US2848351A - Nitric acid pickling process - Google Patents

Description

Unite I and possess only limited adherence.

States NITRIC ACID PICKLING PROCESS Ernest R. Boiler, Marion, Ind., and Lowell D. Eubank, South Euclid, and John W. Robinson, Cleveland, Ohio, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application September 28, 1945 Serial No. 619,265

1 Claim. c1. 117--51 This invention relates to the preparation of metallic uranium surfaces for the application of coatings. It is particularly concerned with the preparation of uranium for coating by dipping in molten metal coating baths.

Metallic uranium articles produced by extrusion, machining, hot rolling, or other methods of working the metal are not in a satisfactory condition for the application of adherent metallic coatings. Coatings applied to such metal surfaces by hot dipping tend to be irregular They have only slight resistance to corrosion and frequently develop pinholes.

The corrosion resistance of metal coatings on uranium seems to depend in a large measure upon the continuity of the inner layer of alloy of uranium with the coating metal. It has been found that coatings which appear to be free from defects sometimes show early failure on exposure to corrosive conditions. Examination of the structure of such coatings has shown that the inner alloy layer has contained numerous pinholes filled with the coating metal. The coatings most resistant to corrosion have shown few or no defects of this type whereas rapid failure is characteristic of coatings containing large numbers of them.

It is an object of the present invention to improve metallic uranium surfaces for the application of coatings. A further object of the invention is to provide a preparatory process as a result of which metallic uranium may be coated by the hot dip method with the production of uniformly adherent corrosion-resistant coatings of various coating metals and alloys. A still further object of the invention is to provide a preparatory process which reduces the tendency of coatings to form pinholes.

We have found that the preparation of metallic uranium for the application of hot dip coatings which will adhere uniformly and firmly and which will not tend to develop pinholes when exposed to corrosive conditions, may be accomplished with nitric acid. Careful control of the nitric acid concentration, temperature, and treatment time is required.

In accordance with the present invention metallic uranium is prepared for coating by treating its surface with aqueous 50% to 70% nitric acid solution at a temperature between 60 C. and 70 C. for between 4 and 6 minutes. The residual acid solution is rinsed from the surface, the surface is dried, and the uranium is then coated in convenient manner.

By employing a nitric acid pretreatment under the specified conditions the pinhole defects in the inner alloy may be kept at a very small number and coatings of superior resistance to corrosion may be obtained.

The metal coating process should follow the nitric acid process promptly to avoid impairment of the surface by atmospheric reaction. Thus it is desirable to rinse and dry the surface rapidly and immediately to coat the article, as by dipping it through an alkali-metal halide flux into a molten coating metal.

atent O The following examples illustrate our invention. Proportions are weight proportions if not otherwise indicated.

EXAMPLE 1 Duplicate specimens of metallic uranium from the same source and having the same previous history were pickled in aqueous nitric acid solutions under various conditions set forth in the table below. Each specimen, after the nitricv acid treatment, was rinsed with water, dried, and coated with a zinc-aluminum alloy containing parts by weight of zinc and 15 parts by weight of aluminum by dipping the specimen through a molten alkali metal chloride flux, comprising 42% LiCl, 53% KCl' and 5% NaCl. In each case the speciment was held in the molten metal until entirely coated with the zinc-aluminum alloy. After the specimens were removed from the molten metal they were placed on rotating, smooth asbestos cement (Transite) rollers until the coatings solidified. Cooling was then allowed to continue in air.

The coatings were stripped by treatment with dilute HNO until the light ZnAl coating was removed, leaving the darker UAl layer exposed. This underlayer was then examined. The results of the examination are set forth in the table.

Table J HNO; Con HNO; Time in HNO Group centration, Temp., min. No. of Pinholcs Wt. percent 0.

50 and 70 60 2, 3. 4, 5, 6, 10.. l0 l00. 50 and 70 60 and 70 2, 3 100. 50 and 70 70 2, 3, 4, 5, 6, 10 Surface rough and pitted. 50 and 70 60 and 70 4, 5, 6 0-l0. 50 and 70 60 and 70 10 10, Cracks.

30 60 2, 3, 4, 5, 6, l0 10 l00. 30 60 and 70 2, 3, 4, 5, 6, 10 10. 30 70 2, 3, 4, 5, 6, 10 Surface rough and pitted.

EXAMPLE 2 A metallic uranium rod, about 1.1 inch in diameter and 4 inches long, was pickled in aqueous 50% nitric acid solution at 6070 C. for five minutes. Following this it was rinsed in clean water and air-dried.

It was then dipped through a flux consisting of 53% potassium chloride, 42% lithium chloride, and 5% sodium chloride into a molten metal bath, comprising 85 parts of zinc (Prime Western Spelter) and 15 parts of aluminum, at 560 C. for three minutes. The rod was Withdrawn from the bath, shaken to remove excess coat ing metal, and rolled on smooth Transite rollers for 2 minutes to solidify the coating. The rod was then allowed to cool in air to room temperature. A smooth, adherent, corrosion resistant coating was thus produced.

EXAMPLE 3 An extruded uranium rod of the type employed in Example 1 was dipped into an aqueous 50% nitric acid solution at 6070 C. for 5 minutes then removed, rinsed, and dried.

It was then dipped directly into an 88% aluminum 12% silicon bath. The period of immersion in the aluminum-silicon bath was 1 /2 minutes and the temperature was 610 C. The product was rolled as in Example 1 until the coating had solidified; it was then cooled in air to room temperature. A smooth, uniform firmly adherent coating of aluminum-silicon alloy over a hard intermediate uranium alloy layer was formed.

EXAMPLE 4 A metallic uranium rod was treated with 50% nitric acid at 65 C. for 5 minutes, rinsed, and dried. The rod was then dipped through a molten alkali-metal chloride flux of the composition employed in Example 1 into a molten metal bath comprising 67% copper and 33% tin maintained at about 800 C. After 45 seconds in this bath the rod was withdrawn and cooled below the melting point of the coating metal. The rod was uniformly coated with a firmly adherent bronze coating.

EXAMPLE 5 A smooth even, firmly adherent coating of zinc was thus formed on the uranium surface.

It will be understood that we intend to include variations and modifications of the invention and that the 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 claim, wherein we claim:

In the application of a metallic coating to a uranium article covered with an oxide film, the improvement which consists of treating said article with aqueous to 70% nitric acid solution at to C. for 4 to 6 minutes, rinsing the acid solution from the uranium article, drying the article, and thereafter promptly passing it through a molten alkali-metal halide flux consisting of 42% LiCl, 53% KCl, and 5% NaCl into a molten metal coating bath consisting of parts by weight zinc and 15 parts by weight aluminum.

References Cited in the file of this patent UNITED STATES PATENTS Co., Ltd., London. Also page 323.