US2709154A - Corrosion resisting coatings - Google Patents

Description

May 24; 1955 fqs m v2,799,154

CORROSION RESISTING COATINGS Filed Dec. 8, 1949 INVENTOR.

Execurmx 4 ATTORNEY United States Patent coRRosroN RESISTING COATINGS Fritz J. Hansgirg, deceased, late of Yonkers, N. 1., by Josephine Maria Hansgirg, executrix, Yonkers, N. Y., assignor to American Electra Metal Corporation, a corporation of Maryland Application December 8, 1949, Serial No. 131,704

2 Claims. (Cl. 204-39) This application is a continuation-in-part of the abancloned application Serial No. 18,885, filed April 5, 1948, by Fritz J. Hansgirg, deceased.

This invention relates to a method for producing by electrodeposition a continuous, and dense corrosionresisting protective coating on a shape-retaining body of hot strength refractory metal, thereby making it possible to use such body within hot oxidizing atmospheres in the interior of an internal combustion engine.

Protective coatings produced by the method of the invention are useful for blades, vanes, nozzles, buckets and turbine wheels of gas turbines; for cylinders, tubes and nozzles of jet propulsion devices; for valves of internal combustion engines and for other purposes where operation temperatures up to about 700 C. and higher under corroding conditions within an oxidizing atmosphere are involved.

Refractory metals such as tungsten and molybdenum, and tungsten and molybdenum alloys have high melting points, and high hot strength properties. However, such refractory metals form volatile oxides when exposed to oxidizing atmospheres at elevated temperatures within the range of 700 C. and higher. For example, molybdenum oxidizes to form a trioxide which sublimes at 795 C. Tungsten oxide volatilizes at somewhat higher temperatures. Therefore, in spite of the refractory character and hot mechanical strength of shaped bodies made of tungsten and molybdenum, they soon deteriorate when operating within a high temperature oxidizing atmosphere.

In accordance with the invention, shape-retaining refractory bodies, such as turbine buckets of molybdenum, tungsten, alloys of molybdenum and tungsten, molybdenum base alloys and tungsten base alloys, are provided with a corrosion resisting coating by immersing the body within a liquid electrolyte bath containing aluminum, and electrodepositing aluminum from the bath on the exterior of the body while maintaining the bath and the body at a high temperature in the range of about 900 to 1100 C. and thereby causing the electrodeposited aluminum to combine with the surface metal of the body into a substantially, dense oxygen-impervious protection coating, including at least one substantially continuous, oxygen-impervious coating layer consisting of an intermetallic compound between the electrodeposited aluminum, and the refractory metal of the body.

it is, therefore, an object of the invention to provide by an economical, simple electrodeposition process a continuous, dense, corrosion-resisting protective coating on a shaped article of hot-strength refractory metal, such as molybdenum, tungsten, molybdenum and tungsten alloys, molybdenum base alloys, and tungsten base alloys, thereby making it possible to operate such corrosion-resisting hot strength shaped bodies within hot oxidizing combustion gases of an internal combustion engine, such as a gas turbine, jet engine, or the like.

The foregoing and further objects of the invention will be apparent from the following description of exemplifications thereof in connection with the accompanying drawings wherein,

Fig. 1 is a vertical cross-sectional view showing in a diagrammatic manner one type of equipment for producing a protective corrosion-resisting coating on a shaped body of refractory metal; and

Fig. 2 is a plan view of the shaped body or article shown in Fig. 1.

Referring to Fig. 1, an aluminum containing electrodeposition bath 24 is held within a graphite crucible 10, shown open at its top. The crucible is embedded in a suitable heat insulating material 11 held within the container 12 of strong heat resistant material, such as ceramic material. The electrolyte 24 is shown arranged to be heated by an induction coil 13, such as copper tubing placed around the container 12. The coil 13 is supplied with the electric heating current of suitable highfrequency from a source connected to the coil terminals 14, 15. A cooling fluid such as water is circulated through the current carrying copper tubing coil 13 to prevent overheating thereof.

The article, or shaped fluid guiding body 16 to be coated is indicated in the form of an air-foil shaped blade or bucket 16 having a root 17 by means of which it is secured within a suitable groove of an engine structure such as the rotor wheel of a gas turbine. The shaped body 16 is suitably held immersed within the bath 24 by an electrically conducting structure indicated in the form of a clamping device 18 provided at the lower end of an electric conductor rod 19 slidably held in a supporting bracket 2% all of electrically conductive material. An electric current lead-in rod 21 of heat-resistant material such as graphite extends through an opening in the container 12, and heat insulation 11 into electric contact engagement with the bottom of the carbon crucible 12, to provide a good current conducting connection to the electrolyte bath 24. Electric insulation (not shown) may be placed around the graphite lead-in rod 21 for insulating it from the heat insulation 11 and container 12.

The graphite leadin rod 21 is shown provided with an external terminal clamp 28 for connecting it through a conductor to the positive terminal 22 of a direct current supply source supplying the electrodeposition current. The other conductor lead-in rod 19 carrying the immersed shaped body 16 is conductively connected through conductor including switch 23 to the negative terminal 29 of the direct current source.

The electrolyte bath is of such composition as to make it possible to electrolytically deposit aluminum on the shaped body 16, while maintaining the electrolyte bath 24 at the proper elevated temperature between about 900 to 1100 C. without excessive evaporization of the molten electrolyte.

In producing the protective coating by the procedure of the invention, the ingredients for the electrolyte bath mixture are molten within the crucible 10 by heating to the desired temperature in the range between about 900 and 1100 C. The article or shaped body 16 is first cleaned to remove surface impurities and oxide films, and the article is then lowered and immersed into the molten electrolyte 24 within vessel 10. With the two lead-in conductors 19 and 21 connected to the negative and positive terminal of the direct current source as shown in Fig. l, the electrodeposition current will flow through the bath upon closing of the switch 23, with the shaped body 16 operating as the cathode and the bath 24 with the crucible 10 as the anode.

With the container or crucible 10 connected through the circuit as the anode, and the article or shaped body arca e;

16 as the cathode, the aluminum contained in the molten electrolyte will move toward the shaped body 16 as the cathode, where they discharge as a deposit of molten aluminum. Since the temperature of the electrolyte is maintained at a desired high level, the deposited molten aluminum combines with the surface metal of the body into a protective coating compound including a continuous intermetallic compound layer between the aluminum and the refractory metal of the shaped body, which protective coating forms a very effective corrosion-resisting protection for the body, and prevents access of oxygen thereto.

A highly effective procedure of the invention for providing shaped bodies of molybdenum and molybdenum base alloys with a protective coating that resists corrosion for prolonged periods of operation Within high temperature oxidizing combustion gases, for instance, within a gas turbine, is to carry on the electrodeposition or" aluminum on the surface of the body at a tempearture of about 1000 C. with a high current density of to 20 amperes per square inch for a short time, such as l to 20 minutes.

The procedures of the invention described herein involving placing a shape-retaining body, such as a turbine bucket, of molybdenum and molybdenum base alloys within an aluminum containing electrolyte bath maintained at a high temperature, such as about 1000 C. and electrolytically depositing aluminum on the exterior of the body causes the aluminum to combine with the underlying body metal into a protective coating containing a continuous dense, oxygen-impervious compound between the metal of the body and the aluminum, which compound is an intermetallic compound of said metals.

The protective coating produced by the procedure of the invention contains the intermetallic compound layer etween the metal of the body and the aluminum coating metal, and a superposed exterior layer which is rich in aluminum and such coating has properties differing from those of the properties of the molybdenum base metal and the aluminum coating metal, and these protective coating layers are not soluble in mineral acids. Accordingly, surplus of free aluminum remaining on the exterior of the protective coating formed by the process of the invention, may be removed by simply dissolving it with dilute hydrochloric, sulphuric, or nitric acid. The protective coating layers so formed on the exterior of the refractory base metal body will not corrode when subjected to high temperature oxidizing combustion gases of 800 to 900 C for long periods of operation.

A satisfactory procedure for producing in accordance with the invention a corrosion-resisting protective coating on the exterior of a shape-retaining body, such as a i turbine bucket, of molybdenum or molybdenum base alloys, is as follows: The shaped molybdenum body is cleaned by dipping in a bath containing about 90% sodium hydroxide and 10% sodium nitrite at a bath temperature of about 230 to 270 C. The so-cleaned washed body is then Washed in slightly acidified hot water. The washed body is then placed within a plating bath of cryolite and sodium fluoride containing 70 mol percent AlFa (calculated), the bath being held within a carbon crucible or container. The deposition bath within the container is maintained at about 980 C. and electrolysis is performed with a direct current of about 10 amperes per square inch surface area of the shaped body from the bath toward the body. The deposition is carried on for 10 to minutes with the shaped body or bodies as the negative electrode and the crucible as the positive electrode. After removal from the deposition bath, the shaped body is placed in a solution of hydrochloric acid to remove any adherent salts and excess of free aluminum thereon, and the shaped body is then sand-blasted. To assure a perfect corrosion-resistant protective coating on the entire exterior of the shaped body, it is again placed in the electrolyte bath, and the electrodeposition of aluminum thereon is carried on for another 10 to 15 minutes with the same current density, and at the same bath temperature. Following this second electrodeposition treatment, the shaped body is again treated with a 20% solution of hydrochloric acid to remove adhering salts and any excess of free aluminum remaining thereon, whereupon the shaped body such as a turbine bucket, is ready for assembly, in the gas turbine in which it is to be used.

X-ray diffraction analysis of the protective coating formed by the procedure of the invention described above on a shaped molybdenum body, shows that it does not have any characteristic refraction lines corresponding to either aluminum or molybdenum, and thus establishes that it constitutes an intermetallic compound of the two metals. Instead of using an aluminum-fluoride electrolyte bath for producing a protective coating by the procedure of the invention, there may be used instead another high temperature electrolyte which is liquid between 970 and 990 C.

Another suitable electrolyte bath for practicing the invention is one consisting of potassium, sodium, and aluminum fluoride, which may be maintained in a molten state at a temperature between 900 to 1000 C., without excessive evaporization.

Another suitable electrolyte bath for practicing the invention, is one consisting of aluminum fluoride and barium fluoride heated to keep it in a molten state in the range between 900 and 1000 C.

Other suitable electrolyte baths for practicing the invention is a eutectic mixture of cryolite and chiolite having a melting point at 1000 C.

A further suitable electrolyte for practicing the invention, is one consisting of about 63% aluminum fluoride and 27% sodium fluoride to which about 10 to 12% aluminum oxide are added, such mixture melting between 800 to l000 C. and forming at such temperature a clear electrolyte from which the aluminum is precipitated through electrolysis on the body 16 operating as a cathode with the aluminum oxide being dissolved at the anode.

1f the dissolved aluminum oxide accumulates at the level of the melt in contact with the air (if no protective atmosphere is used), the aluminum oxide is moved into the melt by the stirring action caused in the melt by the high-frequency induction heating terminals. An-

H other way for removing surplus of the aluminum coating metal from the exterior of the protective coating is to pass a reverse current through the bath 24 with the shaped body 16 as anode, and the molten electrolyte bath 24, and crucible 10 as the cathode. The current density of the reversed current can be regulated by adjusting the tap 27 on the resistor 26. In this manner surplus aluminum which has not been combined with the surface metal of the shaped body is redissolved in the electrolyte bath 24. Thereupon switch 30 is opened, and the shaped body 16 is removed from the electrolyte with the desired protective coating formed thereon.

By procedures of the invention similar to those described above, shaped bodies of tungsten, molybdenumtungsten alloys and tungsten base alloys containing or more of the molybdenum or tungsten base, may be provided with a corrosion resistant protective coating. Suitable base alloys of molybdenum and tungsten are those containing as balance titanium, zirconium, chromium and boron.

The novel principles of the invention described above will suggest various modifications thereof. It should, therefore, be understood that the invention is not limited to any of the exemplifications hereinbefore described.

What is clairned is:

l. The method of providing a shape-retaining body of a base metal selected from the group consisting of molybdenum, tungsten, molybdenum base alloys and tungsten base alloys with a protective coating which will remain substantially free of corrosion when exposed to high temperature combustion gases within the interior of a combustion engine which method comprises placing the body in a molten electrolyte bath containing aluminum compounds and maintained at a high temperature between about 900 C. and 1100 C., pass ing a high current density electric current from an anode in said bath to said body as a cathode, held heated in said bath at said high temperature, and depositing aluminum on the exterior of said body and thereby causing the deposited aluminum to combine with the base metal of the body into a protective coating containing at least one continuous, dense, oxygen-impervious intermetallic compound layer between the base metal and aluminum, and removing substantially all free aluminum remaining on the exterior of said body.

2. The method of providing a shape-retaining body of a base metal selected from the group consisting of molybdenum, tungsten, molybdenum base alloys and tungsten base alloys with a protective coating which will remain substantially free of corrosion when exposed to high temperature combustion gases within the interior of a combustion engine which method comprises placing the body in a molten electrolyte bath containing aluminum compounds and maintained at a high temperature between about 900 C. and 1100 C., passing a high current density electric current from an anode in said bath to said body as a cathode, held heated in said bath at said high temperature, and depositing aluminum on the exterior of said body and thereby causing the deposited aluminum to combine with the base metal of the body into a protective coating containing at least one continuous, dense, oxygen-impervious intermetallic compound layer between the base metal and aluminum, and continuing the electrodeposition of aluminum on the exterior of said body until the intermetallic compound layer of the protective coating on the exterior of said body has reached the thickness of at least two to five microns, and removing substantially all free aluminum remaining on the exterior of said body.

References Cited in the file of this patent UNITED STATES PATENTS l,488,553 Peacock Apr. 1, 1924 1,562,164 Harris Nov. 17, 1925 1,933,319 Driggs et a1. 'Oct. 31, 1933 2,082,622 Fink June 1, 1937 2,109,887 Mattasotti Mar. 1, 1938 2,497,119 Fink Feb. 14, 1950 2,555,372 Ramage June 5, 1951 FOREIGN PATENTS 876,759 France Aug. 17, 1942

Claims (1)

1. THE METHOD OF PROVIDING A SHAPE-RETAINING BODY OF A BASE METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, TUNGSTEN, MOLYBDENUM BASE ALLOYS AND TUNGSTEN BASE ALLOYS WITH A PROTECTIVE COATING WHICH WILL REMAIN SUBSTANTIALLY FREE OF CORROSION WHEN EXPOSED TO HIGH TEMPERATURE COMBUSTION GASES WITHIN THE INTERIOR OF A COMBUSTION ENGINE WHICH METHOD COMPRISES PLACING THE BODY IN A MOLTEN ELECTROLYTE BATH CONTAINING ALUMINUM COMPOUNDS AND MAINTAINED AT A HIGH TEMPERATURE BETWEEN ABOUT 900* C. AND 1100* C., PASSING A HIGH CURRENT DENSITY ELECTRIC CURRENT FROM AN ANODE IN SAID BATH TO SAID BODY AS A CATHODE, HELD HEATED IN SAID BATH AT SAID HIGH TEMPERATURE, AND DEPOSITING ALUMINUM ON THE EXTERIOR OF SAID BODY AND THEREBY CAUSING THE DEPOSITED ALUMINUM TO COMBINE WITH THE BASE METAL OF THE BODY INTO A PROTECTIVE COATING CONTAINING AT LEAST ONE CONTINUOUS, DENSE, OXYGEN-IMPERVIOUS INTERMETALLIC COMPOUND LAYER BETWEEN THE BASE METAL AND ALUMINUM, AND THE REMOVING SUBSTANTIALLY ALL FREE ALUMINUM REMAINING ON THE EXTERIOR OF SAID BODY.

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