US2549411A - Electrolytic stripping of nickel coatings from ferrous metals - Google Patents

Description

Patented Apr. 17, 1951 ELECTROLYTIC STRIPPING OF NICKEL COATINGS FROM FERROUS METALS Henderson M. Bell, I11, Cleveland Heights, and Ernest W. Schweikher, Chagrin Falls, Ohio No Drawing. Application December 6, 1946, Serial No. 714,636

3 Claims. (Cl. 204-146) This invention relates to stripping nickel coatings; and-it comprises a process wherein a ferrous metal article having a nickel coating is immersed in a stripping bath, comprising an ammonium or nitrogen base salt of nitric acid or chromic acid in a concentration advantageously ranging from about 50 to 300 grams of said salt per liter; said bath operating usually at a temperature within the range of about 25 to 55 C. and having a pH within the range of about 3 to 6; followed by passing through said bath from said metal article to a suitable cathode a direct current having a current density within the range of about 20 to 400 amperes per square foot, whereby the nickel coating is stripped without damage to the ferrous base; all as more fully hereinafter set forth and as claimed.

It is frequently necessary in the plating art to strip defective metal coatings from ferrous metal bases. While several stripping methods are known, which give more or, less satisfaction in the case of other metals, the stripping of nickel has always presented great difficulties. Three methods have hitherto been used for thispur pose. One of these makes use of a stripping bath containing sulfuric acid at'a density ranging from about 52 to 56 B. The nickel plated parts are made anodes in this stripping bath. A second method makes use of a concentrated neutral solution of sodium nitrate, this bath being operated in the same manner except for the use of higher current densities. The third method makes use of fuming nitric acid without the aid of the elec tric current. All of these methods have serious technical and economic disadvantages and are hazardous in operation.

The sulfuric acid bath has found the widest use up to this time. However the operation of this bath is much slower than is desired. For example, a nickel deposit that has required from 10 to 40 minutes for its application will need from 30 to 120 minutes for its removal by this bath. The action of this bath can be accelerated by dilution but, if this is done, the attack of the acid upon the base metal is aggravated. This is a step in the wrong direction, for, notwithstanding the use of addition agents to inhibit the etching action of the acid, light gauge cold rolled steels are generally etched to a, degree which makes it impossible to repolish and satisfactorily replate them. Furthermore, an insoluble nickel sludge is formed which must be removed, resulting in large and uneconomic losses of the stripping bath. The disadvantages of the sulfuric acid bath are found also in the sodium nitrate bath, which is even slower in operation.

The fuming nitric acid bath has not been widely used because of the obvious health hazards and J the dangers involved in using so corrosivev an acid.

It is an object of the present invention to provide a stripping bath containing an electrolyte which will remove nickel coatings of commercial hazard in its use and operation. It isa still further object to provide astripping composition which' upon, solution in water is ready for use without the necessity of any adjustment.

We have discovered that the foregoing and other objects of our invention can be attained by the use of an aqueous stripping bath comprising a nitrogen. base salt of nitric acid or chromic acid, suflicient acid being present in said bath to produce a pH of about 6.0 or below. The ferrous netalarticle whose nickel coating is to be stripped is dipped into said bath and its coat 7 ing removed by the passage of a direct current from the article'to a suitable cathode.

We have found that it is necessary to have present in our stripping bath the nitrate or chromate anion in a concentration not substantially less than about 10 grams per liter. Other acids may be present whose anodic decomposition products are relatively inert or non-activating towards ferrous metals. For example we have found that sulfates and acetates do not unfavorably affect the operation of our bath-s, while chlorides cause the etching of the ferrous metal, owing to the activating effect of their anode decomposition product, i. e. chlorine. Chromates and nitrates ofnitrogen bases selected from a class consisting of ammonium hydroxide, guanidine, diethanolamine, triethanolamine, trimethyl amine, ethylene diamine, urea, benzyl amine, aniline, methylphenyl amine, dimethylphenyl amine, diphenylmethyl amine etc. are suitable.

Our stripping baths can be made by mixing the I amine or ammonia with nitric or chromic acid While the tendency to etch is likewise reduced, the work must nevertheless generally be polished before replatin'g.

in an aqueous medium. For example, a bath can be made by adding nitric or chromic acid and ammonia or an amine to Water, to form the corresponding ammonium or substituted ammonium salt. It is also possible and usually more advantageous in cases Where the salts are commercially available, to add these salts directly to the bath rather than adding their reactive components. The resulting baths are operative over a wide range of concentrations and temperatures but for best results the concentrations, temperatures and pH values must be held within relatively close limits.

We have found that if the concentration of the salt is held within the range of from about 10 g./l. (1.3 oz./gal.) to saturation, good stripping results may be obtained. However, if the concentration is below about 50 g./l. (6.7 oz./gal.) the conductivity of the solution is so low that unduly high voltages are required. On the other hand, solutions more concentrated than about 300 g./l.j (40 oz./gal.), while operating quite well, do not exhibit sufficiently greater conductivity or stripping speed to justify the additional cost of preparation and maintenance. In the preferred range, it will be found that a bath containing about 200 g./l. (26.6 oz./gal.) of the salt will most generally prove satisfactory.

In the preferred concentration range, the speed at which the deposit is removed depends upon the temperature and pH of the bath and the anode current density. If the bath temperature is above about 130 F.this limit varying somewhat with type and quantity of salt and with aciditythere may be some tendency for anodic steel to etch on prolonged immersion. It is not desirable to chill the bath, as its resistivity at low temperatures is high enough to require uneconomically high voltages. As a result of these factors temperatures between 80 F. and 130 F. are most suitable.

We have discovered that, in order to avoid etching of the ferrous'base metal, it is essential that the stripping bath be maintained at least slightly on the acid'side of 'the neutral point. Thus, we have found that a neutral stripping bath containing ammonium nitrate'causes some etching of steel when'the latter is merely immersed in the bath for any length of time. But surprisingly we have discovered that this etching is eliminated by slightly acidifying the bath and passing an electric current therethrough from the steel to a suitable cathode. Itwould', of course, have been expected that either or both of these changes would have' accelerated the etching. The fact is, however, that the'com bined effect of the electric current and a slightly acid bath sharply diminishes the etching eife'ct At pH values only slightly on the acid side of the neutral point, that'is, at a pH of 6.0 or above, we have found that the stripping rate isfqu'ite' within a pH range of about 3.0 to 6. This avoids both difficulties.

In operation the pH of the stripping bath tends to increase; hence adjustment of the pH from time to time is necessary. The pH can be controlled within the desired limits by occasional addition of, an acid or'an acid salt. It. is usually suitable materialsjare'l synth t ej resins or the: vinyl or vinylidene groups,"poly s'tyrene', polyethyladvisable to add in this manner the acid whose salt is present in the bath in order to be certain that the passivating character of the bath shall be preserved. But we have found that it is also feasible to adjust the pH, especially the initial pH, by the addition of sulfuric acid or sodium acid sulfate. This reduces the operating cost but, if such an acid is used during operation of the bath, tests should be made from time to time to be certain that the concentration of the passivating anion in the bath does not fall substantially below about 10 g./l. since the passivating acid is used up in the process. When the passivating acid is added to control the pH, however, usually it is not necessary to test the concentration of this acid.

A very convenient way of starting the operation of one of our stripping baths is to use a mixture of dry chemicals which when dissolved in water in a predetermined quantity will produce a stripping bath of the correct pH and concentration. The acid constituent employed in such compositions must, of course, be dry. Chromic acid and sodium acid sulfate are the most convenient to Thus a mixture of ammonium nitrate with either chromic acid or sodium acid sulfate can be' employed and a mixtureiof, a Il'll-i monium or a substituted ammonium chromate with potassium bisulfateor chromic acid is equal; ly satisfactory.

We have found no upper limit to the current, density that may be used. Densities as high as 528 a./sq. ft, (56.7 a./sq. dm.) work quite as, well as densities as low as 20 a./sq. ft. (2 .1 a./sq. dm.).

Very high current densities will of course demandhigh voltages and the spray from the cathodesis likely to be excessive, with loss of stripping. solution. Onthe'other hand, too low an anode current density results in an unduly extended stripping time. Moreover, very low current'densities,

or no current density atall may result in etching of the work; Suitable current densities may be obtained from customary direct current sources,

ing conditions for our stripping "bathfinay' be tabulated as follows:

Concentration of Salt, 50 g ./l. (6.7.o z,/gal.). to.30.0.

g./l. 40 oz. ga1.) pI-I, 3.0 .to 6.0 Temperature, 25? C. (77?. F.)- to 5530, (131? Current densities, 20 a./sq. ft. to, 00 a./s q. ft; (or. 2.0 a./sq. dm. to 40 a,/sq.'d1n,)

A tank made of corrosion-resistant material .or.

lined with such, materiallis requiredfmmongj ene or other plastics, and, glass. It'is necessary merely to dissolve. the salts in, water in such a I tank, connectthe worr' b the positive pole of a generator or othersourceof cur rent, and immerse it' the s olution, the nega} tive W e u ent u e .bQiflQQQHIEQWd;

to an inert qeth eaj ft el' qn iq l or. x: ample, mmersed in he ,$Q l .QI!-

Our invention canbeexplained more fully, by,

reference to the following-specific examples which CBSS.

Example 1 A solution is made up containing:

nrnnos g./1.

NaHSO4 enough tolower the pH to 4.0

In this solution nickel having a thickness of 0.0016 inch was completely stripped in 9 minutes at 3.3 -a./sq. in. current density from polished steel, leaving it bright, and in condition tobe replated without rebuifing.

Example 2 I Triethanolamine I. 90 g./l. HNOs enough to lower the pH to 4.0

In'this solution, nickel having a thickness of 0.0016 inch was completely stripped in minutes at l".3"'a./sq. "in. current density] from polished steel,"1eaving' it bright, and in condition to be replated without rebufling.

, Example'3 ole/1.

NHQOH enough to raise the pI-Iito 4.0

HNO3 adding during, stripping to maintain pH Example ,4

'lojo g./1. rmos enough to pH to 4.0

In this solution, nickel having a thickness of 0.00125 inch was completely stripped in 40 minutes at a current density of 0.75 a./sq. in. from polished steel, leaving it bright, and in-condition to be replated without repolishing..

Example 5 NH4NO3 200 g./1. NaHSO4 0.6 g./1.

lower the Example 6 Urea 150 g./l. CrOs 7 g./1. HNO3 enough to lower the pH to 5.0

In this solution, nickel having a thickness of 0.00185 inch was completely stripped in 14 minutes at a current density of 1.3 a./sq. in. from polished steel, leaving it bright and in condition to be replaced without repolishing.

Example 7 Guanidine carbonate 110 g./l. HNO3 ml./l.

This is equivalent to a 150 g./l. solution of guanidine nitrate. Because of the lesser solubility of guanidine nitrate at lower temperatures, the bath was maintained at -130 F. The pH was kept between 3.5 and 6.0 by occasional additions of HNOs. In this solution, nickel having a thickness .of- 0.00175;inch was completely stripped in- 13. minutes at a current density of 1.8 a./sq. in-

from polished steel, leaving it bright and in condition to be replated without repolishing.

Example 8 Ethylene diamine (100%) 18.75 g./l.

CIO3 31.25 {3/1.

HNOs enough to lower the pH to 5.5

In this solution, nickel having a thickness of 0.0017 inch was completely stripped in 27 minutes at a current density of 0.75 a./sq. in. from pol- In this solution, nickel having a thickness of 0.0005 inch was completely stripped in 10 minutes at a current density of 1.6 a./sq. in. from polished steel, leaving itcoated with a soft thick black film Most of the'film could be swabbed off, and after a 10 second dip in diluted HCl, the piece Was found to be bright and in condition to be replated without repolishing.

It is believed evident from the above that dry mixes can be'made from the solid components of the baths illustrated in the above examples which, when dissolved in water, will produce stripping baths within our invention. Thus chromic acid and sodium bisulfate, for example, can be used I as the solid acid constituents while nitrogen base nitrates, chromates, dichromates, carbonates etc. can be used as the nitrogen-containing constituent. V

A specific example of a dry mix which, upon solution in water, will produce one of our stripping baths, is a mixture of ammonium nitrate and sodium bisulfate in the proportions of 99.7 parts of the nitrate to 0.3 part of the bisulfate. This mixture, when dissolved in Water to produce a solution containing approximately 200 grams per liter, produces a pH in the neighborhood of 3. Another dry mix can be made by mixing together 99.98 parts of ammonium nitrate with 0.02 part of chromic acid. This mix, when dissolved in water to produce a solution containing approximately grams per' liter, produces a pH of about 4.2

It is of interest to note that the action which takes place during the stripping operation of our invention differs to some extent-at least in appearance-depending upon the nitrogen compound employed. For example, in the stripping of nickel in accordance with Example 1, a film of heavy liquid appears to fall away from the surface of the anode at the start of the process. The surface of the nickel becomes black. No gas is evolved from the anode. After a comparatively short time, breaks occur in the nickel surface and oxygen is evolved Where the underlying steel is exposed. The stripped areas expand and the edges of the nickel adjacent these areas start to curl. After a further short time, the 'bits of blackened nickel are dislodged from the anode by the evolving gas. When all the nickel islands are either eaten up or floated away, oxygen is liberated all over the anode area. Electrolysis may be continued for a considerable time after all the nickel is gone, without any important change in the appearance of the anode. In the case of Example 6, the nickel does not blacken,

angers but holes are eaten into it, stopping at-thesteel;

and these holes expand, eating away at the edges' ofthe remaining. nickel.

It will. be: appreciated that the chief merits of our process are (1) that it will rapidly dissolve nickel and (2) that iron can be subjected to it for extendedperiods of time without damage. We know of no other stripping process'that possesses both advantages.

While we have described what we consider to be the most advantageous embodiments ofour process, it is evident, of course, thatvarious specific details which have been described can be varied without departing fromthe purview of our invention. The procedure to be used in making up our stripping baths will depend, of course, uponthe relative cost and. availability of the inredients. ample, and the cost of the amine plus the: cost ofv the oxidizingacid is lowerxthan the cost oftheamine salt, it would be preferable to make the stripping baths by adding, the amine audthecxidizing acid separately to the water thus forming the amine. salt in situ. This is a very satisfactory way to proceed since, if the amine is added first the oxidizing. acid can be added until the pH of the bath is within the desired range. On the other hand, if the amine salt is added to the water, itis then usually necessary to'adjust the pH by the addition of'an acid or acid salt. Any conventional type of electroplating or electrostripping apparatus can be used in our process and conventional stripping procedures can be used; Other modifications of'our. inventionwhich fall within the scope of the following claims will be immediately evident to those skilled in this art.

What we claim is:

1. In the. stripping of nickel from. ferrous ing a concentration rangingfrom' about 50' to 300' grams per liter. and a pH'of about 3.0 to.6.0.

2'. In. the stripping of nickel from ferrous:

metalS the process which comprises inserting a;

If an. amine. salt is to be used, for ex-- 8 ferrous metal article coated with nickel in a stripping bath andpassing a direct current from said article, serving as anode, to a cathode; said stripping bath consisting substantially of' arr aqueous solution of asalt whose anion is that of an oxidizing-passivating acid selected from a class consisting of nitric" and chromic acids and whose: cation is that of a nitrogen base selected froma class consisting of ammonium hydroxide,

"guanidine, diethanolamine, triethanolamine, trimethyl amine, ethylene diamine, urea, benzyl amine, aniline, methylphenyl amine, dimethylphenyl amine and diphenyhnethyl amine, said bathihaving. a concentration of said salt ranging fromabout to 300 grams per liter and a pH within the range of from about'3.0 to'6.0'.

3. In the strip ing of nickel from ferrous metals; the process which comprises inserting a ferrous metal article coated with nickel in a stripping bath and assing an electric current from said article, serving; as anode, to a cathode; said stripping bath consisting substantially of an aqueous solution. of ammonium chromate having aconcentration ranging from about 50 to 300 grams per liter and a pH of from about 3.0 to 6.0'.

HENDERSON M. BELL ERNEST SCHWEIKHER.

B EFERENCES CITED The following references are of record in ..the"- file of. this patent:

UNITED STATES Number Name Date 908,937 Bayliss et a1 Jan. 5, 1909' 1,314,842: Weeks.. Sept. 2, 1919 2,241,585 Day May 13, 1941 OTHER REFERENCES The Monthly Review of The-American Elec-- troplaters Society, July 1945,.pages 673, 679.

Metal Cleaning and. Finishing, Nov. 1933,-

Claims (1)

1. 2. IN THE STRIPPING OF NICKEL FROM FERROUS METALS, THE PROCESS WHICH COMPRISES INSERTING A FERROUS METAL ARTICLE COATED WITH NICKEL IN A STRIPPING BATH AND PASSING A DIRECT CURRENT FROM SAID ARTICLE, SERVING AS ANODE, TO A CATHODE; SAID STRIPPING BATH CONSISTING SUBSTANTIALLY OF AN AQUEOUS SOLUTION OF A SALT WHOSE ANION IS THAT OF AN OXIDIZING-PASSIVATING ACID SELECTED FROM A CLASS CONSISTING OF NITRIC AND CHROMIC ACIDS AND WHOSE CATION IS THE OF A NITROGEN BASE SELECTED FROM A CLASS CONSISTING OF AMMONIUM HYDROXIDE, GUANIDINE, DIETHANOLAMINE, TRIETHANOLAMINE, TRIMETHYL AMINE, EHTYLENE DIAMINE, UREA, BENZYL AMINE, ANILINE, METHYPHENYL AMINE, DIMETHYLPHENYL AMINE AND DIPHENYLMETNYL AMINE, SAID BATH HAVING A CONCENTRATION OF SAID SALT RANGING FROM ABOUT 50 TO 300 GRAMS PER LITER AND A PH WITHIN THE RANGE OF FROM ABOUT 3.0 TO 6.0.