US3334033A - Chromium plating - Google Patents

Description

United States Patent 3,334,033 CHROMIUM PLA'HNG Edward A. Romanowski, Troy, and Henry Brown, Huntington Woods, Mich., assignors to The Udylite Corporation, Warren, Mich., a corporation of Delaware No Drawing. Filed Aug. 6, 1965, Ser. No. 477.937 22 Claims. (Cl. 20451) This application is a continuation-in-part of our c0- pendin-g application, Ser. No. 395,892, now abandoned, filed Sept. 11, 19 64.

This invention relates to the electrodeposition of chromium from aqueous acidic exavalent chromium solutions, and especially to the use of saturation concentrations of certain. rare earth salts in solutions containing the sulfate ion to make possible improved chromium plate from operationally simplified baths. The invention also relates to the use of fluorocarbon acids in these improved chromic acid baths to provide baths which give further improved covering power in recessed areas (low current density areas). Not only is the chromium covering power improved, but no strong discoloration or iridescent films are formed where the chromium plate leaves off in the very low current density areas.

In its broad aspects, the invention contemplates the use of saturation concentrations of certain rare earth salts in chromium plating baths containing from about 100 to about 500 grams per liter of chromic acid with a chromic acid to sulfate ion ratio of from about 75 to 1 to about 300 to 1. With a highly preferred embodiment, the bath contains saturation concentrations of strontium sulfate, and still further improvements can be obtained by including certain fluorocarbon acids in the bath.

The salts useful in the plating baths of this invention are fluorine-containing salts of certain rare earth metals includingsalts of praseodymiurn, neodymium, lanthanum, samarium and gadolinium. As used herein, the term rare earth is also used to include yttrium. Though this latter element is a Group IIIb element, it has properties similar to and is found associated with rare earths in nature. The useful salts are the fluorides or complex fluorides of these metals as hereinafter defined.

The chief commercial source of the rare earth metals is the naturally occurring phosphate ore known as monazite. In addition to the rare earth metals, this ore also contains among others, thorium and yttrium. The nomenclature and separation techniques involved in the refining of the monazite ore are defined in a publication of the Lindsay Division of The American Potash and Chemical Corporation entitled, Thorium, Rare Earth and Yttrium Chemicals.

It has now been found that fluorides and/ or complex fluorides of certain rare earth mixtures derived from the monazite ore are particularly useful for the purpose of this invention. These mixtures as defined in the above publication are termed, didymium salt, neodymium salt and lanthanum salt. Didymium refers to the mixture of rare earths obtained after removal of cerium and thorium from the natural mixture of rare earths found in the monazite ore. The approximate composition of the mixture, in terms of the oxides, is 40-45% La O 812'% Freon, Nd203, $111 03, Gd203, and trace amounts of other rare earth metal oxides.

The didymium mixture may be further processed and a lanthanum portion removed therefrom. This fraction, mainly lanthanum oxide, may also be used to provide the desired fluoride salts. Similarly, the remainder of the didymium mixture from which the lanthanum has been removed can be converted to the fluorides or complex fluorides and included in the baths of this invention. This mixture, termed as neodymium salts by the 3,334,033 Patented Aug. 1, 1967 Lindsay publication has the following composition: 65- 70% Nd O 1216% Pr O 1013% Sm O 35% Gd O and trace amounts of other rare earth oxides.

The neodymium salts can be further refined to provide a variety of mixtures of the remaining rare earth metals or the individual rare earth metals can be obtained therefrom. While the fluorides of any of these mixtures or the fluorides of the remaining individual rare earths can be used in the baths of this invention, the cost of further separation becomes increasingly higher. Thus the preferred salts of this invention are the fluorine containing salts of didymium, neodymium and lanthanum as defined above. These mixtures as the oxide, the carbonate, chloride, fluoride, etc., are available commercially.

The trivalent rare earth ions and especially the didymium ions form a fluoride (a mixture of LaF NdF PrF SmF GdF3) which was found to have the desired low solubility in dilute or concentrated chromic .acid solutions at temperatures in the range of 2080 C.

Tetravalent ceric fluoride (or trivalent cerous fluoride which is oxidized to the tetravalent form at the lead anode) is too soluble in chormic acid solutions and when used in saturation concentrations contributes excessive fluoride ions resulting in a diminution in covering power of the chromium plate.

It was found that saturation concentrations of mixtures such as didymium fluoride, neodymium fluoride and lanthanum fluoride as well as the individual rare earth metal fluorides enumerated above gave excellent results by providing ideal controlled concentrations of fluoride or complex fluoride anions in chromium plating baths having a chromic acid to sulfate ion ratio of from about to 1 to about 300 to 1. With a preferred bath, the sulfate ion is present in a saturation concentration as strontium sulfate and the chromic acid to sulfate ion ratio is from about to 1 to about 250 to 1. With such solutions, there is no need of using suppressing salts for the fluoride or complex fluoride as required by prior art teachings.

As previously pointed out, the invention contemplates the use of certain rare earth metal fluorides, and more particularly didymium, lanthanum or neodymium fluorides or complex fluorides of these rare earths such as fluosilicates, fiuoborates, fluoaluminates, fluotitanates, fluozirconates, etc., in chromic acid plating baths. These fluorides, or complex fluorides may be added as such to the plating bath. Alternatively, they may be formed in situ by adding salts of these metals, preferably the carbonate or hydrated oxides (hydroxides) to chromium plating baths containing a more soluble fluoride or complex fluoride. As used hereinafter, unless otherwise stated, the term fluoride shall be understood to also include complex fluorides as defined above.

In order to electrodeposit chromium from acidic hexavalent chromium solutions, it is well known that small amounts of certain anions such as sulfate or fluoride ions termed catalyst ions must be present. Actually some sulfate ion is required in the bath before fluoride ions cooperate to act as a catalyst. The use of mixed catalyst anions, such as combinations of sulfate ions with fluoride ions, or with boric acid has been previously suggested in chromium plating baths. In this connection, reference is made to US. Patents 1,844,751, 1,864,013, 1,864,014, 1,952,793, 2,042,611, 2,063,197, 2,640,021, 2,640,022 and 2,952,590. In particular, reference is made to Lukens US. 2,042,611, Passal US. 2,640,021 and Stareck US.

2,640,022 and 2,952,590 for the development of the selfcontaining up to about 200 grams per liter of CrO Passal improved on this by adding sparingly soluble potassium silicofluoride (fluosilicate) to the bath containing saturation concentrations of strontium sulfate. Stareck made further improvements by additionally using mixed suppressing agents such as strontium chromate and potassium dichromate to better control the concentrations of the sulfate and silicofluoride ions respectively by common ion effects. Stareck also employed fluoaluminate, fluotitamate and fluozirconate ions instead of fluosilicate to use with controlled sulfate ion concentrations.

The standard or conventional chromium bath employing only the sulfate anion as catalyst and used for plating on nickel, iron, yellow brass or copper has utilized a chromic acid anhydride (CrO to sulfate ion ratio of about 100 to 1. Thus, in a 200 grams per liter CrO bath, 2 grams per liter of sulfate ion would be used. If one uses instead a CrO to $0.; ratio of 200 to 1 or higher and plates on top of nickel, copper, yellow brass or steel, instead of chromium plate, only an iridescent non-metallic chromium chromate (rainbow) film is obtained when dead electrical entry is used.

A specific example is a bath containing about 340 grams/ liter of chromic acid and saturated with strontium sulfate at about 50 C. If chromium plating is attempted on top of a freshly plated bright nickel surface using a dead entry into such a bath, no chromium plating is obtained, but only a non-metallic iridescent film of a basic chromic chromate results. The term dead entry refers to the technique wherein the plating current is turned on only after the nickel plated work is immersed in the bath.

It has now been found that saturation concentrations of certain rare earth fluorides as defined above are unusual in cooperating with the sulfate ion in the chromic acid baths to produce bright chromium plate of exceptional covering power on nickel, brass, copper and steel. Using such a bath, it is possible to deposit bright chromium plate over a very wide cathode current density range. Outstanding results are obtained using saturation concentrations of the rare earth fluorides in combination with saturation concentrations of strontium sulfate.

The rare earth fluoride is used in saturation concentrations and preferably an excess should be present undissolved in the bath. From 5 to grams/liter are more than sufficient to provide this excess. Saturation concentrations of the rare earth fluorides in the chromic acid baths do not cause any harmful effect in conjunction with saturation concentrations of strontium sulfate, such as greatly diminishing the covering and throwing power of the chromium deposit and producing dull white areas in the high current density chromium plate. In this very important respect, the saturation concentrations of rare earth fluorides such as didymium or neodymium fluoride act quite differently than saturation concentrations of other inorganic fluorides which were tested, such as calcium fluoride, strontium fluoride, ceric or cerous fluoride, potassium or sodium fluosilicate (silicofluoride), etc. These latter fluorides and silicofluorides when used in saturation concentrations in chromic acid solutions with saturation concentrations of strontium sulfate diminish the covering and throwing power of the chromium plate and cause white areas in the high current density chromium plate. It was for this reason that with prior art baths, suppressing agents such as potassium dichromate were required in conjunction with saturation concentrations of fluorides such as potassium silicofluoride. In addition to the disadvantages of handling another material and introducing one more variable into the bath, the presence of high concentrations of potassium ions tend to salt out the important and valuable anti-misting agent, perfluoro n-octyl sulfonic acid, which is often present in chromic acid baths. With the baths of this invention, the saltingout problem is totally eliminated in that the need for agents to suppress the fluoride ion is totally eliminated.

The covering power of the chromium plate is maximum when it is plated over a bright nickel deposit obtained from a freshly prepared bath or a bath that has been treated with activated carbon. The chromium plate applied to such bright nickel has excellent covering power and is the least susceptible to whitish streaks. In contrast, when the nickel bath has been heavily used and has not been treated with activated carbon for long periods of time, the covering power of the chromium plate is definitely diminished. The bright nickel deposits from the latter nickel plating baths are more passive. This condition often requires a higher concentration of sulfate ion or fluoride ion in the chromium plating bath than would otherwise be necessary to obtain a good bright chromium plate free of stained low current density areas. These higher concentrations of catalyst ions, however, reduce the covering power of the chromium plate. Nevertheless, it is often necessary despite reduction of the covering power to use higher catalyst concentrations to eliminate or minimize staining in the low current density areas. For this purpose, a very serious problem is presented in using relatively soluble fluorides such as ceric fluoride, ceric fluoborate, calcium fluoride, etc., to supply extra fluoride ions to the bath to reduce the passivity of the nickel plate. Not only is it difficult to analyze for the proper amount of fluoride in the bath, but if an excess is inadvertently added, it is diflicult to reduce the concentration to the proper level without resorting to dilution of the bath.

With baths of the present invention, it is not possible to add an excess of fluoride ion because of the limited solubility of the particular rare earth fluorides of this invention. It is thus possible to keep a constant concentration of fluoride ion in the chromium plating bath, and to increase the concentration of sulfate ion to take care of excessive passivity of the nickel plate. The sulfate ion concentration can easily be determined, and if an excess is accidentally used, it is easily reduced by addition of strontium chromate or carbonate. In general, saturation concentrations of strontium sulfate provide the proper concentration of sulfate ion to cooperate with fluoride ions provided by the saturation concentration of the particular rare earth fluorides described above. This is especially true for the lower range of bath concentrations of chromic acid.

The problem of chromium plating on a relatively passive bright nickel can be further minimized by including in the chromium plating bath from about 0.5 to 5 grams per liter of an aliphatic or cycloaliphatic fluorocarbon acid such as fluorocarbon sulfonic acid or a fluorocarbon phosponic acid. Examples of these activators include perfluorocyclohexyl sulfonic acid, perfluoro para methyl cyclohexyl sulfonic acid, perfluoro para ethyl cyclohexyl sulfonic acid, perfluoro succinic acid, perfluoro methyl sulfonic acid or a fluoroalkyl phosphonic acid such as H(CF CF ),,PO(OH) where m=1 to 3 inclusive. These activators not only are effective with the more passive nickel surfaces to minimize gray in the high current density areas, but also act to increase the chromium coverage.

The acidic hexavalent chromium plating baths may be made up from straight chromic acid anhydride or chromic acid, or from mixtures with dichromates, chromates, and polychromates. It is generally preferred to use straight chromic acid or chromic acid anhydride. The presence of cations such as Na, K, -Li, Mg and Ca are best kept low in concentration, especially K and Na.

Below are listed several examples of the chromic acid baths of this invention. Where strontium sulfate is employed at saturation concentrations in the chromium plating baths, strontium chromate, bichromate or carbonate can also be added to partially suppress the sulfate ion concentration. This is often desirable in the low metal baths such as those employing about to about 200 grams/ liter of chromic acid anhydride.

5 EXAMPLE I 150 to 340 grams/ liter of chromic acid anhydride (CRO' Saturation concentrations of SrSO (excess present), a ratio of chromic acid to sulfate of about 120 to 1 to about 250 to 1-SrCrO or srCr O at to 10 grams/ liter Saturation concentrations of didymium fluoride (2 to 10 grams/liter, an excess is present), or the fluosilicate Temperature--115130 F. (46-55 C.)

EXAMPLE II 150 to 400 grams/liter of CrO Saturation concentrations of SrSO, (excess present), a ratio of chromic acid to sulfate ion of about 120 to 1 to about 250 to 1 Saturation concentrations of didymium fluoride or fluosilicate or fluoborate (2 to 6 grams/liter, an excess is present) 1 to grams/liter of perfluoro para ethyl cyclohexyl sulfonic acid Temperature-105140 F.

EXAMPLE III EXAMPLE IV 200 to 400 grams/liter CrO Saturation concentrations of neodymium fluoride or flu'osilicate (2 to grams/liter, an excess is present) Saturation concentrations of strontium sulfate, 120 to 1 to about 300 to 1 of chromic acid to sulfate ion 1 to 5 grams/liter of perfluoro para ethyl cyclohexyl sulfonic acid 0 to 6 grams/liter of CF SO H and/or where n=an integer from 1 to 3 inclusive Temperature105-140 F.

EXAMPLE V 200 to 400 grams/liter chromic acid anhydride 0.5 to 4 grams/ liter (saturation concentration) of didymium fluoride 4 to 6 grams/liter strontium sulfate (saturation concentration) 4 to 10 grams/ liter of strontium chromate 0.5 to 5 grams/liter of perfluoro p-ethyl cyclohexyl sulfonic acid 0 to 5 grams/liter of H(CF CF PO (OH) where 11:1

to 3 inclusive Temperaturel-O0-140 F.

EXAMPLE VI 100 to 500 grams/liter chromic acid anhydride 1 to 5 grams/liter (saturation concentration of didymium fluoride) 0.3 to 4 grams/liter sulfuric acid (an amount sufficient to have a CrO /SO ratio of about 120-300 to 1) Temperature100-l40 F.

Examples VII-IX which follow are included to show by direct comparison the superiority of the plating baths of this invention as compared with a bath utilizing a relatively soluble metal fluoride. With these examples, three 3" x 4" brass panels were first plated in an identical manner using a standard bright nickel bath. One panel was then plated in a chromium plating bath of this invention utilizing didymium fluoride (Example VII). The second panel was plated in the identical chrome bath of Example VII, but wherein didymium fluoride was replaced with cerium tetrafluoride (Example VIII). The third panel Was plated in a bath as in Example VIII, but wherein the 80., concentration was decreased to provide a CrO /SO ratio of 30 0/ 1 (Example IX). The chromium plating was carried out in a standard Hull Cell. This cell is widely used by the plating industry to investigate and compare the throwing (or covering) power of various plating solutions. The test panel is placed at an oblique angle to the anode and plated for a period of 5 minutes. The percentage of the panel plated provides a direct measurement of the covering power of the plating bath. These tests are especially useful in comparing the covering power of several different baths of the same metal.

EXAMPLE VII 300 grams/ liter CrO 10 grams/liter didymium fluoride (greater than the saturation concentration) 4 grams/liter strontium sulfate (greater than the saturation concentration) Temperaturel20 F.

After 5 minutes of plating, approximately of the panel was plated with a clear, brilliant plate.

EXAMPLE VIII EXAMPLE IX 300 grams/ liter CrO 10 grams/ liter CeF 1 gram/ liter H 80 Temperaturel20 F.

With this example, the CrO /SO ratio was increased to about 300 to 1 in an attempt to improve the covering power of the bath of Example VIII. While approximately 70% of the panel was plated after 5 minutes, the plate was a poor quality showing whitish cloudy areas throughout. Such a plate is definitely not acceptable for a commercial application on bright nickel.

EXAMPLE X to 300 grams/liter CrO 0.5 gram/liter up to saturation concentration of didymium fluosilicate, didymium fluoride or didymium fluoborate Strontium sulfate or H 80 in amounts to give CrO S0 ratios of from about 75 to 1 to about 300 to 1.

Another advantage of the baths of this invention is that there is negligible etching of unplated steel. For example, a steel plate immersed for a week in these chromium baths containing the slightly soluble didymium or neodymium fluorides or fluosilicates present in saturation concentrations is affected to a far less degree compared to other chromium baths containing the more soluble fluorides and/or fluosilicates. This is very important in hard chrome (thick chromium) plating applications.

The particular rare earth fluoride or fluorides can be added directly to the bath as such, which is the preferred method, or formed from the carbonate or the hydrated rare earth oxide. Instead of using the particular rare earth hydroxide or carbonate, the chromate, bichromate, or the sulfate of the particular rare earths used in this invention can be formed into the fluorides, fluosilicates, fluoaluminates, fluoborates, etc., by reaction with hydrofluoric acid, fluosilici-c acid, etc. If excess sulfate ions are present they can be reduced in concentration by adding strontium chromate or carbonate or hydroxide.

We claim:

1. A bath for the electrodeposition of chromium plate comprising about 100 to 500 grams per liter of chromic acid, sulfate ion in a concentration sufficient to have a chromic acid to sulfate ion ratio of from about 75 to 1 to 300 to l, and saturation concentrations of fluorinecontaining salt of a rare earth metal selected from the group consisting of neodymium, praseodymium, lanthanum, gadolinium, sarnarium, yttrium, and mixtures of said salts.

2. A bath in accordance with claim 1 containing saturation concentrations of strontium sulfate.

3. A bath in accordance with claim 1 wherein said salt is a didymium salt.

4. A bath in accordance with claim 1 wherein said salt is didymium trifiuoride.

5. A bath in accordance with claim 1 wherein said salt is didymium fiuosilicate.

6. A bath in accordance with claim 1 wherein said salt is a neodymium salt.

7. A bath in accordance with claim 1 wherein said salt is neodymium fluosilicate.

8. A bath in accordance with claim 1 wherein said salt is a lanthanum salt.

9. A bath in accordance with claim 1 wherein said salt is predominantly lanthanum fiuosilicate.

10. A bath in accordance with claim 1 additionally containing a saturated fluorocarbon sulfonic acid.

11. A bath in accordance with claim 1 additionally containing a saturated fluorocarbon phosphonic acid.

12. A method of electrodepositing chromium which comprises electrolyzing an aqueous acidic hexavalent chromium solution containing about 100 to 500 grams per liter of chromic acid, sulfate ion in a concentration sufficient to have a chromic acid to sulfate ion ratio of from about to 1 to about 300 to 1, and saturation concentrations of a fluorine-containing salt of a rare earth metal selected from the group consisting of neodymium, prasedymium, lanthanum, gadolinium, samarium, yttrium and mixtures of said salts.

13. A method in accordance with claim 12 wherein said bath contains saturation concentrations of strontium sulfate.

14. A method in accordance with claim 12 wherein said salt is a didymium salt.

15. A method in accordance with said salt is didymium trifluoride.

16. A method in accordance with said salt is didymium fluosilicate.

17. A method in accordance with said salt is a neodymium salt.

18. A method in accordance with said salt is neodymium fluosilicatc.

19. A method in accordance with said salt is a lanthanum salt.

20. A method in accordance with claim 12 wherein said salt is predominantly lanthanum fiuosilicate.

21. A method in accordance with claim 12 additionally containing a saturated fluorocarbon sulfonic acid.

22. A method in accordance with claim 12 additionally containing a saturated fluorocarbon phosphonic acid.

claim 12 wherein claim 12 wherein claim 12 wherein claim 12 wherein claim 12 wherein References Cited UNITED STATES PATENTS 1,864,014 6/1932 Ewing 20451 1,881,885 10/1932 Noble et al 204-51 2,640,022 5/1953 Stareck 2045l 2,787,588 4/1957 Stareck et al 204-51 2,950,234 8/1960 Johnson et a1. 204-5l FOREIGN PATENTS 882,936 7/1953 Germany.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

Claims (1)

1. A BATH FOR THE ELECTRODEPOSITION OF CHROMIUM PLATE COMPRISING ABOUT 100 TO 500 GRAMS PER LITER OF CHROMIC ACID, SULFATE ION IN A CONCENTRATION SUFFICIENT TO HAVE A CHROMIC ACID TO SULFATE RATIO OF FROM ABOUT 75 TO 1 TO 300 TO 1, AND SATURATION CONCENTRATION OF FLUORINECONTAINING SALT OF A RARE EARTH METAL SELECTED FROM THE GROUP CONSISTING OF NEODYMIUM, PRASEODYMIUM, LANTHANUM, GADOLINIUM, SAMARIUM, YTTRIUM, AND MIXTURES OF SAID SALTS.