EP0309031A1 - Baths and process for chemically polishing copper or copper alloy surfaces - Google Patents

Abstract

Baths for the chemical polishing of copper or copper alloy surfaces, comprising, in aqueous solution, hydrogen peroxide, chloride ions, phosphoric acid and phosphate and hydrogen phosphate salts in controlled amounts to give the bath a pH between 1.25 and 3.

Description

The subject of the present invention is the composition of baths for the chemical polishing of copper or copper alloy surfaces.

Chemical polishing of metal surfaces is a well-known technique (electrolytic and chemical polishing of metals - W.J. Mc G. TEGART - Dunod - 1960 - p. 122 et seq.); it consists of treating the metal surfaces to be polished with oxidizing baths.

For the chemical polishing of copper and its alloys, aqueous baths of orthophosphoric acid, nitric acid and acetic acid were used (dito: pages 135 and 136). These baths require high working temperatures, of the order of 50 to 80 ° C and intense mechanical stirring. They also attack the metal at high speed, limiting the duration of polishing to less than five minutes. These particularities of these known baths constitute disadvantages: on the one hand, their use is accompanied by toxic gaseous emissions, on the other hand, their high speed of action and the need to subject them to mechanical agitation makes control difficult and random polishing. To overcome these disadvantages, aqueous baths have been proposed comprising hydrogen peroxide and a mixture of nitric, phosphoric and hydrochloric acids. These aqueous baths allow lower working temperatures, of the order of 25 to 35 ° C and their metal attack speed is between 2.5 and 5 microns per minute (Electroplating -October 1953 - 6 - pages 360 to 367 (pages 363 and 364)). The speed of action of these known baths on metal is nevertheless still excessive for certain applications. In particular, it makes them unusable for polishing the internal face of large vessel walls, such as boilers, autoclaves or crystallizers. The time required for filling and emptying such tanks being generally much greater than the duration of the optimum chemical polishing treatment, it indeed becomes impossible to obtain a uniform polish of the wall, certain areas of the latter being insufficiently others are deeply corroded. These known baths are also ineffective for polishing surfaces in contact with which the renewal of the bath is difficult, since this results in abrupt modifications to the local compositions of the bath.

Baths have also been proposed comprising, in aqueous solution, phosphoric acid, hydrogen peroxide, hydrochloric acid and 2,6-di-tert-butyl-4-N, N-dimethyl-aminomethylphenol (SU-A-1211338 (ORG. PHYS. CHEM. INST.)). These known baths would have the property of better stability, but they imply working temperatures of at least 50 ° C., their speed of action is too fast and they do not allow polishing of regular quality.

The invention aims to remedy the aforementioned drawbacks of known polishing baths, by providing novel bath compositions for the chemical polishing of copper and copper alloys, which have a moderate speed of action, do not require a working temperature. excessive nor intense mechanical agitation and ensure polishes of higher quality than that of polishes obtained with known baths.

The invention therefore relates to baths for the chemical polishing of copper or copper alloy surfaces, which comprise, in aqueous solution, hydrogen peroxide, chloride ions and a mixture of phosphoric acid, phosphate ions and hydrogen phosphate ions in respective amounts adjusted to give the aqueous solution a pH value of between 1.25 and 3.

In the baths according to the invention, the hydrogen peroxide serves as an oxidizing agent for the metal to be polished.

The function of chloride ions is to protect the metal from anarchic local corrosion during the polishing treatment. They can be used in the form of all water-soluble compounds such as hydrochloric acid or alkali metal chloride. Sodium chloride is preferred.

The phosphate and hydrogen phosphate ions are anions of general formula:
(H _x PO₄) ^{(3-x) -}
where x is between 0 and 2.

The baths according to the invention can contain mixtures of these anions. These can be used in the form of any water-soluble inorganic compounds, such as alkali metal salts.

According to the invention, the respective amounts of phosphoric acid and of phosphate and hydrogen phosphate anions are chosen so as to give the aqueous solution of the bath, a pH value of between 1.25 and 3, as obtained. by mathematical calculation from the contents of phosphoric acid and of phosphate and hydrogen phosphate anions in the aqueous solution. This imposed pH value differs from the actual value actually measured which depends in particular on the content of hydrogen peroxide and chloride ions in the aqueous solution. Thereafter, unless otherwise stated, the pH values mentioned will be calculated theoretical values as defined above.

où [(H_x PO₄)^(3-x)-] désigne la concentration, exprimée en mole par litre, du constituant (H_x PO₄)^(3-x)- dans la solution aqueuse du bain.By achieving the above-mentioned pH value in the aqueous solution of the bath, the respective contents of hydrogen peroxide, chloride ions, phosphoric acid and phosphate and hydrogen phosphate salts are chosen according to the nature of the metal treated, the working temperature and the desired duration for the polishing treatment. Baths which are well suited in the majority of applications are those in which the hydrogen peroxide content is between 1 and 6 moles per liter of the aqueous solution and the chloride ion content is between 10⁻⁴ and 1 mole per liter. It suits moreover, to select the respective contents of phosphoric acid and of phosphate and hydrogenphosphate anions so that their sum is between 10⁻⁴ and 1 mole per liter of the aqueous solution of the bath, the optimum contents being those for which we have in addition :

where [(H _x PO₄) ^{(3-x) -} ] denotes the concentration, expressed in moles per liter, of the constituent (H _x PO₄) ^{(3-x) -} in the aqueous solution of the bath.

The best results are obtained when the respective contents of phosphoric acid, phosphate ions and hydrogen phosphate ions are chosen so that the following conditions are met simultaneously:

Preferred baths are those in which the aqueous solution has a pH value of between 1.65 and 2.35 and comprises:
- hydrogen peroxide in an amount between 3 and 5 moles / l;
- chloride ions in an amount between 5.10⁻³ and 5.10⁻² mole / l;
- phosphoric acid and the phosphate and hydrogen phosphate ions in respective molar quantities in accordance with the following relationships:

The aqueous solution of the baths according to the invention may contain, in usual proportions, additives commonly present in aqueous baths for the chemical polishing of metals, for example surfactants, viscosity regulators and stabilizers of peroxide. 'hydrogen.

The chemical polishing baths according to the invention make it possible to produce surface polishes of excellent quality, in particular greater than that of the polishes obtained with the polishing baths described in document SU-A-1211338. A great advantage of the polishing baths according to the invention lies in their ability, after adaptation of the respective concentrations of their constituents, to carry out polishing at moderate speed of action, which can be distributed over several hours, so as to allow uniform polishing. large surfaces or hard to reach surfaces.

The baths according to the invention are suitable for polishing all surfaces of copper or copper alloys, such as brass and bronze, for example.

The invention therefore also relates to a method for polishing the surface of a copper or copper alloy object, according to which the surface to be polished is brought into contact with a polishing bath according to the invention.

In the process according to the invention, the polishing bath can be used at all temperatures and pressures for which there is no risk of degrading its constituents. However, it has proved advantageous to use the bath at atmospheric pressure, at a temperature above 20 ° C and below 80 ° C, temperatures between 30 and 60 ° C being preferred.

The contacting of the metal surface with the bath can be carried out in any suitable manner, for example by immersion.

In the method according to the invention, the contact time of the surface to be polished with the bath must be sufficient to achieve effective polishing of the surface; however, it cannot exceed a critical value beyond which local corrosions may appear on the surface. The optimum contact time depends on many parameters such as the metal or alloy constituting the surface to be polished, the configuration and initial roughness thereof, the composition of the bath, the working temperature, the possible turbulence of the bath. in contact with the surface, the ratio between the area of the metal surface to be polished and the volume of bath used; it must be determined in each particular case by routine laboratory work.

In a preferred embodiment of the method according to the invention, the surface to be polished is kept in contact with the bath for a time sufficient to carry out an attack on the metal to a depth at least equal to 10 microns, preferably between 20 and 50 microns. The duration of the treatment of the surface with the bath is thus, in most cases, between 1 and 5 hours.

• La figure 1 reproduit, à grande échelle, le profil d'une surface en cuivre, avant polissage;
• La figure 2 montre le profil de la même surface en cuivre, après polissage conformément au procédé selon l'invention;
• La figure 3 montre le profil d'une surface en cuivre analogue à celle de la figure 1, après qu'elle ait été soumise à un procédé de polissage conforme à celui décrit dans le document SU-A-1211338.

The advantage of the invention will be highlighted on reading the application examples which are given below, with reference to the accompanying drawings.

• Figure 1 reproduces, on a large scale, the profile of a copper surface, before polishing;
• FIG. 2 shows the profile of the same copper surface, after polishing according to the method according to the invention;
• FIG. 3 shows the profile of a copper surface similar to that of FIG. 1, after it has been subjected to a polishing process in accordance with that described in document SU-A-1211338.

Example 1 (according to the invention)

A copper plate, 10 cm² in area, was immersed in 500 cm³ of a bath at about 40 ° C, containing, per liter:
- 3.5 moles of hydrogen peroxide,
- 0.012 mole of hydrochloric acid,
- 0.088 mole of phosphoric acid,
- 0.021 mole of hydrogen phosphate ions (HPO₄) ²⁻,
- 0.013 mole of phosphate ions (PO₄) ³⁻.

This bath had a pH value of 2.32.

The plate initially had an arithmetic mean roughness R _a = 0.40 micron.

During the treatment, which lasted 3 hours, the average depth of attack of the plate was 30 microns. At the end of the 3 hours of treatment, the roughness had fallen to 0.06 microns.

Example 2 (reference)

The test of Example 1 was repeated under operating conditions in accordance with those described in Example 7 of document SU-A-1211388:
- Composition of the polishing bath:
. 9.4 moles of hydrogen peroxide per liter,
. 0.584 mole of phosphoric acid per liter,
. 0.047 mole of hydrochloric acid per liter,
. 0.04 g of 2,6-di-tert-butyl-4-N, N-dimethylaminomethylphenol per liter,
. pH = 1.05;
- Working temperature: 50 ° C;
- Duration of treatment: 15 minutes.

The average attack depth of the plate was 60 microns; at the end of the polishing treatment, the arithmetic mean roughness of the surface was measured: R _a = 0.080 micron.

Figures 1, 2 and 3 show the profile of the plate surface, respectively,
- before polishing,
- after the polishing of Example 1,
- After the polishing of Example 2.

In these figures, the abscissa scale expresses the length of the surface, in mm and the ordinate scale expresses the relief of the surface, in microns.

A comparison of Figures 2 and 3 immediately shows the progress made by the invention in the quality of polishing.

Claims (10)

1 - Baths for the chemical polishing of copper or copper alloy surfaces, comprising, in aqueous solution, hydrogen peroxide, chloride ions and a mixture of phosphoric acid, phosphate ions and hydrogen phosphate ions in respective quantities adjusted to give the aqueous solution a pH value of between 1.25 and 3. 2 - Baths according to claim 1, characterized in that the sum of the respective molar quantities of phosphoric acid, phosphate ions and hydrogen phosphate salts in the aqueous solution is between 10⁻⁴ and 1 mole per liter of solution. 3 - Baths according to claim 2, characterized in that the respective molar amounts of phosphoric acid, phosphate ions and hydrogen phosphate salts in the aqueous solution are such that there are:

where [(H _x PO₄) ^{(3-x) -} ] denotes the concentration, expressed in moles / l, of the constituent (H _x PO₄) ^{(3-x) -} in the aqueous solution. 4 - Baths according to claim 3, characterized in that the aqueous solution comprises hydrogen peroxide in an amount between 1 and 6 moles / l and chloride ions in an amount between 10⁻⁴ and 1 mole / l. 5 - Baths according to claim 4, characterized in that the aqueous solution has a pH value of between 1.65 and 2.35 and comprises:
- hydrogen peroxide in an amount between 3 and 5 moles / l;
- chloride ions in an amount between 5.10⁻³ and 5.10⁻² mole / l;
- phosphoric acid and the phosphate and hydrogen phosphate ions in respective molar quantities in accordance with the relationships:

6 - Baths according to any one of claims 1 to 5, characterized in that the chloride ions are used in the aqueous solution in the form of alkali metal chloride and the phosphate and hydrogen phosphate salts are used at l state of alkali metal salts of phosphoric acid. 7 - Process for polishing a surface of copper or copper alloy, according to which the surface is brought into contact with a chemical polishing bath according to any one of claims 1 to 6. 8 - Process according to claim 7, characterized in that the surface is kept in contact with the bath for a time sufficient to carry out an attack on the metal to a depth at least equal to 10 microns. 9 - Process according to claim 8, characterized in that the surface is kept in contact with the bath for a time sufficient to carry out an attack on the metal to a depth between 20 and 50 microns. 10 - Process according to claim 8 or 9, characterized in that the bath temperature is adjusted between 30 and 60 ° C and it is kept in contact with the surface for a time between 1 and 5 hours.

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