CA1201411A - Rejuvenation of trivalent chromium electrolyte - Google Patents

Abstract

ABSTRACT

The invention is concerned with a process for rejuvenating an aqueous acidic trivalent chromium electrolyte which has been impaired in effectiveness due to the contamination by excessive quantities of hexavalent chromium, the electrolyte containing triva-lent chromium ions, a complexing agent for maintaining the trivalent chromium ions in solution, halide ions, ammonium ions and hydrogen ions to provide a pH on the acid side. The process of the invention comprises the step of adding to the electrolyte a reducing agent comprising vanadium ions in at least an amount sufficient to reduce the concentration of hexavalent chromium ions to a level which is not in excess of 0.4 grams/liter.
The invention enables one to restore the plating efficiency and throwing power of a chromium electro-plating bath while avoiding the costly and time consum-ing step of dumping and replacing the electrolyte.

Description

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This is a divisional application of Canadian application No. 389,254, filed November 2, 1981,which relates to a trivalent chromium electrolyte and a process employing vanadium reducing agent.
Chromium electroplating baths are in wide-spread commercial use for applying protective and decorative platings to metal substrates. For the most part, commercial chromium plating solutions heretofore used employ hexavalent chromium derived from compounds such as chromic acid, for example, as the source of the chromium constituent. Such hexavalent chromium electroplating solutions have long been characteri~ed as having limited covering power and excessive gassing particularly around apertures in the parts being plated which can result in incomplete coverage. Such hexavalent chromium plating solutions are also quite sensitive to current interruptions resulting in so-called "whitewashing" of the deposit.
Because of these and other problems including the relative toxicity of hexavalent chromium, and associated waste disposal problems, extensive work has been conducted in recent years to develop chromium electrolytes incorporating trivalent chromium providing numerous benefits over the hexavalent chromium electro-lytes heretofore known. According to the invention of theparent application, a trivalent chromium electrolyte and process for depositing chromium platings have been '`'``

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discovered by whieh bright ehromium deposits are pro-dueed having a color equivalent to that obtained from hexavalent ehromium baths. The eleetrolyte and proeess of the parent application further provide electro-plating employing eurrent densities whieh vary over awide range without producing the burning associated with deposits plated from hexavalent ehromium plating baths; in whieh the eleetrolyte eomposition minimizes or eliminates the evolution of mist or noxious odors during the plating proeess; the electrolyte and process provide for excellent coverage of the substrate and good throwing power; current interruptions during the electroplating eyele do not adversely affeet the chromium deposit enabling parts to be withdrawn from the electrolyte, inspeeted, and thereafter returned to the bath for eontinuation of the electroplating cycle;
the electrolyte employs low eoneentrations of chromium thereby redueing the loss of ehromium due to drag-out;
and waste disposal of the ehromium is faeilitated in that the trivalent ehromium ean readily be preeipitated from the waste solutions by the addition of alkaline substances to raise the pH to about 8 or above.
The electrolyte of the parent application further ineorporates a redueing agent to prevent the formation of detrimental concentrations of hexavalent ehromium during bath operation whieh heretofore has interfered with the efficient electrodeposition of lZ019~

chromium from trivalent chromium plating baths in-cluding the reduction in the efficiency and covering power of the bath. In some instances, the buildup of detrimental hexavalent chromium has occurred to the extent that a cessation in electrodeposition of chromium has occurred necessitating a dumping and replacement of the electrolyte. It has now been found, in accordance with the invention of the present divi-sional application, that the addition of the reducing agent according to the electrolyte herein disclosed effects a rejuvenation of an electrolyte contaminated with excessive hexavalent chromium restoring the plating efficiency and throwing power of such a bath and avoid-ing the costly and time consuming step of dumping and replacing the electrolyte.
The benefits and advantages of the invention in accordance with the composition aspect thereof are achieved by an aqueous acidic electrolyte containing as its essential constituents, controlled amounts of trivalent chromium, a complexing agent present in an amount sufficient to form a chromium complex, halide ions, ammonium ions and a reducing agent com-prising vanadium ions present in an amount effective to maintain the concentration of hexavalent chromium ions at a level below that at which continued optimum efficiency and throwing power of the electroplating bath is maintained. More particularly, the electro-lyte can broadly contain about 0.2 to about 0.8 lZ0~4~:1 molar trivalent chromium ions, a formate and/or acetate complexing agent present in an amount in relationship to the concentration of the chromium constituent and typically present in a molar ratio of complexing agent to chromium ions of about 1:1 to about 3:1, a bath soluble and compatible vanadium salt present in a con-centration to provide a vanadium ion concentration of at least about 0.015 grams per liter (g/1) up to about 6.3 g/l as a reducing agent for any hexavalent chromium formed during the electroplating process, ammonium ions as a secondary complexing agent present in a molar ratio of ammonium to chromium of about 2,0:1 to about 11:1, halide ions, preferably chloride and bromide ions present in a molar ratio of halide to chromium ions o~
about 0.8:1 to about 10:1; one or a combination of bath soluble salts to increase bath conductivity com-prising compatible simple salts of strong acids such as hydrochloric or sulfuric acid and alkaline earth, alkali and ammonium salts thereof of which sodium fluoborate comprises a preferred conductivity salt, and hydrogen ions present to provide an acidic electro-lyte having a pH of about 2.5 up to about 5.5.

The electrolyte may optionally, but prefer-ably, also contain a buffering agent such as boric acid typically present in a concentration up to about 1 molar, a wetting agent present in small but effective amounts of the types conventionally employed in chromium or nickel plating baths as well as controlled effective 12~)i4 11:

amounts of anti-foaming agents. Additionally, the bath may incorporate other dissolved metals as an optional constituent including iron, cobalt, nickel, manganese, tungsten or the like in such instances in which a chromium alloy deposit is desired.
In accordance with a process aspect of the invention, the electrodeposition of chromium on a conductive substrate is performed employing the electrolyte at a temperature ranging from about 15 to about 45C. The substrate is cathodically charged and the chromium is deposited at current densities ranging from about 50 to about 250 amperes per square foot ~ASF) usually employing insoluble anodes such as carbon, platinized titanium or platinum. The substrate, prior to chromium plating, is subjected to conventional pretreat-ments and preferably is provided with a nickel plate over which the chromium deposit is applied.
In accordance with a further process aspect of the invention, electrolytes of the trivalent chromium type which have been rendered inoperative or inefficient due to the accumulation of hexavalent chromium ions, are rejuvenated by the addition of controlled effective amounts of the vanadium reducing agent to reduce the hexavalent chromium concentration to levels below about 25 100 parts per million (ppm), and preferably below 50 ppm at which efficient chromium plating can be resumed.
Accordingly, the present divisional application is directed toward a process for rejuvenating an aqueous ,, ~ lZ0141~

acidic trivalent chromium electrolyte whieh has been impaired in effectiveness due to the contamination by excessive quantities of hexavalent chromium, the electro-lyte containing trivalent chromium ions, a complexing agent for maintaining the trivalent chromium ions in solution, halide ions, ammonium ions and hydrogen ions to provide a pH on the acid side, which process comprises the step of adding to the electrolyte a reducing agent comprising vanadium ions in at least an amount sufficient to reduce the concentration of hexavalent chromium ions to a level which is not in excess of 0.4 grams/liter.
Additional benefits and advantages of the in-ventions of both parent and divisional applications will become apparent upon a reading of the following descrip-tion of the preferred embodiments and the specificexamples provided.
In accordance with the composition aspect of the invention, the trivalent chromium e~ectrolyte con-tains, as one of its essential constituents, trivalent chromium ions which may broadly range from about 0 2 to about 0.8 molar, and preferably from about 0O4 to about 0.6 molar. Concentrations of trivalent chromium below about 0.2 molar have been found to provide poor throwing power and poor coverage in some instances whereas, concen-trations in excess of about 0.8 molar have in someinstances resulted in precipitation of the chromium cons-tituent in the form of complex compounds. For this reason, it is preferred to maintain the trivalent chromium --"` 120'~4i~

ion concentration within a range of about 0.2 to about 0.8 molar, and preferably from about 0.4 to about 0.6 molar.

The trivalent chromium ions can be introduced in the form of any simple aqueous soluble and compatible salt such as chromium chloride hexahydrate, chromium sulfate, and the like. Preferably, the chromium ions are intro-duced as chromium sulfate for economic considerations.
A second essential constituent of the electro-lyte is a complexing agent for complexing the chromium constituent present maintaining it in solution. The complexing agent employed should be sufficiently stable and bound to the chromium ions to permit electro-deposition thereof as well as to allow precipitation of the chromium during waste treatment of the effluents.

The complexing agent may comprise formate ions, acetate ions or mixtures of the two of which the formate ion is preferred. The complexing agent can be employed in concentrations ranging from about 0.2 up to about 2.4 molar as a function of the trivalent chromium ions present. The complexing agent is normally employed in a molar ratio of complexing agent to chromium ions of from about 1:1 up to about 3:1 with ratios of about 1.5:1 to about 2:1 being preferred. Excessive amounts of the complexing agent such as formate ions are unde-sirable since such excesses have been found in someinstances to cause precipitation of tne chromium con-stituent as complex compounds.

A third essential constituent of the electro---` lZOl~

lyte comprises a reducing agent in the form of bath soluble and compatible vanadium salts present in an amount to provide a vanadium ion coneentration of at least about 0.015 g/l up to about 6.3 g/l. Exeess amounts of vanadium do appear to adversely effect the operation of the electrolyte in some instances causing dark striations in the plate deposit and a reduction in the plating rate. Typieally and preferably, vanadium eoneentrations of from about 0.2 up to about 1 g/l are satisfactory to maintain the hexavalent chromium concentration in the electrolyte below about 100 ppm, and more usually from about 0 up to about 50 ppm at which optimum efficiency of the bath is attained.
The vanadium reducing agent is introduced into the electrolyte by any one of a variety of vanadium salts including those of only minimal solubility in whieh event mixtures of sueh salts are employed to achieve tne rec~uired eoncentration. The vanadium salt may eomprise any one of a variety of salts which do not adversely effect the chromium deposit and inelude, for example, sodium metavanadate (NaVO3); sodium orthovana~
date (Na3VO4, Na3VO4-10H2O, Na3VO4-16H20); sodium pyro-vanadate (Na4V2O7); vanadium pentoxide (V205); vanadyl sulfate (VOSO4); vanadium trioxide (V203); vanadium di-tri or tetra ehloride (VC12, VC13, VC14); vanadium tri-fluoride (VF3.3H2O); vanadium tetrafluoride (VF4);
vanadium pentafluoride (VF5); vanadium oxy bromide (VOBr); vanadium oxy di- or tri-bromide (VOBr2, VOBr3);

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vanadium tribromide (VBr3), ammonium metavanddate (NH4VO3); ammonium vanadium sulfate (NH4V(SO4)2.12H2O):
lithium metavanadate (LiVO3.2H2O; potassium metavana-date (~VO3): thallium pyrovanadate (Tl4VO7), thallium metavanadate (TlVO3), as well as mixtures thereof.
In as much as the trivalent chromium salts, complexing agent, and vanadium salts do not provide adequate bath conductivity by themselves, it is prefer-red to further incorporate in the electrolyte control-led amounts of conductivity additives which typicallycomprise salts of alkali metal or alkaline earth metals and strong acids such as hydrochloric acid and sulfuric acid, as well as the acids themselves. The inclusion of such conductivity additives is well known in the art and their use minimizes power dissipation during the elec-troplating operation. Typical conductivity additives include potassium and sodium sulfates and chlorid~s as well as ammonium chloride and ammonium sulfate. A par-ticularly satisfactory conductivity additi~e is fluobo-ric acid and the alkali metal, alkaline earth metal andammonium bath soluble fluoborate salts which introduce the fluoborate ion in the bath and which has been found to further enhance the chromium deposit~ Such fluobo-rate additives are preferably employed to provide a fluoborate ion concentration of from about 4 to about 300 g/l. It is also typical to employ the metal salts of sulfamic and methane sulfonic acid as a conductivity salt either alone or in combination with inorganic con-~Z014~l1 ductivity salts. Such conductivity salts or mixtures thereof are usually employed in amounts up to about 300 g/l or higher to achieve the requisite electrolyte con-ductivity and optimum chromium deposition.
It has also been observed that ammonium ions in the electrolyte are heneficial in enhancing the reducing efficiency of the vanadium constituent for converting hexavalent chromium formed to the trivalent state. Particularly satisfactory results are achieved 10 at molar ratios of total ammonium ion to chromium ion ranging from about 2.0:1 up to about 11:1, and preferably, from about 3:1 to about 7:1. The ammonium ions can in part be introduced as the ammonium salt of the complexing agent such as ammonium formate, for 15 example, as well as in the form of supplemental con-ductivity salts.
The effectiveness of the vanadium reducing agent in controlling hexavalent chromium formation is also enhanced by the presence of halide ions in the 20 bath of which chloride and bromide ions are preferred.
The use of a combination of chloride and bromide ions also inhibits the evolution of chlorine at the anode.
While iodine can also be employed as the halide con-stituent, its relatively higher cost and low solubility 25 render it less desirable than chloride and bromide.
Generally, halide concentrations of at least about 15 g/l have been found necessary to achieve sustained efficient electrolyte operation. More particularly, the 0~9Ll~

halide concentration is controlled in relationship to the chromium concentration present and is controlled at a molar ratio o~ about 0.8:1 up to about 10:1 halide to chromium, with a molar ratio of about 2:1 to about 4:1 being preferred.
In addition to the foregoing constituents, the bath optionally but preferably also contains a buffering agent in an amount of about 0.15 molar up to bath solubility, which amounts typically range up to about 1 molar. Preferably the concentration of the buffering agent is controlled from about 0.45 to about 0.75 molar calculated as boric acid. The use of boric acid as well as the alkali metal and ammonium salts thereof as the buffering agent also is effective to introduce borate ions in the electrolyte which have been found to improve the covering power of the electro-lyte. In accordance with a preferred practice, the borate ion concentration in the bath is controlled at a level of at least about 10 g/l. The upper level is not critical and concentrations as high as 60 g/l or higher can be employed without any apparent harmful effect.
The bath further incorporates as an optional but preferred constituent, a wetting agent or mixture of wetting agents of any of the types conventionally employed in nickel and hexavalent chromium electrolytes~
Such wetting agents or surfactants may be anionic or , ., 0~L41~

cationic and are selected from those which are compatible with the electrolyte and which do not adversely affect the electrodeposition performance of the chromium constituent. Typically, wetting agents which can be satisfactorily employed include sulphosuccinates or sodium lauryl sulfate and alkyl ether sulfates alone or in comhination with other compatible anti-foaming agents such as octyl alcohol, for example. The presence of such wetting agents has been found to produce a clear chromium deposit while eliminating dark mottled deposits and providing for improved coverage in low current density areas. While relatively high concen-trations of such wettiny agents are not particularly harmful, concentrations greater than about 1 gram per liter have been found in some instances to produce a hazy deposit. Accordingly, the wetting agent when employed is usually controlled at concentrations less than about 1 g/l, with amounts of about 0.05 to about 1 g/l being typical,, It is also contemplated that the electrolyte can contain other metals including iron, manganese, and the like in concentrations of from 0 up to saturation or at levels below saturation at which no adverse effect on the electrolyte occurs in such instances in which it is desired to deposit chromium alloy platings. When iron is employed, it is usually preferred to maintain the concentration of iron at levels below about 0.5 g/l.
The electrolyte further contains a hydrogen ~zo~

ion concentration sufficient to render the electrolyte acidic~ The concentration of the hydrogen ion is broadly controlled to provide a pH of fxom about 2.5 up to about 5.5 while a pH range of about 3.5 to 4.0 is particularly satisfactory. The initial adjustment of the electrolyte to within the desired pH range can be achieved by the addition of any suitable acid or base compatible with the bath constitutents of which h~dro-chloric or sulfuric acid and/or ammonium or sodium carbonate or hydroxide are preferred. During plating, the electrolyte has a tendency to become more acidic and appropriate pH adjustments are effected so as to maintain the pH within an optimum range for the parti-cular bath components and concentrations used as well as the nature of the substrate to be plated; this can be done by the addition of-alkali..me..tal and ammonium hydroxides and carbonates of which the ammonium salts are preferred in that they simultaneously replenish the ammonium constitutent in the bath.
In accordance with the process aspect of the invention, the electrolyte as hereinabove des-cribed is employed at an operating temperature ranging from about 15 to about 45C, preferably about 20~ to about 35C. Current densities during electroplating 25 can range from about 50 to 250 ASF with densities of ~'.

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about 75 to about 125 ASF being more typical. The electrolyte can be employed to plate chromium on con-ventional ferrous or nickel substrates and on stainless steel as well as nonferrous substrates such as aluminum and zinc. The electrolyte can also be employed for chromium plating plastic substrates which have been - 13a -.
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subjected to a suitable pretreatment according to well-known techniques to provide an electrically con-auctive coating thereover such as a nickel or copper layer. Such plastics include ABS, polyolefin, PVC, and phenol-formaldehyde polymers. The work pieces to be plated are subjected to conventional pretreatments in accordance with prior art practic-es ana the process is particularly effective to deposit chromium platings on c~nductive substrates which have been subjected to a prior nickel plating operation.
During the electroplating operation, the work pieces are cathodically charged and the bath incorporates a suitable anode of a material which will not adversely ~f~ect and is compatible with the electrolyte composition. For this purpose anodes of an inert material such as carbon, for example, are preferred although other inert anodes of platinized titanium or platinum can also be employed. When a chromium-iron alloy is to be deposited, the anode may suitably be comprised of iron which itself will serve as a source of the iron ions in the bath.
In accordance with a further aspect of the process of the invention, a rejuvenation of a tri-valent electrolyte which has been rendered ineffective or inoperative due to the high concentration of hexa-valent chromium ions is achieved by the addition of a controlled effective amount of the vanadium reducing agent. Depending upon the specific composition of the ~L2(~

trivalent electrolyte, it may also be necessary to add or adjust other constituents in the bath within the broad usable or preferred ranges as hereinbefore specified to achieve optimum plating performance. For example, the rejuvenant may comprise a concentrate containing a suitable vanadium salt--in further combina-tion with halide salts, ammonium salts, borates, and conductivity salts as may be desired or required. The addition of the vanadium reducing agent can be effected as a dry salt or as an aqueous concentrate in the presence of agitation to achieve uniform mi~ing. The time necessary to restore the electrolyte to efficient operation will vary depending upon the concentration of the detrimental hexavalent chromium present and will usually range from a period of only five minutes up to about two or more hours. The rejuvenation treatment can also advantageously employ an electrolytic treatment of the bath following addition of the rejuvenant by subjecting the bath to a low current density of about 10 to about 30 ASF for a period of about 30 minutes to about 24 hours to effect a conditioning or so-called "dummying" of the bath before commercial plating operations are resumed. The concentration of the vanadium ions to achieve rejuvenation can range within the same limits as previously defined for the operating electrolyte.
In order to further illustrate the composition and process of the invention, the following specific -` 120141~L

examples are provided. It will be understood that the examples are provided for illustrative purposes and are not intended to be limiting of the invention as herein disclosed and as set forth in the subjoined claims.
A series of trivalent chromium electrolytes are prepared having compositions as set forth in Table 1.

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The particular se~uence of addition of the bath constituents during bath make-up is not critical in achieving satisfactory performance. In all of the examples with the exception of Examples 34 and 35, the trivalent chromium ions are introduced in the form of chromium sulfate. In Examples 34 an'd 35, the trivalent chromium constituent is introduced employing chromium chIoride hexahydrate. In each of the examples, the surfactant employed comprises a mixture of dihexyl 10 ester of sodium sulfo succinic acid and sodium sulfate derivative of 2-ethyl-1-hexanol. The operating tempera-ture of the exemplary electrolytes is from 70 to about 80F (21-27C) at cathode current densities of from about 100 to about 250 ASF and an anode current density of about 50 ASF. The electrolytes are employed using a graphite anode at an anode to cathode ratio of about

2:1. The electroplating bath is operated employing a mild air and/or mechanical agitation. It has been found advantageous in some of the examplary bath formulations 20 to subject the bath to an electrolytic preconditioning at a low current density, e.g. about 10 to about 30 ASF
for a period up to about 24 hours to achieve satisfactory plating performance at the higher normal operating current densities.
Each of Examples 1-36 employed under the foregoing conditions produced full bright and uniform chromium de-posits having good to excellent coverage over the current density ranges employed including good coverage in the deep ~L20~1~4~1 recess areas of the J-type panels employed for test plating.

-This example demonstrates the effectiveness of the vanadium compound for rejuvenating trivalent chromium electrolytes which have been rendered un-acceptable or inoperative because of-an increase in hexavalent chromium concentration to an undesirable level. It has been found by test that the progressive build-up of hexavalent chromium concentration will eventaully produce a skipping of the chromium plate and ultimately will result in the prevention of any chromium plate deposit. Such tests employing typical trivalent chromium electrolytes to which hexavalent chromium is intentionally added has evidenced that a concentration of about 0.47 g/l of hexavalent chromium results in plating deposits having large patches of dark chromium plate and smaller areas which are entirely unplated. As the hexavalent chromium concentration is further increased to about O.S5 g/l according to such tests, further deposition of chromium on the substrate is completely prevented. The hexavalent chromium concentration at which plating ceases will vary some-what depending upon the specific composition of the electrolyte.

3.20~

In order to demonstrate a rejuvenation of a hexavalent chromium contaminated electrolyte, a triva-lent chromium bath is prepared having the following composition:
InqredientConcentration, q/l Sodium fluoborate 110 Ammonium Chloride 90 Boric Acid 50 Ammonium formate 50 Cr~3 ions 26 Surfactant 0.1 The bath is adjusted to a pH between about 3.5 and 4.0 at a temperature of about 80 to about 90~F.
S-shaped nickel plated test panels are plated in the bath at a curxent density of about 100 ASF. After each test run, the concentration of hexavalent chromium ions is increased from substantially 0 in the original bath by increments of about 0.1 g/l by the addition of chromic acid. No detrimental effects in the chromium plating of the test panels were observed through the range of hexavalent chromium concentration of from 0.1 up to 0.4 g/l. However, as the hexavalent chromium concen-tration was increased a~ove 0.4 g/l large dark chromium deposits along with small areas devoid of any chromium deposit were observed on the test panels. As the concentration of hexavalent chromium attained a level of 0.55 g/l no further chromium deposit could be plated on the test panel.
Under such circumstances, it has heretofore been common practice to dump the bath containing high hexavalent chromium necessitating a make-up of a new bath which constitutes a costl~ and ~ime consuming operation.
To demonstrate the rejuvenation aspects of 10 the present invention, vanadium ions were added in increments of about 0.55 g/l to the bath containing 0.55 g/l hexavalent chromium ions and a Plating of the test panels was resumed under the conditons as previously described. The addition of 0.55 g/l of vanadium ions 15 corresponds to 2.6 g/l of vanadyl sulfate and corres-ponds to an incremental weight ratio addition of vanadium ions to hexavalent chromium ions of about 1:1.
The initial addition of 0.55 g/l vanadium ions to the bath contaminated with 0.55 g/l hexavalent chromium ions resulted in a restoration of the efficiency of the chromium plating bath producing a good chromium deposit of good color and coverage although hexavalent chromium ions were still detected as being present in the bath.

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The further addition of 0.55 g/l vanadium ions produced a further improvement in the chromium deposit and analysis indicates the presence of a small amount of hexavalent chromium in the bath.
Finally, the addition of a further 0.55 g/l vanadium ions for a total of 1.65 gjl vanadium ions to the bath xesulted in an excel~ent chromium deposit and an analysis for hexavalent chromium was negative. These test results clearly demonstrate the efficacy of 10 vanadium as a rejuvenating agent for contaminated trivalent chromium plating bathsO

In order to further demonstrate the process for rejuvenating trivalent chromium baths contaminated 15 with hexavalent chromium, a trivalent chromium plating bath is prepared of the composition as described in Example 37 to which 1.65 g/l of hexavalent chromium is aaded corresponding to a concentration approximately three times the amount at which tests indicated a 20 deposition of chromium ceased.
A test panel is plated under conditions as previously described in Example 37 clearly evidencing complete failure to deposit any chromium on the test panel. Thereafter, 4.95 g/l of vanadium ions corres-25 ponding to 23.5 g/l of vanadyl sulfate is added to the ~2~L4~;

bath which is calculated to reduce all of the hexavalent chromium present to the trivalent state.
Following the addition of the vanadium reju-venation agent, the bath under agitation was permitted to stand for approximately ten minutes after which a test panel was plated under the conditions as previous-ly described in ~xample 37 It was observed that the test panel exhibited a trace o~ chromium plate on the surface thereof.
After waiting a total of forty-five minutes following th~ vanadium addition to -the bath, a second test panel is plated evidencing an improved chromium plating with an increase in thickness and better appearance.
The bath is thereafter electrolyæed at a low current density of about 30 ASF for an additional three hours and a third test panel is plated. The chromium deposit is observed to he fully bright, of good color, with some thin deposit in low current density areas.
The bath is further electrolyzed at a low current density of 30 ASF for an additional seventeen hour period after which a fourth test panel is plated resulting in a chromium deposit of good thickness, fully bright with thin areas in the low current densities.
The test solution is replenished to return it to the concentration of the constituents as originally provided prior to the hexavalent chromium and vanadium addition including the addition of 3 g/l of trivalent _ -25-114~

chromium ions and a fifth test panel is plated. The re-sultant panel is observed to have a fully bright chromium plating of good color with substantially complete coverage over the entire surface thereof including low current density areas.
It should be appreciated that the efficacy of the vanadium compound to rejuvenate trivalent chro-mium baths contaminated with hexavalent chromium is apparent for a wide variety of such trivalent chromium electrolytes and is not specifically restricted to the electrolyte as set forth in Examples 37 and 38.

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for rejuvenating an aqueous acidic trivalent chromium electrolyte which has been impaired in effectiveness due to the contamination by excessive quantities of hexavalent chromium, said electrolyte containing trivalent chromium ions, a complexing agent for maintaining the trivalent chromium ions in solution, halide ions, ammonium ions and hydro-gen ions to provide a pH on the acid side, said process comprising the step of adding to said electrolyte a reducing agent comprising vanadium ions in at least an amount sufficient to reduce the concentration of hexavalent chromium ions to a level which is not in excess of 0.4 grams/liter.

2. The process as defined in claim 1, in which said vanadium ions added are of a valence of 4+.

3. The process as defined in claim 1, in which said vanadium ions are added in an amount of about 0.015 to about 6.3 g/l.

4. The process as defined in claim 1, in which said vanadium ions are added in an amount of about 0.2 to about 1 g/l.

5. The process as defined in claim 1, in which said vanadium ions are added in an amount to reduce the hexavalent chromium ion concentration to a level below about 100 ppm.

6. The process as defined in claim 1, in which said vanadium ions are added in an amount to reduce the hexavalent chromium ion concentration to a level below about 50 ppm.

7. The process as defined in claim 1, in which said vanadium ions are introduced in the form of electrolyte soluble and compatible vanadium salts.

8. The process as defined in claim 1, including the further step of electrolyzing the electrolyte following the addition of said vanadium ions at a moderate current density to accelerate reduction of said hexavalent chromium ions by said vanadium ions.