GB2124258A

GB2124258A - Purifying trivalent chromium electroplating baths

Info

Publication number: GB2124258A
Application number: GB08320069A
Authority: GB
Inventors: Walter J Wieczerniak
Original assignee: Occidental Chemical Corp
Current assignee: Occidental Chemical Corp
Priority date: 1982-07-29
Filing date: 1983-07-26
Publication date: 1984-02-15
Also published as: JPS5941500A; AU542415B2; DE3327011C2; GB2124258B; JPS6158560B2; GB8320069D0; DE3327011A1; CA1213558A; AU1712983A; FR2537164A1

Abstract

An aqueous trivalent chromium electrolyte containing a detrimental amount of contaminating metal ions (e.g. ions of Ni, Zn, Fe, Cu or Pb) is purified by adding dimethyldithiocarbamate ions, diethyldithiocarbamate ions, or a mixture thereof in an amount effective to precipitate at least a portion of the contaminating metal ions to reduce their concentration in the electrolyte to a level at which satisfactory performance of the electrolyte is restored and thereafter removing the precipitate from the electrolyte. The purified electrolyte does not produce the commercially unsatisfactory, dark streaky or spongy deposits which are obtained from the contaminated electrolyte.

Description

SPECIFICATION Method of purifying trivalent chromium electroplating baths The present invention broadly relates to trivalent chromium electrolytes, and more particularly to a method of purifying and restoring the performance of such electrolytes which have been rendered deficient in producing commercially acceptable chromium electrodeposits due to the progressive accumulation and increase in concentration of contaminating metal ions such as nickel, zinc, copper and lead during normal commercial operation of the bath.When one or combinations of such metal ion impurities attain concentration levels which adversely affect the performance of the electrolyte, the resultant chromium electrodeposit is rendered commercially objectionable due to the presence of black streaks, clouds and hazes which is sometimes further accompanied by a spongy deposit and a loss or reduction in the covering power of the electrolyte.

In order to overcome the detrimental effect of such metal ion contamination in trivalent chromium electroplating baths, it has heretofore been proposed in accordance with United States Patent No. 4,038,160 to add small amounts of water soluble ferrocyanide compounds to the electrolyte to effect a precipitation of such contaminating metal ions which can conventiently be removed by filtration. While the purification technique as disclosed in the aforementioned United States patent has been found effective in many instances, the presence of residual amounts of the ferrocyanide precipitating agent adversely affects the performance of the electrolyte rendering the bath inoperative for producing commercially acceptable chromium deposits.

When an access of the ferrocyanide agent is present in the bath, a further treatment is required to remove such excess which can be performed by the intentional addition of contaminating metal ions to effect a precipitation of the residual ferrocyanide compound. It will be appreciated that the treatment proposed in the aforementioned United States patent requires precise analysis of the contaminants in the bath in order that a precise quantity of the precipitating agent can be added which is not only tedious and time consuming but also requires skilled operating personnel. The toxic nature of the ferrocyanide precipitate requires special waste disposal measures which also is objectionable.

It has also been proposed to effect an electrolytic purification of contaminated trivalent chromium electrolytes employing a cathode on which a codeposition of such contaminating metal ions is effected by electrolyzing the bath over a prolonged time period. While such an electrolytic purification technique is somewhat effective for reducing copper ion contamination, it has been found relatively ineffective for removing nickel and zinc ions and only partially effective for removing iron.

It has heretofore been suggested to employ dithiocarbamates for precipitating heavy metals from electroplating rinse waters employing a pH generally on the alkaline side. Surprisingly, in accordance with the discovery comprising the present invention, it has been found that selected dithiocarbamate compounds can be employed for purifiying aqueous acidic trivalent chromium electrolytes which preferentially react with and precipitate the detrimental metal ions found in trivalent chromium electrolytes without significantly removing the chromium ions which are present in concentrations of about 50 times or more greater than the level of contaminating metal ions present in the bath.Moreover, such trivalent chromium electrolytes frequently contain metal ions such as vanadium as a desirable constituent for controliing and reducing the formation of detrimental hexavalent chromium ions which also surprisingly are not significantly removed by the use of such selected dithiocarba mate additives. Moreover, trivalent chromium electrolytes are on the acid side usually ranging in pH from about 1.5 up to about 5 which is substantially below the alkaline pH range heretofore proposed for treatment of electroplating rinse waters.

The unexpected results of the present invention are further evidenced by the fact that certain dithiocarbamate compounds such as dibutyldithiocarbamate are unsatisfactory.

Similarly, dimethyl glyoxime, a compound well known as a precipitating agent for nickel ions, has been found ineffective for precipitating appreciable amounts of contaminating nickel ions from trivalent chromium baths and, moreover, the precipitate formed is of a gelatinous character which tends to cling to the tank walls and work piece surfaces causing unacceptable plate deposits and a fouling of the filtration equipment.

Similar sulphide containing compounds such as, for example, sodium thiocarbonate (Na2CS3) and sodium thiocyanate (NaSCN) when incorporated in trivalent chromium electrolytes cause serious defects in the chromium deposit including dark blue to black discolouration of low current density recess areas and also significantly reduce the covering power of the bath.

The benefits and advantages of the present invention are achieved by employing dimethyldithiocarbamate or diethyldithiocarbamate ions or a mixture thereof as a purifying agent for trivalent chromium electrolytes by which a selective preferential reaction and precipitation of detrimental contaminating ions is effected producing a precipitate of a somewhat crystalline character which can readily be removed and separated from the treated electrolyte by conventional filtration equipment. Due to the instability of dimethyldithiocarbamic acid and diethyldithiocarbamic acid, it is preferred to introduce the corresponding ions into the bath as bath soluble alkali metal and ammonium salts such as the sodium salt for economic considerations.The additive agent is preferably introduced in the form of a concentrated aqueous solution to facilitate dispersion in the electrolyte.

The specific quantity of the purifying agent added during the purification treatment will vary depending upon the magnitude of contaminating metal ions present in the electrolyte. The quantity employed is therefore calculated to be sufficient to at least remove a sufficient quantity of such metal ion contaminants to reduce their concentration to a level at which the performance of the electrolyte is restored. Typically, concentrations of the precipitating agent relative to the concentration of contaminating metal ions in an amount of about 8:1 g/l is employed.

Additional benefits and advantages of the present invention will become apparent upon a reading of the Description of the Preferred Embodiments taken in conjunction with the specific examples provided.

The purification method of the present invention is particularly effective for purifying trivalent chromium electrolytes which generally contain trivalent chromium ions in a concentration of about 0.2 to about 0.8 molar, complexing agents for complexing the chromium ions to maintain them in solution such as formate ions, acetate ions or mixtures thereof present in concentrations to provide a molar ration of complexing agent to chromium ions usually of about 1:1 to about 3:1, and conductivity salts such as the salts or alkali metal or alkaline earth metals and strong acids such as hydrochloric acid and sulphuric acid present in amounts up to about 300 g/l or higher to achieve the requisite conductively. Among such conductivity salts is fluoboric acid as well as the alkali metal, alkaline earth metal and ammonium bath soluble salts thereof.Such trivalent chromium electrolytes can optionally and preferably further contain ammonium ions present at molar ratios of total ammonium ion to chromium ion of from about 2:1 to about 11:1 as well as halide ions including chloride and bromide ions present at a molar ratio of about 0.8:1 up to about 10:1 of halide ions to chromium ions present. Metallic reducing agents of the types disclosed in published United Kingdom Patent Application Serial No. 2 086 939, the substance of which is incorporated herein by reference, comprising vanadium ions are also desirably included in amounts to provide a vanadium ion concentration of from about 0.015 up to about 6.3 g/l.

Such trivalent chromium electrolytes can also optionally and advantageously contain buffering agents of which boric acid present in concentrations of about 0.15 molar up to bath solubility has been found particularly satisfactory.

Wetting agents can also be advantageously employed of the general types used in nickel and hexavalent chromium electrolytes in concentrations of from about 0.05 up to about 1 9/1.

Known trivalent chromium electrolytes also contain hydrogen ions to provide a pH on the acid side generally ranging from about 2.5 up to about 5.5.

The beneficial results of the present invention can also be obtained employing the purification additive in trivalent chromium electrolytes of the types as generally and specifically described in United States Patents No. 3,954,574; 4,107,004; 4,169,022 and 4,196,063, the teachings of which are incorporated herein by reference.

During conventional commerical operation employing such trivalent chromium electrolytes, a progressive contamination of the electrolyte occurs as a result of drag-in, dissolution in the electrolyte of unprotected surfaces of the receptacles or tanks containing the electrolyte, dissolution of the metallic surfaces of the work pieces being plated, dissolution of exposed portions of the work racks during immersion in the electrolyte, as well as contamination from the water and chemicals employed for replenishing and makeup of the electrolyte. As a result, a progressive buildup in the concentration of contaminating metal ions such as ions of nickel, zinc, iron, copper and lead occurs which, by experimentation, has indicated that concentrations of nickel ions in amounts of about 1 50 ppm or higher are harmful and cause defects in the chromium electrodeposit.While the presence of iron ions in amounts up to about 500 ppm are beneficial to the trivalent chromium electrolyte in that they tend to promote coverage of the chromium deposit, concentrations of above about 1,000 ppm (above 1 g/l) are generally harmful to the chromium deposit. Similarly, concentrations of copper ions in amounts above about 1 5 ppm and zinc ions above about 10 ppm and higher are harmful. When combinations of such metal ions are present in the bath, the harmful effects of the individual ions are cumulative and lower concentrations of the individual metal ions produce defects in the chromium deposit which is evidenced by the presence of black streaks, clouds, and hazes.

Under severe contaminating conditions, the covering power of the electrolyte is also adversely affected.

in accordance with the present invention, dimethyldithiocarbamate and/or diethlydithiocarbamate ions preferably in the form of an aqueous concentrate of the alkali metal and ammonium salts thereof of which the sodium salt constitutes the preferred material, are introduced into the contaminated trivalent chromium electrolyte in an amount sufficient to precipitate at least a portion of such metal ions reducing their level to an innocuous concentration at which satisfactory plating performance is restored. The non-toxic precipitate can readly be disposed of by conventional disposal techniques in contrast to the toxic precipitate produced with ferrocyanide compounds.Unlike the use of ferrocyanide precipitating agents, excess amounts of the purifying agent of the present invention do not detrimentally effect the quality of the chromium deposit and any unreacted quantities of the purifying agent remaining in the electrolyte following the treatment is progressively decomposed and/or removed during the normal electrolysis of the electrolyte during conventional plating operations. The purifying agent is preferably introduced into the electrolyte in the form of a concentrated aqueous solution to facilitate uniform dispersion thereof in the presence of agitation as opposed to the addition in the form of a dry bath soluble powder. The liquid concentrate can usually contain about 30 percent by weight of the purification agent and is usually adjusted to provide a pH of above about 8, preferably above about 9 to provide stability during storage.When commercial dimethyldithiocarbamate and diethyldithiocarbamate products are employed, it is usually desirable to carbon filter or otherwise purify such commercial mixtures to remove any undesirable reaction byproducts therein which may adversely affect the performance of the electrolyte following the purification treatment.

The purification agent of the present invention is useful over a wide pH operating range.

Generally, trivalent chromium electrolytes operate at a pH ranging from about 2 to about 5.5 and more typically, from about 3 to about 4. The purifying agent of the present invention operates well within the foregoing pH ranges as well as at at pH as low as about 1.5.

In order to further iilustrate the present invention, the following specific examples are provided. It will be understood that the examples are provided for illustrative purposes and are not intended to be limiting of the scope of the invention as herein described and as set forth in the subjoined claims.

Example 1 An aqueous acidic trivalent chromium test electrolyte was prepared having the following composition: Ingredient Concentration Cr+6 24.2 g/l NH4COOH 44.0 g/l NaBF4 55.0 g/l NH4CI 1 50.0 gIl H3BO3 57.1 g/l VOSO4 1.0 g/l Wetting agent* 2.0 cc/l *Wetting agent comprises equivalent of 0.1344 g/l dihexyl ester of sodium sulphosuccinic acid and 0.244 g/l of sodium sulfate derivative of 2-ethyl-1 -hexanol.

Contamination of the electrolyte with iron, copper and nickel ions was effected by the addition of the corresponding sulphate salts to produce an iron ion concentration of about 0.312 g/l, a copper ion concentration of about 0.032 g/l and a nickel ion concentration of about 0.110 g/l.

To 1 litre of the foregiong electrolyte containing the contaminating metal ions, 2.5 g/l of sodium diethyldithiocarbamate trihydrate was added and the resultant solution was stirred for approximately 10 minutes at 750F (240C) at a pH of about 3.2 and then allowed to stand quiescent for a period of 2 hours. The resultant solution was then filtered through a carbon filter and analyzed for metallic contamination. Upon analysis, the iron ions concentration was found to be 0.168 g/l; the copper ion concentration was 0.012 g/l; the nickel ion concentration was found to be 0.042 g/l. Accordingly, about 46.2 percent of the contaminating copper ions were removed; about 62.5 percent of the contaminating copper ions were removed; about 62 percent of the contaminating nickel ions were removed.

Example 2 A 400 millilitre sample of the trivalent chromium test electrolyte as described in Example 1 was adjusted by the addition of contaminating metal salts to provide an iron ion concentration of about 0.312 g/l, a nickel ion concentration of about 0.120 g/l and a copper ion concentration of about 0.080 g/l. To the electrolyte containing the dissolved contaminating metal ions, 3.5 g/l of sodium diethyldithiocarbamate trihydrate were added and the solution stirred for approxiamtely one-half hour at 120"F (49 ) at a pH of about 3. The solution was then allowed to stand quiescent for about 2 hours at 750F (240C) whereafter it was filtered through a carbon filter.Analysis of the filtrate revealed that the iron ion concentration was reduced to about 0.072 g/l, the nickel ion concentration was reduced to about 0.010 g/l while the copper ion concentration was nil.

Accordingly, about 77 percent of the iron ion contamination was removed, about 91.7 percent of the nickel ion contamination was removed while substantially 100 percent of the copper ion concentration was removed.

Example 3 In order to evaluate the effect of pH on the effectiveness of the purifying agent of the present invention, the same test electrolyte as described in Example 2 containing the same level of contaminating metal ions was raised to a pH of about 4 by the addition of ammonium hydroxide.

The solution was thereafter treated in the same manner as described in Example 2. Analysis of the filtrate revealed that the iron ion concentration in the treated electrolyte was reduced to about 0.012 g/l, and the nickel ion and copper ion concentrations were nil. Accordingly, about 96.2 percent of the iron ions were removed while substantially 100 percent of the nickel and copper ions were removed. The results of Example 3 indicate that the additive agent is of an increased effectiveness at a pH of 4 in comparison to a pH of about 3 as employed in Example 2.

Example 4 A trivalent chromium test electrolyte was prepared of a composition similar to that described in Example 1 with the exception that the trivalent chromium ion concentration was 21.7 girl, the ammonium formate concentration was about 51.0 g/l and the boric acid concentration was about 50.8 gjl with the remaining ingredients the same as described in Example 1. The electrolyte was adjusted for a pH of about 3.5 and the concentration of contaminating ions was as follows: iron ions about 0.298 g/l; nickel ions about 0.188 g/l and zinc ions about 0.047 g/l.

A test panel which had been plated in a conventional Watts-type bright nickel bath at a current density of about 45 ASF (4.95 ASD) at 1 450F (630C) for a period of 10 minutes to provide a bright nickel deposit of about 0.3 mii thickness, after water rinsing was plated in the foregoing contaminated trivalent chromium electrolyte for a period of 3 minutes at 750F (24"C) and a cathode current density of about 100 ASF) 11 ASD). The chromium deposit was iridescent in the intermediate current density areas and had black streaks in the high current density areas. The chromium deposit was considered unacceptable from a commercial standpoint due to the high level of contaminating metal ions therein.

The contaminated trivalent chromium electrolyte was thereafter treated by the addition of 3.8 g/l of sodium diethyldithiocarbamate trihydrate for a period of 1 hour at 750F (240 C) with continuous stirring. The electrolyte after settling, was thereafter filtered through a carbon filter and the filtrate was analyzed for residual metal ion contaminants present in the treated electrolyte. Upon analysis, the iron ion concentration was found to be about 0.164 g/l, the nickel ion concentration was nil and the zinc ion concentration was found to be about 0.0004 g/l. Accordingly, about 45 percent of the iron ions were removed, about 100 percent of the nickel ions were removed while about 99 percent of the zinc ion contaminant was removed.

A nickel plated test panel of the type hereinabove described was then plated in the treated and filtered solution under the same conditions as employed for the untreated solution.

The chromium deposit was overall bright and of normal and uniform appearance. The plating deposit was considered entirely commercially acceptable. The plating test further indicates that the presence of excess sodium diethyldithiocarbamate in the electrolyte following the treatment does not adversely affect the performance of the trivalent chromium plating solution. This example clearly evidences the effectiveness of the present invention in rejuvenating a metal ion contaminated trivalent chromium electrolyte the performance of which has been rendered commercially unsatisfactory whereby the electrolyte is restored to provide satisfactory chromium deposits.

Example 5 An experimental treatment of a commercial trivalent chromium electrolyte comprising a 4,000 gallon (18200 litre) bath was made having a normal composition corresponding to that described in Example 1. The performance of the electrolyte had become impaired due to the accumulation of iron and nickel contaminating ions during normal electroplating operations.

Analyses of samples of the electrolyte before treatment and following treatment were made for iron ions, nickel ions and vanadium ions present as a reducing agent. The results of the analyses are as follows: Before After Treatment Treatment Fe 1.310 g/l 1.000 g/l Ni 0.501 g/l 0.351 g/l V 0.238 g/l 0.238 g/l The purification treatment of the contaminated electrolyte was carried out by the addition of 30 gallons (136.5 litres) of a solution containing a concentration of about 350 g/l of sodium diethyldithiocarbamate. While the bath was not being electorlyzed and during continuous filtration, the solution of additive agent was added. Immediately following the addition of the aqueous additive solution, a dark precipitate formed which, surprising, instead of increasing filter back pressure actually resulted in a reduction in filter back pressure. As a result, the precipitate was easily removed and satisfactory operation of the electrolyte was restored within about 1 hour after treatment providing commerically satisfactory chromium deposits.

The precipitate recovered by the filtration was analyzed and found to contain about 66.7 mol percent iron diethytdithiocarbamate and 33.3 mol percent nickel diethyldithiocarbamate. The selectivity of the additive agent for extracting contaminating metal ions is evidenced by the fact that no vanadium and no chromium was extracted from the bath during treatment. Similar tests employing ferrocyanide precipitating agents.

evidences a significant removal of both chromium and vanadium which are desirable constituents in the trivalent chromium electrolyte.

Claims

1. A method for purifying an aquoues trivalent chromium electrolyte containing a detrimental amount of contaminating metal ions which comprises the steps of adding, to the contaminated electrolyte, dimethyldithiocarbamate ions, diethyldithiocarbamate ions, or a mixture thereof in an amount effective to precipitate at least a portion of the detrimental contaminating metal ions to reduce their concentration in the electrolyte to a level at which satisfactory performance of the electrolyte is restored and thereafter removing the precipitate from the electrolyte.

2. A method as claimed in Claim 1 in which the dimethyldithiocarbamate ions and or diethyldithiocarbamate ions are added in the form of electrolyte soluble and compatible alkali metal or ammonium salts thereof.

3. A method as claimed in Claim 1 in which the dimethyldithiocarbamate ions and/or diethyldithiocarbamate ions are added as the sodium salt thereof.

4. A method as claimed in Claim 1,2 or 3, in which the dimethyldithiocarbamate ions and/or diethyldithiocarbamate ions are added in an aqueous solution.

5. A method as claimed in any one of Claims 1 to 4, including the further step of uniformly distributing the purifying agent thgroughout the electrolyte.

6. A method as claimed in any one of Claims 1 to 5, in which the step of removing the precipitate from the electrolyte is performed by filtration.

7. A method as claimed in any one of Claims 1 to 6, in which the dimethylthiocarbamate ions, diethylthiocarbamate ions or the mixture thereof ("the purifying agent") is added to the electrolyte in a weight ratio of 8:1 purifying agent to contaminating metal ions present.

8. A method for purifying an aqueous trivalent chromium electrolyte substantially as disclosed in any of the Examples.

9. An aqueous trivalent chromium electrolyte whenever purified by a method as claimed in any one of Claims 1 to 8.

10. A workpiece whenever plated by means of an electrolyte as claimed in Claim 9.