DE1278797B - Bath and process for the galvanic coating of metals with iridium - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. Cl .:

Number:
File number:
Registration date:
Lay-out day:

C 23b

German class: 48 a-5/24

P 12 78 797.2-45 (J 27615)

March 3, 1965

26th September 1968

The invention relates to an aqueous bath containing halogen acid and a method for electroplating Coating of metals with iridium, if necessary with the arrangement of an intermediate layer.

Iridium is an exceptionally hard white metal with excellent resistance to corrosion in aqueous media, concentrated acids, aqua regia and most halogens. However, this metal has not yet been used industrially. Various methods for electrodeposition of iridium were FH Reid in "Metallurgical Reviews", 1963, Vol ₁ 8, pp 196-199 and 207 to 208 tested. However, none of the baths mentioned is suitable for use on an industrial scale.

Furthermore, in the journal "Journal of the Electrochemical Society", January 1962, pp. 61 to 63, Process for the deposition of iridium from aqueous halogen acid-containing baths described. Experiments with baths containing hydrochloric acid are reported in detail, and it is stated that this is also the case when using sodium and potassium chlorides and bromoiridates, but not with trichlorides and tetrachlorides or tribromides and tetrabromides of iridium precipitates are generated as could. However, recent attempts have shown that the known aqueous halogen acid containing Baths not only result in a low deposition rate, but this after the start of the experiment also decreases rapidly. In addition, the coatings produced are of poor quality. this This also applies to iodine baths, which are also unstable, while fluorine baths are unsuitable because of their aggressiveness are.

The invention is based on the surprising finding that solutions of iridium in hydrobromic acid are excellently suited for the galvanic coating of metals with iridium. Such solutions can be prepared by dissolving anhydrous or hydrous iridium dioxide in hydrobromic acid. Although the form in which iridium and bromide are present in such solutions is still uncertain, it can be assumed that the superiority of the bromic acid-containing baths is due to the fact that the iridium is present in them as a cation complex IrBr _x , while in others containing halogen acid Baths forms an anion complex of _{the formula H 2 IrA 6} , in which A is a halogen but not bromine. More specifically, the invention consists in a bath consisting essentially of a solution of iridium in hydrobromic acid and tetravalent iridium in a bath and a method for electroplating metals with iridium

Applicant:

International Nickel Limited, London

Representative:

Dipl.-Ing. H. Sauerland, patent attorney,

4000 Düsseldorf, Cecilienallee 76

Named as inventor:
Colin John Nelson Tyrrell,
Ilford, Essex (UK)

Claimed priority:

Great Britain March 4, 1964 (9172)

in an amount of at least 0.5 g / l and 0.1 to 1.0 mol / l of free hydrobromic acid.

Although an iridium content of 0.5 g / l is sufficient, it is preferably 5 to 10 g / l. Higher concentrations of iridium up to 20 g / l can be used, although the stability of the bath is lower while there is a smaller increase in the deposition rate.

Iridium can be electroplated on various metals including copper, brass, nickel, soft Separate steel, molybdenum and titanium. Metals like molybdenum and titanium that are not affected by the electrolyte attacked can be overdrawn directly. The other metals must, however, for example be protected by a coating of palladium or preferably gold, otherwise no adhesive coatings result. The surface condition of the base metal is of great importance, so that careful preparation of the surfaces is essential in order to achieve good adhesion. Very bright surfaces are not always suitable for direct coating with a thick layer of iridium, so it is preferable to etch the surfaces, around the surface covering and anything to eliminate existing passive film. Such a pretreatment should be performed on a base body made of titanium or molybdenum.

When electroplating with the bath according to the invention, the cathode current density and the initial degree of acidity of the electrolyte are of decisive importance. The current density must be at least 0.14 A / dm ² , since it is lower

809 618/433

Current density, the rate of deposition is very weak and the base body is partially attacked. If the cathode current density increases to values above 0.14 A / dm ² , the deposition rate also increases to a maximum and then decreases again, while the efficiency of the cathode decreases progressively. For these reasons, the current density preferably does not ^{exceed 0.35 A / dm 2.} Nevertheless, the current density can be significantly higher; and generally vary in the range of 0.14 to 12 A / dm ² with little effect on the quality of the coating. At current densities above 12 A / dm ² , the bath becomes unstable and black precipitates appear.

If not at least 0.1 mol / l free hydrobromic acid are present, the bath will gradually decompose. However, if the bathroom is more than Contains 1.0 mol / l of free hydrobromic acid, the rate of deposition decreases very rapidly.

The influence of temperature is slight in the range from room temperature to 60 ° C. The rate of deposition is exceptionally low in this temperature range, but increases rapidly above 60 ° C to a maximum at 75 ° C. At temperatures above 80 ° C it then drops again quickly. The optimum concentration was found to be 5 g / l iridium and 0.1 mol / l hydrobromic acid and a temperature of 70 to 80 ° C., preferably 75 ° C., and a current density of 0.15 A / dm ^{2 were} found to be the optimum working conditions . For most of the base metals, the cathodic current efficiency is about 65% under these conditions, but not more than 45% for titanium. Insoluble anodes are used, which are suitably made of platinum or iridium.

The baths can also contain other ingredients. Small amounts of gloss-producing or Agents that increase conductivity are harmless, but also without any particular advantage. Reducing agent have no effect on the rate of deposition and should be avoided will.

Below are some examples of the electrolytic Deposition indicated:

example 1

A bath containing 8.0 g / l iridium and 0.3 mol / l free hydrobromic acid was prepared by dissolving 4.66 g of iridium dioxide in 20 ml of aqueous hydrobromic acid (46 weight percent HBr) and diluting to 500 ml. With this bath, iridium was deposited on a cleaned titanium cathode. The following procedure was used for cleaning: the titanium cathode was immersed in an aqueous solution containing 3% each of nitric acid and hydrofluoric acid, and then placed in concentrated hydrochloric acid (specific weight 1.18) for 16 hours. After removing the dirt layer created by this treatment, the titanium surface was subjected to a short cathode cleaning treatment before it was introduced into the electroplating bath. An iridium anode was used for the deposition. The bath temperature was 75 ° C. and the cathode current density was 0.16 A / dm ² . Under these conditions, an iridium layer 2.5 microns thick resulted in 2.5 hours with a cathode current efficiency of 45.3%. An increase in the current density to 0.65 A / dm ² led to a decrease in the cathode current yield to 15%. The time required for an iridium layer 2.5 microns thick to form increased to 4 hours.

Example 2

In a bath according to Example 1, iridium was deposited electrolytically on a titanium cathode which had been pretreated as in Example 1. At different bath temperatures, the cathode current density was 2 A / dm ^{2 in} each case. The deposition rate as a function of the bath temperature is shown in the table below:

Table I.

Bath temperature Precipitation ( ⁰ Q (Microns per hour) 25th 0.020 40 0.034 50 0.051 60 0.16 70 0.37 80 0.67 (the bathroom became unstable)

Example 3

This example reproduces the results obtained with a bath as described in example 1 is, when electroplating cathodes made of five different metals, each at one temperature of 75 ° C using an iridium anode.

The sheet metal strips made of copper, brass, nickel and soft steel were after Degrease briefly immersed in a 5% sulfuric acid, then washed with water and with provided with a thin coating of gold, which is electrolytically deposited from an acidic gold-cyanide bath, buffered to pH 4 was generated. The fifth metal was a molybdenum sheet used that after degreasing and subsequent etching for 7.5 minutes in a 90 ° C hot aqueous Solution of 10% sodium hydroxide and 10% potassium ferricyanide was immersed. The following The table shows the results:

Table Π

cathode

Time
(Hours) cathode current density
(A / dm ² )

Cathode current efficiency

C / o)

Layer thickness
(Micron)

copper

Brass

nickel

Soft steel
Molybdenum ...

1
1
1
1
6th

0.16
0.16
0.16
0.16
0.16

61.9

61.8

69.2

0.76
0.76
1.09
0.56
5.13

Claims (5)

Patent claims: 1. Aqueous bath containing halogen acid for electroplating metals with Iridium, characterized in that it consists essentially of a solution of iridium in hydrobromic acid, the tetravalent iridium in an amount of at least 0.5 g / l and 0.1 to 1.0 mol / l of free hydrobromic acid. 2. Bath according to claim 1, characterized in that its iridium content is 5 to 10 g / l. 3. A method for the galvanic coating of metals with iridium using a bath according to claim 1 and 2, characterized in that it is carried out at a bath temperature of 70 to 80 ° C and with a current density of 0.14 to 0.35 A / dm ² will. 4. The method according to claim 3, characterized in that when plating copper, Brass, nickel or soft steel, an intermediate layer of palladium or gold is applied will. 5. The method according to claim 3, characterized in that when coating titanium or molybdenum the surfaces are etched beforehand. Considered publications:
Journal of the Electrochemical Society, January 1962, pp. 61-63. 809 618/433 9.68 ® Bundesdruckerei Berlin