GB2171114A - Trivalent chromium electroplating baths and rejuvenation thereof - Google Patents

Abstract

The operational life of trivalent chromium electroplating baths is extended by the presence or addition of a compound of formula <IMAGE> when R1, R2 and R3 are H or C1-C6 alkyl and X is ammonium or compatible metal such as sodium or chromium. Optionally there is also present or added a compound of formula <IMAGE> (or its ammonium or compatible metal salt) in which A is an aromatic radical, R4 and R5 are H or C1-C6 are alkyl or a -CO- linkage to A or to a further group <IMAGE> In a further aspect, possibly but not essentially to be combined with the above, performance is improved by presence of a C4-or C5-dialkyl sulphosuccinate.

Description

SPECIFICATION Improvements in electroplating This invention relates to decorative chromium electro plating of metallic surfaces.

The process predominantly used for such plating involves hexavalent chromium and sulphuric acid. It provides environmental and health hazards and is not an electrically efficient procedure, having poorthrowing power and limited covering power. It is also intolerant of current interruptions and is highly toxic.

It is known therefore to utilise chromium electroplating baths based on trivalent chromium. Conventionally, such baths utilise trivalent chromium ion in a completely complexed form, for example using shortchain aliphatic fatty acids or othe components as complexing agents. However, the plate produced is darker than that produced using hexavalent chromium in the bath, or, if it is not darker in the initial stages of bath use, darker plating is produced after a period of use. Hitherto, there has only been a limited commercial exploitation of such baths despite extensive studies of such systems.

For example Patent No. 1 562 188, describes a chromium plating bath using trivalent chromium ions which are fully complexed with such shortchain aliphatic carboxylic acid groups, and which also contain ammonium ion and bromide. Examples of the carboxylic acid groups are formate and acetate. A major problem which has to be overcome in the development of a process for plating chromium from the trivalent state, is the facile oxidation of Chromium (III) at an insoluble anodeto Chromium (Vl),which is a harmful contaminant. In the presence of bromide, the carboxylic acid anions are oxidised in preference to chromium (III).

More recently a totally different approach has been adopted: surprisingly, it has been discovered that with the use of certain specified sulphur compounds an amount less than full complexing amount of complexant need be used. The solutions used in such a process aretypically still greenish in colour, from the trivalent chromium ion and can be electrolysed to give a good bright electrodeposit as described in the European PatentApplication 58044.

We have now established further operating criteria for such solutions which surprisingly are relevant to both forms oftrivalent chromium usage, that is to say either ofthe fully complexed form utilising shortchain aliphatic carboxylic acids, or partly complexed form using certain specified sulphur compounds. In plating shop practice both systems have atendencyto deteriorate after a period of use. The exact period is not always possible to forecast, but is often about three months use. (It can be considerably less if the bath is used very intensively). In any event, once the bath has deteriorated it gives plate below decorative specification because of its unacceptable appearance, that is to say, its dark colour.The present invention is concerned with the additionto, or presence in, the bath of one or more additives which prevents this deterioration, orwhich overcomes or inhibits deterioration once present, and thus permits trivalent chromium baths of both types to be used for extended or indefinite periods.

In one aspect the invention consists in a method of operating a trivalent chromium electroplating bath in which there is present in, or added to, the bath a water-solublecompoundformula:

in which R1, R2 and R3 are chosen from hydrogen and C1 to C6 alkyl groups, and in which Xis ammonium or a bath-compatible metal.

Acompatible metal can be (a), an alkali metal, preferably sodium or (b) chromium itself or (c) any other metal giving a soluble salt and either affecting nor affected by the electrodeposition conditions.

R1, R2 and R3can all be hydrogen: in such a case, most preferably the compound is sodium vinyl sulphonate. This is a compound already used in nickel plating, but not hitherto known for use chromium plating. In particular it was not hitherto suspected that it could be used in trivalent chromium plating to increase the effective acceptable lifetime of the bath orto reclaim the performance of a bath which had deteriorated to give dark plating.

The amount of such material to be used in the bath, will generally range from 0.1 to 10 gram per litre. If amounts above 1 gram per litre are used it is sensible to watch carefully for harmful side effects depending upon the exact method of plating used. The preferred amount of such material in the bath is about 0.5 grams per litre. In practice, over a period of time after the amount presentslowly decreases. We have found that below 0.1 grams per litre concentration the material exerts little discernable effect, and the recurrence of darkened plating is a positivesignal to improve the bath. Preferably, of course preventive maintenance by additions at predetemined intervals will be utilised.

In afurtherand particularly preferred form of the invention, the compound above is utilised in conjunc tion with a watersoluble compound ofthe formula below or its ammonium orwatersolublebath- compatible metal salt:

in which A is the residue of an aromatic structure, possibly alkylsubstituted, and in particular a benzene ortoluene structure and at least one of R4 and R5 is H or low (C,-C6) alkyl with any remaining R4 or R5 being a cyclic -CO- linkage to the aromatic group Aor a further group

The primary example of such a sulphonide-type compound is saccharin whose formula is

used as such or as a bath-compatible salt.

However, we envisage also that as such a compound could be used: paratoluene sulphonamide (p) CH3-C6H4 - SO2NH2, bis (benzene sulphonide) C6H5-SO2-NH-SO2-C6H5 bis (p-toluene)sulphonimide (p)CH3-C6H4-SO2-NH-SO2-CH6H4-CH3 etc.

Such sulphonides are preferably also used in the form oftheirsodium salts for water solubility.

The amount of such material to be used can be from 0.5grams per litre to saturation.

It will be appreciated that, since the preferred form of the present invention involves the initial presence or subsequent addition of both the vinyl sulphonate and sulphonidetypes of compounds as discussed above, a further aspect of the present invention is constituted by an additive having dissolved therein compatible amounts of each of the said types of compounds forjoint addition to a trivalent chromium plating bath either prior to plating at predetermined intervals, or on requirement.

As another valuable preferred feature ofthe present invention, the bath contains ayetfurther component. Itwill have been observedthatthe present invention is concerned with the maintenance of plating baths as used under long-term industrial conditions ratherthan laboratory conditions. This additional preferred feature is similarly concerned with such longtermindustrial usage of baths, and thus has a relationshiptothefeaturesdiscussed above.

It is commonplace in electroplating bath to incorporate a surfactant compound to ensure thatthe surface ofthe bath is such that articles immersed in them for plating becorhyyoperlywetted. A typical surfactant which is widely used in practice, especially in the field of nickel plating, is known undertheTrade Mark "Tergitol 08" and is a 2-ethylhexysulphate material.

Such a material has been proposed for use hitherto in trivalent chromium baths. Under laboratory condi- tions it is quite satisfactory, but in long term use we have found it is not fully satisfatory.

We have now established a particularly preferred surfactantforatrivalent chromium bath.

In a further aspect, the present invention accordingly consists in a trivalent chromium bath ofthe nature described above which further contains as asurfac tantmaterial added on initial makeup,for preventive maintenance, or on requirement, a dialkyl sulphosuccinate in which the alkyl esterifying group contains 4 or5carbon atoms.

Three specific surfactants, namely the diamylsulphosuccinate, the dibutyl sulphosuccinate, the amyll butyl sulphosuccinate are envisaged. Normally, these would be used as the sodium salt, butthere should be no objection to using either of the potassium orammonium salts.

In a further aspect ofthe invention, the bath can utilisethe novel surfactants perse i.e. whateverthe other components. Thus, the invention further consists in a method of operating a trivalent chromium electroplating bath in which the bath contains as a surfactant material, whether added on initial makeup orforpreventive maintenance, oron requirement, a dialkyl sulphosuccinate in which the alkyl esterifying group contains 4 to 5 carbon atoms.

Such surfactants are again used because of the longterm deterioration ofthe electroplating bath.

After lengthy periods of use ofthetrivalentchromium, not only does the colour ofthe plate become darker but there are patches of white deposit known to the trade as "whitewashing". This is a well known problem, and is connected with the absence of surfactants. We have found that if the conventional "TERGITOL 08" type of surfactant is used for a trivalent chromium bath such "whitewashing" can appear after as little as one month of average industrial use. By alternative addition atthattime of the surfactant as defined above, or more preferably by utilising such surfactant as part ofthe initial bath makeup instead ofthe "Tergitol" type of surfactant, the whitewashing is no longerapparentandthe bath can remain free of such defects for up to twelve months.

It will be apparentthatthe invention as claimed extends not only to baths containing the additive materials discussed above but also to an additive containing in addition to the previously defined sulphonate and sulphonamide compounds a surfactant material ofthe sulphosuccinatetype defined above.

Since the invention is concerned with plating not on the laboratory scale but upon the morelong-term industrial scale,the additions ofthe specified materials can be made either at the outset (in which case their particular qualities may not be apparentfor a lengthy period) or as part of a regular preventive procedure (in which case their particular effect is only evidentfrom absence of defects) oras a curative measure ifthe bath shouid start to deteriorate.

Thefollowing exampledemonstratesthe use of these varous additives.

EXAMPLE A plating bath was made up from "Chrometan"to give0.1 molartrivalentchromium ionsto which was added boric acid (60 grams per litre), potassium sulphate (100 pergrams per litre) malicacid (10 grams per litre) and thiourea (30 milligrams). To this was added, as a surfactant, the well known "TERGITOL 08" type of material, i.e. 2-ethylhexylsulphate (400 milligrams per lite). The pH ofthe solution was 3.7.

The solution was heated to 50"C until there was no change in the visible absorption spectra. This ensured that while there was insufficientcomplexing agent presentfor a complete reaction with all of the chromium, the amount of reaction available had at least reached equilibrium.

Thereafterthe pH was readjusted to 3.7.

An electroplating plant utilising such a solution was operated to produce a decorative chromium coating on nickel plated substrates.

After a period of one months extensive use it was observedthatthe quality of plating was beginning to deteriorate, in that it was of a darkercolourand that samples of plating occasionally had white stains thereupon.

To the bath was added 3 grams per litre of saccharin and 0.5 grams per litre of sodium vinylsul phonate. It was found that th is cured the problems with colour, and to some extent it cured the white staining problem.

Eventually, however, the problems with the white staining recurred and persisted to the extent that the bath was discarded although the plating produced was still acceptably light in colour even after six months use.

By way of comparison, the above bath was completely duplicated except that the TERGITOL 08 type surfactant was replaced at the outset by the diamylsulphosuccinate surfactant.

After about one month the same additions of saccharin and sodium vinylsulphonate were made.

Occasional further additions of all three types of compound were made by way of preventive maintenance. The bath as described did not give any white staining on the plate, and operated for over one year without any plating defects.

Various modifications may be made within the scope of the invention. For example, instead of or as partial replacementforthe sulphonamide compound such as saccharin there may be used the polynuclear sulphonic acids with at least one sulphonic acid group e.g. thenapthalene sulphonic acids, sulphonated directly on a ring structure, and optionally furthersubstituted with one or more C1 to C6 alkyl groups: orthe low molecularweight polymers of such polynuclear sulphonic acids with formaldehyde e.g. methylene bis (naphthalene mono-di- ortrisulphonic acids); orwatersoluble bath compatible salts of any ofthe above especially alkali metal salts such as sodium.

Claims (21)

1. Amethodofoperating atrivalentchromium electroplating bath in which there is present in, or added to, the bath a water-soluble compound of formula:

in which R1, R2 and R3 are chosen from hydrogen and C1 to C6 alkyl groups, and in which Xis ammonium or a bath-compatible metal.

2. A method as claimed in claim 1 in which the bath-compatible metal isan alkali metal.

3. A method as claimed in claim 1 in which the bath-compatible metal is sodium.

4. A method as claimed in claim 1 in which the bath-compatible metal is chromium.

5. A method as claimed in claim 1 in which R" R2 and R3 are all hydrogen.

6. A method as claimed in claim 5 in which the said water soluble compound is sodium vinyl sulphonate.

7. A method as claimed in claim 1 in which the amount of the said water soluble compound used is from 0.1 to 10 grams per litre.

8. A method as claimed in claim 1 in which the saidwatersolublecompound isutilisedinconjunc- tionwithawatersolublecompoundoftheformula

or its ammonium or other water-soluble bathcompatible metal salts in which A is the residue of an aromatic structure, and at least one of R4 and R5 is H or low C1 to C6 alkyl with any remaining R4or R5 being a cyclic -CO- linkage to the aromatic residue A orto afurthergroup

9. A method as claimed in claim 8 in which the aromatic structure A is alkyl substituted.

10. A method as claimed in claim 8 in which the aromatic structure is a benzene ortoluene structure.

11. A method as claimed in claim 10 in which the said compound is saccharin used as such or as a bath compatible salt.

12. A method as claimed in claim 10 in which the said compound is paratoluene sulphonamide, bis (benzene sulphonimide) or bis (p-toluene sulphonimide, ora sodium saltthereof.

13. A method as claimed in claim 8 in which from 0.5 grams per litre to saturation of said additional compound is used.

14. A method as claimed in claim 1 inwhichthe bath further contains as a surfactant material added on initial make up, or for preventive maintenance, or on requirement, a dialkylsulphosuccinate is which the alkyl esterifying group contains 4 or 5 carbon atoms.

15. A method as claimed in claim 14 in which the dialkylsulphosuccinate is diamysulphosuccinate, dibutylsulphosuccinate, or amyl/butylsu I phosuccin- ate, used as such or as the potassium, sodium or ammonium salt.

16. A method as claimed in claim 8 in which the bath further contains as a surfactant material added on initial make up, or for preventive maintenance, or on requirement, a dialkylsuiphosuccinate is which the alkyl esterifying group contains 4 or 5 carbon atoms.

17. A method as claimed in claim 16 in which the dialkylsulphosuccinate is diamysulphosuccinate, dibutylsulphosuccinate, oramyl/butylsulphosuccinate, used as such or as the potassium, sodium or ammonium salt.

18. A method of operating a trivalent chromium electroplating bath in which the bath contains as a surfactant material, whether added on initial makeup, orfor preventive maintenance, oron requirement, a dialkyl sulphosuccinate in which the alkyl esterifying group contains 4to 5 carbon atoms.

19. A method as claimed in claim 18 in which the sulphosuccinate is diamylsulphoccinate, dibutylsulphosuccinate, oramyl/butyl sulphosuccinate, used as such oras a potassium sodium or ammomium salt.

20. An additive formulation having dissolved therein compatible amounts of each of: (a) a water soluble compound offormula

in which R1, R2and R3 are chosen from hydrogen and C1 to C6 alkyl groups and in which Xis ammonium or a bath-compatible metal, and (b) awater-solublecompound oftheformula

in which A is the residue of an aromatic structure, and at least one of the R4and R5 is H or low C1 to C6 alkyl with any remaining R4 or R5 being a cyclic -CO linkagetothearomaticresidueorto afurthergroup componentforrrrjoint addition to a trivalent chro miumplating bath either priorto plating, or at at predetermined intervals oron requirement.

21. An additive formulation as claimed in claim 20 further containing a material dialkyl sulphosuccinate surfactant.