EP0206386B1 - Baths and process for chemically polishing stainless-steel surfaces - Google Patents

Abstract

Baths for chemical polishing of stainless steel surfaces, comprising, in aqueous solution, a mixture of hydrochloric acid, phosphoric acid and nitric acid, ferricyanide complex ions and an additive capable of decomposing nitrous acid. The baths can be used in particular for chemical polishing of heat exchangers with a very large exchange surface.

Description

The present invention relates to the composition of baths for the chemical polishing of stainless steel 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 baths of mineral acids. For the chemical polishing of austenitic stainless steels, baths are generally used comprising a mixture, in aqueous solution, of hydrochloric, phosphoric and nitric acids (patent US-A-2662814). To improve the quality of the polishing, it is usual to incorporate in these baths suitable additives such as surfactants, 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.

These known polishing baths have the particularity of attacking the metal at very high speed. A polishing treatment of a stainless steel surface with such baths can generally not exceed a few minutes, on pain of causing local corrosions. 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 zones 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 changes in the local compositions of the bath. They are not suitable for polishing installations, in which the surface area to be polished is very high compared to the space available for the bath, for example heat exchangers with very large exchange surface.

Document FR-A-2463820 discloses chemical polishing baths comprising a mixture of hydrochloric, nitric and phosphoric acids and a thiourea. It teaches that thiourea has the function of accelerating the speed of chemical polishing.

The present invention aims 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 avoid setting works with multiple additives and produces excellent quality polishes, especially in the case where the surface area to be polished is very high compared to the space available for the bath.

The invention therefore relates to baths for the chemical polishing of stainless 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 and an additive capable of breaking down nitrous acid.

In the baths according to the invention, the ferricyanide complex ions are complex cyanides of general formula Fe ^III (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 all dissolved compounds such as, for example, hexacyanoferric acid (111), ammonium ferricyanide and ferricyanides of alkali and alkaline earth metals. Preferred compounds are the alkali metal ferricyanides, potassium ferricyanide being especially recommended.

The additive capable of decomposing nitrous acid has the function of decomposing at least part of the nitrous acid which is formed during the polishing of a steel surface, the nitrous acid being the consequence of an oxidation of ferrous ions released into the bath during polishing. In principle, the additive can be chosen from all organic and inorganic substances which are capable of breaking down nitrous acid in an aqueous medium; it should be chosen from among the substances which do not attack the polishing steel and for which the products of the reaction with nitrous acid do not attack the polishing steel. Preference is given to substances which are soluble in the aqueous solution containing the mixture of acids. Sulfuric acid, hydroxylamine, hydrazine, hydrogen peroxide, acetone, urea and primary, secondary and tertiary amines are examples of substances which can be used for the bath additive according to the invention. 'invention. The nitrogen compounds constitute a class of substances which are especially advantageous for the bath additive according to the invention; examples of nitrogen compounds are urea and its derivatives, in particular thiourea and ureines.

Urea is a preferred nitrogen compound, according to the invention.

In the chemical polishing baths according to the invention, the respective contents of phosphoric acid, hydrochloric acid, nitric acid and, in ferricyanide complex ions are chosen according to the nature of the metal treated, the working temperature and the desired duration for the trai polishing. The content of additive capable of breaking down nitrous acid depends on various parameters, such as the nature of said additive, the respective contents of hydrochloric acid, phosphoric acid, nitric acid and ferricyanide complex ions, the volume of the bath used. , the configuration of the metal surface to be polished and the nature of the metal. It has been found that, all other things remaining equal, the optimum content of additive in the polishing baths according to the invention is proportional to the depth of attack of the bath in the metal and to the ratio between the area of the metal surface to polish and the volume of the bath used.

• - entre 0,5 et 10, de préférence entre 1 et 8, moles d'acide chlorhydrique par litre,
• - entre 0,01 et 2,5, de préférence entre 0,05 et 1,5 mole d'acide phosphorique par litre,
• - entre 0,001 et 1,5, de préférence 0,005 et 1, mole d'acide nitrique par litre,
• - entre 0,3 x 10 -⁶ et 0,3 x 10-², de préférence entre 0,3 x 10-⁵ et 0,3 x 10-³ ion-gramme de ferricyanure par litre, et
• - une quantité d'additif (exprimée en mole par litre du bain) défine par la relation
  
  où : S désigne l'aire (exprimée en m²) de la surface métallique à polir;
  • V désigne le volume (exprimé en m3) du bain mis en oeuvre;
  • λe désigne la profondeur moyenne (exprimée en micromètre) d'attaque de la surface métallique à polir par le bain;
  • k est un facteur de proportionnalité
    
    compris entre 10-⁸ et 10-², de préférence entre 10-⁷ et 1₀-_3.

In general, baths in accordance with the invention which are very suitable for carrying out the chemical polishing of surfaces made of austenitic stainless steels such as, for example, those alloyed with chromium and / or nickel, in a time between 2 and 24 hours are those containing

• - between 0.5 and 10, preferably between 1 and 8, moles of hydrochloric acid per liter,
• between 0.01 and 2.5, preferably between 0.05 and 1.5 moles of phosphoric acid per liter,
• - between 0.001 and 1.5, preferably 0.005 and 1, mole of nitric acid per liter,
• - between 0.3 x 10 - ⁶ and 0.3 x 10- ² , preferably between 0.3 x 10- ⁵ and 0.3 x 10- ³ gram ion of ferricyanide per liter, and
• - an amount of additive (expressed in moles per liter of the bath) defined by the relationship
  
  where: S denotes the area (expressed in m ² ) of the metal surface to be polished;
  • V denotes the volume (expressed in m3) of the bath used;
  • λe denotes the average depth (expressed in micrometres) of attack of the metal surface to be polished by the bath;
  • k is a proportionality factor
    
    between 10- ⁸ and 10- ^2, preferably between 10 ^-7 and 1 _0-3.

• - de l'acide chlorhydrique à raison de 2,5 à 5 moles par litre,
• - de l'acide phosphorique à raison de 0,1 à 1 mole par litre,
• - de l'acide nitrique à raison de 0,01 à 0,5 mole par litre, et
• - du ferricyanure de potassium, à raison de 0,1 x 10- ⁴ à 0,2 x 10-³ molécule-gramme par litre, et
• - de l'urée, à titre d'additif capable de décomposer l'acide nitreux, en une quantité, exprimée en moles par litre, définie par la relation précitée dans laquelle k est compris entre 10-⁷ et 10-4.

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:

• - hydrochloric acid at a rate of 2.5 to 5 moles per liter,
• - phosphoric acid at a rate of 0.1 to 1 mole per liter,
• - nitric acid at a rate of 0.01 to 0.5 moles per liter, and
• - potassium ferricyanide, at a rate of 0.1 x 10- ⁴ to 0.2 x 10- ³ molecule-grams per liter, and
• - urea as an additive capable of decomposing nitrous acid, in an amount, expressed in moles per liter as defined by the above equation wherein k is between 10 ^-7 and 10-4.

• - 1:3 pondéral, dans le cas d'agents tensioactifs de la classe des chlorures d'alkylpyridinium;
• - 1:1 pondéral, dans le cas d'agents tensioactifs de la classe des alkylphénols;
• - 1:1 molaire, dans le cas d'épaississants choisis parmi ies 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, relative to the complex cyanide, which do not respectively exceed:

• - 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 alkylpyridinium chloride, alkylphenol and cellulose ether.

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. They are especially well suited for polishing metal surfaces with a very large area, compared to the space available for the bath. For example, they find an interesting application for the polishing of metal surfaces whose area (expressed in m ² ) is at least equal to 3 times, preferably greater than 8 times the volume (expressed in m ³ ) of the polishing bath which is in contact with it, such as, for example, heat exchangers with a very large exchange surface. The performance of the baths according to the invention is not limited by a maximum value of the ratio between the area of the surface to be polished and the volume of the bath used, this ratio, expressed in m − ¹ , which can for example reach 20 and more.

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 stainless 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.

In accordance with a particular embodiment of the method according to the invention, after having brought the metal surface into contact with the bath, an additional addition of nitric acid and additive capable of decomposing nitrous acid is carried out. Alternatively, several successful additions can be made sifs or a continuous addition of nitric acid and said additive, as the polishing progresses.

In a preferred embodiment of the method according to the invention, the bath is carried out in situ in contact with the metal surface to be polished. For this purpose, the metal surface is first brought into contact with an aqueous solution containing hydrochloric acid, phosphoric acid, nitric acid and the additive capable of decomposing nitrous acid, then the ions are added. ferricyanide complexes 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 aqueous solution and the moment when the ferricyanide complex ions are added to said solution, so that it corresponds to a etching of the surface by the solution, of a depth between 0.1 and 6 micrometers, preferably between 0.5 and 4 micrometers. Alternatively, after adding the ferricyanide complex ions to the solution, additional additions of nitric acid and additive capable of breaking down nitrous acid can be made, as discussed above.

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 corrosion may appear on the surface, unless additional nitric acid and an additive capable of decomposing nitrous acid are added in accordance with to the particular embodiment of the method, described above. The optimum contact time of the surface to be polished with the bath or the importance of the additional addition of nitric acid and additive capable of breaking down nitrous acid depend on many parameters such as the composition of the steel of the surface to be polished, its initial configuration and roughness, 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.

The invention will be explained by the examples, the description of which will follow.

Example 1

• 2,7 moles d'acide chlorhydrique,
• 0,3 mole d'acide phosphorique,
• 0,06 mole d'acide nitrique,
• 30 mg de ferricyanure de potassium.

A 20 m2 area plate, in stainless steel grade ASTM-316L (steel alloyed with chromium (16.0 to 18.0%), nickel (10.0 to 14.0%) and molybdenum (2 , 0 to 3.0%)) was immersed in 1 m ³ of a bath containing, per liter:

• 2.7 moles of hydrochloric acid,
• 0.3 moles of phosphoric acid,
• 0.06 mole of nitric acid,
• 30 mg potassium ferricyanide.

Immediately after immersing the plate in the bath, a continuous addition of an aqueous nitric acid solution was carried out at the rate of 0.50 mole of nitric acid per hour. We also made two successive additions of 2 kg of urea, respectively after 4 hours and 6 hours of treatment. After 8 hours of treatment, an average depth of attack of the plate by the bath of 108 micrometers was measured. At this time, the plate was removed from the bath, washed with deionized water and dried. It had a smooth and shiny appearance.

Example 2

• 0,6 mole d'acide phosphorique,
• 0,03 mole d'acide nitrique,
• 100 mg de ferricyanure de potassium.

A 427 cm2 area plate, in ASTM-304L grade stainless steel (steel alloyed with chromium (18.0 to 20.0%) and nickel (8.0 to 12.0%)) was immersed in 935 cm3 of a bath at 55 _° C., containing, per liter: 4.5 moles of hydrochloric acid,

• 0.6 moles of phosphoric acid,
• 0.03 mole of nitric acid,
• 100 mg of potassium ferricyanide.

Every 90 minutes, 0.03 g of nitric acid per liter of the bath and 1.4 g of urea per liter of the bath were added to the bath. After 7 hours of treatment, an average depth of attack of the metal by the bath equal to 117 micrometers was measured. At this time, the plate was removed from the bath, washed with water and dried. It had a smooth and shiny appearance.

Claims (10)

1. Baths for chemical polishing of stainless steel surfaces, comprising, in aqueous solution, a mixture of hydrochloric acid, phosphoric acid, and nitric acid, which are characterized in that they comprise, in the aqueous solution, ferricyanide complex ions and an additive capable of decomposing nitrous acid.

2. Baths according to Claim 1, characterized in that the ferricyanide complex ions are present in the form of potassium ferricyanide and the additive is chosen from urea and urea derivatives.

3. Baths according to Claim 1 or 2, characterized in that the aqueous solution comprises

between 0.5 and 10 moles of hydrochloric acid per litre,

- between 0.01 and 2.5 moles of phosphoric acid per litre,

- between 0.001 and 1.5 moles of nitric acid per litre,

- between 0.3 x 10-6 and 0.3 x 10-2 gram-ions of ferricyanide per litre, and

- a quantity of additive (expressed in moles per litre of bath) of between 10-⁸ and 10-² times the product V-¹.S._Âe,
in which:

S denotes the area (expressed in m²) of the metal surface to be polished;

V denotes the volume (expressed in m³) of the bath employed;

_Ae denotes the mean depth (expressed in micrometres) of attack by the bath on the metal surface to be polished.

4. Baths according to Claim 3, characterized in that the aqueous solution comprises

- between 1 and 8 moles of hydrochloric acid per litre,

- between 0.05 and 1.5 moles of phosphoric acid per litre,

- between 0.005 and 1 mole of nitric acid per litre,

- between 0.3 x 10-5 and 0.3 x 10-³ gram-ions of ferricyanide per litre, and

- a quantity of additive (expressed in moles per litre of bath) of between 10-⁷ and 10-³ times the abovementioned product V-¹.S._Âe.

5. Baths according to Claim 4, characterized in that the aqueous solution comprises:

between 2.5 and 5 moles of hydrochloric acid per litre,

between 0.1 and 1 mole of phosphoric acid per litre,

between 0.01 and 0.5 moles of nitric acid per litre,

between 0.1 x 10-⁴ and 0.2 x 10-³ gram-molecules of potassium ferricyanide per litre, and

as an additive capable of decomposing nitrous acid, urea in a quantity, expressed in moles per litre, between 10-⁷ and 10-⁴ times the abovementioned product V-ⁱ.S._Ae.

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

7. Process for polishing a stainless steel surface, according to which the surface is placed in contact with a chemical polishing bath, characterized in that a bath according to any one of Claims 1 to 6 is employed.

8. Process according to Claim 7, characterized in that the surface is first placed in contact with an aqueous solution containing hydrochloric acid, phosphoric acid, nitric acid and an additive capable of decomposing nitrous acid, and then ferricyanide complex ions are added to the solution.

9. Process according to Claim 8, characterized in that the time period between the time when the surface is placed in contact with the solution and the time when the ferricyanide complex ions are added to the solution is adjusted so that it corresponds to an attack by the solution on the surface to a depth of between 0.1 and 6 micrometers.

10. Process according to any one of Claims 7 to 9, characterized in that, while the steel surface is in contact with the bath, supplementary nitric acid and additive capable of decomposing nitrous acid are added to the latter.