US2653134A - Process for removing scale from zirconium metal and alloys thereof - Google Patents

Description

Patented Sept. 22, 1953 EXANHNLK PROCESS FOR REMOVING SCALE FROM ZIR- CONIUM METAL AND ALLOYS THEREOF Elmer Donald Billing and Gordon Lowell Frederic, Albany, reg., assignors to the United States of America as represented by the Secretary oi the Interior No Drawing. Application October 10, 1950,

Serial No. 189,472 I 1 Claim.

(Granted under Title 35, U. S. Code (1952),

sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to us of of any royalty thereon in accordance with the provisions of the act of April 30, 1928 (ch. 460, 45 stat. L. 467).

This invention relates to pickling solutions for cleaning and removing scale from zirconium metal and alloys thereof, and more particularly, from zirconium shaped objects which have been hot worked whereby they have become coated with a tenacious layer of oxide.

In the hot working of zirconium objects the metallic zirconium, including alloys thereof, reacts with atmospheric gases and becomes coated with a tenaciously adherent scale which must be removed before final finishing. As is customary in the metal pickling art, the objects are degreased and then pickled in an acidic solution designed to dissolve the scale but not substantially to affect the metallic object. In the case of zirconium and its alloys 'it has been exceptionally diihcult to remove the scale without at the same time pocking or otherwise adversely affecting the sheet, bar or other shaped object of zirconium.

Accordingly, this invention has for an object the pickling of metallic objects of zirconium and its alloys without substantially affecting the metallic shaped object. Another object of this invention is to provide a pickling bath containing a certain preferred proportion of reagents which accomplishes the maximum cleaning or pickling with minimum destruction of the metallic zirconium itself. Obviously, to be effective, the pickling bath must selectively dissolve the undesired scale components while having substantially no adverse action on the metallic suband nitrate in contact with metallic lead.

Peculiarly enough, the presence of metallic lead in the pickling bath is highly desirable, inasmuch as the highly successful pickling operation of this invention operates to advantage in its presence. Apparently there exists a cooperative relationship between the etching or pickling components, the metallic lead and the zirconium metal being pickled, such that the scale loosening and removal is more complete in the presence 2 of lead, while the undesired attack upon the metallic zirconium itself is substantially inhibited. Thus solution, etching and pocking of the metallic zirconium object being pickled is substantially prevented.

In carrying out this invention, there may be employed any of the usual forms of zirconium requiring a pickling treatment, such as hot rolled bars, sheets, rods, or the like, formed of pure zirconium or its usual alloys containing a substantial proportion of zirconium, such as for example, the alloy containing from 95 to 99 percent zirconium, the balance being substantially hafnium. If desired, the object of zirconium to be descaled or otherwise cleaned by pickling can be subjected to a preliminary sand blasting operation as is known to those skilled in the art, but oneof the significant advantages of this invention lies in the fact that preliminary sand blasting may be dispensed with entirely in suitable cases.

A preferred pickling bath is made up containing about wrcentby vplun e of 60 percent hydrofluoric ac1d,about'3 percent by weight of lead nitrate, about 3 erc nt by vglume of 70.percent nitric acid, a su s antial excess of free metallic lead over that required by solution in the acid reagents, and the balance being substantially water. The exact proportions of reagents in the pickling bath can be varied considerably, but it is important that a substantial concentration of lead ions, fluoride ions and nitrate ions be present in contact with metallic lead in an aqueous acidic solution having a pH lying between about pH 0.2 and pH 3.5. During the pickling treatment a substantial proportion of the metallic lead present passes into solution which apparently cooperates to inhibit substantial solution of the zirconium metal itself.

The following example illustrates how the invention may be carried out, but is to be considered illustrative and not limitative:

Emampla-A sheet of zirconium which had been hot rolled in air at 650 C. was employed as a test specimen, inasmuch as it contained a very hard tenacious black oxide on its surface. The test specimen was immersed in a plastic tank containing an aqueous solution of pickling reagents. The reagent solution was made up to contain water and 3 percent by volume of 60 percent hydrofluoric acid, 3 percent by weight of lead nitrate, 3 percent by volume of percent nitric acid and strips of metallic lead, totaling about 10 percent by weight of the test specimen to be treated, were placed in the solution.

The zirconium sheet was first cleaned by sand blasting and then degreased by solvent treatment and soap and water as is known to those skilled in the art. Thereupon, the sheet was immersed in the pickling solution until a black spongy precipitate was formed on the surface thereof. The sheet was then removed from the bath and rinsed in clear water to remove the precipitate and pickling solution. During the rinsing gentle rubbing with a sponge or soft cloth completely removed the precipitate and a bright smooth finish of metallic zirconium was obtained. The object was later buffed to a final high polish.

It will be apparent that this invention has pro vided a completely satisfactory way to remove tenacious scale and oxide films from hot worked zirconium objects, including the alloys thereof. Peculiarly enough, the presence of metallic lead in the pickling bath cooperates to facilitate scale removal and minimize solution of metallic zirconium itself, while at the same time substantially inhibiting the pocking or pitting familiar to those skilled in this art. This invention also provides desirable proportions of reagents for accomplishing the most rapid and complete pickling while minimizing undesired side reactions.

Since many apparently differing embodiments of this invention will occur to those skilled in the art, the foregoing description and example should be considered as illustrative only and not as limiting the invention to the precise details disclosed and described.

What is claimed is:

A process for removing scale from metallic articles of zirconium and its alloys that consists in subjecting the article to the action of a pickling bath consisting of about 32; by volume of hydrofluoric acid, 3% by volume of 79% nitric acid, 93-94% by vanes of water with about'3'%' by weight of lead nitrate, while maintaining free metalic lead. in the bath, and maintaining such articles in contact with said bath until the scale is loosened and removable by washing, and thereafter washing and rinsing the articles in water.

E. DON DILLING. GORDON LOWELL FREDERIC.

References Cited in the file of this patent UNITED STATES PATENTS Number