US3186070A - Protective coatings and process for producing the same - Google Patents

Description

June 1, 1965 a. D. oxx, JR 3,186,070

PROTECTIVE COATINGS AND PROCESS FOR PRODUCING THE SAME Filed July 3, 1961 Weight Gain (mg /cm Time Hours Fig. 2. lnvenfor Gordon 0. Oxx, Jr.

His A horney- United States Patent Office 3,185,070 Patented June 1, 1965 3,186,070 PROTECTIVE COATINGS AND PROCESS FOR PRODUCING THE SAME Gordon D. Oxx, In, Shaker Heights, Ohio, assignor to General Electric Company, a corporation of New York Filed July 3, 1961, Set. No. 121,419 11 Claims. ((11. 29-194) This invention relates to bodies formed of columbium and columbium-base alloys and more particularly to protective coatings for such bodies and to processes for protective coatings.

Rapid advances in the technological field dealing with apparatus, such as gas turbines and the like, have resulted in greatly expanded need for refractory metals and materials capable of retaining high strength at elevated temperatures in environments which are often destructive. In the case of gas turbines, many of the component parts must be capable of withstanding severely oxidizing conditions for reasonable lengths of time. Columbium and alloys of columbium have great potential for structural applications at elevated temperatures, not only because of their high strength-to-weight ratio, but also because of their relatively low rates of oxidation as compared with other fabricable refractory metals and alloys. However, even though the oxidation rates of columbium and columbium-base alloys are comparatively low, they still oxidize at rates two to three times greater than acceptable. For this reason, protection of the exposed surfaces of these alloys is required for practical applications.

Attempts have been made in the past to protect columbium and columbium-base alloys from oxidizing environments by enclosing them in protective containers or jackets or by reacting the surface of the metal with another substance to form a protective compound. Various metals have been reacted with the exposed surfaces of columbium and columbium-base alloys, and included among this group is aluminum and aluminum alloys reportedly containing small additions of materials such as beryllium, cerium, chromium, copper, cobalt, magnesium, molybdenum, silicon, titanium, vanadium, and zinc. However, adequate long-time (over 100 hours) protection of the metals with which this invention is concerned has not been obtained since the coatings formed by reaction with aluminum have, for the most part, been brittle and subject to cracking upon thermal cycling.

A principal object of this invention is to provide columbium or columbium-base alloy bodies having protective coatings rendering them suitable for use at elevated temperatures in oxidizing environments.

Another object of this invention is to provide columbium or columbium-base alloy bodies having protective coatings consisting principally of the columbium-aluminum compounds, Cb Al or Cb Al.

An additional object of this invention is to provide a protective coating for columbium or columbium-base alloy bodies, which coating consists principally of the compounds CbgAl and/ or Cb Al .and containing further additions which improve the oxidation resistance of the compounds.

A further object of this invention is to provide a novel process for producing a protective coating on the exposed surfaces of columbium or columbium-base alloy bodies, rendering these bodies suitable for use at elevated temperatures in oxidizing environments.

A still further object of this invention is to provide a process for producing a columbium aluminide protective coating on the exposed surfaces of columbium or columbium-base alloyed bodies.

Additional objects and advantages of this invention will be in part obvious and in part explained by reference to the accompanying specification and drawings.

In the drawings:

FIG. 1 is a graph showing the oxidation rate of selected coating compositions as a function of time; and

FIG. 2 is an enlarged cross section of a coating of this invention showing the reaction products.

Generally, a body according to this invention comprises a columbium or columbium alloy base which has a protective layer formed integrally on an exposed surface thereof. The protective layer or coating is composed of one of the compounds Cb Al or CbgAl or some combination of these two compounds and may contain additional alloying ingredients such as chromium, silicon, or titanium dissolved in the compound to further improve the oxidation resistance.

The process of the invention, which is important in obtaining a coating having the required protective properties, generally comprises preparing an alloy of aluminum with one of the metals, silver, tin or lead, the aluminum normally being present in amounts of from about 17 to 23 weight percent, immersing the columbium or columbiumbase alloyed body in the molten bath and retaining it in the bath for a period of time sufficient to react the aluminum therewith. When a coating of proper thickness has been obtained, the body is removed from the bath. If desired, the aluminum-silver, tin or lead bath may contain appropriate amounts of chromium, silicon, or titanium or alloys thereof which will be alloyed with the columbium aluminide formed on the surface of the base article.

Suitable materials which may be alloyed with columbium comprise tungsten, molybdenum, tantalum, zirconium, and titanium, this list not being complete but given only for purposes of illustrating suitable columbium-base alloys. It is, of course, desirable that the percentage of alloying ingredient added to the columbium not exceed an amount which would preclude the proper formation of a protective coating on the body.

Columbium containing amounts of tungsten up to 15 weight percent; molybdenum, 5 weight percent; tantalum, 33 weight percent; titanium, 8 weight percent; and minor amounts of zirconium have been found to be suitable for high temperature use.

Three compounds are formed by the reaction of aluminum with columbium. These compounds are CbAl which has a melting point in the vicinity of 1620 Q; Cb Al, with a melting point of 1700 C.; and Cb Al, which has a melting point on the order of 2260 C. There are eutectics at 656 C., 1520" C. and 1640" C. between A1 and CbAl CbAl and Cb Al, and CbzAl and Cb Al, respectively. On the surface of an unalloyed columbium sample dipped in aluminum of activity essentially equal to one, all three of these phases will form, and on alloys of columbium, it is probable that even more compounds will be present. By reacting columbium with alumium only, thus causing an activity of one, at a temperature below 1620 C., it is thermodynamically impossible to avoid formation of CbAl as the outer layer. It has been found that the presence of CbAl is extremely harmful to the coating, this compound resulting in the formation of cracks and otherwise having relatively poor corrosion resistance. It has now unexpectedly been found that by lowering the activity of the reacting aluminum, it is possible to avoid formation of CbAl and form a protective coating consisting only of Cb Al, Cb Al, or some combination of these compounds, plus possibly some additional complex compounds where modifying alloying agents have been included.

By combining the aluminum with a metal from the group consisting of silver, tin and lead, these metals being substantially inert with respect to columbium and columbium-base alloys, it is possible to lower the activity of the aluminum and obtain only the compounds Cb Al and CbgAl. Thus, when columbium or columbium-base bodies are dipped into a molten bath containing alumium, in percentages not exceeding about 23 weight percent, the compounds Cb Al and Cb Al are formed as an integral coating on the exposed surface of the base metal. It is also possible if the aluminum content is within the lower end of the range that the coating will consist essentially of the high temperature compound Cb3A1.

The general process used in forming a protective coating on the exposed surfaces of columbium and columbium-base alloy bodies is as follows. A suitable base is prepared and immersed in a bath containing from about 17 to 23 weight percent aluminum, balance substantially all a metal which will dissolve the aluminum and yet be non-reactive with the base material. It has been found that silver, tin and lead are extremely well suited for this purpose since aluminum is soluble in each and since none of these materials is reactive with columbium. Immersion of the base piece within the molten bath is maintained only for a time sufi'icient to develop a coating of desired thickness. Generally, the temperatures of the molten baths are in the vicinity of 1150 C. to 1250 C., although the particular temperature selected is not especially critical. Of course, it will be understood that the higher temperatures make possible the use of shorter immersion times, while lower temperatures necessitate slightly longer immersion times. After the article has been immersed in the molten bath for the required time, it is removed and is then ready for use.

Illustrating the properties of columbium aluminide protective coatings developed according to the present invention, unalloyed columbium test specimens were dipped into a molten silver-aluminum alloy at 1200 C. and allowed to react to form a coating. The molten alloys used contained 10, 20, 25 and 35 percent aluminum, balance substantially all silver plus 5 percent silicon.

The samples were then tested by holding in flowing air at 2372 F. and periodically recording weight change. The oxidation rates of the respective coatings are indicated in FIG. 1, where weight gain per unit area for each sample is plotted against time. While the Ai-5 Si-85 Ag sample, curve 10, showed the best life (69 hours), the Al-5 Si-75 Ag sample, curve 11, was significantly better with regard to oxidation rate. The 20 hours difference in life of these two samples is probably a consequence of differences in coating efficiency and thickness, in thermal cycling during test and in conditions of test. The rate ditference is of technical significance, however. Under proper conditions, the coating that is a consequence of reaction of the 20 Al-5 Si-75 Ag bath with columbium has the best oxidation resistance and potentially the longest life. Curve 12 indicates that high aluminum content, composition Al5 Si- Ag, results in a coating having poor oxidation rate and low life.

Similar comparisons were made between samples reacted with 20 Al-80 Ag and 25 Al- Ag baths. The rates were higher than in the case of the respective baths with the silicon additions but the relative positions were the same. In this case, the 20 Al- Ag bath gave a life of 125 hours at 1300" F. and a rate of about one-fifth that of the 25 Al-75 Ag bath. The latter showed a life of 53 hours.

As a consequence of the above, it is concluded that the 20 Al bath clearly represents the optimum composition with regard to oxidation resistance.

Another important requirement for columbium coatings is that in addition to preventing oxidation, they must prevent penetration of oxygen or nitrogen into the columbium metal. Contamination of the metal by these gases causes embrittlement. The effectiveness of the 20 percent aluminum-silver alloy coating in preventing this was checked by attempting to bend the specimen at room temperature. The sample bent more than without cracking.

Additional specimens of unalloyed columbium were dipped into molten baths containing alloying agents such as chromium, silicon and titanium in addition to the aluminum and silver. The properties of these coatings, together with the temperature at which they were tested in flowing air are shown in the following table:

Table 1 Composition Temp.

of Life test (hrs.) Al Cr Si Ti Ag C) 20 O 2 0 BaL 1, 260 G9 20 5 0 0 Bal l, 260 205 20 5 2 0 Bal 1, 260 393 20 5 0 5 Bal 1, 260 476 20 0 0 0 Bal- 1, 300 125 25 0 0 0 Bal- 1, 300 53 10 0 5 0 Del 1, 300 62 20 0 5 0 Bal.-. 1, 300 49 2O 5 0 5 Bal 1, 370 5 20 5 0 10 Bal--- 1,370 1 25 0 0 0 BaL 1, 370 36 20 5 2 0 Bal 1, 370 51 20 5 5 0 B :11 1, 370 20 5 0 0 Ball 1, 370 44 The data set forth in the preceding table clearly indicates that additions in the bath of about 5 percent chromium have the most potent effect on increasing the life of the specimen. Bath compositions on the order of 20 percent aluminum, 5 percent chromium plus 5 percent titanium give the best protection at 1260" C. The 20 percent aluminum, 5 percent chromium and 2 percent silicon is particularly effective in increasing the life of the specimen at 1370 C. In all of the samples measured, the coatings consisted principally of the two compounds Cb Al and Cb3A1. Although it is possible that some minor percentage of CbAl was present, it was felt to be insignificant in its effect upon the protective coating. After holding in air for hours at 1300 C., the sample dipped in the 20 Al-80 Ag bath showed a microstructure as in FIG. 2. The bottom of this figure, area 15, is the columbium substrate. The coating appears as two layersa thin layer 16 next to the columbium (probably CbgAl) and heavy, continuous outer layer 17 that is probably Cb Al. X-ray diffraction analyses on this sample indicated the presence of Cb, Cb Al and Cb Al, but no CbAl Although this invention has been described in connection with specific examples, these examples are meant to be illustrative of rather than limitations on the broad scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An article of manufacture comprising, a body formed of a metal from the group consisting of columbium and columbium-base alloys, and a protective coating consisting essentially of the compounds Cb Al and Cb Al integral with an exposed surface of said body to protect it against oxidation.

2. An article of manufacture comprising, a body formed of a metal from the group consisting of columbium and columbium-base alloys, and a protective coating consisting essentially of the compounl Cb Al integral with an exposed surface of said body to protect it against oxidation.

3. An article of manufacture comprising, a body formed of a metal from the group consisting of columbium and columbium-base alloys, and a protective coating consisting essentially of the compound Cb Al integral with an exposed surface of said body to protect it against oxidation.

4. An article of manufacture comprising, a body formed of a metal from the group consisting of columbium and columbium-base alloys, and a protective coating consisting essentially of a compound selected from the group consisting of Cb Al and Cb Al, and up to 15 weight percent of a metal selected from the group consisting of chromium, silicon, titanium, and combinations thereof dissolved in said selected compound, said protective coating being integral with an exposed surface of said body to protect it against oxidation.

5. An article as defined in claim 4 wherein up to 5 weight percent chromium is dissolved in said selected compound.

6. An article as defined in claim 4 wherein up to weight percent titanium is dissolved in said selected compound.

7. An article as defined in claim 4 wherein from 1 to 5 weight percent chromium and 1 to 5 weight percent titanium are dissolved in said selected compound.

8. An article as defined in claim 7 wherein 5 weight percent chromium and 5 weight percent titanium are dissolved in said selected compound. 1

9. An article as defined in claim 4 wherein from 1 to 5 weight percent chromium and from 1 to 5 Weight percent silicon are dissolved in said selected compound.

10. An article as definedin claim 9 wherein 5 weight percent chromium and 5 weight percent silicon are dissolved in said selected compound.

11. An article of manufacture comprising, a body formed of a metal from the group consisting of columbium and columbium-base alloys, and a protective coating con- 6 sisting essentially of a compound selected from the group consisting of Cb2A1 and Cb Al, and from 2 to 5 weight percent silicon dissolved in said selected compound, said protective coating being integral with an exposed surface of said body to protect it against oxidation.

References Cited by the Examiner UNITED STATES PATENTS 2,770,031 11/56 Schluchter 29-197 2,774,686 12/56 Hodge 117-114 2,947,069 8/60 Carlson et al. \29197 2,991,197 7/61 Sandoz et a1 117-131 2,992,135 7/61 Finlay 117-114 3,055,088 9/62 Cox 1 29-194 3,057,048 10/62 Harakis 29197 X 3,078,554 2/63 Carlson 29-497 OTHER REFERENCES Zinc Coatings for Protection of Columbium from Oxidationat Elevated Temperatures, Klopp et a1. (Order PB 161 238 from OTS, US. Dept. of Commerce, Washington 25, DC).

DAVID L. RECK, Primary Examiner.

RICHARD D. NEVIUS, HYLAND BIZOT, Examiners.

Claims (1)

1. AN ARTICLE OF MANUFACTURE COMPRISING, A BODY FORMED OF A METAL FROM THE GROUP CONSISTING OF COLUMBIUM AND COLUMBIUM-BASE ALLOYS, AND A PROTECTIVE COATING CONSISTING ESSENTIALLY OF THE COMPOUNDS CB2AL AND CB2AL

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