US2894884A - Method of applying nickel coatings on uranium - Google Patents

Description

METHOD "OF APPLYING NICKEL COATINGS N URANIUM No Drawing. Original application January 9, 1945, Serial No. 572,093. Divided and this application Octo- 'ber 17,1950, Serial'No.'190,671

7 Claims. (Cl. 204-15) This. invention relates to the application of. protective coatings on metallic uranium. The products produced in practlcing this invention are useful in a neutronic reactor of the type described in the co-pending application of Fermi et al., S. N. 568,904, filed December 19, 1944, now Patent No. 2,708,656, dated May 17,

Metallic uranium is an easily oxidizable metal. which is readily corroded by atmospheric oxygen and by aqueous solutions. In order to protect the metal from the injurious effects of such media, it is desirable to provide a coating of some material which is less easily acted upon.

Many common protective coating materials are unsuitable for coating uranium; lead forms a pyrophoric alloy with uranium and for this reason its use is objectionable; some metals, such as tin, permit diffusion of the uranium through coatings of these metals with consequent impairment of their protective value; other metals form very brittle alloys with the uranium,- containing many cracks and crevices which reduce their effectiveness.

It is an object of the present invention to provide protective coatings for uranium which are not subject to the defects enumerated above. A further object is to provide a barrier coating for uranium over which it is possible to apply various protective coatings that normally would be unsuitable because of their behavior toward the base metal. A further object of the invention is to'provide highly protective electrodeposited coatings on metallic uranium.

' In accordance with the present invention metallic uranium is protectively coated with nickel, preferably but not necessarily applied to the uranium directly.

The metallic uranium preferably is prepared for the application of the nickel coating by an etchant treatment of the metal surface with an aqueous etching solution containing chloride ions, for example, a hydrochloric acid solution or a molten hydrate of ferric chloride. Particularly satisfactory preparatory treatments of this type involve anodic pickling of the metal in aqueous trichloracetic acid solution, or aqueous phosphoric acid solution containing about /2.% of hydrochloric acid, to remove a /2 to one mil layer of the surface metal. Prior to the etchant treatment the metal may be cleaned-by sand-blasting, pumice-scrubbing, treatingwith organic solvents to remove grease or oily impurities or treating with a chemical solvent for the oxide film on the metal. Aqueous nitric acid solutions containing from 30% to 55% HNO by Weight are especial- 1y useful for this purpose. An additional short treatment with this reagent should be applied after the etchant treatment in order to remove the coating of oxide or oxychloride from the etched metal surfaces. 7 The nickel coating may be applied by electroplating from a conventional acidic nickel electroplating bath, such as anaqueous nickel sulfate bath.

The nickel coatings of the invention may vary from V 2,894,884 Patented July 14, 1959 exceedingly thin coatings, of the order of 0.000015 inch, up to 0.001 inch or thicker. The thickness of the coatiug may be varied to suit the subsequent treatment of the metal. Thus the thinnest coatings have been found to be especially suitable as bases for chromium electrodeposits. Nickel electroplating baths have much higher throwing power than chromium electroplating baths and consequently are more effective for coating fissures such as cracks, pipes and other surface irregularities. Electroplated nickel coatings protect the uranium from the adverse effects of chromic acid so that more satisfactory chromium platings are obtainable from the standpoint of uniformity and completeness of protection. Since the nickel is required for only transitory protection of the uranium during the chromium plating operation, very thin coatings are suitable for accomplishing the purpose.

The nickel platings retard the diffusion 'of uranium into aluminum or aluminum-silicon brazing alloys and are therefore valuable in preventing the formation of the brittle uranium-aluminum alloys which normally are formed at the interface of the uranium and aluminum or aluminum-silicon when no barrier is provided. The nickel platings are accordingly of benefit in the application of such metals as protective coatings or as bonding means, for example, in the bonding of aluminum sheaths to. the uranium. They serve a similar purpose when zinc or zinc-tin solder is used asthe bonding agent. For use in this manner heavier nickel electro-platings are more desirable, such as platings between 0.0003 inch and 0.001 inch thick. In applying such coatings by hot-dipping, the nickel plated article should be immersed at a temperature and for a time just long enough to wet and uniformly coat the surface.

While very adherent nickel electrodeposits may be produced upon uranium when a suitable preparatory treatment is employed for etching the uranium, their adher ence maybe still further improved by subjecting them to an alloying treatment. Such a treatment involves heating the electroplated uranium for a short time at a temperature above 662 C. The heating may be effected by immersion of the electroplated article in a molten bath such as a bath of molten sodium chloride, potassium chloride, or lithium chloride, or a mixture thereof maintained at a temperature between 700 C. and 800 C. The alloying may also be effected by inductive heating or by radiant heating in a vacuum or in an inert gas. By appropriate control of the time and temperature of heating, the extent of alloying and depth of penetration of the nickel may be controlled to provide nickel alloy layers at the uranium-nickel interface varying all the way from exceedingly thin layers up to layers many times the thickness of the plating. Thus by appropriate heat treatment it is possible to provide straight nickel protective platings bonded to the uranium by nickel-uranium alloy layers of various thicknesses, or to provide very thick layers composed entirely of alloy. By the application of alternate platings of copper and nickel on top of the initial nickel platings and subsequent heat treatment, coatings may be prepared comprising a bonding layer of uranium-nickel alloy beneath a cupronickel alloy layer firmly bonded thereto.

The nickel electrodeposits of the invention may be used as bases for subsequent zinc, tin, or lead hot-dip coatings or electroplatings. Composite platings may be rendered highly impervious by a short heat treatment slightly above the melting point of the secondary plating to promote diffusion into the pores of the underlying nickel layer. Heat treatments below the melting point of the secondary plating also may be used. to improve the coatings. Thus zinc platings on the nickel may be heat treated at 300 C. for 2 or 3 hours.

An extruded uranium rod about /8 inch in diameter is first dipped for one minute in aqueous 50% HNO solution at 25 C., and then rinsed with water. Itis immediately dipped in aqueous 36% HCl solution until the surface is completely black (about 40 seconds). It is then removed from the HCl solution, rinsed with water and dipped for seconds in 50% HNO solution.

The rod, which now has a gray, uniformly etched appearance, is again rinsed briefly with water and then immediately electroplated by placing it in an aqueous electroplating bath, which contains per liter, 240 grams of NiSO .7H O, 45 grams of NiCl .6H O, and 30 grams of H BO and is at a temperature of 40 C., and passing a current through the solution with the rod as cathode at an initial current density of 75 amperes per square foot for one minute and then at a current density of amperes per square foot for 3 minutes.

A smooth, uniform, firmly adherent nickel plating is formed on the surface of the metal.

The nickel electroplating may be alloyed with the uranium red by heating for afew minutes in an induction furnace to raisethe surface temperature to 800 C. and form an integral uranium-nickel alloy coating on:

the rod.

Example 2 A machined uranium rod about 8 inches long and 1 /3 inches in diameter is prepared for electroplating by dipping in trichlorethene to remove grease, sand-blasting lightly, dipping in aqueous 35 HNO by weight at 30 C. for 4'minutes, rinsing with cold water, etching by anodic treatment in aqueous 50% trichloracetic acid solution at between 50 and 60 C. for 10 minutes at 50 amperes'per square foot current density, rinsing with cold water, dipping for 4 minutes in cold aqueous 35% HNO solution to clean the etched'metal surface, then rinsing in cold water.

' The cleaned etched metal rod is immediately electroplated in a nickel sulfate electroplating bath of the composition employed in Example 1 at a current density of amperes per square foot at 40 C. until a nickel plating about 0.0005 inch thick is formed (about 25 minutes).

The nickel-plated rod is dipped directly in a sodiummodified alloy of 88% aluminum and 12% silicon at 640 C. for 50 seconds and then brazed by an alloy of the same composition to an aluminum sheath at about 600 C. A firm bond is formed between the rod and the sheath with no noticeable brittleness.

Example 3 An extruded uranium rod prepared for plating in the manner described in Example 2 is electroplated in a nickel sulfate electroplating bath of the same composition for about 5 minutes.

The lightly plated rod is then rinsed in water and electroplated in an aqueous electroplating bath containing 400 grams per liter of chromic acid, at 50 C. until a chromium plating 0.0001 inch thick has been built up (about 6 minutes). A concentric lead anode comprising an expanded metal cylinder having a diameter about 2 inches greater than the rod is used. A smooth uniform adherent coating is thus produced over the entire cylindrical surface of the rod with all of the fine crevices on the metal surface covered by a continuous coating of chromium on nickel.

Example 4 comprises etching the uranium in an aqueous solution con- 4 per bath, rinsed, and again electroplated in the nickel bath for 10 minutes.

This coating provides good protection for the metallic uranium. However, byimmersing it in a molten bath of potassium chloride lithium chloride eutectic at 750 C. for between 1 and 2 minutes, an alloyed coating of cupro-nickel over uranium-nickel alloy is obtained having excellent adherence and the corrosion resistance characteristicof cupro-nickel alloys. The copper improves the barrier effect of the coating to diffusion of uranium as described in greater detail and claimed in my US. patent application Serial No. 571,673 entitled Copper Coatings and Method of Applying, filed January 6, 1945, now Patent No. 2,854,737, dated October 7, 1958.

Example 5 A uranium rod is nickel electroplated and then electroplated with a single layer of copper as in the. first two electroplating steps of Example 4.

function of preventing uranium-nickel alloy, which at the rolling temperature is relatively soft, from adhering to the tube.

This application is a divisional application of my application Ser. No. 572,093, filed January 9, 1945, now

Patent No. 2,854,738 dated October 7, 1958.

It will be understood that I intendvto include variationsand modifications of the invention and that the t preceding examples are illustrations only and in no wise to be construed as limitations, upon the invention, the

, scope of which is defined in the appended claims, wherecomprises anodically etching the uranium in an etch in an aqueous phosphoric acid solution containing chloride ions, cleaning the etched uranium in aqueous nitric acid solution, and promptly electroplating the cleaned uranium in a nickel sulfate electroplating bath.

2. The method of protecting a metallic uranium article, which comprises etching the uranium in an aqueous etching solution containing chloride ions, electroplating the etched uranium article with a nickel first coating and dipping the nickel-plated article into a molten metal bath comprising principally aluminum.

3. The method of protecting a metallic uranium article, which comprises etching the uranium in an aqueous solution containing chloride ions electroplating the etched uranium with a nickel first coating and dipping the nickel plated article into a molten metal bath comprising a low melting aluminum-silicon alloy for a sufl'icient time to form a coating of aluminum-silicon alloy on the article.

4. The method of protecting a metallic uranium article, which comprises etching the uranium in an aqueous etching solution containing chloride ions, electroplating the etched uranium with a nickel first coating, dipping the nickel-plated article into a molten metal bath comprising a low melting aluminum-silicon alloy for a sufficient time to form a second coating of aluminum-silicon alloy on the article, and subsequently brazing the coated article to an aluminum sheath with a low melting aluminum-silicon alloy.

5. The method of protectively coating uranium which taining chloride ions, electroplating a coating of nickel on the etched uranium and heating the nickel plated uranium by immersion thereof in a moltenbath composed of a material selected from the groupconsisting of sodium chloride, potassium chloride, lithium chloride, and mixtures thereof, maintained at a temperature of between 700 C. and 800 C., for a time sufiicient to alloy the nickel and uranium and form an integral pro- The rod is then rinsed and rolled in an evacuated tube at 750 C. for

and subsequently applying a metallic protective coating 1 thereover.

7. The method of claim 6 wherein the acidic solution containing chloride ions comprises hydrochloric acid.

References Cited in the file of this patent UNITED STATES PATENTS Hall July 27, 1915 6 1,256,954 Travers Feb. 19, 1918 1,975,818 Work Oct. 9, 1934 2,162,789 Raub June 20, 1939 2,250,556 Stareck July 9, 1941 2,429,222 Ehrhardt et a1. Oct. 21, 1947 2,443,651 Cannizzaro June 22, 1948 2,555,372 Ramage June 5, 1951 OTHER REFERENCES Transactions of the Electrochemical Society, vol. 66 (1934), page 41.

Modern Electroplating, published by The Electrochemical Society, N.Y.C., 1942. Pages 235, 236, 237, 238, 260 and 261.

Claims (1)

1. THE METHOD OF PROCTECTIVELY COATING URANIUM, WHICH COMPRISES ANODICALLY ETCHING THE URANIUM IN AN ETCH IN AN AQUEOUS PHOSPHORIC ACID SOLUTION CONTAINING CHLORIDE IONS, CLEANING THE ETCHED URANIUM IN AQUEOUS NITRIC ACID SOLUTION, AND PROMPTLY ELECTROPLATING THE CLEANED URANIUM IN A NICKEL SULFATE ELECTROPLATING BATH.