US2436244A - Metalworking and strippingplating process - Google Patents

Description

' copper as electrodeposit.

. pounds Patented Feb. 17, 1948 METALWORKING AND STRIPPING- PLATING PROCESS Harry L. Benner and Robert R. Bair, Niagara Falls, N. Y., assignors to E. I. (in mours & Company, Wilmington, poration of Delaware Pont de Ne- Del., a cor- No Drawing. Application September 25, 1943, Serial No. 503,840

4 Claims.

This invention relates to removing coatings of copper or copper alloys from metal articles coated therewith.

This application is a continuation-in-part of our copending application Serial No. 443,496, filed May 18. 1942, which has become abandoned.

In the metal working industries it is ofen desired to remove the copper or copper alloy coatings from metal bases coated therewith. In some cases steel articles are given a temporary copper coating which must later be removed, as in metal drawing operations where the coating serves as drawing lubricant and in selective case hardening, where the coating serves as a stopoff to prevent carburization of selected areas. Copper stripping methods are employed by electroplaters to remove defective coatings preparatory to replating. Scrap metal coated with copper or copper alloy may be treated-to recover the copper and prepare the scrap for remelting, so as to recover both the base metal and the cop per contained in the scrap.

The removal of the copper coating is usually accomplished by chemical or electrochemical means. I-Ieretofore the methods utilized for removing the copper coating have not been well adapted to permit efiicient recovery of the copper from the stripping in loss of the copper. The electrochemical methods which have been utilized, in which the work is the anode, have not been adapted to efficiently plate out copper on the cathode. The electrolytes which have been adapted for efficient and complete removal of the copper have not been efficient electroplating solutions and therefore could not be efliciently used for recovery of the Hence, most of the prior methods in prior use have recovered copper in that have to be smelted or otherwise treated to recover metallic copper. Also, copper stripping methods heretofore used often result in excessive attack on the base metal, causing waste or damage.

An object of the present invention is to provide an improved method for stripping copper and copper alloys from steel and other metal bases coated therewith. Another object is to remove coppper coatings from steel articles which have been copper coated and then case hardened, so as to completely remove the copper, to leave a clean, unblemished steel surface. Another object is to remove copper and other copper alloys, especiallygilding metal, from a steel base and to recover the copper or alloy in the solution often resulting,

combined form, producing copper com-- metallic form without substantial change in composition. Another object is to electrolytically strip copper and copper alloy coatings from metal bases and simultaneously deposit the copper or alloy as a smooth, dense electrolytic deposit of high quality. Still other objects will be apparent from the following description of our invention.

In our copending application, Serial No. 443,496 we have described and claimed an electrolytic copper stripping process which efficiently and completely removes copper coatings from the base metal and simultaneously recovers the copper as an electrodeposit of high quality. In that process the work to be stripped serves as anode and the electrolyte is a copper cyanide solution comprising a solution of a double alkali metal copper cyanide; for example, sodium copper cyanide, Na2CU CN)s. The amount of free cyanide in solution, the temperature thereof are controlled within certain limits, to wit: the free alkali metal cyanide concentration is maintained at an amount equivalent to 0.5 to 1.25 ounces per gallon of so dium cyanide, the pH of the solution is maintained at between 10.9 and 13.0, the tempera ture of the solution is maintained at from about 0., up to the boiling point of the solution and the electric current is maintained at not higher than 6 volts. The copper concentration is kept at that equivalent to not less than 2 ounces per gallon of copper cyanide (CuCN). When these conditions are maintained, the solu-- tion acts as a very efficient electrolytic coppe: stripping bath which will rapidly and completely strip copper deposits from steel or other metal bases, and at the same time serves as an efficient electroplating bath, depositing on the cathode smooth, dense copper deposits of excellent quality.

While the above-described process is very satisfactory for most copper stripping operations, and has proven generally superior to prior methods, some difiiculties have been experienced in using it to strip copper from carburized steel articles in selective case hardening operations and in removing gilding metal coatings from gilded steel scrap. In the case hardening operations the removal of the copper coating from the carburized steel often leaves a dark copper stain on the work. Chemical after-treatments to remove this stain tend to attack and corrode the steel. In stripping the gilding metal coatings; difiicult'y has been experienced in efliciently using the high current densities required for fast and ecflnomthe pH of the solution, and

shown in the above table.

ical operation. Gilding metal is essentially a zinc-copper alloy containing a relatively low zinc content, e. g., around 10% by weight. The pres ence of the zinc tends to cause polarization of the anode, especially at high current densities, which interferes with the rapid and complete removal of the copper alloy coating and electrodeposition of alloy of unchanged composition. Also, there is some tendency for exposed steel surfaces on the anode to go intosolution. which is obviously undesirable.

We have discovered that these defects in our copper stripping process can be overcome by the presence of a small amount of phosphate ions in the bath in practicing the process of our copending application Serial No. 443,496, and by other means, as more fully described hereinafter. The objects of the present invention therefore also include means for improving the process of the aforesaid copending application so as to strip copper from carburized steel articles without leaving stains Or other blemishes and to more efficiently strip and remove gilding metal, brass and other copper alloys from metal gases.

In practicing our invention We may utilize the solution and mode of operation described in the aforesaid copending application, except that a source of phosphate ions is added to the stripping bath. Phosphoric acid or its soluble salts may be used for this purpose. Generally we prefer to add trisodium phosphate hydrate (NasP04.l2H2O) but the other alkali metal phosphates may be used, including the phosphates of potassium and. lithium. Satisfactory results may be obtained by adding the phosphate ion component in amounts equivalent to 2 to 16 oz. per gal. of trisodium phosphate hydrate (Na3PO4J2H2O), preferably about 8 ozs. per gallon.

The following tabulation shows the preferred range of bath composition and operating conditions and a typical example.

Preferred Range Example Bath Ingredients CuCN 12 z. per 11.... 6 02s. per gal. NaCN (total). to 17.2 02 per gal. 9.1 025. per gal. NaCN (free) 0.6 to 4 oz. per gal-.. 2.5 ozs. per gal. NSzPOAJZ H2O (or it 2 to 16 oz. per gal 8 ozs. per gal.

equivalent).

Operating Conditions Bath Temperature pH of Bath Anode Current Den- 40 to 90 C Up tof 135 amps. per

80 C. 12.0. 95 amps. per sq. ft.

51 y. Cathode Current Den- 1 tit; 50 amps. per sq. 50. amps. per sq. It.

s Voltage Up to 2.5 volts 1.5 to 2 volts.

operation but this is not necessary.

In one method of practicing our invention, clean gilding metal scrap is made anode, utilizing the preferred bath and operating conditions The voltage is constantly maintained at 1.5 to 2 volts. Near the end of the operation, as thesteel surface becomes exposed, the current decreases in proportion. When the stripping is complete, the current will be reduced to less than 2 amps. per sq. ft. on the bare steel anodic surface, the voltage being maintained constant. The gilding metal is completely removed from the anode, substantially without solution of the steel base. The cathodic deposit is. a smooth, dense. homogeneous alloy having substantially the same composition as that of the original coating on the anode. To obtain these results it is essential that the pH be maintained'at all times between 10.9 and 12.3. If the pH goes below 10.9, an excessive amount of zinc tends to build up in the solution. At a pH above 12.3 polarization of the gilding metal surface may occur and exposed steel surfaces may carry an excessive proportion of the current. Throughout this specification and in the appended claims, pH values are those obtained by electrometric determination.

In deplating articles coated with unalloyed copper, the same conditions are employed, except that the pH may vary from 10.9 to 13.0, if desired, Polished steel articles, which have been partly copper plated, then carburized, are effectively and completely deplated by this process and the deplated steel surface has its original luster and is free from any stains or other blemish.

The following examples further illustrate our invention:

Example 1 This example shows the effect of the phosphate ions in preventing anode polarization on a gilding metal anode.

The bath had the following composition:

Ozs. per gal Cul'JN 3 5 NaCN 6.1 NaaCOa 3.0

Example 2 The following bath was electrolyzed at 1.5 volts and a bath temperature of 82 0., with one gilding metal anode and one steel anode:

Ozs. er a1. CuCN p 6.0 NaCN 9.1 Na3PO4.12H2O 5.5 pH adjusted to 12.1

using the following bath, operated at 82 C. and 1.5 volts:

Ozs. per gal." CuCN 10.0 NaCN 13.5 Na3PO4.12H2O 7.0

pH adjusted 5:11:11:III: 12.1,

Example 4 Gilding metal plated steel scrap was anodically deplated in the following bath electrolyzed at 2 volts and a bath temperature of 82 C.:

Oz. per gal. CuCN 3.5 NaCN 5.1 Na2CO3 3.0

pH adjusted to 12.25.

The anodic current density was 25 amps. per sq. ft. The addition of 1 oz. per gal. of Na3PO4.12HzO increased the anodic current density to 40 amps. per sq. ft. The addition of a second oz. per gal. of NaaPOalZI-IzO further increased the anodic current density to 60 amps. per sq. ft.

Example 5 A coil of crimped gilding metal coated sheet steel weighing 3.99 lbs. was deplated by anodic treatment in a copper cyanide electrolyte containing trisodium phosphate. During electrolysis the bath temperature was maintained at about 80 C. and the pH close to 12.0. The bath was analyzed from time to time and the following is a typical analysis:

Oz. per gal. CuCN- 6.0 Zn(CN)2 2.4 NaCN (total) 13.25 NaCN (free) 2.57 Na3PO4.12H2O 5.5

Example 6 Steel parts were electroplated with copper so that only a portion of each part was coppered. The coppered parts were carburized in a conventional gas carburizing furnace and these were completely deplated by anodic treatment in the following electrolyte operated at a bath temperature of 82 0., at 2 volts and an initial anode current density of 90 amps, per sq. ft.:

Oz. per gal. CuCN 10.0 NaCN p 13.5 NcPO4.12I-I2O 7.0

The deplated parts were clean, with no evidence of staining or corrosion.

Example 7 6 following bath operated at 2 volts and a bath temperature of 82 C. until completely deplated:

Oz. per gal. 6.0

CuCN NaCN 9.1 Na3PO4.12HzO 5.5

pH adjusted to 12.1.

copper or gilding metal, it will be apparent that it is likewise adapted for various other metal working operations, and that many modifications of the invention may be made without departing from the scope of the invention. Our strippingplating process is well adapted for stripping copper plate in operations other than metal drawing and selective case hardening. If desired, work to be stripped may be used as anodes in various copper plating operations. Our process may be used to strip coatings of various copper alloys other than gilding metal from steel or other metal bases, providing the alloy coating is composed at least preponderantly of copper, for example, yellow brass and bronze.

In using our process for deplating in metal drawing and selective case hardening operations, the cathodic deposits obtained or the parts being deplated may be used as anode to electroplate the copper coatings in succeeding lots of 'work.

The copper thus is used over and over, so that a relatively small amount of copper serves for operations upon large quantities of steel. The amount of copper accidentally lost may be kept down to a very low figure by careful operation. Any small losses which may occur can readily be made up, for example, by inserting a few small copper anodes in the electrolyte during the stripping operation, by previously giving the cathodes a copger strike, or by adding copper cyanide to the ath.

If it is desired to recover the stripped copper or copper alloy in massive form rather than as an electrocoating, the stripping process may be carried out by using copper or alloy dummy cathodes. Our stripping solution has been found to be well adapted to plate out exceedingly thick, dense, nonporous deposits of electrolytic copper and is well adapted for the recovery of copper or copper alloy in the form of thick sheets or plates. Thus, the stripped metal may be substantially completely recovered in the form of a thick, dense electrodeposit of high purity, suitable for use as electroplating anodes or other commercial uses.

While we often prefer to use a relatively dilute solution for the stripping-plating bath, e. g., equivalent to 3 to 6 ounces per gallon of CuCN, excellent results also can be obtained with more concentrated solutions, for example, solutions containing up to 20 ounces per gallon of copper cyanide. Our invention is not necessarily restricted to solutions containing only copper and alkali metal cyanides and carbonates, but other commonly known conducting salts, addition agents and the like which are generally suitable in copper cyanide plating solutions can be added as desired. In general, however, we have found no advantage in adding other materials and in some cases some slight disadvantage, so far as efficient stripping of the anode is concerned. For example, we generally prefer to avoid adding alkali metal hydroxide to the bath. Therefore, we prefer to utilize a solution consisting of a copper alkali metal double cyanide containing phosphate ions, with or without alkali metal carbonate, which solution may contain relatively small amounts of acidic ions such as sulfate, chloride, phosphate, tartrate, citrate, 'or the like, which are necessarily introduced if the corresponding acids are added in order to adjust or control the pH of the solution. In preparing the both, we may use sodium cyanide, potassium cyanide or both, as well as other alkali metal cyanides. Potassium cyanide often gives somewhat better results than sodium cyanide.

Control of pH, free cyanide, voltage and temperature as described above is important, and is essential to insure that efficient stripping and electroplating occur simultaneously in the same bath. For example, we have found that free cyanide is essential to maintain high current efficiency at both cathode and anode; and that to insure rapid and complete stripping of copper from the cathode, the free cyanide content should not be permitted to fall below the equivalent of 0.5 ounce per gallon of sodium cyanide, preferably not below 0.6 ounce per gallon. If the free cyanide content is too high, excessive cyanide decomposition tends to occur by hydrolysis; we have found that the cyanide decomposition is practically negligible if the free cyanide content is not more than that equivalent to about 1.25 ounces per gallon of sodium cyanide and relatively small below 4-5 ounces per gallon.

The phosphate ion content may vary over a wide range, up to a concentration equivalent to 16 ounces per gallon of the hydrate:

We generally prefer to use at least 2 ounces per gallon, as smaller concentrations usually have too small effect to be useful in most operations. However, the effect of the phosphate at the low concentration is proportional to its concentration, and if a proportionally small effect is desired, concentrations below 2 ounces per gallon down to a fraction of one ounce per gallon may be used.

Control of pH is important in our bath to obtain efiicient and complete copper stripping of the anode. For example, we have found that conventional cyanide copper plating baths having pH higher than 13 do not strip the copper with the rapidity necessary in our process and tend to result in incomplete stripping. This has been found to be true of copper cyanide baths which are excellent electroplating baths, in which the rate of anode corrosion and anode current efficiencies are very satisfactory for ordinary electroplating purposes. In order to obtain the required efficient and complete stripping, we have found'that the pH of our bath must not rise above 13.0. On the other hand, in stripping copper from steel or other ferrous metal articles. a low pH tends to cause dissolution of the ferrous base of the anode. We have found that no appreciable dissolution of the ferrous metal occurs when the pH of ouizbath is maintained not lower than 10.9. As mentioned hereinbefore, in using our process for stripping gilding metal or other copper alloys such asbrass and the like, the pH should preferably be kept below 12.3 in

order to prevent any polarizing of the gliding metal or copper alloy surfaces as well as to maintain a low percentage of current flowing from exposed steel surfaces.

We have found that control of voltage, coupled with the pH control is an important factor in efiicient operation when the stripping has proceeded to the point where the steel base is exposed or in stripping ferrous articles which are only partly covered with copper or alloy, e. g., from selective case hardening operations. Within the above-designated range of pH control, when the voltage is kept low, the bulk of the electric current in our bath will be carried by the copper portions of the anode surface. In fact, at a voltage around 2 to 2.5 volts or lower, little or no current is carried by the steel surface, so long as any copper or copper alloy is left on the anodes. At higher voltages, up to 6 volts, the amount of current carried by the steel becomes appreciable but still is relatively small, less than 10% of the total current passed through the cell. This effect is advantageous in that it concentrates the electric current on the anodic copper or alloy, so that substantially all of the current is effectively used to dissolve the cuprous coating. Further, it avoids anodic oxidation, which would cause decomposition of cyanide. If the voltage rises above about 6 volts. anodic oxidation tends to become appreciable and tends to occur also on anodic copper surfaces. Above 6 volts, a film of copper oxide usually tends to form on the anode, which seriously interferes with solution of the copper. If desired, the voltage may be reduced much lower, even to a small fraction of one volt. At such low voltage, the current is very etficient- 1y utilized, but of course the amount of current passed is correspondingly low and consequently the operation is slower. For practical operation we generally prefer to operate in the range of 1.0 to 2.5 volts.

A hot bath is preferred for the most rapid and complete stripping and We have found a temperature of about 70 C. or higher gives the most efficient operation. While the temperature may be permitted to rise to the boiling point of the bath with good results, there is a greater tendency towards cyanide decomposition at the higher temperatures. Hence, We prefer to operate at a temperature which favors efiicient stripping, yet low enough to avoid cyanide decomposition. In general, we have found the optimum bath temperature to lie in the range '75 to C.

For best results, we prefer to agitate or circulate the bath or to move the electrodes, or both, so as to maintain the effect of a stream of electrolyte flowing over both anode and cathode surfaces. 7 This condition favors the employment of high current densities, and, for example, when the bath is rapidly circulated over the electrode surfaces, We can operate at anode current densities as high as 135 amperes per square foot, and cathode current densities up to amperes per square foot or even higher, to obtain satisfactory smooth, dense cathodic deposit and rapid, efiicient and complete removal of copper or alloy from the anode.

While our stripping-plating process is especially adapted to strip copper and its alloys from iron, steel, cast iron, ferro alloys or other ferrous base, it can also be used for stripping copper and copper alloys from other metal bases, for example, from tin, nickel, bronze, brass, chromium, etc. When the base is soluble in the cyanide solution, it is, of course, impossible to avoid dissolving more or less of the base metal. We prefer to utilize the invention for stripping copper and its alloys from metals like the ferrous metals, which are sub- 'stantially insoluble in our stripping-plating bath.

We claim:

1. In a metal treating process, the steps comprising copper plating a metal article other than a copper article, subjecting said article to at least one metal treating process and simultaneously removing copper from the so-treated article and plating copper onto another untreated article by constituting said treated article as anode and said untreated article as cathode in a cyanide copper plating solution comprising essentially dissolved alkali metal copper cyanide in a concentration equivalent to about 2 to 20 ounces per gallon of copper cyanide (CuCN), free alkali metal cyanide equivalent to 0.6 to 4 ounces per gallon of sodium cyanide and phosphate ions equivalent to 2 to 16 ounces per gallon of Na3PO4.12H2O, while maintaining said solution at a pH of 10.9 to 13.0 and at a temperature of about 75 to 85 C. and maintaining the voltage of the electric current at about 1 to 2.5 volts.

2. In a process for selective case hardening, the steps comprising applying a coating of copper to a portion of a steel article, subjecting the coated article to a carburizing treatment, and thereafter stripping the copper coating from said article by anodic treatment in an electrolyte comprising essentially dissolved alkali metal copper cyanide in a concentration equivalent to 2 to 20 ounces per gallon of copper cyanide (CuCN), free alkali metal cyanide equivalent to 0.5 to 4 ounces per gallon of sodium cyanide, and phosphate ions equivalent to 2 to 16 ounces per gallon of Na3PO4.12H2O, while maintaining said electrolyte at a pH of 10.9 to 13.0 and at a temperature of about 75 to 85 C. and maintaining the voltage of the electric current at about 1 to 2.5 volts.

3. The process for removing and recovering a copper-zinc alloy from steel coated therewith which comprises subjecting the coated steel to anodic treatment in an aqueous electrolyte comprising essentially dissolved alkali metal copper cyanide in a concentration equivalent to 2 to 20 ounces per gallon of copper cyanid (CuCN), free alkali metal cyanide equivalent to 0.5 to 4 ounces per gallon of sodium cyanide, and phosphate ions equivalent to 2 to 16 ounces per gallon of Na3PO4.12H2O, while maintaining said electrolyte at a pH of 10.9 to 12.3 and at a temperature of about to C. and maintaining the voltage of the electric current at about 1 to 2.5 volts.

4. The process for removing and recovering comprises subjecting the coated steel to anodic treatment in an aqueous electrolyte comprising essentially dissolved alkali metal copper cyanide in a concentration equivalent to 3 to 6 ounces per gallon of copper cyanide (CuCN), free alkali metal cyanide equivalent to 0.5 to 4 ounces per gallon of sodium cyanide, and phosphate ions equivalent to 2 to 16 ounces per gallon of NasPOalZHzO, while maintaining said electrolyte at a pH of 10.9 to 12.3 and at a temperature of. about 75 to 85 C. and maintaining the voltage of the electric current at about 1 to 2.5

I volts.

HARRY L. BENNER. ROBERT R. BAIR.

REFERENCES CITED The following references are of record in the file of this patent:

Metal Industry, vol. 23, No. 11, Nov. 1925, page 452.

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