US2976225A - Electrodeposition of nickel - Google Patents

Description

' line coumar'in.

United States Patent Oihce t, 2,976,225 Patented Mar. 21, 1961 ELECTRODEPOSITION F NICKEL Donald Gardner Foulke, Watchung, William B. Stoddard, Jr., Matawan, Otto Kardos, Red Bank, and Walter B.

Kleiner, Plainfield, NJ., assignors to Hanson-Van Winkle-Manning Company, a corporation of New Jersey No Drawing. Filed Mar. 3, 1959, Ser. No. 196,760

1 Claim. (Cl. 204-49) tive for promoting the formation of ductile electroplates which are much smoother than the base metal plated in such a bath. This ability of an electroplating bath to improve on the smoothness of the basis metal is known as leveling ability, and can be used industrially to reduce or eliminate mechanical, chemical, or electrochemical finishing of the base metal. Nickel electroplates formed from plating baths containing a bromocournarin in accordance with the invention also possess a notable resistance to corrosion.

We have found that in all instances the use of a bromosubstituted coumarin in the bath exerts a pronounced leveling effect on the electroplate formed during the plating operation and, moreover, that an economical and substantially optimum concentration of the bromo-substituted coumarin at which maximum leveling is obtained may be easily maintained during prolonged plating operations no matter what bath temperature is used by continuously circulating the bath through a filter pad containing the bromo-substituted coumarin.

Although the electrodeposition of nickel from nickel electroplating baths containing coumarin or a chlorocoumarin generally results in a deposit which is smoother than the base metal plated in such baths, the leveling ability of coumarin and of the chlorocournarinsare at a maximum at concentrations below their saturation points in the bath. Any further increase or decrease above or below this optimum level of concentration results in a sharp diminution in leveling. Since all 1,2-benzopyrone compounds undergo cathodic reduction during the electroplating operation, this optimum concentration of coumarin (or of chlorocoumarin) in the bath must be constantly maintained over the duration of the plating operation in order to secure maximum benefit of the leveling effect of such compound.

To maintain the concentration of coumarin at a high value in the plating bath, it has been proposed heretofore to circulate the electrolyte continuously through a filter pad consisting of diatomaceous earth mixed with crystal- Most commercial bright nickel plating processes, however, are operated at bath temperatures far in excess of the melting point of coumarin to obtain high plating speed, and consequently circulating the hot plating solution through the filter pad invariably melts the coumarin contained in the pad and introduces molten coumarin into the plating tank where it separates out as an oily phase, the net result of which is a substantial loss of the leveling ability of the bath.

When, however, a bromo-substituted coumarin is used in the plating bath and is continuously added to the solution by circulating the bath through a filter pad containing it, it is impossible to supersaturate the plating solution with the bromocoumarin. compound no matter at what temperature the bath is operated, since the maximum solubility of this compound in the electrolyte is that at which substantially maximum leveling is obtained. The

melting points of the bromo-substituted coumarins are appreciably higher than the highest temperatures used in most commercial bright nickel plating processes, and consequently there is no objectionable introduction of molten compound into the bath even at the maximum commercial plating temperatures. substituted coumarin in the plating solution, it is possible to employ both higher temperatures and higher current densities and to complete the plating operation more rapidly than can be done by using either coumarin or a chlorocoumarin in the bath.

The common structural feature of the bromo-substituted coumarins is the presence of at least one bromine atom in the 1,2-benzopyrone nucleus in a position which neither sterically hinders nor impedes the approach of the compound to the cathode. Only relatively small quantities of the bromo-substituted coumarins are required in a plating bath, for, in general, concentrations as low as 25 milligrams per liter have been found to promote effective leveling. In many cases, however, at least 50 milligrams per liter of the bromo-substituted coumarin should be employed to secure a high leveling effect in the bath. To obtain maximum leveling, the concentration of the'bromosubstituted coumarin in the bath should be maintained at or near its saturation point.

Any bromo-substituted coumarin which is capable of being dissolved by acid and which does not undergo decomposition upon protonation may be selected for inclusion in the plating solution. Particularly satisfactory results have been obtained using 3-bromocoumarin, the structure of which is represented by the following formula:

Br l0 0 The leveling effect of 3-bromocoumarin in nickel plating baths has been found to be especially pronounced when the bath is operated at temperatures ranging from about F. to about 180 F. and at a pH from 3.0 to 4.5. In addition to being notably smoother than the base metal, electrodeposits of nickel formed from a plating bath containing 3-bromocouma1in have also been found to be exceptionally resistant to corrosion, even when the electroplate is subjected to the most severe accelerated corrosion tests.

A preferred process according to this invention for producing well-leveled nickel deposits possessing notable resistance to corrosion comprises electrodepositing nickel from an aqueous acidic solution of at least one nickel salt in which there is dissolved from 25 milligrams per liter to a saturation concentration of 3-bromocoumarin. For most effective leveling, the bath should be operated at a temperature above 125 F. There is no critical upper limit on the bath temperature insofar as its leveling ability is concerned, and it may therefore be operated at temperatures as high as the boiling point of the plating solution. For most purposes, however, the maximum leveling and optimum economy of operation are attained when the temperature of the bath is maintained in the range between F. and F.

The following examples are illustrative of the effectiveness with which bromo-substituted coumarins may be used in a nickel electroplating bath in accordance with the invention. In each example, the electrodeposi-t was formed from 21 Watts plating bath having the following basic composition:

Grams per liter Nickel sulfate, NiSO -7H O 300 Nickel chloride, NiCl -6H O 45 Boric acid, H BO 41.25

By using a bromo-' Plating operations in each example were carried out in an open vessel on either steel or polished stainless steel cathodes, using vigorous air agitation, a bath pH from 4.0 to 4.5, a bath temperature of 140 F., and an average current density of 60 amperes per square foot. The average thickness of each electrodeposit was 0.025 millimeter (0.001 inch).

Example I An electrodeposit of nickel was formed in an open vessel on panels of steel, using the basic Watts bath described above. The deposit formed at a bath temperature of 140 F. and at a pH of 4.0 was matte and slightly stressed. Upon adding 80 milligrams per liter of 3- bromocoumarin to the bath, a brighter nickel electrodeposit was formed under the same plating conditions. Using the Chrysler foil ductility test (in which the foil is removed from the plated panel and then bent between the jaws of a micrometer until it cracks, foil ductility being defined as the ratio of the foil thickness to the micrometer reading when cracking first occurs), the foil ductility of the plated panel was found to be 0.5 which is theoretically the maximum value that can be obtained for ductile nickel electrodeposits.

Example II Using a bath temperature of 160 F. at a pH of 4.5 with vigorous air agitation, a semibright nickel electrodeposit was formed at -a current density of 60 amperes per square foot on a roughened steel panel from a basic Watts nickel plating bath described above and to which had been added 100 milligrams per liter of S-bromocoumarin. By continuously circulating the bath through a filter pad containing 3-bromocoumarin, the concentra tion of 3-bromocoumarin in the bath was maintained at its saturation point (about 100 milligrams per liter) during the entire plating operation. The difference in roughness values of the electroplated and the unplated (roughened) steel panel was 68 percent, indicating that the 3-bromocoumar-in exerted a pronounced leveling effect on the bath during the plating operation. Decreasing the concentration of 3-bromocoumarin in the bath to 50 and milligrams per liter, respectively, resulted in a slight but proportionate decrease in the brightness and smoothness of the electroplate.

Example III To illustrate the improved corrosion resistance exhibited by electrodeposits formed from plating baths containing bromo-substituted coumarins in accordance with the invention, a duplex-plated panel was produced from the basic Watts plating bath in which there was dissolved 100 milligrams per liter of 3-bromocoumarin. As

before, plating operations were carried out in an open vessel on a steel panel, using substantially the same conditions as previously described, the panel receiving a duplex coating having an average thickness of 1.5 mils, bright being 0.3 mil.

The plated panel was coated with a synthetic soil consisting of a paste prepared by dispersing 0.035 gram of cupric nitrate, 0.165 gram of ferric chloride, 1.0 gram of ammonium chloride, and 30.0 grams of kaolin in 50.0 milliliters of distilled water. After the panel had been uniformly coated with a layer of the synthetic soil, the coated panel was exposed to a constant humidity of about 90 percent at a temperature of 100 F. for a period of 24 hours, following which the synthetic soil was removed by washing and the panel then immersed in boiling water for an additional 24 hours. Upon removal from the boiling water, the panel was found to be relatively free from pitting, the Corrodekotc rating being 9.8 as determined by the oflicial Corrodekote test of the American Electroplaters Society. This rating, which is graded on a logarithmic scale from zero (negligible corrosion resistance) to 10 (complete corrosion resistance) represents an exceptionally high resistance to corrosion of the electroplate tested.

In the foregoing examples of the invention, 3-bromocouramin was successfully employed in a standard Watts nickel electroplating bath. Similar advantages are also attained when 3-bromocoumarin and other bromo-substituted coumarins are used in other types of aqueous acidic nickel electroplating baths. For example, the bromo-substituted coumarins are beneficial when used in straight nickel sulfate baths, in straight nickel chloride baths, and in various other nickel plating baths based on using nickel formate, nickel sulfamate, or

nickel fluoborate as the nickel salt which is dissolved in the aqueous acidic solvent, and consequently the invention is applicable to electroplating from any aqueous acidic solution of one or more nickel salts.

We claim:

A process for producing a corrosion-resistant nickel electrodeposit substantially smoother than the basis metal to which it is applied which comprises electrodepositing nickel from an aqueous acidic solution of at least one nickel salt at a temperature in the range from about F. to about F. and at a pH from 3.0 to 4.5 while maintaining a saturation concentration of 3-bromocoumarin in the solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,635,076 Du Rose Apr. 14, 1953