AU477103B2 - Acidic and neutral zinc electrolytes - Google Patents

Description

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The present invention is concerned with acidic and neutral zinc electrolytes for the electrodeposition of glossy zinc coatings.

Acidic zinc electrolytes producing glossy deposits are of great practical importance, because the decontamination of their waste liquor to avoid environmental pollution is freer from problems than is that of the highly toxic alkaline cyanidic electr( lytes. A prerequisite for this purpose is, however, that the acidic electrolytes should contain no complex-formers that prevent or make difficult precipitation of the zinc. In the absence of these complex-formers'simple neutralization suffices to precipi- tate the zinc metal quantitatively as the hydroxide.

Such electrolytes generally require, however, an addition of ammonium salts, in order to achieve the practical requirements of fairly satisfactory gloss formatiou and also strong adhesion of the zinc deposits.

Thus, the acidic zinc electrolytes proposed in German Patent Specifications B 1,263,445, DA& No. 1,269,855, -DA-No. .a-No. 1,521,029 and DS No. 1,904,319 all contain large amounts of ammonium salts.

This content of armmonium salts considerably complicates the purification of the waste liquors, as the ammonium ions, on the one hand, can be removed quantitatively only at great expense and, on the other hand, even anall amounts thereof are sufficient to have a toxic action on fish.

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Accordingly, the problem hitherto has been to provide an

acidic zinc electrolyte which, while avoiding the disadvantages

of the kno.wn electrolytes, enables glossy zinc coatings to be

electrodeposited and the waste liquors to be kept free from toxic

substances, and this problem has now been solved by the electrolyte

of the present invention.

The present invention accordingly provides an acidic or neutral zinc electrolyte containing a carboxylic or sulphonic

acid of a five- or six-membered heterocyclic compound containing

a nitrogen hetero atom, which compound may contain a fused-on

benzene group, or a salt of such an acid. The electrolyte may contain a single compound or two or more compounds selected from such carboxylic and sulphonic acids and salts thereof.

The present invention also provides a process for coating a

surface with zinc, wherein the surface is coated by electrodeposi- tion from an electrolyte of the present invention.

Acids and salts of the kind defined above are especially

acids of the general formula

X]m

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in which N represents an insubstituted pyridine group or a pyridine group substituted by one or more substituc-ats (which when th are tw;o or mrore present may be the same or dif-fecrent) select from alkyl, alkenyl, alkoxy and hydroxyl groups, or ropresen a quinoline, pyridone, isoquinoline, pyrrole, piperidine, pyrrolidine or pyrrolidinone group, R represents a saturated or unsaturated aliphatic hydrocarbon group containing /o 3 carbon atons. X represents an -SO 3H or -COOH group, n represents 0, 1, 2 or 3, and re represents 1 or.2, A/sts of such acids with inorganitc or organic bases. In the general formula there is to be understood for the meaning of the symbol R a saturated or unsaturated aliphatic hydrocarbon group containing to 3 carbon atoms, for example a methylene, ethylene, propylene, vinylene or vinylidene group. As alkyl, alkenyl and alkoxy derivatives of the pyridine group there may be mentioned, for examnple, .mthyl-, ethyl-, methoxy- and vinyl-pyridine. Inor.-anic andl organic bases suitable for salt formation are, for examnple, .alkali'metal hydroxides snd alkaline earth metal hydroxides, for example sodiumr, hydroxide, potass5ium hydroxide and calcium hydroxide, zinc hydroxide and N-containing primary, secondary and also tertiary bases.

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Acids of thc above defined class are, for example, those listed in the following Table:-

Tabl.e A Pyridine-3-acetic acid, pyridine-3-sulphonic acid, pyridine-3- carboxylic acid, pyridie-2,6-dicarboxylic acid, pyridine-3,4- dicarboxylic acid, pyridine-3,5-dicarboxylic acid, pyridine-4- carboxylic acid, pyridine-3-propionic acid, pyridine-2, 3-dicarboxylic acid, 4-pyridone-2, 6-dicarboxylic acid, carboxylic acid, 4-methyl-pvrl,-.inc-3-car, boxylic acid, 4-ethyl- pyrridine-3-carboxylic acid, 2,4-dimethyl-nicotinic acid, 2,6- diiethyl-nicotinic acid, 2,3, acid, 2-vinyl-nicotinic acid, 3-(3-pyridyl)--acrylic acid, 2p- pyridyl-benzoic acid, P-pyridoyl-acetic acid, a-I§6-miethyl- pyridyl-(3) ]-acrylic acid, 6-hydroxy-nicotinic acid., 4-oxy-2, 6- dimethyl-nicotinic acid, 4-oxy-pyridine-3-carboxylic acid, 6- methoxy-pyridine-3-carboxylic acid, quinoline-3-carboxylic acid, quinoline-4-carboxylic acid, piperidine-p-carbcxylic acid, P-[Y-piperidyl]-carboxylic acid, L-pyrrolidincne-2-carboxylic acid and I-pyrrolidine-2-carboxylic acid. These compounds are known or can be made by methods known per ise. The total quantities in w;hich the acids and salts defined above maay be added to the electrolytes arc approxim~ately 0.01 to 10.0 erams per litre, a.-Ld preferably 0.1 to 1.0 gramn per litre. As an'electrolyte there may in Seneral1 be usod an aqueous solution of zinc chloride hzving- the folloiwing; concentration: Zinc chloride 30 to 150 grans per litre.

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Instead of zinc chloride, there may be used other zinc salts, for example zinc sulphate, zinc acetate and zinc fluoroborate.

As conductive salts there are added to the electrolytes the

additives normally used for this purpose, for example sodium chloride, potassium chloride, sodium sulphate and others. The concentrations used for this purpose are approximately 50 to 250

grams per litre.

Furthermore, the electrolytes may contain boric acid or

small amounts of aliphatic and/or aromatic carboxylic acids or

salts thereof, for example acetic acid, benzoic acid or salicylic

acid.

Although not absolutely necessary, there may be added to the electrolytes the usual compounds capable of acting as gloss-

formers, among .whichthere may be mentioned more especially carbon

compounds, for example acetophenone, benzal-acetone, benzoyl-

acetone, 3-acetyl-coumarin, 3-acetyl-pyridine and meta-

hydroxybenzaldehyde, which markedly improve the levelling action

and cause highly glossy coatings to be produced. The acid and salt additives used in accordance with the present invention and the aforesaid carbonyl-compounds appear to have a synergistic effect in the electrolytes, because the improve

ment in action, for example in the deposition in the range of high current densities, far exceeds the expected additive effect.

The carbonyl-compounds are used at concentrations from approximate 0.01 to 2.0 gra:s per litre, and preferably the mixing ratio is

from 1 to 10 parts by weight of the defined acids and salts to 1

part by weight of the carbonyl-compounds.

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As further additives there may be used other usual gloss- formers, for example thio-compounds, compounds of high molecular weight and/or aromatic aldehydes, and also non-foaming wetting

agents, especially anionic alkylol sulphates and ethoxylated amphoteric compounds, the latter in concentrations from 0.1 to

grams per litre, and preferably from 1.0 to 100 grais per

litre. The conditions of operation of the zinc electrolytes of the

present invention are as follows: pH-value: 3.0 to 7.0, preferably 4.5 to Temperature: 15 to 3500, preferably 20 to 3000.

Current density: 0.1 to 20 amperes per dm2 The electrolytes operate especially advantageously at a pH-

value within the range of from 4.5 to 5.5, but it is possible to operate in the region of neutrality without an appreciable decrease

in efficiency. It is to be understood that small amounts of ammonium ions

may be added, if desired, Thus, the electrolytes of the present invention advantageously contain no complex-former and no or only

a small amount of ammonium ions. The electrolytes of the present invention are advantageous not only :in relation to the treatment of the waste liquor. By the addition of the defined acids and salts the quality of the

deposited coatings is considerably improved. with regard to their

gloss effect, and it is possible to work with relatively low con-

centrations of zinc and high current densities.

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The electrolytes of the present invention are suitable for the gloss-galvanization of materials that are customarily galvar including cast iron, in rack, drum or bell devices. The following Examples illustrate the invention. All the electrolyes described in the Examples are aqueous electrolytes. Examnle 1 70.0 grams per litre of zinc chloride, 210.0 of potassium chloride, 20.0 of boric acid, of potassium benzoate,

of wetting agent (a mixture of anionic

alkylol sulphates and ethoxylated amphoteric wetti agents) and 0.4 gram per litre of pyridine-3-acetic acid. pH-value: From this electrolyte deposition was carried out at room temperature and with movement of the electrolyte over a range of current densities from 0.04 to 10 amperes per dm2 and uniformly good glossy, ductile and strongly adherent zinc coatings of any desired thickness were obtained,

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Example 2

70.0 grams per litre of zinc chloride, 210.0 of potassium chloride, 20.0 of boric acid, of wetting agent (a mixture as described-

in Example 1), 0.2 gram per litre of benzoyl-acetone and 0.3 of pyridine-3-sulphonic acid. pH-value: 5.3. Deposition was carried out at room temperature from 0.1 to 8. amperes per dm 2 There were obtained highly glossy zinc coatings. Example 3

34.0 grams per litre of zinc chloride, 149.0 of'potassium chloride, 20.0 of boric acid,

f" of potassium benzoate, of wetting agent (a mixture as described

in Example 1) and 0.2 gram per litre of benzal-acetone. pH-value:

From this electrolyte moderately glossy zinc coatings could be deposited only in the narrow range of 0 to 3 amperes per dm2 As compared with this, the above electrolyte with an addition of 0.1 gram per litre of pyridine-3-carboxylic acid deposited good glossy zinc coatings over a range of current densities from 0 to 8 amperes per dm2 at room temperature.

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Exarole 4

100.0 grams per litre of zinc chloride, 210.0 of potassium chloride, 2 Sof ,4-dihydrozybenzoic acid, potassi

salt, 20.0 of boric acid,

of wetting agent (a mixture as descril

in Example 1), 0.2 gram per litre of benzal-acetone and 0.2 of pyridine-3-carboxylic acid. pH-value:

Highly glossy zinc coatings were deposited from this electrolyte up to current densities of at least 15 anperes per dm with any desired thickness of layer.

Exmnple

90.0 grams per litre of zinc chloride, 350.0 of potassium chloride, 20.0 of boric acid,

S" of potassium salicylate,

of wetting agent (a mixture as describe(

in Example 1) and 0.3 gram per litre of benzoyl-acetone, pH-value: 5.3.

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From this electrolyte glossy zinc coatings could be deposited only up to a maxinum current density of about 5 amperes per dm 2 When 0.4 gram per litre of pyridine-3-carboxylic acid was added to this electrolyte, the current density range for obtaining a highly glossy zinc deposit with especially good distribution in depth was extended to at least 15 amperes per dm 2

Exam'ole 6 70.0 grams per litre of zinc chloride 35 grams per litre

of zinc), 210.0 of potassium chloride, 20.0 of boric acid, of wetting agent (a mixture as described

in Example 1) and gram per litre of 3-acetyl-pyridine. pH-value: This electrolyte deposited glossy zinc coatings in a current density range from 0 to about 5 amperes per dm 2 Above about amperes per dm 2 zinc was deposited only as a poorly adhering grey-black powder. As compared iiith this, there was deposited from this electro- lyte, after the addition of 0.4 gram per litre of pyridine-3- sulphonic acid, well adhering and highly glossy zinc coatings at current densities up to at least 15 amperes per dm 2 with which a speed of deposition of approximately 4 pm per minute was attainable.

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Claims (1)

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   cl Th:s .~ina :r~e the invention are as follows:

   1. An acidic or neutral zinc electrolyte containing, a carboxyli4 or ,-ulphonic acid of a five- or six-membored heterocyclic co)mipound containing a nitrogen hoctero atom, which compound may contain a fused-on benzene group, or a salt of such an acid. 2. An electrolyte as claimed in claim 1, containing a single compound selected from a carboxylic acid of a five- or six- membered heterocyclic compound containing a nitrogen hetero atom, which heterocyclic compound may contain a fused-on benzene group, a suiphionic acid of such a heterocyclic compound and salts of such carboxylic and suiphonic acids. 3. An electrolyte as claimed in claim 1, containing tw.o or more comnpounds selected from carboxylic acids of a fi-,c- or. six- membered hetero cyclic compound containing a nitrcgcn hetero atom, which heterocyclic compound may contain a fused-on benzene group, suJlphonic acids of such a heterocyclic compound and saJtts of such carboxylic and suiphonic acids. 4. An electrolyte as claimed in any one of clains 1 to 3,- containing an acid of the general formula

   N-

   in which

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   N represents an uncubstitutcd pyridine group or a pyridino group substituted by one or more substituents selected from alkyl, alkenyl, alkoxy and hydroxyl groups, or represents a quinoline, isoquinoline, pyridone, pyrrole, piperidine, pyrrolidine or pyrrolidinone group, R represents a saturated or unsaturated aliphatic hydrocarbon group containing ,to 3 carbon atoms, X represents an -SO.H or -CO011 group,

   n represents 0, 1, 2 or 3, and m represents 1 or 2, and/or a salt of such an acid with an inorganic or organic base. An electrolyte as claimed in any one of claims 1 to 4, containing any one of the acids listed in the TableLherein and/or a salt of such an acid. 6. An electrolyte as claimed in any one of claims 1 to wherein the acid(s) and/or the salt(s) is/are present in a total amount of approximately 0.01 to 10.0 grams per litre. 7. An electrolyte as claimed in claim 6, wherein the acid(s) and/or the salt(s) is/are present in a total amount of 0.1 to gram per litre.

   8. An electrolyte as claimed in any one of clairms 1 to 7, which also contains a carbonyl-compound capable of acting as a gloss-former.

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   9. An electrolyte as claimed in claim 8, wherein the carbonyl-

   compound is acetophenone, benzal-acetone, benzoyl-acetone,

   3-acetyl-coumarin, 3-acetyl-pyridine or meta-hydroxybenzal- dehyde.

   An electrolyte as claimed in any one of claims 1 to 9,

   which contains no complex-former and no or only a small

   amount of ammonium ions.

   11. An electrolyte as claimed in any one of claims 1 to containing zinc chloride.

   12. An electrolyte as claimed in any one of claims 1 to 11, having a pH-value within the range of from 3.0 to

   13. An electrolyte as claimed in claim 12, having a pH- value within the range of from 4.5 to

   14. An electrolyte as claimed in claim 1, having a composi-

   tion substantially as described in any one of Examples 1 to 6 herein.

   A process for coating a surface with zinc, wherein the surface is coated by electrodeposition from an electrolyte

   as claimed in any one of claims 1 to 14, the conditions

   of electrodeposition being as herein defined.

   16. A process as claimed in claim 15 substantially as

   herein described with reference to any one of the Examples 1 to 6 herein.

   17. A surface whenever coated by the process claimed in either one of claims 15 or 16.

   DATED this 19th day of December, 1973 SCHERING AKTIENGESELLSCHAFT

   By Its Patent Attorneys,

   ARTHUR S. CAVE CO. 14 4 77,103