Process For Etching Beryllium

Abstract

A process for chemical milling of beryllium and high beryllium alloys is described wherein the beryllium is subjected to an aqueous solution of an acid selected from the group consisting of sulfuric acid, oxalic acid, and ammonium acid fluoride plus sufficient hexavalent chromium ion to prevent channeling. In a preferred embodiment sulfuric acid is present in the range of 5-15 percent chromium ion is present in the range of 7-10 percent CrO.sub.3 ; sodium nitrate is present up to about 1.3 percent; and hydrofluoric acid is present up to about 2 percent.

Description

BACKGROUND

The CHEM-MILL (Registered Trademark of North American Rockwell Corporation) selective etching process is a widely used industrial technique wherein a portion of a structural part is masked with an inert material and another portion of the part is exposed. The exposed area is then subjected to action of an etching solution to uniformly remove metal from the exposed area thereby changing the geometry of a part. Beryllium is a structural material having excellent high temperature properties, very low weight and exceptionally high modulus of elasticity. These desirable properties are offset by a relative difficulty in machining beryllium by conventional mechanical techniques. It is therefore, highly desirable to provide a means for chemical milling of beryllium.

Beryllium is soluble in most mineral acids, however, this alone is not sufficient for most chemical milling operations since it is usually necessary that the finished part have a good surface finish and that the part be uniformly etched in all areas. In addition it is necessary to have an etch rate sufficiently fast to be economical and yet sufficiently slow that carefully controlled tolerances can be achieved. The ability to achieve these characteristics in a chemical milling process is dependent on selection of an etching bath having a composition suitable for a material being etched. In beryllium, in particular, care must be taken to avoid channeling or threading which are names given to a phenomenom wherein the etch rate is higher at the boundary, and a shallow swale is found at the boundary after etching.

Advantages of this invention will be obvious from the matter set forth hereinafter or may be learned by practice of the invention, the same being realized by means of the composition, processes and steps pointed out hereinafter.

SUMMARY OF THE INVENTION

There is provided in practice of this invention according to a preferred embodiment, a process for etching beryllium by subjecting the surface to action of an aqueous solution of an acid selected from the group consisting of sulfuric acid, oxalic acid, and ammonium acid fluoride in a concentration sufficient to maintain a chemical milling etch rate, and sufficient hexavalent chromium ion to prevent channeling.

DISCLOSURE

Referring now in detail to the composition of the etching bath, it is preferred that an acid selected from the group consisting of sulfuric acid, oxalic acid, and ammonium acid fluoride be employed in the range of from about 5-15 percent by weight. If less than about 5 percent of acid is employed it is found that the etching is unreasonably slow, if present at all; and above about 15 percent it is found that uneven etching is obtained over a surface, with resultant peaks or pits in the surface.

It is also preferred that hexavalent chromium be present in the range of about 7-10 percent CrO.sub.3 to inhibit channeling. If less than about 7 percent CrO.sub.3 is present channeling may occur at the edges; further the etching action is excessively fast and control of the tolerance of the part becomes difficult. In addition, significant heating occurs during rapid etching and uneven etching in other areas of the exposed surface may be obtained. Excessive heating of the etchant solution may also cause health hazards due to the toxic nature of beryllium compounds. It is preferred that the concentration of CrO.sub.3 be less than about 10 percent since larger amounts of hexavalent chromium unduly passivate the beryllium surface and reduce the etching rate below any economically feasible level.

Another constituent of the etchant is hydrofluoric acid in a concentration up to about 2 percent by weight. The presence of hydrofluoric acid in the etching bath eliminates smut from the surface thereby promoting more even etching and minimizing cleaning operations subsequent to etching in the chemical milling bath. A concentration of hydrofluoric acid higher than about 2 percent in combination with the other acids may give an excessively high etching rate for controlled chemical milling.

Another constituent of the etching bath is sodium nitrate in a concentration up to about 2 percent. The sodium nitrate substantially inhibits pitting of the beryllium surface during chemical milling and when substantial amounts of beryllium are to be removed a better surface finish is thereby obtained. When only thin layers of beryllium are to be removed, the sodium nitrate can be safely omitted from the etchant. The concentration is preferably limited to about 2 percent sodium nitrate since higher concentrations unduly inhibit etching for production efficiency.

It is preferred to maintain the beryllium concentration in the etchant bath at less than about 0.1 pound of beryllium per gallon of solution since the dissolved beryllium inhibits etching to an undue extent and chemical milling baths having a higher concentration of beryllium are substantially unusable because of a lowered etch rate.

If desired conventional acid compatible wetting agents in small quantities can be included in the bath to assure good wetting and smooth etched surfaces as is often done in chemical milling baths.

A particularly preferred composition of chemical milling etchant for beryllium comprises an aqueous solution having 5-15 percent by weight of sulfuric acid, 1-1.3 percent sodium nitrate, hexavalent chromium in the range of 7-10 percent CrO.sub.3 and hydrofluoric acid in a concentration of about 2 percent. An etchant in this range employed for chemical milling of beryllium is economical, and uniformly provides a good surface finish, a satisfactory etch rate and freedom from channeling, pitting, and smut.

It is preferred to employ a chemical milling bath for beryllium in the temperature range from approximately room temperature to about 140.degree. F. with the temperature and concentration of the etching bath being maintained in a range to give an etch rate in the range of about 5 to about 15 mil per hour. (1 mil = 0.001 inch). An etchant having a (maximum) concentration of sulfuric acid of about 15 percent in a (minimum) concentration of hexavalent chromium of about 7 percent CrO.sub.3 at 140.degree. F. (maximum) has an etch rate of about 1 mil per minute which is considered excessive for chemical milling of beryllium for most applications since close tolerance control is difficult to achieve with rates this rapid. It will be apparent that either lower concentration of sulfuric acid or lower temperatures or higher chromium ion concentration will reduce the etch rate to the preferred range. It is preferred that the etch rate be greater than about 5 mil per hour for best economy since it is found that lower rates give no substantial increase in the tolerance achievable, and further, a tendency toward channeling at the edges of etched areas is noted. The upper etch rate can be maintained at any value compatible with personnel safety and which will give acceptable tolerances. Generally, it is found preferable to maintain an etch rate less than 15 mil/hour to maintain good dimensional control and provide the best surface finish.

Suitable maskants for the beryllium in the described etching bath include adhesively bonded lead tape, conventional vinyl type maskants, conventional neoprene type maskants, and butyl-neoprene type maskants such as described in U.S. Pat. No. 3,325,320 entitled "Method of Selectively Etching Metals" by C. C. Shepherd and assigned to North American Rockwell Corporation, assignee of this application, any of which is suitable for use in the relatively low temperature acid baths.

After chemical milling a chemical polish can be employed, if desired, to enhance the appearance of the part. A substantial luster is obtained by brief exposure to a solution comprising approximately 90 percent phosphoric acid; 5 percent sulfuric acid and 5 chromic acid.

In order to make about 61/2 gallons of etchant 1,050 grams of CrO.sub.3 and 150 grams of NaNO.sub.3 were dissolved in about 23.8 liters of water and about 815 ml. of 98 percent sulfuric acid was added. Sheets of beryllium were masked with a neoprene type maskant and a grid of scribe lines was made in the maskant on one side of the sheet. Alternate squares of the maskant were peeled to expose a "checkerboard" pattern on the sheet with three-quarter-inch squares. The sheet was immersed vertically in etchant at slightly about room temperature to yield an etched depth of about 0.030 inch. The resultant surface was smooth and uniform and there was no evidence of channeling.

Etchants of the compositions shown in Table I also provide good smooth surface finishes, no channeling and a satisfactory etch rate, the exact etch rate being determined by temperature of the bath. In any of these compositions, the etch rate is increased slightly by addition of up to 2 percent hydrofluoric acid, however, the principal effect of hydrofluoric acid is elimination of smut during etching.

TABLE I

H.sub.2 SO.sub.4 NH.sub.4 HF.sub.2 C.sub.2 H.sub.2 O.sub.4.sup. . 2H.sub.2 O CrO.sub.3 NaNO.sub.3 15% -- -- 9% 1-1/2% 5 -- 5% 7 -- 5 -- -- 8 2 -- 7-1/2% -- 10 2-1/2 10 -- -- 10 -- -- 5 -- 9 1-1/2 5 5 -- 9 1-1/2 9 -- -- 18 3 -- -- 12 7 1

it is to be understood that the above described examples are merely illustrative of application of the principles of the invention. Those skilled in the art may readily devise other variations that will embody the principles of the invention. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

What is claimed is:

1. A process for chemical milling beryllium and beryllium alloys comprising:

subjecting the beryllium to an aqueous solution consisting essentially of sulfuric acid in the range of about 5 to 15 percent by weight;

hexavalent chromium in the range of about 7 to 10 percent by weight CrO.sub.3 ;

nitrate ion in the range of about 1 to 1.3 percent by weight of NaNO.sub.3 ;

hydrofluoric acid in the range up to about 2 percent by weight; and

wherein said aqueous solution has a concentration of beryllium of less than about 0.1 pound per gallon of solution and the etch rate is in excess of about 5 mil/hour.

2. A process for chemical milling beryllium and beryllium alloys comprising subjecting the beryllium to an aqueous solution of an acid selected from the group consisting of sulfuric acid, oxalic acid and ammonium acid fluoride in the range of 5 to 15 percent by weight, hexavalent chromium in the range of 7 to 10 percent by weight of CrO.sub.3, hydrofluoric acid in the range of up to 2 percent by weight and less than 0.1 pound of beryllium per gallon.

3. A process as described in claim 2 wherein the temperature of said aqueous solution is about 70.degree. F. to 140.degree. F.

4. A process as described in claim 3 wherein the etch rate is 5 to 15 mils per hour.

5. A process as described in claim 4 wherein said aqueous solution further comprises nitrate ion in the range of up to about 2 percent by weight of NaNO.sub.3.

6. An aqueous chemical milling solution consisting essentially of sulfuric acid in the range of 5 to 15 percent by weight, hexavalent chromium in the range of 7 to 10 percent by weight of CrO.sub.3, nitrate ion in the range of 1 to 1.3 percent by weight of NaNO.sub.3, hydrofluoric acid in the range up to 2 percent by weight and less than 0.1 pound of beryllium per gallon.

7. An aqueous chemical milling solution consisting essentially of an acid selected from the group consisting of sulfuric acid, oxalic acid, and ammonium acid fluoride in the range of 5 to 15 percent by weight, hexavalent chromium in the range of 7 to 10 percent by weight of CrO.sub.3, hydrofluoric acid in the range of up to 2 percent by weight and less than 0.1 pound of beryllium per gallon.

8. An aqueous chemical milling solution as described in claim 7 and further including nitrate ion in the range of up to 2 percent by weight of NaNO.sub.3.