United States Patent Office Patented May 7, 1957 Oxidizing Bright Dips, Chromate Type Protective Dips and Phosphatizing Dips
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2,791,554 United States Patent Office Patented May 7, 1957 oxidizing bright dips, chromate type protective dips and phosphatizing dips. - These objects are accomplished briefly by the use of 2,791,554 - - aqueous cyanide zinc baths that contain bath-soluble 5 reaction products of epichlorhydrin with ammonia or METHOD of ELECTRODEPOSITING ZINC with primary amines either alone or for Some purposes John B. Winters, Westlake, Ohio, assignor to Ann F. Hull in combination with an aromatic aldehyde selected from and Adrian Medert, Cleveland, Ohio, as coir stees the group consisting of oxyheterocyclic aldehydes, phenyl No Drawing. Application September 22, 1954, ethers and methoxybenzaldehydes. Such reaction prod Serial No. 457,797 - ... O ucts may be known as polyepoxyamines and will be so designated hereafter. 6. Claims. (Cl. 204-55) While my novel brighteners are effective in any com mercially operable zinc cyanide plating bath, best re sults are obtained with the approximate bath composi This invention relates to the electrodeposition of zinc, tion and operating conditions shown in Table 1, which and is more particularly directed to a new and improved can be varied as required for special purposes in ac bath composition and process for producing bright Zinc cordance with the known skill in the art. deposits wherein the cyanide bath contains in solution Table 1 bath-soluble reaction products of epichlorhydrin with Bath composition: Grams per liter ammonia or primary amines. - - - - Zinc plated from cyanide baths is widely employed Zinc cyanide------------------------------ 60 as a coating to protect steel base metals from corro Sodium cyanide---------------------------- 42 sion and to provide corrosion resistant undercoats for Sodium hydroxide-------------------------- 80 organic and other protective finishes which are applied thereover. A zinc coating upon an article plated from Ratio of NaON (free E. combined)-27 a cyanide plating bath is dull gray or dark in color, Operating conditions: relatively crystalline in structure, is poor in appearance, Temperature-about.-----. 80° F. - stains readily and accordingly such coating has a very Cathode current density--. 20 amps. per sq. ft. limited field of application. It has long been common Anodes----------------- Zinc of suitable purity. practice to obtain a coating of improved physical char The plating bath is prepared by dissolving the con acteristics, particularly in appearance, by incorporating stituents in water, after which the bath should be puri in the cyanide plating bath various organic and inor fied by adding about one-half gram of sodium sulphide ganic substances which are generally referred to as addi per liter of bath dissolved in water, or the bath may be tion agents or brighteners. - treated by stirring about two grams of zinc dust per liter Heretofore it has been the common practice in ob of bath for a few hours, then settling and decanting or taining bright zinc deposits from cyanide plating baths filtering. - - - to use as addition agents a combination of organic and I have found that addition agents used for cyanide inorganic compounds, and optimum results depended zinc plating baths in accordance with the prior art are on maintaining a pre-determined balance between the subject to certain disadvantages. For example, the use concentrations of the organic and inorganic constituents. of aromatic aldehydes requires very frequent additions Accordingly the prior practice required relatively close because of their instability in the bath, and the same control over a plurality of the constituents of the plat disadvantage applies to colloids such as gelatin. The ing bath by chemical analysis, or other means if ana use of polyvinyl alcohol is quite common, but this ma lytical methods were not feasible. In the commercial terial is subject to the disadvantages of salting out of operation of plating baths employing such addition the bath, forming an undesirable scum on the bath Sur agents, numerous drawbacks were encountered by reason face, producing an undesirable yellow color to the bright of such addition agents which were in addition to those zinc deposit, and furthermore is not uniform in its com brought about by any unbalanced relationship between position. the organic and inorganic substances. For example, one The above disadvantages are entirely overcome with of the disadvantages of the combination of organic-in 50 my new and novel bath-soluble polyepoxyamines. These organic addition agents is that the inorganic constituent polyepoxamines are unusually stable in the plating bath, is a soluble metal compound, the metal of which co their solubility is more than sufficient to preclude their deposits with the zinc, greatly to the detriment of some being salted out, they do not produce yellow deposits of its physical properties, such as ductility and corro upon a plated article and they can be readily produced sion resistance. Such co-deposition causes stains and 5 5 in batches that are uniform in effect when used in a undesirable appearance of the coated surface upon the plating bath. The addition of bath-soluble polyepox subsequent application of surface treatments which are amines also greatly improves the covering power of cy commonly used to passivate the zinc surface or to pro anide zinc plating baths to the extent that recesses in duce thereon a tarnish or corrosion resistant film. The the article which cannot be covered with the metal under purer the zinc deposit upon the surface of the article 60 ordinary operating conditions are readily covered if such the better it is for corrosion protection and for accept addition agents are present. ing subsequent surface treatments designed to produce ihe polyepoxamines may be prepared by any one of passive or protective films such as the chromate type or a number of well known and established procedures. However, for both maximum effectiveness and bath sta phosphate coatings to promote lacquer adhesion. bility, the preparation of the polyepoxamine in accord It is among the objects of my invention to provide ance with the following procedure has been found most an addition agent for zinc cyanide plating bath compo desirable. , - - - - sitions by means of which bright, smooth and relatively Using a jacketed stainless steel reaction vessel, 10 gal pure zinc deposits can be obtained therefrom. A further lons of water and 56 pounds of aqueous ammonia (29%) object of my invention is to provide zinc plating baths are combined and cooled to a temperature commensu and a plating process employing the same which produce 70 rate - with the ability of the apparatus to convey away bright zinc deposits that are uniformly responsive to heat, I have found that a temperature of from 6 to 10 - - - - - - - - - -s :- 2,791,554 - - - - - - 3 4. degrees is the most desirable. Five gallons of epichlor can be subjected to any of the many chromate type dips hydrin are then added to the vessel. Cold water circu for producing passive surfaces or protective coatings with lated within the jacket serves to convey away the heat uniform and satisfactory results. Likewise such bright of reaction and to maintain the substances within the Zinc deposits will take a uniform phosphatizing treat vessel within a desirable temperature range. An initial, 5 ment by any of the widely used commercial processes very slow reaction or condensation takes place which for producing undercoats on zinc for organic finishes and leads to an intermediate product. This is later poly the like. - - - - merized at a controlled rate. The evidence of such a My novel brighteners greatly improve the plating char slow reaction is continuous liberation of heat and a con acteristics of the zinc cyanide bath in which they are used tinuous drop in pH from about 12.5 to 8.0 as determined 10 by increasing both its covering power and throwing power, by a glass electrode. and by providing a broad range of current densities over After the initial reaction has been completed which which bright deposits can be plated. While my novel takes approximately a period of three hours, polymeri brighteners are self-sufficient in producing bright, smooth, zation is promoted by gradual elevation of the tempera relatively pure zinc electrodeposits from zinc cyanide plat ture to about 101 degrees centigrade or the reflux tem ing baths containing them in solution, there are certain perature. After maintaining the high temperature for conditions encountered, for example, in barrel plating, about three additional hours, the desired degree of poly where the use of an aromatic aldehyde such as anisic merization is reached and the resulting polyepoxyamine aldehyde or heliotropin, in conjunction with my novel is suitable for addition to zinc cyanide plating baths. The brighteners produces superior results mainly from the product made in accordance with this procedure possesses standpoint of coverage on low current density areas with a relatively low degree of polymerization which is most improvement in brightness thereon. Accordingly, under effective in the zinc plating bath within the range of Such conditions I find it advantageous to add about A0 to current densities from about 5 to 125 amperes per square 1 gram of such aromatic aldehyde per liter of zinc cya foot. nide bath containing my novel brightener. Experi Another procedure for preparing polyepoxyamines ments have proven that the action of my novel Suitable for use as an additive for zinc plating baths con brighteners with such compounds is synergistic as identi sists in adding epichlorhydrin slowly to the diluted am cal results can not be obtained by the use of either alone. monia at higher temperatures. Using the same type of Similarly, if desired for some applications, a metal bright reaction vessel, 10 gallons of water and 56 pounds of ening agent such as a compound of molybdenum or aqueous ammonia (29%) are combined and heated to chromium or others given in United States Patent No.