US2614975A

US2614975A - Electroplating of zinc and electrolyte therefor

Info

Publication number: US2614975A
Application number: US208079A
Authority: US
Inventors: Allan E Chester
Original assignee: Poor and Co
Current assignee: Poor and Co
Priority date: 1951-01-26
Filing date: 1951-01-26
Publication date: 1952-10-21
Anticipated expiration: 1969-10-21
Also published as: DE951605C

Description

Patented Oct. 21, 195 2 ELECTROPLATING OF ZINC AND ELEC- TROLYTE THEREFOR Allan E. Chester, Highland Park, 111., assignor to Poor & Company, Chicago, 111., a corporation of Delaware No Drawing.

Application January 26, 1951,

' Serial No. 208,079

9 Claims. 1

This invention relates to a new and improved method of electroplating zinc from an acid zinc lioath and to a new and improved electrolyte there- One of the objects of the invention is to provide a new and improved type of electrolyte for electroplating zinc from acid zinc plating baths.

Another object of the invention is to provide anew and improved method of electroplating zinc wherein electroplated articles having a bright into the electrolytes certain chemical compositions or derivatives thereof which greatly assist in the deposition of bright zinc plates.

These and other objects and features of the invention are accomplished by incorporating into acid zinc sulfate electrolyte baths the product of the reaction of thiourea with zinc oxide and an aldonic acid, preferably gluconic acid. When zinc is electroplated from such baths onto the customary base materials. the resultant products have a greatly improved brightness of finish.

The invention will be illustrated by the following examples showing a method for preparing the addition agent and the preparation of the plating bath according to the invention. It will be understood that the invention is not limited to the disclosure in the examples.

Emample I Thiourea, an aqueous solution of 50% gluconic acid and zinc oxide are mixed together in the following amounts:

Pounds Thiourea 9.16 Gluconic acid (50% concentration) 575.00 Zinc oxide (less than 0. lead) 63.845

In mixing the foregoing ingredients, the gluconic acid was first added to a stainless steel tank and mixed with the thiourea with agitation. The zinc oxide was then added slowly at the rate of 50 pounds every 20 minutes. No heat is required since the reaction takes place exothermically. When the addition is complete and foaming subsides the batch is brought to a temperature of 200 F. and maintained at this temperature for 20 minutes. While regulating the temperature between 185 F. and 195 F. the batch is cooked approximately 2 hours or until the viscosity of 96 ml. (milliliters) of the product is 75-77 seconds. The viscosity determination is made with a 100 2 ml. pipette No. 715 calibrated to deliver 100 ml. of water in 45 seconds at 20 C.

The reaction mixture is tested at intervals. When the viscosity is seconds or less, 1 hour is allowed between tests. When the viscosity is '70 seconds tests are made every half hour. When the viscosity is 72 seconds tests are made every 20- minutes. At 65 seconds the pH of a 10% aqueous solution of the reaction mixture should be 6.0 to 6.5. If the pH is below 6.0 more zinc oxide is added to bring it up to the required range. 'If the pH is above 6.5 more gluconic acid is added. At 75-77 seconds viscosity the reaction mixture is poured into trays in layers of about A; to inch thickness. After 24 hours the product is placed on a galvanized screen rack to air dry. When'it is dry it is milled to break it up into solid particles of the desired size. The resultant product is readily soluble in acidic aqueous solutions having a pH within the range of 1.5 to 6.0.

Example II This example illustrates the preparation of acid zinc plating baths according to the invention. The composition prepared as in Example I when added in proportions of 1 to 3 ounces per gallon to an acid zinc sulfate plating bath containing to 450 grams per liter of zinc sulfate (ZHSO4.'7H2O) produces a brightening eifect at pI-Is within the range of 1.5 to 6.0. This plating bath will operate near a pH of 6.0 without the formation of Zinc or iron precipitates and produces high specular reflectivity of the plated metal going up to mirror brightness, depending upon the quantity of the zinc I gluconatethiourea composition employed. Excellent deposits are obtained over a wide range of current densities from 5 amperes per square foot to 5000 amperes per square foot, the current density employed being dependent upon the agitation available. 1

The compositions are especially effective in the electrodeposition of zinc from acidic baths in which the zinc is principally present as zinc sulfate. The zinc sulfate baths can also contain additions of zinc chloride, preferably not more than 20 to 100 grams per liter.

In plating from acid zinc baths according to the invention it is sometimes desirable to employ a small amount of aluminum sulfate in the bath, for example, to 2 ounces per gallon. The aluminum sulfate forms a film of colloidal alumina in the vicinity of the anode and prevents excessive acid attack on the zinc anode. Another optional ingredient which may be added to the bath is sugars, such as dextrose, which apparently produce a refinement in the crystalline structure of the zinc and have some whitening effect. Still another optional ingredient is sodium fluoride which acts as a whitening and grain refining agent at the cathode and apparently causes more uniform solution of zinc at the anode while tending to prevent sludge formation. Other ingredients such as sodium acetate may also be employed but are not required.

Electroplating baths of the type described herein may be used as baths for plating piecework, that is, for batch operations, or may be employed in continuous plating operations.

If the electroplating baths are used in continuous plating operations the makeup bath may be prepared as described in Example II and thereafter the gluconate which is lost due to drag-out is replenished by adding to grams per liter of zinc gluconate at intervals as predetermined by tests of the bath. The zinc gluconate may be prepared as described in Example I by omitting the thiourea. It can also be prepared as described in my copending application Serial No. 180,500, filed August 19, 1950.

The plating baths may be operated at conventional temperatures within the range of 70 F. to 160 F., preferably not higher than 140 F.

'In the preparation of the compositions employed in the present electrolyte other aldonic acids may be used instead of gluconic acid, as for example, mannonic, arabonic, galactonic and xylonic. The invention contemplates the use of one or more of these acids or mixtures thereof, including mixtures of the lactone forms of these acids. Commercial gluconic acid is available as a 50% aqueous solution of approximately 99% gluconic acid and 1% glucose. The delta gamma lacton form of gluconic acid is particularly effective in producing compositions which act as brighteners for acid zinc plating baths. It is significant that the same brightening effects cannot be obtained by adding zinc gluconate and thiourea separately to baths of the type described.

The relative proportions of the reactants employed is subject to variation. For the purpose of the invention it is preferable to determine the proportions by the pH test given in Example I. The reaction between zinc oxide and gluconic acid to form zinc gluconate requires approximately 1 mole of zinc oxide to 2 moles of gluconic acid but an excess of either reactant may be present in the final product. The manner in which the thiourea reacts is not clearly understood and the quantities employed may be varied but preferably correspond to 0.025 to 1.0 mole of thiourea per mole of gluconic acid. The preferred molar ratio of thiourea to zinc oxide to aldonic acid is approximately 1:6:12. The molar ratio of thiourea to aldonic acid should be at least 1:24 and preferably not more than 1:6. The molar ratio of zinc oxide to aldonic acid should be at least 1:4 and prefer.- ably about 1:2.

In addition to the brightening effect of the compositions of the invention in acid zinc plating baths, these compositions have a pronounced anti-burn effect. Hence, they make it possible to operate the baths at very high current densities with less burning of the plated article.

The term acid zinc sulfate bath is used herein to describe acidic plating baths in which the zinc is present principally as zinc sulfate but can also be present in minor amounts in one or more other forms.

The compositions disclosed herein are further disclosed in my copending application Serial No. 199,582 filed December 6, 1950.

The invention is hereby claimed as follows:

1. An aqueous acid zinc sulfate plating bath comprising an acid zinc sulfate electrolyte and a reaction product of thiourea, zinc oxide and an aldonic acid.

2. An aqueous acid zinc sulfate plating bath comprising an acid zinc sulfate electrolyte and a reaction product of thiourea, zinc oxide and gluconic acid.

3. An aqueous acid zinc sulfate plating bath comprising an acid zinc sulfate electrolyte and a reaction product of thiourea, zinc oxide and an aldonic acid in proportions of about 1 to 3 ounces per gallon of plating bath.

1. An aqueous acid Zinc sulfate plating bath comprising an acid zinc sulfate electrolyte and a bath soluble reaction product of thiourea, zinc oxide and gluconic acid in proportions of about 1 to 3 ounces per gallon of plating bath.

5. In a process for the electrodeposition of zinc the step which comprises electrodepositing zinc from an acid zinc sulfate plating bath in the presence of a product of the reaction of thiourea, zinc oxide and an aldonic acid.

6. In a process for the electrodeposition of zinc the step which comprises electrodepositing zinc from an acid zinc sulfate plating bath in the presence of a product of the reaction of thiourea, zinc oxide and gluconic acid.

7.1m a process for the electrodeposition of zinc the step which comprises electrodepositing Zinc from an acid zinc sulfate plating bath in the presence of a product of the reaction of thiourea, zinc oxide and an aldonic acid, said product being present in proportions of about 1 to 3 ounces per gallon of plating bath.

8. The process for continuously electrodepositing zinc which comprises depositing zinc from an acid zinc sulfate plating bath in the presence of about 1 to 3 ounces per gallon of plating bath of the product of the reaction of thiourea, zinc oxide and gluconic acid and replenishing the consumed gluconate by continuously adding zinc gluconate to the bath.

9. The process for electrodepositing zinc which comprises depositing zinc from an acid zinc sulfate plating bath containing about 1 to 3 ounces per gallon of the plating bath of the bath soluble product of the reaction of about 9 parts by weight of thiourea, about 575 parts by weight of gluconic acid (50% concentration) and about 64 parts by weight of zinc oxide, said product having a viscosity of about to 77 seconds as determined on a ml. pipette calibrated to pass 100 ml. of Water in 45 seconds at 20 C.

ALLAN E. CHESTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS