US3573120A

US3573120A - Uranium etchant and method

Info

Publication number: US3573120A
Application number: US757977A
Authority: US
Inventors: Forrest B Waldrop; Max J Bezik
Original assignee: US Atomic Energy Commission (AEC)
Current assignee: US Atomic Energy Commission (AEC)
Priority date: 1968-09-06
Filing date: 1968-09-06
Publication date: 1971-03-30
Anticipated expiration: 1988-03-30
Also published as: GB1225103A; DE1944377A1; FR2017505A1

Abstract

METALLIC URANIUM AND ALLOYS THEREOF ARE PROVIDED WITH UNIFORMLY ETCHED SURFACES FOR FACILITATING THE RECEPTION OF ADHERENT PROTECTIVE COATINGS. THE ETCHANT CONSISTS OF AN AQUEOUS SOLUTION OF EITHER FERRIC CHLORIDE OR FERRIC CHLORIDE AND NICKEL CHLORIDE.

Description

March 30,",1971 WALDRQP ET AL 3,573,120

URANIUM ETCHANT AND METHOD Filed Sept. 6. 1968 Fe C| '6H conc. (moles/liter) URANIUM REMOVAL IN ONE FI'NUTE (mils) Fig.1

2R 4 ,2 i 3 5w m; 2 2 3% 1 2; 4/ :0- 0

TEMPERATURE (*c) Fig.

1N VENTORS.

Forrest B. Wq/drop BY Max J. Bez/k United States Patent 01' lice U.'S. Cl. 15618 2 Claims ABSTRACT OF THE DISCLOSURE Metallic uranium and alloys thereof are provided with uniformly etched surfaces for facilitating the reception of adherent protective coatings. The etchant consists of an aqueous solution of either ferric chloride or ferric chloride and nickel chloride.

Structures or articles of metallic uranium and uranium alloys are customarily provided with a protective sheath or coating, usually an electro-deposit of nickel, for minimizing the health hazards associated with the handling of uranium and for protecting the structures from deleterious corrosion. The preparation of the uranium surfaces for receiving a firmly adherent coating necessarily includes the important step of contacting the surfaces with an etchant for roughening or otherwise preparing these surfaces to facilitate the formation of a tenacious mechanical bond between the coating and the uranium. The etchant should be capable of uniformly etching the entire contacted surfaces while effecting minimal material removal.

The prior art is replete with uranium etching solutions, such as, for example, an aqueous solution containing nickel chloride and nitric acid, an ethyl alcohol-strannous; chloride solution and an anodic pickle in a phosphoric acid-hydrochloric acid solution, a nickel chloride-nitric acid-hydrochloric acid solution, and a stannous chloridenitric acid solution. While these previously known etchants have provided uranium with surfaces receptive to the protective coating, they each, for one reason or another, exhibit shortcomings or drawbacks which detract from their usefulness. For example, the nickel chloride-nitric acidwater etchant suffers a changing etching rate due to the variance in the chloride and nitrate contents which is caused by the interaction of the two ions. This interaction occurs even when the etchant is not being used. Relatively rapid etching rates encountered by using some of the previous etchants create an undesirable condition since very precise timing schedules are required to prevent excessive etching which results not only in excessive surface roughness and material removal, but also in non-uniformly etched surfaces. Another disadvantage of rapid etching rates is due to significant temperature increases during the etching operation that cause the uranium to heat up and continue to etch with adhering etchant when removed from the etchant bath and before the water rinsing step. The useable life of previous etchants as Well as the etching rates of these etchants is significantly influenced by the uranium concentrations in the etchant since previous etchants with uranium concentrations of over about 6 ounces per gallon of the etchant were rendered virtually useless.

It is the aim of the present invention to provide an etching solution for uranium and uranium alloys which substantially minimizes or obviates the above and other shortcomings or drawbacks suffered by previously known etchants. In accordance with this invention structures of metallic uranium or uranium alloys are uniformly etched with minimal material removal in a novel etching solution 3,573,129 Patented Mar. 30, 1971 consisting of an aqueous solution of ferric chloride or ferric chloride and nickel chloride.

An object of the present invention is to provide a new and improved etching solution for etching uranium or uranium alloys in preparation for the reception of protective coatings particularly electro-deposited nickel plate.

Another object of the present invention is to provide for the etching of uranium and uranium alloy surfaces in a uniform manner and with minimal material removal.

A further object of the present invention is to provide a uranium and uranium alloy etchant consisting of an aqueous solution of ferric chloride or ferric chloride and nickel chloride and capable of providing a relatively slow etching rate for permitting better control over the etching operation.

Other and further objects of the invention will be obvious upon an understanding of the following description 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.

In the accompanying drawing:

FIG. 1 is a graph showing the etching rates on metallic uranium when aqueous ferric chloride solutions of various concentrations are employed; and

FIG. 2 is a graph showing the effect of temperature on the rate of etching metallic uranium in a 5-molar ferric chloride solution.

Described generally, the present invention relates to a new and improved chemical solution for etching uranium or uranium alloys for the purpose of preparing such alloys for the reception of a protective coating, particularly electrolytically or electrolessly deposited nickel. The etching solution envisioned herein consists of a ferric chloridewater solution which has been found to effect uniform etching of exposed uranium or uranium alloy surfaces at a moderate or relatively slow rate for providing a desirable surface to which the nickel coating will adhere tenaciously. The etching solution is prepared by admixing ferric chloride hexahydrate (FeCl -6H O) in water. An alternative aqueous solution can be prepared by utilizing a mixed chloride solution consisting of nickel chloride (NiCl and the ferric chloride hexahydrate.

As shown in FIG. 1, the concentration of ferric chloride hexahydrate in moles per liter of water has a considerable influence upon the etching rate of uranium. Thus, in order to provide a satisfactory etching rate as well as uniform etching, the concentration of the ferric chloride in the etchant is preferably in a range of about 4.8 to 5.5 moles per liter of water. At higher FeCl concentrations than those in the preferred range, the etching rate slows down until it is virtually non-existent in a saturated solution at 30 C. However, with FeCl concentrations less than about 4.8 M, the etching rate and the temperature of the etching solution significantly increase until the etching rate is almost catastrophic at a concentration of approximately 4.1 M. Further dilution of the etching bath below 4.1 M effects a decrease in the etching rate until it is again in the practical range. While etching solutions of these lower FeCl concentrations may be usable it has been found that these solutions do not provide the uniformity in the etched surface or dependability of the etching solutions with FeCl concentrations in the preferred range.

The etching solution of the present invention is not dependent upon the use of elevated temperatures such as required of many previously utilized etchants in that excellent results have been achieved in a relatively low temperature range of about 3060 C. As shown in FIG. 2, the etching rates of a 5.2-molar FeCl solution are fairly uniform between 30 and 60 C. However, at temperatures below about 30 C., the etching solution tends to become viscous, which may cause some crystallization to take place and thereby impair the etching properties of the solution. At temperatures greater than about 60 C., on the other hand, the etching rate increases rapidly and detracts from the desirable etching rate provided by the cooler solutions.

Perhaps one of the more desirable features of the aqueous FeCl etching solution is the control factor provided by the relatively slow etching rate which permits close regulation over the surface finish and the quantity of material removed by the etchant. When employing the etching solution of the present invention, in the preferred concentration range the etching rate is fairly rapid when the uranium piece is first immersed in the etching bath but immediately slows down, and after approximately two minutes the etching rate is considerably slower so that a total etching time of about to minutes is required for effecting a desirable surface finish on the uranium piece. It has been found that the surface finish of a uranium piece is properly etched when the quantity of material removed from the surface corresponds to a depth of about 0.6 to about 1.0 mil. The characteristics (centerline averages in microinches) of the surface finish on uranium pieces etched in a 5.2-molar ferric chloride etching bath are more clearly illustrated by the following table.

TABLE I.SURFACE FINISH OF URANIUM E'ICHED IN CHLORIDE SOLUTIONS Surface finish Etching conditions (microinches OLA) Tern era- Time Before After Etchant ture( C.) (minutes) etching etching The mixed chloride solution, i.e., the nickel chlorideferric chloride solution briefly described above, affords satisfactory etching results, but may not be quite as desirable as the ferric chloride solution in that the rate of uranium removal in the mixed chloride bath is substantially greater than that in the ferric chloride bath. The concentration of the chlorides in this mixed bath has been found to be most desirable when the solution is prepared with about 3.0 M NiCl and 1.5 M FeCl As shown in the following tables, the concentration of the mixed chloride solution has a considerable effect upon the rate of material removal.

TABLE III.EFFECT OF TEMPERATURE ON THE RATE OF ETCHING OF URANIUM IN NICKEL CHLORID RIC CHLORIDE SOLUTIONS Molar concentration Temper- Material Etch time ature removed Ni Fe (minutes) C.) (mils) TABLE II.EFFECT OF CONCENTRATION ON THE RATE OF ETCHING OF URANIUM IN A NICKEL CHLORIDE- FERRIC CHLORIDE SOLUTION [3.0 M NiClz+1.5 M FeCla] Material Etch time removed Temperature C.) (minutes) (mils) 4 faces by immersing the etched uranium in a suitable iron solvent such as nitric acid.

In order to provide a better understanding of the present invention an example of a typical etching operation on metallic uranium is set forth below. In this example the the uranium is preferably cleansed of contaminants and oxides prior to the etching treatment by utilizing conventional cleaning mechanisms such as grease solvents, detergents, sand blasting, removal of oxides with acids, etc.

EXAMPLE A metallic uranium disk was initially prepared for etching in the manner described above and then immersed in a ferric chloride etching solution prepared by admixing 1.4 grams ferric chloride hexahydrate per milliliter water (5.2 M FeCl concentration). After the uranium disk was in the solution for 16 minutes with the latter at 30 C., the disk was removed and rinsed with distilled water. The disk was then pickled in a nitric acid solution (35 weight percent at room temperature) for a duration of 8 minutes to remove iron smut formed on the surface. The material removed from the surface of the uranium disk was 0.588 mil, the average rate of removal being 0.037 mil per minute. The etched uranium disk was then provided with an electrolytically deposited nickel plate.

The ferric chloride etching solution of the present invention produces a uniformly etched surface on metallic uranium which is fairly smooth but sufiiciently rough to effect the required mechanical bonding with the nickel plate. In fact, the etching-solution of the present invention tends to improve already rough surfaces rather than increase the roughness of such surfaces. Inasmuch as this novel etching solution has a significantly slower etching rate than previously known etchants, the operational control factor substantially expedites the formation of uniformly etched surfaces. Additionally, the novel bath of the present invention exhibits a significantly longer useable life than the previous etchants due to a higher tolerance for uranium concentrations in the solution and a lower dependence of the etching rate upon temperature. For example, satisfactory etching of metallic uranium is accomplished without objectionable variances in the etching conditions even when uranium concentrations in excess of 10 ounces per gallon of solution are encountered. The effect of temperature on the rate of etching is small in the usual operating range of 3060 C., particularly when comparing the effect of the excessive etching rates encountered by the previously known solutions when subjected to elevated temperatures which are often required or occur during the etching operation.

A uranium piece etched in the ferric chloride etchant of the present invention was nickel plated, given a highhumidity corrosion test (20 hours at 86 C. in 100 percent relative humidity), and then recycled twice to demonstrate the effectiveness of the present etching solution. This recycling consists of stripping off the nickel plate in a cyanide solution and then cleaning the uranium surface in nitric acid but not re-etching. The part was then replated and corrosion tested again. No corrosion was found to be present after any of the plating and exposure tests.

As briefly described above, the ferric chloride etching solution of the present invention is useable with uranium alloys; for example, the uranium alloy consisting of 90 percent uranium and 10 percent molybdenum. A uniform surface etch of this uranium alloy was achieved by using an etching solution substantially less concentrated than that described above for metallic uranium. With this alloy the solution found to be satisfactory consisted of 180 grams of ferric chloride hexahydrate and 140 cubic centimeters of water. The etching was accomplished at 30 C. for a duration of approximately 4 minutes. The surface finish provided on this alloy was -180 microinches CLA. Other alloys of uranium such as uranium-2 percent molybdenum may be similarly etched in the FeCl solution.

It will be seen that the present invention provides a unique etching solution which is capable of uniformly etching uranium with minimal material removal without utilizing nitric, hydrochloric, or phosphoric acid. Uranium structures treated with the etching solution of the present invention have not shown any failure-tO-etch areas on the surfaces of the uranium structures, which substantially overcomes the problem continually encountered in etching operations using previously known etchants. Further, there has been no instance where the parts have not been etched uniformly.

As various changes may be made in the etching solutions, etching temperatures, and etching times 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. A method of preparing articles of uranium metal and alloys thereof for the reception of a protective metal coating, comprising the steps of immersing the article in an etching bath consisting of a water-chloride solution with the chloride being selected from the group consisting of ferric chloride hexahydrate with the ferric chloride in the Water-chloride solution being in the range of about 4.8 to about 5.5 moles per liter or nickel chloride and ferric chloride hexahydrate, maintaining the etching bath at a temperature in a range of about to C. during immersion of the article in the etching bath, maintaining the article in the bath for a duration sufficient to uniformly etch exposed surfaces of the uranium article, removing the article from the bath, rinsing the article in water, and thereafter immersing the article in a solution containing an iron solvent for removing from the article surfaces iron values deposited thereon during the immersion in the bath.

2. The method claimed in claim 1, wherein said duration is in a range of about 10 to 15 minutes.

References Cited UNITED STATES PATENTS 2,886,420 5/1959 Jones et a1 156-18 JACOB H. STEINBERG, Primary Examiner U.S. Cl. X.R.