EP2352860A1

EP2352860A1 - Method for the surface treatment of stainless steel

Info

Publication number: EP2352860A1
Application number: EP09737363A
Authority: EP
Inventors: Olaf Boehme; Siegfried Piesslinger-Schweiger
Original assignee: Poligrat GmbH
Current assignee: Poligrat GmbH
Priority date: 2008-10-29
Filing date: 2009-10-15
Publication date: 2011-08-10
Anticipated expiration: 2029-10-15
Also published as: JP5586614B2; WO2010049065A1; CA2741704A1; JP2012506951A; EP2352860B1; EP2182095A1; HUE026531T2; US20110309296A1; AU2009310094A1; US9115433B2; ES2559387T3

Abstract

A method for the surface treatment of stainless steel is described, wherein thermal oxide layers are brought into contact with a composition which dissolves iron ions out of the oxide layers.

Description

METHOD FOR THE SURFACE TREATMENT OF NON-STAINLESS STEEL

The present invention relates to a process for surface treatment of stainless steel instead of pickling. In this process, scale and tarnish in the area of welds and heat-treated surfaces are converted into corrosion-resistant oxide layers. The aim of this process is improved corrosion resistance without metal removal taking place. In this process, the stainless steel surface is treated with an aqueous or pasty solution / mixture. The mixture usually comprises a combination of complexing agents and an oxidizing agent.

STATE OF THE ART

Stainless steel, which is also often referred to as stainless steel, is an iron alloy which, in addition to iron and chromium, may also contain other elements such as nickel, molybdenum, titanium, copper and others. An essential component of the stainless steel alloys whose treatment is the subject of the present invention is the element chromium, which is present in a minimum concentration of about 12 wt .-%, in order to ensure increased resistance of the steel to corrosion. The chromium present in the alloy reacts with oxygen from the environment at the surface to form an oxide layer on the surface of the material. From a chromium content of about 12 wt .-% in the alloy of the material in question, the resulting chromium oxide can reliably form a dense layer on the surface and thus protects the workpiece from corrosion. This protective layer is called a passive layer.

Such a passive layer is usually about 10 molecule layers thick and contains not only chromium oxide but above all iron oxide in a concentration of 10 - 55 wt .-%. The lower the proportion of iron oxide in the passive layer, the higher its chemical resistance. A thermal treatment of stainless steel in an oxidizing atmosphere at temperatures above 200 ⁰ C causes a progressive thermal oxidation of the material to form an oxide layer consisting essentially of oxides of in

5 of the alloy existing metals and their quantitative ratio to each other substantially corresponds to the quantitative ratio of the metals in the alloy. Depending on the alloy, the thermal oxide layers therefore contain up to about 87% by weight of iron oxides. These oxide layers increase in thickness with increasing temperature and treatment time and lead to discoloration up to black or gray lo coverings. These are known as tinder and temper colors.

Such oxide layers, which contain significantly more iron oxide than chromium oxide, are not resistant to corrosion, which is why the stainless steel in these areas is not sufficiently resistant to corrosion for general use. In wet environments, the iron oxide reacts with water to form iron hydroxide and forms rust.

The reliable and complete removal of scale and tarnish from stainless steel surfaces is a mandatory requirement for the subsequent formation of an intact passive layer, which is the corrosion resistance of the stainless steel

2o causes. In the prior art, the removal of thermal oxides is carried out either by mechanical cleaning by means of grinding, brushing or blasting or by chemical or electrolytic pickling. The mechanical methods have the disadvantage that the cleaning effect is not complete and sufficient and not reach hard to reach areas such as corners, column 5 and cavities. Small and delicate workpieces are easily damaged.

Electrolytic pickling uses aqueous mixtures of mineral acids, which lead under the action of direct current to anodic removal of the top metal layer 0 by electrochemical dissolution, wherein the overlying oxide layers are eliminated with. These methods can only be used with thin oxide layers that are permeable to DC and electrolyte. Furthermore, they require a lot of effort in terms of plant technology. They work with hazardous substances, which also produce wastewater containing heavy metals, which requires extensive treatment and disposal. Chemical pickling chemically dissolves the oxide layers and the metal of the topmost material layer, creating a metallically clean surface. On top of this, a homogeneous passive layer can be formed which effectively protects the material from corrosion. Chemical pickling allows the entire surface of workpieces to be treated, including hard-to-reach areas. A disadvantage is the fact that the dissolution of the oxides and the material extremely aggressive and hazardous chemicals are required, which pose a high risk to humans and the environment.

The essential constituents of chemical stains for stainless steel are hydrofluoric acid (HF) or fluorides of salts of hydrofluoric acid, which form hydrofluoric acid in aqueous solution, as well as oxidants such as nitric acid or hydrogen peroxide. Hydrofluoric acid is extremely poisonous and can lead to death even with relatively little skin contact. Nitric acid releases toxic nitrous gases during pickling, which have a strong lung damaging effect. Hydrofluoric acid and nitric acid are gaseous acids, which is why the air in the working environment has to be sucked off and treated separately. The personnel employed for chemical pickling must wear appropriate protective clothing and, where appropriate, respiratory protection and is subject to constant medical supervision.

20 The manufacture, transportation, storage and use of chemicals used for chemical pickling are subject to strict safety regulations. The wastewater produced by chemical pickling contains in a high concentration the acids contained in the pickle as well as the heavy metals contained in the alloy such as chromium, iron, nickel and molybdenum. They must be chemically treated with care for disposal 5 and the resulting solids should be dumped as hazardous waste. Used pickling solutions are to be disposed of as hazardous waste.

From the rapidly increasing use of stainless steel in all areas of life and industry and the increasing need for pickling, there is an urgent need for a similar process to the pickling process for stainless steel, which is harmless to man and the environment.

The present invention is a chemical process for the treatment of the surface of stainless steel, harmless to humans and the environment and respectable 5 let the achievable corrosion resistance of the method according to the prior art at least equivalent, but in many cases clearly superior. DESCRIPTION OF THE INVENTION

All previously used methods for pickling stainless steel have the basis to remove the existing oxide layers including the uppermost layer of material so that subsequently can form on the cleaned metal surface under the influence of oxygen from the environment gradually a passive layer of the desired quality.

The present invention selects a new, previously unused path to solve the problem:

The invention is based on the idea that it is basically not necessary to eliminate the existing thermal oxide layers. Instead, it is also sufficient to reduce the concentration of iron oxides in the thermal oxide layers so far that the thermal oxide layers result in a concentration ratio of chromium oxides to iron oxides, which corresponds at least to that of intact passive layers. In order to be able to selectively remove iron oxides from the thermal oxide layers, an agent is needed whose chemical affinity is more selective to iron than the affinity of iron for oxygen. This makes it possible that the iron oxides are split. The iron can then be selectively removed from the thermal oxide layers.

Surprisingly, it has been shown that, for example, an aqueous solution with a special combination of organic complexing agents in combination with an oxidizing agent has these sought-after properties. The process according to the invention makes it possible selectively to remove the iron from all oxidation states of the iron, with the exception of hematite. However, hematite is chemically sufficiently stable even under corrosive conditions so that residual hematite residues have no negative impact on the corrosion resistance of stainless steel.

The invention thus provides a process for surface treatment of stainless steel, in which the thermal oxide layers are brought into contact with a composition which causes a selective dissolution of iron ions from the oxide layer. To understand the present invention, two conditions seem to be decisive. On the one hand, the surfaces which are treated according to the invention are stainless steel surfaces which have so-called thermal oxide layers. Thermal oxide layers are just not such oxide-containing layers that usually serve to passivate the stainless steel surface. Rather, the thermal oxide layers are unwanted and disruptive oxide layers which, on the one hand, lead to discoloration and, on the other hand, are themselves susceptible to corrosion or increase the susceptibility to corrosion of a stainless steel surface. Therefore, it is always necessary according to the state of the art that thermal oxide layers are removed in the context of corrosion resistance-improving measures. The present invention differs not only conceptually but also fundamentally from the prior art with regard to the aqueous solution actually used. Namely, according to the prior art, there is a pickling, ie ablation, treatment of the stainless steel surface, with the result that the thermal oxide layers are completely removed. Such a method is described for example in the utility model DE 92 14 890 U1 of the applicant in which the stainless steel surfaces are treated with a solution containing phosphoric acid and sulfuric acid in an erosive manner until neither scale residues nor discoloration around a weld seam can be observed.

According to the invention but not pickled and removed. Therefore, a possible acid content in an aqueous solution according to the invention is always kept so that no significant removal takes place. This means, for example, in the case of the stronger acids nitric acid / sulfuric acid / phosphoric acid, that according to the invention it is possible to work without these acids. However, smaller amounts can be tolerated as long as there is no significant removal of the thermal oxide layers. According to the invention weaker acids are used in smaller concentrations, for example citric acid with up to 5 wt .-%.

Solutions which can be used according to the invention already suggest DE 10 2007 010 538 A1 of the present Applicant for another method. However, this process involves protecting a stainless steel surface from undesired colorings by pretreating with an optimized aqueous passivating solution, which may be caused by the stainless steel surface being exposed to temperatures at which thermal oxide layers first form. So this process just wants exactly the thermal oxide layers prevent, which are treated according to the present inventive method.

Thermal oxide layers according to the invention are scale and tarnish, as they usually occur during thermal treatment or when welding stainless stainless steel. These surface layers are regularly recognized by the fact that they lead to a discoloration of the surface. The surface may then have a straw-yellow coloration, but may, depending on the duration and intensity of the thermal treatment of the surface, even in brown and blue shades.

In the case of high-alloyed steels, the following tempering colors and layer thicknesses are regularly observed at an annealing (elevated temperature) of about 350 ^° C. to> 1200 ^° C. .:

The chemical composition of the solution or mixture used according to the invention is now chosen such that, on the one hand, it can not be measured

Removal of the surface comes, but on the other hand, a dissolution of the iron ions from the oxide layer takes place on the surface. As a result, the method according to the invention thus acts in a similar way to a pickling method, in which, unlike pickling according to the prior art, the thermal oxide layers are not detached. Therefore, it is also possible according to the invention to use such mixtures which do not entail the disadvantages of the pickling baths used in the prior art. The dissolution of the iron ions from the surface preferably takes place selectively. By "selective" herein is meant that the complexing agent has a stronger affinity (i.e., greater complexing) to the iron than to the other components in the thermal oxide layers (eg, chromium or nickel).

Usually, the solutions / mixtures according to the invention comprise a combination of a complexing agent for iron and an oxidizing agent. The complexing agent is usually such a compound that can complex iron ions in aqueous solution. In particular, hydroxycarboxylic acids, phosphonic acids and organic nitrosulfonic acids can be used as complexing agents.

The preferred complexing agents are multidentate complexing agents. These multimetallic complexing agents can form chelate complexes with iron ions. This makes it possible to increase the ratio of chromium oxide to iron oxide in the thermal oxide layers.

Examples of suitable complexing agents furthermore include hydroxycarboxylic acids which have 1, 2 or 3 hydroxyl groups and 1, 2 or 3 carboxyl groups or their salts. A particularly suitable hydroxycarboxylic acid is citric acid. Another suitable complexing agent is the phosphonic acid having the general formula R'-PO (OH) ₂ , where R 'is a monovalent alkyl, hydroxyalkyl or aminoalkyl radical. Diphosphonic acid of the general formula R "[- PO (OH) ₂ ] 2 can also be used according to the invention, where R" is a divalent alkyl, hydroxyalkyl or aminoalkyl radical. Instead of or in addition to these phosphonic acids and / or

Diphosphonic acids can also be used one or more salts of these phosphonic acids or diphosphonic acids. A particularly preferred example of such an acid is 1-hydroxyethane-1,1-diphosphonic acid (HEDP) or its salt. Further suitable complexing agents are the organic nitrosulfonic acids, for example nitroalkylsulfonic acids, nitroarylsulfonic acids or salts thereof. A particularly preferred nitroarylsulfonic acid is the meta-nitrobenzenesulfonic acid. The substituted or unsubstituted alkyl or aryl radicals used should be selected such that the acid or salt has sufficient solubility in the aqueous solution / mixture. Therefore, a hydrocarbon chain preferably has not more than twelve carbon atoms. The compositions of the invention may additionally contain oxidizing agents. Suitable oxidizing agents are, for example, nitrates, peroxo compounds, iodates and cerium (IV) compounds in the form of the respective acids or of the water-soluble salts. Examples of peroxo compounds are peroxides, persulfates, perborates or else percarboxylates, such as peracetate. The oxidizing agents may be used alone or in the form of mixtures.

As stainless steel or stainless steel is considered in the context of the invention, an iron alloy containing a considerable proportion, for example, about 13 wt .-% and more chromium.

The solutions / mixtures according to the invention may additionally comprise one or more wetting agents which reduce the surface tension of the aqueous compositions. Examples of suitable wetting agents are, for example, the nitroalkyl- or nitroarylsulfonic acids already described in connection with complexing agents, or also alkylglycols of the general formula H- (O-CHR-CH ₂ ) _n -OH, where R is hydrogen or an alkyl radical with 1 , 2 or 3 carbon atoms, and n is preferably an integer between 1 and 5, for example 2 or 3.

A particularly suitable composition which can be used for the surface treatment according to the invention has the following composition:

0.5-10 wt .-%, in particular 3.0-5.0 wt .-%, of at least one hydroxycarboxylic acid having 1-3 hydroxyl and 1-3 carboxyl groups or their salt (s),

0.2-5.0 wt .-%, in particular 0.5-3.0 wt .-%, of at least one phosphonic acid of the general formula R'-PO (OH) ₂ or its salt (s), where R ' is a monovalent alkyl, hydroxylalkyl or aminoalkyl radical, and / or the general formula R "[- PO (OH) ₂ ] ₂ or its salt (e), where R" is a divalent alkyl, hydroxyalkyl or aminoalkyl radical,

0.1-5.0 wt .-%, in particular 0.5-3.0 wt .-%, of at least one nitroaryl or nitroalkylsulfonic acid or its salt (s), 0.05-1.0 wt .-% , in particular 0.1-0.5 wt .-%, of at least one alkyl glycol of the general formula H- (O-CHR-CH ₂ ) _n -OH, wherein R is hydrogen or an alkyl radical having 1-3 carbon atoms and n is 1-5, and 0.2-20% by weight, in particular 0.5-15% by weight, of an oxidizing agent, the remainder of the composition being water.

In addition, further wetting agents in a concentration of between 0.02 and 2.0% by weight, preferably between 0.05 and 1.0, can be additionally used in these compositions

Wt .-%, to be added. In addition, these compositions may optionally contain one or more thickening agents. Suitable thickeners are, for example, methyl cellulose and kieselguhr. Such a thickener serves to increase the viscosity of the mixture.

The inventive method is carried out regularly at a temperature between room temperature and 95 ⁰ C. But there are also other temperatures conceivable, while it must always be ensured that there is no significant removal of the treated thermal oxide layers.

The surface treatment according to the invention is usually carried out over a period of time which can be between 0.5 and 7 hours.

Following the surface treatment, the stainless steel surface is periodically rinsed with water, usually with deionized water rinse.

The pickling process can be carried out as a liquid by spraying, trickling, wiping or - slightly thickened by a suitable thickener (methylcellulose) - as spreadable paste in the dipping bath or by applying to the surfaces to be cleaned.

The application temperature is in the immersion bath is preferably 50 ⁰ C to 95 ⁰ C, preferably from 50 ⁰ C to 70 ⁰ C. The treatment time is 3 to 5 hours depending on the degree of scaling, the alloy to be treated and the temperature employed. At lower temperatures, the exposure time may be longer. Temperatures below 50 ^° C are only used in applications outside the bath. The temperature of the baths should be at least 50 ⁰ C permanently, to prevent biological degradation of the bath liquid.

The present invention therefore also relates to the use of a complexing agent for iron-containing composition, which in essence is free of hydrofluoric acid and / or of fluoride ions and other halide ions and mineral acids, for the surface treatment of a workpiece made of stainless steel, said surface having thermal oxide layers.

With regard to the complexing agent, the other constituents of the composition and the procedure for the surface treatment, the prerequisites and features previously established and specified for the method apply.

For example, the phrase "substantially free of hydrofluoric acid and / or fluoride ions" means that the composition does not contain hydrofluoric acid in an amount present in conventional mordants, but preferably the composition is completely free of hydrofluoric acid no mineral acids.

EXAMPLES

Example 1:

Six 1 mm thick 1.4301 stainless steel plates (AISI 304) with an austenitic structure and with a cold-rolled surface were cut from a sheet and then welded in pairs to form three samples (A B and C). The samples were then alkaline degreased, rinsed with deionized water and dried.

Sample A was left untreated.

Sample B was immersed in a pickling solution consisting of 5% by weight of hydrofluoric acid and 15% by weight of nitric acid, the remainder water, for 3 hours at room temperature, then rinsed with deionized water and allowed to stand for 30 minutes Room temperature in a passivation solution consisting of 20 wt .-% nitric acid, balance water, immersed. Finally, the sample was rinsed with deionized water and dried.

Sample C was composed of a solution

3.3% citric acid 2.1% nitroalkylsulfonic acid 3.5% hydroxyethanediphosphonic acid 0.2% ethylene glycol

0.1% wetting agent 25% magnesium nitrate ^■ 6 H2O balance deionized water

immersed for 3 hours at 70 ⁰ C, then rinsed with deionized water and dried.

The pitting potential against an Ag / AgCl electrode in millivolts was then measured potentiodynamically on samples A, B and C in an artificial seawater containing 20,000 ppm of chloride. The measurement was carried out at three points in the area of the blank, non-heat-affected area, in the area of the meeinflußzone and in the region of the weld. The results are shown in table.

Table 1: Pitting potential on material 1.4301

Dbe Uninfluenced heat welding seam

Area of influence area

A 370 280 150

B 350 320 340

C 500 400 350

Example 2

Six 1.2 mm thick 1.4571 (AISI 316 Ti) stainless steel sheets with a cold rolled surface were cut from a sheet and then welded in pairs to three (D ₁ E and F). The samples were then alkaline degreased, deionized Water rinsed and dried.

Sample D was left untreated,

Sample E was treated as Sample B

Sample F was treated as Sample C

On the samples D ₁ E and F, the corrosion potential was then measured according to Example 1 in the area of the bare, not heat-affected area, in the heat-affected zone and at the weld. The results are shown in Table 2

Table 2: Pitting potential on material 1.4571

Sample Uninfluenced heat-weld zone area of influence

D 480 400 400 E 520 550 550

F 700 650 630

Both examples show that the pitting potentials (samples C and F) obtained on the treated surfaces by the method according to the invention are equal to or higher compared to the prior art pickling method (samples B and E).

The process according to the invention has a number of significant advantages over the prior art pickling processes:

• The chemicals used are not hazardous substances. Manufacturing, transportation, storage and use are not subject to any restrictions or special

Regulations. The method according to the invention can be used everywhere and does not require special protective measures for humans and the environment.

• The chemicals used are biodegradable and, like the rinsing water used, require no special effort for disposal.

• The chemicals used do not pollute the air due to gases or odors.

The method according to the invention selectively dissolves iron exclusively from the already existing thermal oxide layer. Chromium, nickel, molybdenum and others

Heavy metals and the metal of the workpiece surface are regularly not dissolved and released. You will not get into the bath chemicals and the rinse water. The amount of iron dissolved from the existing oxide layer is so low that the legal limits for iron in the rinse water are far from being reached. The rinse water therefore requires no special

Treatment and there are no solid, heavy metal pollutants for disposal.

• The amount of dissolved iron is too low to cause the baths to become depleted by metal enrichment. The replacement of chemicals with fresh chemicals adhered to and removed from the treated surfaces keeps the concentration of iron in the bath well below 10% of the critical concentrations. ration. A recovery of the chemicals by evaporation with circulation of the rinse water is easily possible.

The method according to the invention does not attack the metal of the workpiece surface, which is why the appearance and quality of the surfaces do not suffer from the treatment.

• Since the existing oxide layer is not removed, the full corrosion resistance of the stainless steel is guaranteed immediately after the treatment, without the need for additional passivation treatment or waiting time to form a new passive layer. The corrosion resistance is guaranteed regardless of the supply of oxygen from the environment immediately.

Claims

claims

A stainless steel surface treatment method wherein thermal oxide layers are treated with a composition characterized in that the thermal oxide layers are contacted with a composition effective to dissolve out iron ions from the thermal oxide layers.

2. The method according to claim 1, characterized in that the composition contains a complexing agent which can form complexes with iron ions.

i5 3. The method according to claim 2, characterized in that the composition additionally contains an oxidizing agent.

4. The method according to any one of claims 1 to 3, characterized in that the composition consists of:

2o - at least one hydroxycarboxylic acid having 1-3 hydroxyl and 1-3 carboxyl groups or their salt (s),

- At least one phosphonic acid of the general structure R'-PO (OH) 2 or its salt (s), wherein R 'is a monovalent alkyl, hydroxyalkyl or amino kylrest, and / or the general structure R "[- PO (OH) ₂ ] 2 or their

25 salt (s), wherein R "is a bivalent alkyl, hydroxyalkyl or aminoalkyl radical, and

- At least one nitroaryl or nitroalkyl sulfonic acid or its salt (s).

5. The method of claim 1, wherein the oxidizing agent comprises at least one compound selected from the group consisting of

Nitrate, peroxide, persulfate, perborate, the percarboxylates, iodate and the cerium (IV) compounds in the form of the respective acids and / or salts.

6. The method according to any one of claims 1 to 5, characterized in that 5 comprises the aqueous solution:

0.5-10% by weight of at least one hydroxycarboxylic acid having 1 to 3 hydroxyl and 1 to 3 carboxyl groups or their salt (s), 0.2-5.0% by weight of at least one phosphonic acid of the general structure R'-PO (OH) ₂ or its salt (e), where R 'is a monovalent alkyl, hydroxyalkyl or aminoalkyl radical, and / or the general structure R "[- PO (OH) ₂ ] ₂ or its salt (e), where R" is a divalent alkyl, hydroxyalkyl or amino

5 alkyl radical,

0.1-5.0% by weight of at least one nitroaryl- or nitroalkylsulfonic acid or its salt (s),

0.05-1.0% by weight of at least one alkyl glycol of the general structure H- (O-CHR-CH ₂ ) _n -OH, where R is hydrogen or an alkyl radical having 1-3 carbon atoms and n is 1 -5 is, and

- 0.2-20 wt .-% of an oxidizing agent, wherein the remainder of the solution is water, which optionally also one or more thickening agents may be added.

i5

7. The method according to any one of claims 1 to 6, characterized in that the thermal oxide layers over a period of 0.5 to 7 hours are brought into contact with the composition.

8. The method according to any one of claims 1 to 7, characterized in that the contacting 20 in a temperature range between room temperature and

95 ⁰ C takes place.

9. Use of a complexing agent for iron-containing composition, which is substantially free of hydrofluoric acid or fluoride ions, for surface treatment of a workpiece 5 made of stainless steel, said surface having thermal oxide layers.