US2075623A - Zinc plating - Google Patents

Description

Patented Mar. 3%,. 1937 UNITED STATES ZINC PLATING Floyd F. Oplinger, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, Inc., Wilmington, Del., a corporation of Delaware No Drawing. Application November 12, 1934, Serial No. 752,704

19 Claims.

This invention relates to the .production of corrosion resistant zinc coatings on metals and more particularly to the production of zinc coat ings by electroplating.

5 Zinc coatings offer excellent corrosion resistant protection for metallic articles, especially for iron or steel surfaces. Protective zinc coatings I may be applied by hot dipping in molten zinc or by electroplating methods. Heretofore, it has been difilcult to produce heavy zinc coatings of high ductility, with the result that when zinc coated articles are drawn or bent during manufacturing processes or when in use, the zinc coating tends to crack or chip, impairing the corroslon resistance of the coating. Thus, the galvanizing process wherein zinc is coated by dipping metal into molten zinc produces a bright, attractive surface but the coating is relatively brittle and tends to crack or chip off when plated articles are bent or hammered in manufacturing processes. A common method of electroplating zinc comprises the use of an electrolyte containing zinc cyanide, alkali metal cyanide and an alkalining agent such as caustic soda. This electroplating method as utilized heretofore results in zinc coatings of low ductility and furthermore, the coating is of a dull color and less attractive in appearance than the galvanized coating. Such dull electroplated coating must be brightened by burnishing or other mechanical treatment when a bright appearance is desired.

An object of the present invention is to provide a method for producing highly ductile electroplated zinc coating on iron, steel or other electrically conductive bases. A further object is to electrodeposit highly ductile zinc coatings which have a bright appearance. Other objects will be apparent from the following description of my invention.

The above objects may be attained in accordance with my invention by preparing an electrolyte solution containing zinc cyanide, alkali metal cyanide and an alkalining agent, e. g., sodium hydroxide, then purifying the electrolyte by methods hereinafter set forth and thereafter employing the purified solution as electrolyte for electroplating zinc while using a cathode current density sufficiently low to obtain a cathode current eiiiciency of at least 90%. The tempera- 5 ture of the electrolyte during the electroplating operation may be either at low temperature or at an elevated temperature, up to the boiling point of the solution. In' some cases, as hereinafter more fully explained, it is preferable to 5:, operate at elevated temperature since this usually allows a higher cathode current density to be employed. When the electroplating process is operated in accordance with my invention at cathode current efliciencies of less than 100% 6 but not lower than 90%, highly ductile deposits are formed which are dull in color or have yellow to brownish coating thereon. Such dull deposits may be transformed to bright plates without loss of ductility by a mild oxidizing treatment, as hereinafter explained. By operating at sufflciently low cathode current densities to attain a cathode current efliciency sufficiently high (1. e. close to 100%) that substantially no hydrogen is evolved at the cathode, bright, crystalline electrodeposits of extremely high ductility are directly obtained without further treatment.

In accordance with my invention, before the electroplating operation is carried out, the electrolyte, which consists of zinc cyanide and alkali metal cyanide and an alkalining agent, first must be purified by one of the hereinafter specified methods. Ordinarily, in zinc plating, the soluble anode is made of high purity zinc. However, I have found that even the best grades of zinc anodes available contain small amounts of impurities andthese impurities may be imparted to the electrolyte during electrolysis if certain precautions are not followed. I have found that my herein described method of bath purification removes such impurities from the electrolyte and I have further discovered that once these impurities are removed, their occurrence in the electrolyte in soluble form is prevented by operating the bath to electroplate zinc in accordance with my herein described method.

One method of accomplishing purification of the bath consists in adding thereto a reducing agent. Among the reducing agents which I have found suitable for this purpose are: sodium bisulfite, sodium thiosulfate, phenyl hydrazine hydrosulfate and various water soluble aldehydes, e. g., formaldehyde. The amount of reducing agent required will depend upon the amount of impurities in the electrolyte and/or in the material used as soluble anode. In general, the amount of reducing agent required to purify the electrolyte will be from 0.25 to 0.5 ounce per gallon; if desired large amounts, e. g., up to 10 ounces per gallon may be used. The addition of the reducing agent appears to cause the impurities to precipitate in an insoluble form; the impurities usually settle to the bottom of the bath.

Another method of purifying the bath in accordance with my invention consists in electrolyzing the solution, preferably using zinc anodes, with low anode current densities, preferably current densities not above about. 15 amperes per square foot, until the impurities in the electrolyte have been removed from solution by precipitation on the anode. Ordinarily, the time required for this method of purification varies from 12 to 72 hours. If the solution is cold, the

anode current density should not be substantially higher than 1'5 amperes per square foot; at temperatures of 60 C. or higher, somewhat higher vention consists in subjecting the electrolyte to the galvanic action produced by immersing zinc anodes in the bath in intimate contact with iron or steel cathodes. This may conveniently be done by placing the solution in a steel tank and hanging zinc anodes in the solution in contact with the sides of the tank. This galvanic action also may be produced by covering the bottom of a steel tank containing the electrolyte with a layer of granulated zinc. The time required for the purification may be shortened by heating the solution to 60 C. or higher. In these electrolytic methods, the impurities precipitate out on the anodes and fall to the bottom of the container.

A still further method of bath purification consists in precipitating a fiocculent, colloidal precipitate in the solution, which precipitate apparently adsorbs the impurities. For example, I have obtained excellent purification by precipitating ferric hydroxide by slowly adding a ferric salt, as ferric sulfate to the alkaline bath, with stirring and then allowing the precipitate to settle. In this method, it is preferable to have substantially all of the iron in the ferric state.

If desired, the bath purification may be accomplished by combination of the above methods,

for example, in addition to the purification by electrolysis or galvanic action, a reducing agent may be added to the electrolyte before plating.

The above described bath purification methods will remove soluble impurities from the bath to produce an electrolyte which will deposit highly ductile zinc by my herein described method. Under the conditions of plating in accordance with my invention, further impurities which exist in the anode or 'in electrolyte material thereafter added to the bath are precipitated during electrolysis, and hence the electroplating operation may be conducted for long periods of time without further bath purification. The bath when purified, may be allowed to stand for considerable periods between plating operations without purification and ordinarily no further purification is required when plating is resumed. The impurities removed or rendered insoluble by the bath purification appear to be soluble compounds of heavy metals such as cadmium and/or lead. The presence of extremely small amounts of such impurities appear to be deleterious in my process and such small amounts are eifectively removed by the above described purification methods.

.By way of illustration, one method, of practicing my invention will be described. Asolution is made up which contains from 6 to 12 ounces per gallon of zinc cyanide, 3 to 6 ounces per gallon of sodium zinc cyanide and 6 to 16 ounces per gallon of sodium hydroxide. This aqueous solution then is purified by one of the above described purification methods, for example by adding thereto a small amount of sodium bisulfite. The solution then is employed as electrolyte for electroplating zinc with soluble zinc anode at low temperature. The anode may be of the high purity zinc commonly used in zinc plating or it may be of zinc containing a small amount of alloyed vmercury. A zinc-mercury anode is preferable to the pure zinc anode because the eifect of the mercury in the former is to facilitate the separation of impurities which deposit on the anode during the electro-plating operation. The small amount of mercury in the zinc anode, e. g., l to 2%, apparently tends to amalgamate with the impurities deposited on the anode and thereby aids such impurities to settle to the bottom of the electrolyte container. During the electroplating operation, the cathode current density is maintained sufilcienty low that a cathode current efliciency of at least 90% is obtained. For this purpose, cathode current densities of 10 to 60 amperes per square foot may be advantageously employed: with these cathode current densities cathodic zinc deposits are somewhat dull and usually are covered with a brownish or yellowish film. At lower cathode current densities, e. g., l to 8 amperes per square foot, a bright crystalline deposit is directly obtained. In either case, whether the deposit is dull or bright, it is highly ductile. The high ductility of the deposit may be demonstrated by bending or folding the electroplated article, which manipulation may be accomplished without cracking, breaking or other failure of the zinc coating. I have thus produced highly ductile deposits on sheet steel which showed no signs of failure on continuous fold and unfolding of the plated sheet until the base metal itself was cracked bythis manipulation.

The dull deposits or those having a brownish film may be transformed into bright deposit by a mild oxidation treatment. The oxidizing treatment may be carried out by merely immersing the plated article in a dilute, acidic solution of an oxidizing agent, preferably at room. temperature, until the desired brightening effect is obtained. Ordinarily, the brightening effect is obtained by immersion of less than one minute, e. g., 5 to 20 seconds. After the brightening treatment, it is preferable to wash the plated article before allowing it to dry. Examples of preferred oxidizing agents suitable for making these brightening solutions are nitric acid, hydrogen peroxide and chromic'oxide. Ex amples of specific brightening solutions which I have found suitable follow:

Solution A Aqueous solution containing Weight of nitric acid.

Solution B Aqueous solution containing:

, Percent by weight Sulfuric acid 2 to 5 Hydrogen peroxide (l-volume) 2 to Solution 0 Aqueous solution containing:

' Ounces per gallon Chromic acid 16 to 32 Sulfuric acid (conc.) 0.16 to 0.32

Solution D /2 to 1% by an anodic electrolytic treatment wherein the plated article is made in the anode in an alkaline electrolyte, preferably an alkaline electrolyte con-- taining sodium cyanide, for a short time, c. g.,--

to seconds.

The brown film often may also be removed by merely allowing the wet, electroplated article to stand in the air for a short time after its removal from the plating bath whereupon the oxidizing l0 effect of the air upon the wet surface causes the plate to brighten.

My invention is further illustrated by the following examples:

EXAMPLE 1 Bath purification by electrolysis Pieces oi sheet steel 2" x 6" were plated in the following solution and operating condition:

Electrolyte: Ounces per gallon Zinc cyanide 10 Sodium cyanide 3 Caustic soda 10 Operating conditions:

Anodes-Electrolytic zinc Ratio of anode to Cathode 2 or 3 to l Cathode current density 15 to 40 amps. per sq. ft. Anode current density 5 to 15 amps. per sq. ft. Bath temperature 20 to C.

35 The resulting electrodeposits were dark, greycolored and unsightly and tarnished and fingerstained readily. These deposits were somewhat brittle and tended to crack and peel from the base metal when the plated sheets were bent.

The solution then was purified by electrolyzing it for 24 hours under the following operating conditions:

@athode current density 1 amperes per square foot Anode current density 5 amperes per square foot Bath temperature--- 20 to 30 C.

As a result of the above electrolysis, the anode became covered with a dark colored, loosely adherent spongy film. Portions of the spongy mass also settled to the bottom of the bath. Portions of the spongy mass were removed from the bath and upon analysis, was found to contain considerable amounts of cadmium and traces oi lead.

After the above described purification by electrolysis, the bath again was employed as'electrolyte to electroplate zinc on various steel articles, under the folowing operating conditions:

Bath temperature--- 20 to 30 (2.

Cathode current density 1 to 100 amperes per sq. ft.

At cathode current densities of 1-8 amperes per square foot, there was no hydrogen evolution at the cathode and a bright crystalline deposit was obtained, which were highly ductile. The high ductility was demonstrated by bending steel wire, coated with 0.001 to 0.003 inch of the electrodeposit, about their own diameter; no cracking or other failure of the deposit occurred during this manipulation.

At cathode current densities of 10 to 100 amperes per square foot, some hydrogen evolution occurred at the cathode and the deposit was cov- "the lower current densities.

Percent by weight Sulfuric acid 2 to 5 Hydrogen peroxide (-volume) 2 to 5 (c) immersing for 10 to 30 seconds at room temperature in an aqueous solution of:

Ounces per gal. Chromic acid 16 to 32 Sulfuric acid 0.16 to 0.32

(d) immersing at room temperature in an aqueous solution of:

Sodium cyanide 2 to 4 ounces per gal, Hydrogen peroxide (100-vol.) 2 to 5% by weight (e) The plated article, wet with the electrolyte, was allowed to stand in the air immediately after its removal from the plating bath for 10 to 30 minutes.

(,f) Reversing the current in the plating bath, to make the plated article the anode and then. electrolyzing for 5 to 10 seconds.

EXAMPLE 2 A number of solutions were made up having the following ingredientsin the concentration limits noted:

Ounces per gallon Zinc cyanide 6 to 16 Sodium cyanide 2 to 8 Sodium hydroxide 6 to 16 These solutions were used to carry out the operations described in Example 1, and substantially the same results were obtained. In each case, deposits from unpurified baths were dark-colored and more or less brittle, while after purification by electrolysis, fiche baths plated highly ductile deposits.

EXAMPLE 3 Effect of reducing agents The following solution was prepared:

Ounces per gal. Zinc cyanide 10 Sodium cyanide 5 Sodium hydroxide 10 This solution was purified by adding thereto 0.25 to 3 ounces per gallon of sodium bisulfite and allowing precipitated material to settle. The solution then was employed as a zinc plating bath, under the following operating conditions;

Anode-Electrolytic zinc Ratio of anode to cathode area 2 to 1 Cathode current density 1 to 40 amps. per sq. ft, Bath temperature 20 to 30 C.

The same results were obtained as under Example 1, that is,

(a) Shiny white deposits having high ductility at cathode current densities of 1 to B-amperes per square foot.

(b) Brownish yellow deposits which could be 5 brightened by dipping in acidic omdizing solutions at cathode current densities of to 40 amperes per square foot.

EXAMPLE 4 10 Purification by galvanic action EXAMPLE 5 Zinc plating solutions were made up containing varying concentrations of zinc cyanide, sodium cyanide and caustic soda. Each bath was purifled before use by the addition of 0.5 ounces per gallon of sodium bisulfite. Pieces of sheet steel two inches by three inches, used as cathodes, were plated in these baths, some at 60 C. bath temperature and others80 C. bath temperature. During each electroplating operation, the electroplating circuit included a copper coulometer for the purpose of determining the cathode current eniciency. Varying cathode current densities were used and the maximum. cathode current density at which a cathode current efficiency of 95% or higher could be obtained was determined at each temperature. With each electrolyte, operating at cathode current efficiencies of 90% or higher, highly ductile deposits of zinc were obtained.

The ductility of these deposits was demonstrated-- by folding the plated sheet upon itself and unfolding; no cracking or other failure'of the zinc plate was caused by this manipulation. The above mentioned maximum cathode current den-- sities are shown in the following table:

Maximum cathode C. D. for gg gi g fgg cathode current efficiency of 1 gr at least 95 percent Zn (ON) NaCN NaOH At 60 C. bath At80..C. bath Ounces Ounces Ounces temperature. tempera per gallon per gallon pergallon AIBF r A/SF:

4 3 e e5 ,1oo:. 4 3 l0 78.. --1m;' 8 3 4 -x'80 8 3 8 Over 130 i Over-200 8 5 8 80 150 1 s 9 s v 40 100. 12 a 12 12 3 16 Over 300 Over 600 EXAMPLE 6 A number of solutions containing zinc cyanide,

sodium cyanide and caustic soda were made up" and to each was added 0.5 ounceper gallon of maximum current density was used whichwould produce bright plate, with substantially no hydro- 7 genevolution at the cathode. .Undertheseconditions, bright, lustrous deposits of'h ighly ductile,

zinc were produced. The maximum current densities, at temperatures of and 90 C. are given in the following table:

The following solution was prepared and by analysis was found to contain:

Ounces per gallon Zinc cyanide 9.45 Sodium cyanide (total) 13.8 Caustic soda 7.0

The solution waselectrolyzed with electrolytic zinc anodes for a period of approximately 48 hours under the following conditions:

Bath temperature C. Cathode current density 60 to 100 amperes per square foot 10 to 20 amperes per square foot Heavy zinc deposits, 0.003 inch to 0.008 inch thick, were produced, which were very ductile and did not separate from the steel on bending. Beautiful shiny white deposits were obtained at cathode current densities of 18 to 100 amperes per square foot on cold rolled sheet steel cathodes,

Anode current density two to three inches in size.

' Exam 8 The following solution was prepared in a steel tank':

. Ounces per gallon Zinc cyanide 10 Sodiumcyanide 4 Caustic sedan 12 Because offv the heat. of solution of the caustic soda, the temperature of the solution when made up was about 60 C. Pure zinc anodes (electrolytic grade) immediately were hung into the solution'in directbontact with the sides of the steeltank. {As a result, considerable hydrogen was evolved on the surface ofithe steel tank which became cathode in contact with the zinc anodes.

After 48 hours the surface of the steel tank was ".visibly coated, chiefly with precipitated impurities. Upon subsequent electrolysis of the solution 'attemperatures above 60 C. and at cathode current densities, at which cathode current efllciencies of to were obtained, heavy zinc deposits having {very high ductility were readily produced on the "cathode.

EXAMPLE 9 Samples oflsteel wire were electroplated from two plating baths, each containing about eight ounces per gallon of zinc cyanide, about four ounces per gallon of sodium cyanide and about p One bath was purified prior to the electroplating opeight ounces per gallon of sodium hydroxide.

eration by the addition of about 0.5 ounce per gallon of sodium bisulfite. The other bath was used without purification treatment. The purified .J bath wasoperated at cathode-current densities at which (a) in one operation, the cathode current emciency was about 95% and (b) in another operation there was substantially no hydrogen evolution at the cathode. The ductility of the coatings obtained was tested by bending the plated and alkalining agent, expressed in equivalents of sodium cyanide and caustic soda, respectively, should be at least equal to the zinc cyanide concentration. Preferably, the concentration of the alkalining agent, expressed as caustic soda wires. The data and results obtained are tabuequivalent, should be equal to or greater than lated below: the zinc cyanide concentration. Examples of preferred electrolyte compositions follow:

Cathode Beth Descriptionoielectro- Bath R 0 D. temlh deposit Bath ZH(ON)I NEON N803:

p-m-fl- .A-pnrliledbeth. A-l 90 Bright, crystalline; l 1 02 g" Oz f" 51110.111 flaihgut 13.03 (J 3 4 3 I10 0 ;008 ng 12 did not crack when 6 12 0% it l i 8 Oil se kg 200 80 Du my color; It is to be understood that baths having difler about o.00 1 inch ent concentration ratios may be used but in such f a mt"!!! cases correspondingly lower cathode current crack when vgire wa s bent densities usually must be employed. For ex- 8 011 so B4 30 30 Du grey com ample, if the sodium cyanide concentration is both. darker than 4-2, either greatly increased or greatly decreased ,-%g fg beyond that shown in the above table in any crackeciandpeeled, one of the baths illustrated, the maximum alggg f sgff sfg l lowable cathode current density will be correbent about itself. spondingly less. Also if the proportion of caustic 3-2 gggg i eggfg gggf soda is greatly decreased from that shown, the

than B-l; very maximum cathode current density will be less brittle but if desired, higher concentrations of caustic cracking badly and geeiing to expose than those given above may be used without deff: gg f'f gg creasing the maximum cathode current density an gle of less than figure. I have found that by carefully proportioning In order to obtain a highly ductile coating of zinc plate in accordance with my invention, the purified electrolyte, is operated at a cathode current density sufliciently low to result in a cathode current efliciency of at least about 90%. The maximum current density which may be used will vary depending upon the bath temperature and the concentration of the ingredients.

Under the abo e stated conditions, the respectlve concentrations of zinc cyanide, alkali metal cyanide and alkalining agent may be varied greatly in practicing my invention. However, it is preferable to have some of theconcentrations of alkali metal cyanide and alkalining agent, expressed in equivalents of sodium cyanide and caustic soda, respectively, equal to at least about 0.8 times the concentration of the zinc cyanide. Also the alkali metal cyanide and alkalining agent each should be present in appreciable amounts; I prefer to use an amount of alkali metal cyanide equivalent to a sodium cyanide concentration equal to at least 0.25 times the zinc cyanide concentration and an alkalining agent concentration equivalent to a caustic soda concentration equal to at least 0.2 times the zinc cyanide concentration. r

In general, as the total concentration of electrolyte ingredients is increased, the maximum current density at which the highly ductile deposits may be obtained is likewise increased, for a given operating temperature. However, I have found that the increase in this maximum current density depends on the relative proportions of the ingredients, as well as the total concentration. For example, for a given zinc cyanide concentration, in order to attain the highest possible ourthe electrolyte ingredients, I may produce highly ductile zinc deposits at cathode current efiiciencies of or better while employing unusually high cathode current densities, e. g., 40 to amperes per square foot with a cold bath and 100 to 500 amperes per square foot or even higher if the bath is heated, e. g. to 60 C. or higher. In order to obtain a highly ductile deposit and utilize such high cathode current densities, the zinc cyanide content of electrolyte preferably is not less than about 8 ounces per gallon, the ratio of sodium cyanide concentration to zinc cyanide concentration must lie between 0.25 and 0.6 and the concentration of alkalining agent must be equivalent to at least 6 ounces per gallon of sodium hydroxide. By operating at high bath temperatures, e. g., 80 C. or higher, the same results may be obtained with a wider range of concentration of the electrolyte ingredients. However, even at these higher temperatures, in order to utilize a cathode current density of 100 amperes per square foot or higher, the zinc cyanide content ordinarily should not be less than 4 ounces per gallon, the alkalining agent should be equivalent to a caustic soda concentration at leastcequal to that of the zinc cyanide and the ratio of the alkali metal cyanide concentration to the zinc cyanide concentration should be substantially as specified above.

I have discovered that if the conditions of the electrolysis are so conducted that the cathode current density is sufliciently low that there is substantially no hydrogen evolution at the cathode, the highly ductile deposit Which is formed on the cathode has a bright and lustrous appearance. To attain this result, it is necessary to operate with a cathode current efliciency close to 100%, e. g. 97 to 100% efliciency. This bright, lustrous, ductile deposit is unique in that it is highly resistant to tarnish and does not readily finger stain when handled. Further, these bright deposits in general are somewhat more ductile than the above described slightly dull but highly ductile deposits produced by operating at around 90 to 95% cathode efllciency. Practically any alkaline zinc .plating bath containing zinc cyanide, sodium cyanide and caustic soda or its equivalent may be used to produce such bright,

lustrous, ductile deposits provided that the conditions are such that substantially no hydrogen is evolved at the cathode. In order to produce the extremely high current efficiency necessary to 10 produce the bright deposit, the cathode current density must be correspondingly lower than is required to electroplate at lower cathode current efilciencies. Thus a solution containing four ounces per gallon of zinc cyanide may have to be operated at cathode current densities as low as l to 5 amperes per square foot in order to produce the bright deposits. In order to utilize a reasonably high current density and thereby decrease the time required for plating a given deposit, I prefer to operate at temperatures of 60 to 80 C. or higher, preferably at the boiling point of the solution. At such temperatures, a bath containing four ounces per gallon of zinc cyanide and suitable amounts of alkali metal cyanides and alkalining agents may be operated at 10 to 50 amperes per square foot current density to produce the bright, ductile plate. At higher zinc cyanide concentrations and with suitable proportions of alkali metal cyanide and alkalining agent and by operating .at bath temperatures of around 90 0., I have been able to produce these lustrous, ductile deposits at cathode current densities as high as 80 amperes per square foot.

In addition to the effects of temperature and electrolyte concentration, the maximum current density which may be used in practicing my invention may also depend more or less on the size and shape of the electrodes and the spacing between anode and cathode. For example, theoretical considerations indicate that a spherical cathode, concentric with a hollow sphere anode would allow higher cathode current densities to be employed with a given solution and temperature, to attain a given cathode current efficiency than any other cathode-anode arrangement. In general, large cathodes require smaller cathode current densities than small ones and deeply recessed cathodes require smaller current densities than fiat pieces.

While, for the purpose of illustration, I have described electrolytes containing sodium cyanide, other alkali metal cyanides, e. g., potassium cyanide may be used equally well. Likewise, I have found that other strong alkalies, e. g., potassium hydroxide, may be used in place of caustic soda as alkalining agent.

I claim:--

1. A process for electroplating zinc comprising preparing an aqueous solution containing zinc cyanide, alkali metal cyanide and an alkalining agent, treating said solution to substantially completely precipitate soluble heavy metal impurities therefrom'and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a cathode current density sufliciently low to obtain a cathode current efiiciency not lower than about 90%.

2. A process for electroplating 'zinc comprising preparing an aqueous solution containing zinc cyanide, alkali metal cyanide and an alkalining agent, treating said solution to substantially completely remove soluble heavy metal impurities therefrom and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a cathode current density sumciently low to prevent substantial hydrogen evolution at the cathode.

3. A process comprising adding to a zinc plating bath which contains zinc cyanide, alkali metal cyanide and an alkalining agent a soluble reducing agent in a concentration not less than that required to substantially completely precipitate heavy metal impurities from said bath and thereafter electrodepositing zinc from said bath at a cathode current density which is sumciently low-to obtain a cathode current efilciency not lower than about 4. 'A process for electroplating zinc comprising preparing an aqueous solution containing zinc cyanide, alkali metal cyanide and an alkali metal hydroxide, treating said solution to precipitate soluble impurities therefrom by adding to said solution sodium bisulflte in a concentration not less than about 0.25 ounce per gallon'and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a cathode current density sufficiently low to obtain a cathode current efllciency not lower than about 90%.

5. A process for electroplating zinc comprising preparing an aqueous solution containing zinc cyanide, alkali metal cyanide and an alkali metal hydroxide, treating said solution to substantially completely precipitate soluble heavy metal impurities therefrom by subjecting said solution to electrolysis with an anode current density of not more than about 20 amperes per square foot when the solution temperature is not lower than 60 C. and not more than about 15 amperes per square root when the solution temperature is below 60 C. and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a cathode current density sufllciently low to obtain a cathode current efliciency not'lower than about 90%.

6. A process for electroplating zinc comprising preparing an aqueous solution containing zinc cyanide, alkali metal cyanide and an alkalining agent, the alkali metal cyanide concentration of, said solution, expressed in equivalents of sodium cyanide, being 0.25 to 0.8 times the zinc cyanide concentration and the sum of the concentrations of the alkali metal cyanide and the alkalining agent being not less than the zinc cyanide concentration,'treating said solution to substantially completely precipitate soluble heavy metal im purities therefrom and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a cathode current density sufliciently low to obtain a cathode current emciency not lower than about 90%.

7. A process for electroplating zinc comprising preparing an aqueous solution containing not less than about 8 ounces. per gallon of zinc cyanide, not less than 3 ounces of sodium cyanide and not less than fi ounces per gallon of sodium hydroxide, the sodium cyanide concentration being not more than 0.6 times the zinc cyanide concentration, treating said solution to precipitate soluble impuritiestherefrom by adding to said solution sodium bisulfite in a con-;

centration not'less than about 0.25 ounce per gallon in a concentration of 0.25 to 1.0 ounce per gallon and therafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a cathode current density sufficiently low to prevent substantial hydrogen evolution at the cathode.

ing preparing an aqueous solution containing zinc cyanide, alkali metal cyanide and an alkalining agent, treating said solution to substantially completely precipitate soluble heavy metal impurities therefrom and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a cathode current density suiliciently low to obtain a cathode current emciency not lower than about 90% and thereafter subjecting the elec troplated articles to a mild oxidation treatment.

9. A process comprising adding to a zinc plating bath which contains zinc cyanide, alkali metal cyanide and an alkali metal hydroxide, a soluble reducing agent in an amount not less than that required to substantially completely remove heavy metal impuritiesrfrom said bath, thereafter electrodepositing zinc from said bath at a current density sufficiently low to obtain a cathode current efllciency not lower than about 90% and treating the resulting electrodeposit with a dilute acidic solution of an oxidizing agent.

10. A process for electroplating zinc comprising an aqueous solution containing zinc cyanide, sodium cyanide and sodium hydroxide, the sodium cyanide concentration of said solution being 0.25 to 0.8 times the zinc cyanide concentration and the sum of the concentrations of the sodium cyanide and sodium hydroxide being not less than the zinc cyanide concentration, treat-- ing said solution to precipitate soluble impurities therefrom by adding to said solution sodium bisulfite in a concentration not less than about 0.25 ounce per gallon and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a temperature not lower than 60 C. with a cathode current density sufficiently low to obtain a cathode current efiiciency not lower than about 90% and thereafter treating the electroplated articles with a dilute solution of hydrogen peroxide.

11. The process comprising treating a zinc plating bath containing zinc cyanide, alkali metal 45 cyanide and an alkalining agent to substantially completely precipitate soluble heavy metal impurities therefrom, thereafter electrodepositing zinc from said bath and subjecting the resulting electrodeposit toa bright dipping operation.

12. The process comprising treating a zinc plating bath containing zinc cyanide, alkali metal cyanide and an alkalining agent to substantially completely precipitate soluble heavy metal impurities therefrom, thereafter electrodepositing zinc from said bath and. treating the resulting electrodeposit with a dilute nitric acid solution.

13. The process comprising adding to a zinc plating bath which contains zinc cyanide, alkali metal cyanide and an alkali metal hydroxide to a soluble reducing agent in a concentration not less than that required to substantially completely precipitate soluble heavy metal impuriies in said bath, thereafter electrodepositing zinc from said bath and treating the resulting electrodeposit with a dilute acidic solution of an oxidizing agent.

14. The process comprising adding to a zinc piating bath which contains zinc cyanide, alkali metal cyanide and an alkali metal hydroxide a soluble bisulfite in a concentration not less than that required to substantially completely precipitate soluble heavy metal impurities in said bath, thereafter electrodeposlting zinc from said bath and treating the resulting electrodeposit with a dilute nitric acid solution.

15. A. process for electroplating zinc comprising preparing an aqueous solution containing zinc cyanide, alkali metal cyanide and an alkalining agent, treating said solution to substantially com-= pletely precipitate soluble heavy metal impuritties therefrom and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a temperature not lower than 60 C. with a cathode current density sufficiently low to obtain a cathode current efficiency not lower than about 90%.

16. A process for electroplating zinc comprising preparing an aqueous solution containing not less than about 8 ounces per gallon of zinc cyanide, a concentration of alkali metal cyanide equivalent to not less than 3 ounces per gallon of sodium cyanide and a concentration of alkalining agent equivalent to not less than about 6 ounces per gallon of sodium hydroxide, the alkali metal cyanide concentration being not more than 0.6 times the zinc cyanide concentration, treating said solution to substantially completely precipitate soluble heavy metal impurities therefrom by adding to said solution sodium bisulflte in a concentration not less than about 0.25 ounces per gallon and thereafter employing the purified solution as electrolyte to electroplate articles of electricall conductive material at a temperature not lower than 60 C. with a cathode current density sufficiently low to obtain a cathode current efilciency not lower than about 90%.

17. A process for electroplating zinc comprising preparing an electrolyte containing zinc cyanide, alkali metal cyanide and an alkalining agent, treating said solution to substantially completely precipitate soluble compounds of heavy metals other than zinc and thereafter electroplating zinc from said solution.

18. A process for electroplating zinc comprising preparing an aqueous solution containing zinc cyanide, alkali metal cyanide and an alkalining agent, treating said solution to precipitate soluble heavy metal impurities therefrom by adding to said solution a soluble reducing agent in a concentration not less than that required to substantially completely precipitate said impurities and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a temperature not lower than 60 C. with a cathode current density sufliciently low to obtain a cathode current efficiency not lower than about 90%.

19. A process for electroplating zinc comprising preparing an aqueous solution containing zinc cyanide, alkali metal cyanide and an alkali metal hydroxide, treating said solution to precipitate soluble heavy metal impurities therefrom by adding to said solution a soluble reducing agent in a concentration not less than that required to substantially completely precipitate said impurities and thereafter employing the purified solution as electrolyte to electroplate articles of electrically conductive material at a temperature not lower than 60 C. with a-cathode current density sufilciently low to obtain a cathode current efficiency not lower than about 90% and thereafter treating the electroplated articles with an acidic solution of an oxidizing agent.

FLOYD F. OPLINGER.