US3751347A

US3751347A - Reduction and control of trivalent chromium in hexavalent chromium processing solutions

Info

Publication number: US3751347A
Application number: US00137307A
Authority: US
Inventors: M Good; J Kraljic; W Hartford
Original assignee: Allied Chemical Corp
Current assignee: Honeywell International Inc
Priority date: 1971-04-26
Filing date: 1971-04-26
Publication date: 1973-08-07
Anticipated expiration: 1990-08-07

Abstract

A PROCESS FOR REDUCING THE QUANTITY OF TRIVALENT CHROMIUM IN HEXAVALENT CHROMIUM PROCESSING SOLUTIONS, WHICH INVOLVES AGITATING A SOLUTION CONTAINING THE TRIVALENT CHROMIUM WITH LEAD DIOXIDE AT A TEMPERATURE OF AT LEAST 40*C. AND SEPARATING THE RESULTING INSOLUBLE LEAD COMPOUNDS FROM THE SOLUTION.

Description

United States Patent US. Cl. 20451 laims ABSTRACT OF THE DISCLOSURE A process for reducing the quantity of trivalent chromium in hexavalent chromium processing solutions, which involves agitating a solution containing the trivalent chromium with lead dioxide at a temperature of at least 40 C. and separating the resulting insoluble lead compounds from the solution.

BACKGROUND OF THE INVENTION Field of the invention Trivalent chromium Cr(III) is often present as an unwanted contaminant in hexavalent chromium processing solutions, as for example, chromium electroplating (plating) solutions, and chromic acid anodizing solutions. Generally, the undesirable trivalent chromium is formed in situ by chemical and/ or electrochemical reduction of the hexavalent chromium and is detrimental to the process and/or product if not at least partially removed or oxidized to the hexavalent state.

Important among those hexavalent chromium proccessing solutions contaminated with trivalent chromium are the plating solutions or baths used for the electrodeposition of metallic chromium and chromium-containing electroplates, for various concentrations of trivalent chromium Cr(III) are found in all operating chromium plating baths.

Low concentrations of Cr(IH) do not interfere with the satisfactory operation of such baths or adversely affect the quality of the chromium deposits produced. Excessive concentrations of Cr(III), however, can significantly increase the power requirements and also lead to a poor chromium plate. It is apparent, therefore, that the Cr(IH) concentration in chromium plating baths should be maintained below certain limits.

Chromium plating techniques differ from those of most other plating procedures in that the anodes employed are not composed of the depositing metal. In chromium plating the anodes are frequently fabricated of lead or of lead alloyed with antimony and/ or tin.

The presence of lead in the anode and the relation between the surface area of the anode to that of the cathode, significantly affect the concentration of Cr(III) in the bath. It has, in fact, long been known that the Cr(III) concentration can effectively be controlled in a chromium plating bath by maintaining a proper ratio of lead anode 'to cathode surface area.

The mechanism responsible for controlling the concentration of Cr(III) is dependent on the formation of a film of lead dioxide (PbO- which is deposited on the surface of the anode during the plating period. Lead and lead alloy anodes are insoluble in chromium plating baths and the PbO film which forms, oxidizes Cr(IH) to hexavalent chromium Cr(VI). The greater the anode surface area, the greater the amount of PbO which forms, and consequently the more Cr(III) that is oxidized. Thus, under ideal conditions, the anode surface area can be adjusted to maintain the Cr(III) concentration within proper limits.

Chromium plating is usually applied to articles that permit the use of a relatively large ratio of anode area to cathode area. In the field of decorative plating, for example, conventional anodes are frequently used and inherently provide a high ratio of anode to cathode area, often as high as about four to one. Under these conditions the concentration of trivalent chromium is kept relatively low when a steady state is established between the reduction of Cr(VI) to Cr(III) at the cathode, and the oxidation of Cr(III) to Cr(VI) at the anode. Plating on the inside of tubes or cylinders, however, necessarily involves a smaller ratio of anode to cathode area than that normally employed in decorative plating. When plating with inside anodes where the surrounding work piece to be plated is the cathode, the surface area of the anode is often only about a quarter of the surface area of the cathode. Such a condition favors the rapid formation of Cr(III). When the Cr(III) content exceeds about 11 grams per liter (g.p.l.), a drop in bath conductivity occurs, requiring a higher electromotive force (EMF) in order to maintain the current density. A high Cr(l'II) content also leads to grayness in the chromium plate, and the development of nodules as the thickness of the plate is increased. In any application of chromium plating of the latter type, control of the trivalent chromium in chromic acid baths becomes particularly important.

Description of the prior art In the past, various methods have been used or sug gested to overcome this problem of excessive Cr(III) concentration. Paul Morisset, Chromium Plating, Robert Draper, Ltd. (1954) refers to a so-called dechromator,

designed to reduced the Cr(III) concentration. This de- I vice employs a small cathode in a porous container which is hung on the cathode bar of an electrolytic chromium plating cell employing a large anode area and operating at high current density. In operation, the Cr(III) in the bulk of the solution is oxidized to the Cr(VI) form while that formed at the cathode is retained in the porous container. Whereas this device offers a partial solution, it is still used in conjunction with an anode of large area. R. Seegmiller and V. A. Lamb, (Proc. A.E.S., 1948, pp. 125-132), also investigated reoxidation of Cr(III) empolying a chromium plating bath at a high cathode and low anode current density, which again, is the result of using an anode with a large surface area relative to that of the cathode. A bath temperature of C. is recommended. The times required to reduce the Cr(IH) content to a reasonable level are impractically long (about 22 hours at 80 C. to as much as 126 hours at 20 C.). From the foregoing it would appear that reoxidation of Cr(IH) is time consuming and that it cannot be carried out under those plating conditions previously discussed, which lead to high Cr(IH) concentrations. Although in the past it had been known that anodes in a plating bath, having a coating of lead dioxide, would oxidize Cr(IH), this oxidation had always been carried out in an electroplating cell, and the oxidation was considered to be es" sentially an electrolytic oxidation.

We have now found that the addition of (PbO to a chromium plating bath containing Cr(III) will oxidize it to Cr(VI), and surprisingly, that it will do so much more rapidly than is possible by the electrolytic method, employing the PhD; coating on the anode, even when the surface area of the anode exceeds that of the cathode by a considerable amount. The addition of PbO to the bath therefore unexpectedly provides a method of rapidly reducing the Cr(III) content to an acceptable level. Whenever the concentration of Cr (III) is of critical importance, additions of PbO may offer the only means of control.

This oxidizing action of the PbO is entirely independent of the flow of current and may be used in those situations where the anode surface is unavoidably less than that of the cathode, and conditions are therefore conducive to the production of the undesirable Cr(HI).

SUMMARY OF THE INVENTION In accordance with the process of the present invention, the quantity of Cr(III) in hexavalent chromium processing solutions is reduced by the method comprising the steps of:

(a) adding PbO to the solution;

(b) agitating the solution at a temperature of at least 40 C. until the Cr(III) in the solution is reduced to the desired concentration; and

(c) removing resulting insoluble lead compounds from the solution.

The method of this invention, as contrasted with prior art methods, provides a rapid and elfective means of reducing the amount of Cr(III) in chromium processing solutions to acceptable levels without the introduction of impurities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of our invention is applicable to any hexavalent chromium processing solution where trivalent chromium is present, or formed in situ, in undesirable quantities. Typical of such processing solutions are chromium plating baths to which the present invention is particularly directed. The following description of the preferred embodiments has therefore been directed to chromium plating baths, although it should be understood that the process may be similarly applied to any hexavalent chromium processing solutions where it is desirable to control and reduce the quantity of trivalent chromium by chemical oxidation, such as chromic acid anodizing solutions.

In carrying out the process of our invention, a quantity of Pb is added to a hexavalent chromium processing solution sufficient to convert trivalent chromium to hexavalent chromium. The solution is agitated at a temperature of at least 40 C. until the desired trivalent chromium content in the solution is reached, and the resulting insoluble lead compounds are removed from the solution. An excess of Pb02 above the stoichiometric amount in terms of the reaction between Pb0 and Cr(III) would not be detrimental to a plating bath, but it would serve no useful purpose and would raise the cost of the treatment. The addition of any quantity of PIbO; less than the stoichiometric amount would correspondingly reduce the amount of Cr(III) present but the amount of reduction would obviously be limited by the quantity of PbO introduced. Preferably, the PbOg added is essentially the stoichiometric quantity based on the oxidation reaction:

The plating bath is maintained at a temperature of at least 40 C., generally at about 40 to 90 C., wth a temperature of about 65 C. to 75 C. being preferred. The solution is agitated until the concentration of trivalent chromium has dropped to an acceptable level. Since PbO' is a dense, rapidly settling powder, the degree of agitation should be sufficiently vigorous as to keep the PbOg in suspension. The period of agitation required to reduce the Cr(HI) to an acceptable level may vary between 5 and 120 minutes with about 50-70 minutes being required at a bath temperature of 65-75 C.

The concentration of Cr(III) in the plating bath should generally not exceed about 11 g.p.l. The total concentration of iron and trivalent chromium, however, should generally not exceed about 22 g.p.l. as Fe. Thus, if

no iron is present, the maximum allowable level of Cr(IH) would be about 11 g.p.l. If the iron concentration was 15 g.p.l., then the allowable Cr(HI) concentration would generally be about 7 g.p.l. These limitations, however, are suggested as a guide. The plating technologist will, through experience, determine the maximum level of Cr(III) which he considers tolerable.

Following the treatment of the bath with P1302 the solution is separated from insolubles comprising accumulated lead compounds by decantation, filtration or other conventional means.

In a specific embodiment of the present invention, the bath liquor can be continuously treated during the plating operation so as to maintain the Cr(III) content at an acceptable level, thus providing a bath having good electrical conductivity and capable of providing a chromium electroplate free of the defects generally attributable to an excessive amount of Cr(III). This may be accomplished by circulating the chromium plating solution through a bed of lead dioxide. The quantity of lead dioxide is not critical since even a small charge of PbO will oxidize a quantity of Cr(III) commensurate with the amount of said charge. In order to provide a bed which will function for an appreciable time without replenishing the P130 however, a quantity of PbO: in excess of the stoichiometric amount is preferred.

The plating bath solution is continuously circulated through the PbO bed and returned to the plating bath. The temperature of that portion of the bath solution which is circulated through the bed is at least 40 C., generally about 40 to C., with a temperature of 65 to 75 C. being preferred. In order to minimize the reaction time when using this procedure, the plating bath solution is prewarmed to 65 75 C. and maintained at this temperature while circulating through the bed, then preferably cooled to essentially the plating bath temperature (generally between 45 and 55 C.), and returned to the plating bath.

The PbO bed may take many forms, it may comprise a column requiring no agitation beyond the circulation of the plating bath solution through the bed, or it may consist of a vessel wherein the rapidly settling lead dioxide is kept suspended either by the incoming stream of bath solution, forcefully pumped into the vessel or by a rotating impeller or both. With such an agitated PbO- bed, separation of the PbO from the recirculating solution may be accomplished by allowing the suspension to pass through a quiescent zone wherein the PbO settles back to the agitated bed, and an essentially clear solution returns to the plating bath. A clarifying filter may be included in the system if desired. By arranging two beds which may be alternately valved into the system, maintenance of a low Cr (III) content in the plating bath may be effected by one bed While the other is being recharged.

The use of lead dioxide as a chemical oxidant offers several advantages over other oxidants; for, unlike most, it can be used in the manner described without introducing contaminants into the plating bath. Although oxidants such as permanganates and persulfates are efiective, they introduce unwanted impurities. In the latter case, their use may raise the sulfate level above that required for acceptable chromium electroplates. Furthermore, an excess of sulfate is contraindicated where it is important to reduce the Cr(III) content, and the excess sulfate would have to be precipitated (as, for example, with barium carbonate). With any of these oxidants, the plating operation cannot be carried on while the trivalent chromium is being oxidized.

Another major benefit that may be derived from the use of PhD; is that cation exchange resins may be employed for the removal of contaminants such as copper and iron. The reduction of trivalent chromium by the use of PbO would improve the efficiency of the resin and make ion exchange more economically attractive than would otherwise be the case.

The following specific examples further illustrate our invention. Parts are by weight except as otherwise noted.

EXAMPLE 1 This example is carried out to experimentally demonstrate the reaction kinetics involved in the reduction of Cr(III) with PbOg- A portion of a typical chromium plating bath having an initial Cr(III) concentration of 8 g.p.l. is first maintained at 0 C. and the stoichiometric weight of Pb0 (55 grams), is added to it. The mixture is agitated for 30 minutes, then cooled rapidly to room temprature and the supernatant liquor decanted and analyzed for its Cr(III) content. (One analytical method for determining the Cr(III) content in a chromium plating bath is a volumetric method employing standard solutions of 0.1 N Ce(SO in sulfuric acid, and 0.05 N NaNO The method is derived from Willard and Young, Trans. Electrochemical Soc. 67 (preprint) 1935.) The same procedure is repeated at the same temperature and with portions of the same typical plating bath, for a period of 60 minutes, and again for a period of 120 minutes. The results are plotted to produce Curve A of the accompanying drawing. Next, a series of determinations are made covering the same time intervals but with the solution maintained at 70 C. Finally a third series of determinations is similarly carried out at 90 C. The curves therfore trace the changes in the Cr(III) concentration with time and temperature. At 50 C. the reaction rate is found to be quite slow and after 120 minutes the concentration of- Cr(III) has decreased from 8 to 4.6 g.p.l. At 70 and 90 C. the reaction rates are much faster, and the curves are essentially superimposed, indicating that the reaction rates at 70 and 90 C. are substantially the same. In both instances, after 1 hour the concentration of Cr(III) has decreased to about 1.5 g.p.l. The reoxidation rate constantly decelerates so that after an additional hour the concentration of Cr(III) has decreased by only 0.5 g.p.l. At 70 0., therefore, the concentration of Cr(III) can be brought to an acceptable level within 60 minutes.

EXAMPLE 2 100 gals. (378 liters) of a chromium plating bath conainting 250 g.p.l. chromic anhydride (CrO 1.0 g.p.l. sulfate (SO and 2.0 g.p.l. fiuosilicate (SiF is found by analysis to contain 14 g.p.l. Cr(III). The bath is heated to 70 C. and 80 lbs. (36.2 kilos) of lead oxide (P1302) are stirred in. The bath is agitated for 60 minutes at 70 C., separated from the lead oxide by decantation and cooled to an operating temperature of about 50 C. Analysis of the clear bath liquor indicates a Cr(III) content of 4.9 g.p.l.

EXAMPLE 3 The same bath as above is put into essentially continuous plating service in a situation wherein the area of the anode is only about one-fourth of that of the cathode, the arrangement being conducive to the formation of a relatively high concentration of Cr(III). A portion of the bath liquor is continuously withdrawn while the plating bath is in operation, at the rate of about three gallons per minute, by means of a pump. It is passed through a heat exchanger wherein the temperature is raised to 70 C., then into a thoroughly agitated bed comprising 1.5 times the stoichiometric quantity of PbO also maintained at about 70 C. The solution then passes into a quiescent zone wherein essentially all of the PbO powder settles back to the bed and the solution passes through a second heat exchanger wherein the temperature is dropped to 50 C., the working temperature of the 6 bath. The essentially clear solution is continuously returned to the bath.

Two such PbO beds are used alternately so that as the concentration of Cr(IH) builds above the limit chosen (11 g.p.l.), the second bed is valved into the system, and the first cut out for recharge. Analysis of the bath liquor indicates a fairly constant Cr(IH) content of 5 g.p.l. As the bed becomes exhausted, the Cr(III) content of the plating bath slowly climbs to 11 g.p.l. at which point the bed is valved out of service.

Although certain preferred embodiments of the invention have been disclosed for purposes of illustration, it will be evident that various changes and modifications may be made therein without departing from the scope and spirit of the invention.

We claim:

1. A method for maintaining the trivalent chromium content of an acid chromium electroplating bath within a concentration range of from about 1 to 11 grams per liter, which consists essentially of the steps of contacting said bath with an amount of lead dioxide suflicient to convert said trivalent chromium to hexavalent chromium while maintaining said bath while in contact with said lead dioxide at a temperature from 40 to 90 C. until the desired trivalent chromium content of the bath is attained, and removing resulting insoluble lead compounds from the bath.-

2. The method of claim 1 wherein the temperature of the solution is maintained between 65 and C.

3. The method of claim 1 wherein the amount of lead dioxide used is essentially the stoichiometric amount with respect to trivalent chromium present in the bath.

4. The method of claim 1 wherein the process is carried out continuously on a portion of the acid chromium electroplating bath withdrawing from said bath, passing said portion through a bed of lead dioxide and returning said treated portion to said bath.

5. The method of claim 4 in which the plating solution is circulated through the lead dioxide bed while the plating bath is in operation.

6. The method of claim 4 in which the portion of the bath withdrawn is adjusted to between 40 and C. before entering the lead dioxide bed and is readjusted to substantially the temperature of the plating bath before being returned to said bath 7. The method of claim 4 in which the portion of the bath withdrawn is warmed to between 65 and 75 C., before entering the lead dioxide bed and is cooled to substantially the temperature of the plating bath before being returned to said bath.

8. The method of claim 4 wherein the bed of lead dioxide is agitated in the presence of the portion of the plating bath being treated.

9. The method of claim 4 wherein the lead dioxide bed contains at least the stoichiometric quantity of PhD; with respect to the Cr(III) present in the bath.

References Cited UNITED STATES PATENTS 743,668 11/1903 Suchy et a1. 204-89 2,600,171 6/1952 Sagen 204-51 2,708,618 5/ 1955 Schwenzfeier 23-57 X FREDERICK C. EDMUNDSON, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,751,347 I Dated August 7, 1973' Inventofls) John Kraljic, Winslow H. Hartford & Millard F. Good It is certified that error appears in the above-#"identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 4, Column 6 line 35, "withdrawing" should be withdrawn Signed and s ealed this 27th day of November 1973.

(SEAL) Attest':

EDWARD MELBTCHERJR. RENE TEGTTY/IIZYER Attesting Officer Actifig coamnissioner of Patents