowed across the exposed faces or surfaces of the uranium coupons for a duration of one hour for producing the protective film on the uranium coupons by the above-described reaction U+=UO 2UO. During this period the furnace temperature was maintained at approximately 625 C. Upon completing the one-hour tr ating period, the furnace heaters were deenergized and the furnace permitted to cool under vacuum. The treated uranium metal coupons, which were coated with a blue film about 0.25 mil thick, were subjected to the corrosion test described above, and after 378 hours no visible deterioration was evident.

The results in the example just described have been confirmed by numerous equivalent experiments. In addition, experiments have been conducted in which some of the treatment parameters have been modified or chang d; for example, uranium coupons prepared for facilitating the reception of the coating as described above are treated in the vacuum furnace in the absence of an oxygen bleed or in the presence of another oxygen source such as urano-uranic oxide having an excess of oxygen U,o +o). The urano-uranic oxide served as a controlled source of oxygen. The following table compares the results obtained when identical uranium coupons were heated in the vacuum furnace (a) with no oxygen intentionally added to the furnace atmosphere, (b) in the presence of dry oxygen liberated from the urano-uranic oxide, and (c) in the presence of dry oyxgen introduced through one or more of the standard leaks. The test bath referred to in the table is air heated to 200 F. with a moisture content corresponding to 100 percent relative humidity.

TABLE.-.A. BRIEF SUMMARY OF CORROSION TEST RESULT$ Average life in before showing Mode of formation, 1 hr. corrosion at 625 0. spots, hr. Comments U 0 as oxygen source Standard leak(s), Oz gas 600 In view of the data in the above table, it is apparent that a long-lived protective film is not obtainable in the absence of oxygen. In fact, it is believed that the shortlived film produced by this technique was formed by the effect of the heat treatment of uranium oxide formed on the uranium coupon during the time it took to transfer the pretreated uranium coupon from the cleaning and drying solutions to the furnace and/or trace amounts of oxygen in the furnace. The protective coatings as prepared by using the oxygen bleeds are superior to those prepared with the urano-uranic oxide since the use of the controlled oxygen bleeds facilitates greater control over the film thicknesses, while the excess oxygen in the urano-uranic oxide can vary from batch to batch. Another important advantage of the oxygen bleeds over the use of the urano-uranic oxide is the control over the time at which the oxygen is most advantageously introduced into the furnace. It has been established that the most desirable time for the introduction of the oxygen is when the furnace has reached the desired equilibrium temperature, preferably 625 C. To this end it is believed that, when using the urano-uranic oxide form of oxygen, at least some of the available oxygen is consumed at lower temperatures and consequently cannot provide the desired protective film.

It will be seen that the present invention provides a unique protective film for metallic uranium which provides protection against corrosion in moist atmospheres, which significantly exceeds corrosion protection provided by previously practiced methods including platings. In fact, the present invention has an advantage over plating in that there is no appreciable removal of surface uranium (only a thin layer of the natural oxide) in the preparation, whereas the plating operation normally requires removal of about 0.7 mil from each side of the uranium article during the pickling step utilized for preparing the uranium for reception of the plating. With the present invention it is only necessary to form a film of 0.2 to 0.3 mil thick in order to obtain several hundred hours of protection under extreme or severe conditions. Since half or more of the film thickness is below the initial metal surface, there will be very little dimensional changes in the treated uranium article over the article prior to treatment.

As various changes may be made in the theory of the protective film formation, film characteristics, and arrangement of the method steps herein without departing from the spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all matter herein is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The method of providing a metallic uranium article with a corrosion-resistant surface of uranium oxide, comprising the steps of removing contaminants from the surface of the uranium article, confining the article in an enclosed volume, evacuating the volume to a pressure substantially less than atmospheric pressure, heating the uranium article to a temperature greater than about 500 C. but less than the temperature at which the uranium changes to beta phase, contacting the uranium article with oxygen at least virtually free of moisture to produce a uranium oxide film on the surface of the article, introducing the oxygen into the volume after heating the uranium article to the first-mentioned temperature, and maintaining the article a about said firstmentioned temperature in the presence of the oxygen for a duration sufficient to provide the film with a thickness in the range of about 0.2 to about 1.0 mil, said film being characterized by being non-wettable by water, by being virtually free of reaction with and impervious to dry air, moist air, and water.

2. The method claimed in claim 1, wherein the rranium oxide film consists essentially of uranium dioxide and lesser oxides with a gradually decreasing gradient in the oxygen content in the film from the outermost surface thereof inwardly towards the interface between the film and metallic uranium, and wherein the uranium oxide adjacent said interface consists essentially of uranium monoxide.

3. The method claimed in claim 1, wherein the step of removing contaminants from the surface of the uranium article includes immersing the latter in an acid bath for removing naturally occurring oxides, the uranium article is transferred into the enclosed volume after the removal of the contaminants therefrom and the enclosed volume is evacuated within durations sufiicient to inhibit the formation of excess oxide on the uranium article prior to the heating thereof, and wherein the pressure substantially less than atmospheric pressure is of a pressure in a range corresponding to about 1 to 10 microns.

4. The method claimed in claim 1, wherein the heating of the uranium article is achieved at a rate sufficient to outgas the article prior to the oxygen contact, and wherein the temperature greater than 500 C. is about 625 C.

5. The method claimed in claim 1, wherein the oxygen contacting the uranium article is of a quantity sufiicient to effect the reaction U+UO 2UO to provide the uranium oxide film with a thickness in said range together with said characteristics.

6. The method claimed in claim 5, wherein the quantity of oxygen contacting the uranium article is less than the quantity sufiicient to effect the reaction U+O UO 7. The method claimed in claim 6, wherein the oxygen is introduced into the volume at a rate sufficient to provide about 900 to about 2700 microns oxygen per cubic foot per hour for about each 7 square inches of surface area on the uranium article.

8. The method claimed in claim 7, wherein the oxygen is approximately 99.9 percent pure.

9. A new article of manufacture comprising a metallic uranium substrate and a corrosion-resinstant uranium oxide film on exposed surfaces of said substrate consisting es sentially of uranium dioxide and lesser oxide and characterized by being non-wettable by water and by having a decreasing gradient in oxygen content from an outermost surface of the film inwardly towards the interface between the film and the surface of said substrate.

10. The article of manufacture claimed in claim 9, wherein the film is of a thickness in a range of about 0.2 to 1.0 mil, and wherein the uranium dioxide has a predominant crystal orientation in the 110 plane parallel to the surface of said substrate.

References Cited FOREIGN PATENTS 858,656 1/1961 Great Britain 148-6.3

RALPH S. KENDALL, Primaly Examiner U.S. Cl. X.R. l4831.5