EP0462646B1 - Baths and process for chemical polishing stainless steel surfaces - Google Patents

Abstract

Baths for the chemical polishing of stainless steel surfaces which comprise a mixture of hydrochloric acid, nitric acid and phosphoric acid, a substituted or unsubstituted hydroxybenzoic acid and an amine in aqueous solution.

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 in treating the metal surfaces to be polished with oxidizing baths. 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 patent US-A-3709824, a composition of a bath for the chemical polishing of stainless steel surfaces is described, comprising, in aqueous solution, a mixture of hydrochloric acid, nitric acid and acid. phosphoric, a viscosity regulator chosen from water-soluble polymers, a surfactant, sulfosalicylic acid as a brightening agent and, optionally, an amino compound.

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, at the risk of causing local corrosion. This high speed of action of the 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.

In patent EP-B-19964 (SOLVAY & Cie), chemical polishing baths with very slow action are described, which therefore avoid the abovementioned drawbacks. These known baths include, in aqueous solution, a mixture of hydrochloric, nitric and phosphoric acids, sulfosalicylic acid, alkylpyridinium chloride and methylcellulose. These known slow-acting polishing baths are designed to work at temperatures at least equal to 40 ° C, generally between 45 and 100 ° C.

Document EP-A-0274776 describes baths for the chemical polishing of stainless steel surfaces comprising, in aqueous solution, a mixture of hydrochloric acid, nitric acid and phosphonic acid, sulfosalicylic acid and a water-soluble abietic compound which preferably consists of a substituted abietamine. These known polishing baths are specially adapted for the slow polishing of stainless steel surfaces, involving several hours of treatment, at temperatures preferably between 50 and 80 ° C.

The invention aims to provide baths designed to carry out slow and effective chemical polishing of stainless steel surfaces, at working temperatures below 50 ° C.

The invention therefore relates to baths for the chemical polishing of stainless steel surfaces, comprising, in aqueous solution, a mixture of hydrochloric acid, nitric acid and phosphoric acid, an optionally substituted hydroxybenzoic acid and an amine. water-soluble primer comprising more than 10 carbon atoms in its molecule, the aqueous solution containing, per liter, from 0.5 to 5 moles of acid hydrochloric acid, from 0.005 to 1 mole of nitric acid and from 0.005 to 1 mole of phosphoric acid.

In the baths according to the invention, hydroxybenzoic acid serves as a brightening agent. It can be unsubstituted such as salicylic acid or substituted such as sulfosalicylic acid. Salicylic acid is preferred.

The amine is selected from water-soluble primary amines whose molecule contains more than 10 carbon atoms, for example between 11 and 20 carbon atoms. Primary alkylamines having 11 to 16 carbon atoms in their molecule are preferred. The baths according to the invention can comprise a mixture of amines. Optimal amine content depends on the nature of the amine selected. As a rule, it is between 0.001 and 1 g per liter of the aqueous solution.

In a particular embodiment of the invention, the aqueous solution of the bath comprises, in addition to the amine, an additive selected from perchloric acid and the water-soluble salts of perchloric acid. The optimum content of this additive is between 0.001 and 0.5 mole per liter of the aqueous solution.

In another embodiment of the baths according to the invention, the aqueous solution contains a water-soluble additive capable of breaking down nitrous acid. The function of this additive is to decompose at least part of the nitrous acid which forms during the polishing of a steel surface, as a consequence of oxidation of ferrous ions released in the bath during polishing. It is preferably selected from urea and its derivatives, such as thiourea and ureines, and its optimum content is between 0.01 and 5 g per liter of the aqueous solution. The baths in accordance with this embodiment of the invention are specially adapted for polishing treatments in which the ratio between the surface in contact with the bath and the volume thereof is greater than 10 m⁻¹.

It is preferable that the baths according to the invention are free from ferricyanide ions and ferrocyanide ions, especially when they do not contain perchloric acid.

• · entre 0,5 et 5 moles d'acide chlorhydrique (de préférence 1-3 moles),
• · entre 0,005 et 1 mole d'acide nitrique (de préférence 0,05-0,5 mole),
• · entre 0,005 et 1 mole d'acide phosphorique (de préférence 0,01-0,5 mole),
• · entre 0,001 et 5 g d'acide hydroxybenzoïque substitué ou non substitué (de préférence 0,005-0,3 g, dans le cas de l'acide non substitué),
• · entre 0,001 et 1 g d'amine (de préférence 0,005 - 0,300g)
• · entre 0 et 0,5 mole de l'additif sélectionné parmi l'acide perchlorique et les sels hydrosolubles de l'acide perchlorique (de préférence 0,001-0,2 mole),
• · entre 0 et 5 g de l'additif capable de décomposer l'acide nitreux (de préférence 0,01 et 5 g).

The adequate weight quantities of the various constituents of the baths according to the invention depend on the grade of the stainless steel subjected to polishing as well as on the polishing conditions, in particular on the profile of the steel object subjected to polishing, on its volume, on the volume of the bath, its temperature and the agitation to which it is possibly subjected. They must therefore be determined in each particular case by routine laboratory tests. Examples of baths in accordance with the invention, suitable for polishing austenitic stainless steels alloyed with chromium and nickel, at temperatures between 20 and 50 ° C. include, per liter of aqueous solution :

• Between 0.5 and 5 moles of hydrochloric acid (preferably 1-3 moles),
• Between 0.005 and 1 mole of nitric acid (preferably 0.05-0.5 mole),
• Between 0.005 and 1 mole of phosphoric acid (preferably 0.01-0.5 mole),
• Between 0.001 and 5 g of substituted or unsubstituted hydroxybenzoic acid (preferably 0.005-0.3 g, in the case of unsubstituted acid),
• Between 0.001 and 1 g of amine (preferably 0.005 - 0.300 g)
• Between 0 and 0.5 mole of the additive selected from perchloric acid and the water-soluble salts of perchloric acid (preferably 0.001-0.2 mole),
• · Between 0 and 5 g of the additive capable of breaking down nitrous acid (preferably 0.01 and 5 g).

ou un radical hydroabiétyle ou déhydroabiétyle.The polishing baths according to the invention may optionally contain additives usually present in baths known for the chemical polishing of metals, for example surfactants, alcohols and viscosity regulators. They can in particular comprise a water-soluble abietic compound which is a chemical compound comprising an abietyl radical of general formula:

or a hydroabietyl or dehydroabietyl radical.

According to the invention, the abietic compound must be soluble in the aqueous solution.

Abietic compounds which can be used in the baths according to the invention are abietamines.

dans laquelle :

• R₁ désigne un radical abiétyle, hydroabiétyle ou déhydroabiétyle défini ci-dessus,
• X₁ désigne un radical comprenant au moins un groupe carbonyle, et
• X₂ désigne un atome d'hydrogène ou un radical comprenant au moins un groupe carbonyle.

Abietamines specially recommended for baths according to the invention are those of general formula:

in which :

• R₁ denotes an abietyl, hydroabietyl or dehydroabietyl radical defined above,
• X₁ denotes a radical comprising at least one carbonyl group, and
• X₂ denotes a hydrogen atom or a radical comprising at least one carbonyl group.

Examples of such abietamines, which are very suitable in the baths according to the invention, are those in which at least one of the radicals X₁ and X₂ is a radical of general formula:

-CH₂-R₂


in which R₂ denotes a linear or cyclic alkyl radical, substituted or unsubstituted, saturated or unsaturated, comprising at least one carbonyl group. Among these compounds, preference is given to those in which the group —CH₂- is linked to a carbonyl group of the radical R₂ by a carbon atom carrying at least one hydrogen atom. Such substituted abietamines and the means of obtaining them are described in GB-A-734665. Examples of abietamines of this type, which can be used in the baths according to the invention, are those in which the alkyl radical R₂ is selected from acetonyl, 2-keto butyl, 4-methyl 2-keto-pentenyl-3, 4- radicals. hydroxy 4-methyl 2-keto pentyl, 2-keto cyclopentyl, 4-hydroxy 2-keto pentenyl-3, 2-keto cyclohexyl, 2,5-diketo hexyl and 2-phenyl 2-keto ethyl.

The baths according to the invention may also contain products of the brand DEHYQUART (Henkel), which are surfactants selected from alkylpyridinium salts and quaternary ammonium salts, comprising alkyl, phenyl or benzyl radicals, substituted or not substituted.

The baths according to the invention are suitable for polishing chemical of all austenitic stainless steel surfaces. They are specially suitable for polishing austenitic steels containing between 16 and 26% by weight of chromium and between 6 and 22% by weight of nickel, such as steels of grades 18/8 and 18/10, optionally containing molybdenum (for example example AISI-304, 304L, 316 and 316L steels). The baths according to the invention have the particularity of polishing such steels at low speed, generally requiring a contact time of between 3 and 12 hours. They can be used at all temperatures between 20 ° C and the boiling temperature. However, they have the remarkable characteristic of having excellent efficiency at temperatures below 50 ° C, generally between 35 and 45 ° C, at normal atmospheric pressure, which facilitates their implementation and simplifies the measures to be taken to ensure the health of the polishing workshops. The baths according to the invention have the additional advantage of performing good quality polishing of welded assemblies according to the rules of the art.

The invention 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, at a temperature between 20 ° C. and the temperature d 'boiling of the bath, for a time between 3 and 12 hours.

In carrying out the method according to the invention, the contacting of the metal surface with the bath can be carried out in any suitable manner, for example by immersion. 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 the bath loses its polishing properties. The optimum contact time depends on many parameters such as the grade of steel, the initial configuration and roughness of the surface to be polished, the composition of the bath, the working temperature, the agitation of the bath in contact with the surface. , the ratio between the area of the surface to be polished and the volume of the bath; it must be determined in each particular case by routine work in the laboratory.

In a preferred embodiment of the method according to the invention, the bath is carried out at a temperature between 20 and 65 ° C, preferably between 35 and 50 ° C, at normal atmospheric pressure, and the surface to be polished in contact with the bath for a time between 5 and 12 hours.

The advantage of the invention will be highlighted on reading the examples set out below.

In the examples, the description of which follows, plates of stainless steel grade 18/10 were used [steel alloyed with chromium (18.0%) and nickel (10.0%) and free from molybdenum].

• · la profondeur moyenne d'attaque du métal, définie par la relation $$\text{Δe =}\frac{\text{10⁴}}{\text{S.d}}\text{.ΔP}$$
  
  où

  S

  désigne l'aire de la plaque (en cm²),

  d

  désigne la masse spécifique du métal (en g/cm³),

  ΔP

  désigne la perte en poids (en g) de la plaque pendant l'immersion dans le bain,

  Δe

  désigne la profondeur d'attaque (µm);

• · la rugosité moyenne arithmétique R_a, qui est la déviation moyenne par rapport à la surface moyenne de la plaque [Encyclopedia of Materials Science and Engineering, Michael B. Bever, Vol. 6, 1986, Pergamon Press, pages 4806 à 4808 (page 4806)] :
  
  les mesures étant effectuées avec un palpeur muni d'une pointe présentant un rayon de courbure de 5 µm et le calculateur travaillant avec une valeur de cut-off égale à 0,25mm;
• · la brillance de la surface sous un angle d'incidence de 20 degrés (selon la norme ASTM D523).

In each example, the plate was immersed in the polishing bath, kept at a substantially constant temperature and subjected to moderate agitation. At the end of the immersion period, the plate was removed from the bath, rinsed with demineralized water and dried. The following parameters were measured:

• · The average depth of attack of the metal, defined by the relation $$\text{Δe =}\frac{\text{10⁴}}{\text{Nd}}\text{.ΔP}$$
  
  or

  S

  designates the area of the plate (in cm²),

  d

  denotes the specific mass of the metal (in g / cm³),

  ΔP

  designates the weight loss (in g) of the plate during immersion in the bath,

  Δe

  denotes the depth of attack (µm);

• · The arithmetic mean roughness R _a , which is the mean deviation from the mean surface of the plate [Encyclopedia of Materials Science and Engineering, Michael B. Bever, Vol. 6, 1986, Pergamon Press, pages 4806 to 4808 (page 4806)]:
  
  the measurements being carried out with a probe fitted with a tip having a radius of curvature of 5 μm and the calculator working with a cut-off value equal to 0.25 mm;
• · The gloss of the surface at an angle of incidence of 20 degrees (according to standard ASTM D523).

First series of examples: Example 1

• · 1,3 mole d'acide chlorhydrique,
• · 0,25 mole d'acide nitrique,
• · 0,15 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique,
• · 0,1 g de tripentylamine,
• · 0,005 mole d'acide perchlorique.

A polishing bath was used which is not in accordance with the invention, comprising, per liter:

• 1.3 moles of hydrochloric acid,
• 0.25 mole of nitric acid,
• 0.15 mole of phosphoric acid,
• 0.1 g salicylic acid,
• 0.1 g of tripentylamine,
• 0.005 mole of perchloric acid.

• . volume du bain : 970 cm³,
• . aire de la surface soumise au polissage : 87,3 cm²,
• . température : 45°C,
• . durée de l'immersion : 3 heures 30 minutes.

The operating conditions were as follows:

• . bath volume: 970 cm³,
• . surface area subjected to polishing: 87.3 cm²,
• . temperature: 45 ° C,
• . duration of the immersion: 3 hours 30 minutes.

• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,29 µm,
  • · après le polissage : 0,21 µm,
• · brillance : 12%.

The following results were noted:

• Arithmetic mean roughness:
  • · Before polishing: 0.29 µm,
  • · After polishing: 0.21 µm,
• Gloss: 12%.

Example 2

• · 1,5 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,2 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique,
• · 0,075 g de trioctylamine,
• · 0,005 mole d'acide perchlorique.

A polishing bath was used which is not in accordance with the invention, comprising, per liter:

• 1.5 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.2 mole of phosphoric acid,
• 0.1 g salicylic acid,
• 0.075 g of trioctylamine,
• 0.005 mole of perchloric acid.

• · volume du bain : 970 cm³,
• · aire de la surface soumise au polissage : 87,3 cm²,
• · température : 45°C,
• · durée de l'immersion : 3 heures 15 minutes.

The operating conditions were as follows:

• Bath volume: 970 cm³,
• · Surface area subject to polishing: 87.3 cm²,
• Temperature: 45 ° C,
• · Duration of the immersion: 3 hours 15 minutes.

• · profondeur d'attaque : 31,3 µm,
• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,29 µm,
  • · après le polissage : 0,18 µm,
• · brillance : 17 %.

The following results were noted:

• Attack depth: 31.3 µm,
• Arithmetic mean roughness:
  • · Before polishing: 0.29 µm,
  • · After polishing: 0.18 µm,
• Gloss: 17%.

Example 3

• · 1,5 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,2 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique,
• · 0,075 g de dihexylamine,
• · 0,005 mole d'acide perchlorique.

A polishing bath was used which is not in accordance with the invention, comprising, per liter:

• 1.5 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.2 mole of phosphoric acid,
• 0.1 g salicylic acid,
• 0.075 g of dihexylamine,
• 0.005 mole of perchloric acid.

• · volume du bain : 970 cm³,
• · aire de la surface soumise au polissage : 87,3 cm²,
• · température : 45°C,
• · durée de l'immersion : 3 heures 25 minutes.

The operating conditions were as follows:

• Bath volume: 970 cm³,
• · Surface area subject to polishing: 87.3 cm²,
• Temperature: 45 ° C,
• · Duration of the immersion: 3 hours 25 minutes.

• · profondeur d'attaque : 27,1 µm,
• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,31 µm,
  • · après le polissage : 0,22 µm,
• · brillance : 13 %.

The following results were noted:

• Attack depth: 27.1 µm,
• Arithmetic mean roughness:
  • · Before polishing: 0.31 µm,
  • · After polishing: 0.22 µm,
• Gloss: 13%.

Example 4

• · 1,5 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,2 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique,
• · 0,075 g de laurylamine,

A polishing bath in accordance with the invention was used, comprising, per liter:

• 1.5 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.2 mole of phosphoric acid,
• 0.1 g salicylic acid,
• 0.075 g of laurylamine,

• · volume du bain : 970 cm³,
• · aire de la surface soumise au polissage : 87,3 cm²,
• · température : 45°C,
• · durée de l'immersion : 3 heures 50 minutes.

The operating conditions were as follows:

• Bath volume: 970 cm³,
• · Surface area subject to polishing: 87.3 cm²,
• Temperature: 45 ° C,
• · Duration of the immersion: 3 hours 50 minutes.

• · profondeur d'attaque : 25,1 µm,
• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,28 µm,
  • · après le polissage : 0,09 µm,
• · brillance : 36 %.

The following results were noted:

• Attack depth: 25.1 µm,
• Arithmetic mean roughness:
  • · Before polishing: 0.28 µm,
  • · After polishing: 0.09 µm,
• Gloss: 36%.

Example 5

• · 1,5 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,2 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique,
• · 0,075 g de laurylamine,
• · 0,005 mole d'acide perchlorique.

A polishing bath in accordance with the invention was used, comprising, per liter:

• 1.5 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.2 mole of phosphoric acid,
• 0.1 g salicylic acid,
• 0.075 g of laurylamine,
• 0.005 mole of perchloric acid.

• · volume du bain : 970 cm³,
• · aire de la surface soumise au polissage : 87,3 cm²,
• · température : 45°C,
• · durée de l'immersion : 4 heures.

The operating conditions were as follows:

• Bath volume: 970 cm³,
• · Surface area subject to polishing: 87.3 cm²,
• Temperature: 45 ° C,
• · Duration of the immersion: 4 hours.

• · profondeur d'attaque : 27,3 µm,
• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,27 µm,
  • · après le polissage : 0,08 µm,
• · brillance : 38 %.

The following results were noted:

• Attack depth: 27.3 µm,
• Arithmetic mean roughness:
  • · Before polishing: 0.27 µm,
  • · After polishing: 0.08 µm,
• Gloss: 38%.

Example 6

• · 1,5 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,2 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique,
• · 0,075 g de laurylamine,
• · 0,005 mole d'acide perchlorique,
• · 0,1 g d'urée.

A polishing bath in accordance with the invention was used, comprising, per liter:

• 1.5 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.2 mole of phosphoric acid,
• 0.1 g salicylic acid,
• 0.075 g of laurylamine,
• 0.005 mole of perchloric acid,
• 0.1 g of urea.

• · volume du bain : 725 cm³,
• · aire de la surface soumise au polissage : 87,3 cm²,
• · température : 45°C,
• · durée de l'immersion : 4 heures.

The operating conditions were as follows:

• Bath volume: 725 cm³,
• · Surface area subject to polishing: 87.3 cm²,
• Temperature: 45 ° C,
• · Duration of the immersion: 4 hours.

• · profondeur d'attaque : 26,2 µm,
• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,22 µm,
  • · après le polissage : 0,07 µm,
• · brillance : 37 %.

The following results were noted:

• Attack depth: 26.2 µm,
• Arithmetic mean roughness:
  • · Before polishing: 0.22 µm,
  • · After polishing: 0.07 µm,
• Gloss: 37%.

Example 7

• · 1,3 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,1 mole d'acide phosphorique,
• · 0,2 g d'acide salicylique,
• · 0,1 g de tridécylamine,
• · 0,005 mole d'acide perchlorique.

A polishing bath was used which is not in accordance with the invention, comprising, per liter:

• 1.3 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.1 mole of phosphoric acid,
• 0.2 g salicylic acid,
• 0.1 g of tridecylamine,
• 0.005 mole of perchloric acid.

• · volume du bain : 930 cm³,
• · aire de la surface soumise au polissage : 84 cm²,
• · température : 45°C,
• · durée de l'immersion : 4 heures 50 minutes.

The operating conditions were as follows:

• Bath volume: 930 cm³,
• · Surface area subject to polishing: 84 cm²,
• Temperature: 45 ° C,
• · Duration of the immersion: 4 hours 50 minutes.

• · profondeur moyenne d'attaque : 38,6 µm,
• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,25 µm,
  • · après le polissage : 0,10 µm,
• · brillance : 33%.

The following results were noted:

• · Average attack depth: 38.6 µm,
• Arithmetic mean roughness:
  • · Before polishing: 0.25 µm,
  • · After polishing: 0.10 µm,
• Gloss: 33%.

Example 8

• · 1,6 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,2 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique,
• · 0,075 g de dodécylamine,
• · 0,005 mole d'acide perchlorique
• · 0,5 g d'urée.

A polishing bath in accordance with the invention, containing, per liter:

• 1.6 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.2 mole of phosphoric acid,
• 0.1 g salicylic acid,
• 0.075 g of dodecylamine,
• 0.005 mole of perchloric acid
• 0.5 g of urea.

• · volume du bain : 1050 cm³,
• · aire de la surface soumise au polissage : 63 cm²,
• · température : 35°C,
• · durée de l'immersion : 8 heures 40 minutes.

The operating conditions were as follows:

• Bath volume: 1050 cm³,
• · Surface area subject to polishing: 63 cm²,
• Temperature: 35 ° C,
• · Duration of the immersion: 8 hours 40 minutes.

• · profondeur moyenne d'attaque : 30 µm,
• · rugosité moyenne arithmétique
  • · avant le polissage : 0,25 µm,
  • · après le polissage : 0,09 µm,
• · Brillance : 28 %.

The following results were noted:

• · Average attack depth: 30 µm,
• Arithmetic mean roughness
  • · Before polishing: 0.25 µm,
  • · After polishing: 0.09 µm,
• · Gloss: 28%.

Example 9

• · 1,7 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,25 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique,
• · 0,050 g de myristylamine,
• · 0,005 mole d'acide perchlorique.

A polishing bath in accordance with the invention was used, comprising, per liter:

• 1.7 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.25 mole of phosphoric acid,
• 0.1 g salicylic acid,
• 0.050 g of myristylamine,
• 0.005 mole of perchloric acid.

• · volume du bain : 970 cm³,
• · aire de la surface soumise au polissage : 87,3 cm²,
• · température : 45°C,
• · durée de l'immersion : 3 heures 50 minutes.

The operating conditions were as follows:

• Bath volume: 970 cm³,
• · Surface area subject to polishing: 87.3 cm²,
• Temperature: 45 ° C,
• · Duration of the immersion: 3 hours 50 minutes.

• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,22 µm,
  • · après le polissage : 0,11 µm,
• · brillance : 21 %.

The following results were noted:

• Arithmetic mean roughness:
  • · Before polishing: 0.22 µm,
  • · After polishing: 0.11 µm,
• Gloss: 21%.

Second series of examples: reference examples

This series of examples relates to tests which have been carried out with baths which are not in accordance with the invention.

Example 10

• · 1,5 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,2 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique.

A polishing bath was used comprising, per liter:

• 1.5 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.2 mole of phosphoric acid,
• 0.1 g salicylic acid.

• · volume du bain : 970 cm³,
• · aire de la surface soumise au polissage : 87,3 cm²,
• · température : 45°C,
• · durée de l'immersion : 3 heures.

The operating conditions were as follows:

• Bath volume: 970 cm³,
• · Surface area subject to polishing: 87.3 cm²,
• Temperature: 45 ° C,
• · Duration of the immersion: 3 hours.

• · profondeur d'attaque : 29 µm,
• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,26 µm,
  • · après le polissage : 0,23 µm,
• · brillance : 7 %.

The following results were noted:

• Attack depth: 29 µm,
• Arithmetic mean roughness:
  • · Before polishing: 0.26 µm,
  • · After polishing: 0.23 µm,
• Gloss: 7%.

Example 11

• · 1,5 mole d'acide chlorhydrique,
• · 0,2 mole d'acide nitrique,
• · 0,2 mole d'acide phosphorique,
• · 0,1 g d'acide salicylique,
• · 0,005 mole d'acide perchlorique.

A polishing bath was used comprising, per liter:

• 1.5 moles of hydrochloric acid,
• 0.2 mole of nitric acid,
• 0.2 mole of phosphoric acid,
• 0.1 g salicylic acid,
• 0.005 mole of perchloric acid.

• · volume du bain : 970 cm³,
• · aire de la surface soumise au polissage : 87,3 cm²,
• · température : 45°C,
• · durée de l'immersion : 3 heures

The operating conditions were as follows:

• Bath volume: 970 cm³,
• · Surface area subject to polishing: 87.3 cm²,
• Temperature: 45 ° C,
• · Duration of the immersion: 3 hours

• · profondeur d'attaque : 31,2 µm,
• · rugosité moyenne arithmétique :
  • · avant le polissage : 0,24 µm,
  • · après le polissage : 0,22 µm,
• · brillance : 8 %.

The following results were noted:

• Attack depth: 31.2 µm,
• Arithmetic mean roughness:
  • · Before polishing: 0.24 µm,
  • · After polishing: 0.22 µm,
• Gloss: 8%.

A comparison of the results obtained in Examples 4,5,6,8 and 9 with those obtained in Examples 1,2,3,7,10 and 11 shows the progress brought by the invention, as regards roughness and the shine obtained after polishing.

Claims (11)

1. Baths for the chemical polishing of stainless steel surfaces, comprising, in aqueous solution, a mixture of hydrochloric acid, nitric acid and phosphoric acid, and a substituted or unsubstituted hydroxybenzoic acid characterized in that they contain, per litre of aqueous solution, from 0.5 to 5 moles of hydrochloric acid, from 0.005 to 1 mole of nitric acid and from 0.005 to 1 mole of phosphoric acid, and in that they comprise in the aqueous solution a water-soluble primary amine comprising more than 10 carbon atoms in its molecule.
2. Baths according to Claim 1, characterized in that the number of carbon atoms in the amine is between 11 and 20.
3. Baths according to Claim 2, characterized in that the amine is an alkylamine comprising between 11 and 16 carbon atoms.
4. Baths according to any one of Claims 1 to 3, characterized in that the amine is present in the aqueous solution in an amount between 1 and 1,000 mg per litre of solution.
5. Baths according to any one of Claims 1 to 4, characterized in that they are free of ferricyanide ions and ferrocyanide ions.
6. Baths according to any one of Claims 1 to 5, characterized in that they comprise an additive selected from perchloric acid and the water-soluble salts of perchloric acid in the aqueous solution.
7. Baths according to any one of Claims 1 to 6, characterized in that they comprise, in the aqueous solution, an additive capable of decomposing nitrous acid,
8. Baths according to Claim 7, characterized in that the additive capable of decomposing nitrous acid is selected from urea and urea derivatives.
9. Baths according to any one of Claims 1 to 8, characterized in that they comprise, per litre of aqueous solution,

   · between 0.5 and 5 moles of hydrochloric acid,

   · between 0.005 and 1 mole of nitric acid,

   · between 0.005 and 1 mole of phosphoric acid,

   · between 0.001 and 5 g of hydroxybenzoic acid,

   · between 0.001 and 1 g of amine,

   · between 0 and 0.5 mole of the additive selected from perchloric acid and the water-soluble salts of perchloric acid,

   · between 0 and 0.500 g of the additive capable of decomposing nitrous acid.
10. Process for the polishing of a stainless steel surface, according to which the surface is brought into contact with a bath for chemical polishing according to any one of Claims 1 to 9, at a temperature of between 20°C and the boiling temperature of the bath, for a period of between 3 and 12 hours.
11. Process according to Claim 10, characterized in that the temperature of the bath is regulated between 35 and 50°C.