This invention relates to a process and composition for electrodeposition of bright decorative chromium. More particularly, it relates to a chromium plating process characterized by highly dilute decorative.chromium electroplating baths.

This invention lies in the discovery that there is a new and totally unexpected region of operation in chromium plating from dilute chromic acid - containing plating baths which yields superior chromium deposits over a wide plating range.

It is an object of this invention to provide a process for electrodepositing bright decorative chromium plate, characterized by its high coverage of low current density areas.

According to one aspect of the invention there is provided a process for electrodepositing a bright decorative chromium plate on a basis metal comprising maintaining an aqueous hexavalent chromium plating bath containing:

• (i) chromic acid;
• (ii) sulphate, the ratio by weight of component (i) calculated as Cr0₃ to component (ii) calculated as SO₄^-- being within a ratio range of 600:1 to 3000:1;
• (iii) 0.05 - 2 g/litre of fluoride and/or complex fluoride calculated correspondingly as F^- and/or complex fluoride anion, and (iv) at least approximately 0.5 g/litre of at least one suitable organic carboxylic acid anion and/or at least one suitable halogenated organic carboxylic acid anion, and electrodepositing a bright decorative chromium plate from the bath onto the basis metal as cathode in the bath.

According to another aspect of the invention there is provided a composition for addition to an aqueous medium to form a chromium plating solution for the electrodeposition of a bright decorative chromium plate on a basis metal, the said composition comprising the following components expressed at parts by weight:

• , (1) 150 - 500 chromic acid calculated as C^r03^;
• (II) 0.05 - 0.85 sulphate calculated as SO₄;
• (III) 0.05 - 2 of fluoride and/or complex fluoride calculated correspondingly as F^- and/or complex fluoride anion, and
• (IV) 0.5 - 32 calculated as acid anion of at least one suitable organic carboxylic acid or salt thereof and/or carboxylic acid andydride and/or at least one suitable halogenated organic carboxylic acid or salt thereof and/or halogenated organic carboxylic acid anhydride, the ratio by weight of component (I) to component (II) being within a ratio range of 600:1 to 3000:1.

According to a further aspect of the invention there is provided a composition for addition to an aqueous medium to form a chromium plating solution for the electrodeposition of a bright decorative chromium plate on a basis metal, the said composition comprising the following components expressed as parts by weight:

• (XI) 20 - 150 chromic acid calculated as C^r03^;
• (XII) 0.01 - 0.25 sulphate calculated as SO₄;
• (XIII) 0.05 - 2 of fluoride and/or complex fluoride - calculated correspondingly as F and/or complex fluoride anion, and
• (XIV) 0.5 - 32 calculated as acid anion of at least one suitable organic carboxylic acid or salt thereof and/or carboxylic acid anhydride and/or at least one suitable organic halogenated carboxylic acid or salt thereof and/or halogenated carboxylic acid anhydride, the ratio by weight of component (XI) to component (XII) being within a ratio range of 600:1 to 3000:1.

The ratio of chromic acid calculated as CrO₃ to fluoride and/or complex fluoride calculated as monofluoride ion or its univalent equivalent preferably lies within a ratio range by weight of 150:1 to 500:1.

Fluoride (F-) is a general term to include fluoride, F^-, and complex fluorides; e.g. BF₄^-, ZrF₆⁼, TiF₆⁼, SiF₆⁼, AlF₆^≡.

The chromium plating bath employed in the practice of this invention may be an aqueous solution containing 20 - 150 g/litre of chromic acid calculated as CrO₃ and 0.01 g/litre - 0.25 g/litre, say 0.12 g/ litre of sulphate ion SO₄⁼, typically added as sodium sulphate and 0.5 g/litre - 2 g/litre of fluoride ion, F typically added as sodium bifluoride. In the practice of this invention, the ratio by weight of CrO₃:SO₄⁼ is maintained at 600 - 3000:1. Preferred ratios of Cr0₃ to fluoride ion, carboxylic acid anion and any heavy metal impurities are:

It is a particular advantage of this invention that the novel results may be attained (a) in a standard, non-self regulating bath as described supra or (b) in a self-regulating bath. Another typical mixed catalyst bath which may be employed may contain 20 - 150 g/litre of chromic acid calculated as Cr0₃ and 0.01 - 0.25 g/litre, say 0.06 g/litre of sulphate S0₄= ion; and 0.1 - 2.0 g/litre say 0.6 g/litre of silicofluoride SiF₆= ion. It will be noted that the ratio as the term is used in this application refers to the ratio by weightwherein each of the quantities is usually expressed in grams.

This invention may also be used in a self-regulating bath, e.g. of the sulphate type, which may contain 20 - 150 g/litre of chromic acid calculated as Cr0₃, and 0.001 to 0.25 g/litre or more of strontium sulphate; plus optionally an additional strontium compound source of excess strontium ion, such as strontium hydroxide, strontium chromate, etc. in amounts to provide 0 - 20 g/litre strontium ion Sr++. The ratio by weight of Cr0₃ to S0₄= is maintained at 600 - 3000:1.

Similarly the fluoride or complex fluoride ion may be self-regulated by use of appropriate compounds of limited solubility. For example the potassium ion may be used to regulate the concentration of the silicofluoride ion; the calcium or cerium ions may be used to regulate the concentration of the fluoride ion. The organic carboxylic acids and/or halogenated organic carboxylic acids which may be added, either as such or e.g. as their anhydrides or salts (typically the sodium salt), to chromium plating baths in the practice of this invention may be chosen from mono and polycarboxylic acids and stable subsituted carboxylic acids. For example, halogen substituted carboxylic acids are stable as are carboxylic acids with sulphonic or sulphate groups therein. Hydroxy substituted carboxylic acids, such as tartaric acid, are not stable and will oxidize in solution, especially when electrolysis is applied. The carboxylic acids may typically include:

• a. aliphatic monocarboxylic acids,
• b. halogenated aliphatic monocarboxylic acids,
• c. aliphatic polycarboxylic acids,
• d. halogenated aliphatic polycarboxylic acids,
• e. aromatic monocarboxylic acids,
• f. halogenated aromatic polycarboxylic acids,
• g. polyhalogenated aromatic monocarboxylic acids, and
• h. polyhalogenated aromatic polycarboxylic acids.

Typical illustrative monohalogenated aliphatic monocarboxylic acids which may be employed may include:

• chloracetic acid
• 2-chloropropionic acid
• 3-bromopropionic acid
• 3-iodopropionic acid
• 2-chlorobutanoic acid
• chloropivalic acid (monochlorinated tertiary pentanoic acid)
• 2-chloropentanoic acid.

Typical illustrative polyhalogenated aliphatic monocarboxylic acids which may be employed may include:

• di or tri chloracetic acid 2,2-dichloropropionic acid 2,2,3-trichloropriopionic acid pentafluoropropionic acid,

Typical illustrative monohalogenated aliphatic dicarboxylic acids which may be employed may include:

• chloromalonic acid 2-chlorosuccinic acid 2-bromosuccinic acid 2-chloroadipic acid,

Typical illustrative polyhalogenated aliphatic dicarboxylic acids which may be employed may include:

• 2,2-dichlorosuccinic acid 2,2-dichloroadipic acid tetrachlorosuccinic acid 2,3-dibromosuccinic acid 3,3-diiodosuccinic acid 3,4-dichloroadipic acid,

Typical illustrative monohalogenated aromatic monocarboxylic acids which may be employed may include:

• 3-chloro-4-sulphobenzoic acid 3-bromo-4-sulphobenzoic acid.

Typical illustrative monohalogenated aromatic dicarboxylic acids which may be employed may include:

• 4-chlorophthalic acid 2-bromoterephthalic acid ,

Typical illustrative polyhalogenated aromatic monocarboxylic acids which may be employed may include:

• 3,S-dichloro-4-sulphobenzoic acid 3,6-dibromo-4-.sulphobenzoic acid.

Typical illustrative polyhalogenated aromatic dicarboxylic acids which may be employed may include:

• 3,4-dichlorophthalic acid 3,4-dibromophthalic acid 4,5-dichlorophthalic acid,

Other acids falling within the scope of this invention will be apparent to those skilled-in-the-art.

The preferred acids include aliphatic dicarboxylic acids and most preferably a halosuccinic acid such as 2-chlorosuccinic acid or 2,2-dichlorosuccinic acid or 2,3-dibromosuccinic acid or a haloadipic acid such as 3,4-dichloroadipic acid.

In the preferred practice of this invention, the carboxylic acid is added to the electroplating bath in amounts from 0.5 up to 32 g/litre, and preferably from 2 to 25 g/litre. The acids employed will preferably be those having a solubility in the plating bath within these ranges. Solubilizing substituents such as sulpho groups may be included in the carboxylic acid in order to increase solubility in the plating bath.

A typical composition which may be premixed, and added to a water solution in which the concentration of SO₄⁼ ion and components including, e,g., SiF₆⁼, may be adjusted separately, may include the compositions indicated in Table I and II below (here as elsewhere, unless otherwise indicated, all parts are parts by weight). It will be apparent that these compositions, like other chromic acid- containing compositions, should preferably be formed, maintained, and stored in a manner to minimize contact with extraneous organic compositions and materials; and preferably they will be formed, stored, and maintained at temperature below 80°C. It will also be apparent that in compositions hereinafter designated as containing "halo-organic acid", halo-organic acid containing at least two carbon atoms is intended and equivalent amounts of anhydride, salt, etc. may be employed, thus yielding appropriate amounts of the desired ion. All amounts are in grams per litre.

A preferred composition may include:

The organic acid may be added as such, as the anhydride, or as the salt, typically as the sodium salt. In the preferred embodiments, the additive may be admixed with the other ingredients to be used to make up the bath.

A specific self-regulating composition may include:

A typical mixed catalyst composition may include:

A preferred mixed catalyst composition may include:

A typical self-regulating composition having both sulfate and silicofluoride may include:

A preferred self-regulating composition may include:

The baths useful in the practice of this invention may be formed by dissolving the above compositions in aqueous medium to form baths containing 20 - 150 g/litre of CrO₃ and corresponding quantities of the other components.

It is found that particularly outstanding results, in terms of handleability, packaging, ease of manufacture, as well as maximum coverage and brilliance of chromium deposit accompanied by a minimum of lead anode corrosion may be obtained when in the compositions of Tables I, III, V, and VII, the organic carboxylic acid or halo-organic acid is an aliphatic dicarboxylic acid; and such compositions are most highly preferred, because of their peculiarly unexpected superiority.

The baths of this invention which may be employed to readily and conveniently electrodeposit chromium plate, are characterized by high coverage and by high throwing power. These baths may be used to deposit chromium onto any basis metal. Outstanding results may be obtained when the basis metal is a metal having an atomic number of 24 - 30. Typical of such basis metals are chromium, manganese, iron, cobalt, nickel, copper, and zinc. Mixture or alloys of these metals may be plated--typically brass, stainless steel, etc. The preferred basis metal may be nickel or a nickel-iron alloy and preferably active nickel.

The preferred active nickel basis metal may be attained by electrodeposition of nickel onto a suitable substrate metal (such as iron).

Active nickel may be nickel which is highly receptive to the deposition thereon of a bright clear decorative plate and which has a surface which may be free of nickel compounds such as oxide. Typically nickel may be active when freshly plated onto a cathode. If not already active, the nickel may be rendered active by cathodic or other reducing treatment prior to the deposition of chromium plate thereon. Preferably this may be effected by maintaining the nickel as cathode in an aqueous electrolyte solution, preferably containing an acid. The preferred acids for use in either electrolytic or non-electrolytic techniques may include acids such as the common mineral acids, e.g. hydrochloric acid or sulphuric acid etc. When the aqueous electrolyte solution is other than acid, it may preferably be followed by an acid dip.

It has been found when the high ratio baths of this invention are used to plate chromium onto bright nickel basis metal that it is advantageous to activate the bright nickel by applying to the cathode to be plated in the bath, a low voltage applied thereto at a time less than about five seconds after immersion and preferably to apply the voltage prior to immersion of the cathode. The low voltage may be sufficient to produce a cathode current density up to about 0.25 - 0.5 times the plating current density. Then the current density may be raised to its full operating value. This technique makes a bright nickel surface more receptive to the deposit of bright chromium from the baths of this invention.

The bath may be preferably at temperature of 30° - 60°C, say 35° - 50°C. A preferred cathode current density may be 0.3 - 40 amperes per square deoimetre (asd) most preferably 0.5 - 20 asd. Plating may be carried out with air or mechanical agitation for any time to obtain a desired thickness, but for decorative plate it is usually b - 10 minutes; and typically one - three minutes may suffice.

During plating in accordance with the process of this invention, there is unexpectedly and surprisingly no appreciable loss of the organic carboxylic acid or halo-organic acid by decomposition over extended periods of time. For example, in tests, 2,2-dichlorosuccinic acid was found still to function satisfactorily after plating had been carried out for 110 ampere hours per litre and even longer.

At the conclusion of the plating time, the cathode will be found to be covered to a remarkable degree with clear, bright, decorative chromium plate. It is a particular feature of this invention that the plate is unexpectedly characterized by its high coverage without the need for conforming anodes. For example, articles containing deep recesses such as zinc based die cast automotive dashboard trim may be plated by the process of this invention (with no conforming anode) to unexpectedly yield a bright, uniform plate on both high and low current density areas. This has not heretofore been possible from such dilute baths.

The plate produced by the process of this invention may be found to be highly satisfactory with respect to its unusually bright, decorative appearance and its resistance to corrosion.

In the following series of illustrative examples, testing and the criteria for the deposits were established in a 534-ml Hull Cell containing 500 ml of solution. A brass Hull Cell panel, 100 mm wide, was first plated with bright nickel, activated, rinsed and inserted in the test solution at the appropriate end. The solution was maintained at a temperature of 42.8° - 43.9°C and 5 amperes were passed through the test solution for three minutes, rinsed, dried and examined. Notations were made as to the distance in mm from the HCD end of each effect. Coverage is defined as this distance for the extent of chromium metal.

Other temperatures of operation are also suitable so long as the appropriate adjustment in applied current is made. That is, higher temperatures of operation require higher currents to produce chromium deposits in the bright range and lower temperatures must utilize lesser currents.

Optimum compositions, too, may be adjusted for changes in plating temperature; e.g., the lower ratios may be more desirable at the elevated temperatures.

In the following series of illustrative examples the stock bath and the plating conditions were the same as in the previous examples, except that electrodeposition was conducted at 43°C..

In the following tests the superior deposits obtained in this new region consist of typically bright chromium metal deposited from zero (0) mm (i.e., the HCD edge of the test panel) to at least 78 mm and this with only barely perceptible hazes or films on the deposit or even in the region beyond where the deposit ends (e.g., 78 - 100 mm)

The combination containing only sulphate and fluoride (i.e., without the carboxylate) is inadequate to produce the desired results. This is borne out by several experimental sequences described below as Examples 1 - 4. In all cases the concentration of chromic acid was 120 g/litre.

Without the carboxylate the deposits were not commercially acceptable because they went from a passive to a filmed state without a commercial deposit in the sequence.

The effect of adding increasing amounts of a carboxylate, acetic acid in this case, is illustrated in Example 5. In each experiment the chromic acid concentration was 120 g/litre, the sulphate concentration was 0.2 g/litre so that (CrO₃)/(SO₄⁼) =600 and the fluoride ion concentration was 0.5 g/litre so that (CrO₃)/(F^-)=240.

Under the conditions maintained in the experimental sequences the values of HOAc expressed in mL/litre are the same as their values expressed in g/litre for Examples 5 and 16.

Two points are brought out in Example 6, that a complex fluoride may be used instead of the simple fluoride ion and that increasing amounts of sulphate lead to increasing amounts of LCD film band, dubbed sulphate film band. (CrO₃) - 120 g/litre (succinic anhydride) = 6.0 (ZrF₆=) = 1.0.

The best results here arise when the CrO₃/BF₄^- ratio lies between 150 and 500.

The following examples illustrate the usefulness with other carboxylates. In each case the following were maintained constant: (CrO₃) 120 g/litre

• (SO₄=) 0.1 g/litre
• (_F-) 0.5 g/litre

The usefulness with even more dilute baths is illustrated in Examples 13 and 14 which produced acceptable deposits.

The above results are in contrast with the test deposit produced by the "optimum" baths of Konishi.^*

• CrO₃ 50 ^g/litre
• H₂SO₄ 0.5 g/litre Coverage 84 mm .
• Na₂SiF₆ 0.5 g/litre In spite of the 30 second immersion after plating to remove films (as specified by Konishi) the test panel showed general blue films starting at about 25 mm and getting heavier at the lower current densities. In addition there was a.heavy sulphate film band extending from 75 - 82 mm. The panel indicated that the deposit was not of commercially acceptable quality.

^*Konishi, S. & Tadagoshi, M., "Metal Finishing" 71 (11) 49-52 (Nov., 1973).

This example illustrates the variability which may be obtained with the sulphate anion when the fluoride and the carboxylate are maintained constant.

• a. An example which illustrates the deleterious effect of heavy metal impurities: To a bath containing chromic acid 120, succinic anhydride 6, sulphate 0.12 and fluoride 0.5 g/litre was added a mixture of ferrous oxalate and oxalic acid which produced ferric ion and chromic ion in equal amounts upon oxidation of the ferrous ion and the oxalate ion. Up to a total of 2 g/1 each of Fe⁺³ and Cr⁺³ the deposits produced under standard test conditions were acceptable. Beyond this value, e.g., at 3 g/l each, the LCD filming became heavy enough to be objectionable.
• b. Under the same circumstance--but without iron, chromic ion up to 4 g/litre caused a little loss in coverage but at 6 g/litre the loss was more substantial and an objectionable film band appeared.
• c. Copper as cupric ion caused no deterioration at 4 g/litre but at 6 - 8 g/litre started to produce significant hazes which were not desirable.