US2380044A - Process for producing electrodeposits - Google Patents

Description

Patented July 10, 1945 Raymond A.

Claims. This invention relates to the electrodeposition of protective and ornamental finishes and is more particularly directed to the electrodeposition of colored deposits which include an oxygen compound of molybdenum. The present invention is further directed to molybdenum baths from which such deposits may be plated and to the deposits.

The prior art is aware of numerous chemical treatments using molybdenum compounds for the chemical coloring of metals, but these treat- Hoflman, Euclid, Ohio, assignmto E. I. du Pont de Nemours ton, Del., a corporation of Delaware No Drawing. Application June Serial No. 342,902

a Company, Wilmingthrowing power and simplicity of operation for the electrodeposition of colored deposits on metals and particularly on aluminum surfaces.

Further objects of my invention will become apments result in the formation of exceedingly thin and evenmonomolecular layers of colored compound. The compounds, moreover, are usually only poorly adherent to .the metal and provide relatively little resistance to corrosion. These chemical coloring processes rarely produce deposits of pleasing appearance and any attempt to improve them by buiflng usually removes the coating entirely.- Such processes have found very limited application and then p are used only because of the absence of anything else which is more satisfactory. I

By far the most satisfactory methods of applying ornamental and protective'flnishes are processes of electrodeposition. By such processes various protective and ornamental metals can be applied under controlled conditions in substantially any desired thickness. These processes have found wide application in the coating of metal surfaces with zinc, chromium, andthe like. Colored deposits such as those ordinarily applied by chemical treatments can not, however, be applied by electrodeposltion. p

The provision of protective and ornamental finishes for aluminum has presented a particularly diflicult problem because aluminum is not readily amenable to the electrodeposition or chemical coloring treatments used for other metals.

' It is an object of the present'invention to provide processes for the electrodeposition of a colored deposit. It is a further object to provide processes for the coating of aluminum. It is a still further object to provide an ornamental and protective coating which may be applied by electrodeposition and which may be buffed or subiected to other surface treatments. It is a still furtherobjeet toprovide plating baths of great cadmium, copper,

Parent hereinafter.

These and other objects of my invention are accomplished by electrodepositing a molybdenum-oxyge'n deposit thru th use of a solution containing a soluble molybdenum compound and by the use of a current density such that a non-metallic coating of the desired characteristics is obtained. Preferably a soluble compound of a second metal is included in the plating bath and,.according to the preferred embodiment of my invention, boric acid is included.

The more specific and preferred aspects of my invention will be set out with mor particularity hereinafter. a

An electroplating bath according to the present invention may contain molybdenum in the form of any of its soluble compounds such as sodium molybdate, potassium molybdate, and ammonium molybdate. Ammoniummolybdat is, not

preferred, however, because it slowly forms insoluble ammonium complexes with an added metal such as nickel. The amount of mo ybdenum may be widely varied though in general it will be found desirable to use a relatively large amount approaching or approximating the of molybdenum compound More particularly it will be found satisfactory to use such an amount of a soluble molybdenum;

. compound as will provide for the bath from about be used such buffer 7 to 150 grams per liter of molybdenum calculated as metal. Still more specifically it will be found desirable in acid baths to use from about 7 to 30 grams liter.

In addition to a molybdenum compound the plating .baths of my invention should contain a buffer compound. A suitably selected bufler compound may be used in either the acid or alkaline baths of this invention more easily to maintain. a constant pH in the way in which buffers have heretofore been used tenance of-pH in various systems. There may tartaric acid, acetic acid, lactic acid, citric acid,

per liter of molybdenum and in alkaline baths from about to grams per.

for the-maincompounds as oxalic acid.

cyanides, and carbonates. By farthe best buffer compound is boric acid which in addition to a buffering action on the pH of the solution appears to exercise. a, solubilizing action upon the bath constituents and promotes the formation to use a pH of about 8.

The electroplating baths of my invention contain a metal in addition to molybdenum and this metal should be selected from the group consisting of nickel, copper, zinc, cadmium, tin, vanadium, titanium, and cobalt. By far the best results are obtained using nickel as the auxiliary metal and if vanadium is to be used the amount should be kept small.

The amount of the auxiliary metal to be used with molybdenum will 'depend upon the particular metal and upon the exact efiect desired. I have found that in general the amount of auxiliary metal should be such that there is about 6 to .25 times as much molybdenum as of the .secondary metal both metals being computed as the metal. In the preferred acid baths of this invention it will more specifically be found desirable to have from about 1 to times as much molybdenum as of the auxiliary metal while still more specifically some of the best results of the invention have been obtained when the molybdenum is present in an amount equivalent to 2% times the amount of the secondary metal. alkaline baths the best results have been obtained when molybdenum was present in an amount equivalent to about 3 times the amount of secondary-metal. If a number of auxiliary metals are used with molybdenum the same general considerations as to total amounts of secondary metals will apply.

The plating baths according to this invention may be modified as are plating baths for metals posits are not permanent but are thin, soft, and nonadherent. Moreover, the color of the deposit is not permanent. It will generally be found that a current density from about 1 to 15 amperes per square foot may be used tho above about 5 amperes per square foot the bath must be agitated for best results. In order to obtain adherent,

dense deposits a current density of about 2 to 5 amperes per square foot should be used while still more specifically the best results are obtained with a current density of from about 2 to 3 amperes per square foot.

The baths of thisinvention havebeen found quite stable upon continued electrolysis and it has been found that they may readily be maintained in operating condition by replenishing materials plated out. .It has alsobeen found that the baths have great throwing power and in fact some ofv them are unique in having a throwing power greater than the theoretical. As a result of'the great throwing power deeply recessed articles may readily be plated without the use of specially shaped anodes.

The deposits obtained from the baths of my invention are adherent upon various metal surfaces such as steel, copper and brass, but the best resultsare obtained upon a zinc, cadmium, tin, or aluminum base. Prior to applying adeposit of this invention the surface to which it is to be applied is preferably etched slightly with an acid such as hydrochloric, nitric, or sulfuric.

The deposits very markedly increase the corrosion resistance of metals to which they are applied and they are quite resistant to abrasion. The colors of the deposits vary from jet black to brown, red-greenfand to iridescent finishes depending upon the exact character of the bath used, and it appears that they are composed largely ,of compounds of molybdenum combined in some manner with oxygen. The deposits also j contain small amounts of the additional metals by the use of various addition compounds and I they are in general amenable to similar handling as to technique. The organic addition agents sometimes found useful in other plating. baths may frequentlybeused to advantage, and I have found, for instance, that glucose or glycollic acid eifect an improvement in the character of the deposits. Other addition'agents' which have been tried include naphthol disulfonic acid, naphthalene sulfonic acid, and phenyl sulfonic acid.

The current density to be used forthe electrodeposition of colored'electrodeposits according to my invention can best be determined for each specific bath by a few simple trials. It will usuallybe found that the current density is a very' important factor in the operation of these baths. If the current densities are substantially too high the deposits wiil be non-adherent, powdery or brittle. Generally it will be found that deposits of fairly satisfactory character will be' used with the molybdenum.

Like other plating solutions the baths of this invention are affected by the temperature of operation. The temperature used has a definite effect upon the quality of deposit produced andalthough the temperature of the bath can be varied frpm room temperature to the boiling point best results are obtained at a temperature of about C. the deposits produced at this preferred temperature being more uniform, ductile, adherent, and glossy than at other substantially different temperatures.

In order that my invention may be better understood reference should be had to the following illustrative examples:

. Example 1 A preferred bath for the production of colored deposits according to my invention was prepared .as follows: I

Boric acid (HaBOa) grams per Men. 48 Sodium molybdate (NaaMoOO do 22.5 Molybdic oxide (M003) do 15 Nickel sulfate (NiSOe-GHaO) ;-do 30 Sodium thiocyanate -ldo 3.8 pH

produced with current densities up to about fifteen amperes per square foot.

Since the character of deposit is aflected by the current density more than ordinary attention will want to be given to the selection of the current density for any particular plating system of this invention. at low current densities the de;

In use the bath was maintained at a temperature of about 70 C. Nickel anodes and carbon anodes were used to supply current to the bath. Black deposits were produced on zinc die castings, zinc plated articles, tin plated articles, and cadmium plated articles all of which were given a slight preliminary etch in hydrochloric acid.

- showing per cent by weight of some of the constituents of a deposit madeat 3 amperes per square foot is as follows:

' Molybdenum asMo 58.10

Oxygen as O 37.46 Nickel as Ni 4.71 Boron as B 1.49

Sodium thiocyanate is used in the bath of this example to remove copper which may be present as an impurity. The effect of copper in causing deposits to be brown in color is so marked that the copper 'must be removed in some manner if jet-black deposits are to be obtained.

The use of sodium thiocyanate has been found a very satisfactory way of removing this copper.

The relative amounts of molybdenum and auxiliary metals have been mentioned above and it is now noted that for the best results the ratio of secondary metal to molybdenum should be held within even closer limits at a given molybdenum concentration. As has been indicated the molybdenum content of the bath should vary between about 'I and 30 grams per liter and the nickel content for baths such as that of this example would vary from about 1.25 to 30 grams per liter when the molybdenum is at 7 grams per liter and the nickel content should vary from to 120 grams per liter when the molybdenum is at 30 grams per liter. With the preferred molybdenum content of about 19 grams per liter grams per liter. Considerations as to the relative amounts of other metals will be similar.

Example 2 A preferred bath for the production of colored deposits according to my invention was prepared using the following ingredients in the indicated amounts:

Boric acid (H:BOa) grams per Men. 75 Ammonium molybdate ((NHoaMoOoedoun Nickel sulfate (NiSOrGHzO) ;do 30 2 square foot and at a temperature between aboutfifty-five and sixty-five degrees centi'grade. Nickel and carbon anodes were used, about ten per cent of the anodes being nickel, and about ninety per cent of them being carbon.

A lustrous, jet black deposit was obtained. Similar deposits were produced on zinc. and

on other base surfaceswith great success. It'

was found desirable when treating ferrous articles to plate them with bright zinc preparatory to the application of the black molybdenum finish, though this was'not essential.

The deposits obtained with this bath at the the nickel content may vary from about 3 to 75 grams .per liter but preferably will be about 7 tained under the conditions of this of the low density of these deposits and the high rate of deposition by weight, the rate of deposition by thickness is very great. The deposit of this example is plated at a rate approximately twenty-one times as fast as nickel. It is to be observed, however, that the rate of deposition falls off after about ten minutes of plating and the maximum thickness of deposit obexample was about 0.001inch.

- The deposits include molybdenum in the form of some oxygen compound, but I do not under- From the. analysis of this particular deposit it will be observed that nickel is also present in minor amount. This maybe combined with the oxygen or with the other materials present in some unknown manner. Boron is presentin. an amount greatly in excess of what would be expected from a mere occlusion, .and it is quite possible that the boronis in some manner electrolytically deposited or else is in some manner co-depositad with the other materials present. These novel deposits containing molybdenum. oxygen, nickel. andboron constitute an importent and preferred embodiment of my invention.

The amount of nickel sulfate present in the bath of this example may be widely varied, but I have found that it is in general most satisfactory to use between about twenty-five and'sixty grams per liter of the nickel'sulfate crystals. With much larger amounts of nickel sulfate the deposits are less black, -verging on a gray color current density used were much lighter in a weight than metal deposits and they were so light in fact that when pieces of the deposit were stripped from the base surface some of them would float in water. was found that-the average density of the deposit was about 1.006 compared to water. By reason Upon determination it' and with amounts much below about twenty-five grams they are likewise less black and are less adherent and less ductile.

The. amount of ammonium molybdate employed is about an optimum under the conditions of this example, but this may be widely varied; In use the bath composition will naturally fluctuate considerably and over aperiod of four weeks of intermittent operation the ammonium molybdate fell to about seventeen grams perliter. At this concentration'the bath was still operating satisfactorily. It is to be observed that under these conditions the nickel sulfate had dropped to twenty-two grams per liter. In general it will be found desirable'in a bath such. as the one of this example to use from about fifteen to fifty grams per liter of ammonium molybdate. It is particularly to be observed that the amount of ammonium molybdate must be adjusted de-" pending upon thespecific additional metals employed together with molybdenum.

The boricacid concentration of" the bath of this example may be widely varied and excellent results have been obtained using from'about thirty-five to eighty grams per liter. It is pre-. ferred to use about 45 grams per liter.

Various additional materials may be added to the bath to adjust its pH or its properties as desired. I have found that by adding a small amount of copper sulfate crystals to the bath of this example brown to mahogany colored deposits are produced. Various copper compounds give similar results and it is noted that four milligrams per liter of copper sulfate (CuS0t 5Ha0) Examples 1 and 2. Copper if present in the bath as an impurity may readily be removed in any suitable way and it may be removed, for instance as in Example 1, by the use ofsodium thiocyanate.

Example 3 Another bath suitable for use according to my invention was prepared withthe following:

Grams per liter Ammonium molybdate ((NHO 2M004) 100 Stannous chloride (SnClz) 30 Sodium hydroxide (NaOH) 75 Glucose 60 i Black deposits were obtained from this bath on hardware; on instrument parts such as telescope,

aluminum and on various other base metals.

The deposits contained about fifty-two per cent of molybdenum and five per cent of tin in addition to oxygen. The bath was operated at a temperature of forty degrees centigrade and it was observed that with higher temperatures the deposits were somewhat more brittle. v v

The ammonium molybdate was varied in this bath from about one hundred to one hundred and fifty grams per liter and satisfactory deposits were obtained.

The tin chloride may be used in widely varying amounts though it is in general desirable that the amount be considerably smaller than the amount of molybdenum. In general from about ten to thirty grams per liter could be used with satisfactory results. The bath of this example had an amount of molybdenum about three times as great as the amount of tin.

The glucose serves as an addition agent and when it is omitted the deposit is less ductile and less adherent. The amount of glucose may be widely varied as is customary in the use of addition agents in plating baths for metals, and the specific amount to use in a particular bath can easily be determined by a few simple trials.

Example 4 Another bath according to my invention was made up as follows:

, Grams per liter Ammonium molybdate 'flNHi) aMOOs) 200 Zinc cyanide (Zn(CN) 2) 60 Sodium cyanide (NaCN); 42 Sodium hydroxide (NaOH) 78 With this bath black deposits of good character were produced on aluminum, zinc, copper, and other metal articles. The bath was operated at a temperature of about thirty' degrees centigrade. It is noted that the .amount of molybdenum present is about three times as great as the amount of zinc.

' The ammonium molybdate was varied in this bath from twenty-five to two hundred grams per liter. 'The zinc cyanide was varied from ten to ninety grams per liter, sodium cyanide from twenty-five to one hundred and fifty grams per liter, and the sodium hydroxide from two to one hundred grams 'per liter. It will be understood that the various constitutents may be consider ably varied in accordance with the practice known to-the art in conjunction with the electrodeposition of metals.

The compositions of this invention may be ofassume. fered to the tradeas dry, powdered mixtures and such composition as that shown in Example 1 constitutes a preferred feature and embodiment 'of my invention. The plating compositions may if desired be oifered as concentrated solutions or even as solutions ready for use.

The electrodeposits of this invention will be found suitable for a wide variety of ornamental and protective applications; Deposits of ,this' invention have been used on hardware such as grass shear handles, stove parts, and builders that one skilled in the art may readily devise numerous solutions and compositions containing molybdenum for the electroplating of molybdenum-oxygen deposits without departing from the spirlt'of my invention.

This application is a continuation-in-part of application Serial Number 235,799, filed-October 19, 1938.

I claim: I

1. In a process for the production of an electro-geposit, the step comprising electrodeposit- 'ing an adherent, abrasion-resistant cathode deposit of an oxygen compound'of molybdenum at a current-density of about from 1 to 15 am-,, peres per square foot from an aqueous acid bath including boric acid, a soluble nickel compound, and a soluble molybdenum compound selected; from the group consisting of sodium molybdate, potassium molybdate, ammonium molybdate, and molybdic oxide, the amount of molybdenum 'present, calculated as metal, being from '1 to 150 grams per liter.

. 2. In a process for the production of an electrodeposit, the step comprising electrodepositing an adherent, abrasion-resistant cathode deposit of an oxygen compound of molybdenum at a current density of about from] i to 15 amperes per square foot from'an aqueous acid bath of pH 2.5 to 5, the bath including boric acid, nickel sulfate, and a soluble molybdenum compound selected from the group consisting of sodium molybdate, potassium molybdatel ammonium molybdate, and molybdic oxide, the amountof molybdenum present,'calculated as metal, being from 7 to 150 grams per-liter and from 1 to 5 times the amount of nickel. a

3. In a process for the production of an elec-. trodeposit, the step comprising electrodepositing an adherent. abrasion-resistant cathode deposit of-an oxygen compound of molybdenum at a current density of about from 1 to 15 amperes liter of nickel in theform of asoluble nickel compound, and about 19 grams per liter of molybdenum in the form of a soluble-"molybdenunf compound selected from the group consisting of sodium molybd'ate, potassium inolybdate, am

monium moybdate, and molybdic oxidc} the 1 to 5 times the amount of nickel.

5. A composition for the electrogieposition of an adherent, abrasion-resistant, molybdenumoxygen cathode deposit, the composition consisting essentially of 35 to 80 parts by weight of boric acid, 7 parts by weight of nickel in the form of nickel sulfate, and 19 parts by weight of molybdenum in the form of a soluble molybdenum compound selected from the group con- 7 sisting of sodium molybdate, potassium molybdate, ammonium molybdate, and molybdic oxide, the amount of molybdenum present,

- calculated as'metal, being from 1 to 5 times the amount of nickel.

RAYMOND A. HOFFMAN.