Historical Articles

September, 1953 issue of Plating

 

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Black Chromium-Base Electroplating

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Presented at the 40th Annual Convention of the American Electroplaters’ Society, June 16, 1953

Martin F. Quaely, Engineer, Westinghouse Electric Corp., Research Department, Lamp Division, Bloomfield, NJ

ABSTRACT
A number of solutions have been developed for the production of black coatings that have good adherence to the base-metal. These deposits will withstand heating in high vacuum to temperatures of over 930° F (500° C) and can be applied to most metals without the use of special equipment. The new black electrodeposits are basically metallic chromium. They also contain vanadium or nickel or both. The deposits consist of finely divided metal and oxide. From a bath containing chromic acid and nickel salts a silvery deposit results, which is changed to black upon treatment with hydrochloric acid. A second bath contains chromic acid, vanadium salts, and nickel salts. A third contains chromic acid and vanadium salts. Black deposits are obtained directly from the latter two baths. A very small amount of a carboxylic acid is added to each solution to extend operating conditions. The baths are operated at current densities of 695-1850 amp/ft²(75-200 amp/dm²), and at temperatures of 86-131° F (30-55° C).

INTRODUCTION
For some time there has been considerable demand for black finishes possessing good adherence, heat resistance, and black-body properties. Particular situations have arisen wherein a black finish was required for electron tube parts, for bimetal heater elements, and for stainless steel parts of various sizes and shapes for industrial applications.

Paints and lacquers of specific compositions have been used from time to time, but such finishes were invariably unsuitable for a number of reasons. For the most part, they did not resist moderately high temperatures, and their thickness could not be controlled.

Fortunately, there are available a number of methods for producing black finishes on metals. Oxide finishes, especially, have found extended use in a large number of industries. Black oxide coatings can be produced by immersion of the metal article to be coated in a suitable oxidizing solution^{1 2 3}. There are also the phosphate type baths ^{4 5} for producing black coatings. In addition, there are a number of electrochemical procedures for obtaining black finishes. It is with this latter type that this paper is concerned.

EXPERIMENTAL
Probably the most durable of all the electrodeposited black finishes are the chromium-base plates. They are stable in vacuum and in air. They have fairly good corrosion resistance, and their mechanical properties are superior to those of other black finishes. For some years, black deposits have been obtained by electrodeposition from a chromic acid solution^{6 7 8}. A similar method has been proposed within the past year by Gilbert and Buhman⁹.

There have been numerous industrial demands for a uniform black coating which would withstand heating in high vacuum, such as prevails in certain parts of electron tubes 750-930° F (400—500° C). Furthermore, the coatings had to have low vapor pressures Black chromium plating was suggested as a method of obtaining the required finish. Experiments were conducted on a number of plating solutions, with a fair degree of success in most cases.

A straight chromium bath was used first. While a black plate was generally obtainable, the process had to be controlled within very narrow limits in order to get an acceptable deposit. It was quite important to provide cooling for the plating solution in order to remove the excess heat produced by the high power input. The temperature of the bath had to be maintained below 77° F (25° C). At higher temperatures the quality of the plate was seriously affected. If the current density was too high, ”treeing” occurred and the deposit was rough and had a tendency to be off color.

In an attempt to obtain some improvement over the straight chromium bath, various metallic ions were added which, it was believed, would increase the stability of the plate from the standpoint of both temperature and corrosion. Among these metal additions were nickel and vanadium.

Chromium-Nickel Electrolyte
When nickel ion was added to the chromium bath and plating was carried on under the conditions usually necessary to obtain a black finish, no black deposit was obtained under a wide range of current densities. The plating bath contained chromic acid and nickel chloride in a ratio of 10 parts chromic acid (CrO3) to 1 part nickel chloride (NiCl2. 6H2O), the actual amounts being chromic acid 26.8 oz/gal (200 g/l), and nickel chloride 2.68 oz/gal (20 g/l). An addition of about 0.65 fluid ounces of glacial acetic acid per gallon of solution (5 ml per liter) was also made to extend operating conditions and prolong the operating life of the solution. A ratio of about 20 parts chromium to 1 part nickel was found to give optimum results.

With the chromium-nickel solution and at a current density of 695-1850 amp/ft² (75-200 amp/dm²), bright and silvery deposits were obtained. As the coatings were applied in heavier thicknesses it was noted that they became coarser in texture. These silvery deposits were fairly hard. A copper rod with an annealed micro-hardness of 91 Knoop (100-gram load) was plated in this bath. The plate had a micro-hardness of 530 Knoop (100-gram load). The deposit was attacked only slowly by cold hydrochloric and sulfuric acids, although it could be dissolved eventually. Nitric acid had practically no effect on the coating. There was no blistering of the plate when heated to dull red heat. However, a slight oxidation of the surface was evident.

Spectrographic analysis of these silvery deposits showed the presence of chromium and nickel. From work done by Brenner et al ¹⁰ at the National Bureau of Standards it would appear that there are undoubtedly chromium oxides present in the deposit. Spectrographic analysis has shown the chromium and nickel to be present in about the same proportions as in the plating bath. Without-the nickel in the bath, no silvery deposit was obtained.

By plating metal parts in this chromium-nickel bath it is possible to build up thicknesses of plate very rapidly. For instance, a inch cop rod, 4 inches long, was plated for five minutes. A plate of 0.001 inch thickness was obtained.

When this silvery deposit was treated with concentrated hydrochloric acid for about 5 to 30 seconds, the silvery appearance disappeared and a black finish remained. This black finish that was obtained is best described as a blue-gray black, in other words, not a jet black. However, it can be used in many instances where a stable black finish is required. It will not smudge or when vigorously rubbed by hand, and it is very adherent and stable.

In order to determine whether nickel sulfate could be substituted for nickel chloride in the above bath, tests were performed in which nickel sulfate was substituted for nickel chloride in equivalent amounts. Under conditions of plating set up by the author, a flaky non-adherent deposit was obtained. Even plating at lower current densities did not yield a satisfactory bright, hard deposit. Thus the chloride ion appears to be a necessary ingredient of this bath in order to obtain bright deposits at the high current densities which are used. Sulfate ion has a deleterious effect on the quality of the plate.

A further condition established in this work for obtaining deposits that are relatively inert to most chemical reagents is that the chromium-nickel content ratio must be kept at about 20 to 1.

Chromium-Nickel-Vanadium Electrolyte
In the past, many attempts have been made to electrodeposit vanadium metal. A report of numerous experiments was made in 1916 by Fischer ¹¹. In all of these experiments, no electroplate was obtained from aqueous solutions of vanadium salts. During the intervening years since 1916 no reports of any significance on the plating of vanadium have appeared in the literature until recently. Sendero and Brenner¹² reported on attempts to deposit vanadium from aqueous solutions at high temperatures (570° F [300° C]) and at high pressures (2000-3000 psi). No vanadium plate was obtained, and only a black deposit of a complex nature was produced in some tests. If there was a deposit at all, adherence was poor in many cases.

In checking through a list of the metallic elements and their oxides, it is found that some of the metals have black oxides. A number of these metals can be electrodeposited, while others cannot, or at least have not been deposited hitherto out of aqueous solutions. Why some of these metals cannot be deposited has not been thoroughly explained as yet although various theories have been advanced from time to time. One line of approach is on an energy basis, wherein the heats of formation of the oxides and salts are considered. Such a line of approach was used by Fischer ¹¹ in his study of the electrolysis of vanadium salts

If the heats of formation of the oxides and salts of certain of the metals are considered (Table I), it is realized that those metals which have low heats of formation of the oxides and salts generally have been plated with relative ease. The metals with high heats of formation of their oxides and salts are not deposited as a metal out of aqueous solutions.

It is known, for instance, that elements such as silver, copper, and nickel are very easily deposited from their salt solutions. The heats of formation of the oxides and salts of these elements are comparatively low; thus, the reducing power at the cathode is high enough to deposit the metal from the ionized salt. On the other hand, metals such as aluminum, titanium, and vanadium, the oxides and salts of which have high heats of formations, are not deposited from aqueous solutions.

Fischer has discussed this question in some detail, especially as it applies to vanadium. According to him, it is possible that, since the oxides of vanadium have such high heats of formation, the reducing effect produced at the cathode by the electric current is insufficient to furnish the energy required to decompose a vanadic salt in aqueous solutions. In general, the energy required to decompose the salts is greater than that needed to decompose the oxides.

Thus, it was believed that if a vanadium salt was added to the bath containing niekel and chromium salts, a black deposit would result, especially since the vanadium would deposit out, not as metal, but most likely as an oxide. Under the conditions of electrodeposition this oxide would probably be the trioxide, since any pentavalent vanadium would be reduced to trivalent vanadion at the cathode.

A bath was made up containing chromic acid 26.8 oz/gal (2CO g/l), nickel chloride 2.68 oz/gal (20 g/l), and vanadium 0.27 oz/gal (2 g/l), added as vanadium nitrate. Seven-tenths of a fluid ounce of glacial acetic acid was added per gallon of solution (6 ml per liter). Upon plating at a current density of 929 amp/ft2 (100 amp/dm2), and at a temperature of approximately 8695° F (30-35° C), a uniform, adherent deposit was obtained. The color was jet black. A steel rod that was black-plated in this bath was heated to redness (about 1380° F [750° C] ) over a gas flame. The rod was quenched in cold water immediately. The only visible change was in the color, from a gray-black to blue-gray black; adherence was perfect. Another iron rod that was black-plated in the same bath was heated in hydrogen for 1/2 hour at 1380° F (750° C). As there was no visible change in the black deposit, the plated rod was further heated for 1/2 hour in hydrogen at 1830° F (1000° C). The deposit was not visibly affected by this high-temperature treatment. There was very good adherence; the deposit was jet-black in color. Attempts to identify, by X-ray diffraction methods, the various constituents of the coating were not successful, chiefly owing to interference by chromium. However, it is believed that deposits from this bath contain finely divided chromium and nickel metals, plus chromium oxide, ad vanadium oxide, probably as trioxide. Spectrographic examination of the coating showed it to contain chromium, nickel, ad vanadium. By microscopic examination all constituents of the deposit were found to be uniformly intermixed and to adhere very strongly to the base metal to produce a compact surface finish. A favorable composition of the electrolyte is 20 parts chromium to 4 parts nickel to 1 part vanadium. Larger quantities of vanadium increase the vanadium oxide content of the deposit.

Chromium-Vanadium Electrolyte
Since the addition of the vanadium salt to the bath containing chromic acid and nickel salts resulted in a solution from which black deposits could be electroplated, the next step in the work on black deposits was to omit the nickel salt from the bath. Thus, solutions were prepared which contained chromic acid and vanadium salts in a ratio of about 20 parts chromium to about 1 part vanadium. Additions were also made to each gallon of solution of 0.4-2.6 fluid ounces (3-20 ml per liter) of an organic acid such as formic, acetic or butyric. One solution that was used contained chromic acid 26.8 oz/gal (200 g/l), ammonium metavanadate 2.68 oz/gal (20 g/l), and acetic acid 0.18 fl oz/gal (6.5 ml/l). Electrodeposition was carried out at a current density of 929 amp/ft² (100 amp/dm²), and at a potential difference of about 12-15 volts. The temperature of the bath was maintained at 95-122° F (35-50° C). Very even, jet-black deposits were obtained.

Spectrographic analysis of the deposits produced from the above bath showed that they contained chromium and vanadium in about the proportions present in the plating solution. . The deposits had more heat resistance than do the other deposits described, and maintained the black color under heat. The acid resistance was about the same for all types of deposits. The deposits obtained with this bath were usually dull black, but under varying conditions, such as a higher temperature and higher current density, enamel-like glossy black coatings were obtained.

Lead anodes were employed most of the time with the above plating solutions, although graphite anodes were used in a few instances. Cooling coils of stainless steel were initially used; however, when they corroded through, lead coils were substituted and gave good service.

APPLICATIONS
The black finishes described above are useful in many applications; as far as is known, they are better than other types of black finishes for certain uses. One application that shows great promise is the coating of anode supports in rotating anode X-ray tubes, wherein good heat transfer is necessary in order to draw off the heat generated in the tube. Lack of proper heat removal sets a limit on the power input of such tubes. Heat transfer is greatly facilitated by the heat-absorbing properties of the black electrodeposit.

These black coatings can also be used on bimetal heat regulator elements, wherein the elements are subjected to a wide range of temperatures. A similar application is on various types of optical instruments.

Metals or conductors upon which electrodeposits usually can be applied are coated readily with these black finishes. Copper, nickel, brass, iron, 18-8 stainless steel, copper base alloys, and iron base alloys have been coated with black chromium-base electroplates. Aluminum has been plated with these coatings after it was first given one of the usual pretreatments. Tantalum and manganese did not take a deposit. Acceptable coatings have been applied on titanium, although not as readily as on metals such as copper.

While very large objects have not been plated in these black chromium baths, it does not appear that there should be any difficulty in doing so. However, the size of the object that can be plated may be limited by the power available, since very large power supplies would be required for large objects. Hollow articles or those having deep recesses are plated in the conventional manner, using conforming anodes.

Although these black finishes can be used for decorative purposes, the* chief application will probably be where their chemical and physical properties, such as corrosion resistance and heat absorption, can be utilized.

SUMMARY
A hard, bright chromium-base electrodeposit has been developed which can be applied at a high rate of deposition. The electrolyte contains chromic acid and nickel chloride plus a carboxylic acid. When this bright deposit is treated with hydrochloric acid for a few seconds, a black finish results. This finish is non smudging, very adherent, and uniform.

Another black, chromium-base electroplate has been developed which is deposited from an electrolyte containing chromic acid, nickel chloride, a vanadium salt, and a carboxylic acid.

A process has been developed for the electrodeposition of uniform, black electrodeposits from an electrolyte containing chromic acid, a vanadium salt, and a carboxylic acid. The coatings have good heat and chemical resistance.

Acknowledgment
The various plating solutions specified in this paper are covered by the author’s patent applications, assigned to the Westinghouse Electric Corporation, now on file at the United States and Canadian Patent offices.

LITERATURE CITED
1. W. R. Meyer, U. S. Pat. 2,364,993, December 12, 1944.
2. W. R. Meyer and G. P. Vincent, Metal Finishing 4S, 613,
3. W. H. Price, Jr., Proc. Am. Electroplaters’ Soc. 31, 1943 87-90.
4. R. R. Tanner and H. J. Lodeesen, U. S. Pat. 1,911,726, May 30, 1933.
5. H. J. Lodeesen, U. S. Pat. 2,272,216, February 10, 1942.
6. Siemens and Halske, A.-., Ger. Pat. 607,420, December 19, 1935.
7. A. Ungelenk, J. Fischer, and H. Endrass, U. S. Pat. 1,975,239, October 2, 1934.
8. Siemensand Halske, A.-G., Fr. Pat. 754,360, November 6,1933.
9. L. O. Gilbert and C. C. Buhman, U. S. Pat. 2,623,847, December 30, 1952.
10. A. Brenner, P. Burkhead, and C. Jennings, J. Research, Natl: Bur. Standards 40, 31-59 (1948? (Research Paper No.
11. S. Fischer, Jr., Trans. Electrochem. Soc. 30, 175-228 (1916).
12. S. Senderoff and A. Brenner, J. Electrochem. Soc. 97, 361366 (1950).
13. Handbook of Chemistry and Physics, C. D. Hodgman ed. 33rd ed. Chemical Rubber Publishing Co., Cleveland (195i).

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DISCUSSION

DR. H. J. WIESNER (Bendix Products Division, Bendix Aviation Corporation, South Bend, Ind.): Dr. Quaely, I would like to ask two questions: First of all, what thicknesses of coatings were applied?

MR. QUAELY Thicknesses as high as five mils were applied.

DR. WIESNER: Do you have any data as to the relative salt spray resistance?

MR. QUAELY We have made no salt spray tests.

MR. LLOYD 0. GILBERT (Rock Island Arsenal, Rock Island, Ill.): I would like to ask three questions: Do you have any data as to the per cent of efficiency of the bath that you are using

MR. QUAELY: No, we have not. I imagine it would be of the same order as a conventional bright chromium bath, probably a little higher because of the higher operating temperature which tends to increase efficiency.

MR. GILBERT: In the vanadium type of solution that you described, do you have any idea of the per cent of vanadium and chromium deposited?

MR. QUAELY As far as we could learn through analysis, the amount of vanadium in the deposit follows fairly closely the amount in the original solution.

MR. GILBERT: One final question: The throwing power of the bath, has that been determined?

MR. QUAELY No, it has not been determined, but conforming anodes were used for intricate parts. However, throwing power would probably approximate that of a conventional chromium plating bath.

MR. FRANK 0. BEUCKMAN (Eastman Kodak Company, Rochester, N. Y.): Can these coatings be applied to the nickel-chromium type stainless steels?

MR. QUAELY: Yes, we have applied them directly to those metals.

MR. BEUCKMAN: Would these coatings be applicable to parts, for instance, like densitometers where a dead black coating is required?

MR. QUAELY: We have used them for that purpose in similar applications where we wanted a dense black coating.

MR. I. L. NEWELL (Henry Souther Engineering Company, Hartford, Conn.): What is the color that you obtained with the third bath?

MR. QUAELY Black—in fact, I would say that it was blacker, if you can distinguish between black and blacker.

DR. FREDERIK S. SCHULTZ (General Electric Company, Cincinnati, Ohio): In the first bath that you have shown with nickel-chromium, did you notice any change in the composition with change in the base metal—that is, the ratio of nickel to chromium?

MR. QUAELY We have not checked into that, although we would not expect it to be any different so long as the composition of the bath did not vary too much.

MR. ISIDORE; FRIEDMAN (Wright Aeronautical Corporation, Wood Ridge, N. J.): What do you mean by good protection at high temperatures—what is the temperature range required?

MR. QUAELY: On the order of 1000° C. Of course, in these cases, we do not expect the finish to remain black.

MR. FRIEDMAN: But it does give oxidation protection ?

MR. QUAELY Yes, it does.

MR. CHARLES GELDZAHLER (Platers Technical Service, Chicago, Ill.): Do thin coatings on the order of ten-millionths give dense black deposits

MR. QUAELY: Yes, but in that case you won’t have good oxidation protection; you have to go over to the heavier deposits to get protection.

MR. GELDZAHLER: Are these thin coatings abrasion resistant much on the order of bright chromium deposits ?

MR. QUAELY: The silvery deposits are definitely more abrasion resistant than the black deposits, and probably approach the hardness values of regular chromium.

MR. GELDZAHLER: What happens after hydrochloric acid treatment—does it become loose?

MR. QUAELY YOU cannot smudge off any of the coating.

MR. GELDZAHLER: In other words, it is abrasion resistant.

MR. QUAELY: Yes.

MR. A. E. DURKIN (General Electric Company, Lynn, Mass.): What high temperature life in hours does the coating have at 1000° C?

MR. QUAELY: We have not done much beyond 20 hours so far. We are aiming to run longer periods of time in future tests. ;

MR. W. B. STEPHENSON (General Electric Company, Evendale, Ohio): What effect does heat cycling have on the coating?

MR. QUAELY: Our limited tests have shown that the coatings will withstand heat cycling.

MR. STEPHENSON: What base metals have you tried?

MR. QUAELY: We have tried iron, copper, brass, stainless steel and various steel alloys.

MR. STEPHENSON: Do you get diffusion on all of those base metals?

MR. QUAELY: On all of those we have had diffusion, and the amount of diffusion varied with time and temperature.

MR. PHILLIP H. EISENBERG (Sylvania Electric Products, Bayside, I. I., N. Y.): What effects, if any, do the various thicknesses have on this deposit, particularly the smoothness, ductility and structure?

MR. QUAELY: If the rate of deposition is not too high, you can obtain smooth coatings in the range of 5 mils. These coatings tend to become rough if you try to obtain thick deposits at too high a rate.

DR. WIESNER: I just wanted to get one point straight in my mind—the question was asked as to whether you apply this coating directly to stainless steel. You said ”yes”, and I wanted to make sure that there was no activation process in between.

MR. QUAELY: Yes, no type of electrochemical treatment was used other than cleaning of the surface to remove any dirt.

DR. WIESNER: Is that 18 - 8 type of stainless?

MR. QUAELY: Yes.

MR. HANS C. SCHLAUPITZ (R. Wallace & Sons Mfg Co., Wallingford, Conn.): Does prolonged heating at elevated temperature reduce the hardnesses of deposit?

MR. QUAELY: Since diffusion will take place to some extent, the hardness will be affected and it will depend upon the hardness of the base metal plus the hardness of the alloy that is formed in the diffusion process; it will not have the original hardness of the coating.

MR. SCHLAUPITZ: Other than diffusion, would you expect any softening?

MR. QUAELY: I would expect some softening.

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