IL36817A - Method for coating heat resistant alloys - Google Patents

Description

nn*oy nvaioao *io»s no»w Bin «303 Method for coating heat resietant alloys CHROMALl-OS* AMEBICAN CORPOMTIOIT This invention relates to the diffusion coating of metals, such as heat resistant superalloys, and, in particular, to a process for producing an adherent oxidation and sulfidation resistant coating comprising chromium and alumi-num on superalloy substrates while avoiding internal oxidation within the coating. The invention is particularly applicable to high nickel alloys for high temperature applications in, for example, jet and other thermal engines or gas turbines. " Metallurgical developments in recent years have indicated the necessity of high nickel and/or high cobalt alloys (sometimes now referred to as "superalloys") having desirable physical properties for various high temperature uses, such as, for example, the manufacture of rotor blades and stator vanes for high temperature gas turbines where operation without failure is desired of the part, such as during prolonged exposure to temperature well above 1500 ¾" and even substantially above the temperature range at which failure or diminution of the strength characteristics may be expected of even high temperature austenitic or nickel or chromium steels.

Although the nickel or cobalt superalloys (and even those based on iron) may exhibit physical properties within a desirable range for a variety of uses, particular-ly when- subjected in use to extremely high temperatures, the combination of such properties as oxidation resistance and/or erosion of sulfidation resistance at the surface of such alloys, the resistance to thermal shock and the strength characteristics may be less than desired for prolonged severe use. As operating temperatures were raised higher and higher, increased amounts of hardeners, such as aluminum and/or titanium, were added to certain nickel-base alloys in order to assure stiffness at the high temperature levels." As the hardeners increased in amount, it was- not unusual to decrease the amount of chromium in such alloys in order to increase high temperature strength. A case in point is a nickel-base alloy referred to in the trade by the designation "B-1900" which comprises by weight 0.1% carbon, 8% chromium, 10% cobalt, 6% molybdenum, 1% titanium, 6% aluminum, 0.015% boron, 0.1% zirconium, 4% tantalum and the balance essentially nickel. A disadvantage of such alloys is that with decreased amounts of chromium, the resistance to oxidation and sulfidation is generally adversely affected. Usually, 18% chromium or better is required for sulfidation resistance.

A chromium-free alloy which has shown particular promise as a jet engine or gas turbine component and which exhibits excellent stress-rupture properties at temperatures as high as 2200°F and above is an alloy comprising about 18% molybdenum, about 8% aluminum and the balance essentially nickel. However, this alloy does not have the desirable oxidation and/or sulfidation resistance and, therefore, its application in the gas turbine field has been severely limited due to the lack of a coating. At 8% aluminum, the alloy contains gamma prime ( ) precipitate (Ni^Al) . The absence of chromium stabilizes the fine precipitate of the gamma prime to relatively high temperatures, for example, to above 22O0°F as compared to 1950°F for such gamma prime strengthened alloys as the alloy bearing the designation "B-1900" referred to hereinabove. The alloy also contains a small amount of carbon, e.g. 0.03% carbon by weight, which appears as molybdenum carbide precipitated throughout the matrix. The superior mechanical properties are attributed to its lack of chromium, to its high molybdenum content, and to the relative absence of grain boundary carbides. However, this alloy must be used in the coated condition in order to withstand oxidation and sulfidation attack in gas turbine environments .

Initial attempts to produce a duplex chromium/ aluminum coating on this alloy in conventional cementation packs were not successful due to internal oxidation occurring within the coating during the initial chromizing step. Broadly speaking, the procedure usually employed was to embed the metal article to be coated in a dry powder pack, including an inert mineral filler (e.g., powdered alumina), a source of chromium to be diffusion coated and a source of a vaporizable halogen material. A typical pack is one containing by weight 25% chromium, 1/4% of a halide energizer (e.g., ammonium iodide) and the balance alumina. The metal article embecBed in such a pack contained within a metal container or retort, the seams of which are sealed by a fusible material, such as low melting silicate glass, to prevent excessive escape of the diffusing material during heating and inhibit introduction of "air in the pack during the thermal cycle, is heated in a known manner to a diffusion temperature and held at temperature for a number of hours to cause diffusion coating of the chromium into the substrate of the article. Thereafter, the chromized article is aluminized, using another pack cementation procedure.

As stated above, the initial chromizing step applied to the high molybdenum-aluminum nickel-base superalloy resulted in localized internal oxidation within the chromium affected zones. This oxidation manifested itself in the form of angular inclusions of aluminum oxide particles within the chromium rich solid solution region of the coating, the size of the inclusions increasing with increasing coating temperature. Generally, the internal oxidation concentrated near the surface of the coating in which the aluminum concentration had been reduced by dissolution of the chromium from 8% alum-inum to a level below 3% by weight. The internal oxidation was not. observed to penetrate through the coating diffusion zones into the base metal where the aluminum level approached 8% by weight. The low chromium, nickel-base superalloys were observed to exhibit the same phenomenon, such as alloys of the type referred to herein as "B-1900".

The aluminum oxide inclusions have an adverse effect on the properties of the alloy in that the presence of inclusions tends to reduce the resistance of the coating to impact and, moreover, the aluminum oxide/coating matrix interfaces introduce regions susceptible to rapid corrosion attack.

While attempts were made to overcome the foregoing problem, none, as far as it is known, has been successful.

It is thus an object of the invention to provide a method of coating metal articles with oxidation and sulfida-tion resistant coatings while inhibiting internal oxidation within the coating.

Another object is to provide a pack cementation method for coating the substrate of an article made of a heat resistant superalloy in which the heat resistant alloy contains a solute metal (e.g., aluminum) in an amount having a higher propensity to oxidize than the primary coating metal (e.g. chromium) in the cementation pack and the solvent metal of the alloy, while substantially avoiding internal oxidation within the coating.

A still further object is to provide a multi-step pack cementation process for producing a multiple oxidation and sulfidation corrosion resistant coating on superalloys substantially free from internal oxidation near the interface of the coating.

The invention also provides a process for coating a molybdenum-aluminum alloy containing major amounts of nicke 1.

These and other objects will more clearly appear when taken in conjunction with the following disclosure and the accompanying drawings, wherein: - Figs. 1 and 2 are reproductions of photomicrographs taken at 400 times magnification showing the internal oxidation which occurs within a coating produced by a method outside of the invention; Fig. 3 is a reproduction of a photomicrograph taken at 400 times magnification showing the same alloy composition with no internal oxidation within the coating produced in accordance with the invention; Fig. 4 is illustrative of a photomicrograph taken at 400 times magnification showirg the extent to which internal oxidation occurs when the pack cementation process of the invention is carried out in retort without an adequate seal; Fig. 5 is a reproduction of a photomicrograph taken at 400 times magnification showing a coating produced using a pack composition outside the invention depicting nickel aluminide build-up, and the attendant entrapment of alumina Fig. 6 is a reproduction, of a photomicrograph taken at 400 times magnification showing the various phases making up a duplex coating produced in accordance with the invention while avoiding the formation of inclusions of aluminum oxide in the coating.

In its broad aspects, the invention is directed to a method for coating by pack cementation the substrate of an article formed of a heat resistant alloy in which the heat resistant alloy contains a solute metal in amounts having a relatively higher propensity to oxidize than the coating metal in the cementation pack and the solvent metal of the base alloy such that normally an internally oxidized structure is produced within the coating during pack cementation comprising an oxide dispersion of the oxidizable solute metal due to the presence of oxygen in the pack. The improvement resides in embedding the alloy article containing the solute metal in a cementation pack comprising a coating metal having a lower propensity to oxidize than t-he solute metal, and then carrying out the cementation process at an elevated coating temperature while maintaining the oxygen in the pack at a partial temperature below the oxygen threshhold level at which the alloy is subject to internal oxidation. .

The solute metal in the alloy to be coated includes those metals the oxides of which have a negative free energy of formation of at least 115,000 calories per gram atom of oxygen at about 25°C, and generally at least 125,000 calories, e.g. 133,000 calories or higher. Examples of such solute metals normally present in superalloys are aluminum, titanium, and the like. The invention is particularly applicable to heat resistant alloys of the nickel-base type containing aluminum and/or titanium hardeners .

The invention is further applicable to pack cementation systems in which chromium is the initial coating metal and in which the solute metal in the alloy is aluminum. In coating an alloy containing aluminum as the solute metal, a preferred method of maintaining the oxygen in the pack to below the threshhold level is to add to the pack an effective amount of a getter whose propensity to oxidize is at least equal to that of the solute metal in the alloy to be coated. A getter found particular ly satisfactory is aluminum powder in effective amounts ranging up to 1.25%, the amount of aluminum being advantage ously less than that amount which interferes with the diffusion of chromium into the substrate and which tends to cause the attendant entrapment of pack material in the coat ing v For example, if excess aluminum is employed, nickel aluminide may tend to form on the substrate which then inhibits diffusion of chromium therein. A range of aluminum content by weight in the pack fo,und particularly advantageous is 0.25% to 0.75%.

As stated hereinabove, the invention is applicable to the coating of a broad range of alloy compositions, such as those containing by weight up to 30% of a metal from the group consisting of Cr, Mo and W, the total of these metals not exceeding 40% with the Cr content preferably not exceeding 10% or 15%, up to 10% by weight of a metal from the group consisting of Cb and Ta; up to 1% C; up to 10% by weight of a metal from the group consisting of Ti and Al, the total amount of these metals not exceeding 12%; up to 2% Mn; up to 2% Si; up to 0.1% B; up to 1% Zr; and the balance at least 50% nickel. As will be appreciated, the iron group metals Fe and/or Co may be substituted for at least part of the nickel. For example, the alloy may contain up to 20% or more of cobalt.

The invention is particularly applicable to the coating of nickel-moly.bdenum-aluminum alloys containing by weight up to 30% molybdenum (more advantageously 10% to 25%), 0.5 to 10% aluminum (more advantageously 4% to 10%), up to 0.1% carbon (e.g. 0.03%), and the balance essentially nickel. A specific example of the alloy (known commercially as "NX-188") .is one containing approximately 17.5% molybdenum, approximately 7.75% aluminum, approximately 0.03% carbon,- and the balance essentially nickel.

CHEMICAL. COMPOSITION, WEIGHT % ALLOY DESIGNATIO C Cr Ni Co Mo W Cb Fe TRW-6A 0.13 6.1 Bal. 7.5 2.0 5.8 0.5 - Mar-M-246 0.15 9.0 Bal . 10.0 2.5 10.0 - - Mar-M-432 0.15 15.4 Bal. 19.7 - 3.0 1.8 - B-1900 0.10 8.0 Bal. 10.0 6.0 - - - Udimet 700 0.08 15.0 Bal. 18.5 5.2 ·· - - - Udimet 500 0.08 18.0 Bal. 18.5 4.0 - - IN-738 0.13 15.9 Bal. 8.5 1.8 2.6 0.9 0.1 IN-792 0.21 12.7 Bal. 9.0 2.0 3.9 - - IN-713C 0.12 12.5 Bal. - 4.2 - 2.0 - The oxygen threshhold level should desirably not exceed 4 parts per million of the gas in the pack.

A chrom zing pack which is particularly advantageous in carrying out the invention is one containing by weight 5% to 15% chromium, 0.25% to 0.75% aluminum, 3% to 5% nickel and the balance essentially an inert diluent, such as a refractory oxide, for example, alumina. The pack has mixed with it an effective amount of a halide energizer, e.g. one-quarter percent. The function of .nickel in the above pack is to maintain the aluminum potential below a level that causes nickel aluminide to form on the alloy, while, at the same time, not suppressing the chromium transfer rate. . J Examples of other halide energizers are ammonium iodide, ammonium bromide, ammonium bifluoride, and similar halides. By employing the pack of the invention in an adequately sealed retort, the internal oxidation of the chrom-ized layer is inhibited. While it is possible through very special care to reduce internal oxidation by means of an appropriate seal, the use of a getter assures maintaining the oxygen level in the pack to below the oxygen threshhold level above .which internal oxidation occurs and thus substantially avoid oxidation. Once an adequate first coating is produced substantially free from internal oxidation, additional coating of other metals can be applied, e.g., aluminum.

Thus, the invention is also applicable to multi-step pack cementation processes. Broadly, a typical multi-step process for coating heat resistant alloys may comprise, providing a first cementation pack containing a first coating metal (e.g. chromium) having a lower propensity to oxidize than the solute metal in the alloy (e.g. aluminum) to be coated, embedding the heat resistant alloy in the pack, carrying out a first cementation process at an elevated coating temperature while maintaining the oxygen in the pack at a partial pressure below the oxygen threshhold level at which the alloy is subject to internal oxidation, whereby the alloy is coated with the first coating metal while substantially avoiding internal oxidation, embedding the coated alloy in a second cementation pack containing at least one other coating metal (e.g. aluminum) and carrying out the second cementation at an elevated temperature.

A cementation pack which may be employed in the second coating step for transferring for example, aluminum as the second coating material, comprises 5% to 40% of a buffering metal (e.g. chromium) , 1.25% to 20% aluminum and the balance essentially an inert diluent containing a small but effective amount of a halide energizer, the amount of aluminum at the higher range being correlated to the higher range of the buffering metal (e.g. chromium), with the lower range of aluminum being correlated to the lower range of the buffering metal. The buffering metal aids in controlling the transfer and the deposition of the aluminum. Examples of other buffering metals are nickel, iron and cobalt .

In carrying out the primary chromizing step, it is preferred that the pack be prepared by first pre-reacting it at an elevated temperature, for example 1850°F for from 1 to 20 hours. The pre-.reacted pack is then reenergized by mixing it with a small but effective amount of ammonium halide, e.g., one-quarter percent ammonium bromide, and the article to be coated then pack chromized at 1900°F to 2200°F for times up to 50 hours, e.g., 30 hours, e.g. 30 hours at 2000°F for the alloy NX-188.

A typical chromizing pack (the first coat) is one prepared from 15% by weight of -20 + 40 mesh Cr, 4% by weight of minus 200 mesh Ni, 3/4 of 1% by weight of aluminum powder of size of minus 325 mesh, one-quarter percent NH^Br and the balance -14 + 28 mesh Α1203· A typical aluminizing pack (the second coat) is 22% by weight of -20 + 40 mesh Cr, 8% by weight of aluminum powder of size of about minus 325 mesh, one-quarter percent of NH4FHF and the balance ^^2°3 of -14 + 28 mesh. The coating with the latter pack is carried out at 1700°F for 20 hours. Broadly speaking, the temperature may range from 1600°F to 2000°F for up to 50 hours, e.g. 10 When the two-step coating procedure is employed to coat the alloy comprising 17.5% Mo-7.75% Al-bal. nickel alloy using the foregoing packs, the final coating on the metal substrate comprises an outer layer of nickel alum-inide phase with dispersed alpha chromium particles (70% Cr-20% Mo) . Moving further inward from the outer layer, the composition of the white precipitated phases changes from chromium-rich behind the aluminizing zone to molybdenum-rich near the substrateΓ The chromized layer is not sufficient by itself to provide the necessary protection against hot corrosion. For example, a chromized layer (5 mil thick) produced in accordance with the invention on an alloy substrate containing 17.5% Mo, 7.75% Al and the balance essentially Ni exhibited good resistance to sulfidation. However, oxidation exposure at 2200°F caused sufficient vaporization of the oxidized chromium surface thereby reducing the life over which the coating afforded sulfidation resistance. On the other hand, when the chromized substrate was aluminized to form nickel aluminide, the resulting duplex coating exhibited excellent oxidation and sulfidation resistance.

As illustrative of the various embodiments of the invention, the following examples are given.

Example 1 In chromizing an element (such as a vane or blade made of an alloy containing 17.5% molybdenum, 7.75% aluminum, 0.03% carbon and the balance essentially nickel, a chromizing pack is first prepared by mixing by weight 15% of -20 + 40 mesh chromium powder, 4% minus 200 mesh nickel powder, 1% minus 325 mesh aluminum powder, 1/4% NH^Br and the balance essentially -14 + 28 mesh AljOg. The mixed powders are pre-reacted at 2100°F for 20 hours and then re energized with 1/4% NH^Br .

The pre-reacted re-energized pack is placed in a retort and the element embedded in the pack. The retort is sealed with a low melting silicate glass composition and the retort then heated in a muffle furnace to a temperature of 2100°F and held at temperature for 30 hours. The retort is thereafter cooled to room temperature and the chromized element (referred to as Test No. 1) is metallographically examined for internal oxidation.

For comparison purposes, two tests (No. 2A and No. 3A) were carried out on elements' of the same composition in which the chromizing pack used comprised by weight 25% chromium powder, 1/4% NH^Br and the balance Al2°3 without the presence of a getter. In Test 2A, the retort was sealed with the low melting silicate glass mentioned hereinabove while in Test 3A, no seal was employed. In ° With regard to Test Nos . 2A and 3A, metallographic examination revealed internal oxidation in the coating as will be apparent from Fig. 1 (Test No. 2A) and Fig. 2 (Test No. 3A) taken at 400 times magnification, it will be noted that the amount of internal oxidation is greater where no seal is employed in the retort. However, in both cases (Figs. 1 and 2), the internal oxidation is substantial.

As regards Test No. 1 in which the glass sealed pack contained- chromium, nickel and aluminum, the presence of aluminum as a getter prevented internal oxidation as will be apparent from Fig. 3 which is free of internal oxidation.

A test conducted (Test No. 4A) in which the chromium-nickel-aluminum pack of the invention was employed (15% Cr, 4% Ni, 1% Al and the balance Al203) , but in which no seal was used in the retort, resulted in a substantial amount of internal oxidation in the coating as will be apparent from Fig. 4 taken at 400 times magnification.

The foregoing tests of Example 1 confirm that un-less the partial pressure of oxygen in the pack is maintained low enough, internal oxidation will occur during chromizing. This is also true even if inadequate quantities of the getter are present or if the seal is inadequate.

A test, Test 5A , was conducted on the Ni-Mo-Al alloy using a chromizing pack containing 15% chromium, 4% seal. The metal substrate was processed at a temperature of 2100°F for 30 hours and the metallographic structure shown in Fig. 5 was obtained. As will be observed from the photomicrograph, a nickel aluminide layer was formed having entrapped therein alumina particles from the pack. Because nickel aluminide was deposited on the substrate, the chromium transfer was suppressed. This type of coating does not provide adequate hot corrosion protection of the alloy at elevated temperatures"ranging up to 2200°F.

Example 7.

An alloy referred to by the designation "B-1900" may similarly be chromized in accordance with the invention while avoiding internal oxidation. This alloy contains nominally 0.1% carbon, 8% chromium, 10% cobalt, 6% molybdenum, 1% titanium, 6% aluminum, 0.15% boron, 0.1% zirconium, 4% tantalum and the balance essentially nickel. As in Example 1, an element of the alloy is embedded in a pre-reacted pack contained in a sealed retor€ comprising by weight 10% of -20 + 40 mesh chromium powder, 3% minus 20 mesh nickel powder, 0.5% minus 325 mesh aluminum, 1/4% NH. I and the balance Α1_0~ of -14 + 28 mesh. As in 4 2 3 Example 1, the mixed powders are pre-reacted at 2100°F for 20 hours and then re-energized with 1/4% of H4I . The pre-reacted re-energized pack is placed in a retort and the ele then sealed. The assembly is heated in a furnace to a temperature of about 1900°F for 30 hours and thereafter cooled to room temperature to produce a chromized element substantially free from internal oxidation.

Example 3 f An element of an alloy referred to by the desig¬ nation TRW-6A and comprising 0.13% carbon, 6.1% chromium, 7.5% cobalt, 2% molybdenum, 5.8% tungsten, 0.5% columbium, 1% titanium, 5.4% aluminum, 0.02% boron, 0.13% zirconium, 9% tantalum, 0.4% hafnium, 0.14% rhenium and the balance essentially nickel is chromized similarly as in Example 1, except that the pre-reacted pack contains 5% chromium, 3% nickel, 0.35% aluminum, 1/4% NH^FHF and the balance alumina.

The assembly of the retort containing the pack with the em¬ bedded element therein is then sealed and heated to about 1925°F and held at temperature for about 30 hours and there¬ after cooled to room temperature to provide a chromized ele¬ ment substantially free from internal oxidation.

As stated hereinabove, once the first metal coating has been deposited on any of the alloys disclosed herein substantially free from internal oxidation, a second pro¬ tective coating can be easily applied. Thus, the alloy of nickel-molybdenum-aluminum coated in Example 1 (Test No. 1) in accordance with the invention may be aluminized by using a pre-reacted pack composition comprising by weight 22% of minus 325 mesh aluminum powder, 1/4% ammonium bifluoride (NH^FHF) and the balance an inert refractory oxide, e.g. -14 + 28 mesh AI2O3. The pack with the embedded element is sealed in a retort and then aluminized at a temperature of 1700°F for 20 hours. A duplex coating is produced comprising a surface layer of nickel aluminide (NiAl) containing a dispersion of alpha chromium (Mo) particles. The sub-surface coating comprises a nickel-chromium solid solution containing aluminum in solid solution, the amount of aluminum remaining substantially constant at 7.5% to 8% at the aluminide interface to the nominal 7.5% or 8% at the base alloy interface. Molybdenum rich phases are formed in the chromium solid solution adjacent the substrate. The foregoing metallographic structure will be apparent by referring to Fig. 6 taken at 400 times magnification.

Referring to Fig. 6, it will be noted that the surface layer is enriched in nickel aluminide, the layer below and adjacent it comprising a layer of chromium rich solid solution. A layer of molybdenum rich phases is interposed between the chromium rich solid solution and the metal substrate comprised of the nickel-molybdenum-alum-inura alloy containing 17.5% molybdenum, 7.75% aluminum and the balance essentially nickel. The foregoing layers are metallurgically bonded to each other and to the sub By producing the foregoing coating using the invention, internal oxidation is substantially avoided and a high quality oxidation and sulfidation resistant coating obtained, particularly on nickel-molybdenum-aluminum alloys containing up to 30% molybdenum, 0.5% to 10% aluminum and the balance essentially nickel.

The temperature which may be employed to alumin-ize the chromized alloy may range from 1600°F to 2000°F for up to 50 hours, e.g. 10 to 30 "hours.

Claims (23)

061 WHAT WE CLAIM IS:

1. A method for coating by pack cementation the substrate of an article formed of a heat resistant nickel-base alloy, wherein said heat resistant alloy contains a solute metal in an amount having a relatively higher propensity to oxidize than the coating metal in the cementation pack and the solvent metal of the heat resistant alloy such that an internally oxidized structure is produced within the coating during pack cementation comprising an oxide dispersion of said oxidizable solute metal due to the presence of oxygen in the pack, characterized by the steps of, embedding the alloy article containing said solute metal in said cementation pack comprising coating metal having a lower propensity to oxidize than the solute metal in the alloy, and then carrying out the cementation process at an elev- . ated coating temperature while maintaining the oxygen in said pack at a partial pressure below the oxygen threshhold level at which the alloy is subject to internal oxidation.

2. The method of claim 1, characterized in that the solute metal in the alloy is that metal whose f formation of the oxide is at least 115 000

3. The method of claim 1 or 2, characterized in that the partial pressure of oxygen in the pack during the cementation process is maintained at below the thresh-hold level by employing in said pack an effective amount of a getter whose propensity to oxidize is at least equal to that of the solute metal in the heat resistant alloy being coated.

4. The method of claims 1 to 3, characterized in that the cementation pack is one in which the coating metal is chromium, wherein the solute metal in the alloy is aluminum, and wherein the getter in the pack is aluminum.

5. The method of claims 1 to 4, wherein the heat resistant alloy contained by weight up. to 30% of a metal from the group consisting of Cr, Mo and W, the total of these metals not exceeding about 40¾ the chromium content of the alloy ranging up to 15%; up to 10% by weight of a metal from the group consisting of Cb and Ta; up to 1% C; up to 10% by weight of a metal from the group consisting of Ti and Al, the total mount of these metals not exceeding 12%; up to 20% Co; up to 2% Mn; up to 2% Si; up to 0.1% B; up to 1% Zr; and the balance at least 50% nickel .

6. A method for coating by pack cementation the substrate of an article formed of a heat resistant alloy containing by weight [about] up to 30% molybdenum, 0.5 to 10% aluminum as an oxidizable solute metal, up to 0.1% C and the balance essentially nickel, the oxidizable solute metal having a relatively higher propensity to oxidize than the coating metal in the cementation pack such that an internally oxidized structure is produced within the coating during pack cementation comprising an oxide dispersion of aluminum due to the presence of oxygen in the pack, character zed in the steps of, embedding said alloy article in said : .·. cementation pack comprising coating metal having a lower propensity to oxidize than the solute metal, and then carrying out the cementation process at an elevated coating temperature while maintaining the oxygen in said pack at a partial pressure below the oxygen threshhold level at which the alloy is internally oxidized.

7. The method of claim 6, wherein the heat resistant alloy contains 10% to 25% molybdenum and 4% to 10% aluminum .

8. The method of claim 7, characte ized in that the alloy contains 17.5% molybdenum, 7.75% aluminum and 0.03% carbon.

9. The method of claims 6 to 8, characterized in that the partial pressure of oxygen in the pack during the cementation process is maintained at below the thresh-hold level by employing in said pack an effective amount of a getter whose propensity to oxidize is at least equal to that of the solute metal in the heat resistant alloy being coated.

10. The method of claim 9, wherein the cementation pack is one in which the coating metal is chromium, and wherein the getter in the pack is aluminum.

11. The method of claim 10, wherein the effective amount of aluminum ranges up to 1.25%.

12. The method of claim 10, wherein the cementation pack contains by weight 5% to 15% chromium, 0.25% to 0.75% aluminum, 3% to 5% nickel, and the balance essentially an inert diluent containing a small but effective amount of a halide energizer.

13. A multi-step pack cementation process for producing a multiple oxidation and sulfidation corrosion resistant coating on heat resistant nickel-base alloys containing a solute metal in amounts having a relatively higher propensity to oxidize than the coating metal in the cementation pack and the solvent metal of the heat resistant alloy such that an internally oxidized structure is produced within the coating during pack cementation comprising an oxide dispersion of said oxidizable solute metal due to the presence of oxygen in the pack, said process characterized in, providing a first cementation pack containing a first coating metal having a lower propensity to oxidize than the solute metal, embedding said heat resistant alloy in said pack, carrying out a first cementation process at an elevated coating temperature while maintaining the oxygen in said pack at a partial pressure below the oxygen thresh- hold level at which the alloy is subject to internal oxidation whereby said alloy is coated with embedding said coated alloy in a second cementation pack containing at least one coating metal, and carrying out said second cementation at an elevated coating temperature.

14. The process of claim 13, characterized in that the heat resistant alloy contains by weight" up to 30% of a metal from the group consisting of Cr, Mo and W, the total of these metals not exceeding about 40%) the chromium content of the alloy ranging up to 15%; up to 10% by weight of a metal from the group consisting of Cb and Ta; up to 1% C; up to 10% by weight of a metal from the group consisting of Ti and Al, the total amount of these metals not exceeding 12%; up to 20% cobalt; up to 2% Mn; up to 2% Si; up to 0.1% B ; up to 1% Zr; and the balance at least 50% nickel.

15. A multi-step pack cementation process for producing a multiple oxidation and sulfidation corrosion resistant coating comprising a top coat of nickel-aluminum on an initial coating comprising chromium diffused into a heat resistant alloy containing by weight up to 30% molybdenum, 0.5% .to 10% aluminum and the balance essentially nickel, said process characterized in, providing a first cementation pack containing chromium and an effective amount of a getter whose propensity to oxidize is at least equal to that of the aluminum in the alloy being coated, embedding said heat resistant alloy in said pack, carrying out a first cementation process at an elevated coating temperature while maintaining the oxygen in said pack at a partial pressure below the oxygen thresh-hold level at which the alloy is internally oxidized, whereby to produce a diffusion coating of chromium on said alloy, embedding said coated alloy in a second cementation pack containing aluminum and a buffering metal for' controlling the transfer and deposition of said aluminum onto the coated alloy, and carrying out said second cementation at an elevated coating temperature, whereby to produce a final coating of nickel-chromium-aluminum.

16. The process of claim 15, characterized in that the buffering metal is chromium.

17. The process of claim 15, characterized in that the heat resistant alloy contains 10% to 25% molybdenum and 4% to 10% aluminum.

18. The process of claim 17, characterized in that the alloy contains 17.5% molybdenum, 7.75% aluminum and approximately 0.03% carbon.

19. The process of claim 15, characterized in that the first cementation pack contains by weight 5% to 15% chromium, 0.25% to 0.75% aluminum, 3% to 5% nickel, and the balance essentially an inert diluent containing a small but effective amount of a halide energizer.

20. The process of claim 15, characterized in that the second cementation pack contains 5% to 40% chromium, 1.25% to 20% aluminum, such that the higher range of aluminum is employed with the higher range of chromium and such that the lower range of aluminum is employed with the lower range of chromium, and the balance essentially an inert diluent containing a small but effective amount of a halide energizer.

21. The process of claim 15, wherein the final coating contains substantial amounts of nickel aluminide.

22. A coated alloy substrate produced in accordance with the method of claim 15.

23. A coated alloy substrate, the substrate comprising up to 30% molybdenum; 0.5% to 10% aluminum and the balance essentially nickel, the coating being formed of a surface layer of nickel aluminide, a chromium rich solid solution below the surface layer, a layer of molybdenum rich phases between the chromium rich layer and the substrate, said layers being metallurgically bonded together .