US3220876A - Aluminum-containing diffusion coating for metals - Google Patents

Description

United States Patent 3,220,876 ALUMHJUM-CGNTAINING DIFFUSION COATING FOR METALS Roger D. Moelier, Simi, Califl, assignor to North American Aviation, Inc.

No Drawing. Filed June 24, 1964, Ser. No. 377,507 13 Claims. (Cl. 117-114) This invention relates to a method of providing an aluminum-containing diffusion coating on a base metal or alloy, and more particularly to a method for providing a codeposited aluminum-silicon diffusion coating on a ferrous metal.

This application is a continuation-in-part of my copending application Serial No. 85,457, filed January 30, 1961, and since abandoned.

Industrial and commercial uses of metals at high temperatures and in corrosive environments have continually increased, and operations requiring metals with improved special properties are steadily rising. For a great many uses, it is essential that metal parts resist oxidation and other chemical surface reactions at high temperatures and abnormal conditions. Further, it is frequently desirable to have metal alloys of variable compositions, for example, a very hard, corrosion-resistant surface, and a base having good Working characteristicsproperties which frequently are not found with an alloy of uniform composition. Metals having corrosion-resistant surfaces at high temperatures are required, for example, for the following items: furnace parts, turbine blades, petroleum refinery equipment, jet engine sheet metal work, superheater tubes, and chemical plant equipment; the list of applications for ferrous metals with corrosion-resistant surfaces is virtually limitless. Thus, a process which can efficiently improve the temperature corrosion properties of metals, particularly ferrous metals, is 'of considerable interest.

The resistance of metals to surface attack on exposure to elevated temperatures depends largely upon the type of scale formed. The physical properties of the film, such as its adherence to the base metal, its porosity, and its tendency to crack, determine the ability of the metal to withstand surface attack. The type of scale formed depends upon chemical properties, such as its composition, density, melting and boiling points.

Diffusion coatings on metals and alloys have been used to change one or more metallurgical properties, for example, to produce an atmospheric oxidation-resistant surface, a surface resistant to the action of specific chemicals, and a Wear and oxidation-resistant surface. It may be convenient to first make a particular piece from an alloy which is in easily formable condition, and then add the diffusion layer coating to obtain the desired property. A diffusion layer, which usually involves formation of an intermetallic compound or a solid solution of the coating metal with the base metal, has distinct advantages over a simple plating layer.

Although a few methods have been developed for the coating of a base metal with a protective layer of aluminum, each of these leaves much to be desired. A Widely used commercial procedure involves cleaning the surface, applying flux, and then hot dipping in molten aluminum. The major problem appears to be the attainment of suitable adherence of an aluminum coat to the base metal.

An object of the present invention is to provide a method of forming an aluminum-containing diffusion coating on base metals and alloys.

Another object is to provide a method wherein the diffusion material may be applied to all surfaces of a base metal independent of the configuration or shape of the base metal.

Another object is to provide a method of forming an alloy having different surface and body characteristics, thereby providing an alloy having surface characteristics of improved hardness and corrosion resistance.

Still another object is to provide a non-electrolytic method of applying a protective aluminum-silicon diffusion layer on base metals, particularly ferrous metals.

A further object is to provide an aluminum-containing surface diffusion coating of a graded alloy of variable composition on a ferrous metal in order to increase its hardness and resistance to high temperature surface corrosion.

In accordance with the present invention, a base metal is provided with an aluminum-containing diffusion coating by placing the base metal in a molten sodium bath containing at least aluminum dissolved therein, and then maintaining the base metal in the bath for a sufficient time for the dissolved aluminum to diffuse into the base metal to form a diffusion coating containing aluminum on the base metal.

In certain preferred aspects of practicing the invention, both aluminum and silicon are dissolved in the molten sodium bath, while maintaining an inert atmosphere, and both are diffused into the base metal to form a codeposited diffusion coating therewith.

It is an essential feature of this invention that the diffusion coating contain at least aluminum, although it is contemplated that other coating materials may be codiffused and codeposited therewith, particularly silicon. Optionally, other additives may be present in the bath so as to enhance the quality of the diffused coating or shorten the time required.

There will usually be diffusion of the coating material into the base metal, with resulting formation of a solid solution, alloy, or intermetallic compound. Diffusion layers with graded boundary layers varying from 0.1 mil up to 60 mils may be obtained, although diffusion coatings typically varying from 0.5 mil to 10 mils are preferred. The diffusion of the coating material into the base metal accounts for the changed metallurgical properties of the resulting article, for instance improved mechanical and chemical properties. The resulting diffusion layer has a number of distinct advantages over a simple plating layer; an atomic bond is formed, and the surface is graded in composition. There is no sharp interface, which is beneficial in preventing spalling or breaking of the coating.

While the mechanism of diffusion transport of the dissolved aluminum or aluminum-silicon into the base metal and the formation and nature of the alloy diffusion layer (e.g., intermetallic compound or solid solution) is not fully understood, it appears that the sodium metal is largely responsible for the success of the process. The molten sodium provides an excellent environment for maintaining the base metal clean of oxide and organic films, not only initially but throughout the coating operation. Further, the sodium provides the vehicle for diffusion material transport and maintains a constant supply of aluminum or aluminum-silicon at the base metal surface.

The present process may be used for applying a diffusion coating containing aluminum on a large number of base metals, singly and in combination, and there are no limitations as to the base metals, except as may be hereinafter indicated. For example, ferrous metals may be coated with aluminum or with a co-coating such as one of aluminum-silicon. While diffusion coatings of aluminum per se or aluminum-silicon are particularly preferred, other materials may be codeposited with aluminum, so that coatings consisting of aluminum-nickel, aluminum-niobium, aluminum-zinc, aluminum-nickelniobium, may be obtained. Since these coating materials diffuse into or react with the base metal, highly complex protective layers or coatings are formed on the base metal, which may consist of binary, ternary, quaternary, and even more complex compounds.

Sodium, which includes alloys thereof such as sodiumpotassium (NaK), is the molten carrier used in the practice of this invention. An inert environment, i.e., a nonoxidizing one, such as may be obtained with a vacuum or by maintaining an inert gaseous atmosphere provided by noble gases such as helium, is maintained over the molten sodium. The diffusion rate into the base metal and the solubility in the sodium of the aluminum and other metals codeposited therewith are functions of temperature, and it is accordingly desirable to maintain the temperature of the molten sodium bath as high as possible. The maximum temperature is limited by the boiling point of the sodium or alloy thereof at a given pressure. A satisfactory temperature range for a sodium bath is about 1000- 1500 F.

The desired concentration of the aluminum and of the codeposited diffusion coating materials in the sodium bath is essentially self-controlled by their solubility in the bath. This may vary from a range of about to 1000 parts per million, and may reach several percent, being markedly dependent upon the temperature of the bath and the properties of the solute. Generally, with respect to aluminum, it is preferred to maintain a small reservoir or pool of molten aluminum in the molten sodium bath so as to insure saturation conditions of the aluminum in the bath.

The aluminum and codeposited materials used to form the diffusion coating can be dissolved in the molten sodium bath in any convenient manner. Preferably, the aluminum is introduced as a pure metal in either powdered or molten form. Where codeposited coatings are used in conjunction with aluminum, they may be introduced as the pure metals per se in either powder or molten form, for example tin, zinc, or silicon or a molten alloy such as aluminum-nickel or other aluminum alloy may be dissolved as such to provide the codeposited coating with the aluminum.

The base metal may be any metal which does not dissolve in molten sodium at an appreciable rate for the time of immersion used. Since sodium is non-reactive with most of the common base metals, the selection of base metal is relatively broad. Of particular interest are the ferrous metals in view of their wide usage and nickel and cobalt base compositions (i.e. the first triad of group VIII). Also of interest are group IVB metals such as titanium, group VB metals such as niobium, and group VIB metals such as chromium, molybdenum, and tungsten. Reference made herein to the various groups of elements refers to the Periodic Chart of the Elements as shown in Handbook of Chemistry, 10th edition, N. A. Lange, editor, McGraw-I-Iill Book Company, New York, 1961, pages 56-57.

The factor which controls the time required for the diffusion coating is the rate of diffusion of the coating metal into the base metal. The diffusion rate is dependent upon such factors as the temperature of the bath, the concentration of coating material in the bath, the metallic structure of the base metal, and circulation of the bath constituents.

Where the sodium bath is held at a temperature approaching its effective lower limit of 1000 F., the diffusion rate is relatively slow and diffusion times of 50 hours or more may be required to give a diffusion layer of several mils on the base metal. Generally, when too long a time is used for the diffusion coating, there is a tendency for the coating to become brittle. Higher bath temperatures resulting in shorter coating times are therefore preferred for most applications. Since the boiling point of sodium is about 1600 F. at which temperature the sodium evaporates at a rapid rate, a temperature of 1500 F. is generally preferred as a maximum working temperature. While a diffusion coating of 0.1 mil may be obtained in as short a time as a minute, coatings of 0.5 mil to 10 mils are preferred for protecting the base metal. Thicker coatings will be used where the coating is subject to a degradative attack in addition to functioning as a protective coating for the underlying base metal. Since approximately 1.5 weight percent aluminum dissolves in molten sodium under optimum conditions, baths containing from 0.5 weight percent aluminum to 4 percent aluminum, based on the weight of sodium, are used for diffusion coating with aluminum alone. Where silicon is codeposited with the aluminum, amounts of silicon varying from 2 to 20 weight percent, based on the weight of the sodium, are added to the molten sodium bath.

Because of the industrial importance of providing a protective coating for a ferrous material, such as mild steel (AISI 1008 steel), codeposited coatings of aluminum and silicon on mild steel represent a preferred feature of this invention, the coating being codeposited from a bath containing preferably from 2 to 5 percent aluminum and from 2 to 20 weight percent silicon based on the weight of the sodium bath. A coating approximately 0.5 mil thick of codeposited aluminum and silicon has been obtained in one minute from a molten sodium bath maintained at a temperature of 1150 F. and containing 10 percent silicon and 4 percent aluminum. Under similar conditions, coatings of 0.75 mil have been obtained in 10 minutes and coatings of about 1.5 mils in 30 minutes. It will of course be understood that the rate of deposition of a diffusion coating is not a linear function but depends upon the relationship and interaction of many factors not fully understood. In actual practice, the rate of diffusion appears to vary logarithmically with time.

Agitation of the molten sodium bath is an important factor that affects the time required to form a satisfactory diffusion coating layer of a given quality and thickness on a base metal. The forming of a satisfactory aluminum diffusion coating presents a particularly difficult problem because of the apparent formation of a diffusion barrier at the liquid-liquid interface between the aluminum and the sodium, presumably caused by an oxide film of aluminum. It is believed that the formation of this thin impermeable alumina film at the interface partitions the liquid aluminum from the liquid sodium thereby preventing proper solution of the former in the latter and thus preventing the subsequent mass transfer and reaction with the surface of the base metal. However, by continuously disrupting the liquid-liquid interface, metallic sodium is brought into contact with metallic aluminum permitting solution, transfer and reaction. One technique for effecting this continuous disruption of the interface is by mechanical agitation of the reaction chamber and its contents, for example by use of rotating and seesaw type capsules. Alternatively, the capsule or container and its contents may be vibrated, or the contents only or the liquid only vibrated, to cause a wave or ripple action at the liquid-liquid interface. Suitably, mechanical movement of a rake or chain through the liquid may be used to cause interface disruption. Also feasible is introduction of an inert gas stream below the liquid-liquid interface to agitate the interface. To obtain the desired interface disruption, the target to be diffusion coated may be maintained in motion in addition to agitation of the molten sodium bath.

A batch process or a continuous diffusion coating technique may be used in the process of this invention. Generally a batch process is preferred because of the greater degree of control feasible with respect to the dominant parameters of the process. Conveniently, for a batch process a container to hold the bath and the specimens to be diffusion coated is fabricated of mild stainless steel tubing of desired diameter cut to desired lengths. An end cap is welded to one end to form a vessel into which the bath materials are placed, and another end cap is welded on the opposite end. Specific capsule configurations may also be used to support some of the constituents, to provide a reservoir, or to house unusually shaped specimens. The specimen to be coated may be placed into the bath loose, suspended on wire, or encased in a screen envelope. Various types of furnaces may be used to house the capsule, the simplest being a static furnace which is temperature controlled to maintain an isothermal environment for the capsule. In other applications, a tube furnace is used inclined at 30 from the horizontal within which the capsule is rotated about its longitudial axis at about 15 rpm. Additionally, a rocking motion may be added to the rotating motion. For this type of application, a tube furnace is used in which the capsule is rotated at 15 rpm. While the furnace and capsule are both rocked through -30 from the horizontal about a mid-point pivot.

An open vessel bath may be used for both a batch and a continuous process. The temperature is maintained by a furnace around the vessel containing the bath. A recirculating argon environment is maintained over the bath solution to prevent any oxidation occurring. Bafiles are installed above the open bath to keep the vapor within the reaction vessel and to reduce the heat transfer to the argon atmosphere in a dry box which is used to house the open vessel.

The following examples are offered to illustrate the scope and practice of this invention in greater detail, and are not intended to be construed as limitations thereof.

Example I A stainless steel capsule 1 /2 inches in diameter by 6 inches long was 'half filled with sodium. About 10 grams of aluminum was added, and the capsule Welded tight under a helium atmosphere in a glove box. The capsule was placed in a rotating jig inside a tubular furnace. The jig was made to rotate at r.p.m. with its axis of rotation at a 30 angle with the horizontal.

The capsule was heated while rotating, for 18 hours at 1100 F., and for 70 hours at 1500 F. The diffusion layer obtained is .004 inch. The Knoop hardness of the diffusion layer (100 gram load) is 417 KHN, while the Knoop hardness of the parent stainless steel is 200 KHN.

Example II The same as Example I, except that aluminum was deposited on carbon steel by heating aluminum dissolved in sodium at 1500 F. for about 50-70 hours. A diffusion coating of iron-aluminum was produced. Following this with a diffusion annealing treatment served to diffuse the aluminum further into the steel and produced a highly protective coating.

Example III A mild steel specimen (AISI 1010l020) was suspended in a stainless steel capsule together with 40 grams of sodium and 10 grams of aluminum. The capsule was rotated for a period of 43 hours at 1450 F. A coating 40 mils thick was obtained. This coating is extremely hard, with a Knoop hardness number of 890 (100 gram load).

A similar coating, 80 mils thick was obtained by treating another mild steel specimen in a bath containing 50 grams sodium and 15 grams aluminum for 7 hours at 1675 F. X-ray fluorescence analysis of this coating showed a major phase of iron-aluminum inter-metallic compound (Fe Al an intermediate phase of chi-molybdenum-chromium-iron (Mo Cr Fe and minor phases repeating the intermediate phase and aluminum-iron (Al Fe).

Example IV Mild steel specimens were suspended in an open vessel in which a recirculating argon environment over the bath solution provided an inert atmosphere. Temperature was maintained by a furnace around the vessel containing the bath. About 10 pounds of sodium filled the vessel to a depth of 5-6 inches, depending upon the operating temperature. Bafiles were installed above the open bath to keep the vapor within the reaction vessel, and to reduce the heat transfer to the argon atmosphere. The bath contained 10 pounds sodium and grams aluminum, the excess amount of aluminum providing a large reserve pool to insure uniform repeatable processing. The aluminum covered the bottom of the processing vessel to a depth to about of an inch and was continuously agitated by wiper blades on the bottom of a rotating basket so as to stir the molten aluminum resting in the bottom of the vessel and break up any oxide film on the aluminum so as to allow pure aluminum to go into solution readily. Runs in this bath at 1500 F. for 3 /2 to 5 /2 hours produced a very heavy diffusion coating on mild steel specimens. Coatings up to 16 mils in thickness were obtained.

Other ferrous base metals such as a low-carbon enameling iron (0.02 percent carbon), a titanium-stabilized steel, and various low carbon proprietary steels were similarly coated using this open bath processing.

Example V Stainless steel specimens (type 304) were aluminized at a temperature of 1500 F. and 1600 F. for 2 hours and 5 hours in a capsule using a rotating-rocking motion. The capsule was rotated at approximately 15 rpm. while the tube furnace containing the capsule and the capsule itself were also rocked through i30 from horizontal about a mid-point pivot. Diffusion coatings of 1 to 3 mils were obtained. The amount of aluminum used varied from 0.05 to 0.1 gram aluminum per square inch of surface coated. The coating was evenly distributed and light grey in color. The Knoop hardness of the base metal varied from 178 to 252. The case (diffusion coating layer) varied from 420 to 1000 KHN, showing a very marked increase in the surface hardness of the diffusion coated specimens.

Aluminizing of types 316 and 321 stainless steel produced very similar results to the diffusion coatings obtained with the type 304 stainless steel.

Example VI A high strength austenitic nickel-chromium steel (nominally 25% Ni, 15% Cr, 2% Ti, 0.02% Mn, 1.3% Mo, 0.3% V, balance Fe) was aluminized in the form of small fasteners and bolts and nuts. A thin aluminum diffusion coating with minimum change in thread dimensions was obtained in runs in a rotating-rocking furnace at 1250 F. for 2 hours. Approximately 5 grams of aluminum was used with 50 grams of sodium. The specimens had uniform smooth coatings with case steps of 0.3 to 0.5 mil.

Example VII A nickel base alloy (Hastelloy N, nominal composition weight percent, C 0.8, Cr 7, Mo 17, Fe 5, Ni balance) was readily aluminized in the sodium bath. Using an open vessel with aluminum maintained at saturation in the bath for 3 /2 hours at 1500 F. resulted in the diffusion coating having a smooth appearance, light grey in color with a case of about 1.1 mils thick.

Nickel cobalt alloys were also run in an open bath containing 9 pounds sodium and grams aluminum at 1500 F. for 3%. hours, resulting in a smooth even case of 3 mils thick. Hardness test results were: parent material 550, case 860 KHN .(100 gram load).

Example VIII A molybdenum specimen (0.05 inch thick, 6.1 grams) was aluminized in a capsule containing 50 grams sodium and 15 grams aluminum run at 1500 F. for 7 hours. A weight gain of 0.49 gram was obtained.

Example IX A columbium (niobium) specimen wasrun at 1675 F. for 7 hours in a bath of 50 grams sodium and 15 grams 7 aluminum. A case 2.6 mils thick was obtained. The case had a hardness of 846 KHN compared with 113 KHN in the parent columbium.

Example X A diffusion-coated case 0.9 mil thick was formed on a tantalum specimen in a bath containing 50 grams sodium and 6 grams aluminum run at 1450 F. for 7 hours.

Example XI Using a zirconium target in a rotating-rocking capsule, a case 1.2 mils was produced at 1500 F. for hours in a bath containing 50 grams sodium and 2 grams aluminum. The aluminized zirconium had a case hardness of 328 KHN compared with the parent material of 153 KHN. In an oxidation test conducted at 1300 F. for 40 hours, the aluminized zirconium gained 0.13 mg. per square centimeter compared with the uncoated zirconium which gained 4.2 mg. per square centimeter.

Example XII Using a mild steel specimen (AISI -15-1020), a capsule test was run at 1500 F. for 5 hours in a bath containing 50 grams sodium, 1 gram silicon, and 2.5 grams aluminum. A case varying in thickness from 2 mils to 4 mils was obtained. This case was found to be more tenacious. than previously siliconized coatings not containing any aluminum.

Example XIII Using a rotating capsule, a solid copper target was immersed in an aluminum-containing sodium bath at 1150 F. for 2 hours. A case 3.5 mils thick was obtained. Oxidation tests at 1350 F. for 50 hours produced no visible change on the protected copper, while the as-received copper oxidized quite rapidly.

Example XIV A nickel target was run in a capsule at 1650 F, for 16 hours in a bath containing 40 grams sodium and 8 grams aluminum. A pleasing appearing 10-mil thick, twophase case was obtained on the nickel base. The hardness of this case was: outer band, 636 KHN; inner band, 846 KHN; parent nickel, 190 KHN.

In an open vessel run in an aluminum-containing sodium bath, a thinner 1.6-mil case was produced on the nickel target after immersion for 4 hours at 1250 F.

Example XV A columbium alloy (Du Pont D-36, 10% Ti, 5% Zr, balance Nb) was used as a target in a capsule containing 40 grams sodium, 5 grams aluminum, and 4 grams silicon. The capsule was rotated and rocked at 1600 F. for 5 hours. The reaction that occurred involved most of the IO-mil thick target material. In a similar run, where the capsule additionally contained 0.5 gram cesium, a sharply defined case 4.5 mils thick was obtained.

Example XVI A tantalum-tungsten alloy (90 Ta-lO W) was immersed in a capsule containing 60 grams sodium and 8.1 grams aluminum and rotated and rocked at 1450 F. for 5 hours. The case depth obtained was 1.4 mils.

Example XVII 8 Example XVIII Capsule tests were run using mild steel specimens (1015-1020 AISI) as targets, the bath containing 40 grams sodium and 3 grams aluminum, with the silicon content varying from 3 grams to 15 grams. The bath was rotated at 15 r.p.m. while heating to a predetermined temperature of 1400" F. and then held at that temperature for 30 minutes to saturate the bath. The capsules were then inverted to submerge the specimens in the bath and rotated for 5 minutes. Capsules were again inverted and removed from the furnace to cool.

Typical results were as follows: In one run in which the bath composition consisted of 3 grams silicon, 3 grams aluminum, and 40 grams sodium, the case obtained was 2.4 mils thick. In another run in which the bath contained 3 grams aluminum, 15 grams silicon and 40 grams sodium, a 5-mil thick case was obtained. In another run, to the bath containing 3 grams aluminum, 15 grams silicon, and 40 grams sodium, was added 3 grams Fe O A 7-mil thick case was obtained.

Spectrographic analysis of a typical 5-mil thick case showed an aluminum content of 520%, a silicon content of 5-15%, and iron as a major constituent.

Both spectrographic and wet analyses were performed on a case chipped away from a steel tab which had been immersed in a capsule in which the bath contained 40 grams sodium, 4 grams silicon, 4 grams aluminum, and 2 grams FeO, for a period of 15 minutes at 1500 F. Spectrographic analysis of the case showed, weight percent, aluminum 20-50, chromium 5.0, silicon 2-10, and iron as a major constituent. In a more precise wet chemical analysis, the iron content was determined as 38.6%, the aluminum content as 35.5%, and the silicon content as 19.0%.

Example XIX Thin coatings of less than '-1 mil in thickness were obtained in a series of runs on tabs of mild steel (AISI 1015). The specimens were immersed in a presaturated solution and rotated at 15 rpm. during the time cycle. The bath composition contained 40 grams sodium, 3 grams a-luminum, and a silicon content varying from 3 grams to 15 grams. In two further runs where 40 grams sodium, 15 grams silicon, and 3 grams of aluminum were used, an additional amount of 3 grams of iron oxide (FeO) was added to one run and 3 grams of nickel oxide added to another run. The heating cycle was at 1000 F. for times varying from 5 minutes to 15 minutes. The case thickness obtained varied from 0.3 to 0.8 mil.

Example XX The resistance of coated and uncoated mild steel specimens was evaluated in a highly corrosive atmosphere simulating that encountered in the exhaust stream of an automotive mufiier. The specimens were suspended for 20 hours above a bath consisting of a dilute solution of hydrobromic and sulfuric acids maintained at 180 F. The specimens were then removed and dried for 4 hours at 250 F. to complete one cycle.

After five cycles, a sample of uncoated 1015 mild steel was very badly attacked. A mild steel specimen which had been immersed in an aluminum-containing sodium bath at 1600 F. for 5 hours showed a weight gain (mg. per square centimeter) of but 0.56. Specimens which had been immersed in a solution of sodium-aluminumsilicon at 1250 F. for 5 minutes showed an average weight loss (mg. per square centimeter) of 0.08. Another specimen which had been immersed in a sodiumaluminum-silicon bath at 1000 F. for 5 minutes showed a weight loss after 5 cycles of but 0.007 mg. per square centimeter.

Thus, while the aluminum-coated mild steel sample was markedly superior to the untreated mild steel specimen, those specimens which had been co-deposited with aluminum and silicon showed superior resistance to corrosion,

compared with only the aluminum-coated specimen, by factors varying from 2- to 20-fold.

It will of course be understood that many variations are possible in the practice of this invention, depending upon the coating thickness desired, the base metal used, and the particular aluminum-containing diffusion coating desired, and these variants are therefore considered to lie within the scope of this invention. Accordingly, the scope of this invention should be determined in accordance with the objects thereof and the appended claims.

I claim:

1. The method of providing a diffusion coating containing aluminum on a base metal selected from the class consisting of groups.IVB, V-B, and the first triad of group VIII of the periodic table which comprises placing the base metal in a molten sodium bath maintained under an inert environment and containing aluminum dissolved therein and maintaining said base metal in said bath until a diflusion coating containing aluminum is obtained on said base metal.

2. The method of providing an aluminum diffusion coating on a ferrous metal which comprises providing a bath of molten sodium under an inert environment, dissolving aluminum therein, and placing said ferrous metal in said molten bath until a diffusion layer of aluminum is obtained on said ferrous metal.

3. The method of claim 2 wherein said bath is maintained at a temperature between 1000 and about 1500" F. at ambient atmospheric pressure.

4. The method of claim 3 wherein a pool of molten aluminum is maintained in said bath while coating said ferrous metal.

5. The method of providing an aluminum coating on niobium which comprises providing a molten sodium bath maintained at a temperature between 1000 and about 1500 F. under an inert gas atmosphere, dissolving aluminum in said sodium, placing said niobium in said sodium bath, and maintaining said niobium in said sodium bath until a diffusion coating of aluminum on said niobium is obtained.

6. The method of providing a diifusion coating of aluminum on molybdenum which comprises providing a molten sodium bath under an inert environment, dissolving aluminum in said bath, and maintaining said molybdenum in said bath until said diffusion coating of aluminum is obtained thereon.

7. The method of providing a diffusion coating on a base metal which comprises dissolving aluminum and silicon in a molten sodium bath maintained under an inert environment, placing said base metal in said bath and maintaining said base metal in said bath until a diffusion coating containing aluminum and silicon is obtained on said base metal.

8. The method of claim 7 wherein said bath is maintained at a temperature between 1000 and about 1500 F. at ambient atmospheric pressure.

9. The method of claim 8 wherein a .pool of molten aluminum is maintained in said bath while coating said base metal.

10. The method of providing a diffusion coating on a ferrous metal which comprises dissolving aluminum and silicon in a molten sodium bath maintained under an inert environment, placing said ferrous metal in said bath maintained at a temperature between 1000 and about 1500 F. and maintaining said ferrous metal in said bath until a diffusion coating containing aluminum and silicon is obtained on said ferrous metal.

11. A method of providing a codeposited diffusion coating of aluminum and silicon on a ferrous base metal which comprises providing a molten sodium bath containing from 2 to 5% aluminum, and from 2 to 20% silicon, by weight of the sodium, said bath being maintained under an inert environment, placing said ferrous metal in said bath, and maintaining said ferrous metal in said bath maintained at a temperature between 1000 and about 1500 F. for a period of time varying from 1 minute to 5 hours, until a difiusion coating of desired thickness containing aluminum and silicon is obtained on said ferrous metal.

12. A bath for providing a codeposited aluminumsilicon diffusion coating on a ferrous base metal which comprises a molten sodium bath containing, by weight of said sodium, from 2 to 20% silicon and from 2 to 5% aluminum.

13. The bath of claim 12 in which excess aluminum is present as a molten pool in said bath.

References Cited by the Examiner UNITED STATES PATENTS 2,351,798 6/ 1944 Alexander 117-22 2,774,868 12/1956 Hodge 117-114 2,848,352 8/1958 Noland et al 117-114 X 2,894,856 7/1959 Schwendemann et al. 117-114 2,914,419 1 1/ 1959 Oganowski 1117-114 X 2,929,740 3/ 1960 Logan 117-114 2,991,197 7/1961 Sandoz 117-114 X 3,073,720 1/1963 Mets 117-131 3,085,028 4/1963 Logan 117-114 3,086,886 4/1963 Kiefier et al 117-114 OTHER REFERENCES Hansen: Constitution of Binary Alloys, Metallurgy and Metallurgical Engineering Series; McGraw-Hill Book Co., 2nd Ed. (1958), page 91.

RICHARD D. NEVIUS, Primary Examiner.

UNITED STATES PATENT OFFiCE CERTIFICATE OF CORRECTION Patent N06 3 ,220 ,876 November 30 1965 Roger D. Moeller It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 10 line 39, for "2,774 ,868" read 2 ,774 686 rmumhwmh Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNE Commissioner of Patent

Claims (1)

1. THE METHOD OF PROVIDING A DIFFUSION COATING CONTAINING ALUMINUM ON A BASE SELECTED FROM THE CLASS CONSISTING OF GROUPS IV-B, V-B, AND THE FIRST TRIAD OF GROUP VIII OF THE PERIODIC TABLE WHICH COMPRISES PLACING THE BASE METAL IN A MOLTEN SODIUM BATH MAINTAINED UNDER AN INERT EVIRONMENT AND CONTAINING A LUMIUM DISSOLVED THEREIN AND MAINTAINING SAID BASE METAL IN SAID BATH UNTIL A DIFLUSION COATING CONTAINING ALUMINUM IS OBTAINED ON SAID BASE METAL.