Patent Application
20070012575
Smooth surfaces of rhodium exhibit a specular reflectivity of approximately 80 percent throughout the visible spectral range, resulting in a brilliant white appearance. Of the precious metals, only silver exhibits a higher reflectivity over this spectral range; but since silver is subject to tarnishing and rhodium is not, rhodium is ordinarily the coating of choice, even as a thin overplate on silver itself.
In electroplating it is common practice to employ various additives, referred to as Brightening Agents, the effect of which is to reduce the average grain size of the electrodeposit. Ordinarily such grain size reduction results in increased specular reflectivity, or brightness, of the deposit which in turn allows the deposit to be plated with high reflectivity to thicknesses greater than possible in the absence of the brightening agent. It sometimes happens, however, that the increase in specular reflectivity resulting from the use of a brightening agent is not uniform across the spectral range of interest. In such a case, the perceived color of the electrodeposit will change, as well as the reflectivity. Since whiteness of appearance in rhodium electrodeposits is considered highly desirable, a suitable brightening agent for rhodium would be one which would increase the specular reflectivity of the deposit while preserving its uniform spectral response in the visible spectral range.
Additives for rhodium electroplating solutions have been discussed by Safranek (1). Only phenolsulfonic acid is cited as a brightening agent. This material had been investigated by Wiesner and Meers (2) who reported its effective concentration range to be 1-1.5 milligrams per gram of rhodium in the plating solution (0.1-0.15 percent w/w). In rhodium plating solutions containing 5.28 grams rhodium metal per liter together with 33 milliliters concentrated sulfuric acid per liter, addition of 0.1 percent w/w of phenolsulfonic acid at 50° C. yields deposits more reflective but somewhat darker in color than deposits from the same solution without additives. Addition of phenolsulfonic acid at ratios of 0.25 percent w/w and over causes the deposit coloration to become progressively darker blue. Phenolsulfonic acid is not believed to have ever been commercialized as a brightening agent for rhodium.
Lead and thallium in small (parts per million) quantities have been used as whitening agents for rhodium electrodeposits. These do not increase the specular reflectivity of the deposits, and they appear to be effective only at low deposit thicknesses. Additionally, selenic acid, sulfamic acid, and magnesium sulfamate are used to control stress in rhodium electrodeposits. The mechanisms by which these operate are not well understood. They do not appear to affect the reflectivity of the deposits.
At this point there does not appear to exist a suitable brightening agent for rhodium electrodeposits, i.e., one which is capable of increasing the specular reflectivity of the deposit while preserving or increasing its perceived whiteness of color. It is an object of this invention to provide such a brightening agent or agents. It is a further object of the invention to provide such a brightening agent or agents capable of allowing rhodium electrodeposits of improved specular reflectivity and perceived whiteness of color to be plated to increased thicknesses as are required for engineering as well as for decorative applications. It is yet a further object that the brightening agent or agents thus provided be capable of operating in all of the electroplating systems commonly employed for rhodium; namely, the phosphate, sulfate, or mixed phosphate-sulfate systems.
This invention relates to aqueous electroplating solutions for the deposition of rhodium and to the use thereof, in which solutions rhodium is contained in the form of a soluble compound of rhodium with phosphate or sulfate; said solutions also containing an excess of phosphoric acid, sulfuric acid, or mixtures thereof; said solutions also containing a nitrogen-bearing heterocylic organic compound at least one nitrogen of which is incorporated into a six-membered aromatic ring.
The nitrogen-bearing heterocyclic organic compounds of this invention include or are derived from pyridine, picoline, pyrimidine, pyridazine, or pyrazine. Compounds of this class brighten rhodium electrodeposits at threshold concentration ratios as low as 0.25 milligram per gram of rhodium (0.025 percent w/w). Typically, maximum whiteness of deposit appearance is maintained up to concentration ratios about three to five times the threshold level, after which the deposit color gradually darkens, beginning at the lowest current densities. Brightness of the deposit is maintained to concentration ratios over 50 times the threshold value. As might be expected, various members of the class produce somewhat different degrees of deposit brightness and whiteness. In all cases tested, however, the basic pattern demonstrated has been the same.
As described above, the present invention is directed to an electroplating solution for obtaining bright white rhodium electrodeposits. In preferred embodiments, the solution comprises rhodium in the form of a soluble sulfate or phosphate compound, together with an excess quantity of sulfuric acid, phosphoric acid, or mixtures of the two, and the improvement over known solutions of this type comprises the addition of one or more nitrogen-containing heterocyclic organic compounds, at least one nitrogen of which is incorporated into a six-membered aromatic ring. Particularly preferred nitrogen-containing heterocyclic compounds include the following:
A. Pyridine and derivatives thereof;
B. Nicotinic acid and derivatives thereof;
C. Isonicotinic acid and derivatives thereof;
D. Nicotinamide and derivatives thereof;
E. Pyridine 3-sulfonic acid and derivatives thereof;
F. 3-Pyridylacrylic acid and derivatives thereof;
G. 2-Aminopyridine and derivatives thereof;
H. 3-Aminopyridine and derivatives thereof;
I. Picoline and derivatives thereof;
J. Picolinic acid and derivatives thereof;
K. Pyrimidine and derivatives thereof;
L. 2-Aminopyrimidine and derivatives thereof;
M. Pyridazine and derivatives thereof;
N. 3,6-Dihydroxypyridazine and derivatives thereof;
O. Pyrazine and derivatives thereof;
P. Pyrazinamide and derivatives thereof;
As used herein, the terms “derivatives thereof” are defined as simple derivatives of the basic structures, particularly as illustrated above. These derivatives replace one or more hydrogen atoms with another group selected from at least the following; halogen (Br, Cl, I, F), amino, nitro, hydroxy, hydroxy-C1-C6 alkyl, methoxy, cyano, benzyloxy, carboxy, benzoyl, N-oxide, mercapto, thiobenzyl, vinyl, phenylethyl, thio, and the like.
The following examples describe various preferred embodiments of the present invention.
Sufficient water was used to form one liter of a solution containing 5.28 grams of rhodium metal in the form of rhodium sulfate, together with 33 milliliters of concentrated sulfuric acid. A 267 ml aliquot of this solution was plated in a Hull cell at 0.5 ampere for 5 minutes at 50° C. using moving-vane agitation. The resulting deposit was white and bright up to an indicated current density of about 7.5 amperes per square foot, above which the deposit was hazy and yellowish in appearance up to the upper current density edge.
A solution was made up as in Example 1 except additionally containing 2.64 milligrams of pyridine. A 267 ml aliquot of this solution was plated in a Hull cell under conditions identical to those described in Example 1. The resulting deposit was white and mirror-bright across the entire panel.
A solution was made up as in Example 1 except additionally containing 13.2 milligrams of pyridine 3-sulfonic acid. A 267 ml aliquot of this solution was plated in a Hull cell under conditions identical to those described in Example 1. The resulting deposit was white and mirror-bright across the entire panel.
A solution was made up as in Example 1 except additionally containing 2.64 milligrams of nicotinamide. A 267 ml aliquot of this solution was plated in a Hull cell under conditions identical to those described in Example 1. The resulting deposit was white and mirror-bright across the entire panel.
A solution was made up as in Example 1 except additionally containing 13.2 milligrams of 2-picoline. A 267 ml aliquot of this solution was plated in a Hull cell under conditions identical to those described in Example 1. The resulting deposit was white and mirror-bright across the entire panel.
A solution was made up as in Example 1 except additionally containing 13.2 milligrams of pyrimidine. A 267 ml aliquot of this solution was plated in a Hull cell under conditions identical to those described in Example 1. The resulting deposit was white and mirror-bright across the entire panel.
A solution was made up as in Example 1 except additionally containing 2.64 milligrams of pyridazine. A 267 ml aliquot of this solution was plated in a Hull cell under conditions identical to those described in Example 1. The resulting deposit was white and mirror-bright across the entire panel.
A solution was made up as in Example 1 except additionally containing 2.64 milligrams of pyrazine. A 267 ml aliquot of this solution was plated in a Hull cell under conditions identical to those described in Example 1. The resulting deposit was white and mirror-bright across the entire panel.
A solution was made up as in Example 1 except containing 10 grams rhodium per liter, rather than 5.28 grams per liter, in the form of rhodium sulfate. A gold-struck brass coupon of area 12.9 cm2 was plated at 10 mA/cm2 for two hours at 50° C. in this solution in a 1 liter beaker with spinbar agitation. The deposit weight was 0.3048 grams, indicating an approximate thickness of 19 micrometers. The deposit appearance was matte grey.
The experiment of Example 9 was repeated using a solution to which had been added 0.13 gram pyridine 3-sulfonic acid per liter. The deposit weight obtained was 0.2138 grams, indicating an approximate thickness of 13 micrometers. The deposit appearance was bright white.
Sufficient water was used to form one liter of a solution containing 2.64 grams of rhodium metal in the form of rhodium phosphate, together with 40 milliliters or concentrated phosphoric acid. A 267 ml aliquot of this solution was plated in a Hull cell at 0.5 ampere for 5 minutes at 50° C. using moving-vane agitation. The resulting deposit was reflective dark grey below about 3 amperes per square foot indicated, and hazy lighter gray at current densities above 3 amperes per square foot.
A solution was made up as in Example 11 except additionally containing 26.4 milligrams of pyridine 3-sulfonic acid. A 267 ml aliquot of this solution was plated in a Hull cell under conditions identical to those described in Example 11. The deposit obtained was white and mirror-bright across the entire panel.
Sufficient water was used to form one liter of a solution containing 2.11 grams rhodium metal in the form of rhodium sulfate, together with 13.2 milliliters of concentrated sulfuric acid and also 26.4 milliliters of concentrated phosphoric acid. A 267 ml aliquot of this solution was plated in a Hull cell at 0.5 ampere for 5 minutes at 50° C., using moving-vane agitation. The resulting deposit was white and bright below about 3 amperes per square foot indicated, and hazy and yellowish in appearance from about 3 amperes per square foot indicated up to the upper current density edge.
A solution was made up as in Example 13 except additionally containing 2.64 milligrams of pyrazinamide. A 267 ml aliquot of this solution was plated in a Hull cell at 0.5 ampere for 5 minutes at 50° C. using moving-vane agitation. The resulting deposit was white and mirror-bright across the entire panel.
It will be understood by those skilled in the art that the Examples cited herein are illustrative of the invention, but that they do not represent the totality of the useful embodiments thereof. Not all derivatives of the nitrogen-containing heterocyclic organic compounds of this invention are readily available, but they can be prepared using well known synthetic methods. For one commercial source for a number of such compounds see, http://www.flint.com.cn/products/Pyridine.htm. All of the derivatives tested herein, however, have been effective for the purposes of this invention.
