EP2147131B1 - Method for the thermochemical passivation of stainless steel - Google Patents

Method for the thermochemical passivation of stainless steel Download PDF

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Publication number
EP2147131B1
EP2147131B1 EP08715970A EP08715970A EP2147131B1 EP 2147131 B1 EP2147131 B1 EP 2147131B1 EP 08715970 A EP08715970 A EP 08715970A EP 08715970 A EP08715970 A EP 08715970A EP 2147131 B1 EP2147131 B1 EP 2147131B1
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stainless steel
process according
heat treatment
weight
salts
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German (de)
French (fr)
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EP2147131A1 (en
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Olaf BÖHME
Siegfried Piesslinger-Schweiger
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Poligrat GmbH
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Poligrat GmbH
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process
    • C23C22/74—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process for obtaining burned-in conversion coatings
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
    • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
    • C23C22/48—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 not containing phosphates, hexavalent chromium compounds, fluorides or complex fluorides, molybdates, tungstates, vanadates or oxalates
    • C23C22/50—Treatment of iron or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/82—After-treatment
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/02—Pretreatment of the material to be coated

Definitions

  • the present invention relates to a novel method of passivating stainless steel surfaces which provides improved corrosion resistance of the treated surfaces as well as increasing the resistance of these surfaces to thermal discoloration.
  • the process consists of a chemical treatment with an aqueous solution comprising complexing agents, a rinsing and a subsequent thermal treatment in a gaseous, oxygen-containing atmosphere.
  • Stainless steel which is often referred to as stainless steel, is an iron alloy which, in addition to iron, may contain a number of other elements such as chromium, nickel, molybdenum, copper and others.
  • the chromium present in the alloy reacts with oxygen from the environment at the surface and forms an oxide layer on the surface of the material.
  • the resulting chromium oxide can reliably form a dense layer on the surface and thus protects the workpiece from corrosion.
  • This protective layer is also called a passive layer.
  • Such a passive layer is usually about 10 molecule layers thick and contains in addition to the chromium oxide especially iron oxide in a concentration of 10-55 wt .-%.
  • the corrosion resistance of the workpiece depends on the content of chromium and other alloying elements such as nickel and molybdenum. These additional alloying elements are added to the stainless steel alloy to form the Corrosion resistance to further improve, if the addition of chromium alone is not able to give the workpiece the desired degree of corrosion resistance or other characteristics.
  • these other corrosion resistance improving elements are expensive and thus increase the cost of producing the stainless steel to a considerable extent.
  • Another known measure to increase corrosion resistance is to increase the ratio of chromium to iron in the passive layer.
  • treatment of the surface with substances that have a high affinity for iron ions and thus are able to selectively dissolve out and bind iron ions from the passive layer is suitable for this purpose.
  • aqueous solutions of complexing agents and / or chelating agents such as citric acid are frequently used, for example, the chromium / iron ratio on bright-rolled or ground stainless steel surfaces of a value of 0.8 to 1.2 before the treatment to a value of 3.0 can increase to 5.0 after treatment.
  • This increased content of chromium oxide causes a correspondingly improved corrosion resistance of the workpiece.
  • thermally grown oxide layers significantly reduce the corrosion resistance of the stainless steel by either preventing the formation of true passive layers or displacing existing passive layers at higher temperatures.
  • thermally generated oxide layers such as the tarnish or scale described above
  • thermally generated oxide layers such as the tarnish or scale described above
  • no method is known in the prior art, which improves the resistance of stainless steel surfaces against thermal discoloration, ie against the formation of such thermally generated oxide layers.
  • the aim of the present invention is a method for passivation of stainless steel surfaces, which in comparison to known passivation method according to the prior art, a significant increase in the corrosion potential, measured as pitting potential according to DIN 50900 causes.
  • the increase in corrosion potential that can be achieved with the methods described herein ranges from +500 mV to +850 mV from the initial state. This makes it possible in many cases, instead of expensive molybdenum or copper-containing materials cheaper To use stainless steel grades that have the required corrosion resistance due to their passivation according to a method of the present invention.
  • the corrosion resistance of the stainless steel surface can be markedly improved both for workpieces made of stainless steel with ferritic and austenitic structures by targeted heat treatment of the surfaces in an oxygen-containing atmosphere.
  • This heat treatment in an oxygen-containing atmosphere is often referred to below as heat treatment or thermal treatment.
  • the stainless steel workpiece is heated to a temperature of at least 80 ° C for a certain period of time.
  • the upper limit of the temperature to be used is given by the temperature at which a thermally induced discoloration of the stainless steel surface begins and varies depending on the quality of stainless steel used.
  • the present invention thus relates to a method for passivating stainless steel, wherein the stainless steel is first subjected to a chemical treatment with an aqueous solution, then rinsed with water and subsequently a heat treatment is carried out.
  • the aqueous solution used in the chemical treatment comprises at least one complexing agent combination and an oxidizing agent.
  • the complexing agent combination consists of those compounds known to complex iron ions in aqueous solution.
  • the invention is based in particular on the observation that only with a combination of the complexing agents a passivation effect is achieved, which satisfies the objectives of the invention.
  • complexing agents are used in particular hydroxcarboxylic acids, phosphonic acids and organic nitrosulfonic acids.
  • the complexing agents used are multidentate complexing agents. These multidentate complexing agents form chelate complexes with the iron ions and therefore contribute to further increasing the ratio of chromium oxide to iron oxide in the passive layer.
  • suitable complexing agents include, for example, hydroxycarboxylic acids which have 2 or 3 hydroxyl groups and 2 or 3 carboxyl groups or salts thereof.
  • a particularly suitable example of such a hydroxycarboxylic acid is citric acid.
  • Another suitable complexing agent is a diphosphonic acid of the general structure R "[- PO (OH) 2 ] 2 , where R" is a divalent alkyl, hydroxyalkyl or aminoalkyl radical.
  • R is a divalent alkyl, hydroxyalkyl or aminoalkyl radical.
  • R is a divalent alkyl, hydroxyalkyl or aminoalkyl radical.
  • a particularly preferred example of such an acid is 1-hydroxyethane-1,1-diphosphonic acid (HEDP) or its salts.
  • HEDP 1-hydroxyethane-1,1-diphosphonic acid
  • nitrosulphonic acids ie the nitroalkylsulphonic acids, nitroarylsulphonic acids or their salts.
  • a particularly preferred nitroarylsulfonic acid is meta- nitrobenzenesulfonic acid.
  • the acid or the salt has sufficient solubility in the aqueous solution.
  • the carbon chains whether linear, branched, cyclic or aromatic, comprise not more than about 12 carbon atoms, more preferably not more than 10 carbon atoms, and most preferably not more than 6 carbon atoms.
  • oxidizing agent Another essential ingredient of the aqueous solution in the chemical treatment is an oxidizing agent.
  • This oxidizing agent must be sufficient to ensure a normal potential of at least +300 mV in the solution.
  • Suitable oxidizing agents include, for example, nitrates, peroxo compounds, iodates and cerium (IV) compounds in the form of the respective acids or the corresponding water-soluble salts.
  • peroxo compounds are peroxides, persulfates, perborates or else percarboxylates, such as peracetate. These oxidizing agents can be used alone or in the form of mixtures.
  • the term "stainless steel” refers to iron alloys containing at least 13 weight percent chromium. Other corrosion resistance improving elements may be included in the alloy.
  • the chemical treatment according to the invention is not to be confused with a conventional pickling process, in which targeted metal from the surface of a metallic Workpiece is removed (see. DE 92 14 890 U1 EP 0596 273 A , and WO 88/00252 A1 ).
  • the inventors of the present application suspect that the special effect of the method according to the invention is attributable to the fact that a passive layer is not first produced, but an already existing passive layer is changed in its composition and structure by the sequence of the method steps according to the invention. But this is rather a theoretical idea that can not be understood in the sense of a limitation of the present method.
  • the aqueous solutions may also comprise one or more wetting agents which reduce the surface tension of the aqueous solution.
  • suitable wetting agents are, for example, the nitroalkyl or nitroaryl sulfonic acids already described in the case of the complexing agents, or also alkyl glycols of the general structure H- (O-CHR-CH 2 ) n -OH, where R is hydrogen or an alkyl radical having 1, 2 or 3 Is carbon atoms, and n is preferably an integer between 1 and 5, for example 2 or 3; or other wetting agents.
  • the at least one hydroxycarboxylic acid comprises citric acid, and / or the at least one diphosphonic acid HEDP, and / or the at least one nitroaryl or nitroalkyl sulfonic acid m- nitrobenzenesulfonic acid, and / or the at least one alkyl glycol, ethylene glycol and / or butyl glycol, and Oxidizing agent nitrate, peroxide, persulfate and / or cerium (IV) ions, in each case in the weight ratios indicated above.
  • compositions may be added to the above compositions in a concentration between 0.02 and 2.0% by weight, preferably between 0.05 and 1.0% by weight.
  • thickening agents may optionally be added to these compositions. These thickening agents, for example diatomaceous earth, can serve to increase the viscosity of the solution.
  • the chemical treatment in aqueous solution is preferably carried out in a dip bath, so that such thickeners can be dispensed with.
  • the aqueous solution preferably has a pH which is less than 7, preferably less than 4. This can be achieved by the aqueous solution containing at least one acid.
  • a preferred method is that at least one of the complexing agents and / or at least one of the oxidizing agents is at least partially added in the form of an acid to the solution.
  • the first step of the treatment according to the present invention in a preferred embodiment, is in an aqueous solution having a temperature of at most about 70 ° C. It is further preferred that the treatment takes place in aqueous solution at a temperature between room temperature and 60 ° C.
  • the chemical treatment in aqueous solution is preferably carried out over a period of at least 60 minutes, for example, the chemical treatment can be carried out with an aqueous solution over a period of 1-4 hours.
  • the workpiece is rinsed with water, preferably deionized water, to remove the passivating solution, and optionally dried, before the workpiece enters the passivation solution
  • Water preferably deionized water
  • Temperature treatment is subjected. This rinsing can be done by spraying or by (possibly repeated) immersion in a dipping bath or by combinations of these rinsing methods.
  • the step of heat treatment is carried out at a temperature of at least 80 ° C in an oxygen-containing atmosphere.
  • the heat treatment is preferably carried out at a temperature in the range between 80 ° C and 280 ° C, in particular at a temperature above 100 ° C and at most 260 ° C.
  • the oxygen-containing atmosphere of the thermal treatment may be air in a preferred embodiment.
  • the oxygen-containing atmosphere is primarily water vapor or a mixture of water vapor and air.
  • Such a steam-containing atmosphere is preferably used at a temperature of at least 100 ° C.
  • the optimum temperature range for the heat treatment depends essentially on the type of stainless steel to be treated. However, this optimum range can easily be determined by a person of ordinary skill in trial experiments.
  • a suitable temperature in a range between 100 ° C and 270 ° C, preferably between 150 ° C and 260 ° C, in particular between 220 ° C and 260 ° C, when the stainless steel is an austenitic steel having a content of about 16-20 wt .-% chromium and about 7-10 wt .-% nickel, such as stainless steel grade 1.4301 (see. FIG. 1 ).
  • the term "essentially none" means that the elements in question, if present at all, are present in a concentration of less than 1% by weight, generally between 0 and 0.1% by weight, in the alloy.
  • This heat treatment should take place over a period of at least 2 minutes (cf. FIG. 3 for quality 1.4301 stainless steel). Preferably, the heat treatment takes place over a period of 15-45 minutes, for example for about 30 minutes. Depending on the quality of the stainless steel, too long a thermal treatment, for example over several hours, can lead to a fall in the corrosion resistance of the treated workpiece.
  • a stainless steel of quality 1.4016 when heated to 140 ° C, ie at a temperature which is in the optimum range for the heat treatment first shows a rapid increase in the pitting potential to values around +1000 mV (cf. FIG. 4 ). However, if such a workpiece is exposed to this temperature for longer periods, the pitting potential drops again to values of about +700 mV. It is therefore important for some types of stainless steel that the heat treatment is carried out no longer than about 90 minutes, preferably not longer than about 60 minutes.
  • Another important advantage of the method described here is that it is not only suitable for significantly increasing the corrosion resistance, measured as the pitting potential according to DIN 50900, compared with the starting state, but that the method is also suitable for the resistance of stainless steel workpieces to increase thermal discoloration.
  • Such an increase in the resistance of stainless steel workpieces or their surfaces against thermal discoloration in their use by a passivation process has not been described and represents a further significant advantage of the invention described herein.
  • the prior art discloses, inter alia, also a method for cleaning and passivating a stainless steel surface, in which a hydroxyacetic acid or citric acid is applied to the surface in an aqueous solution (cf. EP 0 776 256 B1 ).
  • a hydroxyacetic acid or citric acid is applied to the surface in an aqueous solution
  • the content of hydroxycarboxylic acid in this process is well below 3.0 wt .-%.
  • the complexes used easily precipitate and are incorporated into the oxide film over the workpiece (see. Paragraph [0032] of the aforementioned EP 0 776 256 B1 ).
  • Example 1 Stainless steel of quality 1.4301
  • the chemical treatment was carried out at 40 ° C for 180 min. Subsequently, the sheet was rinsed with deionized water and dried in air.
  • the pitting potential of plate B was measured at +750 mV, an increase of +200 mV compared to the initial state.
  • sheet D was treated in a passivation solution whose composition is described in Example 1. The treatment takes place at room temperature (+ 22 ° C) for a duration of 2.5 h. Subsequently, the plate was rinsed clean with deionized water, dried in air and the pitting potential measured at +520 mV, an increase from the initial state by +150 mV.
  • this example shows that when exceeding the optimal temperature range for a certain quality stainless steel corrosion resistance decreases again, but still higher than before the passivation treatment.

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Abstract

The present invention relates to a process for improving the heat and corrosion resistance of stainless steel by means of a novel passivation process. This process comprises a chemical treatment with an aqueous solution comprising a complexing agent combination of at least one oxidant, a subsequent rinsing and a subsequent treatment at elevated temperature in an oxygen-containing atmosphere. The stainless steel surfaces obtained according to the invention have a homogeneous passive layer having increased chemical resistance and resistance to thermal discoloration.

Description

Die vorliegende Erfindung betrifft ein neuartiges Verfahren zum Passivieren von Edelstahloberflächen, das eine verbesserte Korrosionsbeständigkeit der behandelten Oberflächen bewirkt, sowie die Widerstandsfähigkeit dieser Oberflächen gegen thermische Verfärbungen erhöhen kann. Das Verfahren besteht aus einer chemischen Behandlung mit einer wässrigen Lösung, die Komplexbildner umfasst, einem Spülen und einer nachfolgenden thermischen Behandlung in einer gasförmigen, sauerstoffhaltigen Atmosphäre.The present invention relates to a novel method of passivating stainless steel surfaces which provides improved corrosion resistance of the treated surfaces as well as increasing the resistance of these surfaces to thermal discoloration. The process consists of a chemical treatment with an aqueous solution comprising complexing agents, a rinsing and a subsequent thermal treatment in a gaseous, oxygen-containing atmosphere.

Stand der TechnikState of the art

Nichtrostender Stahl, der häufig auch als Edelstahl bezeichnet wird, ist eine Eisenlegierung, die neben Eisen eine Reihe weiterer Elemente wie etwa Chrom, Nickel, Molybdän, Kupfer und andere enthalten kann. Wesentlicher Bestandteil der Edelstahllegierungen, deren Behandlung Gegenstand der vorliegenden Erfindung ist, ist das Element Chrom, das in einer Mindestkonzentration von etwa 13 Gew.-% vorliegt, um eine erhöhte Korrosionsbeständigkeit des Stahls sicherzustellen. Das Chrom, das in der Legierung vorhanden ist, reagiert dabei an der Oberfläche mit Sauerstoff aus der Umgebung und bildet eine Oxidschicht an der Oberfläche des Werkstoffs. Ab einem Chromgehalt von etwa 13 Gew.-% der Legierung des betreffenden Werkstücks kann das dabei entstehende Chromoxid zuverlässig eine dichte Schicht an der Oberfläche bilden und schützt somit das Werkstück vor Korrosion. Diese Schutzschicht wird auch als Passivschicht bezeichnet.Stainless steel, which is often referred to as stainless steel, is an iron alloy which, in addition to iron, may contain a number of other elements such as chromium, nickel, molybdenum, copper and others. An essential component of the stainless steel alloys, the treatment of which is the subject of the present invention, is the element chromium, which is present in a minimum concentration of about 13 wt .-%, to ensure increased corrosion resistance of the steel. The chromium present in the alloy reacts with oxygen from the environment at the surface and forms an oxide layer on the surface of the material. From a chromium content of about 13 wt .-% of the alloy of the relevant workpiece, the resulting chromium oxide can reliably form a dense layer on the surface and thus protects the workpiece from corrosion. This protective layer is also called a passive layer.

Eine solche Passivschicht ist in der Regel etwa 10 Moleküllagen dick und enthält neben dem Chromoxid vor allem Eisenoxid in einer Konzentration von 10-55 Gew.-%. Je geringer der Anteil an Eisenoxid in der Passivschicht ist, desto höher ist die chemische Beständigkeit der Oberfläche. Soweit nichts anderes festgehalten ist, beziehen sich alle hier angegebenen Prozentwerte jeweils auf das Gesamtgewicht der jeweiligen Zusammensetzungen des Edelstahls, der Lösungen, usw.Such a passive layer is usually about 10 molecule layers thick and contains in addition to the chromium oxide especially iron oxide in a concentration of 10-55 wt .-%. The lower the content of iron oxide in the passive layer, the higher the chemical resistance of the surface. Unless otherwise stated, all percentages given herein are based on the total weight of the respective compositions of the stainless steel, the solutions, etc.

Die Korrosionsbeständigkeit des Werkstücks hängt vom Gehalt an Chrom und weiteren Legierungselementen wie etwa Nickel und Molybdän ab. Diese weiteren Legierungselemente werden der Edelstahllegierung zugesetzt, um die Korrosionsbeständigkeit weiter zu verbessern, falls die Zugabe von Chrom allein nicht in der Lage ist, dem Werkstück den gewünschten Grad an Korrosionsbeständigkeit oder andere Merkmale zu verleihen. Diese weiteren, die Korrosionsbeständigkeit verbessernden Elemente sind jedoch teuer und erhöhen somit die Kosten der Herstellung des Edelstahls in nicht unerheblichem Ausmaß.The corrosion resistance of the workpiece depends on the content of chromium and other alloying elements such as nickel and molybdenum. These additional alloying elements are added to the stainless steel alloy to form the Corrosion resistance to further improve, if the addition of chromium alone is not able to give the workpiece the desired degree of corrosion resistance or other characteristics. However, these other corrosion resistance improving elements are expensive and thus increase the cost of producing the stainless steel to a considerable extent.

Eine Alternative zur Verwendung dieser teuren weiteren Elemente besteht in der Ausbildung einer möglichst fehlerfreien und dichten Passivschicht auf der Oberfläche des Edelstahlwerkstücks, die ein möglichst hohes Verhältnis von Chrom zu Eisen in der Passivschicht aufweist. Eine solche fehlerfreie und dichte Passivschicht ist ebenfalls in der Lage, die Korrosionsbeständigkeit des Werkstücks deutlich zu erhöhen. Um eine rasche Ausbildung einer solchen fehlerfreien und dichten Passivschicht zu fördern, werden üblicherweise "Passivierungsverfahren" eingesetzt, das heißt, die Oberflächen der Edelstahlwerkstücke werden mit oxidierenden Medien behandelt. Üblich ist dabei eine Behandlung mit verdünnter Salpetersäure oder Wasserstoffperoxid oder Phosphorsäure, die häufig nach einem Beizen der Oberfläche durchgeführt wird. Ein passivierungsverfahren für Edelstahl ohne Salpetersäure, Soudern mit H2SO4, HF und H2O2 ist aus US 5 354 383 A bekannt.An alternative to the use of these expensive further elements is the formation of a defect-free and dense passive layer on the surface of the stainless steel workpiece, which has the highest possible ratio of chromium to iron in the passive layer. Such a defect-free and dense passive layer is also able to significantly increase the corrosion resistance of the workpiece. In order to promote a rapid formation of such a defect-free and dense passive layer, "passivation processes" are usually used, that is to say the surfaces of the stainless steel workpieces are treated with oxidizing media. A treatment with dilute nitric acid or hydrogen peroxide or phosphoric acid, which is frequently carried out after a pickling of the surface, is usual. A passivation process for stainless steel without nitric acid, souders with H 2 SO 4 , HF and H 2 O 2 is out US 5,354,383 A known.

Eine weitere bekannte Maßnahme zur Erhöhung der Korrosionsbeständigkeit ist die Erhöhung des Verhältnisses von Chrom zu Eisen in der Passivschicht. Hierzu eignet sich etwa die Behandlung der Oberfläche mit Substanzen, die eine hohe Affinität zu Eisenionen haben und somit in der Lage sind, selektiv Eisenionen aus der Passivschicht herauszulösen und zu binden. Häufig werden dafür wässrige Lösungen von Komplexbildnern und/oder Chelatbildnern wie beispielsweise Zitronensäure eingesetzt, die etwa das Chrom/Eisen-Verhältnis auf walzblanken oder geschliffenen Edelstahloberflächen von einem Wert von 0,8 bis 1,2 vor der Behandlung auf einen Wert von 3,0 bis 5,0 nach der Behandlung erhöhen können. Dieser erhöhte Gehalt an Chromoxid bewirkt eine entsprechend verbesserte Korrosionsbeständigkeit des Werkstücks.Another known measure to increase corrosion resistance is to increase the ratio of chromium to iron in the passive layer. For example, treatment of the surface with substances that have a high affinity for iron ions and thus are able to selectively dissolve out and bind iron ions from the passive layer is suitable for this purpose. For this purpose, aqueous solutions of complexing agents and / or chelating agents such as citric acid are frequently used, for example, the chromium / iron ratio on bright-rolled or ground stainless steel surfaces of a value of 0.8 to 1.2 before the treatment to a value of 3.0 can increase to 5.0 after treatment. This increased content of chromium oxide causes a correspondingly improved corrosion resistance of the workpiece.

Mit diesen hier beschriebenen, bekannten Maßnahmen können Verbesserungen der Korrosionsbeständigkeit von Edelstahlwerkstücken, gemessen anhand des Lochfraßpotentials dieser Werkstücke, von +100 mV bis bestenfalls +400 mV im Vergleich zum Ausgangszustand erzielt werden, in Abhängigkeit von der Zusammensetzung und der Oberflächenqualität des behandelten Edelstahls, sowie den eingesetzten Passivierungsverfahren. Neben der Korrosionsbeständigkeit ist für die Verwendung von Edelstahl häufig auch dessen Temperaturbeständigkeit von Bedeutung. Wird Edelstahl an Luft über eine kritische Temperatur hinaus erwärmt, beginnt sich die Oberfläche zu verfärben. Diese Verfärbung beginnt in der Regel mit einer strohgelben Farbe, die bei höheren Temperaturen in braune und blaue Farbtöne übergehen kann. Ursache für diese Verfärbungen, die auch als Anlassfarben bezeichnet werden, sind Lichtinterferenzen an einer Oxidschicht zunehmender Dicke. Die kritische Temperatur, bei der die Verfärbung beginnt, hängt von der jeweiligen Legierung, dem Gefüge und der Oberflächenqualität des Edelstahlwerkstücks ab. Sie liegt häufig im Bereich von etwa 160 bis 180°C und ist umso höher, je höher die Korrosionsbeständigkeit der Edelstahls ist.With these known measures described herein, improvements in the corrosion resistance of stainless steel workpieces, as measured by the pitting potential of these workpieces, can be achieved from +100 mV to at best +400 mV compared to the initial state, depending on the composition and surface quality of the treated stainless steel the passivation procedure used. In addition to the corrosion resistance, the temperature resistance is often important for the use of stainless steel. If stainless steel is heated in air above a critical temperature, the surface begins to discolor. This discoloration usually begins with a straw-yellow color, which can turn into brown and blue shades at higher temperatures. The cause of these discolorations, which are also referred to as tempering colors, are light interferences on an oxide layer of increasing thickness. The critical temperature at which discoloration begins depends on the particular alloy, microstructure and surface quality of the stainless steel workpiece. It is often in the range of about 160 to 180 ° C and is higher, the higher the corrosion resistance of stainless steel.

Diese thermisch erzeugten Oxidschichten sind nicht nur unschön, sie weisen auch - verglichen mit den echten Passivschichten, wie sie hier zuvor beschrieben sind - eine erheblich geringere chemische Widerstandsfähigkeit auf. Solche thermisch erzeugten Oxidschichten reduzieren die Korrosionsbeständigkeit des Edelstahls in erheblichem Umfang, indem sie entweder die Ausbildung echter Passivschichten verhindern oder bei höheren Temperaturen bestehende Passivschichten verdrängen.Not only are these thermally generated oxide layers unpleasant, they also have significantly less chemical resistance than the true passive layers described hereinbefore. Such thermally grown oxide layers significantly reduce the corrosion resistance of the stainless steel by either preventing the formation of true passive layers or displacing existing passive layers at higher temperatures.

Es ist daher äußerst wichtig, die Edelstahloberflächen vor dem Gebrauch von etwaig vorhandenen thermisch erzeugten Oxidschichten zu reinigen und die Bildung solcher thermisch erzeugten Oxidschichten im Betrieb zu vermeiden.It is therefore extremely important to clean the stainless steel surfaces prior to use of any existing thermally generated oxide layers and to avoid the formation of such thermally generated oxide layers during operation.

Die Beseitigung thermisch erzeugter Oxidschichten, wie der oben beschriebenen Anlauffarben oder Zunder, erfolgt in der Praxis entweder mechanisch durch Strahlen, Schleifen oder Bürsten der Oberfläche, oder chemisch durch Beizen oder Elektropolieren. Es ist jedoch im bisherigen Stand der Technik kein Verfahren bekannt, das die Widerstandsfähigkeit von Edelstahloberflächen gegen thermische Verfärbungen, also gegen die Bildung solcher thermisch erzeugten Oxidschichten verbessert.The removal of thermally generated oxide layers, such as the tarnish or scale described above, takes place in practice either mechanically by blasting, grinding or brushing the surface, or chemically by pickling or electropolishing. However, no method is known in the prior art, which improves the resistance of stainless steel surfaces against thermal discoloration, ie against the formation of such thermally generated oxide layers.

Ziel der hier vorliegenden Erfindung ist ein Verfahren zum Passivieren von Edelstahloberflächen, das im Vergleich zu bekannten Passivierungsverfahren gemäß dem Stand der Technik eine deutliche Erhöhung des Korrosionspotentials, gemessen als Lochfraßpotential gemäß DIN 50900, bewirkt. Die Erhöhung des Korrosionspotentials, die mit den hier beschriebenen Verfahren erreicht werden kann, liegt im Bereich von +500 mV bis +850 mV gegenüber dem Ausgangszustand. Damit ist es in vielen Fällen möglich, anstelle teurer Molybdän- oder Kupfer-haltiger Werkstoffe günstigere Edelstahlqualitäten einzusetzen, die aufgrund ihrer Passivierung gemäß einem Verfahren der vorliegenden Erfindung die geforderte Korrosionsbeständigkeit besitzen.The aim of the present invention is a method for passivation of stainless steel surfaces, which in comparison to known passivation method according to the prior art, a significant increase in the corrosion potential, measured as pitting potential according to DIN 50900 causes. The increase in corrosion potential that can be achieved with the methods described herein ranges from +500 mV to +850 mV from the initial state. This makes it possible in many cases, instead of expensive molybdenum or copper-containing materials cheaper To use stainless steel grades that have the required corrosion resistance due to their passivation according to a method of the present invention.

Beschreibung der AbbildungenDescription of the pictures

  • Figur 1 zeigt das Lochfraßpotential von unbehandeltem und chemisch behandeltem Edelstahl der Qualität 1.4301 nach jeweils 30 Minuten Wärmebehandlung bei den angegebenen Temperaturen. FIG. 1 shows the pitting potential of untreated and chemically treated stainless steel grade 1.4301 after each 30 minutes of heat treatment at the indicated temperatures.
  • Figur 2 zeigt das Lochfraßpotential von unbehandeltem und chemisch behandeltem Edelstahl der Qualität 1.4016 nach jeweils 30 Minuten Wärmebehandlung bei den angegebenen Temperaturen. FIG. 2 shows the pitting potential of untreated and chemically treated stainless steel grade 1.4016 after every 30 minutes of heat treatment at the indicated temperatures.
  • Figur 3 zeigt das Lochfraßpotential von Edelstahl der Qualität 1.4301 im Zeitverlauf einer Wärmebehandlung bei 140°C. FIG. 3 shows the pitting potential of grade 1.4301 stainless steel over time at 140 ° C.
  • Figur 4 zeigt das Lochfraßpotential von Edelstahl der Qualität 1.4016 im Zeitverlauf einer Wärmebehandlung bei 140°C. FIG. 4 shows the pitting potential of grade 1.4016 stainless steel over time at 140 ° C.
Beschreibung der ErfindungDescription of the invention

Überraschend zeigte sich, dass durch eine gezielte Wärmebehandlung der Oberflächen in einer sauerstoffhaltigen Atmosphäre die Korrosionsbeständigkeit der Edelstahloberfläche sowohl bei Werkstücken aus Edelstahl mit ferritischer, als auch bei solchen mit austenitischer Struktur merklich verbessert werden kann. Diese Wärmebehandlung in einer sauerstoffhaltigen Atmosphäre wird im Folgenden häufig auch als Wärmebehandlung bzw. thermische Behandlung bezeichnet. Dazu wird das Edelstahlwerkstück für einen gewissen Zeitraum auf eine Temperatur von mindestens 80°C erwärmt. Die Obergrenze der anzuwendenden Temperatur ist durch die Temperatur gegeben, bei der eine thermisch bedingte Verfärbung der Edelstahloberfläche einsetzt und ist je nach verwendeter Edelstahlqualität unterschiedlich. Wird diese Obergrenze des Temperaturbereiches überschritten und somit ein Temperaturbereich erreicht, in dem es zu einer thermischen Verfärbung des Edelstahl kommt, fällt die Korrosionsbeständigkeit des behandelten Werkstücks wieder ab. Mit einer geeigneten Wärmebehandlung lässt sich das Lochfraßpotential gemäß DIN 50900 häufig um etwa +100 bis +150 mV und sogar um bis zu etwa +200 mV und mehr erhöhen.Surprisingly, it has been found that the corrosion resistance of the stainless steel surface can be markedly improved both for workpieces made of stainless steel with ferritic and austenitic structures by targeted heat treatment of the surfaces in an oxygen-containing atmosphere. This heat treatment in an oxygen-containing atmosphere is often referred to below as heat treatment or thermal treatment. For this purpose, the stainless steel workpiece is heated to a temperature of at least 80 ° C for a certain period of time. The upper limit of the temperature to be used is given by the temperature at which a thermally induced discoloration of the stainless steel surface begins and varies depending on the quality of stainless steel used. If this upper limit of the temperature range is exceeded and thus reaches a temperature range in which there is a thermal discoloration of the stainless steel, the corrosion resistance of the treated workpiece drops again. With a suitable Heat treatment can often increase the pitting potential in accordance with DIN 50900 by about +100 to +150 mV and even by up to about +200 mV and more.

Es war ebenfalls überraschend, dass eine Vorbehandlung der Edelstahloberflächen vor dieser Wärmebehandlung mit einer optimierten wässrigen Passivierungslösung zu einer weiteren, teilweise drastischen Erhöhung des Lochfraßpotentials führen kann. Diese Vorbehandlung in einer wässrigen Passivierungslösung wird im Folgenden häufig auch als chemische Behandlung bezeichnet. So wurde etwa in Versuchen mit Edelstählen der Qualität 1.4016 (18% Chrom, ferritisches Gefüge) und anschließender Wärmebehandlung eine Erhöhung des Lochfraßpotentials um +500 bis +550 mV gegenüber dem Ausgangszustand erzielt. Bei Versuchen mit Edelstählen der Qualität 1.4301 (18% Chrom, 8% Nickel, austenitisches Gefüge) und anschließender Wärmebehandlung konnte sogar eine Erhöhung des Lochfraßpotentials um etwa +850 mV und mehr im Vergleich zum Ausgangszustand erzielt werden. Dieser Zuwachs an Korrosionsbeständigkeit kann somit sogar oberhalb der Werte liegen, die sie sich aus der Summe der Erhöhungen des Lochfraßpotentials der jeweiligen einzelnen Behandlungen ergeben, so dass hier offenbar eine synergistische Wirkung der chemischen und der thermischen Behandlungen beobachtet werden kann.It was also surprising that pretreatment of the stainless steel surfaces prior to this heat treatment with an optimized aqueous passivation solution can lead to a further, sometimes drastic increase in the pitting potential. This pretreatment in an aqueous passivation solution is often referred to below as a chemical treatment. Thus, for example, in tests with stainless steels of quality 1.4016 (18% chromium, ferritic microstructure) and subsequent heat treatment an increase of the pitting potential by +500 to +550 mV compared to the initial state was achieved. In tests with stainless steels of quality 1.4301 (18% chromium, 8% nickel, austenitic structure) and subsequent heat treatment, it was even possible to increase the pitting potential by about +850 mV and more compared to the initial state. This increase in corrosion resistance may thus even be above the values which result from the sum of the increases in the pitting potential of the respective individual treatments, so that a synergistic effect of the chemical and thermal treatments can obviously be observed here.

Die vorliegende Erfindung betrifft somit ein Verfahren zum Passivieren von Edelstahl, wobei der Edelstahl zunächst einer chemischen Behandlung mit einer wässrigen Lösung unterzogen wird, anschließend mit Wasser gespült und nachfolgend eine Wärmebehandlung durchgeführt wird. Die bei der chemischen Behandlung eingesetzte wässerige Lösung umfasst mindestens eine Komplexbildnerkombination und ein Oxidationsmittel. Die Komplexbildnerkombination besteht aus solchen Verbindungen, die bekanntermaßen Eisenionen in wässeriger Lösung komplexieren können. Die Erfindung geht insbesondere auf die Beobachtung zurück, dass erst mit einer Kombination der Komplexbildner eine Passivierungswirkung erreicht wird, die den erfindungsgemäßen Zielen genügt. Als Komplexbildner werden insbesondere eingesetzt Hydroxcarbonsäuren, Phosphonsäuren sowie organische Nitrosulfonsäuren.The present invention thus relates to a method for passivating stainless steel, wherein the stainless steel is first subjected to a chemical treatment with an aqueous solution, then rinsed with water and subsequently a heat treatment is carried out. The aqueous solution used in the chemical treatment comprises at least one complexing agent combination and an oxidizing agent. The complexing agent combination consists of those compounds known to complex iron ions in aqueous solution. The invention is based in particular on the observation that only with a combination of the complexing agents a passivation effect is achieved, which satisfies the objectives of the invention. As complexing agents are used in particular hydroxcarboxylic acids, phosphonic acids and organic nitrosulfonic acids.

Als Komplexbildner werden mehrzähnige Komplexbildner eingesetzt. Diese mehrzähnigen Komplexbildner bilden mit den Eisenionen Chelatkomplexe und tragen deshalb dazu bei, das Verhältnis Chromoxid zu Eisenoxid in der Passivschicht weiter zu erhöhen.The complexing agents used are multidentate complexing agents. These multidentate complexing agents form chelate complexes with the iron ions and therefore contribute to further increasing the ratio of chromium oxide to iron oxide in the passive layer.

Beispiele für geeignete Komplexbildner umfassen etwa Hydroxycarbonsäuren, die 2 oder 3 Hydroxylgruppen und 2 oder 3 Carboxylgruppen aufweisen bzw. deren Salze. Ein besonders geeignetes Beispiel für eine solche Hydroxycarbonsäure ist Zitronensäure. Ein weiterer geeigneter Komplexbildner ist eine Diphosphonsäure der allgemeinen Struktur R"[-PO(OH)2]2, wobei R" ein divalenter Alkyl-, Hydroxyalkyl- oder Aminoalkylrest ist. Anstelle von oder zusätzlich zu diesen Diphosphonsäuren können auch ein oder mehrere Salze dieser Diphosphonsäuren eingesetzt werden. Ein besonders bevorzugtes Beispiel einer solchen Säure ist 1-Hydroxyethan-1,1-diphosphonsäure (HEDP) bzw. deren Salze. Weitere geeignete Komplexbildner gehören zur Klasse der organischen Nitrosulfonsäuren, also der Nitroalkylsulfonsäuren, Nitroarylsulfonsäuren, bzw. deren Salze. Eine besonders bevorzugte Nitroarylsulfonsäure ist meta-Nitrobenzolsulfonsäure. Bei den hier erwähnten substituierten oder nicht substituierten Alkyl- oder Arylresten bzw. den Kohlenstoffgerüsten der Verbindungen ist darauf zu achten, dass die Säure bzw. das Salz eine ausreichende Löslichkeit in der wässrigen Lösung besitzen. Aus diesem Grund wird bevorzugt, dass die Kohlenstoffketten, ob linear, verzweigt, ringförmig oder aromatisch, nicht mehr als etwa 12 Kohlenstoffatome, insbesondere nicht mehr als 10 Kohlenstoffatome, und am meisten bevorzugt nicht mehr als 6 Kohlenstoffatome umfassen.Examples of suitable complexing agents include, for example, hydroxycarboxylic acids which have 2 or 3 hydroxyl groups and 2 or 3 carboxyl groups or salts thereof. A particularly suitable example of such a hydroxycarboxylic acid is citric acid. Another suitable complexing agent is a diphosphonic acid of the general structure R "[- PO (OH) 2 ] 2 , where R" is a divalent alkyl, hydroxyalkyl or aminoalkyl radical. Instead of or in addition to these diphosphonic acids, it is also possible to use one or more salts of these diphosphonic acids. A particularly preferred example of such an acid is 1-hydroxyethane-1,1-diphosphonic acid (HEDP) or its salts. Further suitable complexing agents belong to the class of the organic nitrosulphonic acids, ie the nitroalkylsulphonic acids, nitroarylsulphonic acids or their salts. A particularly preferred nitroarylsulfonic acid is meta- nitrobenzenesulfonic acid. In the case of the substituted or unsubstituted alkyl or aryl radicals or the carbon skeletons of the compounds mentioned here, care must be taken that the acid or the salt has sufficient solubility in the aqueous solution. For this reason, it is preferred that the carbon chains, whether linear, branched, cyclic or aromatic, comprise not more than about 12 carbon atoms, more preferably not more than 10 carbon atoms, and most preferably not more than 6 carbon atoms.

Ein weiterer essentieller Bestandteil der wässerigen Lösung bei der chemischen Behandlung ist ein Oxidationsmittel. Dieses Oxidationsmittel muss ausreichen, um in der Lösung ein Normalpotential von mindestens +300 mV zu gewährleisten. Geeignete Oxidationsmittel umfassen beispielsweise Nitrate, Peroxoverbindungen, Iodate und Cer(IV)-Verbindungen in Form der jeweiligen Säuren bzw. der entsprechenden wasserlöslichen Salze. Beispiele für Peroxoverbindungen sind Peroxide, Persulfate, Perborate oder auch Percarboxylate wie etwa Peracetat. Diese Oxidationsmittel können allein oder in Form von Gemischen verwendet werden.Another essential ingredient of the aqueous solution in the chemical treatment is an oxidizing agent. This oxidizing agent must be sufficient to ensure a normal potential of at least +300 mV in the solution. Suitable oxidizing agents include, for example, nitrates, peroxo compounds, iodates and cerium (IV) compounds in the form of the respective acids or the corresponding water-soluble salts. Examples of peroxo compounds are peroxides, persulfates, perborates or else percarboxylates, such as peracetate. These oxidizing agents can be used alone or in the form of mixtures.

Der Begriff "Edelstahl", wie er hier verwendet wird, richtet sich auf Eisenlegierungen, die einen Gehalt von mindestens 13 Gew.-% Chrom aufweisen. Weitere, die Korrosionsbeständigkeit verbessernde Elemente können in der Legierung enthalten sein.As used herein, the term "stainless steel" refers to iron alloys containing at least 13 weight percent chromium. Other corrosion resistance improving elements may be included in the alloy.

Die erfindungsgemäße chemische Behandlung ist nicht zu verwechseln mit einem üblichen Beizverfahren, bei dem gezielt Metall von der Oberfläche eines metallischen Werkstücks abgetragen wird (vgl. DE 92 14 890 U1 EP 0596 273 A , und WO 88/00252 A1 ). Die Erfinder der vorliegenden Anmeldung vermuten, dass die besondere Wirkung des erfindungsgemäßen Verfahrens darauf zurückzuführen ist, dass nicht zunächst eine Passivschicht erzeugt wird, sondern eine bereits vorliegende Passivschicht in ihrer Zusammensetzung und Struktur durch die Abfolge der erfindungsgemäßen Verfahrensschritte verändert wird. Dies ist aber eher eine theoretische Vorstellung, die nicht im Sinne eine Beschränkung des vorliegenden Verfahrens verstanden werden kann.The chemical treatment according to the invention is not to be confused with a conventional pickling process, in which targeted metal from the surface of a metallic Workpiece is removed (see. DE 92 14 890 U1 EP 0596 273 A , and WO 88/00252 A1 ). The inventors of the present application suspect that the special effect of the method according to the invention is attributable to the fact that a passive layer is not first produced, but an already existing passive layer is changed in its composition and structure by the sequence of the method steps according to the invention. But this is rather a theoretical idea that can not be understood in the sense of a limitation of the present method.

Die wässrigen Lösungen können außerdem noch ein oder mehrere Netzmittel umfassen, die die Oberflächenspannung der wässrigen Lösung herabsetzen. Beispiele für geeignete Netzmittel sind etwa die bereits bei den Komplexbildnern beschriebenen Nitroalkyl- bzw. Nitroarylsulfonsäuren, oder auch Alkylglykole der allgemeinen Struktur H-(O-CHR-CH2)n-OH, wobei R Wasserstoff oder ein Alkylrest mit 1, 2 oder 3 Kohlenstoffatomen ist, und n vorzugsweise eine ganze Zahl zwischen 1 und 5 ist, beispielsweise 2 oder 3; oder andere Netzmittel.The aqueous solutions may also comprise one or more wetting agents which reduce the surface tension of the aqueous solution. Examples of suitable wetting agents are, for example, the nitroalkyl or nitroaryl sulfonic acids already described in the case of the complexing agents, or also alkyl glycols of the general structure H- (O-CHR-CH 2 ) n -OH, where R is hydrogen or an alkyl radical having 1, 2 or 3 Is carbon atoms, and n is preferably an integer between 1 and 5, for example 2 or 3; or other wetting agents.

Ein besonders geeignetes Beispiel für eine wässrige Lösung, wie sie im ersten Schritt der Behandlung gemäß der vorliegenden Erfindung verwendet werden kann, umfasst folgende Zusammensetzung:

  • 0,5-10 Gew.-%, insbesondere 3,0-5,0 Gew.-%, mindestens einer Hydroxycarbonsäure mit 2-3 Hydroxyl- und 2-3 Carboxylgruppen bzw. deren Salz(e),
  • 0,5-5,0 Gew.-%, mindestens einer Diphosphonsäure der
    der allgemeinen Struktur R"[-PO(OH)2]2 bzw. deren Salz(e), wobei R" ein divalenter Alkyl-, Hydroxyalkyl- oder Aminoalkylrest ist,
  • 0,1-5,0 Gew.-%, insbesondere 0,5-3,0 Gew.-%, mindestens einer Nitroaryl- oder Nitroalkylsulfonsäure bzw. deren Salz(e),
  • 0,05-1,0 Gew.-%, insbesondere 0,1-0,5 Gew.-%, mindestens eines Alkylglykols der allgemeinen Struktur H-(O-CHR-CH2)n-OH, wobei R Wasserstoff oder ein Alkylrest mit 1-3 Kohlenstoffatomen ist und n 1-5 ist, und
  • 0,2-20 Gew.-%, insbesondere 0,5-15 Gew.-%, eines Oxidationsmittels, das ausreicht, um in der Lösung ein Normalpotential von mindestens +300 mV zu gewährleisten,
    wobei der Rest der Lösung Wasser ist. Die hier angegebenen Prozentsätze beziehen sich auf die jeweiligen Reinsubstanzen bzw. Ionen. Werden Salze oder Zusammensetzungen verwendet, die weitere Substanzen enthalten, wie etwa Gegenionen, Kristallwasser, Lösungsmittel, etc., sind entsprechend höhere Gewichtsanteile einzusetzen.
A particularly suitable example of an aqueous solution which can be used in the first step of the treatment according to the present invention comprises the following composition:
  • 0.5-10 wt .-%, in particular 3.0-5.0 wt .-%, of at least one hydroxycarboxylic acid having 2-3 hydroxyl and 2-3 carboxyl groups or their salt (s),
  • 0.5-5.0 wt .-%, of at least one diphosphonic acid of
    the general structure 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 Alkylglykols the general structure H- (O-CHR-CH 2 ) n -OH, wherein R is hydrogen or a Is alkyl of 1-3 carbon atoms and n is 1-5, and
  • 0.2-20 wt .-%, in particular 0.5-15 wt .-%, of an oxidizing agent which is sufficient to ensure a normal potential of at least +300 mV in the solution,
    the remainder of the solution being water. The percentages given here refer to the respective pure substances or ions. Salts or compositions containing other substances, such as counterions, are used. Crystal water, solvents, etc., are to use correspondingly higher weight fractions.

In einer besonders bevorzugten Ausführungsform umfasst die mindestens eine Hydroxycarbonsäure Zitronensäure, und/oder die mindestens eine Diphosphonsäure HEDP, und/oder die mindestens eine Nitroaryl- oder Nitroalkylsulfonsäure m-Nitrobenzolsulfonsäure, und/oder das mindestens eine Alkylglykol Ethylenglykol und/oder Butylglykol, sowie das Oxidationsmittel Nitrat, Peroxid, Persulfat und/oder Cer(IV)-Ionen, jeweils in den oben angegebenen Gewichtsverhältnissen.In a particularly preferred embodiment, the at least one hydroxycarboxylic acid comprises citric acid, and / or the at least one diphosphonic acid HEDP, and / or the at least one nitroaryl or nitroalkyl sulfonic acid m- nitrobenzenesulfonic acid, and / or the at least one alkyl glycol, ethylene glycol and / or butyl glycol, and Oxidizing agent nitrate, peroxide, persulfate and / or cerium (IV) ions, in each case in the weight ratios indicated above.

Den obigen Zusammensetzungen können gegebenenfalls weitere Netzmittel in einer Konzentration zwischen 0,02 und 2,0 Gew.-%, vorzugsweise zwischen 0,05 und 1,0 Gew.-% zugegeben werden. Außerdem können diesen Zusammensetzungen gegebenenfalls noch ein oder mehrere Verdickungsmittel zugesetzt sein. Diese Verdickungsmittel, beispielsweise Kieselgur, können dazu dienen, die Viskosität der Lösung zu erhöhen. Bevorzugt wird die chemische Behandlung in wässriger Lösung jedoch in einem Tauchbad durchgeführt, so dass auf solche Verdickungsmittel verzichtet werden kann.If desired, further wetting agents may be added to the above compositions in a concentration between 0.02 and 2.0% by weight, preferably between 0.05 and 1.0% by weight. In addition, one or more thickening agents may optionally be added to these compositions. These thickening agents, for example diatomaceous earth, can serve to increase the viscosity of the solution. However, the chemical treatment in aqueous solution is preferably carried out in a dip bath, so that such thickeners can be dispensed with.

Die wässrige Lösung hat vorzugsweise einen pH-Wert, der unter 7, vorzugsweise unter 4 liegt. Dies kann dadurch erzielt werden, dass die wässrige Lösung mindestens eine Säure enthält. Ein bevorzugtes Verfahren besteht darin, dass mindestens einer der Komplexbildner und/oder mindestens eines der Oxidationsmittel zumindest teilweise in Form einer Säure der Lösung zugegeben wird.The aqueous solution preferably has a pH which is less than 7, preferably less than 4. This can be achieved by the aqueous solution containing at least one acid. A preferred method is that at least one of the complexing agents and / or at least one of the oxidizing agents is at least partially added in the form of an acid to the solution.

Der erste Schritt der Behandlung gemäß der vorliegenden Erfindung erfolgt gemäß einer bevorzugten Ausführungsform in einer wässrigen Lösung, die eine Temperatur von höchstens etwa 70°C aufweist. Weiter bevorzugt wird, dass die Behandlung in wässriger Lösung bei einer Temperatur zwischen Raumtemperatur und 60°C erfolgt. Die chemische Behandlung in wässriger Lösung erfolgt vorzugsweise über einen Zeitraum von mindestens 60 min, beispielsweise kann die chemische Behandlung mit einer wässrigen Lösung über einen Zeitraum von 1-4 h erfolgen.The first step of the treatment according to the present invention, in a preferred embodiment, is in an aqueous solution having a temperature of at most about 70 ° C. It is further preferred that the treatment takes place in aqueous solution at a temperature between room temperature and 60 ° C. The chemical treatment in aqueous solution is preferably carried out over a period of at least 60 minutes, for example, the chemical treatment can be carried out with an aqueous solution over a period of 1-4 hours.

Im Anschluss an die Behandlung mit einer wässrigen Passivierungslösung wird das Werkstück mit Wasser, vorzugsweise entionisiertem Wasser, gespült, um die Passivierungslösung zu entfernen, und wahlweise getrocknet, bevor das Werkstück der Temperaturbehandlung unterzogen wird. Dieses Spülen kann durch Besprühen oder durch (gegebenenfalls mehrmaliges) Eintauchen in ein Tauchbad oder durch Kombinationen dieser Spülverfahren erfolgen.Following treatment with an aqueous passivating solution, the workpiece is rinsed with water, preferably deionized water, to remove the passivating solution, and optionally dried, before the workpiece enters the passivation solution Temperature treatment is subjected. This rinsing can be done by spraying or by (possibly repeated) immersion in a dipping bath or by combinations of these rinsing methods.

Der Schritt der Wärmebehandlung erfolgt bei einer Temperatur von mindestens 80°C in einer sauerstoffhaltigen Atmosphäre. Die Wärmebehandlung erfolgt vorzugsweise bei einer Temperatur im Bereich zwischen 80°C und 280°C, insbesondere bei einer Temperatur oberhalb von 100°C und von höchstens 260°C.The step of heat treatment is carried out at a temperature of at least 80 ° C in an oxygen-containing atmosphere. The heat treatment is preferably carried out at a temperature in the range between 80 ° C and 280 ° C, in particular at a temperature above 100 ° C and at most 260 ° C.

Die sauerstoffhaltige Atmosphäre der thermischen Behandlung kann in einer bevorzugten Ausführungsform Luft sein. In anderen Ausführungsformen dieser Erfindung ist die sauerstoffhaltige Atmosphäre vor allem Wasserdampf bzw. ein Gemisch aus Wasserdampf und Luft. Eine solche Wasserdampf enthaltende Atmosphäre wird vorzugsweise bei einer Temperatur von mindestens 100°C verwendet.The oxygen-containing atmosphere of the thermal treatment may be air in a preferred embodiment. In other embodiments of this invention, the oxygen-containing atmosphere is primarily water vapor or a mixture of water vapor and air. Such a steam-containing atmosphere is preferably used at a temperature of at least 100 ° C.

Der optimale Temperaturbereich für die Wärmebehandlung hängt wesentlich von der Art des zu behandelnden Edelstahls ab. Dieser optimale Bereich lässt sich jedoch von einen Durchschnittsfachmann einfach in Probeversuchen bestimmen.The optimum temperature range for the heat treatment depends essentially on the type of stainless steel to be treated. However, this optimum range can easily be determined by a person of ordinary skill in trial experiments.

Beispielsweise liegt eine geeignete Temperatur in einem Bereich zwischen 100°C und 270°C, vorzugsweise zwischen 150°C und 260°C, insbesondere zwischen 220°C und 260°C, wenn der Edelstahl ein austenitischer Stahl ist, der einen Gehalt von etwa 16-20 Gew.-% Chrom und etwa 7-10 Gew.-% Nickel aufweist, wie beispielsweise Edelstahl der Qualität 1.4301 (vgl. Figur 1).For example, a suitable temperature in a range between 100 ° C and 270 ° C, preferably between 150 ° C and 260 ° C, in particular between 220 ° C and 260 ° C, when the stainless steel is an austenitic steel having a content of about 16-20 wt .-% chromium and about 7-10 wt .-% nickel, such as stainless steel grade 1.4301 (see. FIG. 1 ).

Ein Edelstahl der Qualität 1.4016 mit einem Chromgehalt von ca. 16-20 Gew.-%, der ansonsten im Wesentlichen keine weiteren, die Korrosionsbeständigkeit erhöhenden Legierungsbestandteile wie etwa Nickel oder Molybdän aufweist, wird mit gutem Ergebnis einer Wärmebehandlung unterzogen, bei der die Temperatur im Bereich zwischen 100°C und 190°C, vorzugsweise zwischen 120°C und 160°C, insbesondere zwischen 130°C und 150°C, liegt (vgl. Figur 2). Der Ausdruck "im Wesentlichen keine" bedeutet dabei, dass die betreffenden Elemente, wenn überhaupt vorhanden, in einer Konzentration von weniger als 1 Gew.-%, in der Regel zwischen 0 und 0,1 Gew.-% in der Legierung vorliegen.A grade 1.4016 stainless steel having a chromium content of about 16-20% by weight, which otherwise has substantially no other corrosion resistance increasing alloying constituents, such as nickel or molybdenum, is heat treated to a good degree with the temperature at Range between 100 ° C and 190 ° C, preferably between 120 ° C and 160 ° C, in particular between 130 ° C and 150 ° C, lies (see. FIG. 2 ). The term "essentially none" means that the elements in question, if present at all, are present in a concentration of less than 1% by weight, generally between 0 and 0.1% by weight, in the alloy.

Diese Wärmehandlung sollte über einen Zeitraum von mindestens 2 min erfolgen (vgl. etwa Figur 3 für Edelstahl der Qualität 1.4301). Vorzugsweise erfolgt die Wärmebehandlung über einen Zeitraum von 15-45 min, beispielsweise für etwa 30 min. Eine zu lange thermische Behandlung, etwa über mehrere Stunden, kann je nach Edelstahlqualität gegebenenfalls dazu führen, dass die Korrosionsbeständigkeit des behandelten Werkstücks wieder sinkt.This heat treatment should take place over a period of at least 2 minutes (cf. FIG. 3 for quality 1.4301 stainless steel). Preferably, the heat treatment takes place over a period of 15-45 minutes, for example for about 30 minutes. Depending on the quality of the stainless steel, too long a thermal treatment, for example over several hours, can lead to a fall in the corrosion resistance of the treated workpiece.

So zeigt beispielsweise ein Edelstahl der Qualität 1.4016 beim Erwärmen auf 140°C, also auf eine Temperatur, die im optimalen Bereich für die Wärmebehandlung liegt, zunächst einen raschen Anstieg des Lochfraßpotentials auf Werte um +1000 mV (vgl. Figur 4). Ist ein solches Werkstück jedoch dieser Temperatur über längere Zeiträume ausgesetzt, so sinkt das Lochfraßpotential wieder auf Werte von etwa +700 mV. Es ist daher bei einigen Edelstahlarten darauf zu achten, dass die Wärmebehandlung nicht länger als etwa 90 min, vorzugsweise nicht länger als etwa 60 min durchgeführt wird.For example, a stainless steel of quality 1.4016 when heated to 140 ° C, ie at a temperature which is in the optimum range for the heat treatment, first shows a rapid increase in the pitting potential to values around +1000 mV (cf. FIG. 4 ). However, if such a workpiece is exposed to this temperature for longer periods, the pitting potential drops again to values of about +700 mV. It is therefore important for some types of stainless steel that the heat treatment is carried out no longer than about 90 minutes, preferably not longer than about 60 minutes.

Ein weiterer wichtiger Vorteil des hier beschriebenen Verfahrens liegt darin, dass es nicht nur geeignet ist, die Korrosionsbeständigkeit, gemessen als das Lochfraßpotential gemäß DIN 50900, im Vergleich zum Ausgangszustand deutlich zu erhöhen, sondern dass das Verfahren auch dazu geeignet ist, die Widerstandsfähigkeit von Edelstahlwerkstücken gegen thermische Verfärbungen zu steigern. Eine solche Erhöhung der Widerstandsfähigkeit von Edelstahlwerkstücken bzw. deren Oberflächen gegen thermische Verfärbungen bei ihrer Benutzung durch ein Passivierungsverfahren ist bisher nicht beschrieben worden und stellt einen weiteren wesentlichen Vorteil der hier beschriebenen Erfindung dar.Another important advantage of the method described here is that it is not only suitable for significantly increasing the corrosion resistance, measured as the pitting potential according to DIN 50900, compared with the starting state, but that the method is also suitable for the resistance of stainless steel workpieces to increase thermal discoloration. Such an increase in the resistance of stainless steel workpieces or their surfaces against thermal discoloration in their use by a passivation process has not been described and represents a further significant advantage of the invention described herein.

Im Stand der Technik wird unter anderem auch ein Verfahren zum Reinigen und Passivieren einer Edelstahloberfläche offenbart, bei der eine Hydroxyessigsäure oder Zitronensäure in einer wässerigen Lösung auf die Oberfläche aufgebracht wird (vgl. EP 0 776 256 B1 ). Der Gehalt an Hydroxycarbonsäure bei diesem Verfahren liegt jedoch deutlich unterhalb von 3,0 Gew.-%. Auch bei diesem Stand der Technik, der im Übrigen die thermische Behandlung des Werkstücks nicht erwähnt, geht es wohl eher darum, eine Passivschicht auf der Werkstoffoberfläche zu bilden, wobei die eingesetzten Komplexe leicht ausfällen und in den Oxidfilm über dem Werkstück eingebaut werden (vgl. Absatz [0032] der zuvor genannten EP 0 776 256 B1 ). Erwähnenswert ist auch die DE 39 91 748 C2 , die im Anschluss an ein elektrochemisches Vorpolieren eines rostfreien Stahlmaterials die Behandlung der polierten Oberfläche mit einem oxidierenden Verfahren in einer oxidierenden Hochtemperaturgasatmosphäre offenbart. Die Temperatur dieses Verfahrensschritts liegt oberhalb von 300°C. Das erfindungsgemäße Verfahren findet üblicherweise bei Temperaturen unterhalb von 300°C statt.The prior art discloses, inter alia, also a method for cleaning and passivating a stainless steel surface, in which a hydroxyacetic acid or citric acid is applied to the surface in an aqueous solution (cf. EP 0 776 256 B1 ). However, the content of hydroxycarboxylic acid in this process is well below 3.0 wt .-%. Also in this prior art, which does not mention the thermal treatment of the workpiece, moreover, it is more likely to form a passive layer on the material surface, the complexes used easily precipitate and are incorporated into the oxide film over the workpiece (see. Paragraph [0032] of the aforementioned EP 0 776 256 B1 ). Worth mentioning is also the DE 39 91 748 C2 which discloses, after electrochemical pre-polishing a stainless steel material, treatment of the polished surface with an oxidizing process in a high temperature oxidizing gas atmosphere. The temperature of this process step is above 300 ° C. The process according to the invention usually takes place at temperatures below 300.degree.

Die Erfindung wird in den folgenden Beispielen näher erläutert. Diese Beispiele stellen jedoch nur mögliche Ausführungsformen des hier beschriebenen Passivierungsverfahrens dar und sollen in keiner Weise eine Beschränkung auf diese Beispiele implizieren.The invention is explained in more detail in the following examples. However, these examples are only possible embodiments of the passivation process described herein and are not intended to imply any restriction to these examples.

BeispieleExamples Beispiel 1: Edelstahl der Qualität 1.4301Example 1: Stainless steel of quality 1.4301

Zwei 1,5 mm dicke Edelstahlbleche (A und B) der Qualität 1.4301 mit austenitischem Gefüge und einem Gehalt von 18 Gew.-% Chrom und 8 Gew.-% Nickel in der Legierung, die eine kalt gewalzte und blank geglühte Oberfläche hatten, wurden im Originalzustand alkalisch entfettet, mit entionisiertem Wasser sauber gespült und getrocknet. Anschließend wurde das Lochfraßpotential gemäß DIN 50900 gemessen. Das Lochfraßpotential betrug bei beiden Blechen im Ausgangszustand +550 mV.Two 1.5 mm thick 1.4301 austenitic stainless steel sheets (A and B) containing 18% by weight of chromium and 8% by weight of nickel in the alloy had a cold rolled and bright annealed surface Alkaline degreased in the original state, rinsed clean with deionized water and dried. Subsequently, the pitting potential was measured according to DIN 50900. The pitting potential for both plates was +550 mV in the initial state.

Blech B wurde anschließend in eine Passivierungslösung folgender Zusammensetzung (in Gew.-%) getaucht:

  • 3,5% Zitronensäure
  • 1,9% m-Nitrobenzotsutfonsäure
  • 3,0% Hydroxyethandiphosphonsäure (HEDP)
  • 0,1% Butylglykol
  • 0,2% Netzmittel
  • 22,1% Magnesiumnitrat·6 H2O
  • ad 100%: entionisiertes Wasser
Sheet B was subsequently immersed in a passivation solution of the following composition (in% by weight):
  • 3.5% citric acid
  • 1.9% m- nitrobenzotsutfonic acid
  • 3.0% hydroxyethanediphosphonic acid (HEDP)
  • 0.1% butyl glycol
  • 0.2% wetting agent
  • 22.1% magnesium nitrate · 6H 2 O
  • ad 100%: deionized water

Die chemische Behandlung erfolgte für 180 min bei 40°C. Anschließend wurde das Blech mit entionisiertem Wasser gespült und an Luft getrocknet.The chemical treatment was carried out at 40 ° C for 180 min. Subsequently, the sheet was rinsed with deionized water and dried in air.

Daraufhin wurde das Lochfraßpotential von Blech B mit +750 mV gemessen, einer Steigerung gegenüber dem Ausgangszustand um +200 mV.Subsequently, the pitting potential of plate B was measured at +750 mV, an increase of +200 mV compared to the initial state.

Anschließend wurden beide Bleche (A und B) für eine Dauer von 30 min in einem Ofen auf 240°C erwärmt. Nach dem Abkühlen zeigte das in der Passivierungslösung behandelte Blech B keine Farbveränderung, während das unbehandelte Blech A strohgelb verfärbt war. Die nachfolgende Messung des Lochfraßpotentials ergab folgende Ergebnisse:

  • Für das nicht chemisch behandelte Blech A:
    • +650 mV und somit eine Verbesserung um +100 mV gegenüber dem Ausgangszustand und ein um -100 mV niedrigerer Wert im Vergleich zu dem chemisch behandelten Blech B vor dessen Wärmebehandlung.
  • Für das chemisch behandelte Blech B:
    • +1450 mV und somit eine Verbesserung um +900 mV gegenüber dem Ausgangszustand und um +700 mV gegenüber dem Wert nach dem Tauchen in die Passivierungslösung sowie um +800 mV gegenüber dem nur wärmebehandelten Blech A.
Subsequently, both sheets (A and B) were heated in an oven at 240 ° C for a period of 30 minutes. After cooling, the sheet B treated in the passivating solution showed no color change, while the untreated sheet A was colored straw yellow. The subsequent measurement of the pitting potential resulted in the following results:
  • For the non-chemically treated sheet A:
    • +650 mV and thus an improvement of +100 mV compared to the initial state and a -100 mV lower value compared to the chemically treated sheet B before its heat treatment.
  • For the chemically treated sheet B:
    • +1450 mV and thus an improvement of +900 mV compared to the initial state and by +700 mV compared to the value after immersion in the passivation solution and by +800 mV compared to the heat-treated sheet metal A.

Beispiel 2: Edelstahl der Qualität 1.4016Example 2: Quality 1.4016 stainless steel

Zwei 1,0 mm dicke Edelstahlbleche (C und D) der Qualität 1.4016 mit ferritischem Gefüge und 18% Chrom in der Legierung, die eine kalt gewalzte und blank geglühte Oberfläche hatten, wurden alkalisch entfettet, mit entionisiertem Wasser gespült und an Luft getrocknet. Daraufhin wurde gemäß DIN 50900 das Lochfraßpotential im Ausgangszustand gemessen. Es betrug bei beiden Blechen C und D +370 mV.Two 1.0 mm thick 1.4016 stainless steel sheets (C and D) with ferritic microstructure and 18% chromium in the alloy, which had a cold rolled and bright annealed surface, were degreased alkaline, rinsed with deionized water, and dried in air. Then, according to DIN 50900, the pitting potential in the initial state was measured. It was C + D +370 mV for both sheets.

Anschließend wurde Blech D in einer Passivierungslösung behandelt, deren Zusammensetzung in Beispiel 1 beschrieben ist. Die Behandlung erfolgt bei Raumtemperatur (+22°C) für eine Dauer von 2,5 h. Anschließend wurde das Blech mit entionisiertem Wasser sauber gespült, an Luft getrocknet und das Lochfraßpotential mit +520 mV gemessen, einer Steigerung gegenüber dem Ausgangszustand um +150 mV.Subsequently, sheet D was treated in a passivation solution whose composition is described in Example 1. The treatment takes place at room temperature (+ 22 ° C) for a duration of 2.5 h. Subsequently, the plate was rinsed clean with deionized water, dried in air and the pitting potential measured at +520 mV, an increase from the initial state by +150 mV.

Anschließend wurden beide Bleche C und D in einem Ofen für eine Dauer von 30 min auf 140°C erwärmt. Nach dem Abkühlen wiesen beide Bleche keine Farbveränderung auf. Bei der Bestimmung des Lochfraßpotentials wurden folgende Ergebnisse erhalten:

  • Für das nicht chemisch behandelte Blech C:
    • +570 mV und damit ein um +200 mV höherer Wert als der des Ausgangszustands und ein um +50 mV höherer Wert im Vergleich zu Blech D nach dessen Behandlung in der Passivierungslösung.
  • Für das zuvor chemisch behandelte Blech D:
    • +900 mV und damit ein um +530 mV höherer Wert als der des Ausgangszustands und ein um +380 mV höherer Wert als nach der Behandlung in der Passivierungslösung, sowie ein um +330 mV höherer Wert als er für das Blech C nach der Wärmebehandlung gemessen wurde.
Subsequently, both sheets C and D were heated in an oven at 140 ° C for a period of 30 minutes. After cooling both panels showed no color change. In determining the pitting potential, the following results were obtained:
  • For the non-chemically treated sheet C:
    • +570 mV and thus a value +200 mV higher than that of the initial state and a value +50 mV higher compared to plate D after its treatment in the passivation solution.
  • For the previously chemically treated sheet D:
    • +900 mV and thus a value +530 mV higher than the starting state and a value +380 mV higher than after the treatment in the passivation solution, and a value +330 mV higher than that measured for the plate C after the heat treatment has been.

Beispiel 3: Edelstahl der Qualität 1.4016Example 3: Quality 1.4016 stainless steel

Zwei Edelstahlbleche (E und F) der Qualität 1.4016 wurden wie in Beispiel 2 vorbehandelt und das Blech F in der Passivierungslösung (Zusammensetzung siehe Beispiel 1) behandelt. Anschließend wurden beide Bleche im Ofen für eine Dauer von 30 min auf 210°C erwärmt. Nach dem Abkühlen wies das nicht in der Passivierungslösung behandelte Blech E eine deutliche strohgelbe Verfärbung auf, wogegen das Blech F keine Farbveränderung zeigte.

  • Das nicht chemisch behandelte Blech E:
    • Das Lochfraßpotential von Blech E lag bei +480 mV und damit um +110 mV höher als im Ausgangszustand, jedoch um 90 mV unterhalb dem Wert, der bei einer thermischen Behandlung im optimalen Bereich erzielt wurde (vgl. Beispiel 2).
  • Das zuvor chemisch behandelte Blech F:
    • Das Lochfraßpotential von Blech F lag bei +520 mV und entsprach damit dem Wert, der vor der Wärmebehandlung gemessen wurde. Dieser Wert liegt jedoch um 380 mV unterhalb dem Lochfraßpotential von +900 mV, das nach einer Behandlung im optimalen Temperaturbereich, das bei ca. +140°C liegt, ermittelt wurde (vgl. Beispiel 2, Blech D), obwohl sich bei Blech F keine temperaturbedingte Verfärbung zeigte.
Two stainless steel sheets (E and F) of quality 1.4016 were pretreated as in Example 2 and the sheet F in the passivation solution (composition see Example 1) treated. Subsequently, both sheets were heated in the oven for a period of 30 min at 210 ° C. After cooling, the sheet E not treated in the passivating solution had a marked straw yellow discoloration, whereas the sheet F showed no color change.
  • The non-chemically treated sheet E:
    • The pitting potential of plate E was +480 mV, which is +110 mV higher than in the initial state, but 90 mV below the value obtained in a thermal treatment in the optimum range (see Example 2).
  • The previously chemically treated metal sheet F:
    • The pitting potential of Sheet F was +520 mV, which was the value measured before the heat treatment. However, this value is about 380 mV below the pitting potential of +900 mV, which was determined after treatment in the optimum temperature range, which is about + 140 ° C (see Example 2, sheet D), although in sheet metal F showed no temperature-related discoloration.

Somit zeigt dieses Beispiel, dass bei einem Überschreiten des für eine bestimmte Edelstahlqualität optimalen Temperaturbereich die Korrosionsbeständigkeit wieder abnimmt, jedoch immer noch höher liegt als vor der Passivierungsbehandlung.Thus, this example shows that when exceeding the optimal temperature range for a certain quality stainless steel corrosion resistance decreases again, but still higher than before the passivation treatment.

Claims (14)

  1. A process for the passivation of stainless steel, wherein the stainless steel is subjected
    - firstly to a chemical treatment with an aqueous solution containing at least one polydentate complexing agent for iron and at least one oxidant, where the oxidant is able to ensure a standard potential of at least +300 mV in the solution,
    - subsequently to a rinsing step with water, and
    - then to a heat treatment at a temperature of at least 80°C in an oxygen-containing atmosphere.
  2. The process according to claim 1, characterized in that a complexing agent combination is used, where the complexing agent combination consists of:
    - at least one hydroxycarboxylic acid having 2-3 hydroxyl groups and 2-3 carboxyl groups or a salt/salts thereof,
    - at least one diphosphonic acid having the general structure R"[-PO(OH)2]2 or a salt/salts thereof, where R" is a divalent alkyl, hydroxyalkyl or aminoalkyl group, and
    - at least one nitroarylsulfonic or nitroalkylsulfonic acid or a salt/salts thereof.
  3. The process according to any of claims 1 to 2, wherein the oxidant comprises at least one compound selected from the group consisting of nitrate, peroxide, persulfate, perborate, the percarboxylates, iodate and cerium (IV) based compounds in the form of the respective acids and/or salts.
  4. The process according to any of claims 1 to 3, characterized in that the aqueous solution comprises:
    - 0.5-10% by weight of at least one hydroxycarboxylic acid having 2-3 hydroxyl groups and 2-3 carboxyl groups or a salt/salts thereof,
    - 0.5-5.0% by weight of at least one phosphonic acid having the general structure R"[-PO(OH)2]2 or a salt/salts thereof, where R" is a divalent alkyl, hydroxyalkyl or aminoalkyl group,
    - 0.1-5.0% by weight of at least one nitroarylsulfonic or nitroalkylsulfonic acid or a salt/salts thereof,
    - 0.05-1.0% by weight of at least one alkyl glycol having the general structure H-(O-CHR-CH2)n-OH, where R is hydrogen or an alkyl radical having 1-3 carbon atoms and n is 1-5, and
    - 0.2-20% by weight of an oxidant which is able to ensure a standard potential of at least +300 mV in the solution,
    where the remainder of the solution is water to which optionally one or more thickeners can also be added.
  5. The process according to any of claims 1 to 4, wherein the chemical treatment in an aqueous solution is carried out at a temperature of not more than 70°C.
  6. The process according to any of claims 1 to 5, characterized in that the chemical treatment in an aqueous solution is carried out for a period of 1-4 hours.
  7. The process according to any of claims 1 to 6, characterized in that the subsequent heat treatment is carried out in an air atmosphere, in a water vapor atmosphere or a mixture of air and water vapor.
  8. The process according to any of claims 1 to 7, characterized in that the subsequent heat treatment is carried out at a temperature in the range from 80°C to 280°C.
  9. The process according to claim 8, characterized in that the temperature of the heat treatment is in the range from 100°C to 270°C, preferably from 150°C to 260°C, when the stainless steel is an austenitic steel which has a content of 16-20% by weight of chromium and 7-10% by weight of nickel.
  10. The process according to claim 9, characterized in that the temperature in the heat treatment is in the range from 100°C to 190°C, preferably from 120°C to 160°C, when the stainless steel is a ferritic steel which has a chromium content of 16-20% by weight and contains essentially no nickel and/or molybdenum.
  11. The process according to any of claims 1 to 10, characterized in that the subsequent heat treatment is carried out for a period of at least 2 minutes.
  12. The process according to any of claims 1 to 11, characterized in that the subsequent heat treatment is carried out for a period of 15-45 minutes.
  13. The use of a process according to any of claims 1 to 12 for increasing the corrosion resistance of stainless steel surfaces.
  14. The use of a process as claimed in any of claims 1 to 13 for increasing the resistance of stainless steel surfaces to thermal discoloration.
EP08715970A 2007-03-05 2008-02-22 Method for the thermochemical passivation of stainless steel Active EP2147131B1 (en)

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EP (1) EP2147131B1 (en)
JP (1) JP5222308B2 (en)
AT (1) ATE520802T1 (en)
DE (1) DE102007010538A1 (en)
ES (1) ES2370088T3 (en)
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CN106637175B (en) * 2016-11-09 2019-02-22 深圳市麦滕医疗器械有限公司 The surface passivation treatment method of surgical operating instrument martensite type anti-bacteria stainless steel
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Publication number Priority date Publication date Assignee Title
DE102014203412A1 (en) 2013-02-25 2014-08-28 Zschimmer & Schwarz Mohsdorf GmbH &Co.KG. Stripping and passivating stainless steel surfaces of e.g. engine cylinder, by contacting an article with formulation comprising mixture of hydroxyethylamino-di(methylene phosphonic acid) and aminoalkyl phosphonic acid and oxidizing agent

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DE102007010538A1 (en) 2008-09-11
PL2147131T3 (en) 2012-01-31
US8430973B2 (en) 2013-04-30
ATE520802T1 (en) 2011-09-15
US20100132844A1 (en) 2010-06-03
EP2147131A1 (en) 2010-01-27
JP2010521581A (en) 2010-06-24
WO2008107082A8 (en) 2008-11-20
ES2370088T3 (en) 2011-12-12
WO2008107082A1 (en) 2008-09-12
JP5222308B2 (en) 2013-06-26

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