US2907702A - Anodes for electroplating bath - Google Patents

Description

ANODES FOR ELECTROPLATING BATH Arnold D. Arnaut, Syosset, and Edward B. Saubestre, Elmhurst, N.Y., assignors, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware No Drawing. Application April 27, 1954 Serial No. 426,046

13 Claims. (Cl. 204-43) This invention relates to a method of operating an alloy plating solution. More particularly, it relates to a high zinc low tin alloy plating solution with soluble alloy anodes. For economic reasons it is important in operating plating solutions that soluble anodes be used. This helps to keep the bath in proper balance at a minimum of cost. As is well known in the art anodes of tin or of tin alloys which are operated in alkaline solutions must be maintained in a polarized state in order to operate effectively. This polarized state consists of a filming of the anode with a tin oxide so as to cause tin to enter the plating solution as stannate. If the polarized state is not maintained the tin will go into solution as stannite rather than stannate. The presence of stannite in a plating solution leads to dark rough deposits. While many techniques have been developed for accomplishing the desired anode polarization it has heretofore been deemed necessary to keep the tin content of a tin zinc alloy anode above 50% in order to maintain such a polarized film. It has been maintained that when the tin content of the anode falls below 50% the current density at which the film is lost spontaneously rises while the current density at which the anode becomes passive falls and thus the desirable working range which lies between these two limiting conditions is very narrow. It has therefore been deemednecessary in the prior art when attempting to plate from solutions in which the tin content is lower than 50% to either make use of insoluble controls and undesired galvanic deposits on the zinc anode.

solution.

It is accordingly an object of this invention to provide These latter deposits eventually stop its disa method of operating a plating solution designed to produce an alloy plate having a high zinc content and a relatively low tin content which will permit the deposition of a. zinc tin alloy under commonly operable conditions While using a soluble alloy anode of the proper zinc to tin ratio to maintain the bath at its optimum conditions. It is a further object of this invention to provide a method of treating the soluble anodes so as to permit 'them to be used at high efficiency while maintaining a polarized film thereon.

It is a still further object of this invention to provide a method of reactivating anodes which have become aged,

thereby ceasing to dissolve at high efficiencies. It has been found that soluble zinc tin alloy anodes can be polarized and used in an economical manner in the zinc tin baths provided that they have a negative free ,cyanidecontent and a higher hydroxide concentration than those traditional in the art. particularly suitable for use in this manner are those The baths which are which are described in the co-pending application, Serial nited States Patent 2,907,702 Patented Oct. 6, 1959 ICC No. 426,047 of April 27, 1954, in the names of Arnaut and Saubestre, filed concurrently herewith. These baths have been especially developed for the plating of a zinc tin alloy which has good corrosion resistant properties and is readily solderable. .These baths differ from the baths of the prior art in that they have a negative free cyanide content. In contrast thereto the baths of the prior art made use of free cyanide. The term free cyanide as understood in the prior art is the difference between the total amount of cyanide present in all forms and that required to complex all of the .zinc present as a complex zinc cyanide radical Zn(CN) A negative free cyanide bath on the other hand is one in which the total cyanide content is less than the amount required to complex all of the zinc present as Zn(CN) The baths of the prior art which made use of positive free cyanide to deposit tin zinc alloys were used successfully for the deposition of alloys containing more than 28% of tin. The negative free cyanide bath of this invention may, however, be used to deposit a solderable zinc alloy which is relatively high in zinc and contains just sufficient tin alloying material to permit ready solderability. Examples of baths which have been found to be stable chemically and permit continued plating operations at high cathode efficiencies (better than and high zinc alloy content (platings having better than 87% zinc) are given by the following formulation:

Example 1.--Using zinc cyanide Zn(CN) 22.5 g./l.

Na Sn0 3H O 67 g./l.

NaCN l2 g./l.

NaOH 32 g./l.

Temperature 65 C.

Cathode current density 20 amp/sq. ft. (optimum), 540 amp./

sq. ft. (useful range).

Example 2.-Using zinc oxide ZnO 15.5 g./l. Na2SHO33H O g./l.

NaCN 30 g./l.

NaOH 20 g./l.

Temperature 65 C. Cathode current density 20 amp/sq. ft. (optimum), 5-40 amp./ sq. ft. (useful range).

Example 3.Usingzinc cyanide Zn(CN) 27 g./l.

K SnO -3H O g./1.

KOH 60 g./l.

KCN 17.5 g./l. Temperatureun 65 C. Cathode current density 50 amp/sq. ft. (optimum), 20-100 ampl/ sq. ft. (useful range).

Example 4.--Using zinc oxide ZnO 18.7 g./l.

K SnO -3H O 100 g./l.

KOH 34 g./l.

KCN 47 g./1.

Temperature 65C.

Cathode current density 50 amp/sq. ft. (optimum) 20-100 amp/sq. ft. (useful range).

These baths are also quite stable and are capable of sustained operation at high cathode efliciency that is better than 85% with alloy platings the zinc content of which is better than 87%.

In formulating the baths of this invention it is, of

course, of prime importance that the resulting bath have a negativefree cyanide content.

This can be assured by keeping .the sodium cyanide content of the bath between zero. and three times the zinc metal content. In those cases in which potassium formulations are being used in making up the bath the total potassium cyanide content should be held between zero and four times the zinc metal content of the bath.

In order to obtain a bath which will function in the desired manner the hydroxide content of the bath should at least be great enough to insure that all of the zinc is complexed, since all of the formulations of necessity lack sufficient cyanide to fully complex the zinc. The

maximum value of the hydroxide content of the bath is independent of the amount of zinc or cyanide present.

'3.43 times the quantity defined as the zinc content in g./l. minus one fourth the total potassium cyanide content in g./l. The maximum value of the potassium hydroxide recommended is also 150 g./l.

It is, of course, understood that other of the components of the bath could also vary. For example, in

accordance with recommended practice the zinc content of the bath should be no less than 5 g./l. of zinc as metal, with a maximum value of 50 g./l. The recommended tin content should be no less than 15 g./ 1. of tin as metal with a maximum value of 75 g./l.

Alloy plates which have been produced from baths prepared in accordance with the invention described herein have been found to have soldering characteristics which are comparable to the tin-zinc alloys, tin, or cadmium and far better than zinc. Tests which confirm this were carried out at soldering temperatures of about 500 F. The corrosion characteristics of the solderable zinc alloys as tested in a salt fog cabinet according to ASTM specifications showed that the deposits would give good sacrificial protection to the steel, the quality of the protection afforded being superior to that provided by earlier known tin-zinc alloys or pure tin and comparable to that provided by zinc. Furthermore, from a cost viewpoint the alloy has a metal cost of about 20% of that of tin and 9% of that of cadmium. This, of course, is another very important economic feature of the alloy plated from this bath.

Alloy anodes having a high zinc content can be operated in the polarized state over a rather wide operating range in the negative free cyanide baths described above. Unlike previous practice, however, the polarized film cannot be detected by studying the visual appearance of the anode .itself as can be done with the conventional tin 2,907,702 M. a n

zinc alloys which are used with the positivefree cyanide baths. These alloy anodes which contain 50% or more of tin turn yellowish green when operated in the polarized state. The high zinc low tin alloys on the other hand are generally of a grayish, dull cast, whether they are polarized or not. The existence of the polarized film must be detected by noting the increased voltage across the cell for a given current after polarization. It has not been found possible to operate such anodes at high efiiciency over a wide operating range except in the baths which are described in the co-pending application of Arnaut and Saubestre.

Since the polarization film is not stable even in the negative free cyanide content baths in the absence of sufficient anode current density, the anodes depolarize everytime the current is shut oif in the plating cell and consequently must be repol-arized before plating operations are started again. It has been found upon repeated polarization of the anode, that the value of the polarizing current and of the depolarizing current decreases and thus for any given set of operating conditions the alloy anodes would soon become passive. This phenomenon might be called aging and has not previously been described. To correct this condition it has been found necessary to control the operating conditions in such manner as'to either minimize the rate of aging or to periodically reactivate the aged anodes. "Dhis reactivation can in accordance with this invention readily be accomplished by reversing the current in the alloy plating solutions and making the anodes cathodic at 20 to 100 amp/sq. ft. for one to five minutes. rent density are inter-related, such that the total reactivation energy applied is 100 amp.-min./sq. ft. This process is one which probably reduces the oxide film which has been formed on the metal to a thickness more suitable for use. After the anode has been reactivated in this manner a polarized surface can readily be reformed and plating can proceed as before. anodes can also be reactivated in other ways which are not quite as convenient. For example, they maybe immersed for about one minute in a 10% by weight solution of HCl or immersed in a 2% by weight solution of H Still another method is to make the anode cathodic at a current density of 50 amp./ sq. ft. in a 10% weight solution of sodium hydroxide for five minutes. By theme of any of these techniques it is possible to reactivate the anodes and particularly with the use of the first mentioned technique of reversing the current it is readily possible to make use of the high zinc content alloys as a soluble anode in a commercially practicable manner.

In alloy plating it has been found to be good practice to minimize the rate of aging. This can be accomplished by frequent reactivation of the anodes and by maintaining good solution agitation around the anode during its use. It has also been found desirable to maintain anode current density at the lowest practicable level consistent with the need to avoid spontaneous depolarization. When using alloy anodes having a zinc content of 75% the current density which is preferred is about 1 amp/sq. ft. in a non-agitated bath. Current densities in the neighborhood of 27 amp/sq. ft. can readily be used in agitated baths of the type described in Example 1. When baths which have been based on the potassium salts such as those described in Example 3 are used current densities of about 20 amp/sq. ft. in non-agitated and 48 amp/sq. ft. in agitated baths can be used. Still another way to minimize the rate of aging is to dummy the solution when plating is not taking place. This should be done at an anode current density greater than the limiting value set forth above. In this way the frequency of repolarizations required is reduced.

While the above description herewith discloses a preferred and practical embodiment of the alloy plating solution of this invention it will be understood that the The time and cur-' The alloy' specific details of construction and arrangement of parts described are by way of illustration and are not to be construed as limiting the scope of the invention.

What is claimed is:

1. In the electroplating of a cathode with a solderable protective zinc-tin alloy from a plating solution containing zinc, tin, cyanide ion and hydroxide ion, the cyanide ion being present in an amount less than that required to complex all of the zinc as the complex zinc radical Zn(CN) the steps including immersing in said plating solution a soluble zinc-tin alloy anode containing less than 50% tin, polarizing said anode by raising the current density on said anode above the normal operating current density for a period sufficient to form an oxide film on said anode, and after a period of electroplating with said anode, reducing the thickness of the oxide film on said anode and repolarizing said anode.

2. The electroplating of a cathode according to claim 1 wherein the step of reducing the thickness of the oxide film is by making said anode cathodic in said plating bath.

3. The electroplating of a cathode according to claim 1 wherein the step of reducing the thickness of the oxide film is by immersing said anode in a dilute mineral acid solution followed by washing of said anode.

4. The electroplating of a cathode according to claim 1 wherein the step of reducing the thickness of the oxide film is by making said anode cathodic in an alkaline plating bath. e

5. In the electroplating of a cathode with a solderable protective zinc-tin alloy from a plating solution containing zinc, tin, cyanide ion and hydroxide ion, the cyanide ion being present in an amount less than that required to complex all of the zinc as the complex zinc radical Zn(CN) the steps including immersing in said plating solution a soluble zinc-tin alloy anode containing less than 50% tin, polarizing said anode by raising the current density on said anode above the normal operating current density for a period suflicient to form an oxide coating on said anode, and after a period of use in electroplating reducing the thickness of said oxide coating by making said anode cathodic in said plating bath with a total reactive energy of approximately 100 amp. min/sq. aft. and repolarizing said anode.

6. The electroplating of a cathode according to claim 1 wherein the step of reducing the thickness of the oxide film is by immersing the anode in a dilute hydrochloric acid solution followed by washing of said anode.

7. The electroplating of a cathode according to claim 1 wherein the step of reducing the thickness of the oxide film is by making the anode cathodic in an alkaline solution at a current density of about 50 amps/sq. ft.

8. The electroplating of a cathode according to claim 1 wherein the step of reducing the thickness of the oxide film is by immersing the anode in a dilute sulfuric acid solution 'followed by washing of said anode.

9. In the electroplating of a cathode with a solderable yet protective zinc-tin alloy from a plating solution containing zinc, tin, cyanide ion and hydroxide ion, the cyanide ion being present in an amount less than that required to complex all of the zinc as the complex zinc radical Zn(CN) the steps including immersing in said plating solution a soluble zinc-tin alloy anode containing less than 50% tin, and polarizing said anode by raising the current density on said anode above the normal operating current density for a period suflicient to form a thin polarizing layer over the surface of said anode such that the tin in said anode can dissolve in said solution substantially only in the stannate form, said layer being visually indistinguishable from normal oxidation on the surface of said anode.

10. For use in the electroplating of a cathode with a solderable zinc-tin alloy from a plating solution containing zinc, tin, cyanide ion and hydroxide ion, the cyanide ion being present in an amount less than that required to complex all of the zinc as the complex zinc radical Zn(CN) a soluble zinc-tin alloy anode containing less than 50% tin and having an oxide coating prepared by polarization in said plating solution, said anode being polarized by raising the current density on said anode above the normal operating current density for a period sufficient to form said oxide coating on said anode, said oxide coating being characterized in that it allows tin contained in said anode to be dissolved in said solution substantially one in the stannate form.

11. For use in the electroplating of a cathode with a solderable zinc-tin alloy from a plating solution containing zinc, tin, cyanide ion and hydroxide ion, the cyanide ion being present in an amount less than that required to complex all of the zinc as the complex zinc radical Zn(CN) a soluble zinc-tin alloy anode containing less than 50 percent tin and adapted to be immersed in said plating solution, said anode having a thin oxide coating prepared by polarization of said anode by raising the current density on said anode while immersed in said plating solution above the normal operating current density for a period sufiicient to form said thin oxide coating on said anode, said oxide coating being characterized in that it allows tin contained in said anode to be dissolved in said solution substantially only in the stannate form.

12. In the electroplating of a cathode with a solderable protective zinc-tin alloy from a plating solution containing zinc, tin, cyanide ion and hydroxide ion, the cyanide ion being present in an amount less than that required to complex all of the zinc as the complex zinc radical Zn(CN) the total amount of cyanide ion and hydroxide ion being sufiicient to complex all of the zinc, the steps including immersing in said plating solution a soluble zinc-tin alloy anode containing less than 50% tin, polarizing said anode by raising the current density on said anode above the normal operating current density for a period sufficient to form a thin oxide coating on said anode, and after a period of use in electroplating reducing the thickness of said oxide coating by making said anode cathodic in said plating bath with a total reactive energy of at least amp. min/sq. ft. and repolarizing said anode.

13. In the electroplating of a cathode with a solderable protective zinc-tin alloy from a plating solution containing zinc, tin, cyanide ion and hydroxide ion, the cyanide ion being present in an amount less than that required to complex all of the zinc as the complex zinc radical Zn(CN) the steps including immersing in said plating solution a soluble zinc-tin alloy anode containing less than 50% tin, and polarizing said anode by raising the current density on said anode above the normal operating current density for a period sufficient to form a thin, polarizing coating on said anode of such character that said anode is soluble in said plating solution and the tin therein dissolves in said plating solution substantially only in the stannate form.

References Cited in the file of this patent UNITED STATES PATENTS 1,904,732 Haneisen et al Apr. 18', 1933 2,675,347 Lowenheim Apr. 13, 1954 FOREIGN PATENTS 548,009 Great Britain Sept. 21, 1942 142,336 Australia July 20, 1951 OTHER REFERENCES Heiman et 211.: Metal Industry, April 1939, pp. 161- 164.

Baierz- Metal Industry, June 1942,1211. 435-438.

Claims (1)

1. IN THE ELECTROPLATING OF A CATHODE WITH A SOLDERABLE PROTECTIVE ZINC-TIN ALLOY FROM A PLATING SOLUTION CONTAINING ZINC, TIN, CYANIDE ION AND HYDROXIDE ION, THE CYANIDE ION BEING PRESENT IN AN AMOUNT LESS THAN THAT REQUIRED TO COMPLEX ALL OF THE ZINC AS THE COMPLEX ZINC RADICAL ZN(CN)4=, THE STEPS INCLUDING IMMERSING IN SAID PLATING SOLUTION A SOLUBLE ZINC-TIN ALLOY ANODE CONTAINING LESS THAN 50% TIN, POLARIZING SAID ANODE BY RAISING THE CURRENT DENSITY ON SAID ANODE ABOVE THE NORMAL OPERATING CURRENT DENSITY FOR A PERIOD SUFFICIENT TO FORM AN OXIDE FILM ON SAID ANODE, AND AFTER A PERIOD OF ELECTROPLATING WITH SAID NODE, REDUCING THE THICKNESS OF THE OXIDE FILM ON SAID ANODE AND REPOLARIZING SAID ANODE.