US3216835A - Synergistic chelate combinations in dilute immersion zincate solutions for treatment of aluminum and aluminum alloys - Google Patents

Description

United States Patent ()flice 3,216,835 Patented Nov. 9, 1965 SYNERGISTIC CHELATE COMBINATIONS IN Dl- LUTE lMMERSlON ZINCATE SOLUTIONS FOR TREATMENT OF ALUMINUM AND ALUMINUM ALLOYS Edward ll. Saubestre, Hamden, Conn., asslgnor to Enthone, incorporated, New Haven, Conn., a corporation of Connecticut No Drawing. Filed Oct. 6, 1960, Ser. No. 60,795 19 Claims. (Cl. 106-1) The present invention relates to improved immersion zincate solutions for the treatment of aluminum and aluminum alloys.

Aluminum and its alloys are difllcuit to plate because of the rapidity with which they form an oxide coating when exposed to air. As a result. special treatments must be employed when plating on aluminum. These treatments include mechanical treatments: chemical etches. especially acid etchcs containing iron. nickel, and manganese salts; alkaline displacement solutions, especially those depositing zinc. brass, and copper: anodizing. especially in phosphoric, sulfuric or chromic acids; electroplating with zinc at low current densities for a few seconds. Of these treatments, the alkaline displacement solutions are generally the most successful commercially. in a summary by W. O. Zelley (J. Electrochem. Soc. 100, 328-332 (1953)), it is pointed out that such solutions are the most practical and economical of the various processes mentioned above. in general, alkaline displacement solutions require less time and equipment. are less critical to control, and are applicable to a wider range of alloys and shapes of parts.

While many metals can be deposited on aluminum by displacement, zinc is the most common. in this case. the process is known as the zincate process.

CONCENTRATED ZiNCATE FORMULAS The conventional zineate solution is usually quite eoncentrated (on the order of g./l. of total salts) and usually contains sodium hydroxide and zinc oxide. The NaOH concentration may advantageously be as high as 800 g./l. since higher NaOli concentrations generally yield thinner and more uniform zine films which is advantageous for subsequent plating operations. At temperatures of -27 C. the immersion is for A-l minute. This yields a film weight of l-7 mgJdmJ, depending on the alloy. Aluminum alloys H00 (28), 3000 (3S). 2024 (248), 5052 (528) and 606i (618), as designated by Reynolds Aluminum Data llooit. published by Reynolds Aluminum Company, Richmond l8. Virginia, were subjected to this experiment. Best results (most uniform film) are obtained by removing the zinc coating in a brief dip in nitric acid, then re-applyiag the zinc coating a sec 0nd time. This is the so-called "double zlneate" process. and is widely used commercially.

Since the formulas above result in an exceptionally high alkalinity, some workers have preferred to use milder aikalis, while retaining the use of very concentrated solutions to obtain thin, uniform zinc coatings. Examples of such milder alkalls are carbonate and tartrate.

AC'iiVATED ZlNCATE FORMULAS During the years, a number of improvements have been made in the conventional zineate formulation. most of them aimed at accelerating the rate of film formation. and the degree of adhesion and uniformity of the zinc coating produced. Two approaches have been used: the addition of an activating metal ion. or the addition of complexing agents for aluminum to hasten the rate of removal of the oxide film on aluminum.

till

mums ZINCATE FORMULAS All of the formulas referred to above involve concentrated solutions (on the order of 500 g./l. of total salts). in addition, the literature contains some reference to more dilute baths. For purposes of convenience, concentration ranges may be classed as follows:

(a) Conventional: Total salts of 3 lb./gal. (350 g./l.)

or more (b) Moderately dilute: Total salts of between 1 and 3 lb./gal. (between 180-360 g./l.)

(c) Dilute: Total salts of 1% lb./gal. or less (180 g./l.

or less) An example of a moderately dilute bath is:

G./l. NaOH ZnSO -7H O 95 NuCN 40 Ntl SO.

it will be noted that the moderately dilute formula given above calls for a higher percentage of complexing agent (cyanide) than that present in conventional or activated conventional formulations. Moderately dilute formttlatioas may also be of the metal activated type:

The above was particularly recommended for plating on copper-silicon aluminum alloys. Among the dilute formulations falling under the previous definition, very few are simple zincatlag baths.

SPECIAL PROCEDURES Some special procedures have been published which involve zineating as an essential step in the preparation of aluminum for subsequent finishing. For example, in one method. the work is first dipped in an alkaline iron solution until it gets dark; the coating is then stripped and the work immersed in a zlncate solution.

in accordance with the present invention, 1 have discovered new and greatly improved formulations for dilute zincatiag baths. By the previous definition. this refers to baths whose total salt content is preferably g./l. or less. Dilute zlncate baths have the following advantages over conventional zincating formulations:

(a) The lower concentration results in much lower viseosities. This minimizes trapping of solution in porous die castings and the like. Such trapped solution is a source of blistering of subsequent cicctrodeposlts.

(b) The resulting lower viscosity leads to lessening drag-out losses from operating tanks. This can be a sizeable economic factor where drag-out is high. as when the shape of the parts being plated is such as to trap substantial quantities of solution.

(c) The more dilute the bath. the lower the initial cost of bath make-up tends to be.

(d) The more dilute zincate baths tend to operate faster than conventional baths. This is an advantage in the operation of fully automated plating installations. it

is also advantageous when plating alloys which tend to form surface oxides rapidly. in the latter case it is necessary to form the zinc coating as rapidly as possible.

Despite the above advantages, dilute zincate formulations have found relatively little commercial application compared to conventional, concentrated formulations. The reason is that the dilute zincate baths have been hard to control with resulting tendency to formation of blisters after subsequent clcctrodcposits have been applied. In addition, any given dilute zincate bath has tended to work for only a limited number of alloys of aluminum. Thus, if a variety of alloys had to be processed, no one given formulation could be reliably utilized.

The purpose of this disclosure is to reveal new formulations for dilute zincate baths which overcome the above disadvantages of dilute baths, while retaining all of their advantages.

The principal shortcoming of previous dilute zincate baths is that the displacement deposit of zinc tends to form so rapidly as to be of unsound structure. Thus, such baths can be used only on some aluminum alloys which are suitable for such baths. Aluminum alloys containing magnesium and silicon, for example, cannot be treated reliably in such formulations. Previous atlcmpts to remedy these defects have centered on the use of metallic activators, such as iron or copper or even lead, or on the use of complcxlng agents such as cyanide. Another approach has been to add small amounts of inorganic oxidizing agents, such as nitrite and/or nitrate.

None of these attempts to overcome the objections to dilute zincate formulations have been fully successful. in particular, the above formulations often cause blistering after subsequent electroplating. Specifically, dilute zincate baths containing cyanide have not been nearly as successful as conventional zincating baths.

1 have found that all of the above objections may be overcome by using dilute zincate formulations containing no complexing agents for zinc or aluminum other than hydroxide and to which chclating agents have been added which have the following properties, either singly, or in synergistic combination:

(a) Capable of chclating both aluminium and zinc.

(b) Capable of chclating strongly enough to prevent an unsound zinc deposit from forming, yet not so strongly as to slow down the deposition rate appreciably.

(c) Capable of mplrlly chclating aluminum at the interface between aluminum and the zincatlng solution.

Contrary to what might be expected by one skilled in the art, i have found that the presence of good complexing agents for aluminum and zinc, such as fluoride and cyanide, is actually deleterious. it is conceivable that this is related to the fact that the min at which such complexes form, and dissociate, is relatively slow.

Also, contrary to what might be expected by one skilled in the art, l have found that it is not necessary to incorporate other metal ions as activators (such as iron, copper, tin or lead). Thus, my invention is characterized by the use of a dilute (preferably 180 g./i. or less of total salts) zincate formulation containing no complexing agents for zinc or aluminum, other than hydroxide, no metal ions other than zinc, and which contains chclating agents for both zinc and aluminum, either singly or in combination, said chelating agents having the property of controlling the activity of zinc and aluminum ions in the zincatlng bath so as to permit deposition of a metallurgieaily sound zinc deposit, yet permitting a sufliclently rapid deposition rate so as to avoid unsound deposits due to oxide film formation on the surface of the aluminum.

A further property of the formulations forming my invention is the ability to plate a sound zine deposit on virtually all commercial aluminum alloys by suitable adiustment of (a) bath concentration, preferably in the range of 60-lti0 g./i. total salt content, (b) temperature, preferably in the range of 45'-200' F., and (c) time of immersion, preferably in the range of 5 seconds to 2 minutes. The formulations which constitute my invention may be used in combination with my conventional treatment cycle for preparing the aluminum surface. By such conventional treatments, i understand use of etching or non-etching (slllcated or otherwise) alkaline cleaners for aluminum; pickles, or etehants, such as those containing nitric acid; nitric-sulfuric acids: nltrlc-sulfurlc-hydro iluorlc acids (this being an illustrative, and not a re- Oil Oil

4 strictive listing); bright dips for aluminum, either acid or alkaline; use of either a single, double, or triple dip in the zincating bath, with intermediate removal of the immersion coating in any of the pickles or bright dips mentioned above.

SCOPE OF DISCLOSURE In the tables which follow, the data are taken from (a) "Tables of Stability Constants (2 volumes), 1. Bjerrum, G. Schwartzenbach, L. D. Sillen, IUPAC (Chem. Soc., London, 1957); (b) The Sequestration of Metals," R. L. Smith (Macmillan, 1959); (e) "The Chemistry of the Coordination Compounds," J. C. Bailar, Jr., Ed., A.C.S. (Reinhold, i956) and (d) "Chemistry of the Metal Chclate Compounds," A. E. Martell, and M. Calvin (Prentice-Hall, 1952).

The numbers given below (unless otherwise stated) are for the common logarithm of the stability constant, k defined as:

where 1 denotes activity, M is the metal ion involved, and X is the chelate ion involved. in a few cases, other k values are indicated:

The cheiates which I discovered to be effective (as additives to the basic dilute zlncate formulations) may be grouped as follows:

These will now be taken up in detail. The tables which are included are illustrative and not restrictive. The tables are largely, though not exclusively, limited to compounds for which stability constant data are available in the above-mentioned references.

1. AMINES A. Mon0mnlncs.-l have discovered as one class of chclating agents, having a log k, zinc stability constant of 4.5 to l8, the use of mono-amines of the general formula Its-N-R where hydrogen constitutes 0-2 of the R members, and the balance of the R members contain 1-4 carbons in each R member and at least one of the R members con- 7 8 C. Poiyamincs.1 also discovered as having similar Table IlACnti.nued chelating action as Classes IA and 1B, the use of polyamincs of the general formula Name Formula 10 :1 Al

5 Clluconate COO- 1.7

Rt Rs CHOU) l"/ IL CH --NR (011) N 0 1: l J. Us H Baoehsrata CO0- CHOU) where hydrogen constitutes 0-4 of the R members, the 000- balance of the R members each containing 1-4 carbons, H at least one of the R members containing a hydroxyor (m l no earboxylic grouping; further, wherein R is either hydrogen or an organic group as in the case of R members; II further, wherein x is 1-4. Examples are given in the following Table 16 no Table Name Formula 1 k M 0Ae\ AeO Dlathylenatrlmulne pentsaretate N-(Clls)sN-(Cl1|)sN 15.1

OAc e0 AeO Trlethyisneletramine N1h-l(Ciislr-Nllls-(Cllsh-Nlis l2.0 i'entnethylenellcsemlne Nll|-l(Cl1slr-Nills-(Cihlg-Nlls 10.2

ll. CARilOXYLiC ACiD OR CORRESPONDING SALT A. a-ffydroxy cnrboxylie.l have found that these compounds having a log k, zinc stability constant of about 1.5 to 4, particularly e-hydroxy cnrboxylic acids (or their salts) of the general formula lit 110-(5-0 0 011 where hydrogen constitutes 0-2 of the R members, and the balance of the R members contain 1-4 carbons in combination with cheiating agents of Classes 1A, 1B, 1C and 111A function to activate the zlncating process. Such carbon-containing R members may be hydrocarbons, such as alkyl groups, but preferably contain oxygen, such as earboxylic, aldehydic, or hydroxyl groups. Examples are given in the following Table A B. Dlcarboxylle.-Thesc compounds having a similar log k; zinc stability constant as Class "A, namely, dicarboxylic acids (or their salts), of the general formula where R contains 0-2 carbons likewise function effectively in the combination. Examples are given in the following C. aand p-Keto CarboxyIlc.-The use of a-lteto carboxyiic acids (or their salts) of the general formula wherein R contains 1-3 carbons in a chain, with or without side chains in the combination produces novel and unexpected chelating results. While not necessary it is preferable that carbons in the 0-position to the CO group in the formula above contain hydroxy or carboxylic groupings. I further claim fi-keto carboxylic acids (or their salts) of the general formula RCOCH -COOH wherein R is defined as above. Examples are given in the following table iii. OTHER OXY-COMPOUNDS A. a-llyrlmx) aivoltol.r.'lhcse compounds having a similar log k, zinc stability constant as Classes "A and HB. namely, the use of a-hydroxy alcohols of the general formula wherein R is preferably hydrogen, but may also represent a chain containing l-3 carbons; wherein R may be hydrogen, or may represent a chain of 1-4 carbons, said carbons preferably. but not necessarily, containing hydroxyi groupings are likewise effective in the combination. Examples are given in the following table Table IIIA Name Formula Propylene glycol (llyenrol Mnuuitoi: florlritol B. {J-I)Iketm's.-These compounds. namely, [Milketones of the general formula a,-co-cu,-co-a,

where R, and R, have 1-3 carbons, with or without side chains have a similar cheinting action in the combination as Classes "A, ill! and "C. An example is given in the following table iii) Table 1118 Name Formula log k Al Acetylaeetone CIh-CO-Cih-CO-Clh 6.1 &0

Urn-9.0. it-HMS; kin-22.3.

C. a-Hydroxy aryi compounds-These compounds.

namely, the use of a-hydroxy aryi compounds of the general formula where R is a side chain containing oxygen, such as OH. -CHO, COOH have the same cheiating effect in the combination as Classes IA, iB, IC and 1118. In addition, other substituents may be present on other positions in the aryl ring, such as SO;,H, -Ci, NO,, etc. Such substituents do not contribute to any essential cheiating feature, but may contribute other desirable features, such as greater solubility in water. 1 further claim substituted phenols of the general formula wherein R is as defined above, and R is an easily hydrolyzed grouping, such as a cnrboxyl grouping. Examples are given in the following table Table INC Name Formula lo; in Al (lataehoi -0il -Oll Ballot-late -0lt H Buliosalleylate Oil (land -0|B- COO- 0Cl0(llis Amlylsalleyiate lt 0ll 8ni|aylaldalty lo.......... LA -Cii0 Oli Bulfasalloyinldnltyrla tLO -0 B -Oll0 ins=i'l.4.

D. Ailzarln derlvallvcs.-Sucit compounds, namely. the use of aiizarin and its derivatives have similar cheiating action in the combination as Class lIlC. Examples are given in the following table Table IIID Name Formula Al Allsarln -80s" Ailsarln Sultanate Oll An examination of the tables will show that relatively little data are available for aluminum, but somewhat more for zine. it will be noted that the stability constants for zinc cheiates fall into two broad categories:

(a) Log k, is 45-18 in Tables I and iiiB-D (b) Log k, is 1.5-4 in Table ii and liiA Qualitative data in the literature indicate a similar classlficatlon for aluminum. There are no quantitative data for Table liiA, but qualitative data from the literature suggest the classification given above.

On the basis of the classification given above, it might at first seem that the chelating agents of Tables i and lilll-i) would be superior to those of Tables ii and HM. However, i have discovered a synergistic relationship between the two above classifications of chelating agents which would not be evident to one skilled in the art of either chelating agents in deposition solutions or of plating on aluminum.

Accordingly, in addition to the agents referred to prevlousiy, and subject to limitations imposed elsewhere, the use of any of the following classes of chelating agents: 1A, 1B, lC, lliB, lllC, iliD (as heretofore defined), singly or in combination, in conjunction with the use of any of the following classes of chelating agents: "A; 11B; 11C; M (as heretofore defined), singly or in combination. i have discovered the surprising and unexpected resuit that zincating formulations containing the two above classes of chelating agents coniunctiveiy are more efficacious than those employing chelating agents drawn from only one of the above two classifications.

i believe that the synergistic action of a combination of chelating ingredients producing the desired zincatlng action is unpredictable and would hypothesize this action, without restriction as to the scope of the specification and claims as follows:

"Zlncating" of aluminum surfaces is a galvanic displacement reaction, wherein zinc ions are reduced to zinc metal, and aluminum is oxidized to aluminum ions. If the free ions were employed (as in acid solutions, for example), the resulting galvanic deposits would be dendritic (i.e., trced." of coarse, and non-coherent erystalline structure). Thus it is necessary to complex or chetill late the ions involved. The complexing agent most com monly employed is hydroxide, i.e., highly alkaline solutions are employed. The stability constants for hydroxide complexes are: log k =i4.9 (Zn); 33.8 (Al). Thus, in theory, the so-cailed zincating" of aluminum surfaces would take place by the following oxidation-reduction reaction:

in practice, however, the rate at which such hydroxide complexes form is slow. Thus, if the "zineating" reaction is too fast, aluminum hydroxide will precipitate on the surface and be incorporated with the zinc deposit, leading to unsatisfactory results. A simple "zincating" formulation, i.e., Na0H+a zinc salt, is satisfactory for conventional concentrated zineating baths. However, the dilute baths operate more rapidly, so rapidly indeed, that hydroxide precipitates form on the surface. Thus, in dilute zincaling formulations, it is necessary to employ cheiating agents which will rapidly cheiate aluminum (to prevent precipitate formation), and which will rapidly liberate zinc ions (to permit the oxidation-reduction reaction to proceed in the first place). it would appear (based on practical results obtained, but not on kinetic or other rate-indicative data) that cheiates of groups II and "IA permit such rapid chelating action. However, their use alone is not the optimum solution of the problem, since the stability constants are low, thus leading to the eventual precipitation of hydroxides somewhere in the general vicinity of the aluminum surface. Convection and diffusion in the solution will then lead to re-deposlt such hydroxides on the surface being plated. By also adding cheiates of types 1 and lllB-D, this problem is overcome since these have much higher stability constants. However, when cheiates of the latter types are used alone, less than optimum results are obtained since their rate of cheiate formation with Al, and liberation of Zn ions is slower (based on practical observations, and not on kinetic data). Thus, there is some risk of hydroxide precipitate formation directly on the surface of the work. While hypothetical, the above model would account for the unexpected synergistic effects noted when both classes of chelating agents are present.

The chelating agents of the present invention are preferably used in fa'muiations containing about 60-180 g./i. of total salts. Of the total salt content, the following are present in the amounts indicated:

(a) An alkali metal hydroxide, preferably sodium hydroxide, about 60 to parts by weight.

(b) A zine salt (such as zinc oxide, zinc sulfate, etc.) about 5.5 to 12 parts by weight based on the zinc content. A preferred economical salt is zinc oxide.

(0) Weight ratio of OH-IZn (as metal): about 2.1- 7.9 (expressed as NaOH/ZnO: 4-15).

(d) A chelating reagent in an amount of from about 5 to 20 parts by weight comprising the combination of (i) from about 5 to percent of at least one water soluble chelating agent having a log k, zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent of at least one water soluble chelating agent having a 'log k; zinc stability constant of about L5 to 4. (3) Optionally, surface active agents may be present to lower surface tension up to about 2 parts by weight.

The examples of formulas corresponding to the above set forth in the following table are purely illustrative and not to be regarded as limiting the scope of the invention as defined in the accompanying claims.

Parts. (wt.)

Acetyl Aretnna. Aretyi sullcylatu (Na).... litulun trlacetate (Na). Catecltol Dlethylene trlantlue peutanretate (Na) Etltyleuedlnmlne tetraaretate n) Ethylene gl col. (llucouatet a) (ilyrerol ltocitulle Halt t orhitol Waiting Agent (anionic).

The preceding examples may be used at temperatures from 45-200 F., with immersion times of sec. to 2 min. The higher the temperature, the shorter the immersion time. The lower the total salt concentration, the shorter the immersion time. The more electrochemically active the aluminum alloy surface, the shorter the immersion time. For most common commercial alloys of aluminum, the optimum conditions are 70-95 F., with immersion times of sec.-l min. As with conventional zincating practice. the zinc coating may be removed by a brief dip in nitric acid, then re-applicd (the "double zincate" process).

in the foregoing specification and the claims to follow, the salts of both the basic and acidic chelating agents. including but not limited to amines and carboxylic acids, are to be regarded as equivalent to the unsubstituted acid and base derivatives.

While i have shown and described a preferred embodiment of my invention, it will be understood that it is not to be limited to all of the details shown. but is capable of modification and variation within the spirit of the invention and within the scope of the claims.

What i claim is:

i. A zincating bath for aluminum metals comprising a dilute aqueous solution of a zinc salt having a total salt concentration of no more than about ltiO grams per liter, an alkaline compound and a chelating reagent capable of chelating both aluminum and 'zinc consisting essentially of the combination of (i) from about 5 to 95 percent by weight of a water soluble chelating agent having a log Ir, zine stability constant of about 4.5 to ill and (2) from about 95 to 5 percent by weight of a water soluble chelating agent having a log k, zine stability constant of about 1.5 to 4.

2. A zincating bath for aluminum metals comprising a dilute aqueous solution of a zinc salt having a total salt concentration of from about 60 to tilt) grams per liter. an alkaline compound and a chelating reagent capable of chelating both aluminum and zinc consisting essentially of the combination of (i) from about 5 to 95 percent by weight of a water soluble chelating agent having a log k, zinc stability constant of about 4.5 to lit and (2) from about 95 to 5 percent by weight of a water soluble chelating agent having a log k, zinc stability constant of about L5 to 4.

3. A zincating bath for preparing aluminum metals for plating comprising a dilute aqueous solution of a zinc salt, an alkaline compound and a chelating reagent comprising, in combination, (i) from about 5 to 95 percent by weight of a compound having a log k, stability constant for zinc of about 4.5 to ill and selected from the class consisting of the monoamines. polyamines and their salts l'i-dikctoncs, a-hydroxyaryl compounds and aiizarin derivatives and their salts, and (2) from about 95 to 5 percent by weight of a compound having a log k, stability constant for zinc of about L5 to 4 and selected from the class consisting of a-hydroxy carboxylic acids and their salts, dicarboxylic acids and their salts, aand fl-keto carboxylic acids and their salts and a-hydroxy alcohols.

4. A zincating bath for aluminum metals as set forth in claim 3, wherein the total salt concentration is approximately 60 to 180 grams per liter.

5. A zincating bath for aluminum metals as set forth in claim 3, wherein the chelating reagent is the combination of an amine and an a-hydroxy carboxylic acid.

6. A zincating bath for aluminum metals as set forth in claim 3, wherein the chelating reagent is the combination of an amine and an m-hydroxy alcohol.

7. A zincating bath for aluminum metals as set forth in claim 3, wherein the chelating reagent is the combination of an a-hydroxyaryl compound and an a-hydroxy carboxylic acid.

8. A zincating bath for aluminum metals as set forth in claim 3, wherein the chelating reagent is the combination of an a-hydroxyaryl compound and an et-hydroxyi alcohol.

9. A zincating composition for preparing aluminum metal surfaces for plating comprising about 60 to parts by weight of an alkali metal hydroxide, about 7 to 15 parts by weight of a zinc salt and about 5 to 20 parts by weight of a chelating reagent capable of chelating both zinc and aluminum consisting essentially of, in combination, (I) from about 5 to percent by weight of a chelating agent having a log k, zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent by weight of a chelating agent having a log k, zinc stability constant of L5 to 4.

10. A zincating composition for preparing aluminum metal surfaces for plating comprising about 60 to 85 parts by weight of an alkali metal hydroxide, about 7 to l5 parts by weight of a zinc salt and about 5 to 20 parts by weight of a chelating reagent capable of chelating both zinc and aluminum consisting essentially of, in combination. (1) from about 5 to 95 percent by weight of a chelating agent having a log k, zinc stability constant of about 4.5 to l8 and (2) from about 95 to 5 percent by weight of a chelating agent having a log k, zinc stability constant of 1.5 to 4, the weight ratio of hydroxyl ions to zinc being about 2.l to 7.9.

II. A zincating composition for preparing aluminum metal surfaces for plating including in combination a zinc salt, an alkali and a mixture of chelating agents consisting essentially of (i) from about 5 to 95 percent by weight of at least one water soluble chelating agent having a log k, zinc stability constant of about 4.5 to 18 and (2) from about 95 to 5 percent by weight of at least one water soluble chelating agent having a log k, zinc stability constant of about 1.5 to 4.

12. A zincating composition for.preparing aluminum metal surfaces for plating including in combination a zinc salt, an alkali and (i) from about 5 to 95 percent by weight of at least one water soluble chelating agent having a log in zinc stability constant of about 4.5 to 18 selected from the class consisting of the monoamines. polyamines and their salts, {s-diketones, a-hydroxyaryi compounds and aiizarin derivatives and their salts, and (2) from about 95 to 5 percent by weight of a chelating agent having a log k, zinc stability constant of about 1.5 to 4 selected from the class consisting of e-hydroxy carboxyiic acids and their salts, dicarboxyiic acids and their salts. aand {s-keto carboxylic acids and their salts and e-hydroxy alcohols.

13. A zincating composition as set forth in claim 12, wherein the chelating reagent is the combination of an amine and an a-hydroxy carboxylic acid.

14. A zincating composition as set forth in claim l2,

wherein the cheiating reagent is the combination of an amine and a dicarboxylic acid.

15. A zincating composition as set forth in claim 12, wherein the cheiating reagent is the combination of an amine and an a-ketc carboxylic acid.

16. A zincating composition as set forth in claim 12, wherein the chelating reagent is the combination of an amine and an a-hydroxy alcohol.

17. A zincating composition as set forth in claim 12, wherein the chelating reagent is the combination of an a-hydroxynryl compound and an a-hydroxy carboxylic acid.

18. A zincating composition as set forth in claim 12, wherein the chelating reagent is the combination of an a-hydroxyaryl compound and an a-hydroxy alcohol.

19. A ziacating composition aa set forth in claim 12, wherein the chelating reagent is the combination of an aiizarin derivative and an a-hydroxy carboxylic acid.

1 6 References Cited by the Examiner UNITED STATES PATENTS 2,650,886 9/1953 Zelley 106-1 2,766,138 10/1956 Talmey 106-1 2,872,346 2/1959 Miller 106-1 OTHER REFERENCES Bersworth Chemical Company, Versenes", Technical Bulletin No. 2, Bersworth Chemical 00., Framingham,

Mass. 1952). Sec. 1, p. 7.

15 MORRIS LIEBMAN, Primary Examiner.

JOSEPH REBOLD, ALEXANDER H. BRODMERKEL,

Examiners.

Claims (1)

1. A ZINCATING BATH FOR ALUMINUM METALS COMPRISING A DILUTE AQUEOUS SOLUTION OF A ZINC SALT HAVING A TOTAL SALT CONCENTRATION OF NO MORE THAN ABOUT 180 GRAMS PER LITER, AN ALKALINE COMPOUND AND A CHELATING REAGENT CAPABLE OF CHELATING BOTH ALUMINUM AND ZINC CONSISTING ESSENTIALLY OF THE COMBINATION OF (1) FROM ABOUT 5 TO 95 PERCENT BY WEIGHT OF A WATER SOLUBLE CHELATING AGENT HAVING A LONG K1 ZINC STABILITY CONSTANT OF ABOUT 4.5 TO 18 AND (2) FROM ABOUT 95 TO 5 PERCENT BY WEIGHT OF A WATER SOLUBLE CHELATING AGENT HAVING A LOG K1 ZINC STABILITY CONSTANT OF ABOUT 1.5 TO 4.