US3343929A - Oxidation-resistant beryllium articles and process of making - Google Patents

Description

United States Patent 3,343,929 OXIDATION-RESISTANT BERYLLIUM ARTICLES AND PROCESS OF MAKING Ray J. Van Thyne, Oak Lawn, and John J. Ransch, Evanston, Ill., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed June 22, 1964, Ser. N 377,141 8 Claims. (Cl. 29-494) This invention relates to the coating of beryllium metal to make it corrosionand oxidation-resistant in air at elevated temperature.

Beryllium metal is being used as the principal construction material for neutron reflectors in nuclear reactors and is also of interest for high-temperature structural applications. In these devices the beryllium is exposed to air of elevated temperature which causes oxidation of the surface of the beryllium and thus deterioration. Conventional metal cladding is not advisable, particularly not for space service reactors, because metals that fulfill all the requirements for satisfactory operation, namely low neutron-capture cross section, compatibility with beryllium at elevated temperature, oxidation resistance at elevated temperature and sufficiently high melting point, could not be found.

Various methods have been investigated to protect the beryllium from oxidation. The most satisfactory method used heretofore is anodizing of the beryllium bodies in an aqueous chromic acid solution using a direct current and subsequent sealing of the anodized beryllium by immersion in boiling water. Although this method provided for improved oxidation resistance, it still was not too satisfactory, because the service life of the coating was limited.

It is an object of this invention to provide a coating on beryllium metal that is chemically and mechanically stable and does not react harmfully with the base metal.

It is another object of this invention to provide a coating on beryllium metal that adheres well to the base metal.

It is also an object of this invention to provide a coating on beryllium metal that is oxidation-resistant and impervious to oxygen migration into the beryllium.

It is a further object of this invention to provide a coating on beryllium metal that melts at a temperature that is sufficiently removed from the melting point of beryllium and melts at a temperature between the melting point of beryllium and the temperature at which it is used so that it can be applied readily by fusion.

It is also an object of this invention to provide a coating on beryllium metal that does not form any brittle intermetallic compounds with the beryllium.

It is another object of this invention to provide a coating on beryllium metal that remains thermally stable for a long period of time.

It is finally an object of this invention to provide a coating on beryllium metal that has a low neutron-capture cross section.

A two-phase alloy of beryllium and silicon was found to have the advantages enumerated above for coating beryllium metal. Silicon is substantially insoluble in solid beryllium and does not form an intermetallic compound with it. The beryllium-silicon system is solid at the service temperature of a reactor, which can range up to about 815 C.

The phase diagram for the binary beryllim-silicon system shows that a eutectic is formed at a silicon content of 33 atomic percent; this eutectic mixture melts at about 1090" C. Beryllium melts at about 1282 C. and silicon at about 1414 C. Because a coating alloy that melts at a temperature higher than does the base metal is "ice not feasible and also because it has to have at least a certain silicon content to obtain the required oxidation resistance, the range of between 33 and 70 atomic percent would be suitable for the silicon content of the two-phase coating. (The phase diagram for beryllium-silicon is published in the book, Constitution of Binary Alloys by M. Hansen, McGraw-Hill, New York, 1958, on page 297.)

Beryllium-silicon coatings containing 35, 50 and 70 atomic percent of silicon were therefore investigated. Those containing 50 atomic percent of silicon gave a better protection against oxidation than those containing only 35 percent. However, the coatings containing 70 percent of silicon resulted in porous coatings. For this reason, the silicon content was considered critical and the invention is seen in beryllium-silicon systems containing from 35 to 50 atomic percent of silicon.

The coatings can be applied by any methods known to those skilled in the art. The inventors have obtained particularly good results with the method now to be described.

The beryllium surface to be coated was first pretreated by removing a 0.005-inch layer by etching in an aqueous solution containing sulfuric, phosphoric and chromic acids in concentrations of about 7 fi. oz./gal.; 121 fl. oz./gal.-; and 1 lb./gal., respectively. Beryllium and silicon were then mixed in the proportion desired and alloyed by arc-melting. The arc-melted mass was then crushed to a particle size of 200 mesh, and the powder thus obtained was slurried in a lacquer-base binder. Known lacquers can be used for this purpose, but the inventors have obtained particularly good results with nitrocellulose lacquer and best results with a solution of one percent by volume of cellulose acetate butyrate in acetone.

The beryllium base to be coated, preferably after edges and corners have been rounded to obtain continuity of the film, was then coated with the slurry by conventional means, for instance by brushing or immersion; the latter was preferred. It was found that the coating should not be thicker than because at greater thickness spalling occurred during firing.

The slurry-coated beryllium article Was then heattreated, first by vacuum-heating, for instance at a pressure of about 3X 10- mm. Hg in a quartz muffle until a temperature of 982 C. was obtained. At about 482 C. the binder volatilized. After the temperature of 982 C. was reached, an inert atmosphere of argon or helium gas of a pressure of between 1 and 3 p.s.i.g. was introduced and the article Was heated for fusion of the coating. The temperature in this step, it Was found, had to be watched closely, a temperature much higher than the liquidus of the coating being disadvantageous. Temperatures below the melting range of the coating did not result in satisfactory wetting of the beryllium base metal by the coating, while temperatures Well above the melting range (liquidus) caused attack of the beryllium base metal by the coating alloy, as will be shown in Example I. These are the reasons for restricting the temperature. The temperature was maintained for two minutes. The temperature used for the beryllium-silicon coatings, at the pressure range given above, was 1150 C. for that containing about 35 atomic percent of silicon, between 1150 and 1165 C. for that containing 50 atomic percent of silicon and for that containing 70 atomic percent of silicon 1177 C.

The following Example I shows the effect of firing temperature on the degree of protection obtained.

EXAMPLE I A 0.1" thick slice each of beryllium metal was used as a test coupon for each of three runs. The coupons were coated, as described above, with a 35 a/o silicon- 65 a/o beryllium mixture. The binder used was the 1% cellulose acetate butyrate solution in acetone, and coating was carried out by immersion. The coated coupons were heated in a quartz muffle to 982 C. in a vacuum of 3 10- mm. Hg. When this temperature was reached, the furnace tube was pressurized to one p.s.i.g. of argon gas and then heated to the final firing temperature. Diderent firing temperatures were used for the three coupons, namely, 1093 C., 1149" C. and 1205" C., respectively. The coating fired at the lowest temperature of 1093 C. did not wet the base metal sufiiciently, while that fired at 1149 C. flowed well and uniformly on the base. The specimen fired at 1205 C. showed a substantial attack of the beryllium base by the coating. These three runs prove the criticality of firing temperature.

The next example illustrates the difference in oxidation resistance with various silicon contents of the coating alloy.

EXAMPLE II Uncoated beryllium was exposed to air at 843 C. and a dew point of 0 C.; it was severely oxidized in less than 24 hours. Two other beryllium specimens were coated, one with an alloy containing 50 a/o of silicon and the other one with an alloy containing 70 a/o of silicon. Both coated specimens were exposed to air under the conditions used for the uncoated beryllium (843 C.; dew point of 0 C.). The alloy coatings containing 50 and 70 a/o of silicon withstood the exposure for 296 and 144 hours, respectively, before failure. Other samples coated with the 50% and 70% alloys were exposed to air at 732 C. at a dew point of 0 C. Whereas both specimens were protected for over 1000 hours, the specimen coated with 50% silicon-beryllium did not show any failure after 1204 hours, but the 70% sample broke down during this last exposure. This proves that a 50% alloy coating is superior to that prepared with a 70% alloy.

The superiority of the 50% silicon alloy coating on beryllium over that with the 35% silicon composition was shown by another set of parallel tests in air at 843 C. A beryllium sample coated with the 35% silicon composition failed after an exposure of 153 hours, whereas the specimen coated with the 50% silicon alloy performed for 234 hours before failure.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process of protecting beryllium metal against oxidation, comprising mixing beryllium and silicon; arc-melting the mixture obtained; cooling the arc-melted product; crushing the product; suspending the product in a lacquer solution to form a slurry; coating a beryllium base to be protected with the slurry to obtain a coating of a maximum thickness of 1'5 inch; heating the coated base at a temperature of between 950 and 1000 C. under reduced pressure: introducing an inert atmosphere of superatmospheric pressure and heating the coated base to about the liquidus of the coating for about two minutes in said inert atmosphere; and cooling the coated base.

2. The process of claim 1 wherein the berylliumsilicon mixture contains 35 atomic percent of silicon and heating in said inert atmosphere is carried out at 1150 C.

3. The process of claim 1 wherein the beryllium-silicon mixture contains atomic percent of silicon and heating in said inert atmosphere is carried out at between 1150 and 1165 C.

4. The process of claim 1 wherein the lacquer solution is an acetone solution containing 1% by volume of cellulose acetate butyrate.

5. The process of claim 1 wherein the base is coated with the slurry by immersion.

6. The process of claim 1 wherein any edges and corners of the base are rounded 01f prior to the application of the coating.

7. As a new article of manufacture, a beryllium base coated with a fired beryllium-silicon alloy containing from 35 to 50 atomic percent of silicon.

8. The article of manufacture of claim 7 wherein the silicon content is approximately 50 atomic percent.

References Cited UNITED STATES PATENTS 3,002,930 10/1961 Robinson et al. 11722 3,196,007 7/1965 Wikle 200 3,275,471 9/1966 Lowell et a1. 117-1351 ALFRED L. LEAVITT, Primary Examiner.

A GRAMALDI, Assistant Examiner.

Claims (2)

1. A PROCESS OF PROTECTING BERYLLIUM METAL AGAINST OXIDATION, COMPRISING MIXING BERYLLIUM AND SILICON; ARC-MELTING THE MIXTURE OBTAINED; COOLING THE ARC-MELTED PRODUCT; CRUSHING THE PRODUCT; SUSPENDING THE PRODUCT IN A LACQUER SOLUTION TO FORM A SLURRY; COATING A BERYLLIUM BASE TO BE PROTECTED WITH THE SLURRY TO OBTAIN A COATING OF A MAXIMUM THICKNESS OF 1/32 INCH; HEATING THE COATED BASE AT A TEMPERATURE OF BETWEEN 950 AND 1000*C. UNDER REDUCED PRESSURE: INTRODUCING AN INERT ATMOSPHERE OF SUPERATMOSPHERIC PRESSURE AND HEATING THE COATED BASE TO ABOUT THE LIQUIDS OF THE COATING FOR ABOUT TWO MINUTES IN SAID INERT ATMOSHPERE; AND COOLING THE COATED BASE.

7. AS A NEW ARTICLE OF MANUFACTURE, A BERYLLIUM BASE COATED WITH A FIRED BERYLLIUM-SILICON ALLOY CONTAINING FROM 35 TO 50 ATOMIC PERCENT OF SILICON.