Electroplating white palladium

© Start of patent period: 1984 02 15 Longest possible duration:

t Issued: 1984 09 25

Inventor:

© dependence:

AT 375 964 © Pamphlets contemplated for distinction from the prior art:

US-PS 4098656 US-PS 4066517 GB-PS 1035850 DE-OS 2943399 (26-0 3-19811

Nr.375964

The invention relates to an electroplating bath for the deposition of white palladium metal on various surfaces. In particular, the invention relates to baths for forming thin coatings of white palladium metal.

As is already known, coatings 5 ​​are formed with gray color when conventional palladium baths are used. On the other hand, rhodium baths are known to form white coatings which are very useful in the jewelry and decoration industry. In view of the relatively high cost of rhodium compared to palladium, it would be highly desirable to be able to obtain a white coating from palladium baths to replace the rhodium coatings currently used. The previous attempts to form a white palladium metal coating 10 were unsuccessful because the coating was not white enough for the intended purposes, for example as a replacement for the conventional white rhodium coatings. It would also be useful for commercial purposes to easily produce thin, white palladium metal coatings.

U.S. Patent No. 3,330,149 mentions the preparation of a white palladium coating. The electroplating bath according to this patent contains palladium chloride, ammonium phosphate, sodium phosphate or ammonia and optionally benzoic acid. The pH of the bath when working is not disclosed, although it is stated that ammonia is boiled off and the liquid, which was alkaline, becomes slightly acidic. As stated, benzoic acid is only optionally used, but it is further disclosed in the said patent that it bleaches the coating and makes it more noticeable on iron and steel.

Electroplating baths intended to improve the gloss of palladium or palladium alloy coatings on metal substrates are also already known, see, for example, US Pat

Nr.4,098,656. According to this patent, the improved gloss is achieved by using an organic whitener of both Class I and Class II in the bath and adjusting a pi of 4.5 to 12.

In the drawing, the whiteness of palladium coatings obtainable according to the invention is explained in comparison with those of the prior art.

According to the invention, it has now been found that thin white palladium metal coatings can be readily obtained from an electroplating bath formed by a bath-soluble palladium source and an ammonium salt, the pi value being about 8 to 10. The use of a phosphate matrix is ​​preferred because it results in an excellent whiteness. However, it is clear that, for example, ammonium sulfate also provides acceptable results.

The invention thus provides a stable aqueous electroplating bath suitable for obtaining thin white palladium metal coatings, characterized in that it consists essentially of a bath soluble source of pure palladium metal free of alloying elements in proportions sufficient in the art Bath 0.1 to about 20 g / 1 palladium, 30 to about 120 g / l of a bath-soluble conductive ammonium salt and a sufficient proportion of ammonium hydroxide to adjust and maintain a pi-value in the bath of about 8 to 10.

With regard to the subject matters of U.S. Patent Nos. 4,098,656, 4,066,517 and GB-PS

No. 1,035,850 were carried out comparative experiments, the results show that according to the invention white palladium coatings were obtained, which are much more similar in appearance rhodium, as can be achieved according to the methods of said three PS-en. These test results are as follows:

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<td rowspan="2">examples game</td><td rowspan="2">source</td><td colspan="4">test conditions</td>

<td>Pd g / i</td><td>Temp. ° C</td><td>P_H</td><td>Dense ASF</td>

<td>1</td><td>Invention Example 1</td><td>2</td><td>22</td><td>9.2</td><td>10</td>

<td>2</td><td>U.S. Patent No. 4,098,656, Ex. 1</td><td>10</td><td>50</td><td>8.5</td><td>5</td>

<td>3</td><td>British Patent No. 1,035,850 Ex. 1</td><td>11</td><td>30</td><td>8.5</td><td>6</td>

<td>4</td><td>U.S. Patent No. 4,066,517, Ex. 1</td><td>10</td><td>37.7</td><td>9.0</td><td>10</td>

<td rowspan="2">examples game</td><td rowspan="2">source</td><td colspan="4">% Reflectance based on% rhodium metal reflectance</td>

<td>400 nm</td><td>500 nm</td><td>600 nm</td><td>700 nm</td>

<td>1</td><td>Invention Example 1</td><td>85</td><td>93</td><td>93</td><td>96</td>

<td>2</td><td>U.S. Patent No. 4,098,656, Ex. 1</td><td>82</td><td>89</td><td>89</td><td>91</td>

<td>3</td><td>British Patent No. 1,035,850 Ex. 1</td><td>73</td><td>82</td><td>84</td><td>85</td>

<td>4</td><td>U.S. Patent No. 4,066,517, Ex. 1</td><td>77</td><td>84</td><td>86</td><td>86</td>

Further experiments were carried out in which the prior art, which gave the results closest to the invention, was compared with the same proportion of palladium in the bath, namely 1 g / l. These results are as follows:

<td rowspan="2">example</td><td colspan="4">% Reflectance based on% rhodium metal reflectance</td>

<td>400 nm</td><td>500 nm</td><td>600 nm</td><td>700 nm</td>

<td>1</td><td>92</td><td>94</td><td>93</td><td>94</td>

<td>3</td><td>73</td><td>82</td><td>88</td><td>91</td>

The above results clearly show that the invention provides palladium coatings that are significantly more similar in appearance to rhodium than any of the prior art coatings.

¹⁰ According to an essential feature of the invention, it is necessary that ammonium ions in the

System as part of the conductive salt and they are also used to adjust the value, preferably to increase the p ^ value to about 9. It has been found that if the bath contains disodium phosphate in place of the ammonium phosphate, the desired white coating is not obtained. Unsatisfactory results were also obtained as the p ^ value¹⁵ was adjusted with sodium or potassium hydroxide. It should be understood, however, that the presence of sodium ions has no adverse effect on the coating since sodium tetraborate is an acceptable buffer for the system.

The sparingly soluble source of palladium metal in the electroplating bath of the invention may be any palladium amine complex such as nitrate, nitrite, chloride, sulphate and sulphite²⁰ complex. Typical such useful complexes are palladium diaminodinitrite and palladamine chloride, with palladium diaminodinitrite being preferred. The palladium content of the plating bath is at least sufficient to deposit palladium on the substrate when the bath is electrolyzed, but is less than that which causes the coating to darken. Typically, the palladium concentration is about 0.1 to 20 g / l, with concentrations of about²⁵ 1 to 6 g / 1 are preferred.

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The conductive salt may be any sparingly soluble ammonium-containing inorganic salt, such as dibasic ammonium phosphate, ammonium sulfate, ammonium chloride and the like. Mixtures of such salts can also be used. The proportion of ammonium salt in the plating bath is at least such as to impart sufficient conductivity to the bath to effect the electrolytic precipitation of palladium and is up to the maximum solubility of the salt in the bath. Typically, the conductive ammonium salt is present at a level of about 30 to 120 g / l, with levels of about 50 to 100 g / l being preferred.

As mentioned above, the third material used in formulating the electroplating bath of the invention is ammonium hydroxide. This compound is used in a proportion sufficient to raise the pH of the bath to the desired range, namely about 8 to 10, preferably about 9 to 9.5. In general, the ammonium hydroxide is used in proportions of about 10 to 50 ml / 1 plating bath.

Buffers such as ammonium biborate, sodium tetraborate, trisodium phosphate and the like can be used to ensure that the desired pi value is maintained during plating in the bath. The proportion of buffering agent or agent used in the plating bath may be from 0 to about 50 g / l, preferably about 10 to 30 g / l.

The temperature of the palladium plating bath can be maintained between room temperature and 71 ° C. To avoid the evolution of excess ammonia from the solution, the plating temperature is preferably below about 54 ° C. For many purposes, working at room temperature is preferred. Current densities of about 0.01 to 5 A / dm² are suitable. In general, current densities of 0.2 to 2 A / dm², preferably about 1 A / dm², be applied.

Another feature of the invention is that only thin palladium coatings are formed to ensure the formation of a white coating. Thus, the coating thickness may vary from about 0.01 to 0.5 gm, preferably from 0.03 to 0.4 gm.

The whiteness characteristic of the invention is determined as the reflectance of white light as measured by spectrophotometric techniques, such as using a Perkin Elmer 559 spectrophotometer and plating the coatings to be tested on 2.5χ 2.5 cm plates, labeled 0 0.01 mm (0.5 mil) copper and then preplated with 0.01 mm (0.5 mil) nickel, hereinafter referred to as nickel plated plates, to eliminate surface defects. The reflectance of white light of these plates is scanned in the transmission mode from 400 to 700 nm against a magnesium oxide reference plate. The scan of the sample coating is then compared to a similar scan of a rhodium coating.

Electroplating baths with a pi value of 9 to 9.5 are as follows:

<td></td><td>component</td><td></td><td></td><td colspan="2">concentration</td>

<td>(A)</td><td>Pd (NH₃)₂(NO₂)₂</td><td>1</td><td>to</td><td>6</td><td>g / 1 (as Pd)</td>

<td>(B)</td><td>conductive salt</td><td>50</td><td>to</td><td>100</td><td>g / 1</td>

<td>(C)</td><td>ammonium hydroxide</td><td>10</td><td>to</td><td>50</td><td>ml / 1</td>

<td>(D)</td><td>buffer</td><td>0</td><td>to</td><td>50</td><td>g / 1</td>

*

Palladiumdiaminodinitrit

The following examples are intended to illustrate the invention in more detail, but without this being intended to be limited thereto.

Example 1: A palladium electrolytic solution was prepared by dissolving the following ingredients in water:

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Component concentration

Palladium diaminodinitrite 2 g / l (as Pd) dibasic ammonium phosphate 95 g / l

Ammonium hydroxide 24 ml / 1

The proportion of ammonium hydroxide used in the above formulation represented the p_ßValue to about 9.2. The plating was done at ambient temperature and with a current density of 1 A / dm² for 45 seconds on a nickel plated plate to give a white palladium coating 0.25 to 0.35 gm thick.

Example 2: A plating bath similar to Example 1 but using a buffer was formulated as follows:

Component concentration

<td>Palladiumdiaminodinitrit</td><td>2</td><td>g / 1 (as Pd)</td>

<td>dibasic ammonium phosphate</td><td>96</td><td>g / 1</td>

<td>ammonium biborate</td><td>25</td><td>g / 1</td>

<td>ammonium hydroxide</td><td>24</td><td>ml / 1</td>

The proportion of ammonium hydroxide used in this formulation represented the p_ßValue also at about 9.2. The plating was done at ambient temperature and with a current density of 1 A / dm² for 45 seconds on a nickel plated plate to give a white palladium coating 0.25 to 0.35 gm in thickness. The ammonium biborate acted as buffer and held the p_ßValue at the desired value.

Example 3: A plating bath similar to Example 2 was formulated as follows except that sodium tetraborate was used as the buffer.

Component concentration

<td>Palladiumdiaminodinitrit</td><td>4</td><td>g / 1 (as Pd)</td>

<td>monobasic ammonium phosphate</td><td>50</td><td>g / 1</td>

<td>ammonium hydroxide</td><td>24</td><td>ml / 1</td>

<td>Sodium tetraborate</td><td>25</td><td>g / 1</td>

The aqueous solution contained sufficient ammonium hydroxide to afford the p_ßValue to 9. The plating was carried out under the conditions of Examples 1 and 2 to give a white palladium coating having a thickness of 0.25 to 0.35 gm.

In the following table, the reflectance of white light of the palladium coatings on the nickel plated plates of Examples 1 to 3 with a rhodium coating on a nickel plated plate and with coatings prepared according to Example 3 of U.S. Patent Nos. 4,098,656 and 3,330,149 (page 1) , Lines 77 to 102, and page 2, lines 1 to 8) were compared. The coatings of said US-PS had a thickness of 0.25 to 0.35 gm. The Perkin-Elmer spectrophotometer and the above test method were used.

Nr.375964

- 6 table

<td rowspan="2">coating</td><td colspan="4">% Reflectance</td>

<td>400 nm</td><td>500 nm</td><td>600 nm</td><td>700 nm</td>

<td>rhodium</td><td>80.5</td><td>85.0</td><td>88.5</td><td>90.5</td>

<td>U.S. Patent No. 4,098,656</td><td>60.0</td><td>71.5</td><td>78.0</td><td>80.5</td>

<td>U.S. Patent No. 3,330,149</td><td>51.5</td><td>60.0</td><td>66.5</td><td>72.0</td>

<td>example 1</td><td>63.5</td><td>75.0</td><td>80.0</td><td>82.5</td>

<td>Example 2</td><td>64.5</td><td>75.5</td><td>81.0</td><td>83 "5</td>

<td>Example 3</td><td>63.0</td><td>74.5</td><td>80.0</td><td>83.0</td>

The above data shows that the electroplating baths of the present invention provide a significantly improved palladium metal coating as compared to the coatings of the two US patents referred to, which relates to the reflectance of white light. The visual difference of whiteness is so significant that when used commercially, it can make the difference between acceptance and rejection.

When the above data is recorded, u.zw. % Reflectance versus wavelength, as in the drawing, the curve obtained shows the significance between the results obtained.

By means of a scanning electron microscope (SEM) micrographs of the coating prepared according to Example 2 and the coatings formed according to the two mentioned US-PS were made. These micrographs show that the coatings according to US Pat. No. 3,330,149 have extensive dendritic deposits as well as a rough surface. The coatings of U.S. Patent No. 4,098,656, while exhibiting somewhat reduced dendrite growth, still possess significant surface roughness. In contrast, the coating of Example 2 is very smooth without any dendritic deposits. This further illustrates the unique properties of the coatings obtained according to the invention and shows the correlation between the smoothness of the coating and the reflectance of white light.