Abstract:
The present invention provides an ammonia-free acid zinc plating bath for electrodepositing bright zinc on a substrate in which the zinc deposit produced thereby is ductile and presents a glossy to bright coating on the substrate over a wide cathodic current density range. The ammonia-free acid zinc plating baths of the invention comprise zinc ions, chloride ions, at least one polyoxyalkylated naphthol, at least one aromatic carboxylic acid or bath-soluble salt thereof, and at least one anionic aromatic sulfonic acid or bath-soluble salt thereof. In addition to the above components, the ammonia-free acid zinc plating baths of the invention may contain at least one nonionic polyoxyethylene compound and at least one aromatic aldehyde, ketone, or mixtures thereof. The ammonia-free acid zinc plating baths of the invention are free from significant amounts of complexing agents that would impede the removal of zinc ions from the baths prior to disposal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an acid plating bath for electrodepositing bright zinc on a substrate in which the zinc deposit produced thereby is ductile and presents a glossy to bright coating on the substrate over a wide cathodic current density range. The invention relates also to a method of electrodepositing bright coatings on a substrate. More particularly this invention relates to improved zinc plating bath compositions which are free or substantially free of ammonium ions, and the method of plating with such baths. 
     Alkaline cyanide-containing zinc baths have been most widely used baths for electrodepositing bright zinc plate on substrates. These baths have various disadvantages due particularly to the cyanide content which is constant danger for the people working with the baths. Moreover, the baths are subject to aging due to the decomposition of the cyanide, and the waste water must be subjected to a costly treatment to destroy the cyanide ions and to precipitate zinc as hydroxide prior to disposal. 
     Alkaline solutions containing complex compounds of zinc and alkaline metal pyrophosphates have been proposed as replacements for cyanide baths and cyanide processes for the electrodeposition of bright zinc. The electrodeposition of zinc using a pyrophosphate bath, however, may result in relatively poor low current density coverage, spore formation, roughness and insufficient brightness. The use of phosphates also may produce waste disposal problems since phosphates are not easily removed and may promote the growth of undesirable aquatic plant life if discharged into streams. These disposal disadvantages limit the acceptance of the pyrophosphate zinc plating bath compositions in industrial applications. 
     The enactment and enforcement of various environmental protection laws, particularly with respect to improving water quality, have made it desirable and necessary to reduce or eliminate discharge of cyanides, phosphates and a number of metal ions contained in the waste water from plating plants. Accordingly, non-polluting bright zinc plating processes have been sought as alternatives to the zinc cyanide and phosphate-containing baths. 
     Acidic plating baths have been known, and these baths are cyanide-free. The acid zinc plating baths are much less toxic than the alkaline zinc plating baths and are preferred where the disposal of waste presents an environmental problem. However, many of the known acid zinc plating baths do not produce bright zinc deposits unless the baths contain complexing agents for zinc. Examples of complexing agents which have been utilized in acid zinc baths include organic zinc complexing agents such as hydroxycarboxylic acids or salts thereof, ethylenediamine tetraacetic acid or salts thereof or similar materials which prevent the precipitation of zinc from the bath as the insoluble hydroxide at higher values of pH. Almost all of the baths of the prior art used for depositing bright zinc coatings within the mild acid range also contain large quantities of ammonium salts such as ammonium chloride. The complex forming properties of these ammonium salts and organic salts which is desirable in the plating bath itself is, however, a problem in disposal of the used acid bath. Such complexing agents impede the removal of zinc ions from the waste electrolyte by precipitation in the weakly alkaline pH range and may require special measures for effecting the precipitation of the metals from the bath prior to disposal. Also, the presence of ammonia in the plating bath is undesirable because of the biochemical oxygen demand caused by ammonia, and the chlorine demand of the ammonia in waste treatment facilities. 
     Acid zinc plating baths which are substantially free of ammonium ions have been described in U.S. Pat. No. 3,928,149. These baths contain, in addition to zinc ions, and an aromatic carbonyl compound, a polypropoxyether having a molecular weight of from about 300 to 1500 and/or a polypropoxy-ethoxy ether having a molecular weight of about 1,000 to about 5,000 and containing up to a maximum of 25% by weight of oxyethylene units. Such baths are reported to produce bright zinc deposits in the absence of ammonium salts. 
     U.S. Pat. No. 3,920,528 also describes electrolytes free from significant amounts of complexing agents that would impede precipitation of zinc ions at weakly alkaline pH. The plating bath described in this patent contains zinc ions, an inert salt for improving the conductivity of the electrolyte, and a brightener. The preferred brighteners are derivatives of nitrogen-containing heterocyclic rings such as pyridine-3-acetic acid and pyridine-3-sulfonic acid. 
     SUMMARY OF THE INVENTION 
     It has now been found that satisfactory bright, level and ductile zinc electrodeposits can be obtained from electroplating baths which are free of ammonia or amines and which contain zinc ions, chloride ions, at least one polyoxyalkylated naphthol, at least one aromatic carboxylic acid or bath-soluble salt thereof, and at least one anionic aromatic sulfonic acid bath-soluble salt thereof. While acid plating baths containing the ingredients mentioned above will produce zinc electrodeposits of good brightness over a wide current density range, the properties of the electrodeposit may be improved further by the incorporation of other additives including additional brightening agents, stress-relieving additives such as nonionic polyoxyethylene compounds, aromatic aldehydes, or ketones, wetting agents, etc. The present invention also comprises methods for the electrodeposition of bright and level zinc deposits from such acid baths as well as additive compositions for forming the aqueous acid zinc plating baths of the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The acidic electroplating baths of the invention are ammonia-free zinc plating baths comprising zinc ions, chloride ions, at least one polyoxyalkylated naphthol, at least one aromatic carboxylic acid or bath-soluble salt thereof, and at least one anionic aromatic sulfonic acid or bath-soluble salt thereof. The polyoxyalkylated naphthols useful in the baths of this invention are obtained by reacting a naphthol with an alkylene oxide such as ethylene oxide and propylene oxide and more particularly, with from about 6 to about 40 moles of ethylene oxide per mole of naphthol. The naphthol reactant may be either α- or β- naphthol and the naphthalene ring may contain various substituents such as the alkyl groups or alkoxy groups, especially lower alkyl and lower alkoxy groups of up to seven carbon atoms each, so long as the polyoxyalkylated naphthol remains bath-soluble. When present, there usually will not be more than two such substituents per polyoxyalkylated naphthol; that is, two lower alkoxy groups, two lower alkyl groups, or a lower alkyl or a lower alkoxy group. The preferred polyoxyalkylated naphthols are ethoxylated naphthols having the formula: ##STR1## wherein y is from about 6 to about 40 and preferably from about 8 to about 20. 
     At least one aromatic carboxylic acid or bath-soluble salt of an aromatic carboxylic acid is included in the zinc plating baths of the invention. Aromatic carboxylic acids such as benzoic acid and salicylic acid are preferred. Examples of bath-soluble salts include the sodium and potassium salts of benzoic acid and salicylic acid. The amount of aromatic carboxylic acid included in the acid zinc plating bath of the invention is an amount which is effective to provide a bright and level zinc deposit which can be determined readily by one skilled in the art. In general, amounts ranging from about 1 to about 10 grams per liter of at least one aromatic carboxylic acid or water-soluble salt thereof have been found to be effective. 
     The acid zinc plating baths of the invention also contain at least one anionic aromatic sulfonic acid or bath-soluble salt thereof. The anionic aromatic sulfonic acid may be a compound obtained by the polycondensation of formaldehyde and an aromatic sulfonic acid which is generally naphthalene sulfonic acid. Condensation products of this type which are useful in the invention have the formula: ##STR2## and/or ##STR3## wherein z is an integer from 1 to 3 and a is an integer from 1 to 14, preferably from 2 to 6. Polycondensation products of this type are known compounds and their production is described in, for example, Houben-Weyl, &#34;Methoden Der Organishen Chemie,&#34; Volume XIV/2, at page 316, and said description is hereby incorporated by reference. The utility of these condensation products in ammonium-containing acid zinc baths is described in U.S. Pat. No. 3,878,069. 
     The general method of preparing these polycondensation products involves reaction of formaldehyde solution with naphthalene sulfonic acid at a temperature of from about 60° to 100° C. until the formaldehyde odor has disappeared. Similar products can be obtained by sulfonation of naphthalene formaldehyde resins. The condensation products obtained in this manner contain two or more naphthalene sulfonic acids linked by methylene bridges which can have from one to three sulfonic acid groups. These anionic aromatic sulfonic acid compounds may be introduced into the plating baths either in their acid form or as the water-soluble salts which may be the sodium or potassium salts. The amount of the anionic polycondensation product included in the acid plating baths of the invention will vary depending upon the other ingredients in the plating baths but should be an amount effective to impart brightness, ductility, and malleability to the zinc deposit obtained from the baths. 
     The anionic aromatic sulfonic acid utilized in the acid zinc plating baths of the invention also may be a water-soluble salt of tetrahydronaphthalene sulfonate such as the sodium or potassium salts. Generally, from about 5 to about 15 grams of the salt of tetrahydronaphthalene sulfonate will be used per liter of plating bath. The presence of the tetrahydronaphthalene sulfonic acid widens the bright current density range in both the high and low current densities and permits the use of rather vigorous air agitation because of its low foaming property. In some applications, it is preferred to use a mixture of the water-soluble salts of tetrahydronaphthalene sulfonic acid with a water-soluble salt of a polycondensation product of formaldehyde and an aromatic sulfonic acid. 
     The properties of the bright and level zinc deposits produced by the plating baths of the invention may be improved by incorporating additional additives. It has been found, for example, that the brightness of the deposit can be improved through the use of at least one nonionic polyoxyethylene compound selected from the compounds of the formula: ##STR4## wherein m may be zero, the sum of m+n is at least 10, x is 1 or 2, and the compound is derived from a reaction of ethylene oxide with a polyol selected from the group consisting of 2,4,7,9-tetramethyl-5-decyne-4,7-diol; polyoxypropylene glycol of a molecular weight of at least about 9,000; and N,N,N&#39;,N&#39;-tetrakis(polyoxypropylene glycol)ethylene diamine of a molecular weight of at least about 500; and wherein R is defined by the selected polyol. The molecular weights of the polyoxypropylene glycol and the ethylene diamine compounds should not be as high as to form a bath-insoluble compound of Formula IV. 
     Nonionic polyoxyethylene compounds of this type, and their utility in acid zinc baths containing ammonium ions are described in U.S. Pat. No. 3,855,085. Such polyoxyethylene compounds are available commercially under the general trade designations &#34;Surfynol&#34; by Air Products and Chemicals, Inc. of Wayne, Pa., and under the designation &#34;Pluronic&#34; or &#34;Tetronic&#34; by BASF Wyandotte Corp. of Wyandotte, Mich. Examples of specific polyoxyethylene condensation products useful in the invention include &#34;Surfynol 465&#34; which is a product obtained by reacting 10 moles of ethylene oxide with 1 mole of tetramethyldecynediol. &#34;Surfynol 485&#34; is the product obtained by reacting 30 moles of ethylene oxide with tetramethyldecynediol. &#34;Pluronic L 35&#34; is a product obtained by reacting 22 moles of ethylene oxide with polypropylene glycol obtained by the condensation of 16 moles of propylene glycol. 
     The brightness of the zinc deposit obtained with the baths and with the process of the invention is most effectively improved if the bath contains at least one aromatic aldehyde, aromatic ketone, or mixtures of aromatic aldehydes and ketones. These supplementary brighteners impart optimum leveling action of specular brightness over a wide plating range. 
     The following compounds will illustrate the types of aromatic aldehydes and aromatic ketones which have been found useful as brighteners in the plating baths of the invention: ortho-chlorobenzaldehyde, para-chlorobenzaldehyde, benzylidene acetone, coumarin, 1,2,3,6-tetrahydrobenzaldehyde, acetophenone, propiophenone, furfurylidine acetone, 3-methoxybenzal acetone, etc. Mixtures of one or more of the above aldehydes with one or more ketones also are useful. When employed in the baths of the invention, the brighteners will be included within the range of from about 0.02 to about 1 gram per liter and preferably from about 0.03 to about 0.5 grams per liter of bath. 
     The above-identified bath additives are included in the ammonia-free aqueous acidic zinc plating baths of the invention. The plating baths of the invention contain free zinc ions and are prepared with water-soluble zinc salts such as zinc chloride, zinc sulfate, zinc fluoroborate, zinc acetate, and/or zinc sulphamate. The zinc ion concentration in the plating baths of the invention may vary over a wide range such as from about 15 to about 75 grams per liter and preferably from about 22 to about 55 grams per liter. The baths containing lower amounts of zinc ion are particularly useful for barrel plating operations where drag-out losses represent a substantial economic problem, and where better plate distribution is desired at lower current densities. For most plating operations, and particularly for rack plating operations, the zinc ion concentration will be higher. 
     The plating baths of the invention also contain conductive salts which generally are salts of alkali metals such as sodium and/or potassium, including the sulfates, chlorides, fluoroborates and acetates, either alone or in combination. Since it is generally preferred that the plating baths of the invention contain from about 75 to about 150 or 200 grams per liter of chloride ion, the alkali metal chlorides preferably are used in combination with zinc chloride in the preparation of the baths of the invention. 
     As mentioned above, boric acid normally is included in the plating baths of the invention. Boric acid serves as a weak buffer to control pH and the cathode film. The boric acid also is helpful in smoothing the zinc deposit and is believed to have a cooperative effect with the leveling agents included in the baths of the invention. The concentration of boric acid in the baths of the invention is not critical and generally will be in the range of from about zero to about 60 grams per liter, preferably from about 30 to 40 grams per liter. 
     The acidic zinc electroplating baths of the invention may be utilized to produce bright ductile zinc deposits on all types of metals and alloys, for example, on zinc die cast, copper, brass, malleable and cast iron, and carbonitrided and high carbon heat-treated steel. The plating baths of the invention may be employed in all types of industrial zinc plating processes including steel plating baths, high-speed plating baths for strip or wire coating, and in barrel plating. 
     The acidic zinc plating bath of the present invention will deposit a bright and ductile zinc deposit on substrates at any conventional temperature such as from about 20° to about 60° C. and, more preferably, from about 20° to about 35° C. The acidity of the bath may vary from a pH of from about 3 or 4 to about 6, but preferably is operated at a pH of from about 5.2 to about 5.7. Hydrochloric acid may be used to lower the pH, and potassium hydroxide may be used to raise the pH when necessary. 
     Two typical ammonia-free acidic zinc plating baths in which the additive compositions of the invention may be added are as follows: 
     
         ______________________________________Bath No. 1______________________________________Zinc chloride         105    g/lPotassium chloride    210    g/lBoric acid            20     g/lpH                    4.7______________________________________ 
    
     This formulation is suitable especially for plating current densities from about 3.0A/dm 2  to 10A/dm 2  although it may be utilized for current densities below 3.0A/dm 2 . 
     
         ______________________________________Bath No. 2______________________________________Zinc chloride         50     g/lPotassium chloride    140    g/lBoric acid            20     g/lpH                    4.7______________________________________ 
    
     This more dilute bath is particularly suited for barrel plating operations where current densities average below 2.0A/dm 2 . 
    
    
     The following specific examples illustrate the utility of the plating baths of the invention and the method of electrodepositing zinc plate from such baths by plating steel Hull cell panels in a 267ml. Hull cell. The baths were prepared as indicated by including the specified additive combination described in Table I in either Bath 1 or Bath 2. Current densities were measured with a Hull cell scale. The panels were plated at 1 or 3 amperes for five minutes from a DC rectifier. The solutions were mechanically agitated. 
     
                       Table 1______________________________________ADDITION AGENT COMPONENT COMBINATIONSIN GRAMS PER LITER OF BATHCOMPONENT          A      B      C    D   E______________________________________Sodium salt oftetrahydronaphthalenesulfonic acid      6.0    6.0    6.0  6.0 6.0Blancol N (a sodium saltof a sulfonated naphthalenecondensate, available commerciallyfrom GAF Corp.)    0.6    0.6    --   0.6 0.6Surfynol 465 (tetramethyldecynediol ethoxylated withten moles of ethyleneoxide, available commerciallyfrom Air Products) 1.5    1.5    1.5  1.5 1.5The reaction product of -naphtholreacted with twelvemoles of ethylene oxide              0.4    0.4    0.4  0.4 0.4Sodium salt of benzoic acid              --     2.0    2.0  2.0 2.0Sodium salt of salicylic acid               2.25  --     --   --   2.25Benzylidene acetone               0.05   0.05   0.05                                 --  --Ortho-chlorobenzaldehyde              --     --     --   0.1 0.1______________________________________ 
    
     EXAMPLE 1 
     Bath No. 1 
     Current across Hull Cell--3 amperes 
     Addition combination A from Table I 
     Results--bright, level zinc plate from below 0.15A/dm 2  to above 12.0A/dm 2   
     EXAMPLE 2 
     Bath No. 1 
     Current--3 amperes 
     Addition combination B 
     Result--bright, level zinc plate from below 0.15A/dm 2  to above 12.0A/dm 2   
     EXAMPLE 3 
     Bath No. 1 
     Current--1 ampere 
     Addition combination C 
     Result--bright, level zinc plate from below 0.05A/dm 2  to above 4.0A/dm 2   
     EXAMPLE 4 
     Bath No. 1 
     Current--3 amperes 
     Addition combination D 
     Result--similar to Example No. 1 
     EXAMPLE 5 
     Bath No. 1 
     Current--3 amperes 
     Addition combination E 
     Result--similar to Example No. 1 
     EXAMPLE 6 
     Bath No. 2 
     Current--1 ampere 
     Addition combination B 
     Result--bright, level zinc plate from below 0.05A/dm 2  to above 4.0A/dm 2  with dull, mat zinc plate on the high current density edge of the Hull Cell panel. 
     In practice, the improved zinc plating baths of the invention may be operated on a continuous or intermittent basis, and from time to time, the components of the bath have to be replenished. The various components may be added singularly as required or may be added in combination. The amounts of the various additive compositions to be added to the plating baths may be varied over a wide range depending on the nature and performance of the zinc plating bath to which the composition is added. Such amounts can be determined readily by one skilled in the art. 
     Another aspect of the invention relates to additive compositions which may be mixtures of various additives without any solvent or carrier or they may be concentrates of bath components in water, alcohols, or mixtures of water and one or more alcohols. The additive compositions will comprise the polyoxyalkylated naphthol, the aromatic carboxylic acid or bath-soluble salt thereof, and at least one anionic aromatic sulfonic acid or bath-soluble salt thereof. In addition to the above-described components, the additive compositions may contain any or all of the additives discussed above such as at least one nonionic polyoxyethylene compound, at least one aromatic aldehyde, ketone or mixtures thereof, as desired. The amounts of the compounds in the additive composition or concentrates will be such that when they are diluted and added to the baths, they will provide the requisite amounts of the components in the bath or the requisite amounts of the components required to replenish the bath. 
     The following additive compositions or concentrates in addition to those listed in Table I illustrate the various combinations of compounds that may be prepared and utilized in accordance with the invention for preparing or maintaining the baths of the invention and/or improving the performance of the baths of the invention. 
     
         ______________________________________                    Parts                    by Weight______________________________________Additive Composition FThe reaction product of β -naphtholreacted with ten moles of ethylene oxide                       5Sodium benzoate            20Blancol N (a sodium salt of a sulfonatednaphthalene formaldehyde concentrateavailable commercially from GAF Corp.)                      10Water                      65Additive Composition GThe ethoxylated β -napthol ofComposition F               5Sodium salt of salicylic acid                      25Blancol N                  10Surfynol 465 (tetramethyl decynediol ethoxylatedwith ten moles of ethylene oxide, availablecommercially from Air Products)                      15Water                      45Additive Composition HEthoxylated β -naphthol of Composition F                       5Sodium salt of benzoic acid                      20Sodium salt of tetrahydronaphthalene sulfonicacid                       15Water                      50Additive Composition IThe ethoxylated β -naphthol ofComposition F               5Sodium salt of salicyclic acid                      25Blancol N                  10Surfynol 465 (tetramethyl decynediol ethoxylatedwith ten moles of ethylene oxide, availablecommercially from Air Products)                      15Benzylidene acetone         1Water                      44______________________________________