EP0516652B1 - Baths and method for chemically polishing stainless steel surfaces - Google Patents

Abstract

Baths for chemically polishing stainless steel surfaces comprising, in an aqueous solution, a mixture of hydrochloric acid, nitric acid and phosphoric acid, an optionally substituted hydroxybenzoic acid, an abietic compound and an additive selected from perchloric acid and the water-soluble salts thereof.

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 phosphoric acid, nitric acid and acid. hydrochloric acid, a viscosity regulator chosen from water-soluble polymers, a surfactant and sulfosalicylic acid as a brightening agent.

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 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 becomes effect impossible to obtain a uniform polish of the wall, certain zones thereof being insufficiently polished, others being deeply corroded. The high speed of action of known chemical polishing baths also makes polishing control difficult.

In document EP-A-274776 (SOLVAY & Cie), chemical polishing baths with very slow action are described, which therefore avoid the abovementioned drawbacks. These known baths comprise, in aqueous solution, a mixture of hydrochloric, nitric and phosphoric acids, sulfosalicylic acid and an abietic compound, such as an abietamine.

These known slow-acting polishing baths are designed to work at temperatures above 50 ° C, generally between 60 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.

Consequently, the invention 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, a compound abietic and an additive selected from perchloric acid and the water-soluble salts of perchloric 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.

ou un radical hydroabiétyle ou déhydroabiétyle.In the context of the invention, abietic compounds are chemical compounds 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 the 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 remainder 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 residue R₂ is selected from acetonyl, 2-keto butyl, 4-methyl 2-keto-pentenyl-3, 4-hydroxy 4 radicals -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.

In the baths according to the invention, the respective amounts of additive selected from perchloric acid and its water-soluble salts, on the one hand, and of abietic compound, on the other hand, should be adjusted to make coadsorption possible. on the surface of the steel to be polished, while avoiding exceeding their solubility product. As a general rule, it is expedient that the baths according to the invention contain, per liter, between 0.001 and 0.5 mole of the additive selected from perchloric acid and its water-soluble salts and between 0.005 and 1 g of the abietic compound.

• . entre 0,5 et 5 moles d'acide chlorhydrique (de préférence 0,5-3 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 0,0005 et 0,5 mole de l'additif sélectionné parmi l'acide perchlorique et ses sels hydrosolubles (de préférence 0,001-0,2 mole),
• . entre 0,005 et 1 g de composé abiétique (de préférence 0,01-0,5 g),
• . 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).

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 25 and 50 ° C. include, per liter of aqueous solution:

• . between 0.5 and 5 moles of hydrochloric acid (preferably 0.5-3 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 0.0005 and 0.5 mole of the additive selected from perchloric acid and its water-soluble salts (preferably 0.001-0.2 mole),
• . between 0.005 and 1 g of abietic compound (preferably 0.01-0.5 g),
• . between 0.001 and 5 g of substituted or unsubstituted hydroxybenzoic acid (preferably 0.005-0.3 g in the case of unsubstituted acid).

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 may in particular include products of the DEHYQUART brand (Henkel), which are surfactants selected from alkylpyridinium salts and quaternary ammonium salts, comprising substituted or unsubstituted alkyl, phenyl or benzyl radicals.

The baths according to the invention are suitable for the chemical polishing of all austenitic stainless steel surfaces. They are specially adapted 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 grade 18/8 and 18/10, free of molybdenum (AISI standard -304 and 304L). The baths according to the invention have the particularity of polishing such steels at low speed, generally requiring a contact time of between 2 and 20 hours, generally between 5 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 placed in contact with a chemical polishing bath according to the invention.

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 the steel, the configuration and the initial roughness of the surface to be polished, the composition of the bath, the working temperature, the possible 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 laboratory work.

In a preferred embodiment of the method according to the invention, the bath is carried out at a temperature between 20 and 50 ° C, preferably between 35 and 45 ° 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
  
  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 (»m),

• . 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 de 5 »m de diamètre et correspondant à une valeur de cut-off égale à 0,25 mm.
• . la brillance de la surface.

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
  
  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 (»m),

• . arithmetic mean roughness R _a , which is the average 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 feeler provided with a point 5 ”m in diameter and corresponding to a cut-off value equal to 0.25 mm.
• . the shine of the surface.

Example 1 (according to the invention)

• . 2,0 moles d'acide chlorhydrique,
• . 0,2 mole d'acide nitrique,
• . 0,2 mole d'acide phosphorique,
• . 0,1 mole d'acide perchlorique,
• . 0,1 g d'acide sulfosalicylique,
• . 0,1 g du produit RODINE 213, qui est un mélange d'abiétamines substituées, d'isopropanol et d'agents tensioactifs (RODINE étant une marque déposée de Amchem Products Inc.).

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

• . 2.0 moles of hydrochloric acid,
• . 0.2 mole of nitric acid,
• . 0.2 mole of phosphoric acid,
• . 0.1 mole of perchloric acid,
• . 0.1 g of sulfosalicylic acid,
• . 0.1 g of the product RODINE 213, which is a mixture of substituted abietamines, isopropanol and surfactants (RODINE being a registered trademark of Amchem Products Inc.).

The operating conditions were as follows: . bath volume 1940 cm³, . surface area subject to polishing 87 cm², . temperature 35 ° C, . duration of immersion 11:00.

The following results were noted: . average depth of attack 27 ”m; . arithmetic mean roughness . before polishing 0.3 ± 0.1 ”m, . after polishing 0.11 ± 0.02 ”m.

The appearance of the surface is satin.

Example 2 (according to the invention)

• . 1,5 mole d'acide chlorhydrique,
• . 0,1 mole d'acide nitrique,
• . 0,2 mole d'acide phosphorique,
• . 0,02 mole d'acide perchlorique,
• . 0,1 g d'acide salicylique,
• . 0,15 g du produit RODINE 213 défini à l'exemple 1.

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

• . 1.5 moles of hydrochloric acid,
• . 0.1 mole of nitric acid,
• . 0.2 mole of phosphoric acid,
• . 0.02 mole of perchloric acid,
• . 0.1 g of salicylic acid,
• . 0.15 g of the product RODINE 213 defined in Example 1.

The operating conditions were as follows: . bath volume 970 cc, . surface area subject to polishing 43 cm², . temperature 35 ° C, . duration of immersion 12 hours.

The following results were noted: . average depth of attack 25 ”m; . arithmetic mean roughness . before polishing 0.3 "m, . after polishing 0.11 ”m. . Gloss [at an angle of 30 degrees (according to ASTM E430)] 28%.

Example 3 (reference test)

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

The test of Example 2 was repeated with a polishing bath not in accordance with the invention, containing neither perchloric acid nor the abietic compound. The bath included, per liter:

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

The operating conditions were as follows: . bath volume 970 cc, . surface area subject to polishing 43 cm², . temperature 35 ° C, . duration of immersion 9 hours, 30 minutes.

• . On observe une augmentation de la rugosité.

The following results were noted: . average depth of attack 25 ”m; . arithmetic mean roughness . before polishing 0.23 "m, . after polishing 0.27 ”m.

• . An increase in roughness is observed.

Claims (10)

1. Baths for the chemical polishing of stainless steel surfaces, comprising, in aqueous solution, a mixture of hydrochloric acid, nitric acid and phosphoric acid, a brightening agent chosen from optionally substituted hydroxybenzoic acids and an abietic compound, which are characterized in that they comprise an additive chosen from perchloric acid and the water-soluble salts of perchloric acid, in a quantity of between 0.001 and 0.5 mol per litre of the solution, the abietic compound being present in a quantity of between 0.005 and 1 g per litre of solution.
2. Baths according to Claim 1, characterized in that the hydroxybenzoic acid is salicylic acid.
3. Baths according to Claim 1, characterized in that the substituted hydroxybenzoic acid is sulphosalicylic acid.
4. Baths according to any one of Claims 1 to 3, characterized in that the abietic compound is a substituted abietamine of general formula:

   

   in which:

   - R₁ denotes an abietyl, hydroabietyl or dehydroabietyl radical,

   - X₁ denotes a radical containing at least one carbonyl group, and

   - X₂ denotes a hydrogen atom or a radical containing at least one carbonyl group.
5. Baths according to Claim 4, characterized in that at least one of the radicals X₁ and X₂ is a radical of general formula:
   
   
   
           -CH₂-R₂
   
   
   
   where R₂ denotes a saturated or unsaturated, substituted or unsubstituted, linear or cyclic alkyl residue containing at least one carbonyl group.
6. Baths according to Claim 5, characterized in that the alkyl residue R₂ is chosen from acetonyl, 2-ketobutyl, 4-methyl-2-keto-3-pentenyl, 4-hydroxy-4-methyl-2-ketopentyl, 2-ketocyclopentyl, 4-hydroxy-2-keto-3-pentenyl, 2-ketocyclohexyl, 2,5-diketohexyl and 2-phenyl-2-ketoethyl residues.
7. Baths according to any one of Claims 1 to 6, 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 0.0005 and 0.5 mole of the additive chosen from perchloric acid and its water-soluble salts,

   - between 0.005 and 1 g of abietic compound, and

   - between 0.001 and 5 g of substituted or unsubstituted hydroxybenzoic acid.
8. Baths according to any one of Claims 1 to 7, for the chemical polishing of surfaces made of austenitic steels.
9. Baths according to Claim 8, for the chemical polishing of surfaces made of austenitic steels alloyed with chromium and nickel and free from molybdenum.
10. Process for the polishing of a stainless steel surface, according to which the surface is brought into contact with a chemical polishing bath, characterized in that a bath in accordance with any one of Claims 1 to 10 is employed, and is maintained at a temperature of between 20 and 50°C.