US3672971A - Bright-zinc plating bath - Google Patents

Abstract

A BRIGHT-ZINC PLATING BATH IN WHICH THE PRIMARY BRIGHTENING AGENT CONSISTS OF A REACTION MIXTURE OF POLYAMINES, SALTS OF THE METALS OF GROUPS V AND VI OF THE PERIODIC TABLE AND SULFUR-RELEASING ORGANIC COMPOUNDS. THE SULFURSPLITTING COMPOUNDS ARE PREFERABLY ALIPHATIC AROMATIC AND HETEROCYCLIC COMPOUNDS CONTAINING SULFUR AND INCLUDE MERCAPTANE, THIOCOMPOUNDS AND THE LIKE.

Description

United States Patent O1 zfice 3,572,971 Patented June 27, 1972 3,672,971 BRIGHT-ZINC PLATING BATH Gerd Senge, Brackwede, and Rolf Sieburg, Eickum, Germany, assignors t Firma Riedel & Co., Bielefeld,

Germany No Drawing. Filed July 7, 1970, Ser. No. 53,038 Claims priority, application Germany, July 15, 1969,

P 19 35 821.5 Int. Cl. C23b 5/10 US. Cl. 204-55 R 6 Claims ABSTRACT OF THE DISCLOSURE A bright-zinc plating bath in which the primary brightening agent consists of a reaction mixture of polyamines, salts of the metals of Groups V and VI of the Periodic Table and sulfur-releasing organic compounds. The sulfursplitting compounds are preferably aliphatic aromatic and heterocyclic compounds containing sulfur and include mercaptane, thiocompounds and the like.

FIELD OF THE INVENTION Our present invention relates to a bright-zinc electrobathing bath, a method of making such a bath, and an electrogalvanizing method for producing a bright-zinc coating and the product made by the latter method; more particularly, the invention relates to a bright-zinc electrogalvanizing system which eliminates many of the disadvantages of earlier electrogalvanizing methods, including toxicity of the bath, the difliculty in processing the waste liquors, and the poor plating efficiency of those baths which are free from the other disadvantages.

BACKGROUND OF THE INVENTION In the electrogalvanization of metal substrates, it is possible to produce a bright-zinc coating using five conventional zinc-plating baths. These conventional baths include a strongly alkaline cyanide-containing bath, a strongly alkaline cyanide-free bath, a weakly alkaline pyrophosphate-containing bath, a weakly acid bath and a strongly acid bath. As will be apparent hereinafter, the choice among these baths has often been a compromise of the disadvantages, inconveniences and necessary evils inherent therein. For example, a strongly alkaline cyanidecontaining bath a strong acid bath have a common disadvantage in that it is difiicult to maintain the stability of the system during plating operation. Even with intense monitoring of the alkali or acid concentrations, deviations are found to occur which affect the nature of the zinc plate, the plating rate and other parameters of the system. The cyanide-containing zinc bath has the further disadvantage that the presence of cyanide salts renders the bath highly toxic and makes it difficult to so treat the bath liquor as to enable its convenient disposal. Aside from the handling problems arising with the use of highly acidic zinc-plating baths, such baths are found to have poor throwing power and to give poor coatings in many instances.

To avoid these major disadvantages in the most common types of plating baths, it has been proposed to provide strongly alkaline cyanide -free electrogalvanizing baths, weakly alkaline pyrophosphate-containing baths and weakly acidic high-conductivity baths, all of which are free from the toxicity characterizing the presence of cyanide compounds although they are incapable of solving other problems which arise in the bright-zinc plating of metal substrates. The pyrophosphate-containing electrolytes are disadvantageous in that the pyrophosphates and other components thereof are recognized widely as waterpollutants even though they may be present in small quantities. Hence the disposal of a pyrophosphate-containing liquor is a major problem.

Furthermore, alkaline cyanide-free electrogalvanizing baths generally include significant quantities of complexing agents or complex formers which are water pollutants of no small degree as well. Here again, disposal of the waste electrolyte is a major problem. Some of these difficulties are not encountered when Weakly acidic zinc-plating baths are used although such baths require extensive pretreatment and after treatment, somewhat in the manner of high-conductivity nickel baths, at correspondingly high cost and effort.

In the cyanide-free alkali zinc-plating bath it has been proposed heretofore to provide brighteners or brightening agents which appear to be effective in yielding a brightzinc coating instead of a coating with a matte-like appearance. Such conventional brighteners include gelatine and aldehydes (e.g. benzaldehyde, vanillin, heliotropin or piperonal and anisaldehyde or anisic aldehyde), compounds generally salts) of metals of Groups VI and VII of the periodic system, especially chromium, molybdenum, tungsten and manganese, and generally compounds of the eighth Group of the Periodic Table, namely, iron, cobalt and nickel. The disadvantage of this system is that the gelatine and aldehydes are unstable in a strongly alkaline medium and sutfer alkali-hydrolysis, therefore rendering the system practically inutile for commercial operations.

Much may also be made of the fact that it is known to provide in zinc electroplating bath, complex formers or complexing agents such as triethanolamine, polyethylenamine, ethylenediaminetetraacetate, triisopropanolamine, dimethylamine, polyamines (such as diethylenetriamine, triethylenetetramine and pentaethylene) and hexamethylenetetramine. Complexing agents of this type have the advantage that they stabilize the stability of the electrolyte although they often give rise to the problem of treatment of the waste electrolyte. This is all the more significant when it is recognized that zinc and other toxic heavy metals may be solubilized as a result of the presence of the complex formers and remain in the electrolyte as the latter is treated as a waste efiluent.

There are also extant brightening systems for brightzinc electroplating which are combinations of ketones (for example, cyclohexanone and diphenylketone) and polyamines, which have been found to be most effective in the higher current-density ranges for producing bright-zinc coatings. A disadvantage of such systems, probably based upon the ketone-component, is that in short order there is a reduction in the brightness and a noticeable reduction in the attainable current density. While investigations have shown that the combination of polyamines with brighteners of the aldehyde type are also effective in the electrodeposition of bright-zinc coatings, the current-density range over which the bright coating is produced, is proportionally narrow and, therefore, difiicult to control.

OBJECTS OF THE INVENTION It is, therefore, the principal object of the present invention to provide a bright-zinc electroplating or electrogalvanizing bath which is free from the difliculties of disposal, is nontoxic, and can produce bright-zinc coatings over a relatively wide current-density range at high efficiency and with good throwing power.

Another object of this invention is to provide an improved bright-zinc electrogalvanizing bath which avoids the difficulties hitherto encountered with conventional baths and yet is able to produce an excellent bright-zinc plating or coating in a highly stable manner with high efliciency.

Still another object of our invention is the provision of an improved method of making a bright-zinc electroplating bath which permits highly efficient zinc plating of a bright or shiny coating with high throwing power and current density and without the toxic difliculties encountered with some of the earlier baths proposed for bright-zinc electrogalvanizing.

It is also an object of our invention to provide an improved method of electroplating metal substrates to yield bright-zinc coatings.

Another object of our invention is found in a plating body in which the above methods and improved plating systems are employed.

The invention also has as its object the provision of an improved brightening composition or brightening agent for bright-zinc electrogalvanizing baths.

SUMMARY OF THE INVENTION These objects and others, which will become apparent hereinafter, are attained, in accordance with the present invention, by the provision of an aqueous alkali cyanidefree bright-zinc electrogalvanizing or electroplating bath which includes as a brightening system, agent or composition a combination of at least three components and, preferably, a reaction mixture thereof, the three components being a polyamine (of the type mentioned earlier and hitherto used as a component of brightening agents for zinc electroplates baths), a salt soluble in the alkali medium of at least one of the metals of Groups V and VI of the Periodic Table and preferably a vanadate or a molybdate, and a sulfur-splitting organic compound, i.e. an organic compound containing sulfur from which the sulfur is readily released.

When reference is made herein to a reaction mixture, it is to be understood that we have found that a mixture of the three components appears to undergo a chemical reaction of the complex-forming or like type. In other words, chemical bonds are created when the three components are combined, which bonds are not found in the three components individually. It has been discovered that it is advantageous to combine the three components prior to their introduction into the bath to obtain maximum benefit from the interaction observed above, although the effect of the brightening system is noticeable to a lesser extent when the three components are introduced individually into the electroplating electrolyte. Reference to the Periodic Table is intended to mean the chart of the periodic arrangement of the elements contained on pages 445 and 446 of the Handbook of Chemistry and Physics, 41 edition, 1959-1960 or the periodic chart of the elements contained on pages 448 and 449 of this publication. When the latter chart is employed, the groups to which the metals should belong are Groups V-B and VI-B. More specifically, the applicable metals are vanadium, chromium, niobium, molybdenum, tantalum and tungsten, although preference is given to vanadium and molybdenum, as discussed above.

The sulfur-splitting compounds most suitable for the purposes of the present invention are those which react most readily with the other components of the reaction mixture, as defined above. These compounds include mercaptans and thio-compounds and are generally describable as the aliphatic, aromatic, aromatic-heterocyclic compounds readily yielding sulfur. The compounds which have been used, in combination or individually, include mercaptothiazoline, mercaptopyridine (C H NSH) thioureas and derivatives thereof, dithiocarbamic acids and salts and derivatives thereof, thioacetamide and thiocarbanilide (diphenylthiourea) According to a more specific feature of this invention, the reaction mixture is reacted at a temperature of about 20 C. in an initial reaction stage and is thereafter subjected to a second-stage reaction at a higher temperature in the presence of hot water. Preferably, the second stage of the reaction or the after reaction is effected at a temperature of 70 C., i.e. at the temperature of the hot water which is introduced into the reaction mixture or 4 vice versa. Following the second reaction stage, the temperature of the reaction mixture is slowly cooled to a temperature of about 20 C. Advantageously, the reaction mixture is cooled progressively to the temperature of 20 C. in the presence of formaldehyde or sodium hydroxide.

According to another feature of this invention, the reaction mixture is present in the electrolyte or plating bath in an amount ranging between 0.01 to 10 parts by weight per parts by weight of the remainder of the bath, although these limits are not critical and it is possible merely to add the reaction mixture in the cyanide-free alkaline plating bath until the desired quality of brightplating is obtained. Similarly the ratios of the three components may range widely and, for example, the polyamine may be present in an amount (by weight) ranging between 1 and 20 times that of the sulphur-containing compound, while the sulphur-containing compounds may be present in an amount (by weight) ranging between 0.5 and 20 times that of the metal salt.

An important advantage of the electrolyte of the present invention is the low proportion of complex formers therein and, consequently, the ease with which the waste liquor can be treated or disposed of. A dilution of 1:100 yields a complexed-zinc concentration which is relatively low, i.e. below 3 mg. per liter.

In the following examples of compositions of the zinc electroplating bath, according to the invention, bright plating of zinc was carried out in each case at a temperature of 10 to 30 C., at intervals of 2 C., at current densities of 0.5 to 4 amperes per/dm. for a treatment time of 10 to 20 minutes. The treatment times were varied at two-minute increments and the current density tests were taken with 0.5 ampere per/dm. increments. In each case, the composition yielded a bright zinc coat upon a metallic substrate electrogalvanized in the bath.

Example I Grams/liter Zinc chloride 50.0 Sodium hydroxide 230.0 Tetraethylenepentamine 2.0 Ammonium molybdate 0.1 Sodium dimethyldithiocarbamate 0.1 Heliotropin or heliotropin-bisulphite 1.0 Water Balance Example II Grams/liter Zinc oxide 30.0 Sodium hydroxide 200.0 Pentaethylenehexamine 4.0 Thiourea 0.2 Mercaptothiazoline 1.0 Sodium diethyldithiocarbamate 0.1 Ammonium molybdate 0.28 35% aqueous solution of formaldehyde 0.4 Vanillin 1.0 Water Balance Example III Grams/liter Crystalline zinc sulphate 130.0 Sodium hydroxide 250.0 Tetraethylenepentamine 2.0 Thiourea 1.0 Triethylenetetramine 1.5 Thioacetamide 1.0 Ammonium vanadate 0.5 Formaldehyde (35% aqueous solution) 0.5 Piperonal 0.4 Vanillin 0.4

Example IV Grams/liter Zinc oxide 30.0 Sodium hydroxide 180.0 Triethylenetetramine 2.0 Ammonium molybdate 0.2 4-mercaptopyridine 0.5 Formaldehyde (35% aqueous solution) 0.5 Piperonal 0.4 Vanillin 0.4

It has been found, surprisingly, that the use of the individual components of the reaction mixture of the present invention, i.e. without prereaction, as described generally above, is significantly less effective in providing a brightzinc coating than the same composition wherein, however, the polyamine, the sulphur-containing organic compound and the metal salt are prereacted prior to introduction into the electrolyte. It appears that the prereaction gives rise to a metalsulphidoamino complex of an as yet undefined composition, which nevertheless is the effective species in promoting the formation of uniform bright zinc coatings within the wide range of current densities and with a high tenacity to the substrate. In fact, the current-efliciency curve of electrogalvanic metallic substrates with bright zinc produced from the baths of Examples LIV, above, appproximates the curves obtained with bright zinc coating from cyanide baths, without however the toxicity associated therewith. This high advantageous currentefliciency characteristic is not, however, obtained when the substances of the reaction mixture are supplied to the electrolyte individually, i.e. without a preliminary reaction. Whereas the prereacted mixture provides an extremely bright coating, the use of the same quantities of the individual components yields a bright coating which is less brilliant, is striated and is flakey.

It will be understood that, as illustrated in the examples, the reaction mixture can be employed together with conventional brighteners as enumerated above. While, as pointed out, the mechanism time which the active species of the reaction mixture is formed is not fully clear, it has been found that deviations from the proportions given above and reaction steps under markedly different conditions do not yield the active species or a reaction mixture which is optimally elfective in the formation of bright zinc coatings. In each of the aforementioned examples, the polyamine, the sulphur-containing organic compounds and the metal salts are reacted at about room temperature, i.e. 20 C. by being brought together for a period of a few minutes and intimately mixed. Thereafter, about 20 ml. of water at 70 C. is introduced into each reaction mixture (second stage reaction) whereupon formaldehyde and caustic soda is added (about 5 grams for the amounts indicated of the reaction mixture). The formaldehyde appears to be an accelerator while the caustic soda functions as a stabilizer. When the formaldehyde is not supplied, the final reaction takes substantially longer. In each case, the electroplating bath is formed by dissolving the zinc salt and the sodium hydroxide in water and adding the reaction mixture to make up the specified compositions. Thereafter, any customary brighteners, if any, may be supplied.

We claim:

1. A method of making an electrolyte for the brightzinc plating of a substrate comprising the steps of interreacting at least one organic polyamine selected from the group which consists of diethylenetriamine, triethylenetetraamine, pentaethylenehexamine, hexamethylenetetramine, and ethylenediamine; at least one aliphatic, aromatic or aromatic-heterocyclic sulphur-containing compound selected from the group which consists of mercaptothiazolines, mercaptopyridines, thioureas, dithiocarbamic acids and salts thereof, thioacetamides and thiocarbanilids; and at least one vanadate or molybdate salt and incorporating the resulting reaction mixture into an alkaline electrolyte containing zinc.

2. The method defined in claim 1 wherein said reaction mixture is subjected to an initial reaction step at a temperature of about 20 C., is thereafter treated with water at a temperature of about 70 C. in a second reaction stage and is cooled to a temperature of about 20 C. slowly.

3. The method defined in claim 2 further comprising the step of treating the reaction mixture with formaldehyde and sodium hydroxide during the cooling thereof.

4. An electrolyte for the bright-zinc plating of a substrate as made by the method defined in claim 1.

5. A method of brighbzinc electroplating a substrate comprising the step of electrodepositing zinc from the electrolyte prepared as defined in claim 1 at a current density of 0.5 to 4 amperes per dm. at a temperature of 10 to 30 C. for a period of 10 to 20 minutes.

6. An aqueous electrolyte as made by the method defined in claim 1 for bright-zinc electroplating and selected from the group which consists of:

Grams/liter Zinc chloride 50.0 Sodium hydroxide 230.0 Tetraethylenepentamine 2.0 Ammonium molybdate 0.1 Sodium dimethyldithiocarbamate 0.1 Heliotropin or helioptropin-bisulphite 1.0 Water Balance Grams/liter Zinc oxide 30.0 Sodium hydroxide 200.0 Pentaethylenehexamine 4.0 Thiourea 0.2

Mercaptothiazoline 1 .0 Sodium diethyldithiocarbamate 0.1 Ammonium molybdate 0.2 35% aqueous solution of formaldehyde 0.4 Vanillin 1.0 Water Balance Grams/liter Crystalline zinc sulphate 130.0 Sodium hydroxide 250.0 Tetraethylenepentamine 2.0 Thiourea 1.0 Triethylenetetramine 1.5 Thioacetamide 1.0 Ammonium vanadate 0.5 Formaldehyde (35% aqueous solution) 0.5 Piperonal 0.4 Water Balance Grams/liter Zinc oxide 30.0 Sodium hydroxide 180.0 Triethylenetetramine 2.0 Ammonium molybdate 0.2 4-mercaptopyridine 0.5 Formaldehyde (35% aqueous solution) 0.5 Piperonal 0.4 Vanillin 0.4- Water Balance References Cited FOREIGN PATENTS 1,091,171 11/1967 Great Britain 204-55 R 1,043,618 9/1966 Great Britain 204-55 R F. C. EDMUNDSON, Primary Examiner