US3413159A - Process of simultaneously cleaning and coating uranium surfaces - Google Patents

Description

United States Patent 3,413,159 PROCESS OF SIMULTANEOUSLY CLEANING AND COATING URANIUM SURFACES Ival O. Salyer, Dayton, and David Gerald Glasgow, Centerville, Ohio, assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Dec. 24, 1964, Ser. No. 421,083

16 Claims. (Cl. 148-614) ABSTRACT OF THE DISCLOSURE Method of protecting a uranium surface by immersing in a bath containing from 0.01% to 5% by weight of an organic acidic material and from 0.5% to 60% by weight of nitric acid in an inert liquid carrier, and removing from the bath with a coating thereon; the organic acidic material may be either alkanoic, epoxyalkanoic, alkanedioic or alkenoic acids, said acids containing from 8 to 22 carbon atoms, aromatic hydro-carbon mono-carboxylic acids which are free of aliphatic unsaturation and contain from 7 to 13 carbon atoms, and partial esters of phosphoric acid with alkanols of from 4 to 12 carbon atoms.

The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

This invention relates to the modification of metal surfaces, and more particularly provides a new and valuable method of simultaneously cleaning and protecting surfaces of uranium.

Most metals are readily susceptible to oxidation and consequent deterioration when exposed to air and/or water. Generally, such metals can be protected by painting them; but in many instances, painted surfaces prevent utilization of the metal for the purposes for which it is intended. This is particularly true in the case of uranium,

wherein many applications require a clean surface of the metal and corrosion is rapid. The art has thus been faced with the problem of preventing this metal from tarnishing and/or corroding during storage while not unduly obstructing its subsequent utility. Cleaning of the tarnished surface before use, e.g., by acid pickling or electropolishing offers a solution only when the metal is to be used within a very short time, e.g., generally within the same day. Therefore, even when the metal is to be stored for a comparatively short time in a moist atmosphere the problem of protecting it from corrosion arises. What is required is a means whereby uranium is cleaned and then made available for future use. The metal should not be allowed to remain unprotected from corrosive agents for any substantial period because the corrosion is progressive, increasing substantially geometrically with storage time. On the other hand, uranium can not be protected from corrosive elements by the use of conventional paints or varnishes since the corrosion proceeds readily underneath such surface coatings and thus coatings of this type are completely ineffectual. Advantageously, the metal should be protected during storage in such a manner that for many uses removal of the protective material prior to fabricating the metal for the intended purpose is unnecessary, e.g., in operations involving laminating or jointing where in the protective coating may actually facilitate, rather than hinder adhesion or wherein the protective coating serves to protect any exposed surfaces of, say, lap joined materials.

The protection of oxidizable metals by coatings generally requires that before the coating is applied the surface to which it is applied be thoroughly free of corrosion scale and other soils. Otherwise, the once-initiated corrosion continues in spite of the top coating, and/or the coating either refuses to adhere, or it peels off. Therefore, the metal must be thoroughly cleaned before it is coated. Such cleaning is generally conducted by acid pickling, burnishing, or electropolishing. However, when these usual methods of cleaning are employed with uranium, sufficient oxidation can occur between the time that the metal is cleaned and the time that it is coated to hinder adequate adhesion of the protective coating. In cleaning by pickling, for example, some oxidation occurs even while the newly cleaned metal is being rinsed and dried. Even though it may be very minute, it is enough to initiate the rapid corrosion of the reactive metal and prevent the formation of a continuous film of a protective coating upon the surface. Discontinuities in the coating thus provide for an easy means of attack by air or water.

Accordingly, an object of this invention is the protection of the uranium against attack by air and/or water. Another object is the protection of uranium during storage. Still another object is the protection of uranium surfaces against corrosion by means of an easily applied and easily removed protective coating. A most important object is the provision of a method whereby a uranium surface can be simultaneously cleaned and protected against subsequent attack by air and/ or water.

These and other objects hereinafter disclosed are provided by the process which comprises immersing metallic uranium into a bath consisting essentially of from 0.01% to 5.0% by weight of an organic acidic material selected from the class consisting of alkanoic, epoxyalkanoic, alkanedioic and alkenoic acids of from 8 to 22 carbon atoms, perfluoroalkanoic acids of from 4 to 18 carbon atoms, aromatic hydrocarbon mono-carboxylic acids which are free of aliphatic unsaturation and contain from 7 to 13 carbon atoms, and partial esters of phosphoric acid with alkanols of from 4 to 12 carbon atoms, and from 0.5% to 60% by weight of nitric acid calculated as HNO with the balance being an inert liquid carrier, and removing from the bath metallic uranium having a protective coating deposited on the surface thereof.

The present process serves to clean the metal surface and to coat the resulting clean surface with a water impermeable coating of the organic acid material. Since the coating is deposited upon the cleaned surface before it has an opportunity to be affected by air, no surface oxidation products hinder the uniform adhesion of the coating. Although the coating of organic acid is uniformly adherent and protective, it is very thin. .When the thus coated metal is about to be used in an application where the coating would interfere, it can be readily removed by dipping into an organic solvent or in aqueous alkali.

It is well known in the art that monolayers or multilayers of various organic materials can be deposited upon some metal substrate by preparing an aqueous bath having a film of the fatty acid on the surface thereof, dipping the metal into the bath and pulling it out of the bath through the film. See, e.g., Katharine B. Blodgett, J. Amer. Chem. Soc., 57, 1007 (1935); Eric Rideal and I. Tadayon, Proc. Royal Soc., A225, 346 (1954) and Harold Schonhorn, J. Polymer Science, Part A, 1, 2343- 2359 (1963). Although, initially, attention was directed primarily at the phenomenon of monomolecular films thus provided, it was found that by repeated, careful dipping and withdrawing through the film of, say, a fatty acid, a great many layers of the film could be built up. However, the tedious manipulation hindered practical application. Moreover, the fact that adherence of the first layer required an extremely clean, smooth metal surface also impeded use of the method for a commercial coating process even with those metals which were not readily oxidized by exposure to air and moisture. In the case of uranium, deposition of a coating of fatty acid by the prior art methods is unfeasible simply because, in spite of very thorough cleaning, and in spite of almost immediate introduction of the thus-cleaned metal into the coating bath, the metal does not remain clean long enough to permit deposition of a coating which is sufficiently continuous and adherent to serve as a protective film against corrosion by air and/ or water.

The use of aqueous nitric acid as a pickling agent for metals, generally, is also known. See, for example, the report Chemical Surface Treatments for Uranium and Their Application to Uranium Technology, by I. K. Gore et al. (Contract W7405eng. 36, with the US Atomic Energy Commission), published as LA-2190 (TID4500, 13th ed. Rev.), wherein treatment with 50 v/o HNO is often referred to as a useful method of removing oxides from uranium surfaces. However, although aqueous nitric acid is, in fact, a very good cleaning agent, it is impracticable, if not impossible to avoid some oxidation of the clean surface before applying a protective coating in a separate bath. Even when the coating step follows the cleaning step almost immediately, sufiicient oxidation occurs in the interim to hinder a satisfactory, uniformly adherent coating.

The present invention is based on the surprising discovery that the cleaning operation and the coating operation can be conducted in one and the same bath. Although the prior art has been so concerned with the effect of pH on deposition of fatty acid films that much care has been directed at operating within very narrow limits of pH, and although nitric acid is well-known to be an oxidant of organic materials, we have found that protective films of the herein defined organic acidic materials can be deposited upon uranium from baths which are so acidic that they serve as pickling agents for the uranium. We hav also found that deposition of the film need not be conducted in any special apparatus, e.g., a Langmuir trough, and that no painstaking technique such as drawing the metal from the bath through a surface film is needed. Such apparatus and techniques may be used in the presently provided one-step cleaning and coating process, but they are not required.

There may be used in the presently employed bath only the quantity of organic acid which is soluble in the mixture of nitric acid and inert liquid carrier, so that no film of the organic acid is formed on the bath, i.e., the organic acid may be present only in the dissolved form. Owing to the high receptivity of the freshly cleaned metal surface which results upon immersion into the nitric acidcontaining bath, the organic acid which is also present in the same bath becomes firmly bound to the metal surface while the metal is immersed in the bath. Since uniform deposition of the coating of organic acid does not depend upon pulling the metal through a surface film of the bath, the metal is simply removed from its bath in any manner which is customarily employed for the removal of solid objects from liquids in which they have been immersed, e.g., the metal may be taken out, with forceps, or the bath liquor may be decanted from the metal. For continuous operation, the organic acid content of the bath should be replenished as it is depleted by deposition on the metal surface.

When it is inconvenient to use the organic acid only in an amount which is soluble in the mixture of nitric acid and carrier, or to maintain a constant concentration of organic acid within the bath, there may be used a quantity of organic acid which exceeds the limit of its solubility in said mixture. In that case, the excess will form a film upon the surface of the bath, which film will diminish in thickness as the dissolved organic acid is depleted. Since deposition of a coating of organic acid upon the metal surface takes place while the metal is immersed in the bath, no precautions have to be observed in removing the coated metal, even though its removal requires penetration of the surface film. Pulling through the surface Ag film will only cause additional, though often unnecessary, build-up of layers of the organic acid upon the metal surface. The coating which has been deposited upon th metal surface while it has been immersed in the bath is extremely adherent; hence, there is little, if any, risk of peeling as a consequence of too rapid a withdrawal.

The time required for deposition of a protective coating will vary with the cleanliness of the metal surface, the concentration of nitric acid, and the nature and concentration of the organic acid. A badly tarnished metal surface will require a longer pickling or cleaning time than will a surface which has been recently cleaned. Hence, the metal should be immersed in the bath for a time sufficient to assure thorough cleaning of the surface. Deposition of the organic acid on the cleaned surface occurs almost immediately, so that multilayers, rather than a monolayer, of the organic acid upon the metal surface are generally obtained within a few minutes or even seconds. Ordinarily, even with old, badly tarnished or corroded surfacesof uranium a sojourn time of from, say, 5 to 20 minutes is sufficient both to clean the surface and to coat it. Manipulation of the metal while in the bath is unnecessary, although, in order to expose all surfaces to the bath liquors within the shortest time, it may be found desirable to suspend the metal as in electroplating or to turn it over should it be merely supported by the base of the container.

When working with badly tarnished or soiled uranium, it may be desirable to pre-clean it before immersion into the organic acid-containing bath. The cleaning may be conducted in known mann r, i.e., by pickling in a mineral acid, by electro-polishing, or by mechanical burnishing. Thereby premature contamination of the bath is avoided, and repeated use of the same bath in a continuous process is facilitated. The noteworthy feature of the present invention is thus not necessarily the elimination of a cleaning step; rather it is the incorporation into the coating bath of sufiicient nitric acid to assure maximum receptivity of the metal surface for the coating of organic acid while simultaneously making the organic acid available throughout the bath liquor rather than only at the surface of the bath.

As disclosed above, a variety of organic acidic materials are presently useful. These include alkanoic, epoxyalkanoic, alkanedioic and alkenoic acids of from 8 to 22 carbon atoms, e.g., caprylic, nonanoic, decanoic, undecanoic, lauric, myristic, palmitic, stearic, behenic, 7,8- epoxyoctanoic, 12,13-epoxytridecanoic, 9,10-epoxyoctadecanoic acid, or Z-octenedioic acid, suben'c acid, sebacic acid, octadecanedioic acid, 4-octenoic acid, IO-decenoic acid, ll-undecylenic acid, oleic acid, and 13-docosenoic acid; perfiuoroalkanoic acids of from 4 to 12 carbon atoms, e.g., perfluorobutyric, perfiuorocaprylic, perfiuoroundecylenic or perfluorolauric acid; aromatic hydrocarbon monocarboxylic acids such as benzoic, 0-, mor p-toluic, o-, mor p-hexylbenzoic, aor fl-naphthoic, or o-, mor p-biphenylcarboxylic acids; and partial esters of phosphoric acid and alkanols of from 4 to 12 carbon atoms, e.g., the diesters such as dibutyl, dihexyl, dinoyl, didodecyl, butyl pentyl or decyl heptyl phosphate or the mono-esters such as mono-'butyl, mono-Z-ethylhexyl or mono-dodecyl phosphate.

As hereinbefore disclosed, the concentration of organic acid in the bath may be from 0.01% to 5.0% by weight. For most purposes, a concentration of, say, from 0.1% to 2.0% is convenient, since thereby the bath provides adequate coating without frequent replenishing in a continuous run. Also, the use of such concentrations permits easy incorporation of the organic acid into the inert liquid carrier, since the organic acids are generally soluble in aqueous nitric acid to the extent required for the coating. A film of the organic acid forms on the bath surface only when the concentration of organic acid exceeds its solubility limit.

The presently useful carrier may be water or any organic liquid, so long as it is inert under the reaction conditions. The presently used organic acids are generally readily soluble in inert, liquid organic solvents, e.g., the lower aliphatic ketones such as acetone or 2-butanone, the lower alkanols such as ethanol or isopropanol, and other solvents such as dioxane, dimethylformamide and dimethyl sulfoxide.

When the metal has been pro-cleaned, so that only a very small quantity of nitric acid is needed in the bath to remove the low amount of oxide which has accumulated on the surface of the metal after pre-cleaning, a quantity of as little as, say, 0.5% by weight of nitric acid calculated as HNO incorporated with a solution of the organic acid in an inert organic solvent for the acid is suflicient to prevent uneven bonding of the organic acid. This is because even so low and amount of nitric acid serves to clean the metal surface of the recently accumulated oxides, which prevent coating of the surface wherever the oxides are present.

For purposes of economy, it is advantageous, of course, to dispense with organic liquid carriers and use water instead. The presently useful organic acids are generally soluble in aqueous nitric acids of varying concentrations, depending upon the strength of the nitric acid. Generally, aqueous nitric acid having an HNO content of from to 60% by weight dissolves the organic acids in the small concentrations in which they are used in the bath. However, for optimum solubility of organic acid and provisions of a smooth, clean and retentive uranium surface, an aqueous nitric acid having an HNO concentration of from to by weight is preferred. A 1:1 by volume ratio of water and concentrated nitric acid of commerce (7071% HNO is conveniently prepared. It is sufiiciently strong to clean even badly oxidized surfaces of uranium, and it dissolves quite readily the small concentrations of organic acids which are used in the present process. However, much more highly concentrated aqueous nitric acid may be used if desired, since even with aqueous nitric acid having an HNO content of up to by weight there is generally no risk of over-pickling. Aqueous nitric acids of such higher concentrations are particularly useful when no pre-pickling has been employed and/or when the higher molecular weight, more difficultly soluble, organic acids are used.

Although as pointed out above, the presently useful organic acids are generally soluble in aqueous nitric acid particularly at the low concentrations, tedious stirring may be required to effect the desired solution, and in initial runs the optimum nitric acid concentrations for most readily dissolving the organic acid which is it desired to use must be experimentally determined. Accordingly, for easy manipulation, it will be found advantageous first to dissolve the organic acid in an inert organic liquid which is a solvent therefor, and then to add the resulting solution to aqueous nitric acid. In this way, the use of large amounts of organic solvent is eliminated since only the amount oforganic solvent required to dissolve the organic acid is needed. When a polar solvent is used, a clear, homogenous bath is generally obtained when the organic solution is added to the aqueous nitric acid.

The invention is further illustrated by, but not limited to, the following examples.

Example 1 A cleaning and coating bath was prepared as follows: To a beaker containing 50 ml. of distilled water there was added 50 ml. of -71% nitric acid. A solution con sisting of 0.2 g. of stearic acid in 20ml. of acetone was then added, and the whole was well-stirred. Five 1" x 0.5 x 0.125" pieces of discolored uranium were immersed in the bath and maintained therefor 15 minutes, turning each over at the end of about the first half of this period without removing from the bath. They were then removed from the bath and stored individually in distilled water contained in polyethylene bottles. At the end of 50 days, one of the pieces was removed. No evidence of corrosion was observed. The other pieces were each removed at the end of 139 days; these also showed no evidence of corrosion.

Example 2 Five 1" x 5" x 0.125" pieces of old used, uranium were pie-cleaned by pickling in 100 ml. of a 1:1 by volume mixture of concentrated (7071%) nitric acid and distilled water for 15 minutes, turning each piece over at about the end of the first 7.5 minutes. The resulting clean pieces were weighed and placed in a bath prepared by adding a solution of 1 g. of stearic acid in 10 ml. of acetone to 200 ml. of a 1:1 by volume of distilled water and concentrated (7071%) nitric acid. At the end of 5 minutes, each piece was turned over in the bath; and at the end of another 5 minutes the pieces were removed, redipped briefly, air-dried, placed on a sheet of polyethylene and stored in an relative humidity bath at a temperature of 100 F. for 40 days. At the end of that time the pieces were again weighed. There was no change in weight; and visually, also, there was no sign of oxidation. On the other hand, pieces which had been only pre-cleaned as described above, but not treated with the stearic acid-containing bath were badly corroded within one day of storage.

Example 3 Four 4.5" x 1.0 x 0.125" bars of uranium having badly corroded surfaces owing to previous use as lap joints were immersed for 10 minutes in 100 ml. of a 1:1 by volume solution of nitric acid (7071%) and distilled water. The bars were then transferred into a fresh 100 ml. portion of the same kind of nitric acid solution, turned over after 7.5 minutes and allowed to remain in the solution for another 7.5 minutes. Without removing the bars, a solution consisting of 1 g. of stearic acid in acetone was added to the nitric acid solution in which the bars were immersed. A film formed upon the surface of the solution. The bars were pulled through the film with forceps, returned to the solution, and again removed by pulling through the film. The bars were now clean and not discolored. Resistance of the bars to attack by water was tested by immersing them in distilled water contained in glass jars and maintained at 140 F. At the end of days there was no evidence of oxidation, and the bars were removed from the water and used for subsequent experimentation.

Example 4 Three uranium pieces (each 1" x 0.5" x 0.125") were pickled clean in a 1:1 by volume mixture of 707l% nitric acid and water, rinsed with distilled water and then immersed in a coating bath consisting of 99 ml. of acetone, 1.0 stearic acid and 1 ml. of 71% nitric acid. Immersion time was 10 minutes, the pieces being turned over after 5 minutes. The pieces were then removed, air-dried, rinsed With a carbon tetrachloride spray and air dried again. After weighing they were immersed completely in distilled Water contained in polyethylene bottles and the bottles were stored in a 140 F. oven. All three pieces were in excellent condition at the end of eighteen days.

On the other hand, when uranium pieces of the same size which had been pickled clean in the same manner were immersed in a bath consisting of 100 ml. of acetone and 1 g. of stearic acid but no nitric acid and the pieces in the bath were allowed to stay in twice as long (total of 30 minutes with turning over of each piece at the end of the first 15 minutes) similar storage of the treated pieces showed pronounced corrosion at the end of 3 days.

Example 5 Three badly discolored, 1.0" x 0.5" x 0.125" bars of uranium were cleaned by immersing them for 15 7 minutes in a bath consisting of 1:1 by volume solution of 71% nitric acid and distilled water, turning the bars over once at about the midpoint of the immersion period. A solution, which had been prepared by dissolving 0.2 g. of sebacic acid in 5 ml. of warm acetone and then cooling to room temperature, was then poured into the bath in which the bars were immersed. The bars were pulled through the top surface of the bath by means of forceps, returned to the bath and then again pulled through the surface. After rinsing them with distilled water, the resulting clean bars were immersed in distilled water contained in a polyethylene bottle and maintained in the water at 140 F. Substantially daily inspection disclosed no evidence of corrosion until the bars had been in the water for 125 days.

Example 6 Five 1 x 0.5" x 0.125" bars of uranium were cleaned by immersing them in aqueous nitric acid for 15 minutes. They were then transferred to a bath which had been prepared by adding a solution consisting of 0.1 g. of perfluorooctanoic acid in 5 ml. of freshly distilled acetone to 200 ml. of 1:1 by volume mixture of 71% nitric acid and water. At the end of 5 minutes the bars were turned over while immersed in the bath, and allowed to remain in the bath for another 5 minutes. They were then slowly removed from the bath with forceps, regripped at another portion, returned to the bath, and then again removed from the bath as before. The bars, which were still free of discoloration, were then stored directly in polyethylene bottles filled with distilled water and maintained at 140 F.

Substantially daily observation of the stored bars revealed no evidence of corrosion of any of the five bars until they had been stored for 93 days.

Example 7 Ten 4.5" x 1.0" x 0.125" used and discolored bars of uranium were cleaned first by immersion in hot, aqueous sodium hydroxide and then in aqueous nitric acid.

A bath was prepared by adding a solution consisting of 0.5 g. of perfiuorooctanoic acid in 5 ml. of acetone to 900 ml. of a 1:1 by volume mixture of water and concentrated (70-71%) nitric acid. The cleaned bars were placed in this bath for minutes, then removed, re-dipped and again removed. After rinsing with water, they were stored in glass bottles which were half-filled with well-aerated water. Half of these were respectively stored in the half-filled bottles by total submersion; the other half were partially exposed to the air in the bottles. At the end of 48 hours, none of the ten bars showed any evidence of corrosion.

Example 8 Five discolored 1" x 0.5" x 0.125" pieces of uranium were pre-cleaned by pickling in a 1:1 by volume mixture of water and 70-71% concentrated nitric acid for minutes, each piece being turned over in a pickling liquor at about the middle of this period.

The clean, bright pieces of uranium thus obtained were immediately weighed and then placed in a coating bath which had been prepared by adding a solution of 0.1 g. of perfluorooctanoic acid in 5 ml. of acetone to 200 ml. of a 1 :1 by volume mixture of distilled water and 7 071% concentrated nitric acid. At the end of 5 minutes each piece was turned over in the bath, and allowed to remain in the bath for another five minutes. The pieces were then removed from the bath, re-dipped briefly, and stored in an 80% relative humidity chamber at 140 F. for 34 days. At the end of that time, there was determined only a 0.005 gram change in weight.

Example 9 This example is like Example 6-, except that in this example there was used heptafluorobutyric acid instead of the perfluorooctanoic acid of Example 6. The operation was separately conducted with 5 difierent pieces of the uranium, as in Example 2. The treated pieces were separately immersed in 140 F water for testing stability to water. Observation at the end of 48 hours disclosed no visible evidence of corrosion.

Example 10 Three uranium coupons having completely oxidized surfaces were cleaned by immersing them for 15 minutes in a bath consisting of ml. of a 1:1 volume mixture of distilled water and concentrated (70-71%) nitric acid, each coupon being turned over after about the first 7 minutes. Without removing the coupons, a solution of 1 g. of benzoic acid in 5 ml. of freshly distilled acetone was added to the bath, whereupon a film formed upon the surface of the bath. Each coupon was pulled out of the bath and then redipped once. After rinsing with distilled water, the coupons were stored in distilled Water in polyethylene bottles at F. The stored coupons were inspected for corrosion at intervals; at the end of 44 days, at which time inspection ceased, the coupons still showed no signs of corrosion.

Example 11 Two pieces of uranium having oxidized surfaces were pickled clean by immersion in 100 ml. of a 1:1 by volume mixture of concentrated (7 071%) nitric acid and distilled water. Then, without removing the pieces, a solution of l g. of bis(2-ethylhexyl) phosphate in 5 ml. of reagent grade acetone was added to the cleaning bath. A surface film formed on the bath. The pieces of uranium were then removed by pulling them out of the bath through this film. They were then returned to the bath and removed in the same manner. After rinsing with distilled water they were stored by total immersion in water contained in polyethylene bottles and maintained at 140 F. At the end of 74 days, at which time inspection ceased, the two treated pieces of uranium showed no sign of corrosion.

Example 12 This example is like Example 11, except that here there was employed IO-undecenoic acid instead of the bis(2- ethylhexyl) phosphate of Example 11. The treated pieces of uranium were found to be free of corrosion when stored for 2 weeks as in Example 11.

Example 13 This example is like Example 11, except that the bis(2- ethylhexyl) phosphate of that example was replaced by 10,11-epoxyundecanoic acid. The treated uranium pieces were stored as in Example 11. Inspection at the end of 77 days, :at which time observation of the treated specimens was discontinued, showed no corrosion of the uranium.

Although, for purposes of comparison, the above-described cleaning and coating baths were generally prepared by first dissolving the organic acid in acetone and then adding the solution to aqueous nitric acid, solvents other than :acetone may be employed. Also, the cleaning baths may be prepared by simply adding the organic acid to aqueous nitric acid and stirring. As shown in Example 4, the major constituent of the bath may be the organic solvent rather than water. It will be realized, of course, that substantially the same concentration of nitric ,acid was employed in most of the above examples in order to illustrate the comparative utility of the various organic acids without introducing any unnecessary variables. It is for this reason, also, that the immersion time was the same in :a number of the experiments shown above.

It is to be understood that although the invention has been described wi h specific reference to particular embodiments thereof, it is not to be so limited since obvious changs and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What We claim is:

1. The method which comprises immersing metallic uranium into a bath consisting essentially of from 0.01% to 5.0% by weight of an organic acidic material selected from the class consisting of alkanoic, epoxyalkanoic, alkanedioic and alkenoic acids, said acids containing from 8 to 22 carbon atoms, aromatic hydrocarbon mono-carboxylic acids which are free of aliphatic unsaturation and contain from 7 to 13 carbon atoms, and partial esters of phosphoric acid with alkanols of from 4 to 12 carbon atoms, and from 0.5% to 60% by weight of nitric acid calculated as HNO with the balance being an inert liquid carrier, "and removing from the bath the metallic uranium having a protective coating deposited on the surface thereof.

2. The method defined in claim 1, further limited in that the organic acidic material is an alkanoic acid of from 8 to 22 carbon atoms.

3. The method defined in claim 1, further limited in that the organic acidic material is an alkanedioic acid of from 8 to 22 carbon atoms.

4. The method defined in claim 1, further limited in that the organic acidic material is an alkenoic acid of from 8 to 22 carbon atoms.

5. The method defined in claim 1, further limited in that the organic acidic material is an aromatic hydrocarbon monocarboxylic acid which is free of aliphatic unsaturation and contains from 7 to 13 carbon atoms.

6. The method defined in claim 1, further limited in that the organic acidic material is a partial ester of phosphoric acid with an alkanol of from 4 to 12 carbon atoms.

7. The method defined in claim 1, further limited in that the organic acidic material is an epoxyalkanoic acid of from 8 to 22 carbon atoms.

8. The method which comprises immersing metallic uranium into a bath consisting essentially of from 0.1 to 2.0% by weight of an alkanoic acid of from 8 to 22 carbon atoms with the balance being a mixture of an inert organic liquid which is a solvent for the alkanoic acid and aqueous nitric acid suflicient to provide a quantity of HNO which is from 0.5% to 60% by weight of the bath, and removing from the bath the metallic uranium having a protective coating deposited on the surface thereof.

9. The method defined in claim 8, further limited in that the alkanoic acid is stearic acid.

10. The method which comprises immersing metallic uranium into a bath consisting essentially of from 0.1% to 2.0% by weight of an alkenoic acid of from 8 to 22 carbon atoms with the balance being a mixture of an inert organic liquid which is a solvent for the alkenoic acid and 10 aqueous nitric acid sufficient to provide a quantity of HN0 which is from 0.5% to by Weight of the bath, and removing from the bath the metallic uranium having a protective coating deposited on the surface thereof.

11. The method defined in claim 10, further limited in that the alkenoic :acid is IO-undecenoic acid.

12. The method which comprises immersing metallic uranium into a bath consisting essentially of from 0.1% to 2.0% by wegiht of an alkanedioic acid of from 8 to 22 carbon atoms with the balance being a mixture of an inert organic liquid which is a solvent for the alkanedioic acid and aqueous nitric acid sufiicient to provide a quantity of HNO which is from 0.5 to 60% by weight of the bath, and removing from the bath the metallic uranium having a protective coating deposited on the surface thereof.

13. The method defined in claim 12, further limited in that the alkanedioic acid is sebacic acid.

14. The method which comprises immersing metallic uranium into a bath consisting essentially of from 0.1% to 2.0% by weight of an epoxyalkanoic acid of from 8 to 22 carbon atoms with the balance being a mixture of an inert organic liquid which is a solvent for the epoxyalkanoic acid and aqueous acid sufiicient to provide a quantity of HNO which is from 0.5% to 60% by weight of the bath, and removing from the bath the metallic uranium having a protective coating deposited on the surface thereof.

15. The method defined in claim 14, further limited in that the epoxyalkanoic acid is 10,11-epoxyundecan0ic acid.

16. The method which comprises immersing metallic uranium into a bath consisting essentially of from 0.1% to 2.0% by weight of stearic acid, and sutficient acetone to dissolve the stearic acid, with the balance being aqueous nitric acid containing 20% to 60% by weight of HNO and removing from the bath metallic uranium having a protective coating deposited on the surface thereof.

References Cited UNITED STATES PATENTS 2,410,688 11/1946 Shapiro 106-14 X 2,982,702 5/1961 Wehrmann 134-41 X 3,282,731 11/1966 Hudson et al 106-14 X 3,284,248 11/1966 Rumberger 148-614 X 3,341,350 9/1967 Anderson et al. 117-127 X ALFRED L. LEAVITT, Primary Examiner.

J. R. BATTEN, IR., Assistant Examiner.