US3649226A

US3649226A - Oxidation-sulfidation resistant articles

Info

Publication number: US3649226A
Application number: US812267A
Authority: US
Inventors: Fred E Lynch; Berl L Carlton
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1969-04-01
Filing date: 1969-04-01
Publication date: 1972-03-14
Anticipated expiration: 1989-03-14
Also published as: GB1277500A

Abstract

High-temperature nickel and cobalt base alloy articles such as turbine buckets are provided with improved oxidation and sulfidation resistance by the provision of a diffused aluminum and manganese layer in the surface of the article. The invention herein described was made in the course of work under a contract or subcontract thereunder with the Department of the Air Force.

Description

United States Patent Lynch et a1.

[ 5] Mar. 14, 1972 Primary Examinerl-lyland Bizot Attorney-Sidney Carter and Peter P. Kozak [57] ABSTRACT High-temperature nickel and cobalt base alloy articles such as turbine buckets are provided with improved oxidation and sulfidation resistance by the provision of a diffused aluminum and manganese layer in the surface of the article. The invention herein described was made in the course of work under a contract or subcontract thereunder with the Department of the Air Force.

4 Claims, 3 Drawing Figures OXIDATION-SULFIDATION RESISTANT ARTICLES This invention relates to metal articles such as turbine blades and nozzle guide vanes adapted for use in high temperature and corrosive environments and more particularly to metal articles of the aforementioned type formed of a nickel or cobalt based alloy and having a diffused surface layer which provides the article with superior oxidation and sulfidation re sistance.

High-temperature alloy components, such as turbine blades and nozzle guide vanes used in gas turbine engines are subjected to extended periods of service at elevated temperatures under variable stress conditions. When such components are formed of certain high-temperature nickel based alloys and cobalt based alloys, they possess excellent strength under most hightemperature service conditions. However, the durability of these turbine buckets and nozzle guide vanes is materially reduced because of inadequate resistance to oxidation and sulfidation.

The term sulfidation" as used herein refers to the attack of nickel or cobalt based alloys by sulfur-bearing compounds such as sulfides and sulfates combined with chlorine. As applied to aircraft engine components, the sulfur-bearing compounds and chlorine come from contaminated atmosphere, the engine fuel, and sea water, or from the reaction of the fuel and water.

It is known that the treatment of the nickel or cobalt based alloys by diffusing aluminum into the surface of the metal provides the articles with a high degree of oxidation resistance. A suitable aluminum diffusion treatment is described in the US. Pat. No. 3,129,069 Hanink et al., assigned to the assignee of the present invention. While the diffused aluminum surface layer described in this patent provides the nickel and cobalt based alloys described therein with outstanding oxidation resistance, it does not provide the alloys with comparable sulfidation resistance.

It is, accordingly, the principal object of this invention to provide articles such as turbine blades formed of high-temperature nickel and cobalt based alloys which have both oxidation and sulfidation resistance to an outstanding degree.

A more specific object of the invention is to provide the surface of nickel or cobalt based alloys with a diffusion layer including both aluminum and manganese in certain suitable proportions improved sulfidation resistance as well as oxidation resistance is achieved.

In general, these and other objectives are accomplished by applying a powdered mixture containing aluminum and manganese as a layer over the surface of the alloy to be diffusion treated and subjecting the alloy to a diffusion heat treatment whereby both the aluminum and manganese are diffused into the alloy surface to a depth of about 0.0004 to 0.006 inch to form complex intermetallic phases in the diffusion layer which affords the alloy surface with outstanding sulfidation and oxidation resistance. Preferably, the proportions of aluminum and manganese in the mixture are such that based on the total aluminum and manganese content, the proportion of aluminum is about to 67 percent with the balance manganese, on a weight basis.

Other objects and advantages will be apparent from the following description of the invention, reference being had to the accompanying drawings in which:

FIG. 1 is an elevation view with parts broken away and in section of a turbine bucket formed of a nickel base alloy provided with an aluminum-manganese diffused surface layer in accordance with the invention.

FIG. 2 is a photomicrograph view of a high-temperature creep-resistant nickel base alloy showing the micrographic structure of the alloy near its working surface after the aluminum-manganese diffusion treatment of this invention.

FIG. 3 is a photomicrographic view of the nickel base alloy shown in FIG. 2 showing the micrographic structure of the alloy near its working surface after it has been exposed to cyclic heating at elevated temperatures to determine sulfidation resistance.

As indicated above, this invention is concerned primarily with articles of manufacture formed of high-temperature nickel or cobalt based alloys which are intended for use at relatively high temperatures in the vicinity of 2,000 F. and in oxidizing and sulfidizing environments. Gas turbine blades or buckets and stator vanes are examples of articles of this type to which this invention is particularly applicable.

In general, the nickel and cobalt based alloys to which this invention is applicable are nickel based alloys containing 40-80 percent by weight nickel, 5-20 percent chromium, and the balance other constituents, and cobalt based alloys containing 45-70 percent cobalt, 15-30 percent chromium, and the balance other constituents. In the case of the nickel based alloys the other constituents may include up to 10 percent molybdenum, up to 5.5 percent titanium, up to 6.5 percent aluminum, up to 3 percent columbium, up to 9 percent tantalum, up to 13.5 percent tungsten, up to 2 percent hafnium, up to 1 percent rhenium, up to 1.5 percent vanadium, up to 20 percent cobalt, up to 3 percent iron, and minor amounts of cesium, boron, zirconium, silicon, manganese, and impurities in the form of sulfur, copper, and phosphorous. In the case of the cobalt based alloys, the other constituents may include up to 7 percent molybdenum, up to 10 percent tantalum, up to 16 percent tungsten, up to 16 percent nickel, up to 3 percent iron, minor amounts of carbon, manganese, silicon, titanium, boron, and zirconium, and impurities in the form of iron, sulfur, phosphorous, and copper.

The following Table I contains examples of suitable high temperature nickel and cobalt based alloys which may be satisfactorily provided with a thin aluminum an manganese diffusion layer in accordance with this invention, the composition being listed in by weight. Alloys Nos. 1, 2 and 3 are nickel based alloys, and No. 4 is a cobalt based alloy.

TABLE I No. I No.2 No.3 No.4 Carbon 0.08-02 0.13-0.17 0.05-0.1 0.55-0.65 Manganese 0.25 max. 0.20 max. 0.1 max. 0.1max

Chromium 13-15 8-10 17.514 18.5 23-245 Cobalt I max. 9-11 14.5-15.5 Remainder Molybdenum 3.8-5.2 2.25-2.75 2.75-3.25

Tungsten 9-11 1.25-1.75 6.5-7.5

Iron 2.5 max. 1 max. 0.5 max. 1.5 max Titanium 0.5-1.25 1.25-1.75 4.75-5.25 0.15-0.25

Aluminum 5.5-6.5 5.25-5.75 2.25-2.75

Silicon 0.5 max. 0.20.max. 0.2 max. 0.4 max.

Sulfur 0.015 max. 0.015 max. 0.015 max.

Copper 0.1 max. 0.1 max. 0.5 max.

Tantalum 1.8-2.8 1.25-l.75 3-4 Boron 0.005-0015 0.01-0.02 0015-0025 0.01 max.

Zirconium 0.05-0.15 0.03-0.08 0.4-0.6

Nickel Remainder Remainder Remainder 9-1 1 Another nickel base alloy which may be advantageously treated in accordance with this invention is disclosed in the US. Pat. No. 2,688,536 Webbere et al. This alloy comprises about 0.06-0.25 percent carbon, 13-17 percent chromium, 46 percent molybdenum, 8-12 percent iron, 1.5-3 percent titanium, 2-4 percent aluminum, 0.0l0.5 percent boron, and the balance substantially all nickel.

Referring more particularly to the drawings, there is shown in FIG. 1 a turbine bucket 10 for a gas turbine engine of the axial flow type. In accordance with this invention, an article such as this turbine bucket 10 is formed ofa nickel base 12 of the type described above which is provided with a surface diffusion layer 14 containing both aluminum and manganese. For the purpose of illustration the thickness of the diffusion layer is considerably exaggerated, the actual thickness being as described elsewhere herein. It is usually unnecessary to provide the manganese-aluminum diffusion layer over the fastening portion 16 of the turbine bucket.

In general, the diffusion layer 14 is formed by first applying a mixture of aluminum and manganese to the surface of the base alloy 10 in particulate form in a manner so as to provide an adherent layer thereof adjacent the alloy surface. The alloy surface and the adherent layer are then heated to a temperature and for a time whereby the aluminum and manganese are both simultaneously diffused into the base metal alloy surface to a depth of about 0.0004 to 0.006 inch to form the diffusion layer 14. Preferably, the diffusion heat treatment is accomplished by first heating the coated article at about 1,300 F for about 1 hour and subsequently heating the article for about 2 hours at about 2, l F. in a hydrogen environment.

Several methods for simultaneously diffusing the aluminum and manganese into the base metal have been found to provide substantially equivalent diffusion surface layers which provide the article with markedly superior sulfidation and oxidation resistance.

ELECTROPI-IORETIC METHOD Example I In general, this method consists of applying a particulate coating of aluminum and manganese to the article to be diffusion coated by suspending metal particles of suitable particle size in an organic dielectric solvent and impressing a direct current of 50 to 500 volts between two electrodes, thus causing the metallic particles to uniformly deposit on the article which is one of the electrodes. Subsequently the coated article is heat treated to diffuse the metal particles into the article surface.

A coating bath was prepared consisting of a solution of 6 parts by weight isoproponal and 4 parts by weight nitromethane to which was added 2.7 grams per liter zein and 0.2 grams per liter cobalt nitrate. Then, 8.25 grams manganese powder per liter of the bath and 16.75 grams aluminum powder per liter of the bath were added to the bath and mixed therein to form a dispersion of the metal powder in the bath. The metal powders were of a I to micron size.

A cast turbine blade specimen cast of the alloy No. l as shown in FIG. 1 of the drawings, Table l, was first cleaned by dry honing, using No. 240 grit or finer aluminum oxide abrasive. Blasting was discontinued when the surface color of the casting changed to a uniform light gray. It was then immersed in the bath and made cathodic. An inert stainless steel anode was immersed in the bath and a current of milliamperes per square inch at 200 volts was applied across the electrodes for about 60 seconds, whereby the powdered metals were caused to migrate to and be deposited uniformly over the blade surface. Sufficient zein was also deposited along with the powders to act as a binder. About 0.07 grams per square inch of the metal powders were deposited on the blade surfaces. A thusly coated blade was heat treated for about 1 hour at l,300 F. and then for an additional 2 horus at 2,100 F. in a hydrogen environment. A diffusion layer 14 of about 0.003 inch in thickness was formed as shown in FIG. 1 ofthe drawings.

A 500X magnification photomicrograph of the diffused layer was made as shown in FIG. 2 and an X-ray diffraction study of the diffusion layer was made. The region of electron microprobe line-scan analysis shows four distinct zones which are described as follows.

The a-b inner zone is very slightly enriched Cr, Mo, Mn, and Fe, and depleted in Ni and Ti with respect to the normal structure of this alloy in the base material adjacent to it. Some intrusion of the matrix phase from the adjacent b-d zone can also be seen.

The b-d or first coating zone contains at least two phases. The matrix phase is enriched in Al, Mn, and Ti, and depleted in Cr, Mo, Fe, and Ni, as compared to the base material. This matrix appears to be the nickel rich side of the Ni Al, phase in which Mn, Cr, and Fe are substitutional for some of the Ni. Ti and Al, comprise the balance of the phase. The included particulate material in this zone appears to be an intermetallic phase containing primarily Cr, M0, and Ni with lesser amounts of Al, Mn, and Fe. Intrusion of the inner zone material a-b into this zone can be observed in the micrograph of FIG. 2.

The d-e or intermediate coating zone appears to be a single phase, but close examination of the photograph indicates the possible presence of an extremely fine (about 0.1 micron) randomly dispersed phase. The element distribution scans show a continued increase of aluminum and manganese across this zone and a corresponding continued decrease of chromium and titanium. The nickel also shows a very slight increase in concentration across the zone. This is believed to be the intermetallic phase based on NiAl having other elements substitutional for both nickel and aluminum across the zone. The phase NiAl has a wide lattitude of composition (nickel 65 to 83 percent and aluminum 35 to 17 percent by weight) explaining the variable composition across the zone.

The ei or outer coating zone comprises at least two and possibly three phases. The matrix phase, on the basis of aluminum and nickel content, appears to be nearly stoichiometric N iAl with some substitution of Mn into the lattice, probably for the Ni. The large particle (i) is a MoCrSi Mn phase. Other particles in this zone appear to be somewhat different in composition, containing Nb, Ti, and/or Al, The extremely fine particles seen near the surface side of this zone are too small to be resolved on the probe.

Thereafter, the diffusion coated bucket was subjected to a test designed to establish the resistance of the coating to both sulfidation and oxidation. in general, the test consists in repeatedly heating and cooling a test specimen in a sulfidation and oxidation environment.

The equipment used consisted of a gas fired furnace having an internal shaft means for supporting and rotating a test specimen therein, spray nozzles for spraying a 1 percent by weight sulfate ion solution of deionized water using sodium sulfate as a source of sulfate ion, and an optical pyrometer. One cycle of the test consisted in heating the specimen to about l,900 F. and then permitting it to cool to 500 F. while spraying it with the aforementioned sulfate ion solution while rotating it.

The diffusion coated turbine blade described above was subjected to 500 cycles as above indicated. The bucket experienced a weight loss of only 0.0488 grams. Visual inspection indicated virtually no corrosive effect and no cracking on the blade surface. To further test the blade for cracking, it was heated to about l,0OO F. in the atmosphere and visually observed in this condition. At this temperature the metal assumes a gold or high-brown color which reveals any cracks on the blade surface. No cracks were observed. Finally, the blade was dipped in Zyglo penetrant and inspected. No cracking was observed.

A 500x magnification photomicrograph of the diffused layer after the sulfidation-oxidation test was made as shown in FIG. 3 and an X-ray diffraction study of the diffusion layer was made. The region of electron microprobe line-scan analysis shows four distinct zones which are described as follows. Referring to FIG. 3, the cone a-b appears to consist of either a single complex oxide or a mixed multioxide. The outer portion of the zone is Mn rich and contains some Fe. Both the Al and Cr content are quite low in this portion and the Ni content appears to be les than a few percent. The inner part of this zone is high in Al and also contains some Cr and Mn. This appears to be a complex Al (Mn-Cr) oxide phase.

The zone b-c is very rich in Ni, and also contains Al, Mn, and Cr. It does not etch like the balance of the matrix material in the diffusion region and seems to be a zone from which the oxide forming elements (Al, Cr, Mn) have diffused, leaving behind a relatively Ni rich (about 80 percent) region. This phase appears to be a solid solution of Al, Mn, and Cr in Ni.

The c-g zone is a complex one containing two or more phases dispersed in a Ni rich matrix. The matrix phase, based on the Al (about 9 percent by weight) content appears to be either the NiAl (about 30 percent by weight Al) or Ni Al (about 13 percent by weight aluminum) phases. The Ni (about 70 percent by weight) content in this phase considered alone would indicate the phase to be NiAl. The phase is seen to contain approximately 70 percent by weight Ni, 9 percent by weight Al, and 4 percent by weight Cr, with the other nominal alloying ingredients associated with Alloy No. 1 (Ti, Nb, Mo, Si, Fe) comprising no more than 2 percent by weight of the total. By difference this means that the matrix phase contains about percent by weight manganese. Particle F" in this zone appears to be a (Cr, Mo, Mn, Ni) intermetallic phase similar to the particulate phase found in the b-d coating zone of the as coated specimen shown in FIG. 2.

The gh zone like the adjacent zone is also multiphase in character. The matrix is nearly the same as in the e-g zone, i.e., based on the Ni Al phase with substitution of Mn and Cr into the lattice. The massive phase (i) is Ni rich (about 65 percent by weight) and contains low aluminum (about 3 percent by weight), nominal chromium (about 3 percent by weight) Mo (about 4 percent by weight) and Mn (about 12 percent by weight) with the balance of the phase being composed of other alloying elements normal to the No. 1 base alloy involved.

The h-j zone shows the extent of Mn diffused into the bulk of the matrix. It can readily be seen that the Mn is substitutional for the Ni in the zone.

The entire procedure of Example No. l was repeated for Alloy No. l where the electrophoretic coating composition contained l6.75 grams per liter of Mn powder and 8.25 grams per liter of aluminum powder. Micrographic examination indicated a similar diffusion structure and after a similar sulfidation and oxidation test, a weight loss of 0.0028 grams was measured with the surface showing no sign of cracking or corrosion failure.

EXAMPLES Nos. 2 and 3 The procedure of Example No. 1 was carried out with the Alloys Nos. 2 and 3 of Table I with similar results indicating similar excellent sulfidation and oxidation resistance properties.

The procedure described in Example No. 1 using Alloy No. l as the base metal was performed with different proportions of manganese and aluminum in the electrophoretic bath with results indicating that with proportions of aluminum of about 18 to 67 percent and the balance manganese, satisfactory sulfidation and corrosion-resistant diffusion coatings are ob tained. When the proportions of these metals are appreciably outside this range, test blades failed the sulfidation-oxidation tests. A test where the manganese content was 16.75 grams per liter with no aluminum, produced a diffusion coating thickness of 0.002 inch, the blade involved a loss of 0.2927 grains in the sulfidation-oxidation test and inspection of the tested blade indicated a failure. A test where the electrophoretic bath contained 25 grams per liter aluminum and no manganese produced a diffusion coating of 0.0004 inch, the blade showed a weight loss of 0.21 10 grams after the sulfidation-oxidation test and the blade failed. A test where the bath contained 40 grams per liter manganese and 10 grams per liter aluminum developed a diffused coating thickness of 0.0004 inch, a weight loss of 0.3440 grams after the sulfidation-oxidation test and the blade failed. A test wherein the manganese content was 33 grams per liter and the aluminum content was 8.25 grams per liter in the coating bath developed a diffused coating thickness of 0.0025 inch, involved a weight loss of 0.3341 grams after the sulfidation-oxidation test and the blade failed.

Other methods for providing the diffusion layer 14 have been found to produce diffusion coatings substantially equivalent to that formed by the method described in Example 1.

PACKED DIFFUSION PROCESS In this method, the article to be diffusion coated is packed in a retort surrounded by a suitable powdered mixture consisting of the metals to be diffused and an activator carrier. The article is then heated in the retort and then in a hydrogen environment to obtain a diffusion coating.

A uniformly mixed composition was prepared using on a weight basis 10 percent aluminum powder, 25 percent manganese powder, 8 percent ammonium chloride, and 57% aluminum oxide. The powders were of about a 5 micron size. The composition was placed in a retort with a turbine blade specimen formed of Alloy No. l to be diffusion coated immersed in the powdered mixture. The packed retort was placed in a furnace and raised to l,350 F. for one hour and then cooled in the retort to a handling temperature. The specimen was then removed from the retort and placed in a rack for the diffusion step. Diffusion was accomplished by placing the rack in a hydrogen atmosphere (25 F., 2. max.) furnace, heated to about 2,100 F. and held at this temperature for 2 hours. The blade was then removed from the furnace and allowed to cool in a draft-free closure. Metallographic examination before and after the sulfidation-oxidation test as described in Example 1 indicated a similar diffusion coating and similar oxidation-sulfidation resistance characteristics.

Similar test were performed with a pack consisting of on a weight basis 10 percent aluminum powder, 8 percent ammonium chloride, 34 percent ferro-manganese (78 percent by weight manganese) and 48 percent aluminum oxide. Satisfactory results were also obtained using a pack consisting of on a weight basis 10 percent aluminum powder, 10 percent manganese powder, 8 percent ammonium chloride, and 72 percent aluminum oxide. Diffusion layer thicknesses in the range of 0.001 to 0.003 inch were obtained in these tests.

SLURRY METHOD In this method an inorganic liquid binder is blended in suitable proportions with aluminum and manganese powders and applied to the surfaces of the article to be coated by dipping or spraying. After drying the coating, the article is heat treated to obtain a diffused coating.

A slurry was prepared by mixing on a weight basis of one part aluminum powder and 4.5 parts manganese powder together with about 1 part of aqueous potassium silicate and a small amount of potassium nitrate as a wetting agent to form a brushable or sprayable slurry. The powders used were of a 1 to 5 micron size. A turbine bucket specimen formed of Alloy No. l was coated with the slurry by dipping it into a stirred bath of the slurry. The specimen was removed and air dried for thirty minutes and then oven baked for about 1 hour at 230 to form a coating bonded to the specimen. The coated specimen was then placed in an oven having an initial temperature of not in excess of 400 F. The furnace temperature was raised to l,325 F. and the specimen was subjected to this temperature for 1 hour. The furnace temperature was then increased to 2, F. and the specimen was subjected to this temperature for 2 hours. The furnace was then allowed to cool to room temperature. Diffused coating thicknesses in a range of 0.001 to 0.003 inch were obtained on the specimen surfaces by this method. This diffusion layer was similar metallurgically to that described in Example 1 and sulfidation-oxidation tests indicated similar results. In another test, a slurry was prepared containing two parts aluminum powder and one part manganese powder. Similar results were obtained.

A suitable slurry for use in this method may be prepared by mixing suitable proportions of manganese powder with Sermetal 222, a product of Teleflex Corp. which contains an alkali metal binder aluminum powder and alumina. Good results are obtained using, for example, 60 grams Sermetal 222 to 90 grams manganese powder.

Cobalt based alloy articles are provided with similar improvement in sulfidation and oxidation resistance by the same methods-described above with the cobalt generally serving the role of the nickel in the above tests and with cobalt forming intermetallic constituents in the diffusion layer as does nickel in the above tests. Specific examples of applicable cobalt based alloys include the Alloy No. 4 of Table I as well as the commercial alloy consisting of on a weight basis 0.50 percent carbon, 0.5 percent manganese, 0.5 percent silicon, 0.02 percent sulfur, 25.5 percent chromium, 10.5 percent nickel, 7.5 percent tungsten, 1.0 percent iron, with the remainder substantially cobalt and an alloy consisting of 0.10 percent carbon, 1.5 percent manganese, 0.20 percent silicon, 0.02 percent phosphorous, 0.02 percent sulfur, 20.0 percent chromium, percent nickel, l5 percent tungsten, 2.0 percent iron, and the balance substantially cobalt.

The above-described tests and other tests have demonstrated that markedly improved sulfidation and oxidation resistance is obtained when the aluminum constitutes by weight to 67 percent of the aluminum-manganese content of the materials diffused into the base metal and when the diffusion layer is present in the range of 0.0004 to 0.006 inch in thickness. Optimum oxidation and sulfidation resistance have been achieved with the aluminum content of about 18 percent and manganese content of 82 percent. The preferred diffusion layer thickness is about 0.003 inch. It has been found that where the diffusion depth is greater than about 0.006 inch, it spalls due to brittleness. Diffusion layer thicknesses of less than 0.0004 do not provide effective corrosion and oxidation resistance. In general, the depth of diffusion obtained is a function of processing parameters and the geometry of the part. Tests for various mechanical properties of the diffusion coated turbine blade specimens indicated no deleterious effects.

Although the invention has been described in terms of certain specific embodiments, it is to be understood that other forms may be adopted within the scope of the invention.

What is claimed is: 1. An oxidation and sulfidation resistant article for use in a high-temperature environment,

said article being formed of an alloy selected from the group consisting of a nickel based alloy and a cobalt based alloy,

said nickel based alloy consisting essentially on a weight basis 40-80 percent nickel, 5-20 percent chromium, up to 10 percent molybdenum, up to 5.5 percent titanium, up to 6.5 percent aluminum, up to 3 percent columbium, up to 9 percent tantalum, up to 13.5 percent tungsten, up to 2 percent hafnium, up to 1 percent rhenium, up to 1.5 percent vanadium, up to 20 percent cobalt, up to 3 percent iron, and up to minor amounts of carbon, boron, zirconium, silicon, and manganese,

said cobalt based alloy consisting essentially on a weight basis 45-70 percent cobalt, 15-30 percent chromium, up to 7 percent molybdenum, up to 10 percent tantalum, up to 16% tungsten, up to 16 percent nickel, and up to minor amounts of carbon, manganese, silicon, titanium, boron, and zirconium,

said article having diffused in the surface of at least a portion thereof the combination consisting initially of aluminum and manganese to form a diffusion layer of about 0.0004 to 0.006 inch in thickness, said aluminum and said manganese being diffusedthroughout said diffusion layer, the proportion of said aluminum to said manganese in said PO-105O UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No.

649,226 March 14, 1972 Dated Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1,

Column 2,

Column 3,

Column 4,

Column 6,

line 44, after "portions" insert whereby TABLE I, Chromium, No. 3 should read line 71, "horus should read hours line 4, after "enriched" insert in line 39, "test" should be tests Signed and sealed this 22nd day of August 1972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. Attesting Officer ROBERT GOT'ISCHALK Commissioner of Patents

Claims

2. Claim 1 wherein said alloy is said nickel based alloy.
3. Claim 1 wherein said article is a turbine bucket, and said alloy is said nickel based alloy.
4. Claim 1 wherein said article is a turbine bucket, said alloy is said nickel based alloy and said layer is about 0.003 inch in thickness and the proportion of said aluminum is about 18 percent.