Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Encased with nonfuel component
Reexamination Certificate
1951-07-19
2004-02-03
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Encased with nonfuel component
C376S417000, C376S457000, C420S470000, C420S473000, C420S560000, C427S006000
Reexamination Certificate
active
06687324
ABSTRACT:
This invention relates to metallic protective coatings for uranium. It is particularly concerned with bronze coatings on uranium for protection of the uranium against corrosion and for use under other metal coatings.
Metallic uranium is highly reactive with oxidizing agents and its use in the presence of air or other oxidizing media requires its protection by some less reactive coating. The metallic uranium has a tendency to alloy with some coating metals, especially at ele-vated temperatures, and this tendency may lead to diffusion of uranium through the coating metal with consequent reduction in resistance or the coating to oxidizing agents.
In the accompanying drawings,
FIGS. 1-3
are photomicro-graphs of cross-sectional views of uranium articles coated by the process of this invention.
The present invention has for its object the provision of coatings which are not only resistant to oxidizing agents and other corrosive media but are also resistant to the diffusion of uranium and consequently retain their protective value over long periods and under widely varying conditions of exposure.
In accordance with the present invention metallic uranium is provided with a protective coating of bronze. The coating preferably is applied by dipping the metallic article into molten bath of the coating metal.
The proportions of copper and tin in the coating may be varied over a wide range. In general it is desirable that the coating contain about 20-75% copper and about 80-25% tin by weight. Other metals may be present in minor proportions. Coating baths having the composition of speculum metal (67% copper, 33% tin) have been employed satisfactorily to provide continuous coatings of this metal. Bronze baths containing copper and tin corresponding to the peri-tectic mixture (47%, copper 53% tin) also have been employed and these baths have been found to be especially advantageous in the application of undercoatings for the application of certain other coating metals by the hot dip process. In the early application of bronze coatings to uranium, it was found that the addition of about 1% of aluminum or ½% to 5% of nickel was advantageous for improving the continuity of the resulting coatings. Later it was determined that, by employing uranium the surface of which has been properly prepared, no additions to the bronze baths are necessary to produce continuous, adherent coatings. A satisfactory preparatory treatment comprises immersing the uranium in 50% to 70% nitric acid at a temperature between 50° and 60° C. for between four and six minutes, rinsing the metal in clean, warm water, and drying immediately before dipping in the molten bronze bath.
The molten bronze bath may be emp
1
oyed with a dry surface but is preferably protected by an alkali-metal chloride flux, as described and claimed in U.S. patent applicatiorn Ser. No. 583,176 filed Mar. 16, 1945 by Lowell D. Eubank.
The optimum temperature for bronze-dipping the metallic uranium depends upon the composition of the bronze being applied. Temperature from 700° to 850° C. have been employed with com-position varying from the peritectic mixture to speculum metal.
The bronze coatings may serve as temporary or permanent protective coatings in the application of coating metals such as zinc, tin, terne, aluminum-silicon alloys, and aluminum. Tin does not coat the speculum metal coatings readily and con-sequently when this metal is employed it is desirable to employ a bronze having a composition nearer that of the peritectic mixture or to follow the dip in speculum metal with a dip in a bronze bath containing a greater tin concentration before applying the pure tin coating. Two-dip coatings of copper-tin peritectic over speculum metal present a more workable surface than the speculum metal alone. Coatings prepared from bronze baths containing 57% copper and 43% tin by weight exhibit properties similar to the coatings applied by dipping first in speculum metal and then in the peritectic mixtures.
The bronze coatings of the invention have been found to be especially suitable as temporary protective undercoatings for the application of aluminum-silicon casting alloy coatings or brazings to uranium. Aluminum-silicon casting alloys are aluminum alloys of silicon in which the aluminum predominates. The principal alloys of this type are those containing 5% to 20% by weight of silicon and the remainder essentially aluminum. The ternary alloys of aluminum, silicon, and sodium with about 10-15% silicon and about 0.1% sodium are commonly preferred. All of these alloys tend to form with metallic uranium a layer of a brittle compound of aluminum, uranium, and silicon at the interface between the metal and the aluminum-silicon coating. This compound layer frequently exhibits cracks which increase in size with the thickness of the compound layer. A compound layer of substantial thickness is undesirable not only because of its characteristic brittleness but because the presence of cracks and fissures substantially impairs the protective value of the coating. By the application of a bronze undercoating prior to dipping metallic uranium in aluminum-silicon alloy, the formation of the brittle compound layer may be inhibited and its thickness held to a value such that the objectionable characteristics do not attain significant proportions.
Bronze undercoats beneath aluminum-silicon alloy coatings have a highly important value in substantially preventing undercutting by acidic water containing oxidizing agents and chloride ions. Thus an aluminum-silicon alloy coating may be completely penetrated and yet continue to afford adequate protection for the underlying metal. This property is characteristic even though the final product contains a copper film only 0.01 to 0.03 mil thick and considerably denuded of tin, beneath the aluminum-silicon alloy.
Aluminum-silicon alloys do not wet bronze coatings on uranium readily and dipping times as long as 30 seconds may be necessary to secure continuous coatings by this method. Such long coating periods may lead to washing off of the bronze and formation of substantial areas of thick, brittle compound layer. By subjecting the bronze-coated metal to a preliminary dip in molten tin, the time required for coating with molten aluminum-silicon alloys may be reduced to a fraction of that required for applying the aluminum-silicon coatings directly to the bronze-coated metal. Thus instead of a 30-second dipping period only a 1- to 5-second dip is necessary.
In certain applications the presence of tin in the outer aluminum-silicon coatings decreases the corrosion resistance of the coatings. For such applications it is desirable to remove excess tin by wiping or centrifuging the tin-coated article immediately after the tin dip so as to remove all excess molten metal, and to renew the aluminum-silicon bath whenever its tin content attains an undesirably high value. Centrifuging may be effected in a basket type or lathe type centrifuge supporting the article either concentrically or eccentrically and using speeds corresponding to force from 100 to 10,000 or more times the force of gravity.
Application of the tin coating at a high temperature, for example a temperature of at least 600° C. and preferably 630-640° C. also is instrumental in providing an exceedingly thin tin coating. This also is necessary for maintaining the uranium article in Beta phase in those cases in which reversion to the Alpha phase would be objectionable. Uranium passes from Alpha to Beta phase when heated above about 650° C. but re-version to Alpha phase during normal coating periods may be inhibited by maintaining temperatures above about 600° C.
By applying a bronze coating from a bath containing about 45-50% copper and about 50-55% tin at a temperature of about 730° C., a tin coating at about 600° C. with subsequent centrifuging in a 12-inch basket at about 640 rpm maximum speed for 5 seconds, and then an aluminum-silicon coating of about 88 parts aluminun and 12 parts silicon at 640° C. for 2 seconds, coatings are obtained which are free from objec-
Boller Ernest R.
Eubank Lowell D.
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