EP0102874B1 - Nickel or cobalt chemical plating bath using reducing agents containing boron or phosphor - Google Patents

Description

The invention relates to a bath for the chemical deposition of nickel and / or cobalt using a boron or phosphorus-based reducing agent.

The technique of chemical deposition of nickel, cobalt, or nickel-cobalt has been widely used by the industry for many years because of the regularity of the deposit which can be obtained whatever the shape of the part, and what '' these are metal parts or plastic parts.

All the baths currently used for the deposition by chemical, that is to say by auto-catalytic, nickel and / or cobalt, whether acid baths or alkaline baths, use a reducing agent based on boron or phosphorus in the form of alkaline hypophosphite or hydrogenated boron derivative. In addition to this reducing agent, they contain one or more salts of the metal or metals to be deposited, one or more complexing agents of this metal (or of these metals) and a stabilizer.

The role of the complexing agent is to keep the metal in solution and to release it as the deposition reaction proceeds.

The stabilizer is essential for a self-catalyzed reaction such as that occurring here, in order to slow down the speed of the deposition reaction and therefore allow the desired regular deposition to be obtained on the part.

The stabilizers used industrially are, for acid baths, mineral or organic derivatives of sulfur and, for alkaline baths, compounds of metals or metalloids of groups Illb, IVb and Vb of the periodic table of the elements, and in particular thallium.

Although the baths thus formed are entirely satisfactory for usual applications, the Applicant has encountered difficulties in using them for particular applications.

He is concerned, among other things, with the development of metal parts, for example turbine blades, intended to operate at very high temperatures and in highly corrosive gases, in particular sulfur gases, and has developed a surface treatment process for protecting such alloys by chromaluminization (French Patent No. 2,278,794), which thermochemical treatment must be carried out on parts previously coated with a nickel deposit, advantageously produced chemically.

However, it has been found that the use, for this chemical deposition, of baths comprising sulfur stabilizers leads to the formation of a deposit containing, in addition to the metal, sulfur, which, at a concentration of the order of 3 to 5% _o by weight of the deposit, can cause serious phenomena of corrosion of the deposit and / or of the coated substrate. Similarly, the use of a bath comprising, as stabilizer, a heavy metal compound such as thallium leads, in the deposit, to the presence of heavy metal which, for a concentration of the order of 5% by weight of the deposit, causes, by diffusion or by coalescence in the deposit and / or in the substrate, a considerable lowering of the mechanical characteristics of the coated alloy.

Furthermore, these chemical metal deposition techniques are used for coating molds for the manufacture of glass containers intended to receive food products. However, the stabilizers currently used cause the presence on the container of toxic elements which can diffuse in the glass and come into contact with the food product. It therefore appeared desirable to the Applicant to be able to provide a chemical deposition bath of nickel and / or cobalt which does not cause the formation of a deposit comprising such toxic elements.

French patent application 2 329 762 proposes a type of stabilizer different from those usually used, consisting of an iodized organic compound. These compounds do not eliminate the drawbacks of the usual stabilizers because the presence of iodine causes effects comparable to the presence of sulfur. The same can be said of other halogens.

The Applicant has therefore sought to develop a chemical nickel and / or cobalt deposition bath comprising a stabilizer which does not lead to the drawbacks, mentioned above, of the previously known stabilizers.

The bath according to the invention is in accordance with claim 1.

Boron, aluminum, carbon, nitrogen, phosphorus, oxygen, fluorine and chlorine are acceptable constituents of the stabilizer, either because they are not included in the deposit, or because they can be eliminated, either because they do not cause a corrosive or toxic effect.

In particular, fluorine and chlorine can be eliminated. If you want to avoid this operation, you must choose a stabilizer that does not include any metalloid from the Vlla group.

It was by properly studying the phenomenon of autocatalysis that the inventors were able to determine the property which the stabilizers had to present in order to obtain the desired goals.

Their attention was first focused on the electronic structure of the deposited metal. Nickel, for example, has a 3d layer which, instead of having 10 electrons, has 8, and a 4s layer which has two electrons and is therefore saturated. To find a certain electronic stability, nickel tends to lose the two electrons of its layer 4s (by oxidative attacks thus leading to the cation Ni ²⁺ ) or to capture two electrons to saturate its layer 3d (chemisorption). During chemical deposition, the reducing agent tends to oxidize on contact with nickel, which in turn tends to reduce. The electrons released by the oxidation of the reducing agent then come to saturate the 3d layer of nickel, but since the existence of a nickel anion is physically impossible, the metal then gives up its excess electrons to the cation closest to it, which hastens to take these additional electrons to be included in the deposit. It is thus an electrochemical reaction catalyzed by the deposit itself.

The inventors then noted that it could be advantageous to use this property to reduce the activity of the deposit, and that, for this purpose, it was advisable to bring together a product having an electronic doublet which can be easily picked up by the nickel and / or cobalt, to saturate the 3d layer of the nickel atom, thus allowing it to adsorb on the metal.

The tests carried out by the Applicant with baths for chemical deposition of nickel and / or cobalt in accordance with the invention have been entirely conclusive.

As compounds satisfying this condition, mention may be made of heterocycles of aromatic nature comprising one or more heteroatoms of nitrogen and / or oxygen, as defined in claim 1.

The stabilizer can thus be chosen from 5-atom nitrogen heterocycles comprising one or more nitrogen atoms, for example pyrrole, indole, purine, pyrazole, triazole, tetrazole and similar compounds.

The stabilizer can also be chosen from nitrogen heterocycles with 6 atoms comprising one or more nitrogen atoms in the ring, for example pyridine, cinnoline, pyridazine, pyrimidine, pyrazine and the like.

EP-A-0 066 656, published after the priority date of this Application, discloses the presence of imidazole, to the exclusion of any other nitrogen heterocycle, in a chemical deposition bath comparable elsewhere to that of invention. This document does not indicate that imidazole plays the role of stabilizer.

Although unsubstituted heterocycles generally give the best results, these heterocycles can be substituted on one or more of their carbon atoms, the substituent possibly being in particular a hydrocarbon radical, an alcohol, carboxylic acid, ether, ester or amine, an aliphatic or aromatic derivative, a halogen atom, or a nitro or nitroso type substituent.

In the case of a heterocycle with several nitrogen atoms, substituents may be present on at least one of them, provided that at least one unsubstituted nitrogen atom remains, otherwise the easily detected electronic doublet would disappear.

Indeed, the inventors have found that N-methylpyridinium chloride does not have stabilizing properties and that a chemical deposition bath containing this compound decomposes very quickly, which is explained by the masking of the electronic doublet by the substituent .

As substituted derivatives, it is also possible to use heterocycles in which at least two adjacent atoms (carbon or nitrogen) carry substituents which together form an aromatic ring, provided that at least one unsubstituted nitrogen atom remains.

As examples of substituted nitrogen heterocycles, mention may be made of nicotinic acid (a-pyridinecarboxylic), cinnolic acid (a. A-pyridine-dicarboxylic), 2-aminoquinoline, riboflavin, acridine.

Another type of compound which can be used as a stabilizer for a nickel and / or cobalt chemical deposition bath according to the invention consists of oxygenated heterocycles of aromatic nature and their substitution derivatives. Mention may be made of heterocycles with 5 atoms, for example furan, unsubstituted or substituted on one or more carbon atoms of the ring by substituents analogous to those defined above for nitrogen heterocycles.

Finally, a third type of compound which can be used as stabilizers, in accordance with the invention, consists of heavy ethers of the R-Q-R 'type. soluble in water, in which R and R 'are aliphatic radicals. Can be considered as heavy ethers those having a molecular mass sufficient not to lower the boiling point of the bath below 70 ° C, minimum temperature of industrial use of most baths. Ethers in which the radicals R and R 'each contain at least 3 carbon atoms are thus suitable.

The stabilizers according to the invention, which have no sulfur, unwanted heavy metal or metalloid, are very satisfactory, in particular for coating parts of alloys intended to operate at very high temperatures or for the manufacture of parts intended to enter into contact with food products.

Furthermore, it can be noted that the type of stabilizer according to the invention acts directly on the metal of the deposit and can therefore be used whatever the reducing agent used. However, in the case where a boron hydride reducing agent is used, for example an alkaline borohydride such as NaBH ₄ or KBH ₄ or an aminoborane such as BH ₃ , R ₂ NH where R is an aliphatic radical, the the addition of a secondary stabilizer is made necessary in order to slow down the activity of the reducing agent, as usual in the chemical deposition baths hitherto used employing primary sulfur or heavy metal stabilizers. For this purpose, a chromium compound is advantageously used for weakly acidic or moderately alkaline baths and an arsenic compound for strongly alkaline baths. These secondary stabilizers also never cause the presence of residues in the deposits obtained.

The baths according to the invention can comprise a single compound or a mixture of stabilizing compounds.

The stabilizer content is between limits varying according to the operating conditions. If the bath contains too small a quantity of stabilizer, spontaneous decomposition of the bath takes place and if, on the other hand, it contains an excess of stabilizer, the speed is then practically zero.

It may be indicated that, for acidic baths, it is preferable that the concentration of the stabilizing bath is between approximately 5.7 · 10-3 and 6.9 10 ⁻³ M and that it is preferably from 1 '' 6.3 · 10 ^-3 M.

For alkaline baths, the stabilizer concentration is advantageously between approximately 2.5 · 10 ^-4 and 3.5 · 10 ^-4 M, and it is preferably of the order of 3 · 10 ^-4 M.

The invention is illustrated below by a few examples of baths which are in no way to be considered as limiting. The concentrations indicated are expressed in mole / liter (molar concentration).

Examples are given of weakly acid baths, weakly alkaline baths and alkaline baths, using boron-based or phosphorus-based reducing agents.

A. Weakly acid baths: 4.0 <pH <7.0 Example 1

• metal salt: nickel chloride hexahydrate 0.13 M
• complexing agents:
  • sodium citrate 4.3 - 10- ² M
  • sodium succinate 4.4 - 10 - ² M
  • sodium acetate 2.4 - 10- ¹ M
• stabilizers:
  • - primary: pyridine 6.3 - 10 ^-3 M
  • - secondary:
    • 20 ml of a solution of 0.5 M in chromium chloride hexahydrate and 0.7 M in citric acid, the whole brought to pH = 6 by ammonia, which provides in the bath chromium chloride hexahydrate (CrCl ₃ , 6H ₂ 0) 9 · 10 ^-3 M and citric acid 1.4 · 10- ² M.

The pH, the value of which is determined by electrometric measurement, is adjusted between 5.0 and 5.5 using dilute ammonia.

The temperature for using such a bath is between 50 and 80 ° C.

It has been tested with an aqueous solution of diethylaminoborane as a reducing agent.

A deposition rate of the order of 10 μm / h was obtained for a diethylaminoborane concentration of 2.5 g / l at a bath temperature of 72 ° C.

Example 2

• sel métallique : chlorure de nickel hexahydraté 0,21 M
• complexants :
  • acide glycolique 3 - 10^-4 M
  • acide lactique 1,6 · 10^-4 M
  • acide succinique 5,5 · 10-² M
• stabilisants :
  • - primaire : pyridine 6,3 - 10^-3 M
  • - secondaire :
    • chlorure de chrome hexahydraté 9 · 10^-3 M
    • acide citrique 1,4 - 10-² M

The bath according to this example differs from the previous bath in particular by the nature of the complexing agents. Its composition is as follows:

• metal salt: 0.21 M nickel chloride hexahydrate
• complexing agents:
  • glycolic acid 3 - 10 ^-4 M
  • lactic acid 1.610 ^-4 M
  • succinic acid 5.5 · 10- ² M
• stabilizers:
  • - primary: pyridine 6.3 - 10 ^-3 M
  • - secondary:
    • chromium chloride hexahydrate 9 · 10 ^-3 M
    • citric acid 1.4 - 10- ² M

The pH of the bath, the value of which is determined by electrometric measurement, is adjusted between 6.0 and 6.5 using dilute ammonia.

The temperature for using such a bath is between 70 and 80 ° C.

It was tested with an aqueous solution of dimethylaminoborane as a reducing agent.

For such a temperature of between 70 and 80 ° C, and for a reducing agent concentration of 4 g / l, a deposition rate of between 10 and 15 μm / h is obtained.

Example 3

A bath identical to that of Example 2 is prepared, except that cobalt chloride hexahydrate in a concentration of 0.2 M is used in place of nickel chloride hexahydrate.

The temperature for using such a bath is between 70 and 80 ° C.

This bath was tested using an aqueous solution of dimethylaminoborane as a reducing agent.

For a reducing agent concentration of 4 g / l, and for a temperature between 70 and 80 ° C, a deposition rate between 10 and 15 µm / h is obtained respectively.

Example 4

A bath identical to that of Example 2 is prepared, except that nickel chloride hexahydrate is used in a concentration of 0.17 M and cobalt chloride hexahydrate in a concentration of 0.042 M at instead of only nickel chloride hexahydrate.

The temperature of use of this bath is between 70 and 80 ° C.

By using an aqueous solution of dimethylaminoborane as reducing agent and operating at a bath temperature of between 70 and 80 ° C., a deposition rate of between 10 and 15 μm / h is obtained respectively.

B. Weakly alkaline bath: 7.0 <pH <9.0

For such baths, good complexing agents consist of ammonium salts.

Example 5

• complexants :
  • acide succinique 0,3 M
  • citrate d'ammonium
  • diammonique 8,8 · 10-² M
• stabilisants :
  • - primaire : pyridine 6,3 · 10^-3 M
  • - secondaire :
    • chlorure de chrome hexahydraté 1 10^-3 M
    • acide citrique 1,4 · 10^-2 M

metal salt: 0.21 M nickel chloride hexahydrate

• complexing agents:
  • 0.3 M succinic acid
  • ammonium citrate
  • 8,8 · 10- ² M
• stabilizers:
  • - primary: pyridine 6.310 ^-3 M
  • - secondary:
    • chromium chloride hexahydrate 1 10 ^-3 M
    • citric acid 1.410 ^-2 M

The pH of the bath, the value of which is determined by electrometric measurement, is adjusted between 7.0 and 8.0 using dilute ammonia.

The bath is tested with, as reducing agent, an aqueous solution of dimethylaminoborane.

For a bath use temperature between 50 and 70 ° C, and for a reducing agent concentration of 4 g / I, a deposition rate of between 5 and 10 μm / h is obtained.

C. Strongly alkaline bath: pH> 9 Example 6

• complexant : éthylène diamine : 1 M
• stabilisants :
  • - primaire : alcool furfurylique : 2,9 . 10-⁴ M
  • - secondaire : acide arsénique : 4,4 · 10^-3 M Pour porter le pH à la valeur appropriée, on utilise de la soude caustique 1 M.

metal salt: nickel chloride hexahydrate: 0.13 M

• complexing agent: ethylene diamine: 1 M
• stabilizers:
  • - primary: furfuryl alcohol: 2.9. 10- ⁴ M
  • - secondary: arsenic acid: 4.4 · 10 ^-3 M To bring the pH to the appropriate value, use caustic soda 1 M.

The pH is then between 13 and 14.

This bath is tested with, as reducing agent, an aqueous solution of sodium borohydride.

Its temperature is maintained between 92 and 95 ° C.

For this temperature range, and for a reducing agent concentration of 0.5 g / l, a deposition rate of between 5 and 10 μm / h is obtained.

Claims (12)

1. A bath for the chemical deposition of nickel and/or of cobalt, comprising a salt of the metal (or of the metals) to be deposited, one or more complexing agents of this metal (or of these metals), a reducing agent comprising boron or phosphorus and a stabilizing agent, characterised in that the stabilizing agent is an organic compound soluble in water, comprising neither sulphur nor any metal or metalloid of the groups lllb (except boron and aluminium), IVb (except carbon), Vb (except nitrogen and phosphorus), Vlb (except oxygen) and Vllb (except fluorine and chlorine), and having an electron pair which can easily be captured by nickel and/or cobalt, selected from among the heterocyclic compounds of an aromatic character comprising one or more heteroatoms of nitrogen and/or of oxygen, excluding imidazole, and the ethers soluble in water corresponding to the formula R-0-R' in which R and R' each comprise at least three atoms of carbon.

2. A bath as claimed in claim 1. characterised in that the stabilizing agent does not comprise any metalloid of the group Vllb.

3. A bath as claimed in one of the Claims 1 and 2, characterised in that the stabilizing agent is selected from among the heterocyclic compounds of an aromatic character comprising a heteroatom of nitrogen.

4. A bath as claimed in claim 3, characterised in that the stabilizing agent is selected from among the heterocyclic compounds with five atoms, the ring of which includes a nitrogen atom, and from among their N- or C-substituted derivatives.

5. A bath as claimed in claim 3, characterised in that the stabilizing agent is selected from among pyrrole and indole.

6. A bath as claimed in claim 3, characterised in that the stabilizing agent is selected from among the heterocyclic compounds with six atoms, the ring of which includes a nitrogen atom, and from among their N- or C-substituted derivatives.

7. A bath as claimed in claim 6, characterised in that the stabilizing agent is pyridine.

8. A bath as claimed in claim 1, characterised in that the stabilizing agent is selected from among the oxygenated heterocyclic compounds with five atoms and their substituted derivatives.

9. A bath as claimed in claim 8, characterised in that the stabilizing agent is furan.

10. A bath as claimed in any one of the preceding claims, characterised in that it comprises a mixture of a plurality of stabilizing compounds.

11. A bath as claimed in any one of the preceding claims, characterised in that, for an acid bath, the stabilizing compound (or compounds) is (are) present in a molar concentration comprised between about 5.7 x 10-³ M and 6.9 x 10-³ M, preferably of the order of 6.3 x 10-3 M.

12. A bath as claimed in any one of the claims 1 to 11, characterised in that, for an alkaline bath, the stabilizing compound (or compounds) is (are) present in a molar concentration comprised between about 2.5 x 10-4 M and 3.5 x 10-⁴ M. preferably of the order of 3 x 10^-4 M.