US3276974A - Anodizing process for the metal beryllium - Google Patents

Description

United States Patent 3,276,974 ANODIZING PROCESS FOR THE METAL BERYLLIUM Richard C. Tyson H, San Jose, Calif., ,assignor, by mesne assignments, to the United ,States of America as represented by the Secretary of the Navy No Drawing. Filed Jan. 18, 1963, Ser. No. 252,538

1 Claim. (Cl; 20432) This invention relates to the metal coating art and has particular reference to the productionof a protective coating upon beryllium during an anodizing treatment thereof.

Beryllium is a metal possessing a remarkable combination of properties when compared to other metals. It exhibits an elastic modulus 40% higher than steel, a strength-weight ratio 30% greater than titanium, a density comparable to magnesium, a 40% conductive capacitance for heat and electricity as that of copper. The main difliculty with beryllium is its quick susceptibility both to stress corrosion cracking and also to galvanic corrosion when subjected to a corrosive atmosphere such as a salt atmosphere.

Stress corrosion cracking is caused by the combined action of a corrosive media and of a residual or applied stress. The path of the cracking follows a trans-granular or inter-granular course at the grain boundaries, slip planes, planes of precipitated constituents or in any such area which differs from the grain and which becomes cathodic or anodic to the grain. Since these zones are in electrical contact with the grain, a galvanic couple is established whereby the metal at the anode becomes soluble. As this electrochemical reaction takes place, an applied or residual stress pulls the metal away from each side of the electrolytic zone thereby creating a fissure. Since new material is being continually exposed, the crack proceeds preferentially throughout the area.

Galvanic corrosion is a localized attack which occurs when dissimilar metals are in contact and exposed to corrosive medium. The dissimilar metals function as short circuited electrodes. The corrosive medium can be a moisture, salt spray or other such atmosphere which acts as an electrolyte. A small galvanic current is induced and the metal having the more anodic potential is dissolved.

Under the prior art many attempts were made to coat beryllium with thin protective layers of suitable metals which would either alleviate or eliminate entirely the above discussed corrosion problem peculiar to the metal beryllium. Such methods comprise among others those of electroplating, roll cladding, deposition from a carboxyl and dip coating. All of the methods employed under the prior art failed to provide the metal beryllium with a corrosion-resistant coating which will give adequate protection at elevated temperature against either galvanic or stress corrosion. This is because of the tendency of beryllium to form hard, brittle inter-metallic compounds with the more common metals which are used for coating metal surfaces. It has been determined that galvanic action takes place at the metal-beryllium faying surface.

In addition to this difiiculty, electroplated beryllium forms brittle diffusion phases at high temperature thereby resulting in the plated metal being separated from the beryllium base. This produces an irregular coating and the resultant voids are diflicult or impossible to bridge during subsequent plating.

Therefore, one of the objects of this invention is to provide an improveed anodizing process for protecting a beryllium surface.

Another object of the invention is to provide a novel process for the production of oxide coatings upon beryl- 3,276,974 Patented Dot. 4, 1966 lium which are substantially poreless and which withstand all types of corrosion.

A still further object of this invention is to provide a process for increasing the corrosion abrasiveness of beryllium.

Other objects and advantages of this invention, it is believed, will be readily apparent from the following detailed description thereof.

Briefly, this invention includes the discovery that the surfaces of beryllium may be provided with an oxide film brought about by carefully controlling the composition of the bath, the bath temperature, the time and the current density during an anodizing electrolytic deposition. Unlike aluminum, beryllium is a sintered metal which makes it difficult to clean prior to being coated. Beryllium has the additional problem of current b-urn due .to its high throwing power while it is in the electrolyte. The present invention is directed to a process for overcoming these difliculties.

The invention will be illustrated by the following non limitative example.

The surface of a beryllium workpiece was first degreased with conventional degreasing agents such as gasoline, or mineral spirits or the like. Beryllium should not be placed in pickling solutions as these solutions will produce preferential dissolution of various inclusions on the surface of beryllium due to its sintered nature thereby leaving voids in the surface. These resultant voids are difi'icult to bridge during the subsequent plating operation.

The workpiece was baked for one hour in a furnace wherein the temperature is maintained between ZOO-220 F. in order to eliminate blistering of the film. It was then Water rinsed and cooled.

It is then given a chemico-electrical treatment in a bath having the following composition and under the following conditions:

Electrolyte 8 oz./gal. of sodium carbonate, 2 oz./gal. of sodium hydroxide in water. The solution to be heated to 130- 145 F.

Procedure The beryllium workpiece was immersed in the bath with the result that the solution of the metal by the alkaline electrolyte commences almost immediately. Direct current of 30-45 .amperes sq. foot was switched on, the beryllium being the cathode. An iron or steel anode was used and procedure in the bath follows electro-plating practice to some extent. The treatment was continued under these conditions for about 1.2 to 20 minutes. The current was then turned off and the beryllium was permitted to soak from 5 to 20 minutes in the solution. The temperature of the solution was maintained at 130-145 F. The current 'was then turned on again and the metal cathode was subjected to the same treatment with the current density being 30 to 45 .amperes per sq. ft. The article was then removed from the bath with the current still on and immediately rinsed in clean water. The surface of the metal now possessed a very bright appearance.

For the following part of the treatment, the metal was subjected to an anodic process in an electrolyte, the composition of which and the conditions of operation being as follows:

Chemical polish After any areas on the workpiece which are not to be anodized were masked, it was dipped for 20 to 50 minutes in a water solution containing by weight of 85% H PO 7% by weight of CrO and 5% by weight of commercial H The temperature of this solution 3 was maintained between 70 F. and 120 F. The-workpiece was again Water rinsed. It was now ready for the anodizing procedure.

- Electrolyte Aqueous solution containing 6 /2 to 8 oz./ gal. of CrO maintained at a temperature of 60 to 75 F.

Procedure The beryllium workpiece was made the anode. The cathode can be made of either iron, steel, graphite, a lead tin alloy or aluminum and because of the high throwing power characteristic of beryllium, the cathode is shaped so as to correspond to the design of the workpiece and placed approximately 2 /2" to 3" away from the workpiece in the electrolyte in order to avoid current burn. Holes are drilled in tthe cathode to permit circulation of the electrolyte during'processing. All of these cathodic preparations are necessary because of the extremely high current density required to anodize beryllium. A current density of 130 to 150 amperes/square foot was required, for a period of 30 minutes to obtain a coating of beryllium oxide .001 to .002 inch thick on the base metal. This is very significant when compared 5 A sealing operation was'performed' on the workpiece by placing it in an aqueous solution of sodium chromate (.12 to .24 ounce/gal. of Na Cr O )-fOr 10 to 15 minutes. The temperature of the solution was maintained between 160 and 185 F.

The film produced according to this method-has a dielectric constant of approximately 6.3 which is better than either magnesium oxide or aluminum oxide. Its Brinell hardness is 112-114 and it was able to withstand a salt spray test in excess of 200 hours without showing any evidence of corrosion or surface deformation.

While the foregoing particular example deals only with pure beryllium, the scope of the invention is not limited thereto and beryllium alloys are included in the materials which are susceptible to beneficial treatment by the process according to .the invention. The above description and example are intended to be illustrative only. Any

modification of or variation therefrom which conforms to the spirit of the invention is intended to be included within the scope of the claim- What is claimed is:

A process for treating the surface of an article ofberyllium, including the steps in sequence of:

(a) degreasing the surface with gasoline;

(b) baking the surface for one hour at a temperatur 4 sodium carbonate and sodium hydroxide in the proportions of about 8 ounces/gal. of the sodium carbonate and 2 ounces/gal. of the sodium hydroxide at a temperature of 130 to 145 F.;

(e) applying direct. current having a density of the order of 30 .to 45 amperes per square foot of the surface through the said sodium carbonate sodium hydroxide solution for 12 to 20 minutes, the beryllium surface being the cathode in the. circuit;

(f) soaking the surface in the said solution 'without the application of the direct current for 5 to 20 minutes;

(g) reapplying the direct current through the said solution with a current density of 30 to 45 amperes per square foot of the surface for 12 to 20 minutes;

(h) water rinsing the surface;

(i) polishing the surface in an aqueous solution consisting essentially of by weight H PO 7% by weight'of CrO and 5% by weight of H for 20 to 50 minutes at a temperature of 70 to F.;

(j) water rinsing the surface;

(k) anodizing the surface in an aqueous soution of chromic oxide containing 6% to 8 ounces per gallon of CrO at a temperature of 60 to 75 F.W1l.h a current density of to amperes per square foot of the surface for a period of 30 minutes; the

r cathode in the anodizing treatment having openings therein and being disposed 2% to 3 inches away from the berylliumanode surface;

'- (1) water rinsing the surface;

(m) and sealing the anodized surface in an aqueous solution containing .12 to .24 ounce per gallon of Na Cr O at a temperature of to F. for 10 to 15 minutes.

References Cited by the Examiner OTHER REFERENCES Graham, Electroplating Engineeering Handbook 1955, Reinhold Pub. Co., ppp143-6, 161, 376-80.

Levin, Trans. Faraday Soc. 54; 935-40 (June 1958). Tech. Proc. Am. Electroplaters Soc. 48; 106-8, 109-11 (1961).

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, Examiner.

L. G. WISE, W. VANSISE, Assistant Examiners.