US1902503A - Process for coating metals - Google Patents

Description

Patented Mar. 21, 1933 UNITED STATES PATENT; OFFICE GOODWIN H. HOWE, OF SCO'IIA, NEW YORK, ASSIGNOB 1'0 GENERAL ELECTRIC COMPANY, A CORPORATION OI NEW YORK PROCESS FOR COATING METALS 1T0 Drawing. Application filed May 29,

The present invention relates broadly to the protection of surfaces of metals which are subject to corrosion or oxidation. More specifically, this invention is concerned with an improved process for coating such metals with a protective layer of chromium.

It has been proposed heretofore to coat the surfaces of metals, either ferrous or nonferrous, which are subject to corrosion or oxidation, with protective metals such as chromium by bringing the coating metal with or without an inert medium, such as aluminum oxide, directly into contact with the surface to be coated and then heating for a requisite period of time at the proper temperature. While this method is successful in effecting a coating of protective metal on the surface of the metal coated, its practice is, nevertheless, limited in certain cases. For example, where objects of varied or complex shape have been involved, a large amount of material has been required to be packed around the object in order to insure complete coating of all the surfaces thereof. Again, the objects must always be packed with more or less care so that the surfaces may receive the proper coating of protective metal. Moreover, it is often inconvenient to handle large amounts of this packing material in coating objects by this method.

It has been recognized that coating with a protective metal in the vapor phase not only facilitates the operation and makes possible and practical the coating of objects with a minimum amount of coating mate rial, but also produces a better coating of protective metal. The coating in the vapor phase with metals of zinc and aluminum has 4 been accomplished heretofore with compara-v tively little difliculty inasmuch as relatively low temperatures could be used in effecting the operation. For example, while zinc melts at about 419 C. and boils at about 45 930 C. its vapor pressure is such that at about 400 C. coating in the vapor phase may be-successfully carried out. Again in the case of aluminum which melts in the neighborhood of 660 0., vapor phase coating 5 is'found practical at about 950 G. How ever, with chromium as the coating metal 1930. Serial No. 457,482.

no such low temperatures can be used in carrying out the coating operation in the vapor phase. Chromium melts in the vicinity of 1615 C. and in order to effect a coating with this metal in the vapor phase it requires a temperature in the neighborhood of 1500 to 1600 C., and heating at this temperature for a prolonged period of time. Obviously, such high temperatures are not practical for commercial application.

I have discovered a practical method whereby I can protect the surfaces of a metal subject to corrosion or oxidation (hereinafter termed the foundation metal), using chromium and effecting the coating in the vapor phase at a temperature considerably lower and in a much shorter period than has heretofore been possible and obtain a smooth, adherent, resistant coatmg of chromium.

According to my invention, the article to be protected is placed in a porous receptacle such as a porous alundum tube, which is then closed at its open ends and within which has also been placed but out of contact with the articl.e,a relatively small amountof chromium. This receptacle is placed in another suitable container such as an iron tube and packed with an inert material such as aluminum oxide and containing a small amount of a carrier. The function of the carrier will 8 be more fully pointed out hereinafter. If desired, some chromium may be added to the inert material but this is not necessary. The container so packed is then placed in a furnace and heated for the requisite period of time, the atmosphere of the furnace being either air or an inert or reducing atmosphere, if desired, although the latter are not necessary to the successful operation of the process. The chromium deposits on the article in the form of a smooth, adherent, resistant protective coating and is ver similar to a coating of chromium obtained y electro-deposition.

As illustrative of the process of my invention, but in no way limiting it, the following is given:

A piece of iron in the form of a wire or rod, the exact shape being of no material consequence, is placed in a porous alundum container togetherwith a small amount of chromium, preferably a few small pieces of chromium. The chromium is at one side of and out of contact with the iron. The alundum container which may be in the form of a piece of tubing, is then plugged at its open ends with asbestos paper. Itis then placed in an iron container such as an iron tube and completely surrounded by an inert medium. This inert medium may consist of such material as aluminum oxide to which may be added, if desired. some chromium. Mixed with this inert material is a small percentage of a carrier. As an example of the carrier which I employ ammonium chloride is cited. A com'enient mixture to use for the inert medium comprises approximately 49% aluminum oxide, 50% powdered chromium, and 1% ammonium chloride. The iron tube packed with this mixture is placed in a cold air furnace and heated to about 1000 C. for about two hours. After this treatment the iron rod has a bright, resistant coating of chromium thereon.

The mixture outside of the alundum tube may consist entirely of an inert material, such as the aluminum oxide, with the carrier and the same results will be obtained as above. The percentage of carrier which may be used may be varied although I have found'that up to approximately 2% by weight of carrier yields the best results, Results with about 5% by weight of the carrier are not satisfactory. The temperature at which the coating takes place may be varied within the approximate limits 950 to 1100 C., although I have found that about 1000 C. is the optimum temperature. The time of heating is considerably shorter than that necessary where the coating is packed around the object to be coated and in general depends on the'size of the object to be protected. Two hours ordinarily gives a good protective layer; and there is usually no object in prolonging the heating much beyond this period as the carrier is usually spent by this time.

\Vhile I do not wish to be limited to the following theory, the best explanation I now have as to the mechanism of the process is as follows:

Assuming that ammonium chloride is used as the carrier for the chromium during heating the ammonium chloride breaks down into ammonia and hydrochloric acid, the latter combining with the metallic chromium to yield chromium chloride which in turn reacts with the foundation metal, iron, for example, the chromium depositing thereon in the form of a smooth adherent, resistant, hard coating. The reactions take place in the vapor phase, the ammonium chloride acting as the carrier for the chromium.

It is evident that the present process gives a convenient and practical way of protecting metallic surfacesagainst corrosion or oxidation, using chromium as the protective metal. The temperature at which the process may be carried out is considerably lower than when no carrier'is used for the chromium, and the time of heating is materially shortened. A much smoother coating of protective metal is obtained by means of the present invention than is obtained by the use of prior processes involving the packing of the coating metal directly on the object coated.

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

1. The method of providing a foundation metal with a protective coating of chromium, which comprises heating the said metal in the presence of, but out of contact with, chromium and with a carrier which is out of contact with both the foundation metal and the chromium.

2. The method of providing a foundation metal with a protective coating of chromium, which comprises heating the said metal in the presence of, but out of contact with, chromium and with ammonium chloride'which is out of contact with both the foundation metal and the chromium.

3. The method of providing a foundation metal with a protective coating of chromium which comprises heating said metal in the presence of, but out of contact with, chromium and with ammonium chloride which is out of contact with both the foundation metal and the chromium, at about 1000 C. until the coating results.

4. The method of providing a foundation metal with a protective coating of chromium, which comprises heating said metal in the presence of, but out of contact with, chromium and with about 2% by weight of a carrier which is out of contact with both the foundation metal and the chromium.

5. The method of providing a foundation metal with a protective coating of chromium, which comprises heating the said metal in the presence of, but out of contact with, chromium at a temperature of about 1000 C. and with a carrier which is out of contact with both the foundation metal and the chromium, said carrier consisting of not more than about 2% by Weight of ammonium chloride.

6. The method of providing a foundation metal with a'protective coating of chromium which comprises placing said metal and chromium within a porous receptacle out of contact with each other, packing theexterior of said receptacle with an inert medium containing a small amount of carrier therein and heating to form a coating on said metal.

7. The method of providing a foundation metal with a protective coating of chromium which comprises placing said metal and chromium within a porous receptacle out of contact with each other, packing the exterior of said receptacle with an inert medium containing a small amount of ammonium chlo- Q ride therein and heating to form a coating on said metal.

8. The method of providing a corrodible foundation metal Wit a protective coating of chromium which comprises placing said metal in a refractory porous receptacle, placing a small amount of chromium in said receptacle out of contact with said metal, placing said receptacle in a metal container and surrounding said receptacle within the metal container with an inert medium com rising a small amount of ammonium chlori e, and heating said receptacle andcontainer to a temperature of about 1000 C. to produce a protective coating of chromium on said metal. In witness whereof I have hereunto set my hand this 28th day of May, 1930.

GOODWIN H. HOWE.