EP0786539A2

EP0786539A2 - High current density zinc organosulfonate electrogalvanizing process and composition

Info

Publication number: EP0786539A2
Application number: EP97100964A
Authority: EP
Inventors: Nicholas M. Martyak; John A. Corsentino; Frederik L. Rohling; Marie M. Kasper
Original assignee: Elf Atochem North America Inc
Current assignee: Arkema Inc
Priority date: 1996-01-26
Filing date: 1997-01-22
Publication date: 1997-07-30
Also published as: JPH09310192A; KR970059314A; EP0786539A3

Abstract

The inventors disclose a process for reducing high current density edge buildup dendrite formation, edge burn, controlling high current density roughness, grain size, and orientation of a zinc coating obtained from an aqueous zinc acidic electrogalvanic coating bath comprising passing a high density current from a zinc anode in the bath to a metal cathode in the bath for a period of time sufficient to deposit a zinc coating on the cathode. The bath contains greater than about 5g/l of a water soluble zinc organosulfonate. A random or block polyoxyalkylene glycol homopolymer or copolymer based on 2 to about 4 carbon atom alkylene oxides. The inventors employ current densities from about 250 to about 4,000 ASF, and optionally, a sulfonated condensation product of naphthalene and formaldehyde, a boron oxide compound, and a lignin compound. The invention also comprises bath compositions.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The field of the invention comprises high current density zinc electroplating bath compositions of matter, and processes utilizing such composition for reducing high current density edge build up dendrite formation and edge burn, controlling high current density roughness, grain size, and crystallographic orientation of a zinc coating obtained from the bath. The automotive, electrical and wire industries employ these coatings.

Description of Related Art

Zinc corrosion resistant coatings electrolytically applied to ferrous metals such as steel find use extensively in industries requiring corrosion resistance, such as the automotive industry.
Zinc provides sacrificial protection for ferrous metals because it is anodic to the substrate which is protected so long as some zinc remains in the area to be protected. The presence of minor pin holes or discontinuities in the deposit is of little significance. Zinc is plated continuously in most industrial processes such as the electrogalvanic coating of continuous steel substrates employed in the automotive and tubular steel industries. Acid chloride and sulfate baths are used extensively because they are capable of higher plating speeds than cyanide coating baths.
They have also displaced cyanide baths because of EPA regulations requiring the reduction or elimination of cyanide in effluents. The chloride baths include neutral chloride baths containing ammonium ions and chelating agents and acid chloride baths having a pH of from about 3.0 to about 5.5 that substitute potassium ions for the ammonium ions used in the neutral baths. Acid baths have largely replaced neutral ones in practice.
The ASTM specification for zinc deposits on ferrous metals call for thicknesses of from about 5 to about 25 µm, depending on the severity of the expected service. ASTMB633-78, Specification For Electrodeposited Coatings Of Zinc On Iron and Steel.
Zinc is deposited from aqueous solutions by virtue of a high hydrogen over voltage since hydrogen preferentially deposits under equilibrium conditions.
Typical continuous plating tanks employed in the automotive industry contain anywhere from about 50,000 to about 300,000 gallons for plating either zinc or a zinc alloy such as a zinc-iron alloy. These plating tanks will accommodate steel rolls about 8 feet in diameter at speeds of anywhere from about 200 to about 850 feet per minute with varying coating weights of from about 20 to about 80 grams/m² and coating thicknesses from about 6 to about 10 µm. The solution flow rate is about 0.5 to about 5 m/sec. Current densities of about 50 to about 250 A/dm² (about 500-2,500 ASF, i.e., amps per square foot) are employed which also contribute to the excess buildup of zinc on the edge of the plated steel. Allowances for such high current density [HCD] plating are made by adjusting the solution conductivity, providing close anode-cathode spacing, and providing a high solution flow rate.
The steel is drawn over conductive rolls to provide adequate contact and prevent the coating solution from reaching the roll. Zinc anodes are immersed in the baths adjacent the coating rolls. In the case of zinc-iron alloy plating operations, separate iron anodes are added to the system.
Plating tanks employed in the tubular steel industry to produce galvanized steel tubing for electrical conduit vary from about 100 gallons to about 50,0000 gallons and employ current densities from about 10 to about 75 A/dm². Solution agitation occurs as a result of passing the substrate through the bath at a rate of from about 0.1 to 1 m/sec which is less than that employed in the automotive industry. Deposit thickness varies from about 0.2 to about 20 µm.
Wire plating proceeds in substantially the same way at from about 10 to about 100 A/dm², with mild solution agitation where the wire is unspooled at one end of the line, cleaned, plated, and spooled at the other end. Line speeds vary in order to obtain different critical deposit thicknesses which vary from about 10 to 100 µm.
Zinc chloride electrolyte plating baths commonly employ soluble anodes in the system. Zinc sulfate electrolyte solutions generally operate at a pH of about 1.2 to about 3 and elevated temperatures anywhere from about 35°C to about 80°C. The low pH generally requires employing insoluble anodes; however, some zinc sulfate solutions may employ zinc anodes.
Excess buildup of zinc at high current densities, however, can occur. If a relatively narrow steel strip is being coated, there may be excess anodes in the system. It is impossible to remove the excess anodes because the next strip to be coated may be larger in size. The mechanics of the line make it too cumbersome to remove and add anodes to accommodate the size of the different substrates being plated.
Another major concern is that HCD processes produce roughness in the form of dendrites at the edge of the steel strip during the coating operation. These dendritic deposits may break off during plating or rinsing. As the electrogalvanized steel is passed over rollers, these loose dendrites become embedded across the coated substrate and subsequently show up as blemishes which are referred to as zinc-pickups. The edges of the steel strip that are coated are also non-uniform in thickness, and burned because of HCD processing. Additionally, HCD processes can cause roughness across the width of the steel strip and change the grain size and crystallographic orientation of the zinc coating. Nonetheless, HCD processes are industrially desirable since production speed is directly related to current density i.e., higher coating line speeds can be obtained at higher current densities.
The surface roughness of the coated steel strip is expressed in "Ra" units whereas the degree of roughness is expressed in "PPI" units or peaks per inch. These parameters are important in that surface roughness promotes paint adhesion and proper PPI values promote retention of oil which is important during forming operations for zinc coated steel that is used in the manufacture of automobile parts or other parts that are subsequently press formed. A rule of thumb is that the Ra and PPI values should be close to that of the substrate. In some instances it is better to have a zinc coating that is rougher than the substrate rather than smoother, and sometimes smoother than the substrate (i.e., slightly less rough than that of the substrate). Accordingly, the Ra value generally should not exceed about 40 micro inches and the PPI value should be anywhere from about 150 to about 225.
A composition has been used to obtain some of these advantages, and is based on an ethylene oxide polymer having a molecular weight of 600 in combination with equal parts of an antidendritic agent which comprises a sulfonated condensation product of naphthalene and formaldehyde. When employing this combination in these proportions, however, it was found that the zinc coating substantially replicated the surface roughness (Ra) and degree of roughness (PPI) of the steel substrate to which the zinc coating was applied. Zinc coatings having a smoother surface than the substrate could not be obtained.
Additionally, it has been found that various crystallographic orientations of the electrodeposited zinc [(002), (110), (102), (100), (101), and (103)] are obtained, but that with some compositions the (101) orientation is favored, and with others the (002) is favored.
As noted, production speed can be increased as current density increases and where current densities presently being employed by industry are at about 1,000 ASF (110 A/dm²) current densities of anywhere from about 1,500 to about 3,000 ASF are being explored in order to obtain higher production rates. Operating at these higher current densities has resulted in unacceptable edge burn, dendritic formation and break off, grain size, problems with obtaining or retention of the (101) or (002) orientation, and unacceptable values for Ra and PPI.
Additionally, many of the additives to the plating bath employed at about 1,000 ASF do not adequately address the foregoing difficulties.
Broadwell, United States Patent No. 905,837 describes the electrodeposition of zinc and alloys containing aluminum or cadmium or like metals having a brightening influence upon zinc. The process utilizes a solution of zinc sulfate in combination with zinc naphthalene di-sulfonate. The alloy is electrodeposited by incorporating a salt of the alloying metal such as aluminum sulfate into the electroplating bath.
Flett, United States Patent No. 2,195,409 describes the use of an alkyl aromatic sulfonic acid in a zinc plating bath containing zinc sulfate and aluminum sulfate.
Creutz, United States Patent No. 4,207,150 describes a non-cyanide zinc electroplating bath based on zinc chloride, sulfate, fluoroborate or acetate with levelling amounts of methane sulfonic acid (sometimes referred to as "MSA") zinc salts in amounts from about 0.005 to 5.0g/l. The coating bath is operated in a pH range of 2.0 to 7.5 and also contains so-called secondary or supporting brighteners consisting of polyethers having a molecular weight from 100 to 1,000,000. Plating is carried out at from 60°F to 140°F at current densities ranging from 5 ASF, to 200 ASF and in a pH range from 2.0 to 7.5.
Wilson, United States Patent No. 5,039,576, describes the use of alkyl sulfonic or polysulfonic acids or salts in combination with a tin and bismuth ion for the electrodeposition of tin-bismuth alloys on a conductive substrate.
United States Patent No. 774,049 describes a process for electrolytically depositing lead peroxide on lead plates from baths containing a sulfonic acid or oxysulfonic acid derivative of methane and its hydroxy-substituted derivatives. These include methylsulfonic acid, methylene disulfonic acid, oxymethylene disulfonic acid and the like.
United States Patent No. 2,313,371 and British Patent No. 555,929 describe tin and tin-lead plating baths containing aromatic sulfones and mono- and poly-sulfonic acids of benzene, phenol and cresol.
United States Patent No. 4,132,610 discloses tin-lead alloy plating baths containing hydroxyalkyl sulfonic acids.
Deresh et al, United States Patent No. 4,849,059 describes a tin, lead, or tin-lead alloy electroplating bath containing free alkane sulfonic acid brightening agents and other compounds.
Pilavov, Russian Patent 1,606,539 describes weekly acidic baths for electrogalvanizing steel containing a condensation copolymer of formaldehyde and 1,5- and 1,8-aminonaphthylalene-sulfonic acid prepared in monoethanolamine. The galvanized steel shows a smaller decrease in ductility compared to that obtained from a conventional bath.
Watanabe et al., U.S. Patent No. 4,877,497 describe an acidic aqueous electrogalvanizing solution containing zinc chloride, ammonium chloride or potassium chloride and a saturated carboxylic acid sodium or potassium salt. The composition inhibits production of anode sludge.
Strom et al., U.S. Patent No. 4,515,663 disclose an aqueous acid electroplating solution for depositing zinc and zinc alloys which contains a comparatively low concentration of boric acid and a polyhydroxy additive containing at least three hydroxyl groups and at least four carbon atoms.
Paneccasio, U.S. Patent No. 4,512,856 discloses zinc plating solutions and methods utilizing ethoxylated/propoxylated polyhydric alcohols as a novel grain-refining agent.
Kohl, U.S. Patent No. 4,379,738 discloses a composition for electroplating zinc from a bath containing antidendritic additives based on phthalic anhydride derived compounds and analogs thereof in combination with polyethoxyalkylphenols.
Arcilesi, U.S. Patent No. 4,137,133 discloses an acid zinc electroplating process and composition containing as cooperating additives, at least one bath soluble substituted or unsubstituted polyether, at least one aliphatic unsaturated acid containing an aromatic or heteroaromatic group and at least one aromatic or N-heteroaromatic aldehyde.
Hildering et al., U.S. Patent No. 3,960,677 describe an acid zinc electroplating bath which includes a carboxy terminated anionic wetting agent and a heterocyclic brightener compound based on furans, thiophenes and thiazoles.
Dubrow et al., U.S. Patent No. 3,957,595 describe zinc electroplating baths which contain a polyquaternary ammonium salt and a monomeric quaternary salt to improve throwing power.

Summary of the Invention

Accordingly, the present invention is directed to a process and composition that substantially obviate one or more of these and other problems due to limitations and disadvantages of the related art.
These and other advantages are obtained according to the present invention which is the provision of a process and composition of matter that substantially obviate one or more of the limitations and disadvantages of the described prior processes and compositions of matter.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and obtained by the process and composition of matter, particularly pointed out in the written description and claims hereof.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention comprises a high current density electrogalvanizing process and composition of matter for reducing high current density edge build up, dendrite formation and edge burn, and controlling high current density roughness, maintaining grain size similar to that currently produced by industry and orientation especially (002) orientation which zinc sulfate baths produce. The invention achieves the foregoing by means of a zinc coating obtained from an aqueous zinc organosulfonate electrogalvanic coating bath. The latter will be referred to herein as such or as the bath, or coating bath, unless otherwise indicated.
The electrogalvanizing process is carried out under conditions and in the manner heretofore described for coating a metal substrate and especially a steel substrate by passing a current from a zinc or insoluble anode immersed in the electrogalvanic coating bath to a metal cathode in the bath for a period of time sufficient to deposit a zinc coating on the cathode.
Passing a high density current from a zinc or insoluble anode in the bath to a metal cathode in the bath for a sufficient time causes a zinc coating to deposit on the cathode where the bath contains greater than about 5 g/l of a water soluble zinc organosulfonate. In a preferred embodiment, the zinc salt in the bath is substantially all zinc organosulfonate. One of the important aspects of the invention comprises the discovery that zinc organosulfonate coating baths do not require additives to eliminate or minimize edge roughness for automotive applications employing a bath pH of greater than about 2.5 to about 5. Dropping the pH to about 2.5 or less than about 2.5 such as a pH of about 1.5 used in other electrogalvanization processes employing zinc sulfate solution requires additives in the baths, as described hereinafter. Proceeding in this way substantially minimizes or eliminates dendrite formation and edge burn and controls high current density roughness, grain size and crystallographic orientation. In either case, the incorporation of brighteners in the baths produce bright zinc coatings.
Accordingly, the invention generally comprises:

(A) Zinc organosulfonate solutions requiring no additives at a solution pH of greater than about 2.5, sometimes referred to as a "blank" solution containing only the zinc organosulfonate to produce acceptable electrogalvanic coatings in automotive, wire, or tubular steel plating applications especially at HCD conditions. The use of zinc organosulfonates such as zinc methane sulfonic acid compared to zinc sulfate allows for higher current density plating yet, as noted, produces an acceptable deposit;
(B) Zinc organosulfonate electrogalvanic baths will include additives at pH levels from about 1.5 to about 2.5 such as that now employed by automotive companies using zinc sulfate. Without the additives, the overall edge roughness is reduced compared to the zinc sulfate solution but the zinc organosulfonate does not eliminate this problem. Additives subsequently described herein when incorporated in the zinc organosulfonate substantially minimize or eliminate edge roughness, especially in HCD processes;
(C) Either of the foregoing processes optionally employ brighteners to deposit bright electrogalvanic coatings, with brighteners including orthochlorobenzaldehyde, benzylidene acetone, and nicotinic acid.

This new zinc organosulfonate solution and especially a zinc methane sulfonic acid solution maintains the grain size and crystallographic orientation that is currently produced from zinc sulfate solutions used in electrogalvanic processes.
The main reason for a zinc organosulfonate such as zinc methane sulfonic acid electrolytes compared to the zinc sulfate electrolyte currently used in the wire and tubular steel plating industries is to increase the current density which is to say the production rate and economies of coating, without sacrificing deposit quality.
The bath additive can comprise glycol compounds based on the lower alkylene oxides, such as those alkylene oxides having from 2 to about 4 carbon atoms and includes not only the polymers thereof but also the copolymers such as the copolymers of ethylene and propylene oxide and/or butylene oxide. The copolymers may be random or block copolymers, where the repeating units of the block copolymers are heteric, or block, or the various combinations of these repeating units known in the art.
In one embodiment, the polyoxyalkylene glycol is preferably substantially water soluble at operating temperatures and may be a polyoxyalkylene glycol ether all-block, block-heteric, heteric-block or heteric-heteric block copolymer where the alkylene units have from 2 to about 4 carbon atoms and may comprise a surfactant which contains hydrophobic and hydrophilic blocks where each block is based on at least oxyethylene groups or oxypropylene groups or mixtures of these groups. Mixtures of copolymers and homopolymers may also be used, especially the two component, three component, or four component mixtures.
Of the various polyether-polyol block-copolymers available, the preferred materials comprise polyoxyalkylene glycol ethers which in the case of surfactants contain hydrophobic and hydrophilic blocks, each block preferably being based on at least oxyethylene groups or oxypropylene groups or mixtures of these groups.
The random or block polyoxyalkylene glycol homopolymer or copolymer in one instance is based on 3 to about 4 carbon atom alkylene oxides having an average molecular weight of from about 300 to about 1,100. An especially suitable alkylene oxide in this regard comprises propylene oxide, and includes polyoxyalkylene glycols comprising propylene oxide homopolymers having an average molecular weight of about 425.
Other random or block polyoxyalkylene glycol homopolymer or copolymers that are used have an average molecular weight of from about 2,000 to about 9,500 and include homopolymers or copolymers based on ethylene oxide, especially ethylene oxide homopolymers having an average molecular weight of about 8,000.
Another class of block or random polyoxyalkylene glycol homopolymers or copolymers used according to the invention has an average molecular weight of from about 570 to about 630 and includes random or block polyoxyalkylene glycol homopolymers or copolymers based on ethylene oxide, especially those having an average molecular weight of about 600.
The bath of the invention may also contain a water-soluble boron oxide compound, a lignin compound, and a sulfonated condensation product of naphthalene and formaldehyde as an antidendritic agent. Mixtures of the foregoing can also be employed whether mixtures of all, or two component, three component, or four component mixtures.

Detailed Description

As noted, the zinc organosulfonate preferably comprises a water soluble compound by which it is meant that the compound is soluble in water at about room temperature (about 20°C) or lower (about 10°C to about 20°C), and preferably from these temperatures up to or slightly below the operating temperature of the bath, and has the formula:

Zn[(R)(SO₃)_x]_y formula (A)

where x has a value from 1 to about 3; and
y has a value from 1 to 2 so that y may be 1 when x is greater than 1.
R is an organo group comprising an alkyl group having from 1 to about 15 carbon atoms and especially 1 to about 7 carbon atoms including the straight chain and branch chain isomers thereof such as methyl, ethyl, propyl, isopropyl, butyl, t-butyl, isobutyl, pentyl, isopentyl, and the like. Hydroxy substituted alkyls, as alkyl is defined herein, are also included. Specific zinc salts in this regard comprise zinc methane sulfonates, zinc ethane sulfonates, zinc propane sulfonates, zinc isopropane sulfonates, zinc butane sulfonates, zinc isobutane sulfonates, zinc t-butane sulfonates, zinc pentane sulfonates, zinc isopentane sulfonates, and the like, as well as the hydroxy substituted compounds thereof. R also includes cyclic, and heterocyclic hydrocarbon substituents such as cycloaliphatic, unsaturated cycloaliphatic, and aromatic groups having from 4 to about 16 carbon atoms and especially from about 6 to about 14 carbon atoms including cyclobutyl, cyclobutenyl, cyclohexyl, cyclohexenyl, cyclohexadienyl, cyclooctanyl, cyclooctadienyl, furanyl, furfuryl, pyranyl, naphthenyl, naphthyl, anthracyl, phenanthryl, and the various alkyl substituted compounds thereof, where alkyl is defined herein, including benzyl, tolyl, and xylyl, as well as the hydroxy substituted compounds thereof. Specific compounds in this regard include zinc cyclohexyl sulfonate, zinc phenyl sulfonate, zinc benzyl sulfonate, and the various zinc naphthalene sulfonates based on 1-naphthalene sulfonic acid, 2-naphthalene sulfonic acid, 1,5-naphthalene disulfonic acid, 1,6-naphthalene disulfonic acid, 2,6-naphthalene disulfonic acid, 2,7-naphthalene disulfonic acid, 1,3,5-naphthalene trisulfonic acid and 1,3,6-naphthalene trisulfonic acid as well as the various hydroxy naphthalene sulfonic acids including both the monosulfonic and disulfonic acids especially those described by Mosher, United States Patent No. 5,427,677 which is incorporated herein by reference. Other sulfonic acids include 1-naphthol-4-sulfonic acid, 1-naphthol-5-sulfonic acid, 2-naphthol-6-sulfonic acid, 2-naphthol-7-sulfonic acid, 2-naphthol-8-sulfonic acid, naphthalene-1.5-disulfonic acid, naphthalene-1.6-disulfonic acid, naphthalene-2.5-disulfonic acid, 1-naphtol-3.6-disulfonic acid, 1-naphtol-3.8-disulfonic acid, 1-naphtol-4.8-disulfonic acid, 2-naphtol-3.6-disulfonic acid, and 2-naphtol-6.8-disulfonic acid. The invention also employs mixtures of zinc salts, based on the foregoing acids, especially the two component, three component, or four component mixtures.
Other sulfonic acids that may be employed and processes for manufacturing zinc salts of these sulfonic acids are described by Obata et al., United States Patent No. 4,673,470; Dohi et al., United States Patent No. 3,905,878, and United States Patent No. 4,132,610; Flett, United States Patent No. 2,195,409; Werntz, United States Patent No. 2,187,338; Tucker, United States Patent No. 2,147,415; Tinker et al., United States Patent No. 2,174,507; Langedjik, United States Patent No. 1,947,652; and Wilson, United States Patent No. 5,039,576 all of which are incorporated herein by reference.
The invention also includes depositing alloys of zinc in lieu of the zinc coating of the present invention, and can employ organosulfonate salts of the alloying metals and zinc organo sulfonates, where in formula (A), the alloying metal will be substituted for "Zn," "y" has a value of 1 up to the valence of the alloying metal, and "x" has the values given above.
These alloys of zinc may be deposited employing an additive to the coating bath. Iron alloys are a common alloy of zinc utilized in zinc-type corrosion protection coatings and the preparation of these type of alloy coatings are also within the scope of the present invention. Any of the other Group VIII metals of the Periodic Table of the Elements may be used in this regard besides iron, and include nickel and cobalt. Group IIB, VB, VIB and VIIB of the Periodic Table of the Elements metals may also be plated with zinc and include by way of example, vanadium, manganese, chromium, and cadmium to form zinc alloys. Mixtures of alloying metals from Group VIII and/or Groups IIB, VB, VIB and VIIB e.g., Cd, Cr and/or Mn may be prepared, especially the two component, or three component, or four component alloys where the total alloying metal is present in the coating in an amount anywhere from about 0.2 to about 20 percent by weight and especially from about 5 to about 15 percent by weight.
The alloys as noted can be prepared by adding an organo sulfonate salt of the alloying metal to the coating bath, where the organo sulfonate is based on the organo sulfonic acids as defined herein. The alloys are also prepared by inserting the alloy metal into the coating baths as an anode in a manner well known in the art. Other salts of alloying metals, based on mineral acids as defined herein, or organic acids may also be used, where the organic acids have anywhere from 1 to about 10 carbon atoms and from 1 to about three carboxyl groups. These include aliphatic or cyclic acids, whether saturated or unsaturated and are well known in the art.
The amount of water soluble zinc organosulfonate employed in the bath is greater than about 5 g/l especially greater than about 6 or 7 g/l up to the saturation point of the zinc organosulfonate in the plating bath at operating temperatures. Concentrations of from about 10 to about 175 g/l, particularly, about 25 to about 165 g/l and especially from about 50 to 150 g/l can be used. Preferably, the concentration of zinc organosulfonate is from about 75 to about 100 g/l. The foregoing concentrations are based on the weight of zinc in the zinc organosulfonate compound in the bath.
The invention includes use of other zinc salts in the bath, so long as the zinc organosulfonate is within the foregoing concentration parameters and exceeds concentrations of greater than about 5 g/l especially greater than about 6 or 7 g/l, up to the saturation point of the combination of zinc organosulfonates and zinc salts in the bath aat operating temperatures. For example, the zinc organosulfonate in the bath containing mixtures of zinc organo sulfonate salts and zinc salts will comprise ranges of alloying materials where the lower end of the range of alloying metal or metals will be 10%, 20% or 50% and the upper end of the range will be 60%, 70%, 80% or 99% based on the total weight of zinc present in the bath. As an example, these ranges can comprise anywhere from about 10% to about 99%, or about 20% to about 99%, and especially from about 50% to about 99% of the alloying metal or metals, based on the total weight of zinc present in the bath.
Preferably, substantially all zinc in the bath (i.e. about 95% to about 100%) is a zinc organosulfonate. The other zinc salts, if employed, comprise the mineral acid salts, such as those mineral acids as defined herein, and particularly include sulfates, chlorides, nitrates, acetates, and fluoroborates, but especially the sulfates and chlorides. The alloys deposited according to the invention can also be produced from salts having the foregoing anions, or mixtures of these alloying salts with alloying metal organo sulfonates, in the same range of ratios as the zinc organo sulfonates to zinc salts.
High current density or HCD as referred to in this aspect of the invention is intended to include currents of from about 250 to about 4,000 ASF or higher. The lower end of this range of current densities comprises 250 ASF, 300 ASF or 1,000 ASF whereas the higher end of the range comprises 3,000 ASF, 3,500 ASF, or 4,000 ASF or higher. Some exemplary ranges include from about 250 to about 3,500 ASF, and particularly from about 300 to about 3,000 ASF and especially from about 1,000 to about 3,000 ASF.
Current densities that employed using a zinc organo sulfonate for the automotive industry coatings comprise from about 1,500 to 2,500 ASF (about 150 to about 250 A/dm²). High current densities employed using the zinc organo sulfonate for the tubular steel and the wire coating industries comprise from about 750 to 1,500 ASF (about 75 to about 150 A/dm²).
The polyoxyalkylene glycols of the present invention preferably are water soluble at operating temperatures and may be polyoxyalkylene glycol ether all-block, block-heteric, heteric-block or heteric-heteric block copolymers where the alkylene units have from 2 to about 4 carbon atoms and may comprise surfactants which contain hydrophobic and hydrophilic blocks where each block is based on at least oxyethylene groups or oxypropylene groups or mixtures of these groups. Mixtures of homopolymers and copolymers may also be used, especially the two component, three component, or four component mixtures.
Of the various polyether-polyol block-copolymers available, one of the preferred materials comprises polyoxyalkylene glycol ethers which in the case of surfactants contain hydrophobic and hydrophilic blocks, each block preferably being based on at least oxyethylene groups or oxypropylene groups or mixtures of these groups.
The most common method of obtaining these materials is by reacting an alkylene oxide such as ethylene oxide with a material that contains at least one reactive hydrogen. Alternative routes include the reaction of the active hydrogen material with a preformed polyglycol or the use of ethylene chlorohydrin instead of an alkylene oxide.
The reacting active hydrogen material must contain at least one active hydrogen preferably alcohols, and optionally acids, amides, mercaptans, alkyl phenols and the like. Primary amines can be used as well.
Especially preferred materials are those obtained by block polymerization techniques. By the careful control of monomer feed and reaction conditions, a series of compounds, e.g., surfactants can be prepared in which such characteristics as the hydrophile-lipophile balance (HLB), wetting and foaming power can be closely and reproducibly controlled. The chemical nature of the initial component employed in the formation of the initial polymer block generally determines the classification of the materials. The initial component does not have to be hydrophobic. In the case of surfactants, hydrophobicity will be derived from one of the two polymer blocks. The chemical nature of the initial component in the formation of the first polymer block generally determines the classification of the materials. Typical starting materials or initial components include monohydric alcohols such as methanol, ethanol, propanol, butanol and the like as well as dihydric materials such as glycol, glycerol, higher polyols, ethylene diamine and the like.
The various classes of materials, suitable for practice of this aspect of the present invention that are surfactants, have been described by Schmolka in "Non-Ionic Surfactants," Surfactant Science Series Vol. 2, Schick, M.J., Ed. Marcel Dekker, Inc., New York, 1967, Chapter 10 which is incorporated herein by reference.
The first and simplest copolymer is that in which each block is homogeneous which is to say a single alkylene oxide is used in the monomer feed during each step in the preparation. These materials are referred to as all-block copolymers. The next classes are termed block-heteric and heteric-block, in which one portion of the molecule is composed of a single alkylene oxide while the other is a mixture of two or more such materials, one of which may be the same as that of the homogeneous block portion of the molecule. In the preparation of such materials, the hetero portion of the molecule will be totally random. The properties of these copolymers will be entirely distinct from those of the pure block copolymers. The other class is that in which both steps in the preparation of the different repeating units involve the addition of mixtures of alkylene oxides and is defined as a heteric-heteric block copolymer.
The block copolymer is typified by a monofunctional starting material such as a monohydric alcohol, acid, mercaptan, secondary amine or N-substituted amides. These materials can generally be illustrated by the following formula:

I-[A_m-B_n]_x (1)

where I is the starting material molecule as described before. The A portion is a repeating unit comprising an alkylene oxide unit in which at least one hydrogen can be replaced by an alkyl group or an aryl group, and m is the degree of polymerization which is usually greater than about 6. The B moiety is the other repeating unit such as oxyethylene with n again being the degree of polymerization. The value of x is the functionality of I. Thus, where I is a monofunctional alcohol or amine, x is 1; where I is a polyfunctional starting material such as a diol (e.g., propylene glycol), x is 2 as is the case with the Pluronic® surfactants. Where I is a tetrafunctional starting material such as ethylenediamine, x will be 4 as is the case with Tetronic® surfactants. Preferred copolymers of this type are the polyoxypropylene-polyoxyethylene block copolymers.
Multifunctional starting materials may also be employed to prepare the homogeneous block copolymers.
In the block-heteric and heteric-block materials either A or B will be a mixture of oxides with the remaining block being a homogeneous block. Where the copolymer is a surfactant, one block will be the hydrophobe and the other the hydrophile and either of the two polymeric units will serve as the water solubilizing unit but the characteristics will differ depending on which is employed. Multifunctional starting materials can also be employed in materials of this type.
The heteric-heteric block copolymers are prepared essentially the same way as discussed previously with the major difference being that the monomer feed for the alkylene oxide in each step is composed of a mixture of two or more materials. The blocks will therefore be random copolymers of the monomer feed. In the case of surfactants, the solubility characteristics will be determined by the relative ratios of potentially water soluble and water insoluble materials.
The average molecular weight of the polyoxyalkylene glycol ether block copolymers based on 3 to about 4 carbon atom alkylene oxides is from about 300 to about 1,000 and especially those having an average molecular weight of about 425. These copolymers, as represented by formula (1) are prepared so that the weight ratio of A to B repeating units will also vary from about 0.4:1 to about 2.5:1, especially from about 0.6:1 to about 1.8:1 and preferably from about 0.8:1 to about 1.2:1.
In one embodiment, these copolymers have the general formula:

RX(CH₂CH₂[CH₂]_yO)_nH (2)

where R has an average molecular weight of from about 200 to about 900, especially from about 300 to about 850 and especially from about 350 to about 400, and where R is usually a typical surfactant hydrophobic group but may also be a polyether such as a polyoxyethylene group, a polyoxypropylene group, or a polyoxybutylene group, or a mixture of polyoxypropylene, polyoxyethylene and polyoxypropylene groups. In the above formula X is either oxygen or nitrogen or another functionality capable of linking the polyoxyalkylene chain to R, and y has a value of 0, 1, or 2. In most cases, n, the average number of alkylene oxide units must be greater than about 5 or about 6. This is especially the case where it is desired to impart sufficient water solubility to make the materials useful.
The invention, in one embodiment, employs low molecular weight polyoxyalkylene glycols based on 3 to about 4 carbon atom alkylene oxides including the homopolymers or copolymers thereof with each other and/or ethylene oxide. The copolymers may be random or block copolymers, where the repeating units of the block copolymers are block or heteric or the various combinations of these repeating units known in the art. The low molecular weight polyoxyalkylene glycol in this regard has a molecular weight from about 300 to about 1,100 and especially from about 325 to about 800 and preferably from about 350 to about 550. Those having an average molecular weight of about 425 are especially useful.
Homopolymers and copolymers based on propylene oxide are preferred, especially homopolymers based on propylene oxide, such as for example, polypropylene glycol 425.
The invention also comprises the use in the bath of a low molecular weight polyoxyalkylene glycol homopolymer or copolymer based on 2 to about 4 carbon atom alkylene oxides, wherein the homopolymer or copolymer has a molecular weight of from about 570 to about 630, and especially one having an average molecular weight of about 600. Homopolymers or copolymers based on ethylene oxide are preferred, especially homopolymers based on ethylene oxide.
High and low molecular weight polyoxyalkylene glycol ether block copolymers utilized according to the present invention are those based on 2 to about 4 carbon atom alkylene oxides. The high molecular weight copolymers may have a molecular weight of from about 2,000 to about 9,500 especially from about 2,000 to about 8,500. Low molecular weight polymers have a molecular weight of from about 570 to about 630. The weight ratio of A to B repeating units will also vary from about 0.4:1 to about 2.5:1, especially from about 0.6:1 to about 1.8:1 and preferably from about 0.8:1 to about 1.2:1. These copolymers have the general formula:

RX(CH₂CH₂O)_nH (3)

R in formula (3) is usually a typical surfactant hydrophobic group but may also be a polyether such as a polyoxyethylene group, a polyoxypropylene group, a polyoxybutylene group, or a mixture of these groups.
In formula (3), X is either oxygen or nitrogen or another functionality capable of linking the polyoxyethylene chain to R. In most cases, n, the average number of oxyethylene units in the oxyethylene group, must be greater than about 5 or about 6. This is especially the case where it is desired to impart sufficient water solubility to make the materials useful. In formula (3), R has an average molecular weight of from about 500 to about 8,000, especially from about 1,000 to about 6,000 and preferably from about 1,200 to about 5,000 for the high molecular weight polyoxyalkylene glycol. The polyoxyalkylene glycol in one embodiment comprises polyethylene glycol or the various copolymers thereof as noted herein and especially a polyethylene glycol having a molecular weight of from about 2,000 to about 9,500 and preferably a polyethylene glycol having an average molecular weight of about 8,000. These compounds include CARBOWAX® PEG 4000 (molec. wt. 3,000-3,700), PEG 6000 (mol. wt. 6,000-7,000) and PEG 8000 sold by Union Carbide Corporation.
The molecular weight of R in formula (3) for the low molecular weight polyoxyalkylene glycols employed is from about 200 to about 600, and especially from about 300 to about 500.
One preferred class of polyoxyalkylene glycol ethers are the non-ionic polyether-polyol block-copolymers. However, other non-ionic block-copolymers useful in the invention can be modified block copolymers using the following as starting materials: (a) alcohols, (b) fatty acids, (c) alkylphenol derivatives, (d) glycerol and its derivatives, (e) fatty amines, (f)-1,4-sorbitan derivatives, (g) castor oil and derivatives, and (h) glycol derivatives.
Molecular weight and average molecular weight, as those terms are used herein, are intended to mean weight average molecular weight.
The grain refiner in one embodiment comprises the foregoing low molecular weight polyoxyalkylene glycol homopolymer or copolymer based on 2 to about 4 carbon atom alkylene oxides, wherein the homopolymer or copolymer has a molecular weight of from about 570 to about 630, and especially one having an average molecular weight of about 600. Homopolymers or copolymers based on ethylene oxide are preferred, especially homopolymers based on ethylene oxide. The antidendritic agent comprises a sulfonated condensation product of naphthalene and formaldehyde.
The polyoxyalkylene glycol is employed in an amount anywhere from about 0.025 to about 1.0 gms/liter and especially from about 0.05 to about 0.2 gms/liter as part of the bath.
The sulfonated condensation product of naphthalene and formaldehyde used as an antidendritic agent e.g., BLANCOL®-N, may be employed in an amount so that it will be present in the bath at from about 25 to about 500 ppm and especially from about 75 to about 150 ppm. The term "ppm" as used throughout will refer to the amount by weight, compared to the weight of the bath.
The ratios of the high molecular weight polyoxyalkylene glycol to the sulfonated condensation product of naphthalene and formaldehyde is anywhere from about 1.5:1 to about 1:1.5 and especially from about 1.2:1 to about 1:1.2.
The pH of the bath may be anywhere from about 1.5 to about 5.5 and especially from greater than about 2.5 to about 5.0 where the bath contains no additives and from about 1.5 to about 2.5 with the additives.
Acids, such as mineral acids. may be added to the bath in order to adjust the pH. These acids include hydrofluoric, hydrochloric, hydrobromic and hydriodic acids. Additionally, these acids comprise nitrogen acids, or sulfur acids may be added to the bath in order to adjust the pH. These acids are well known in the art and include inter alia, nitric or nitrous acids as well as sulfuric, sulfurous, oleum, thiosulfuric, dithionous, metasulfuric, dithionic, pyrosulfuric, or persulfuric acid and the like. Hydrochloric acid, nitric acid and sulfuric acid are preferred because of their commercial availability.
Mixtures of acids within each class, or from different classes of the foregoing halogen, nitrogen, and sulfur acids may also be used, especially the two component, three component, or four component mixtures.
The bath is operated at a temperature of from about 100°F to about 170°F, and especially from about 120°F to about 160°F.
The low molecular weight polyoxyalkylene glycol homopolymer or copolymer, based on alkylene oxides having anywhere from 2 to about 4 carbon atoms and especially ethylene oxide polymer homopolymers and copolymers, employed according to one embodiment of the invention are used in an amount anywhere from about 25 to about 500 ppm and especially from about 75 to about 200 ppm.
The bath may also include a water-soluble boron oxide compound such as boric acid or an alkali metal borate (where the alkali metals are defined herein) or a fluoroborate including the alkali metal fluoroborates, where the alkali metals include those of Group IA of The Periodic Table of Elements, especially sodium, potassium, and lithium, as well as the ammonium and organo nitrogen art-known equivalents thereof.
The water-soluble boron oxide compound is employed in an amount anywhere from about 10 to about 70 gms/liter and especially from about 30 to about 40 gms/liter of the coating bath. Boric acid is especially suitable in this regard.
The bath may also contain a lignin compound such as vanillin which is an aldehyde derived from lignin. Additionally, lignin sulfate or other lignin salts known in the art may be employed. These lignin compounds are brighteners and are used in those applications where a bright finish is desired.
The lignin compound or other brighteners may be employed in an amount anywhere from about 0.002 to about 0.01 gms/liter and especially from about 0.03 to about 0.05 gms/liter of the coating bath.
Other brighteners include orthochlorobenzaldehyde, nicotinic acid and benzylidene acetone either of which provide good results at concentrations noted herein as well as from about 5 to about 100mg/l based on the bath.
In comparative experiments, no edge buildup at high current densities using zinc methane sulfonate as the electrolyte occurred at high current densities (up to 300 ADM²) compared to a conventional zinc sulfate or zinc chloride bath.
The inventive process and composition allows operation of the zinc methane sulfonate baths at higher pH values compared to zinc sulfate. The latter is operated at pH levels of about 1.5 to about 2.0 whereas zinc methane sulfonate baths operate best at a pH greater than about 2.0, preferably a pH in the range greater than about 2.0 to about 3 or about 5. The high pH causes less dissolution of the steel substrate prior to zinc plating.
Standard practice to minimize edge buildup or to increase operating current densities using zinc sulfate or zinc chloride solutions comprises increasing concentration of zinc in the plating solution to levels of about 100-150g/l. However, this results in considerable more dragout for waste treatment. Using zinc methane sulfonate or other water soluble zinc organosulfonates as described herein, permits operation at from about 75 to about 100g/l zinc thereby requiring less waste treatment.
The foregoing will be appreciated taking into account typical dragout occurring in commercial electrogalvanizing processes which run anywhere from about 2 to about 5% by volume per day of the zinc solution. This translates to anywhere from about 2,000 to 5,000 gallons per day (about 8,000 to about 20,000 L) for a 100,000 gallon tank which requires treatment. Where the solution contains 125g/l as zinc metal in the sulfate solution it will be appreciated at 20,000 L contains 2.5⁶ gm of zinc (i.e., 5,500 pounds of zinc metal). Using zinc methane sulfonic acid solutions at 75 gms/liter reduces the amount of zinc in the waste stream to only 3,300 pounds, roughly a 40% reduction in waste requiring treatment.
The following Examples are illustrative.
Example 1. Automobile - A Zn(MSA)₂ solution was prepared by dissolving zinc carbonate in methane sulfonic acid so the final zinc (as zinc metal) was 75 g/L. The pH was adjusted to pH 3.5 - 4.0 with methane sulfonic acid or sodium bicarbonate. Steel strips were plated using a rotating cathode spinning at 1000 RPM. The cathodic current density was 150 A/dm². A soluble zinc anode was used. The solution was operated at 60°C. The zinc thickness on the cathode was eight microns. Microscopic analysis showed no high current density dendrites at the edge of the steel. The composition and process has application in producing galvanized steel for the automobile industry.
Example 2. Alloys - A Zn(MSA)₂ solution was prepared by dissolving zinc carbonate in methane sulfonic acid so the final zinc (as zinc metal) was 75 g/L. The pH was adjusted to pH 3.5 - 4.0 with methane sulfonic acid or sodium bicarbonate. Nickel was added as Ni(MSA)₂ to provide a 12% by weight alloy of Ni with Zn in the final coating. Steel strips were plated using a rotating cathode spinning at 1000RPM. The cathodic density was 150 A/dm². A soluble zinc anode was used. The solution was operated at 60°C. The zinc thickness on the cathode was eight microns. Microscopic analysis showed no high current density dendrites at the edge of the steel. The deposit was an alloy coating of zinc and nickel and can be used in the automotive, wire or tubular steel industries.
Example 3. Tubular Steel - A Zn(MSA)₂ solution was prepared by dissolving zinc carbonate in methane sulfonic acid so the final zinc (as zinc metal) was 35 g/L. The pH was adjusted to pH 3.5 - 4.0 with methane sulfonic acid or sodium bicarbonate. Steel mandrels (e.g. drill rods 3/8" diameter) were used as the cathode to simualte the plating of steel tubes. The solution was mechanically agitated and operated 45°C, pH 3.0. The cathodic current density was 25 A/dm². The deposit was smooth and has application in the galvanized tubular steel industry.
Example 4. Wire Plating - A Zn(MSA)₂ solution was prepared by dissolving zinc carbonate in methane sulfonic acid so the final zinc (as zinc metal) was 50 g/L. The pH was adjusted to pH 3.5 - 4.0 with methane sulfonic acid or sodium bicarbonate. Wire of varying diameters were used as the cathode. The thickness of the zinc varied from twenty-five to fifty microns. The solution was mechanically agitated and operated at 50°C, pH 3.5. The cathodic current density was 25 A/dm². The deposit was bright and smooth.
Example 5. Bright Plating - A Zn(MSA)₂ solution was prepared by dissolving zinc carbonate in methane sulfonic acid so the final zinc (as zinc metal) was 35 g/L. The pH was adjusted to pH 3.5 - 4.0 with methane sulfonic acid or sodium bicarbonate. Steel mandrels (e.g. drill rods 3/8" diameter) were used as the cathode to simulate the plating of steel tubes. The solution was mechanically agitated and operated at 45°C, pH 3.0. The solution contained 25 mg/L benzylidene acetone and 10 mg/L nicotinic acid. The cathodic current density was 25 A/dm². The deposit was bright and smooth.
Although the examples describe the electrogalvanizing process as one that is conducted on a steel substrate, any conductive metal substrate may be employed whether a pure metal or a metal alloy and include other iron-alloy substrates or metals or alloys based on Groups IB, IIB, IIIA, IVA, IVB, VA, VB, VIB or VIIB, of the Periodic Table of the Elements the alloys comprising combinations of two or more of these metals and especially the two or three or four component combinations of metals. The total alloying metal is present in the substrate in an amount anywhere from about 0.1 to about 30 percent by weight and especially from about 2 to about 20 percent by weight.
Because some compositions of the invention include certain compounds that may combine and/or react after mixing with other compounds in the composition of the invention, or the coating bath incorporating the composition or compounds, and the subsequent analysis or identification of the compounds in the composition is either difficult or impossible, another aspect of the invention includes the product of the process comprising combining the various compounds of the inventive composition with one another and/or the coating bath for providing HCD electrogalvinization according to the invention.
It will be apparent to those skilled in the art that various modifications and variations can be made to the composition and process of the invention without departing from the spirit or scope of the invention. It is intended that these modifications and variations of this invention are to be included as part of the invention, provided they come within the scope of the appended claims and their equivalents.

Claims

A process for reducing high current density edge build up dendrite formation and edge burn and controlling high current density roughness, grain size and orientation of a zinc coating obtained from an aqueous zinc acidic electrogalvanic coating bath, said process comprising passing a high density current from an anode in said bath to a metal cathode in said bath for a period of time sufficient to deposit a zinc coating on said cathode, wherein said bath contains greater than about 5 g/l of a water soluble zinc organosulfonate where the pH of said bath is greater than about 2.5.
The process of claim 1 where the pH of said bath is from about 1.5 to about 2.5, and includes a random or block polyoxyalkylene glycol homopolymer or copolymer based on 2 to about 4 carbon atom alkylene oxides.
The process according to claim 1 or claim 2 wherein the current density is from about 250 to about 4,000 ASF.
The process of claim 2 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer is based on 3 to about 4 carbon atom alkylene oxides and has an average molecular weight of from about 300 to about 1,100.
The process of claim 4 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer is based on propylene oxide.
The process of claim 4 wherein said polyoxyalkylene glycol comprises a propylene oxide homopolymer having an average molecular weight of about 425.
The process of claim 2 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer has an average molecular weight of from about 2,000 to about 9,500.
The process of claim 7 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer is based on ethylene oxide.
The process of claim 8 wherein said polyoxyalkylene glycol comprises an ethylene oxide homopolymer having an average molecular weight of about 8,000.
The process of claim 2 wherein said polyoxyalkylene glycol homopolymer or copolymer has an average molecular weight of from about 570 to about 630.
The process of claim 10 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer is based on ethylene oxide.
The process of claim 11 wherein said polyoxyalkylene glycol homopolymer or copolymer comprises an ethylene oxide polymer having an average molecular weight of about 600.
The process of claim 1 or claim 2 wherein said bath also contains a water-soluble boron oxide compound.
The process of claim 1 or claim 2 wherein said bath also contains a lignin compound.
The process of claim 1 or claim 2 wherein said bath contains a sulfonated condensation product of naphthalene and formaldehyde as an antidendritic agent.
The process of any one of claims 1, 2, 4, 7 or 10 wherein said zinc organosulfonate comprises a zinc methane sulfonate.
The process of claim 1 where said bath contains a zinc salt other than said zinc organo sulfonate.
The process of claim 1 where said zinc coating comprises a zinc alloy and said bath contains at least one salt of nickel, iron, cobalt, manganese, chromium, cadmium and vanadium to produce said zinc alloy coating.
A composition for reducing high current density edge buildup dendrite formation and edge burn and controlling high current density roughness, grain size and orientation of a zinc coating obtained in an aqueous zinc acidic electrogalvanic coating bath, wherein said bath comprises greater than about 5 g/l of a water soluble zinc organosulfonate where the pH of said bath is greater than about 2.5.
The composition of claim 19 where the pH of said bath is from about 1.5 to about 2.5, and includes a random or block polyoxyalkylene glycol homopolymer or copolymer based on 2 to about 4 carbon atom alkylene oxides.
The composition of claim 20 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer is based on 3 to about 4 carbon atom alkylene oxides and has an average molecular weight of from about 300 to about 1,100.
The composition of claim 21 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer is based on propylene oxide.
The composition of claim 21 wherein said polyoxyalkylene glycol comprises a propylene oxide homopolymer having an average molecular weight of about 425.
The composition of claim 20 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer has an average molecular weight of from about 2,000 to about 9,500.
The composition of claim 24 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer is based on ethylene oxide.
The composition of claim 25 wherein said polyoxyalkylene glycol comprises an ethylene oxide homopolymer having an average molecular weight of about 8,000.
The composition of claim 20 wherein said polyoxyalkylene glycol homopolymer or copolymer has an average molecular weight of from about 570 to about 630.
The composition of claim 27 wherein said random or block polyoxyalkylene glycol homopolymer or copolymer is based on ethylene oxide.
The composition of claim 28 wherein said polyoxyalkylene glycol homopolymer or copolymer comprises an ethylene oxide polymer having an average molecular weight of about 600.
The composition of claim 20 wherein said composition also contains a water-soluble boron oxide compound.
The composition of claim 20 wherein said composition also contains a lignin compound.
The composition of claim 20 wherein said bath contains a sulfonated condensation product of naphthalene and formaldehyde as an antidendritic agent.
The composition of any one of claims 19, 20, 21, 24 or 27 wherein said zinc organosulfonate comprises a zinc methane sulfonate.
The composition of claim 19 of said baths contains a zinc salt other than said zinc organo sulfonate.
The composition of claim 19 wherein said zinc coating comprises a zinc alloy, and said bath contains at least one salt of nickel, iron, cobalt, manganese, chromium, cadmium, and vanadium to produce said zinc alloy coating.