EP0193239B1 - Compositions and process for chemically polishing steel surfaces - Google Patents

Abstract

The chemical polishing of steel surfaces is carried out by bringing the latter into contact with a bath comprising, in aqueous solution, hydrochloric acid, phosphoric acid, nitric acid and ferricyanide complex ions. The invention is in particular applicable to the polishing of vessels of large dimensions made from stainless steel alloyed with chromium and with nickel.

Description

The present invention relates to the composition of baths for the chemical polishing of steel surfaces, in particular stainless steel.

Chemical polishing of metal surfaces, which is a well known technique (electrolytic and chemical polishing of metals - WJ Mc G. TEGART - Dunod - 1960 - p. 122 et seq.), Consists in treating the metal surfaces to be polished with baths mineral acids (generally a mixture, in aqueous solution, of hydrochloric, phosphoric and nitric acids) containing suitable additives such as surface-active agents, viscosity regulators and brighteners. Thus, in US-A-3,709,824, there is provided a composition of a bath for the chemical polishing of stainless steel surfaces, comprising, in aqueous solution, a mixture of phosphoric acid, nitric acid and d hydrochloric acid, a viscosity regulator chosen from water-soluble polymers, a surfactant and sulfosalicylic acid as a brightening agent. This known polishing bath has proven to be very effective. However, it has the disadvantage of containing several organic additives, which add to the cost, complicate its implementation and constitute a source of pollution during the rejection of the spent bath.

A bath of simpler composition is known, for the chemical polishing of steels, this bath comprising, in aqueous solution, a mixture of hydrochloric acid, phosphoric acid and nitric acid, potassium ferrocyanide and the surfactant OP- 7 which is a nonionic surfactant from the family of alkylphenols (Central Patents Index, Basic Abstracts Journal, section E, Derwent Publications Ltd, London, abbreviated 13581 U-EM: Soviet author's certificate SU-344 035). In use, this known bath has however proved powerless to achieve uniform polishing, of good quality.

The known polishing baths which have just been described have the common characteristic of attacking the metal at very high speed. A polishing treatment of a steel surface with such baths can generally not exceed a few minutes, at the risk of causing local corrosion. This high speed of action of known polishing baths is a drawback, because it makes them unusable for certain applications, in particular for polishing the internal face of the walls of large tanks, 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. The high speed of action of known chemical polishing baths also makes polishing control difficult. 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.

The object of the present invention is to remedy the aforementioned drawbacks of known polishing baths, by providing bath compositions for the chemical polishing of austenitic stainless steel surfaces, in particular steel alloyed with chromium and nickel, which make it possible to avoid the use of multiple additives, which are effective in all types of application including the polishing of large surfaces or surfaces that are difficult to access and which make it possible to produce polishes of excellent quality.

The invention therefore relates to baths for the chemical polishing of steel surfaces, comprising, in aqueous solution, a mixture of hydrochloric acid, phosphoric acid and nitric acid; according to the invention, the baths comprise, in the aqueous solution, ferricyanide complex ions.

In the baths according to the invention, the ferricyanide complex ions are complex cyanides of general formula [Fe ^lil (CN) _6] ^3- , also called hexacyanoferrates (III) (Encyclopedia of Chemical Technology - Kirk-Othmer - John Wiley & Sons , Inc. - 1967 - Vol. 12 - pages 25, 26, 31, 32). They can be present in the aqueous solution in the form of any dissolved compounds such as, for example, hexacyanoferric acid (III), ammonium ferricyanide and ferricyanides of alkali and alkaline earth metals. Preferred compounds are the alkali metal ferricyanides, potassium ferricyanide being especially recommended.

• - de l'acide chlorhydrique à raison de 2,5 à 5 moles,
• - de l'acide phosphorique à raison de 0,1 à 0,2 mole,
• - de l'acide nitrique à raison de 0,01 à 0,1 mole, et
• - du ferricyanure de potassium à raison de 0,3 x 10-5 à 2 x 10-⁴ ion-gramme.

In the chemical polishing baths according to the invention, the respective contents of phosphoric, hydrochloric and nitric acids, and of ferricyanide complex ions are chosen according to the nature of the metal treated, the working temperature and the duration desired for the polishing treatment. Baths in accordance with the invention which are very suitable for ensuring the chemical polishing of stainless steel surfaces alloyed with chromium and / or nickel, in a time of between 2 and 24 hours are those in which the aqueous solution comprises, per liter, between 1 and 6 moles of hydrochloric acid, between 0.05 and 0.3 mole of phosphoric acid, between 0.005 and 0.5 mole of nitric acid and between 0.3 x 10- ⁶ and 0.3 ion- gram of ferricyanide. In these baths, it is advantageous that the content of ferricyanide ions does not exceed 0.1 gram ion per liter, preferably 0.001 gram ion per liter. Specially recommended baths are those in which the overall molarity of the mixture of acids in the aqueous solution is between 1 and 7, preferably 2 and 6. The molarities between 2.5 and 5 are the most advantageous in the majority of applications. Preferred baths are those in which the aqueous solution comprises, per liter:

• - hydrochloric acid in an amount of 2.5 to 5 moles,
• - phosphoric acid in an amount of 0.1 to 0.2 mol,
• - nitric acid in an amount of 0.01 to 0.1 mole, and
• - potassium ferricyanide at a rate of 0.3 x 10-5 to 2 x 10- ⁴ gram ion.

• - de l'acide chlorhydrique, à raison d'au moins 3 moles,
• - de l'acide phosphorique, à raison d'au moins 0,10 mole,
• - de l'acide nitrique, à raison d'au moins 0,05 mole, et
• - du ferricyanure de potassium à raison de 0,3 x 10-⁴ à 1 _X 10-³ mole.

Baths recommended for slow polishing at low temperature, not exceeding 30 ° C, generally at room temperature from 15 to 25 ° C, are those in which the aqueous solution comprises, per liter:

• - hydrochloric acid, at least 3 moles,
• - phosphoric acid, at a rate of at least 0.10 mol,
• - nitric acid, at a rate of at least 0.05 mol, and
• - potassium ferricyanide at a rate of 0.3 × 10 ^-4 to 1 _× 10 ^-3 mole.

• - 1 : 3 pondéral, dans le cas d'agents tensioactifs de la classe des chlorures dlalkylpyridinium;
• - 1 : 1 pondéral, dans le cas d'agents tensioactifs de la classe des alkylphénols;
• - 1 : 1 molaire, dans le cas d'épaississants choisis parmi les éthers de cellulose.

The baths according to the invention may optionally contain additives usually present in baths known for the chemical polishing of metals, such as, for example, surfactants, corrosion inhibitors, viscosity regulators and brighteners. Where appropriate, it is preferred that the baths contain these additives in relative amounts, with respect to the complex cyanide, which do not exceed respectively:

• - 1: 3 by weight, in the case of surfactants from the class of alkylpyridinium chlorides;
• - 1: 1 by weight, in the case of surfactants from the class of alkylphenols;
• - 1: 1 molar, in the case of thickeners chosen from cellulose ethers.

Preferred baths are those which are essentially free of such additives.

All other things being equal, it has been observed that the substitution of ferricyanide ions, in accordance with the invention, for ferrocyanide ions in the known bath described above (Central Patent Index, Basic Abstracts Journal, section E, Derwent Publications Ltd., London, abbreviated 13581 U-EM: Soviet author's certificate SU-344 035) results in a considerable improvement in the quality of the polishing. Another 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. They are especially well suited for polishing metal surfaces whose area (expressed in m ² ) can range up to approximately six times the volume (expressed in m ³ ) of the polishing bath which is in contact with it.

In a particular embodiment of the polishing baths according to the invention, these contain anions selected from the bromide, iodide and thiocyanate ions. These anions can generally be used in the state of alkali metal bromide, iodide or thiocyanate, the latter advantageously being sodium. The baths according to this embodiment of the invention have proved to be particularly advantageous for the polishing of welded assemblies, at the level of the weld beads. The best results are obtained for contents of bromide anion, iodide or thiocyanate comprised between 10-5 and 1 gram-ion per liter, the contents of between 10- ⁴ and 10- ² gram-ion per liter being preferred.

The baths according to the invention are suitable for polishing all austenitic stainless steel surfaces. They find a particularly advantageous application in the polishing of austenitic stainless steels alloyed with chromium and nickel, in particular those containing between 12 and 26% of chromium and between 6 and 22% of nickel, such as steels 18/8 and 18/10 , for example.

The invention therefore also relates to a method for polishing a steel surface, according to which the surface is brought into contact with a chemical polishing bath according to the invention.

In the process according to the invention, a prefabricated bath can be used, in contact with which the metal surface to be polished is then placed.

However, it is preferred to act in accordance with a particular embodiment of the method according to the invention which consists in forming the polishing bath in situ in contact with the metal surface to be polished. To this end, in accordance with this embodiment of the method according to the invention, the metal surface is first brought into contact with an aqueous solution of hydrochloric acid, phosphoric acid and nitric acid, then added complex ferricyanide ions to the solution, while it is in contact with the metal surface. In the implementation of this embodiment of the method according to the invention, it is advantageous to wait until the metal surface has undergone a substantial attack by the acid solution, before adding the ferricyanide ions thereto; in practice, it is advantageously possible to adjust the time interval between the moment when the surface to be polished is brought into contact with the acid solution and the moment when the ferricyanide complex ions are added to said solution, so that there is corresponds to an attack on the surface by the solution, with a depth of between 0.1 and 6 microns, preferably between 0.5 and 4 microns.

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 the bath used; it must be determined in each particular case by routine laboratory work.

In a particular embodiment of the process according to the invention, using a slow-action polishing bath, the surface to be polished is brought into contact with the aqueous acid solution at a temperature between 15 and 70 ° C. preferably 20 and 55 ° C., the ferricyanide complex ions are added to said solution after having maintained the surface in contact for at least 15 minutes, and then the maintenance of the surface in contact with the resulting bath is continued for a time at least equal 1 hour. In this embodiment of the method according to the invention, the addition of the ferricyanide complex ions to the acid solution can for example be carried out after the surface to be polished has been kept in contact with said solution for a time between 30 and 60 minutes and the resulting bath can then be kept in contact with the surface for a time between 6 and 24 hours.

The advantage of the invention will be highlighted on reading the application examples given below.

Example 1

A chemical polishing bath in accordance with the invention was used to polish the internal face of a spherical stainless steel tank with a capacity of 6 m ³ and a diameter of 2.4 m, made of stainless steel of grade ASTM-316L , which is a steel alloyed with chromium (16.0 to 18.0% nickel (10.0 to 14.0% and molybdenum (2.0 to 3.0% (Engineering techniques - Metallurgy -M 323A-8 (Table G) - July 1983).

The bath used had the following composition: Mixture of acids:

Additive: potassium ferricyanide 20 mg / 1.

The bath was introduced into the tank, at a rate of 3.9 per dm ² of surface to be polished, and it was subjected to a stirring movement therein by means of a three-blade agitator, while maintaining its temperature between 45 and 50 ° C.

After 9 hours of treatment, the tank was drained and rinsed with demineralized water.

At the end of the treatment, the entire interior surface of the tank was found to be shiny and uniformly smooth to the touch.

Example 2

A chemical polishing bath in accordance with the invention was used to polish the external surface of the tubes of a tubular heat exchanger made of stainless steel 1.9 m in diameter and 6 m in length. This exchanger was equipped with 455 tubes of 50 mm in diameter and 9 baffles made, like the ferrule, in stainless steel grade ASTM-304L, which is a steel alloyed with chromium (18.0 to 20.0%) and Nickel (8.0 to 12.0%) (Techniques de l'Ingénieur Métallurgie-M.323A - 8 (Table G) July 1983).

The bath used had the following composition: Mixture of acids in aqueous solution

Additive: potassium ferricyanide 30 mg / I

This bath was introduced into a shuttle circuit at the rate of 1.3 per dm ² of surface to be polished and it was subjected to a linear passage movement over the surfaces to be treated by means of a circulation pump, while maintaining its temperature between 45 and 50 C by injecting steam inside the tubes.

After 6 hours of treatment, the circuit was drained and rinsed with demineralized water.

This bath made it possible to chemically polish, almost perfectly, the outside surface of the tube bundle.

Claims (11)

1. Baths for the chemical polishing of steel surfaces which baths comprise, in aqueous solution, a mixture of hydrochloric acid, phosphoric acid and nitric acid, characterized in that the baths comprise ferricyanide complex ions in the aqueous solution.

2. Baths according to claim 1, characterized in that the aqueous solution comprises, per litre, between 1 and 6 mol of hydrochloric acid, between 0.05 and 0.3 mol of phosphoric acid, between 0.005 and 0.5 mol of nitric acid and between 0.3 x 10-⁶ and 1 x 10-³ gram-ion of ferricyanide.

3. Baths according to claim 2, characterized in that the aqueous solution comprises, per litre, between 2.5 and 5 mol of hydrochloric acid, between 0.1 and 0.2 mol of phosphoric acid, between 0.01 and 0.1 mol of nitric acid and between 0.3 x 10-⁵ and 2 x 10-⁴ gram-ion of ferricyanide.

4. Baths according to claim 2, characterized in that the aqueous solution comprises, per litre, at least 3 mol of hydrochloric acid, at least 0.10 mol of phosphoric acid, at least 0.05 mol of nitric acid and between 0.3 x 10-⁴ and 10-3 mol of potassium ferricyanide.

5. Baths according to any one of claims 1 to 4, characterized in that the ferricyanide complex ions are present in the bath in the form of potassium ferricyanide.

6. Baths according to any one of claims 1 to 5, characterized in that the global molarity of the mixture of acids in the aqueous solution is between 2 and 6.

7. Baths according to any one of claims 1 to 6, characterized in that they contain a surfactant of the class of the alkylpyridinium chlorides, and/or a viscosity regulator of the class of the cellulose ethers, in relative amounts, in relation to the cyanide complex, equal to at most 1 : 3 by weight in the case of the surfactant and 1 : 1, in molar terms, in the case of the viscosity regulator.

8. Baths according to any one of claims 1 to 7, characterized in that the aqueous solution moreover contains anions selected from among the bromide, iodide and thiocyanate ions, in an amount of between 10-⁵ and 1 gram-ion per litre.

9. Process for the polishing of a steel surface, according to which the surface is brought into contact with a chemical polishing bath, characterized in that a bath according to any one of claims 1 to 8 is employed.

10. Process according to claim 9, characterized in that the surface is brought into contact with an aqueous solution of hydrochloric acid, phosphoric acid and nitric acid and ferricyanide complex ions are then added to the solution.

11. Process according to claim 10, characterized in that the time interval between the instant at which the surface is brought into contact with the solution and the instant at which the ferricyanide complex ions are added to the said solution is adjusted so that it corresponds to an attack on the surface by the solution to a depth of between 0.1 and 6 microns.