US3046205A - Nickel-aluminum alloy coatings - Google Patents

Description

3,046,205 NICKEL-ALUMINUM ALLOY COATINGS Dwight E. Couch, Washington, D.C., Harold Shapiro,

Hyattsville, and Abner Brenner, Chevy Chase, Md., and

Jean H. Connor, Washington, D.C., assignors to the United States of America as represented by the Secretary of the Navy No Drawing. Fiied July 23, 1959, Ser. No. 829,156 2 Claims. (Cl. 204-37) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention is a continuation-in-part of copending application Serial No. 665,616, filed June 13, 1957, and relates generally to methods of producing a corrosion and oxidation resistant coating that is uniform in thickness, can be produced in any desired thickness and has no free or unalloyed aluminum therein. Such coatings can be produced on metallic surfaces for increasing wear resistance of sliding or reciprocating parts of machines and the like, such as bearing surfaces, pistons, piston rings, cylinder Walls and gun mechanisms. More particularly, the invention pertains to coatings on base metals of nickel alurninum alloys and methods of applying these coatings so that a layer of unalloyed nickel remains between the nickel-aluminum layer and the base.

In extensive use at present on metal surfaces subject to sliding riction are coatings of chromium. This type of coating, while useful in many respects, has been found lacking in uniformity of metal distribution and the metal loses its room-temperature hardness if heated to 1000 C.

Nickel-aluminum alloys are known to have desirable properties for use as hard, strong, corrosion-resistant structural materials or castings.

Objects of the invention, therefore, are to develop methods for controllably depositing aluminum on a nickel surface thereby permitting alloying of the aluminum deposit with the nickel surface whereby unalloyed nickel will remain beneath the layer of nickel aluminide which is formed, to provide an electrolytically deposited metal coating of improved hardness, to provide a coating which will retain its room-temperature hardness after heating up to 1000 C., and cooling, and to provide a coating more corrosion resistant than either nickel, aluminum or chromium which, when applied to a base metal, will be distributed uniformly over the entire base metal surface.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description.

Nickel-aluminum intermetallic compounds have been found to possess many desirable properties over the separate forming metals, among the chief of which are a substantial increase in melting point, stability at high temperatures, good modulus-of-rupture strength, excellent oxidation resistance and hardness. With these desirable properties, however, nickel aluminide, in massive form, is diflicult to fabricate and too brittle for practical applications. To overcome these difiiculties for coating use, the nickel aluminide is formed directly on the base metal surface after the necessary shaping operations have been completed. This is accomplished by electrodeposition process steps and several modes of accomplishing this result will now be described.

In the first method, nickel is deposited on the base metal in the conventional Way, as by electrodeposition from baths containing nickel sulfate or nickel chloride. If the nickel when deposited is severely soiled, degreasing of the article with subsequent pumice scrubbing thereof is required prior to aluminum plating. Unless the nickel is markedly soiled no precleaning is necessary. Aluminum is then deposited on the nickel plate by electrodeposition from a fused salt bath consisting of an aluminum halide and an alkali halide, as, for example, aluminum chloride and sodium chloride in the ratio of upwards of 1.5 moles of aluminum chloride to 1 mole of sodium chloride. Satisfactory performance results from operation at l to 4 amperes/sq. dm. and at a temperature in the range of 200 C. Once prepared the bath must be kept molten, for if allowed to solidify it is very difficult to remelt without breaking the vessel containing the bath. The time of treatment varies depending on the thickness of plating desired but it most commonly varies between 5 minutes and 1 hour. Very good, coherent aluminum deposits up to 0.8 mil thick can be produced by this method using aluminum (2S) anodes. When thicker deposits are desired smooth deposits of aluminum up to 0.002 inch thick can be obtained by using tungsten anodes.

After the aluminum has been deposited on the nickel coat the article is dried and placed in a furnace preferably of the electric type such as a high frequency induction or resistance furnace. Heating may be conducted in air or in an inert atmosphere, such as in helium or nitrogen, at a temperature in the range from 550 to 750 C. for periods up to about 20 hours. At this temperature all the aluminum alloys with the nickel to form a nickel aluminum alloy layer over the unalloyed portion of the nickel layer. This arrangement is accomplished by limiting the deposit of aluminum to that of a layer having a thickness less than the thickness of the nickel layer. Actually, the optimum deposit thicknesses are an aluminum deposit approximately onefourth the thickness of the nickel layer whereby all the aluminum is alloyed with the nickel forming a layer of nickel aluminides exceeding in thickness the prior aluminum deposit but still leaving a portion (about one-half) of the original nickel layer in unalloyed condition.

Actually, it has been found that the oxidizing effect of heatin in air is beneficial in that there is a marked increase in resistance to salt spray corrosion. In case the alloy is formed by heating in an inert atmosphere the oxidation will in any event occur during normal subsequent high temperature applications.

The first method, as above described, is particularly useful Where the thickness of the aluminum deposit is less than 0.001 inch or Where the object being coated might be damaged by heating to too high a temperature. However, as stated above thicker deposits can be obtained by the use of tungsten anodes.

In the second method aluminum is electrically deposited on the nickel coat from a fused salt bath operated at a temperature of about 660 to 1000 C., i.e., above the melting point of aluminum, the alloy being formed simultaneously with the plating operation due to the heat of the bath. A suitable bath is cryolite at a temperature of 1000 C. or cryolite dissolved in alkali halides such as sodium chloride and potassium chloride at a temperature ranging from 660 to 900 C. Using cryolite with the bath temperature at about 1000 C., and a current flow of 50 amperes per square decimeter surface, a flow of 2 to 3 minutes will produce a coat of about 0.0005 inch thickness. A coat thickness of 0.001 to 0.002 inch is produced by a current of 20 to 25 amperes per square decimeter flowing for 15 to 30 minutes. For deposits over 0.002 inch 15 to 20 amperes per square deciineter flowing for about 1 hour are required.

In the operation of the potassium chloride, sodium chloride, cryolite bath although the bath may be operated in the temperature range stated (660900 C.), at the higher temperature the graphite anodes oxidize rather rapidly if an inert atmosphere is not used while at the lower temperature the aluminum diffuses slowly into the nickel. A series of runs has shown that a temperature range of about 700 to 800 C. gives a satisfactory compromise of these factors. When the bath is used in this temperature range, aluminum deposits 0.25 mil thick are produced in minutes at a current density of 7 amperes/sq. dm. To produce thicker deposits of aluminum at a rate not in excess of the rate of diffusion of the aluminum into the nickel the following schedule should be employed in using this bath in the optimum temperature range.

Maximum time permissible with- Approximate Current density, amp/dm. out producing thickness of free aluminum aluminum on the surface, deposit, mils minutes Since the bath used to electrodeposit the minimum is operated at temperatures higher than the tempering temperature of steel, when the base material is steel or a steel alloy, heat treatment becomes necessary to restore the temper of this base material after the nickel aluminide layer has been formed.

Samples to be plated were placed in the bath and allowed to hang there for to seconds to reach bath temperature before the current was turned on. Optimum concentrations for the chloride-cryolite bath are: sodium chloride 440 gms., potassium chloride 560 gms. and cryolite 150 gms. Optimum operating conditions exist at a current density of 2 to 10 amps/sq. dm. and in the temperature range of 700-800 C.

In order to avoid producing excess unalloyed aluminum on the surface it is often advantageous to employ varying current densities. Thus, in case of a chloride-cryolite bath operated at 700 C. deposits of 1 mil thick can be prepared by plating at 7 amp/sq. dm. for 5 minutes and then reducing the current density to 34 amp/sq. dm. and plating for to 40 minutes. Deposits of 1.8 mils can be produced in two hours by the following schedule: 7 amp/sq. dm., 5 minutes; 4 amp/sq. dm., 20 minutes; 2 amp./ sq. dm., 95 minutes. These plating rates can be increased if the bath temperature is raised above 700 C.

An excessive rate of deposition of aluminum is to be avoided since at bath temperature this aluminum is molten and nickel dissolves in molten aluminum. The problem created is that of depletion of nickel from the nickel layer since the nickel goes into solution in the molten aluminum and the aluminum collects into beads which subsequently flow into the bath carrying the dissolved nickel along. This results in a reduction in the amount of nickel available for alloying.

This is in fact one of the basic disadvantages of the hotdip process known in the prior art. As an indication, some 4" x 6" samples were plated with about 0.001 inch of nickel and accurately weighed. These samples were then sent to a commercial firm for the deposition thereon of aluminum by the hot-dip process. All samples actually lost weight as a result of the dipping process as indicated by the following:

Wt. of

Wt. of nickel base coated (gins) base Wt. of Al deposit (gins) The nickel aluminide, as produced by the described methods, has a hardness varying from 700 to 1050 Vickers at room temperature depending on the percentage of aluminum in the given nickel aluminide. This fact makes this compound particularly useful for wear resistance In pistons, piston rings, cylinder walls, gun mechanism, sliding parts of machine and the like. In addition, the room temperature hardness of the compound is not affected by heating to 1000 C. whereas, in the case of chromium the hardness is destroyed.

Further, the nickel aluminide is markedly oxidation resistant even at 1100 C. there being only slight surface oxidation. This oxidation resistance is apparent when comparison is made with nickel in which, over a 400 hour period, there is a gain in weight at 1100 C. of 0.05 gram per cm. as compared with an 0.01 gram gain for nickel aluminide at the same temperature and for the same time period. Since nickel and chromium have approximately the same oxidation properties, it is apparent that the oxidation resistance of nickel aluminide exceeds that of chromium, also. Since the described methods all involve clectrodeposition, the coating thickness may be readily controlled, values ranging from 0.0002 to over 0.002 inch being readily obtainable.

In addition to the beneficial results already set forth the results of salt spray corrosion tests (set forth below) on a series of steel alloy panels coated with nickel aluminum alloy clearly indicate the excellent protection afforded in this respect by the present invention. Further, these tests indicate the superior nature of coatings in which a layer of nickel remains between the base and the nickel aluminide layer.

Percentage of surface covered Thickness of Thickness of with rust after Ni in inches Al in inches a minimum of 75 hrs. of salt spray 12%. over 50%.

less ihan 1%.

None.

1 Oxidized.

While any of the structural metals may be used as the base material, in common use for this purpose are steel, nickel or copper.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A method of coating an article which comprises the steps in sequence of electrolytically depositing in a controlled manner a layer of nickel over the surface of the base metal of said article, electrolytically depositing a layer of aluminum from .0001 to .0018 inch thick over said nickel layer in such manner as to be about one-fourth the thickness of said nickel layer, heating the coated article at a temperature in excess of 550 whereby said aluminum alloys With less than the total thickness of said nickel layer thereby providing said article with a layer of nickel covered by a protective surface layer of nickel aluminide, said layer of nickel aluminide being free of contamination from the base metal.

2. The method according to claim 1 wherein the heating step is conducted in an inert atmosphere at a temperature in the range from 550 to 750 C.

References Cited in the file of this patent UNITED STATES PATENTS Armstrong Mar. 7, 1939 Nachtman June 25, 1946 Renzoni Nov. 16, 1948 Rhodes Oct. 13, 1953 Whitfield et a1. June 29, 1954 Hansgirg May 24, 1955 Lewiston July 14, 1959

Claims (1)

1. A METHOD OF COATING AN ARTICLE WHICH COMPRISES THE STEPS IN SEQUENCE OF ELECTROLYTICALLY DEPOSITING IN A CONTROLLED MANNER A LAYER OF NICKEL OVER THE SURFACE OF THE BASE METAL OF SAID ARTICLE, ELECTROLYTICALLY DEPOSITING A LAYER OF ALUMINUM FROM .0001 TO .0018 INCH THICK OVER SAID NICKLE LAYER IN SUCH MANNER AS TO BE ABOUT ONE-FOURTH THE THICKNESS OF SAID NICKEL LAYER, HEATING THE COATED ARTICLE AT A TEMPERATURE IN EXCESS OF 550* WHEREBY SAID ALUMINUM ALLOYS WITH LESS THAN THE TOTAL THICKNESS OF SAID NICKEL LAYER THEREBY PROVIDING SAID ARTICLE WITH A LAYER OF NICKEL COVERED BY A PROTECTIVE SURFACE LAYER OF NICKEL ALUMINIDE, SAID LAYEER OF NICKEL ALUMINIDE BEING FREE OF CONTAMINATION FROM THE BASE METAL.