EP0425013A1 - Baths and process for chemical polishing of stainless steel surfaces - Google Patents

Abstract

Baths for chemical polishing of surfaces made of stainless steel, comprising, in aqueous solution, a mixture of hydrochloric acid and nitric acid, a quinone derivative, a surfactant and a brightening agent chosen from naphthalenes and quinolines substituted by at least one hydroxyl group and a sulphonate group. The baths are suitable for chemical polishing of surfaces made of austenitic steel in contact with which they are kept in a stagnant state.

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 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, 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 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 documents EP-B-19964, EP-A-193239, EP-A-206386 and EP-A-274776 (SOLVAY & Cie), chemical polishing baths with very slow action are described, which therefore avoid the disadvantages cited above. The baths described in these four documents comprise, in aqueous solution, a mixture of hydrochloric, nitric and phosphoric acids, combined with suitable additives. Thus, the baths described in document EP-A-19964 include sulfosalicylic acid, alkylpyridinium chloride and methylcellulose. The baths described in document EP-A-193239 comprise ferricyanide complex ions and optionally bromide, iodide or thiocyanate ions. The baths described in document EP-A-206 386 comprise complex ferricyanide ions, as well as urea or a urea derivative. The baths described in document EP-A-274776 comprise an abietic compound, such as an abietamine.

These known slow-action polishing baths have the particularity of having to be the seat of controlled agitation during their use for polishing a steel surface. The optimum parameters of agitation which should be carried out depend on various factors, in particular on the geometry and the dimensions of the surface subjected to polishing, which can cause difficulties in certain cases.

The invention aims to solve these difficulties associated with known polishing baths, by providing baths designed to carry out slow and effective chemical polishing of stainless steel surfaces, without requiring stirring.

Consequently, the invention relates to baths for the chemical polishing of stainless steel surfaces, comprising, in aqueous solution, a mixture of hydrochloric acid and nitric acid, a quinone derivative, a surfactant and a brightening agent selected from naphthalenes and quinolines substituted with at least one hydroxy group and one sulfonate group.

The term quinone derivative is intended to denote both the quinones and the quinone derivatives. The choice of the quinone derivative is not critical, provided that it is soluble in the aqueous solution of acids. Preferred examples of quinone derivatives which can be used in the context of the invention are benzoquinone and its derivatives such as tetrachlorobenzoquinone, unsubstituted hydroxybenzenes such as hydroquinone, pyrogallol, pyrocatechol and substituted hydroxybenzenes, such as chlorohydroquinone and 2,5-dihydroxy 1,4-disulfonate benzene.

The function of the surfactant is to ensure a homogeneous distribution of the bath over the metal surface subjected to polishing. It is preferably chosen from polyalkoxylated phenols substituted, on the phenol group, by at least one alkyl, aryl, alkylaryl or arylalkyl radical; TRITON products (registered trademark of ROHM & HAAS CO) constitute examples of surfactants of this type, usable in the baths according to the invention.

The brightener is selected from naphthalenes substituted by at least one hydroxy group and a sulfonate group and quinolines substituted by at least one hydroxy group and a sulfonate group. The brightening agent is selected from those of these compounds, which form water-soluble complexes with the ferric ions released in the bath. Examples of such compounds are described in Soviet Electrochemistry 14, (1978) 8, pages 1011-1017 (Bershadskaya et al: "Choice of brightener for chemical polishing of stainless steel"). 1,8-dihydroxy 3,6-disulfonate naphthalene, 2,3-dihydroxy 7-sulfonate naphthalene, 4-hydroxy 2-sulfonate 6-amino naphthalene and 8-hydroxy 5-sulfonate quinoline are well suited.

In the baths according to the invention, the mixture of hydrochloric acid and nitric acid may optionally contain other mineral acids commonly used in chemical polishing baths, for example phosphoric acid and / or acid sulfuric.

In a particular embodiment of the baths according to the invention, they also contain an additive capable of breaking down nitrous acid. In this embodiment of the invention, said additive has the function of decomposing at least part of the nitrous acid which is generated in the bath during the polishing of a steel surface. In principle, this additive can be chosen from all organic and inorganic substances which are capable of breaking down nitrous acid in an aqueous medium. Examples of such additives are the bromide, iodide and thiocyanate ions, sulfamic acid, hydroxylamine, hydrazine, acetone, primary, secondary and tertiary amines, urea and urea derivatives.

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 and its temperature; 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, nickel and molybdenum include, per liter of aqueous solution:
. between 1.5 and 5 moles of hydrochloric acid,
. between 0.05 and 0.5 mole of nitric acid,
. between 0 and 1 mole of phosphoric acid,
. between 0 and 1 mole of sulfuric acid,
. between 0.001 and 0.2 mole of the quinone derivative,
. between 0.001 and 0.05 mole of the brightening agent,
. between 0 and 0.7 mole of the additive capable of breaking down nitrous acid,
. between 0.5 and 10 ml of the surfactant.

The baths according to the invention are suitable for the chemical polishing of all surfaces in austenitic stainless steel alloyed with molybdenum. They are specially suitable for polishing austenitic steels alloyed with chromium, nickel and molybdenum, in particular those containing between 12 and 26% by weight of chromium, between 6 and 22% by weight of nickel and between 1 and 6% by weight of molybdenum. The baths according to the invention have the particularity of carrying out the polishing of such steels at slow speed, generally requiring a contact time of between 2 and 60 hours, generally between 10 and 50 hours. They have the additional particularity of carrying out polishing in the stagnant state, that is to say that they do not need to be the site of agitation in contact with the treated metal surfaces. These features of the baths according to the invention give them the advantage of considerably simplifying the polishing operations and, consequently, of reducing the cost thereof.

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, the bath being maintained in a stagnant state on contact. from the surface.

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

The examples whose description follows relate to chemical polishing tests using a few baths in accordance with the invention. In the tests, stainless steel plates of grade AISI-316L were used (steel alloyed with chromium (16.0 to 18.0%), nickel (10.0 to 14.0%) and molybdenum (2 , 0 to 3.0%)], with an initial arithmetic mean roughness (before polishing) of between 0.35 and 0.45 µm.

In each test, the plate was immersed in the polishing bath, the volume (V) of which was chosen as a function of the total lateral surface (S) of the plate, so as to produce a ratio
S / V = 9 m⁻¹

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)) :

. la brillance de la surface sous un angle de 60 degrés (selon la norme ASTM D523).Throughout the duration of the immersion, the bath was maintained in a substantially stagnant state and its temperature was kept constant. 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

where S denotes 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)
. 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 gloss of the surface at an angle of 60 degrees (according to standard ASTM D523).

Example 1

A polishing bath in accordance with the invention was used, comprising, per liter:
. 2.3 moles of hydrochloric acid,
. 0.3 moles of phosphoric acid,
. 0.1 mole of nitric acid,
. 0.05 mole of sulfamic acid,
. 10 g of pyrogallol,
. 6 g of 1,8-dihydroxy 3,6-disulfonate naphthalene,
. 5 ml of TRITON X 305 surfactant (registered trademark of ROHM & HAAS CO).

The immersion time of the plate in the bath was 41 hours and the temperature of the bath was maintained at 45 ° C. The following results were noted after polishing:
. average depth of attack: 23.0 µm;
. arithmetic mean roughness: 0.24 µm;
. gloss: 26.8%.

Example 2

Composition of the bath (for one liter):
. 2.7 moles of hydrochloric acid,
. 0.1 mole of phosphoric acid,
. 0.1 mole of nitric acid,
. 0.1 mole of sulfuric acid,
. 0.03 mole of potassium iodide,
. 0.5 g of pyrocatechol,
. 1 g of chlorohydroquinone,
. 7 g of 2,3-dihydroxy 7-sulfonate naphthalene,
. 0.5 g of 8-hydroxy 5-sulfonate quinoline,
. 3 ml of TRITON X 102 surfactant (registered trademark of ROHM & HAAS CO).
Duration of the immersion: 17 hours.
Bath temperature: 45 ° C.
Polishing results:
. average depth of attack: 30.2 µm,
. arithmetic mean roughness: 0.25 µm,
. gloss: 18.6%.

Example 3

Composition of the bath (for one liter):
. 3 moles of hydrochloric acid,
. 0.1 mole of nitric acid,
. 0.2 mole of sulfuric acid,
. 1 g of hydroquinone,
. 8 g of 4-hydroxy 2-sulfonate 6-amino naphthalene,
. 0.25 g of benzoylacetone,
. 1 ml of TRITON DF 16 surfactant (registered trademark of ROHM & HAAS CO),
. 5 ml of TRITON X 102 surfactant (registered trademark of ROHM & HAAS CO),
. 5 ml of methanol.
Duration of the immersion: 17 hours, 30 minutes.
Bath temperature: 45 ° C.
Polishing results:
. average attack depth: 26.0 µm,
. arithmetic mean roughness: 0.15 µm,
. gloss: 48.1%.

Example 4

Composition of the bath (for one liter):
. 2.3 moles of hydrochloric acid,
. 0.1 mole of nitric acid,
. 0.2 mole of phosphoric acid,
. 0.05 mole of thiourea,
. 2 g of chlorohydroquinone,
. 3 g of 2,3-dihydroxy 7-sulfonate naphthalene,
. 3 ml of TRITON X 102 surfactant (registered trademark of ROHM & HAAS CO).
Duration of the immersion: 48 hours.
Bath temperature: 45 ° C.
Polishing results:
. average attack depth: 11.1 µm,
. arithmetic mean roughness: 0.18 µm,
. gloss: 18.0%.

Example 5

Composition of the bath (for one liter):
. 3.2 moles of hydrochloric acid,
. 0.1 mole of nitric acid,
. 0.1 mole of sulfuric acid,
. 0.05 mole of hexylamine,
. 1.25 g of 2,5-dihydroxy 1,4-disulfonate benzene,
. 0.25 g of salicylic acid,
. 1 g of 4-aminophenol.
Duration of the immersion: 8 hours.
Bath temperature: 45 ° C.
Polishing results:
. average depth of attack: 33.1 µm,
. arithmetic mean roughness: 0.25 µm,
. gloss: 25.4%.

Example 6

Composition of the bath (for one liter):
. 4 moles of hydrochloric acid,
. 0.3 moles of nitric acid,
. 0.1 mole of sulfuric acid,
. 0.2 mole of urea,
. 0.5 g of tetrachlorobenzene,
. 0.5 g of chlorohydroquinone,
. 5 g of 2,3-dihydroxy 7-sulfonate naphthalene,
. 3 g of 1,8-dihydroxy 3,6-disulfonate naphthalene,
. 0.5 g of 8-hydroxy 5-sulfonate quinoline,
. 3 ml of TRITON X 305 surfactant (registered trademark of ROHM & HAAS CO).
Duration of the immersion: 25 hours.
Bath temperature: 25 ° C.
Polishing results:
. average depth of attack: 16.1 µm,
. arithmetic mean roughness: 0.27 µm,
. gloss: 12.2%.

Example 7

Composition of the bath (for one liter):
. 3.2 moles of hydrochloric acid,
. 0.2 moles of nitric acid,
. 0.2 mole of phosphoric acid,
. 0.1 mole of sulfuric acid,
. 0.1 mole of urea,
. 10 g of pyrogallol,
. 1.5 g of hydroquinone,
. 0.5 g of 4-hydroxy 2-sulfonate 6-amino naphthalene,
. 5 ml of TRITON X 165 surfactant (registered trademark of ROHM & HAAS CO).
Duration of the immersion: 32 hours.
Bath temperature: 25 ° C.
Polishing results:
. average attack depth: 9.5 µm,
. arithmetic mean roughness: 0.22 µm,
. gloss: 26.1%.

Claims (10)

1 - Baths for the chemical polishing of stainless steel surfaces, comprising, in aqueous solution, a mixture of hydrochloric acid and nitric acid, characterized in that they comprise, in the aqueous solution, a quinone derivative, a surfactant and a brightener selected from naphthalenes and quinolines substituted with at least one hydroxy group and one sulfonate group. 2 - Baths according to claim 1, characterized in that the quinone derivative is selected from substituted or unsubstituted hydroxybenzenes. 3 - Baths according to claim 1 or 2, characterized in that the surfactant is a polyalkoxylated phenol substituted, on the phenol group, by at least one alkyl, aryl, alkylaryl or arylalkyl radical. 4 - Baths according to any one of claims 1 to 3, characterized in that the brightening agent is selected from 1,8-dihydroxy 3,6-disulfonate naphthalene, 2,3-dihydroxy 7-sulfonate naphthalene, 4-hydroxy 2-sulfonate 6-amino naphthalene and 8-hydroxy 5-sulfonate quinoline. 5 - Baths according to any one of claims 1 to 4, characterized in that they further comprise, in the aqueous solution, an additive capable of breaking down nitrous acid. 6 - Baths according to claim 5, characterized in that the additive capable of breaking down nitrous acid is selected from urea, urea derivatives and sulfamic acid. 7 - Baths according to any one of claims 1 to 6, characterized in that the mixture of acids additionally contains phosphoric acid and / or sulfuric acid. 8 - Baths according to any one of claims 1 to 7, characterized in that they comprise, per liter of the solution aqueous,
. between 1.5 and 5 moles of hydrochloric acid,
. between 0.05 and 0.5 mole of nitric acid,
. between 0 and 1 mole of phosphoric acid,
. between 0 and 1 mole of sulfuric acid,
. between 0.001 and 0.2 mole of the quinone derivative,
. between 0.001 and 0.05 mole of the brightening agent,
. between 0 and 0.7 mole of the additive capable of breaking down nitrous acid,
. between 0.5 and 10 ml of the surfactant. 9 - Baths according to any one of claims 1 to 8, for polishing surfaces of austenitic steels alloyed with nickel, chromium and molybdenum. 10 - Method for polishing a stainless steel surface, according to which the surface is brought into contact with a chemical polishing bath, characterized in that a bath is used in accordance with any one of claims 1 to 7, and the bath is maintained in a stagnant state in contact with the surface.