US3365327A - Vapor diffusion coating containing aluminum-chromium-silicon - Google Patents

Description

Jan. 23, 19-68 PUYEAR ET AL 3,365,327

NG CONTAINING ALUMINUM-CHROMIUMSILICON VAPOR DIFFUSION COATI Filed April 14, 1965 2 Sheets-Shqet l HOLLOWAY ROBERT B. PUYEAR JACK H A r TORNEV Jan. 23, 1968 R. B. PUYEAR ETAL 3,365,327

VAPOR DIFFUSION COATING CONTAINING ALUMINUM-CHROMIUM-SILICON Filed April 14, 1965 2 Sheets-Sheet 2 INVENTORS JACK H. HOLLOWAY ROBERT B.PUYEAR ATTORNEY United States Patent Ofilice 3,365,327 Patented Jan. 23, 19 68 3,365,327 VAPOR DIFFUSION COATING CONTAINING ALUMINUM-CHROMIUM SILICON Robert B. Puyear, Kokomo, Ind., and Jack H. Holloway,

Marshalltown, Iowa, assignors to Union Carbide Corporation, a corporation of New York Filed Apr. 14, 1965, Ser. No. 448,082 4 Claims. (Cl. 117-1072) A method in accordance with the present invention for producing a coated metallic article comprises embedding the articles to be coated in a particulated mixture of elemental chromium, elemental aluminum, silicon carbide and halide carrier material; and heating the mixture and embedded article with the exclusion of air at an elevated temperature whereby a diffused metallic coating is provided on the embedded article.

In the practice of the present invention, the operating equipment described in U.S. Patent 3,079,276 can be effectively employed. For example, a charge of finely divided aluminum, finely divided chromium, finely divided silicon carbide and finely divided halide carrier is blended and placed in a heat-resistant retort and the articles to be coated are cleaned, if necessary, and embedded in the charge.

Table I shows the compositional ranges, and sizing for the charge in accordance with the present invention and Table II lists exemplary compositions which have been coated by the process of the present invention. Suitable carrier materials are ammonium halides, aluminum halides and nickel halides.

TABLE I.COMPOSITION OF CHARGE MATERIAL, WEIGHT PERCENT Broad Range Preferred Range Optimum Charge Material Type Particle Particle Particle Size U.S. Content, Size US. Content, Size U.S. Content, Screen Percent Screen Percent Screen Percent Mesh Mesh Mesh Atomized metal powder -100 4-10 200 4-8 200 5 Electrolytic metal powder 100 -30 200 10-20 250 Commercially available 58-86 150 71-85 -240 79. 4 6-1 grade 0. 2-1 50 0. 2-0. 5 0. 6

applications, including gas turbine and oil refinery equipment, articles coated by the processes of the aforedescribed patents and patent application, were attacked when exposed to sulfur-containing media at elevated temperatures. Consequently, in view of the superior performance of diffused metallic coatings generally, e.g. oxidation resistance, it would be of significant industrial benefit if dilfused coatings could be provided which are resistant to chemical corrosion, particularly in sulfur-containing media at elevated temperatures.

It is therefore an object of the present invention to provide a vapor-diffusion coating for metallic articles which is resistant to attack in sulfur-containing media at elevated temperatures.

It is another object of the present invention to pro vide a vapor diffusion coating for metallic articles which is strong and oxidation resistant in addition to being resistant to corrosion in sulfur-containing media.

Other objects will be apparent from the following de scription and claims taken in conjunction with the drawing in which:

FIGURES 1 and 4 respectively show an uncoated metal article before and after exposure to sulfur-containing media at elevated temperatures;

FIGURES 2 and 5 respectively show metal articles, coated by a commercial process, before and after exposure to sulfur-containing media at elevated temperatures and FIGURES 3 and 6 respectively show metal articles coated by the process of the present invention before and after exposure to sulfur-containing media at elevated temperatures.

The original magnification of the photographs of FIG- URES 1 through 6 is 400x.

TABLE II.-IYPICAL ALLOYS COATED BY THE PROCESS OF THIS INVENTION [Nominal compositions, in weight percent] I May also contain other modifying elements such as: carbon, silicon, boron, zirconium, titanium, copper.

2 Contains thoria dispersed within the nickel matrix.

After the charge and articles to be coated are in place, and with air exhausted from the retort, the retort is suitably sealed and the contents are heated at a temperature between about 1850 F. and 2l0 0 F., preferably 2000 F. for a time sufiicient to provide the desired coating thickness. A suitable process operating time is 5 to 20 hours with 7 hours at 2000 F. being preferred.

After the process has operated for a time sufficient to provide the desired coating, the retort is cooled to about 200 F. or lower and the coated aritcles recovered and cleaned, e.g., with a bristle brush to remove any loose charge material.

The coated articles thus prepared have smooth uniform coatings containing chromium, aluminum and silicon and are resistant to attack from sulfur-containing media. Also, the coated articles have excellent oxidation resistance and very good mechanical properties as hereinafter more specifically indicated.

The following Example I will more fully illustrate the present invention.

EXAMPLE I An article in the form of cast disk inch in diameter by inch thick having a composition corresponding to alloy N-l of Table II (13% Cr, 4.5% Mo, Al, 2.5% Cb-t-Ta, bal. Ni) was lightly grit blasted, washed with water and thoroughly dried. A layer about two inches thick of the preferred charge material shown in Table I (5% aluminum, 15% chromium, 79.4% silicon carbide, 0.4% ammonium bromide, 0.2% aluminum chloride) was placed in the bottom of a retort of the type disclosed in U.S. Patent 3,079,276 and the article to be coated was placed on this layer and completely covered with additional charge. The retort was then covered and sealed with a fusible silicate material and the retort and charge were heated to 2000 F. During heating of the charge to temperature, air was driven from the retort and passed out through the fusible silicate seal. The retort and charge were maintained at 2000 F. for 7 hours after which they were cooled to 200 F. and the metal article removed, cleaned with a bristle brush and washed with dilute ammonium hydroxide solution.

The resultant article had a smooth, uniform chromiumaluminum-silicon coating about 3 mils thick.

The above procedure was repeated with articles in the form of castings and wrought articles having compositions corresponding to N6, C-4 and C-1 as shown in Table 1. In all instances, the articles were provided with a smooth, uniform chromium-aluminum-silicon-containing coating. Coating thickness tended to vary depending upon composition of the specific alloys being coated. For example, on cobalt base alloys there was a 1 to 3 mil coating; on nickel base alloys there was a 2 to 4 mil coating and on iron base alloys there was a 1 to 10 mil coating.

To determine the suitability of the coated articles for industrial applications various tests were conducted as follows:

Resistance to sulfur attack The article to be tested was suspended about half immersed for one hour at 1650 F. in a molten salt solution of 99.5% sodium sulfate, 0.5% sodium chloride. The ex- 4 tent of sulfidation attack on the test article was recorded as weight change in milligrams per square centimeter. Table III shows the comparative results obtained for coated and uncoated specimens.

1 Severely attacked, resulting in an adherent layer of corrosion product 60 Oxidation test The article to be tested was mounted on a hub which was rotated at 1725 r.p.m. and which traveled between a furnace (2100 F.) and a water spray quenching station at preset intervals. Sixty seconds exposure in the furnace and 90 seconds cooling in the water spray constituted one cycle and the test specimens were subjected to different numbers of cycles as shown in Table IV. Oxidation resistance was measured and recorded as average depth of intergranular attack and weight loss.

Alloy C-4, an excellent high temperature engineering material, but which is generally not employed in oxidizing environments, was additionally tested by exposure to flowing air at 2000 F. for extended periods. The results, in terms of weight change are shown in Table IVa.

TABLE IV.-*OXIDATION TESTS AT 300 CYCLES Average Average Weight Depth of Alloy Condition Loss in Intcrgrauular Milligrams Attack of Base Metal 52 3 mils. 36 Nil. 169 3 mils.

53 Nil.

UncoatetL 1, 371 Over 3 mils. Coated. 47 Nil.

Uncoatcd. 65 3 mils.

.l Coated. +31 (gain) Nil.

TABLE IVa.OXIDATION TESTS AT 2,000 F.

Weight Change in rug/cm.

Alloy Condition 4 8 16 32 6 1 100 hrs hrs. hrs. hrs. hrs. hrs.

C 4 Coated +.6 +1.2 +1.8 +2.1 +1.2 .3 C-4 Uncoatetlqi. +1.2 3.99 Severe oxidation Mechanical properties Tensile strength and stress rupture tests were conducted following standard techniques and the results are tabulated in Tables V and VI. While it is generally considered that the mechanical properties of alloys are substantially lessened by diffusion coating processing, it can be seen that articles coated by the present invention have very high strength.

TABLE V.-SHORT-TIME TENSILE DATA [Articles coated by the process of this invention (average of 3 tests, as-cast specimens)] Ultimate Yield Reduc- Temp., Tensile Strength Elongation in Alloy Condition F. Strength, at 0.2% tion, Area,

p.s.i. Offset, Percent Percent p.s.i.

Room 147, 200 121, 600 9 8 1, 300 132, 600 102, 8 9 Room 123, 100 106, 800 4 4 1, 500 114, 500 88, 200 10 16 Room 102, 900 66, 400 10 12 Room 113, 300 74, 700 3 4 1, 050 87, 800 47, 800 l9 l8 1, 050 98, 600 43, 500 0 9 C-4 Coated Room 103, 800 82, 100 4 5 TABLE VI.STRESS-RUPTURE DATA Alloy Test 'Iem- Stress, p.s.i. Life, Hours Elongation,

perature, F. Percent Fatigue A Krouse Rotating Beam Testing Machine was used under standard operating conditions for this test. The number of repeated reversed bending stresses to which the specimen was subjected at high speed until failure were measured and recorded. The average results are shown in Table VII.

TABLE VII Alloy N-l Stress p.s.i 25,000 Rotating speed, r.p.m. 8,000 Average cycles to failure:

Coated this invention 1 300,000,000 Uncoated as cast 11,540,150

1 Testing discontinued, no failures.

To further demonstrate the improved industrial utility of the present invention, alloys N-l, N-6 and C-4 in the form of cast disks inch in diameter by A inch thick were coated following the procedure of copending patent application 317,833, now U.S. Patent No. 3,325,305 referred to herein as PC1. This coating procedure comprised embedding the article in a mixture of 49.5 percent prealloyed 70% Fe30% Al material, 49.5 percent silicon carbide, 0.8 percent ammonium bromide and 0.2 percent aluminum chloride. The prealloyed material was prepared as described in U.S. Patent No. 3,079,276 and was particulated to about minus 20 mesh.

The silicon carbide was also particulated to about minus 60 mesh. The two halide carrier materials which were used were commercially available CP grade chemicals particulated to about minus 100 mesh and finer. The four ingredients were mixed in a twin-cone-type blender for about one hour. The alloy article was lightly grit blasted prior to embedding in the mixture and placed in a retort as described in US. Patent No. 3,079,276 in such a manner that it was completely embedded in the above-mentioned mixture.

The retort was then placed in a furnace and heated to about 1700 F. for 18 hours with the exclusion of air. After the heating step, the retort was permitted to cool to below about 200 F. and the article was removed from the retort and brushed with a soft bristile brush to remove any adhering particles of the mixture. The coated article was then washed in a dilute ammonium hydroxide solution and rinsed in clear water and dried.

Articles produced in the foregoing manner were then tested for resistance to sulfur attack in the same manner as before described, i.e., suspended half-immersed for one hour at 1650 F. in a molten salt solution of 99.5% sodium sulfate, 0.5% sodium chloride. The results of the test together with data from Table III relating to coated articles of the present invention are shown in Table VIII. A comparison of the data of Table VIII shows the superiority of articles of the present invention with respect to sulfidation attack.

TABLE VIII.SULFIDATION ATTACK ON SELECTED SPE CIMEN S Weight Change in mg./cm.

Coated by the Alloy No. Base Metal Process of- Uncoated Specimens PC-l This Invention 1 Severely attacked resulting in adherent layer of corrosion product as shown in Figure 4.

In connection with Table VIII reference is taken to the photographs of the drawing wherein FIGURE 1 shows an uncoated N-1 alloy specimen and FIGURE 4 shows the same specimen after being subjected to the previously described sulfidation attack test. The dark grey area 10 in FIGURE 4 indicates the corrosion product formed on the uncoated metal and the light grey area 11 indicates sub-surface'attack. FIGURE 2 shows a specimen of N1 alloy coated according to the PC1 technique and FIG- URE 5 shows a similarly coated specimen after being subjected to the described sulfidation attack test. The numeral 12 in FIGURE 2 indicates the original coating depth and 12 in FIGURE 5 indicates the coating remaining after the sulfidation attack test. FIGURE 3 shows a specimen of N-l alloy coated by the procedure of Example I of this invention and FIGURE 6 shows a similarly coated specimen after being subjected to the same sulfidation attack test. The numeral 14 in FIGURE 3 indicates the original coating depth and 14 in FIGURE 6 shows the coating after the test.

As can be seen, from Table VIII and the photographs of FIGURES 1 through 6, the material coated in accordance with the present invention provides very superior resistance to sulfidation attack. Also, as previously noted and as shown in Tables IV and Wu, articles coated in accordance with the present invention are provided with excellent oxidation resistance and Tables V and VI show that articles coated in accordance with the present invention have very good mechanical properties. Table VII additionally clearly shows that coated articles in accordance with the present invention have superior resistance to fatigue.

With particular regard to the practice of the process of the present invention it has been found that the composition ranges for the charge mixture as set forth in Table I are critical in that variations from these proportions result in unsuitable coatings. It was found that a ratio of about 3 to 1, chromium to aluminum, is preferred for optimum results. Elemental chromium (at least 99% Cr) in the form of fine powder preferably about minus 200 mesh, and elemental aluminum (at least 99% Al) in the form of fine powder, preferably about minus 200 mesh, as available commercially, are required instead of prealloyed charge materials, in order to obtain smooth uniform coatings. Also, the use of inert fillers and as A1 0 clays and silica in place of silicon carbide is not suitable in the practice of the present invention and such materials are to be avoided since they cause the formation of rough surfaces and caking of the charge. Further, silicon carbide is required as a charge constituent in the present process so that the coating produced will contain silicon, ranging from about 0.2 to about 1%, which has been found to improve the properties of the coating by stabilizing the same and thus retarding diffusion of the coating into the substrate metal during subsequent high temperature use. Particulated commercially pure silicon carbide, preferably minus 200 mesh, is available commercially.

Table IX shows, by way of specific examples, the criticality of the processing conditions of the present invention.

TABLE IX.--DIFFUSION COATING TESTING RESULTS Compounds as Carrier Agen ts Charge Material Mixture, Weight Percent Containing Halide NiAI FeCr

Heating Remarks Time, Temper-a A110 1 Hours i ture, F.

18 1. 850 Rough surface; mixture caked.

30 18 1, 8') .i 0. 35 18 1. 850 Rough surface; adherent particles. 32 1?; 1. 850 Rough surface; mixture cakes. 33 18 i 1.850 Rough surface. 17 1, 750 Mixture coating caked severely. 17 1, 750 Mixture caked; rough coating surface;

hardened. 43 16 1,900 Mixture caked; rough coating surface.

7 2. 000 N0 caking; smooth coating surface. 16 1, 900 No coking; smooth surface. 16 1. 900 Do. 1? 1, 750 DO.

A further advantage of the present invention is that the process operating temperatures coincide with the temperatnre generally required in the heat treatment of cohalt-base alloys and it has been found that the heat treatment and diffusion coating can be performed simultaneously.

While the foregoing specification has been directed to the difi'usion coating of nickel base, cobalt base and iron base alloys, other metallic substrates which are capable of being diffusion coated at temperatures of l8502l00 F. can be processed in the manner disclosed. For example, cast and wrought articles have been successfully coated. Further, copper may be coated with the process of this invention at a temperature between 1150 and 1450 F. for 2 to 6 hours.

Mesh sizes referred to herein are United States Sieve Series.

What is claimed is:

1. A method for diiiusion coating metallic articles comprising:

(1) embedding the article to be coated in a particulated charge material consisting essentially of about 4 to 10 percent elemental aluminum, about 10 to 30 percent elemental chromium, about 58 to 86 percent silicon carbide and about 0.2 to 1 percent of a halide carrier material, and

(2) heating the charge material and embedded article with the exclusion of air at an elevated temperature for a time sufficient to form an aluminum-chromiumsilicon-containing coating.

2. A method for diffusion coating metallic articles comprising:

(1) embedding the article to be coated in a particulated charge material consisting essentially of 4 to 8 percent elemental aluminum, 10 to 20 percent elemental chromium, 71 to 85 percent silicon carbide and 0.2 to 0.5 percent of a halide carrier material.

(2) heating the charge material and embedded article with the exclusion of air at a temperature between 1850 F. and 2100 F. for about 5 to 20 hours to form an aluminurn-chrornium-silicon-containing coat- 3-. A method for diffusion coating metallic articles comprising:

(1) embedding the article to be coated in a particulated charge material consisting essentially of about 5 percent elemontal aluminum, about 15 percent elemental chromium, about 79 percent silicon carbide and about 0.6 percent of a halide carrier material (2) heating the charge material and embedded article with the exclusion of air at a temperature of about 2000 F. for about 7 hours to form an aluminum chromium-silicon-containing coating.

4. A metallic article having a protective aluminum chromium-silicon-containing coating formed by heating the articie with the exclusion of air at a temperature between about 1850 F. and 2150 F. in a particulatcd charge material consisting essentially of 4 to 10 percent elemental aluminum, 10 to 30 percent elemental chromium, about 58 to 86 percent silicon carbide and 0.2 to 1 percent of a halide carrier material.

References Cited UNITED STATES PATENTS 3,095,316 6/1963 Hartwig 117--107.2 3,254,969 6/1966 Bungardt ll7-107.2 3,257,230 6/1966 Wachtell et a1. 117107.2 3,298,858 1/1967 Ashikari 117107.2

ALFRED L. LEAVITT, Primary Examiner.

A. GOLIAN, Assistant Examiner.

Claims (1)

1. METHOD FOR DIFFUSION COATING METALLIC ARTICLES COMPRISING: (1) EMBEDDING THE ARTICLE TO BE COATED IN A PARTICULATED CHARGE MATERIAL CONSISTING ESSENTALLY OF ABOUT 4 TO 10 PERCENT ELEMENTAL ALUMINUM, ABOUT 10 TO 30 PERCENT ELEMENTAL CHROMIUM, ABOUT 58 TO 86 PERCENT SILICON CARBIDE AND ABOUT 0.2 TO 1 PERCENT OF A HALIDE CARRIER MATERIAL, AND (2) HEATING THE CHARGE MATERIAL AND EMBEDDED ARTICLE WITH THE EXCLUSION OF AIR AT AN ELEVATED TEMPERATURE FOR A TIME SUFFICIENT TO FORM AN ALUMINUM-CHROMIUMSILICON-CONTAINING COATING.

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