EP0274776B1 - Baths and process for chemically polishing stainless steel surfaces - Google Patents

Abstract

Baths comprising, in aqueous solution, a mixture of hydrochloric acid, phosphoric acid, nitric acid and sulphosalicylic acid and at least one abietic compound.

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, 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 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 is 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 polished, others being 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. 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 a very large exchange surface.

In patent EP-B 19964 (SOLVAY & Cie), chemical polishing baths with very slow action are proposed, which therefore avoid the abovementioned drawbacks. These baths consist of a mixture, in adequate proportions, of hydrochloric, phosphoric and nitric acids, sulfosalicylic acid, alkylpyridinium chloride and methylcellulose. They have been found to be especially suitable for the slow polishing of homogeneous surfaces in austenitic stainless steel, in particular of grade ASTM-304L or ASTM-316L which are steels alloyed with chromium and nickel.

We have now found other bath compositions for the slow chemical polishing of stainless steel surfaces, in particular of austenitic shade, which not only make it possible to obtain a higher polish, but also to simultaneously polish steel surfaces of different shades, as it can for example occur in riveted, welded or bolted assemblies, without risking galvanic corrosion.

dans laquelle :

• R₁ désigne un radical abiétyle de formule générale :
  
  ou un radical hydroabiétyle ou déhydroabiétyle,
• 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, ladite abiétamine étant soluble dans la solution aqueuse.

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, and sulfosalicylic acid; according to the invention, the baths comprise at least one abietamine of general formula:

in which :

• R₁ denotes an abietyl radical of general formula:
  
  or a hydroabietyl or dehydroabietyl radical,
• 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, said abietamine being soluble in the aqueous solution.

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, substituted or unsubstituted, saturated or unsaturated alkyl residue, comprising at least minus 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-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 chemical polishing baths according to the invention, the respective contents of hydrochloric, phosphoric and nitric acids, of sulfosalicylic acid and of abietic compound are chosen according to the nature of the metal treated, the working temperature and the desired duration. for polishing treatment. Baths according to the invention which are very suitable for ensuring the chemical polishing of surfaces of stainless steel 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 0.01 and 1 mole of phosphoric acid, between 0.005 and 0.5 mole of nitric acid, between 0.05 and 20 g of sulfosalicylic acid and between 0.001 and 3 g of the abietic compound. Especially advantageous baths are those in which the overall molarity of the mixture of hydrochloric, phosphoric and nitric acids is between 1 and 7, preferably 1.8 and 6, the content of sulfosalicylic acid, expressed in g / l of the solution aqueous, being between 0.05 and 1.5 times the overall normality of the mixture of hydrochloric, phosphoric and nitric acids.

In a particular embodiment of the baths according to the invention, these also comprise an alcohol, an ether or an ether-alcohol dissolved in the aqueous solution. The optimum amount of this additional compound in the aqueous solution of bath depends on various parameters, in particular of said additional compound, of the abietic compound selected and of the contents of the constituents of the bath. In practice, contents of this additional compound of between 0.001 and 10 g per liter of aqueous solution are very suitable.

All other things remaining equal, the baths according to this embodiment of the invention make it possible to produce surface polishes of better quality, characterized in particular by a higher gloss.

The polishing baths according to the invention may optionally contain additives usually present in known baths for the chemical polishing of metals, for example surfactants and viscosity regulators.

• entre 2 et 5 moles d'acide chlorhydrique,
• entre 0,02 et 0,5 mole d'acide phosphorique,
• entre 0,01 et 0,2 mole d'acide nitrique,
• entre 0,1 et 10 g d'acide sulfosalicylique,
• entre 0,010 et 2,5 g du composé abiétique,
• entre 0,002 et 5 g d'alcool, d'éther ou d'éther-alcool,
• entre 0,005 et 6 g d'agent tensio-actif.

Baths according to the invention which are specially adapted or slow polishing of austenitic stainless steel surfaces include, per liter of aqueous solution of the bath:

• between 2 and 5 moles of hydrochloric acid,
• between 0.02 and 0.5 mole of phosphoric acid,
• between 0.01 and 0.2 mole of nitric acid,
• between 0.1 and 10 g of sulfosalicylic acid,
• between 0.010 and 2.5 g of the abietic compound,
• between 0.002 and 5 g of alcohol, ether or ether-alcohol,
• between 0.005 and 6 g of surfactant.

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 areas or areas that are difficult to access. 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 polishing metal surfaces whose area (expressed in m²) is between 1 and 10 times the volume (expressed in m³) of the polishing bath which is in contact with it.

The baths according to the invention are suitable for polishing all surfaces of austenitic stainless steel. 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, between 6 and 30% of nickel and between 0 and 6% of molybdenum, such as 18/8 and 18/10 / 2.5 steels, for example.

The baths according to the invention also find an interesting application in the polishing of steel assemblies of different shades, in particular welded assemblies, without causing local galvanic corrosion of the assembly. By way of example, they allow the polishing of welded assemblies combining elements in steels alloyed with chromium and nickel of grade 18/8 (ASTM-304 and 304L) with elements in steels alloyed with chromium, nickel and molybdenum of grade 18/10 / 2.5 (ASTM-316 and 316L).

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.

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 25 ° C and below 100 ° C, temperatures between 40 (advantageously 50) and 80 ° C being preferred.

The contact 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, he 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 method according to the invention, after having brought the metal surface into contact with the bath, a supplement of nitric acid is added to the bath to regenerate it. The addition of the nitric acid supplement can be carried out continuously or at intervals. It is adjusted to keep the nitric acid content in the bath permanently within a range of values compatible with optimum polishing of the metal surface.

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

Example 1

• 3 moles d'acide chlorhydrique,
• 0,15 mole d'acide phosphorique,
• 0,01 mole d'acide nitrique,
• 0,5 g d'acide sulfosalicylique,
• 1 g du produit "RODINE 213" (Amchem Products Inc.) (mélange d'abiétamines substituées, d'isopropanol et d'agents tensio-actifs).

A plate of 21.50 cm² of area, in stainless steel grade ASTM-304L (steel alloyed with chromium (18.0 to 20.0%) and nickel (8.0 to 12.0%)) was immersed in 500 cm³ of a 55 ° C bath, containing, per liter:

• 3 moles of hydrochloric acid,
• 0.15 mole of phosphoric acid,
• 0.01 mole of nitric acid,
• 0.5 g of sulfosalicylic acid,
• 1 g of the product "RODINE 213" (Amchem Products Inc.) (mixture of substituted abietamines, isopropanol and surfactants).

The plate initially had an arithmetic mean roughness R _a = 0.27 micron.

• brillance : 22 %
• R_a : 0,13 micron.

After 13 hours of treatment (the bath being maintained continuously with stirring), an average depth of attack of the metal by the bath equal to 20 microns was measured. At this time, the plate was removed from the bath, washed with water and dried. It had a smooth and shiny appearance. We measured its gloss (ASTM E430 standard) and its arithmetic mean roughness (R _a ):

• gloss: 22%
• R _a : 0.13 micron.

Example 2

The test described in Example 1 was repeated under the following conditions:

ASTM-316L grade stainless steel plate (steel alloyed with chromium (16.0-18.0%), nickel (10.0-14.0%) and molybdenum (2.0-3.0%) ).

Surface of the plate: 50 cm².

• 2,7 moles d'acide chlorhydrique,
• 0,2 mole d'acide phosphorique,
• 0,02 mole d'acide nitrique,
• 1 g d'acide sulfosalicylique,
• 1 g du produit "RODINE 213",
• 2 g du produit "TRITON N101" (Rohm & Haas Co.) (mélange de surfactants à base d'alkylaryl polyéther alcools).

Composition of the polishing bath (quantities reduced to 1 l of the bath):

• 2.7 moles of hydrochloric acid,
• 0.2 mole of phosphoric acid,
• 0.02 mole of nitric acid,
• 1 g of sulfosalicylic acid,
• 1 g of the product "RODINE 213",
• 2 g of the product "TRITON N101" (Rohm & Haas Co.) (mixture of surfactants based on alkylaryl polyether alcohols).

Bath temperature: 65 ° C.

Bath volume: 1250 cm³.

Duration of treatment: 6 hours.

The plate had an initial roughness R _a = 0.28 micron.

• une profondeur d'attaque : 22 microns,
• une brillance de 35 %,
• une rugosité R_a = 0,09 micron.

At the end of the treatment, we measured:

• a depth of attack: 22 microns,
• 35% gloss,
• a roughness R _a = 0.09 micron.

Example 3

The test described in Example 1 was repeated under the following conditions:
ASTM-430 grade martensitic stainless steel plate (chrome alloy steel (16.0-18.0%), with nickel and carbon contents of 0.75% and 0.12% respectively).

Surface of the plate: 22.50 cm².

• 2,7 moles d'acide chlorhydrique,
• 0,2 mole d'acide phosphorique,
• 0,02 mole d'acide nitrique,
• 3 g d'acide sulfosalicylique,
• 1 g du produit "RODINE 213" (défini à l'exemple 1),
• 2 g du produit "TRITON N101" (défini à l'exemple 2).

Composition of the polishing bath (quantities reduced to 1 l of bath):

• 2.7 moles of hydrochloric acid,
• 0.2 mole of phosphoric acid,
• 0.02 mole of nitric acid,
• 3 g of sulfosalicylic acid,
• 1 g of the product "RODINE 213" (defined in Example 1),
• 2 g of the product "TRITON N101" (defined in Example 2).

Bath temperature: 65 ° C.

Bath volume: 500 cm³.

Duration of treatment: 10 hours.

The plate had an initial roughness R _a = 0.70 micron.

• une profondeur d'attaque : 25 microns,
• une brillance de 12 %
• une rugosité R_a = 0,22 micron.

At the end of the treatment, we measured:

• a depth of attack: 25 microns,
• 12% shine
• a roughness R _a = 0.22 micron.

Example 4

The test described in Example 1 was repeated under the following conditions:

• une plaque de 30 cm² d'aire, en acier de nuance ASTM-304L,
• une plaque de 30 cm² d'aire, en acier de nuance ASTM-316L.

Assembly of two rectangular stainless steel plates, welded along a common edge:

• a plate of 30 cm² of area, in steel of grade ASTM-304L,
• a plate of 30 cm² of area, in steel of grade ASTM-316L.

• 4 moles d'acide chlorhydrique,
• 0,1 mole d'acide phosphorique,
• 0,02 mole d'acide nitrique,
• 1 g d'acide sulfosalicylique,
• 1 g du produit "RODINE 213",
• 1,3 cm³ d'un mélange d'isopropanol et de n-propanol.

Composition of the polishing bath (quantities reduced to 1 l of bath):

• 4 moles of hydrochloric acid,
• 0.1 mole of phosphoric acid,
• 0.02 mole of nitric acid,
• 1 g of sulfosalicylic acid,
• 1 g of the product "RODINE 213",
• 1.3 cm³ of a mixture of isopropanol and n-propanol.

Bath temperature: 65 ° C.

Bath volume: 1000 cm³.

Duration of treatment: 10 hours.

• plaque ASTM-304L : R_a = 0,29 micron,
• plaque ASTM-316L : R_a = 0,28 micron.

The plates of the assembly had the following initial roughness:

• ASTM-304L plate: R _a = 0.29 micron,
• ASTM-316L plate: R _a = 0.28 micron.

• plaque ASTM-304L : . profondeur d'attaque : 27 microns,
  • . brillance : 21 %,
  • . rugosité R_a = 0,13 micron,
• plaque ASTM-316L : . profondeur d'attaque : 22 microns,
  • . brillance : 22 %,
  • . rugosité R_a = 0,11 micron.

The assembly, collected at the end of the treatment, presented a smooth and shiny appearance, without any trace of corrosion. For each plate, the depth of attack, the gloss and the roughness were measured:

• ASTM-304L plate:. depth of attack: 27 microns,
  • . gloss: 21%,
  • . roughness R _a = 0.13 micron,
• ASTM-316L plate:. depth of attack: 22 microns,
  • . gloss: 22%,
  • . roughness R _a = 0.11 micron.

Claims (9)

1. Baths for the chemical polishing of stainless steel surfaces, comprising, in aqueous solution, a mixture of hydrochloric acid, phosphoric acid and nitric acid, and of sulphosalicylic acid, characterized in that they comprise at least one 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, the said substituted abietamine being soluble in the aqueous solution.
2. Baths according to Claim 1, 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, straight-chain or cyclic alkyl moiety containing at least one carbonyl group.
3. Baths according to Claim 2, characterized in that, in the radical of general formula:
   
   
   
           -CH₂-R₂
   
   
   
   the group -CH₂- is linked to a carbonyl group of the R₂ moiety via a carbon atom carrying at least one hydrogen atom.
4. Baths according to Claim 3, characterized in that the alkyl moiety 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 moieties.
5. Baths according to any one of Claims 1 to 4, characterized in that they additionally comprise an alcohol, an ether or an ether-alcohol dissolved in the aqueous solution.
6. Baths according to Claim 5, characterized in that they contain, per litre of aqueous solution,

   - between 2 and 5 moles of hydrochloric acid,

   - between 0.02 and 0.5 mole of phosphoric acid,

   - between 0.01 and 0.5 mole of nitric acid,

   - between 0.1 and 10 g of sulphosalicylic acid,

   - between 0.01 and 2.5 g of the abietic compound,

   - between 0.002 and 5 g of alcohol, ether or etheralcohol, and

   - between 0.005 and 6 g of a surface-active agent.
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 bath is employed at a temperature of between 40 and 80°C.
9. Process according to Claim 7 or 8, characterized in that after the steel surface has been placed in contact with the bath, a complementary quantity of nitric acid is added to the latter.