Nickel Aluminide Coated Low Alloy Stainless Steel

Abstract

Low alloy austenitic stainless steel is protected from oxidative corrosion and erosion at high temperatures in the presence of exhaust gases from internal combustion engines by flame-spraying a layer between about 0.003 inches and about 0.007 inches thick of an alloy of 80 percent nickel and 20 percent chromium on the steel base, and thereafter converting the layer to an intermetallic chromium containing nickel aluminide coating by a low temperature aluminum cementation process. A cementation pack comprising 96 percent aluminum oxide, 2 percent aluminum, 1 percent sodium chloride, and 1 percent ammonium bifluoride is placed on the nickel-chromium layer in an inert atmosphere for a period between four hours and 24 hours at 1400.degree.F to 1600.degree.F to form an intermetallic chromium containing nickel aluminide coating containing 40 to 60 atomic percent aluminum. The coated steel is highly resistant to oxidation corrosion and erosion by high temperature exhaust gases from internal combustion engines. It is particularly useful in the construction of thermal reactors for exhaust systems in automobiles for the control of pollution from exhausts.

Description

ORIGIN OF THE INVENTION

The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to a coating for low alloy austenitic stainless steel to render the steel highly resistant to oxidation corrosion and erosion from exhaust gases in internal combustion engines.

The engine exhaust of automotive internal combustion engines contains carbon monoxide, carbon dioxide, nitrogen dioxide, unburned hydrocarbons, sulfur compounds, and other corrosive products which are exhausted into the atmosphere in the absence of means for controlling such emissions. Recently federal regulations have been imposed which set progressively lower allowable engine exhaust levels for such combustion products. Minor adjustments in carburetion and spark control and the like are no longer satisfactory in meeting such requirements. One way such engine emissions can be reduced further in the future in order to meet the requirements of controlling agencies is by the utilization of thermal exhaust converters or reactors. Such thermal reactors replace the exhaust manifold.

In some reactors, secondary air is introduced into the reactor in order to promote the complete combustion of unburned hydrocarbons and carbon monoxide. In order to achieve the desired ends, many components of the thermal reactor must operate at temperatures on the order of 1900.degree.F with the possibility that if the ignition means fails, the temperature may increase up to 2300.degree.F.

It is a practical requirement of the automobile industry that such reactors have useful lives of 2000 hours or more, which translates to 50,000 miles of driving at an average speed of 25 miles per hour. Under such stringent conditions of severe temperatures, exhaust atmosphere, and term of life requirements, the oxidation corrosion and erosion as well as the chemical attack by lead and sulfur compounds on unprotected steel components are very significant.

Since such thermal exhaust reactors are required in large quantities, for example, up to fifteen million units a year at the present automotive industry production rate, it is essential that such thermal reactors be capable of being produced at the minimum possible cost. One approach to avoid utilization of expensive components is to use protective coatings on the metal materials used in such thermal reactors.

DESCRIPTION OF THE PRIOR ART

It is known, of course, that materials such as extremely high alloy steels are resistant to oxidation corrosion and erosion and thus fulfill the technical requirements for materials in such thermal exhaust reactors. The use of high alloy steels would be extremely expensive for such applications, however, because of the vast amounts of metal such as nickel that are required. It is known that low alloy steels possess moderately high temperature strengths but that they are not resistant to degradation by the environmental conditions in the thermal exhaust reactors.

In U.S. Pat. No. 3,481,715, Whalen et al., there is disclosed a sealing member which is moderately resistant to oxidation at high temperatures in which the steel base is coated with an intermediate layer of an alloy of nickel with aluminum or chromium applied by a plasma gun, to which a surface layer of material comprising nickel oxide and calcium fluoride is applied using a plasma gun. The nickel with aluminum or chromium layer serves to improve the adhesion of the oxide surface layer.

U.S. Pat. No. 3,338,733, Rowady, discloses metallic surface coatings resistant to corrosion and wear such as that to which internal combustion valves are exposed. In this case a steel base is first preheated to 1300.degree. to 1800.degree.F and a nickel-chromium alloy is applied by flame-spraying. The coated object is then subjected to a first diffusion step at temperatures of 1800.degree. to 2250.degree.F to promote alloying between the coated alloy and the base metal. The coating subsequently may be coated with an aluminum layer by first preheating the coated base to 1200.degree. to 1400.degree.F and then thereafter dipping the article in a molten bath of aluminum or applying the aluminum by a flame-spraying technique. After the aluminum coating has been applied, the object is then heated to 1500.degree.F to 2100.degree.F to diffuse and alloy the aluminum into the nickel-chromium alloy.

SUMMARY OF THE INVENTION

The present invention provides a low alloy austenitic stainless steel article highly resistant to oxidation corrosion and erosion by a coating which comprises a chromium containing nickel aluminide applied by the process of this invention. Low alloy stainless steel material coated according to this invention exhibits oxidation resistance equal to or better than the oxidation resistance obtained on many highly alloyed steels either uncoated, or coated with expensive alloys and other compounds, many applied by complex processes.

A relatively low cost stock article suitable for use in constructing thermal exhaust reactors is provided by this invention by using low alloy austenitic stainless steel. It provides resistance to corrosion using steels having only a relatively small amount of nickel.

According to this invention, an oxidation corrosion and erosion resistant article is provided by depositing on a low nickel alloy austenitic stainless steel base a layer between about 0.003 to 0.007 inch, preferably 0.005 inch, thick of a nickel-chromium alloy such as 80 percent nickel-20 percent chromium, by means of flame-spraying or plasma spraying, then converting the nickel-chromium alloy layer to one comprising an intermetallic compound, as opposed to an alloy, of chromium-containing nickel aluminide by a low temperature aluminum pack cementation process at 1400.degree. - 1600.degree.F for four to 24 hours which in a single process step also serves to diffusion bond the coating to the steel substrate. The chromium-containing nickel aluminide coating contains from about 40 to about 60 atomic percent of aluminum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Test specimens of austenitic 304 stainless steel (Fe-18Cr-8Ni-0.008C) were cut from 1/16 inch thick sheet stock. One set were 3/4 inch .times. 11/2 inches, and another were 1 inch .times. 2 inches. Prior to coating according to this invention the specimens were ball milled in a ceramic mill with no. 36 alumina grit and water and thereafter vapor blasted with 100 mesh alumina, degreased, and then ultrasonically cleaned in trichloroethylene.

The samples are then coated with a 0.005 inch layer of 80 percent nickel-20 percent chromium by flame-spraying, using a conventional spray gun adapted for spraying nickel-chromium alloy fed in wire form. The thickness of the layer can be between 0.003 and 0.007 inches. Layers having thicknesses at the lower end of the range have less stresses while the thicker layers have longer lives.

The nickel-chromium layer is then converted to a chromium containing nickel aluminide intermetallic compound by a pack cementation step by treating the layer with a high activity aluminum pack. The pack employed comprises 96 percent aluminum oxide, 2 percent aluminum, 1 percent sodium chloride, and 1 percent ammonium bifluoride. The coating is treated in a furnace under an inert atmosphere such as argon for from four to 24 hours, preferably for 16 hours, at 1600.degree.F. The process deposits approximately 15 mg. of aluminum per square centimeter which reacts to form a chromium enriched nickel aluminide intermetallic protective layer on the low alloy austenitic stainless steel and at the same time forms a diffusion bond between the coating and the substrate.

Essentially no diffusion bonding takes place during the flame-spraying application of the nickel-chromium layer. The bond between the alloy layer and the steel base at this stage is itself primarily a mechanical bond. Following the aluminum cementation, the chromium-nickel alloy layer has been converted to a chromium-containing nickel aluminide intermetallic compound coating which is diffusion bonded to the steel substrate.

The 3/4 inch .times. 11/2 inches samples are then placed in a horizontal multitube furnace on high purity recrystallized alumina boats. Air is passed over the specimens at the rate of two cubic feet per hour. One set of specimens is heated for 100 hours at 2000.degree.F, with cycling to room temperature. The cycles consist of 20 hours at test temperature followed by cooling to room temperature. At room temperature the specimens are lightly brushed to remove any oxide scale and weighed to 0.05 mg. per square centimeter accuracy. A second set of specimens is similarly cycled for 600 hours at 1800.degree.F. Excellent resistance to oxidation corrosion and erosion was evident from visual examination, x-ray diffraction, microscopic and metallographic analysis. Furthermore, there was no loss of weight of the samples, but a slight weight gain, for example, the specimens gain about 3 mg. per square centimeter after 600 hours exposure to the 1800.degree.F cycling tests. During all of the tests, the metal temperature is measured by thermocouples which are mounted on adjacent uncoated specimens.

The 1 inch .times. 2 inches coated specimens are tested in an experimental automotive thermal reactor connected to a high performance 472 cubic inch V-8 automobile engine with air injection and carburetion. Leaded gasoline is used as a fuel and the engine is operated to provide test specimen cycles of 10 minutes at 1900.degree.F, 5 minutes at 1200.degree.F, and repeat. After 170 cycles, the specimens exhibit no significant deterioration and gain approximately 0.1 gram. All coatings after testing exhibit excellent weight stability, coating preservation and metallographic stability. The specimens when examined visually appear almost unchanged from the as coated condition.

The coatings obtained according to the process of the present invention impart excellent high temperature corrosion and erosion resistance to low alloy austenitic stainless steel. The protection of the low alloy austenitic stainless steel is achieved by a process which involves primarily only two steps as opposed to other methods of applying different oxidation resistant coatings which are often quite complex. The invention is particularly advantageous in providing an article suitable for utilization in automotive thermal reactors which are necessary to reduce exhaust pollution in automobiles. Significantly the present invention provides an excellent material for such reactors at relatively low cost and a low consumption of expensive nickel metal. By way of illustration, in order to provide 15 million thermal exhaust reactors annually utilizing a high alloy stainless steel such as aluminum coated Inco 800 containing 32 percent nickel, about 15 percent of the total nickel consumed annually in the United States would be required, whereas utilizing material according to the present invention about 7 percent of the nickel consumed annually would be required. This is based upon the use of 41/2 square feet of material 1.5 mm. thick for a typical thermal core reactor.

While the invention has been explained by a detailed description of a specific embodiment, it is understood that various modifications and substitutions can be made within the scope of the appended claims which are intended to include equivalents of such embodiments.

Claims

What is claimed is:

1. An article of manufacture resistant to oxidative corrosion and erosion at high temperatures comprising an austenitic stainless steel base having a nickel-chromium alloy coating with a thickness of about 0.005 inch forming a diffused modified bonded junction with the stainless steel base, said alloy coating having aluminum diffused thereinto to form nickel aluminide which increases from the base surface to the outer surface so that said outer surface comprises substantially all nickel aluminide and chromium.

2. An article of manufacture as claimed in claim 1 wherein said coating is produced by applying aluminum by means of low temperature pack cementation to a flame-sprayed layer between about 0.003 inch and about 0.007 inch thick of 80 percent nickel-20 percent chromium alloy on said base, said aluminum reacting with said layer to form said nickel aluminide.

3. An article of manufacture as claimed in claim 2 wherein said austenitic stainless steel is a low nickel alloy stainless steel, and said aluminum is applied from a cementation pack comprising 96 percent aluminum oxide, 2 percent aluminum, 1 percent sodium chloride, and 1 percent ammonium bifluoride.

4. An article of manufacture as claimed in claim 2 wherein said aluminum is applied in an inert atmosphere at about 1400.degree. to 1600.degree.F.

5. An austenitic stainless steel substrate having a flame sprayed alloy coating of 80 percent nickel and 20 percent chromium with a minimum thickness of 0.003 inch, said alloy coating having aluminum diffused thereinto to form nickel aluminide which increases from the stainless steel surface to the outer surface thereof so that said outer surface comprises substantially all nickel aluminide and chromium, said coating forming a diffused modified bonded junction with the stainless steel substrate.

6. An article as claimed in claim 5 wherein said flame-sprayed layer is about 0.005 inch thick.