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Historical Articles

October, 1953 issue of Plating

 


Plating on Molybdenum


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

A. Korbelak, Editor, Plating, Newark, N.J.

 

INTRODUCTION
Among the metals that are used in this country, there are only two whose domestic sources can meet the ever hungry demands of industry. One of the two is molybdenum, often called ”moly” in a shortened version of its chemical name. A silvery white and extremely hard metal with a melting point that ranks fifth highest (see Table I) among the metallic elements, it attracted automatically the probing interest of design engineers in their search for materials of construction for heat engines and other high temperature equipment. One major supplier of the metal reported1 an increase of 1500 per cent in its production rate over a period of a few years. This figure which dramatically points up the transition of molybdenum from a rare metal status to a vital one in the country’s economy is further supplemented by another report2 which placed the demand for the metal at 500 tons per week. With the increased demand there have been developed a series of manufacturing methods which have wiped out numerous restrictions on ingot sizes, complexity of fabricated shapes and, in addition, with resultant cost reductions.

A conventional method for manufacturing the metal involves the pressing, at pressures of about 20 tons per square inch, a powder obtained by the hydrogen reduction of either ammonium molybdate or molybdenum trioxide.

Chief uses of molybdenum are: a hardening ingredient for alloy steels, mandrel material for tungsten lamp filaments, electronic tube components, welding tip electrodes, electrodes for glass melting furnaces, heating elements for high temperature electric furnaces, mercury vapor light sources, tap extractors, crucibles and as a bonding agent in spray metallizing.

DISCUSSION
As new uses in the high temperature field began to develop, the physical properties of the metal were translated from cold chemical statistics to every day practical applications. The simple statement in chemical texts that molybdenum combines with oxygen to form a number of oxides, does not reveal the full nature of one truly irksome property of the metal. That property is its instability in air at elevated temperatures. Ramage reported3 that oxides begin to form at a temperature as low as 842°F (450°C.) and that about 1292°F (700°C) oxide sublimation in the form of heavy white fumes is quite rapid. Parke reported2 that about 1832°F (1000°C), the rate of high temperature corrosion of molybdenum in slowly flowing air is between 0.0003 and 0.0008 inch per minute. Jones, Spretnak and Speiser observed4 that the rate of oxidation is more rapid in still or slowly moving air than in rapidly moving air.

Fig. 1. illustrates the result of a 15 hour exposure at 1600°F (870°C) in a high velocity air stream. The test bracket was plated with a coating of chromium 0.002 inch thick to retard its oxidation.

The practical effect of this property is that the metal cannot be used above 1292°F (700°C) in an oxidizing atmosphere. Thus, is emphasized the necessity for a protective coating for molybdenum in such high temperature applications if advantage is to be taken of its combination of attractive properties of high strength and hardness coupled with its domestic abundance.

Cladding of molybdenum has been used for some time in the electronic tube industry which utilizes platinum clad rod and wire in the construction of various designs. However, even with such an expensive jacket, there remains the problem of overcoming the diffusion into the core or base of the cladding. Once the surface of such a material becomes rich in molybdenum its behavior pattern follows that of the pure metal. The use of ceramic coatings for protection suffers from the poor mechanical strength of such layers ad the added objection of the eventual devitrification of some ceramic formulations.

METHODS OF COATING MOLYBDENUM
The application of protective coatings on special shapes by other methods is best summarized by the following:

Pack Method: A method of packing molybdenum pieces in powdered chromium, nickel and other metals followed by heat treatment in high temperature controlled atmosphere furnaces has been used with some success. The biggest drawback to this procedure is that special equipment is necessary and because of the limitations of furnace sizes, large sheets or complex shapes can not be accommodated.

Reduced-Oxide Method: Nickel oxide in a powdered form, dispersed in a suitable vehicle such as lacquer has been used in some applications. The coating may be brushed, dipped or sprayed onto the article which is then heat treated in a reducing atmosphere furnace. The vehicle is burned off, and the reduced oxide film layer remains. This process, in a modified form, is reported by Hansen and Mantell5 as being used by an electronic tube manufacturer. The modified process involves the application of a coating of nickel and silver oxides which mixture is then reduced by treatment in hydrogen for fifteen minutes at 1742° F (950° C). Other metallic films may be applied to the resultant layer of this jacketing treatment by electroplating. As with the pack treatment method, furnace size limitations and special equipment needs prevent a wider use of this procedure.

Gas Plating Method: The use of gas chromizing techniques have been reported by Lustman and Mosher6. This process involved the use of a vapor of a metal halide generated through the action of hydrochloric acid and hydrogen on chromium particles in contact with molybdenum. The deposition of metal layers by the thermal decomposition of volatile metallic compounds, such as carbonyls in an inert atmosphere is covered7 by U. S. Patents of recent issue.

Diffusion Method: Powdered metal, such as nickel, in a suitable vehicle such as amyl acetate has been applied to molybdenum. The coated item is then heat-treated in which process the vehicle is destroyed, leaving the metal on the surface. A treatment time of 30 minutes at 1832° F (1000° C) results in a diffusion-bonded article of good quality. Objection to this process arises from the roughness of the coating produced and from the requirement of special equipment.

A variation of this method involves the application of a layer of nickel by electroplating—followed up by the atmosphere heat-treatment. The molybdenum in this modified procedure is electropickled anodically at 10 volts in sulfuric acid of concentrations ranging from 1-1 to full strength, rinsed in water, dipped in a mildly alkaline solution, again rinsed in water, dipped in a 10 percent (by volume) solution of sulfuric acid, water rinsed and then transferred to a warm 110-130° F (45-55° C) Watts’ nickel bath. Coatings over 0.00025 inch in thickness were found to separate from the molybdenum upon heat-treatment in an inert atmosphere.

Table I. Melting Points of Refractory Metals
Metal
Melting Points
Centigrade
Fahrenheit
Tungsten
3370
6100
Rhenium
3170
5740
Tantalum
2850
5160
Osmium
2700
4890
Molybdenum
2620
4750

This separation, caused by differences in thermal expansion of the two metals was uniform and concentric on rods and in the form of blistered areas on sheet material. Best results were obtained by bringing up the furnace temperature with the articles being heat treated. Objections to the diffusion method again arise from the necessity for special heat-treating equipment and the possible change in physical characteristics of the molybdenum through alloying with other metals.

Plating Method: The possibilities of developing a good process through plating alone were sufficiently attractive to bring about a study into electroplated coatings on molybdenum. The need for an inexpensive, simple method for coating molybdenum existed even prior to the heat engine era.

Where heat engine applications stimulated a study into the plating of higher melting point metals, electronic applications for ease in brazing or soldering and superior high frequency current conductivity promoted the approach in the field of lower melting point metals.
Conventional cleaning and plating techniques were tried—a wide assortment of pickling solutions, ac-dc treatments, all of which failed to produce any sort of adherent plated coating. Results were poorest with alkaline plating solutions, probably because of the solubility of the molybdenum in such media causing the formation of interfering films on the metal surface. Using such solutions it was seen in operations performed in glass beakers that blisters formed practically immediately in the plating process. The best results,

Fig. 1. The result of a 15 hours’ exposure test, at 1600° F in a high velocity air stream, of a chromium plated molybdenum bracket  Fig. 2. The result of a 15 hours’ exposure test, at 1600° F in a high velocity air stream, of a chromium plated plus ceramic coated molybdenum bracket

appearance wise, were obtained with chromium and nickel plating solutions. With nickel deposits it was found readily that the adherence was only superficial —but could be made entirely satisfactory by heat treating in hydrogen or an inert atmosphere as noted in the section immediately above. With the extra pressure being put forth by the demands of heat engine designers, whose service temperature limits seemingly spiral in a never ending upward climb, the plating approach was levelled at the deposition of chromium because of its properties of higher melting point and hardness. Porosity, the then generally acknowledged structural weakness of plated chromium and the poor distribution of the deposit were overcome for a time by the application on the plated surface of a slurry of powdered glass in alcohol. The glass was fused after evaporation of the alcohol. However, tests that were run on articles so treated showed failure at fairly short exposure periods, though they proved to be a definite improvement over a straight chromium coating. Fig. 2 shows no failure at 15 hours in a high velocity air stream at 1500° F (815° C) though the ceramic layer has been badly distorted.

Fig. 3. Cross-sectional view of a test piece plated with alternate layers of chromium and nickel (400X)

At this stage in the development work, attempts were made to apply, by electrodeposition, a coating whose composition approached that of Nichrome V—80 per cent nickel, 20 per cent chromium. The excellent service life of this material when used as a resistance heating element in laboratory furnaces that operate at 1832° F (1000° C) and over dictated the attempt in this direction. Accordingly, alternate layers of chromium and nickel were plated on molybdenum to a thickness that totaled 0.003 inch. Fig. 3 is a photomicrograph in cross-section of a test piece prepared in the manner described. The test piece was then packed in chromium powder and heat treated in hydrogen at 2516° F (1380° C) for five hours. Fig. 4 shows a photomicrograph of a cross-section of a specimen taken after the hydrogen firing operation. Studies of these pictorial data showed an interesting and as yet unexplained phenomenon—chromium apparently does not alloy with molybdenum except when in direct contact with aluminum oxide. Fig. 5 shows the same type coating as shown in Fig. 4 after heat treating as above, but in contact with powdered alundum. This singular property was demonstrated in repeat experiments and led to the development of what may be called the chromium strike-bonding technique for the plating of adherent coatings on molybdenum.

Chromium Strike-Barrier Process: It was evident at this time that a chromium plate on properly cleaned metal produced an adherence that would withstand severe bending tests. Proceeding from this fact—and prompted by the requirements of other applications such as brazing, soldering, surface conductivity and others it was a logical step towards the adoption of a chromium strike technique. In this procedure, chromium was plated in the form of a 30 to 60 second flash coating, followed by appropriate rinses and a 30-60 second treatment in a Wood nickel chloride solution. Any conventional plated coating then may be applied on the nickel surface.

Specimens of sheet material were plated using this method with an added nickel coating of 0.0005 inch. Then they were bent repeatedly to the point of fracture of the sheet. Examination of the sheared edges produced no evidence of flaking of the plated layers. In addition to the adherence obtained, there was the benefit of a barrier layer which prevented the unwanted diffusion of other metals, a property of particular importance in certain applications which will be detailed in another section of this paper. Since chromium coatings alone resulted in short oxidation lives in high temperature applications the use of a plated metal which offered more in the way of ease of application, freedom from porosity and better rate of deposition was adopted. The behavior of nickel approaches that of noble metals in many corrosive media and was thus the metal of choice in this application.

CHROMIUM-STRIKE CYCLE
The exact cycle that was found to work effectively is as follows:

1. Electroclean anodically in sulfuric acid solution.
2. Rinse in water.
3. Rinse in an alkaline solution.
4. Rinse in water.
5. Rinse in 10 per cent sulfuric acid
6. Rinse in water.
7. Chromium strike.
8. Rinse in water.
9. Chloride nickel strike.
10. Nickel or any other electroplate with pre-rinsing as required.
11. Rinse and dry.

A detailed explanation of the steps in the cycle is provided below with some modifications for special conditions being indicated.

Step 1. The molybdenum first is cleaned free of surface contaminants. Any films of oil or grease are removed by conventional degreasing methods. If the parts are heavily oxidized from an operation involving the cementing or sealing of glass members to the metal it is pre-cleaned preferably in a 10 per cent solution of sodium hydroxide used at room temperature. Treatment in this solution is effected with alternating current at about 25 volts for a period of time about 20 seconds. If the parts are not badly oxidized the step detailed immediately above may be omitted and the following one used directly after degreasing. The part is cleaned anodically in a solution of sulfuric acid ranging from concentrated down to 2 parts acid—one part water. Voltage requirements vary from 24 volts at the higher concentration to 10 volts at the lower values. Current densities range from 100-300 amperes per square foot. Treatment time of approximately 30 seconds is adequate. The molybdenum will be electropolished to a bright mirror finish by this operation and a film of blue oxide will form on the metal. Contamination of the acid with copper will cause the formation of a brown smut on the parts which will interfere with the following plating steps. Build up in molybdenum metal was not analyzed for to a fixed amount since acid bath life was found to last through the processing of thousands of parts. However, if the same acid solution is being used for stripping of nickel or the electropickling of nickel parts a high molybdenum content will result in pitting of the nickel. Temperature variations of the acid solution from 70° F to 90° F (20-32° C) were found to produce equally good results.

Step 2. This is a simple rinse which removes most of the readily soluble blue molybdenum oxide.

Step 3. Traces of oxide which are not always removed by water rinsing are thoroughly dissolved by immersion in practically any alkaline solution. The author used a number of proprietary cleaners with equal success and since spent alkaline cleaners are available generally, it is recommended that such material be used. Solutions of tri-sodium phosphate and sodium hydroxide also may be used. Elimination of this step will cause erratic results where maximum adherence is needed.

Fig. 4. Photomicrograph of piece prepared in a manner similar to the one shown in Fig. 3. Shown here is the diffused layer that resulted after a high temperature hydrogen baking operation (480X) Fig. 5. Cross-sectional view of result of hydrogen baking operation when plated layers were in contact with alundum (400.)

Steps 4, 5, 6 and 8. These are conventional good practice as generally employed in cleaning and plating cycles.

Step 7. Chromium strike in a standard chromium plating solution for about 60 seconds at approximately 150 amperes per square foot is adequate. Heavier strike coatings also may be employed. It was found that a solution heavily contaminated with copper and iron did not cover molybdenum parts in lower current density areas. The temperature at which the chromium solution is operated was found to be important. Peeling of the deposit was found to occur when the bath temperature falls below 120° F (50° C).

Step 9. The nickel-chloride strike bath must be used to get adherence on the chromium film. Copper impurities will result in dark to pink coloration in low current density areas and may result in blistering of subsequently plated coatings. A current density in this step of about 50 amperes per: square foot produces good results, though as low as 25 amperes per square foot have been found satisfactory.

Step 10. Any desired plate may be applied to the molybdenum: processed as described. Intermediate rinses in water after the chloride strike may be omitted if a final nickel coating is applied.

An analysis as to why the chromium strike technique works in bonding electrodeposits to molybdenum may be partially answered for either, and probably, for both of the following reasons: Only two of the plateable metals, iron and chromium have structures that match the body centered cubic crystal structure of molybdenum. Also the thermal expansion coefficient of chromium is closer to molybdenum than that of the other plateable metals. See Table II.

Table II. Coefficients of Thermal Expansion
Metal
x 10-6
Molybdenum
4.9
Chromium
6.8
Iron
11.4
Nickel
13.0

Dubpernell furnished information that tends to confirm this theory. He stated8 that the chromium structure changes when the deposit is applied from a solution operated below 120° F (50° C). The peeling of plated coatings using the chromium strike technique, but at a chromium plating temperature below 120° F has been observed by the author and by Swift9. On the other hand, the crystal matching theory appears to be set back by a report from Friedman who stated10 he had poor success adherence-wise using iron electrodeposits on molybdenum. This may have been because of the iron plating solution used since Vaaler, Snavely and Faust11 reported good adherence of iron deposits on molybdenum.

Application of the Method: There are numerous applications for parts of molybdenum processed as described. Since the method does not involve any special equipment it can be utilized by most shops. Of currently topical importance are those applications involving heat engines.: Certain temperature recording or signalling devices can be made of structurally strong molybdenum and, being plated with a diffusion barrier, protection can be extensive.

Resistance welding tips are more intimately joined to holders through the use of such a plated coating. A copper plated surface layer assures a good brazed

Fig. 6. High-wattage mercury vapor lamp, one of the items on which the chromium strike technique has been used successfully Figs. 7, 8. Electronic tube components which have been plated with adherent coatings through use of the chromium strike technique joint with all voids eliminated along with potential hot spots that tend to reduce tip life.


High wattage mercury vapor short-arc light sources (see Fig. 6) utilizing a series of glasses graded for thermal expansion characteristics to obtain a vacuum seal with quartz envelopes, have been using the method rather successfully. Without the protective electroplate threaded sections of the device would smoke away during glass sealing so that an original 10/32 ‘thread would accept an 8/32 connector, and poorly at that.

Electronic applications are numerous. (Figs. 7, 8). Soft soldering of small components becomes a simple job. Brazing of complex and large sized units at temperatures close to 1832° F (1000° C) have been produced repeatedly.

The problem of skin effect is now an easy hurdle for molybdenum plated with copper, silver or gold—a procedure which previously could not be done economically.

In the construction of electronic tubes, grid wires of platinized molybdenum are employed to obtain desired thermionic emissivity characteristics. Diffusion of the platinum coating into the molybdenum core results in tube failure occasioned by high secondary emission. To maintain the superior emission suppression property of platinum for a longer time it is suggested that a material utilizing a chromium barrier will be successful.

Fig. 9. Electrodes of molybdenum such as are used in glass melting furnaces

Certain glass melting furnaces operate more efficiently with electrodes of molybdenum. (Fig.9). However, service life is seriously shortened by oxidation at the air-glass level. A means of solving this problem is offered through use of the chromium-bond-barrier plating method. Since metals other than platinum tend to discolor optical glass melts—a layer of platinum fixed over and around the plated molybdenum may prove to be the answer in this troublesome application.

The method may also find uses in the plating of molybdenum alloy steels. Where peeling or blistering is encountered on high molybdenum steels—it is suggested that the chromium strike technique be employed as a means of overcoming poor adherence.

ACKNOWLEDGMENTS
Certain portions of the subject matter outlined above have been set forth in the author’s application pending before the U. S. Patent office and assigned to the Westinghouse Electric Corporation. The author also wishes to acknowledge the assistance and co-operation of the personnel of the Engineering Departments of that company where the photographs used to illustrate this paper were prepared.

REFERENCES
1. J. Gelok, ”Molybdenum—Practical Structul al Material” Westinghouse Eng. 15159 (Sept. 1947).
2. R. M. Parke, ”Molybdenum, A New High Temperature Metal”, Metal Progress 60, 1, 81-96 (July 191).
3. J. H. Ramage, private communication, Dec. 12, 194.
4. E. S. Jones, J. W. Spretnak and R. Speiser, ”A Study of the Oxidation Cl2aracteristics of Molvbdenum at Elevated Temperatures”, First Technical Report, NR Project Nr 034 404, Contract N6 ONR 22528.
5. R. M. Hansen and C. L. Mantell, ”Adherent Electroplating on Molybdenum”, manuscript submitted for publication to the Electrochemical Society. (May 1953).
6. B. Lustman and D. R. Mosher, ”Gas Chromizing of Molybdenum”, Westinghouse Electric Corp. Research Report R94402-18-A (May 12, 194`’).
7. U. S. Patents 2,332,309 and 2,344,138.
8. G. Dubpernell—private communication.
9. G. P. Swift, private communication.
10. I. Friedman, private communication.
11. L. E. Vaaler, C. A. Snavely and C. L. Faust, ”Iltroductory Plating Studies on Protecting Molybdemm from High Temperature Oxidation.” Battelle Memoral Institute Report 831.


Discussion

MR. MORTON SCHWARTZ (Surface Alloys Engineering Corp., Los Angeles, Calif.): The formation of ”blue oxides” in the anodic acid treatment is important from our experience. If the brownish films are formed then difficulties arise resulting in non-adhesion.

MR. KORBELAK: We experienced the same results and did some work which led us to believe that metallic impurities in the acid solution such as copper caused more frequent formation of brown oxide films.

DR. CLOYD A. SNAVELY (Battelle Memorial Institute, Columbus, Ohio): I would like to add a few comments to Mr. Korbelak’s excellent presentation. This problem of plating on molybdenum is very important to electroplaters because a few wrong moves can lead to failures which give a bad name to the whole field of plating. I hope every plater who attempts to plate on molybdenum will have a copy of Mr. Korbelak’s paper or other good reference material on hand.

In work at Battelle we confirmed Mr. Korbelak’s findings that a flash of chromium is very helpful in obtaining adhesion of nickel plate on molybdenum.

In testing chromium and nickel-plated molybdenum .specimens sometimes we found failures which could not be explained until we sectioned the specimens and examined them metallographically. Often the molybdenum contained long stringers of oxide which were exposed at the surface. The electroplate did not bridge these inclusions and failure started around them. At one point in our work, we had to stop and wait for an entirely new supply of molybdenum because of these difficulties.

An interesting phenomenon that Mr. Korbelak might have observed in his composite nickel and chromium plates is that the chromium, being a strong oxide former, will extract oxygen from the nickel layers. There seems to be a very decided advantage to alternating chromium and nickel Apparently, the nickel holds up much better if it is deoxidized. Healing of pores may occur more readily in this condition.

I would also like to point out the difference in testing conditions used in various investigations of plating on molybdenum. Some tests have required 1000 hours at an elevated temperature, and others only 100 hours at temperature. It would be helpful if the testing procedures were made more definite than they are at present.

MR. KORBELAK: I see Mr. Friedman in the audience. I know he is familiar with current testing techniques, and he may wish to add a few remarks.

MR. ISIDORE FRIEDMAN (Wright Aeronautical Division, Curtiss-Wright Corporation, Wood-Ridge, N. J.): First, I would like to commend Mr. Korbelak on having presented such an excellent paper. Now we, too, have had these variations in requirements for temperature-time exposures. Changes in temperature requirements from 1800° F on up, and exposure times from 25 hours to 500 hours are not uncommon.

In connection with the comparison of composite nickel-chromium coatings against straight chromium, it is interesting to note that for a deposit of the same thickness, that is 0.001 inch, we were able to get 14 hours exposure time at 2000° F, as compared to 10 hours for a straight chromium deposit at the same temperature. :;

DR. SNAVELY: The work we did at Battelle was recently released in an AEC report. Dr. L. E. Vaaler of our staff developed a ferricyanide etch, which seemed to be very helpful. Otherwise, the results were about the same as Mr. Korbelak’s.

MR. KORBELAK: Were you able to confirm the barrier property we found in this work with chromium?

DR. SNAVELY: NO. I didn’t mention this because our test temperatures were somewhat different from yours. After treatment at 1800° F, we definitely observed diffusion of chromium into the molybdenum. I doubt that any aluminum oxide was present in our work. Nickel, of course, diffused much more rapidly. The diffusion areas in both cases appeared to have undesirable physical properties.

MR. EDWARD F. KOETSCH, JR. (Springfield Armory, Springfield, Mass.): I would like to ask if you have had any occasion to strip the nickel from molybdenum, and if you have, how do you do it?

MR. KORBELAK (holding up sample): This piece is a graded seal of several high temperature glasses joined to quartz. The technique described in the text was used in the assembly of the unit, and as a result of the high temperatures used during that assembly operation, the ends were heavily oxidized. The coating was stripped off with anodic sulfuric acid treatment, then the ends were replated using the chromium strike technique.

DR. FREDERIK S. SCHULTZ (General Electric Co., Cincinnati, Ohio): I would like to add a comment to Dr. Snavely’s remarks: The life of these coatings depends very much on how you test them and the method of test should depend on the end use. For instance, a nickel coating 3 to 5 mils thick might last 800 to 1000 hours on static oxidation at 1800° F. If cooled intermittently its life will be much less because the protective oxide will spall off due to the fact that NiO is oxidized to Ni2O3 at approximately 400° C and at 600° C goes back to NiO. There is a volume change which accompanies this reaction causing the loss of the protective oxide.
I would like to ask you a question: Have you studied any oxidation rates on these coatings—that is, the nickel-chromium combinations?

MR. KORBELAK: Early in these high temperature studies of coatings on molybdenum, when the requirements of the Services called for 24 hours at 1800° F, we were able to obtain a life of 300 hours with a 3 mil nickel coating on a chromium base. Without the chromium, life at this temperature was but 24 hours.

DR. SCHULTZ: DO they have an alloy that will not recrystallize at these temperatures?

MR. KORBELAK: None that I know of.

MR. STANLEY J. KLIMA (Sperry Gyroscope Co., Great Neck, N. Y.): What would be the best material to use for racking molybdenum parts plated by the procedure outlined in your paper?

MR. KORBELAK: TO prevent copper contamination of the anodic etch, simple contact points comprising phosphor-bronze connected through a sleeving arrangement to tungsten tips were used; then the entire contact except for the tungsten tip was coated with a stop-off lacquer; such a contact works well and incidentally also works beautifully in the electropolishing of stainless steel parts where copper contamination would be troublesome.

MR. KLIMA: The next question is in regard to the chromium bath—what is the upper operating temperature limit?

MR. KORBELAK: I suggest that you use your conventional operating temperatures, but would not go below 120° F. If you operate at 130, I would continue to work at that temperature. I would add to that answer, following what Mr. Dow said here this morning, that the possibilities of crack free chromium in this application certainly should prove interesting and now that initial data have been presented, further work in this field should result in some interesting figures.

MR. KLIMA: With reference to chromium-nickel or chromium-nickel-copper multi-layer plating on molybdenum, how critical is the plating thickness of the nickel-copper to prevent the formation and diffusion of chrome oxide when the parts are assembled by means of silver or gold brazing operations in a tank hydrogen atmosphere furnace where water vapor is present?

MR. KORBELAK: That would depend on the initial thickness of chromium and the treating time and water vapor present in the hydrogen. A recent article in PLATING (November, 1952, Page 1222.) listed data which are similar to those involved in your question.

MR. KLIMA: In Step 1 of your procedure, is the concentration of sulfuric acid expressed as two parts acid to one part water by volume or weight?

MR. KORBELAK: Volume.

MR. KLIMA: With regard to cleaning oxidized molybdenum prior to the plating procedure outlined in your paper, would it be good practice to first clean molybdenum parts in a hydrogen atmosphere furnace to remove trapped oxygen or to reduce oxides? If so, how dry must the hydrogen atmosphere be, i.e., dew point -60° C or could tank hydrogen be used?

MR. KORBELAK: If you were going to do any work involving sizeable pieces that represent a lot of money and time to assemble, a good safety step would be to pretreat your molybdenum piece in hydrogen with a conventional amount of moisture content. Any occluded foreign material would generally clean up in a 30 minute treatment of 1800° F. Longer or shorter treating times should be established by trial runs.

MR. KLIMA:’ What type of bath did Mr. Friedman use to obtain iron electrodeposits on molybdenum? ‘

MR. FRIEDMAN: That was a standard ferrous chloride, calcium chloride bath.

DR. SNAVELY: Iron in contact with molybdenum is very bad where elevated-temperature oxidation is a matter of concern. T certainly would not recommend iron plate on molybdenum, even though the lattice parameters of the two metals are in the right range to expect an adherent plate.

MR. EVERETT F. CARTER (Sylvania Electric Inc., Towanda, Pa.): Dr. Snavely mentioned this, but we didn’t get any response to speak of. In your work on electroplating of molybdenum, did you find that molybdenum base material from different processes would tend to give you various degrees of success in your plating line?

MR. KORBELAK: The quality of the molybdenum definitely fits into the picture.

MR. CARTER: In your work, actually, most of the molybdenum you used, I take it for granted, was Westinghouse?

Mr. KORBELAK: Yes.

MR. CARTER: And that work had not been done on molybdenum produced from other sources?

MR. KORBELAK: The work described in the paper was limited to Westinghouse material. Other investigators in other units of the company may have used molybdenum from outside sources.

MR. W. B. STEPHENSON, JR. (General Electric Co., Evendale, Ohio): How do you overcome racking marks on your molybdenum? We find a lot of times we get excellent results on panels we have plated, except where we have racked them and although we try to reverse them at times, we have plating troubles.

MR. KORBELAK: On the large cylinder you saw, we shifted the contacts about 45° under water, using rubber gloves, by simply twisting it on the rack without taking it off the rack. Then we applied another chromium strike on the first chromium strike, which dulled the plate. The same shifting technique was followed through in the other plating operations.

MR. STEPHENSON: You do not get any peeling between the two chromium layers?

MR. KORBELAK: No.

MR. STEPHENSON: What metal did you use in contact?

MR, KORBELAK: Tungsten tipped phosphor bronze, or phosphor-bronze alone.

MR. STEPHENSON: To what do you attribute the blistering of nickel plate when plated directly on molybdenum?

MR. KORBELAK: Molybdenum is oxidized in transport through air and since the oxides are not soluble in the nickel plating solution, they represent an interfering film which prevents good bonding.

MR. STEPHENSON: You feel that the chromium will remove the oxides prior to the beginning of the plating action?

MR. KORBELAK: Yes

MR. STEPHENSON: In heat treating in hydrogen, is the temperature critical—about what range do you finish it?

MR. KORBELAK: I take that question to mean where we diffuse nickel into the molybdenum directly?

MR. STEPHENSON: No, you made some statement of plating with chromium, and then nickel over the chromium,, and then various other plates; then you heated the whole thing.

MR. KORBELAK: That type of coating was treated at the melting point of the nickel-chrome alloy, 1380° C.

MR. STEPHENSON: Do you find perhaps lower temperatures give you better results or worse?

MR. KORBELAK: They were not tried.

MR. E. R. BOWERMAN (Sylvania Electric Products, Inc., Bayside, N. Y.): Communicated. Our laboratory has found that when the anodic sulfuric acid is warm, as when a new solution is prepared, the chromium frequently is non-adherent. This trouble disappears after the acid solution is cooled to room temperature and adherent deposits are obtained. Have you observed any such behavior?

MR. KORBELAK: No.




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