EP1013800B1 - Process for pickling stainless steel - Google Patents

Process for pickling stainless steel Download PDF

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Publication number
EP1013800B1
EP1013800B1 EP99123620A EP99123620A EP1013800B1 EP 1013800 B1 EP1013800 B1 EP 1013800B1 EP 99123620 A EP99123620 A EP 99123620A EP 99123620 A EP99123620 A EP 99123620A EP 1013800 B1 EP1013800 B1 EP 1013800B1
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Prior art keywords
pickling
added
process according
acid
electrolyte solution
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German (de)
French (fr)
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EP1013800A2 (en
EP1013800A3 (en
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Jovan Dipl.-Ing. Dr. Starcevic
Dietfried Gamsriegler
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Andritz AG
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Andritz AG
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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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C23G1/081—Iron or steel solutions containing H2SO4
    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C23G1/086—Iron or steel solutions containing HF
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
    • C25F1/02—Pickling; Descaling
    • C25F1/04—Pickling; Descaling in solution
    • C25F1/06—Iron or steel

Definitions

  • the invention relates to a method for chemical and / or electrochemical pickling of stainless steel in a nitric acid-free and acidic liquid.
  • Fe 3+ salts sulfate, chloride, nitrate or fluoride salts
  • Chromate detoxification is very costly. As reducing agent mostly Na 2 S 2 O 5 or similar sulfur compounds are used.
  • the aim of the invention is the disadvantages of both methods, in particular the costly "chromate detoxification", to avoid.
  • the invention is therefore characterized in that spent electrolyte solution is metered in from electrochemical neutral salt pickling lines, preferably sodium sulphate pickling lines.
  • electrochemical neutral salt pickling lines preferably sodium sulphate pickling lines.
  • a favorable development of the invention is characterized in that that the redox potential in the pickling liquid by the amount of added Electrolyte solution is set to a predetermined value, wherein the reduction-oxidation potential (redox potential) of the pickling liquid measured and the amount of spent electrolyte solution dosed in accordance with electrochemical Natriumsulfatbeizlinien accordingly can be.
  • the pickling effect for the Stainless steel band can be adjusted in the acid pickling liquid.
  • An advantageous embodiment of the invention is characterized in that that in a new batch of acid pickling a divalent and a trivalent iron salt, preferably iron sulfates, may be added.
  • An advantageous development of the invention is characterized in that that, in addition to the spent electrolyte, another oxidizing agent, preferably hydrogen peroxide, fed to the pickling liquid becomes. Should the amount of accumulated spent neutral electrolyte amount are not sufficient to the desired or necessary redox potential for To adjust the pickling process, so this redox potential in a simple Be achieved manner.
  • another oxidizing agent preferably hydrogen peroxide
  • a favorable embodiment of the invention is characterized in that that inhibitors are added. In this way, the corrosion attack be better controlled on the pickling material.
  • a favorable development of the invention is characterized in that that acidic and neutral electrochemical etching stages with each other be combined, the used electrolyte solution from the neutral electrolyte stage can be added to the acidic stage.
  • the advantage of the method can be achieved in a pickling line.
  • An advantageous embodiment of the invention is characterized in that the acidic pickling liquid is neutralized by means of sodium hydroxide (NaOH) or sodium carbonate (Na 2 CO 3 ), that subsequently the resulting metal hydroxides are separated off, preferably filtered off, for example by means of microfiltration, and that the resulting sodium sulfate (Na 2 SO 4 ) is recycled again as a neutral electrolyte.
  • NaOH sodium hydroxide
  • Na 2 CO 3 sodium carbonate
  • Na 2 SO 4 sodium sulfate
  • a favorable development of the invention is characterized in that that the consumed electrolyte solution of one or more cold strip pickling line (s) removed and the nitric acid-free and acidic pickling liquid one or more hot strip pickling line (s) is added.
  • This can be the For a stainless steel plant cheapest option in terms of investment costs and operating costs are chosen.
  • the spent electrolyte solution contains the trivalent iron in the form of suspended hydroxide sludge (Fe (OH) 3 ) and contains the hexavalent chromium (Cr 6+ ), which reacts directly with the bivalent iron (Fe 2+ ) produced by the pickling process, thereby causing Fe 3+ and Cr 3+ arise.
  • the electrolytic treatment with a charge density of 200 A * sec / dm 2 was carried out in the acidified electrolytic solution in which the redox potential was set to 440 mV, and then the sample sheet was further electrochemically treated in the neutral electrolyte.
  • This combined pickling method has the advantage over a conventional neutral electrolyte treatment that with the same charge density of, for example, 600 A * sec / dm 2, about 20% more pickling loss could be achieved with a slightly lower gloss.
  • a large-scale plant consisting of six electrochemical pickling cells can be optimized so that the first two cells can be operated with acidic electrolyte at exactly set redox potential and the subsequent four with neutral electrolyte.
  • the resulting advantages are: higher pickling losses in the electrochemical pickling part, ie shorter after-treatment in the following mixed chemical acid (HF + HNO 3 ) pickling part.
  • the final product has the same gloss level.
  • the most important advantage, however, is that the entire chromate-containing neutral electrolyte solution can be recycled and thus no longer incur the costs for the Cr 6+ reduction.
  • a further advantage of this optimization is that the sodium sulfate solution (Na 2 SO 4 ) required for pickling in the neutral pH range is obtained by neutralization of the acidic electrolyte solution (H 2 SO 4 , Na 2 SO 4 , Fe 2+ , Fe 3+ , Cr 3+ , Ni 2+ ) with sodium hydroxide (NaOH) or sodium carbonate (Na 2 CO 3 ) and subsequent separation of the metal hydroxides (Fe (OH) 2 , Fe (OH) 3 , Cr (OH) 3 , Ni (OH) 2 ) by filtration, can be recovered.
  • the filtration is advantageously carried out as microfiltration.
  • the experiments are only examples of process control as defined in the claims.

Abstract

Chemical and/or electrochemical pickling of high grade steel is carried out in an acidic liquid that contains no nitric acid. Consumed electrolyte solution from electrochemical neutral salt pickling lines, preferably sodium sulphate pickling lines, is added.

Description

Die Erfindung betrifft ein Verfahren zum chemischen und/oder elektrochemischen Beizen von Edelstahl in einer salpetersäurefreien und sauren Flüssigkeit.The invention relates to a method for chemical and / or electrochemical pickling of stainless steel in a nitric acid-free and acidic liquid.

Beim Beizen von Edelstahlwarmband muß nach dem Warmwalz- und Glühprozeß das Band vom Zunder befreit werden. Dieser Beizvorgang wird in der Regel in Mischsäure (Flußsäure (= HF) und Salpetersäure (= HNO3)) durchgeführt. Hierbei entstehen jedoch nitrose Gase (NOx), so daß in den letzten Jahren zunehmend "salpetersäurefreie Beizverfahren" entwickelt wurden. Das Prinzip aller dieser Beizverfahren besteht darin, daß die Salpetersäure durch eine andere mineralische Säure (Schwefelsäure, Salzsäure) in welcher das Redoxpotential durch den Anteil der Eisenionen verschiedener Wertigkeit (Fe2+/Fe3+) genau eingestellt ist, substituiert wird. Beim Beizen geht Eisen als zweiwertiges lon in Lösung und das dreiwertige Eisenion wird entweder durch teilweise Oxidation von Fe2+ oder durch Zugabe von Fe3+ - Salzen hergestellt.When pickling stainless steel hot strip, the strip must be freed of scale after the hot rolling and annealing process. This pickling process is usually carried out in mixed acid (hydrofluoric acid (= HF) and nitric acid (= HNO 3 )). However, this nitrous gases (NO x ), so that in recent years increasingly "nitric acid-free pickling" were developed. The principle of all these pickling is that the nitric acid is substituted by another mineral acid (sulfuric acid, hydrochloric acid) in which the redox potential is precisely adjusted by the proportion of iron ions of different valence (Fe 2+ / Fe 3+ ). Upon pickling, iron is dissolved as a bivalent ion, and the trivalent iron ion is produced either by partial oxidation of Fe 2+ or by addition of Fe 3+ salts.

Es sind Verfahren bekannt, bei denen die Herstellung der dreiwertigen Eisenionen durch Oxidation des Fe2+ mit Wasserstoffperoxid (H2O2) erzeugt werden.Methods are known in which the preparation of trivalent iron ions by oxidation of Fe 2+ with hydrogen peroxide (H 2 O 2 ) are produced.

Bei anderen Verfahren werden Fe3+-Salze (Sulfat-, Chlorid-, Nitrat- oder Fluoridsalze) ins Beizbad zugesetzt.In other processes Fe 3+ salts (sulfate, chloride, nitrate or fluoride salts) are added to the pickling bath.

Nachdem das gebeizte Warmband durch das Walzen zu dünneren Blechstärken reduziert wurde muß es nochmals geglüht werden. Hierbei entsteht eine dünne Zunderschicht, welche auf eine "schonende Art" entfernt werden muß, ohne dabei die Oberflächenqualität (Glanz) zu zerstören. Weltweit hat sich für das Edelstahlkaltband das elektrochemische Beizverfahren in Natriumsulfatlösungen (Neutralelektrolyt) durchgesetzt. Bei diesem neutralen (pH = 7 bis 4) Beizverfahren entsteht das sehr toxische Chromat (Cr6+). Die verbrauchten Beizbäder werden daher in einer separaten Behandlung entgiftet, d.h. Cr6+ wird zu Cr3+ reduziert, bevor sie neutralisiert und die Metallhydroxide abfiltriert werden können.After the pickled hot strip has been reduced by rolling to thinner sheet thicknesses, it must be annealed again. This results in a thin scale layer, which must be removed in a "gentle way", without destroying the surface quality (gloss). The electrochemical pickling process in sodium sulphate solutions (neutral electrolyte) has become established throughout the world for stainless steel cold strip. This neutral (pH = 7 to 4) pickling process produces the very toxic chromate (Cr 6+ ). The spent pickling baths are therefore detoxified in a separate treatment, ie Cr 6+ is reduced to Cr 3+ before it can be neutralized and the metal hydroxides filtered off.

Die Chromat-Entgiftung ist sehr kostenintensiv. Als Reduktionsmittel werden meistens Na2S2O5 oder ähnliche Schwefelverbindungen eingesetzt.Chromate detoxification is very costly. As reducing agent mostly Na 2 S 2 O 5 or similar sulfur compounds are used.

Die Reduktion erfolgt nach folgender Formel: 4 H2CrO4 + 3 Na2S2O5 + 3 H2SO4 => 2 Cr2(SO4)3 + 3 Na2SO4 + 7 H2O The reduction takes place according to the following formula: 4 H 2 CrO 4 + 3 Na 2 S 2 O 5 + 3 H 2 SO 4 => 2 Cr 2 (SO 4 ) 3 + 3 Na 2 SO 4 + 7 H 2 O

Die Reduktion kann auch mit zweiwertigem Eisensulfat FeSO4 durchgeführt werden: 2 H2CrO4 + 6 FeSO4 + 6 H2SO4 => 2 Cr2(SO4)3 + 3 Fe2(SO4)3 + 8 H2O The reduction can also be carried out with bivalent iron sulfate FeSO 4 : 2 H 2 CrO 4 + 6 FeSO 4 + 6 H 2 SO 4 => 2 Cr 2 (SO 4 ) 3 + 3 Fe 2 (SO 4 ) 3 + 8 H 2 O

Ziel der Erfindung ist es, die Nachteile beider Verfahren, insbesondere die kostenintensive "Chromatentgiftung", zu vermeiden.The aim of the invention is the disadvantages of both methods, in particular the costly "chromate detoxification", to avoid.

Die Erfindung ist daher dadurch gekennzeichnet, daß verbrauchte Elektrolytlösung aus elektrochemischen Neutralsalzbeizlinien, bevorzugt Natriumsulfatbeizlinien, zudosiert wird. Damit kann einerseits das Chromat ohne zusätzliche kostenintensive Reduktionsmittel reduziert ("entgiftet") werden und andererseits eine Oxidation des Eisens von Fe2+ zu Fe3+ erreicht werden.The invention is therefore characterized in that spent electrolyte solution is metered in from electrochemical neutral salt pickling lines, preferably sodium sulphate pickling lines. Thus, on the one hand, the chromate can be reduced ("detoxified") without additional cost-intensive reducing agents and, on the other hand, an oxidation of the iron from Fe 2+ to Fe 3+ can be achieved.

Eine günstige Weiterbildung der Erfindung ist dadurch gekennzeichnet, daß das Redoxpotential in der Beizflüssigkeit durch die Menge an zugegebener Elektrolytlösung auf einen vorgegebenen Wert eingestellt wird, wobei das Reduktions-Oxidationspotential (Redoxpotential) der Beizflüssigkeit gemessen und die Menge der verbrauchten Elektrolytlösung aus elektrochemischen Natriumsulfatbeizlinien entsprechend zudosiert werden kann. Damit kann in vorteilhafter Weise die Beizwirkung für das Edelstahlband in der sauren Beizflüssigkeit eingestellt werden.A favorable development of the invention is characterized in that that the redox potential in the pickling liquid by the amount of added Electrolyte solution is set to a predetermined value, wherein the reduction-oxidation potential (redox potential) of the pickling liquid measured and the amount of spent electrolyte solution dosed in accordance with electrochemical Natriumsulfatbeizlinien accordingly can be. Thus, advantageously, the pickling effect for the Stainless steel band can be adjusted in the acid pickling liquid.

Eine vorteilhafte Ausgestaltung der Erfindung ist dadurch gekennzeichnet, daß bei einem Neuansatz der sauren Beizflüssigkeit ein zweiwertiges und ein dreiwertiges Eisensalz, vorzugsweise Eisensulfate, zugesetzt werden.An advantageous embodiment of the invention is characterized in that that in a new batch of acid pickling a divalent and a trivalent iron salt, preferably iron sulfates, may be added.

Damit kann in günstiger Weise ein Start für das Verfahren erreicht werden.This can be achieved in a favorable manner, a start for the process.

Eine vorteilhafte Weiterbildung der Erfindung ist dadurch gekennzeichnet, daß zusätzlich zum verbrauchten Elektrolyten ein anderes Oxidationsmittel, vorzugsweise Wasserstoffperoxid, der Beizflüssigkeit zugeführt wird. Sollte die Menge an anfallender verbrauchter Neutralelektrolytmenge nicht ausreichen, um das gewünschte bzw. notwendige Redoxpotential für den Beizvorgang einzustellen, kann so dieses Redoxpotential in einfacher Weise erzielt werden.An advantageous development of the invention is characterized in that that, in addition to the spent electrolyte, another oxidizing agent, preferably hydrogen peroxide, fed to the pickling liquid becomes. Should the amount of accumulated spent neutral electrolyte amount are not sufficient to the desired or necessary redox potential for To adjust the pickling process, so this redox potential in a simple Be achieved manner.

Eine günstige Ausgestaltung der Erfindung ist dadurch gekennzeichnet, daß Inhibitoren zugesetzt werden. Auf diese Weise kann der Korrosionsangriff auf das Beizgut besser kontrolliert werden.A favorable embodiment of the invention is characterized in that that inhibitors are added. In this way, the corrosion attack be better controlled on the pickling material.

Eine günstige Weiterbildung der Erfindung ist dadurch gekennzeichnet, daß saure und neutrale elektrochemische Beizstufen miteinander kombiniert werden, wobei die gebrauchte Elektrolytlösung aus der Neutralelektrolytstufe in die saure Stufe zudosiert werden kann. Durch diese Kombination läßt sich der Vorteil des Verfahrens in einer Beizlinie erzielen.A favorable development of the invention is characterized in that that acidic and neutral electrochemical etching stages with each other be combined, the used electrolyte solution from the neutral electrolyte stage can be added to the acidic stage. Through this Combination, the advantage of the method can be achieved in a pickling line.

Eine vorteilhafte Ausgestaltung der Erfindung ist dadurch gekennzeichnet, daß die saure Beizflüssigkeit mittels Natronlauge (NaOH) oder Natriumcarbonat (Na2CO3) neutralisiert wird, daß anschließend die entstandenen Metallhydroxide abgetrennt, vorzugsweise abfiltriert werden, beispielsweise mittels einer Mikrofiltration, und daß das entstandene Natriumsulfat (Na2SO4) wieder als Neutralelektrolyt rückgeführt wird. Damit kann neben der Neutralisation und Regeneration der sauren Beizflüssigkeit auch der Neutralelektrolyt regeneriert werden, was zu erheblichen Kosteneinsparungen führt.An advantageous embodiment of the invention is characterized in that the acidic pickling liquid is neutralized by means of sodium hydroxide (NaOH) or sodium carbonate (Na 2 CO 3 ), that subsequently the resulting metal hydroxides are separated off, preferably filtered off, for example by means of microfiltration, and that the resulting sodium sulfate (Na 2 SO 4 ) is recycled again as a neutral electrolyte. Thus, in addition to the neutralization and regeneration of the acid pickling liquid and the neutral electrolyte can be regenerated, resulting in significant cost savings.

Eine günstige Weiterbildung der Erfindung ist dadurch gekennzeichnet, daß die verbrauchte Elektrolytlösung einer oder mehreren Kaltbandbeizlinie(n) entnommen und der salpetersäurefreien und sauren Beizflüssigkeit einer oder mehrerer Warmbandbeizlinie(n) zudosiert wird. Damit kann die für ein Edelstahlwerk günstigste Variante hinsichtlich Investitionskosten und Betriebskosten gewählt werden.A favorable development of the invention is characterized in that that the consumed electrolyte solution of one or more cold strip pickling line (s) removed and the nitric acid-free and acidic pickling liquid one or more hot strip pickling line (s) is added. This can be the For a stainless steel plant cheapest option in terms of investment costs and operating costs are chosen.

Die Erfindung wird nun anhand von Ausführungsbeispielen näher erläutert.The invention will now be explained in more detail with reference to exemplary embodiments.

Versuch 1:Trial 1:

Am Ende einer langen Testserie über das elektrolytische Beizen von Edelstahlkaltbändern hatte man eine verbrauchte Elektrolytlösung mit Metallgehalten, wie sie auf Großanlagen üblich sind. Die Cr6+ Konzentration betrug 4.8 g/l. ( Das Eisen (Fe3+) und Nickel (Ni2+) liegen in Form von suspendiertem Hydroxidschlamm und das Chrom liegt in Form von Chromat (Cr6+), d.h. chemisch gelöst, vor.)At the end of a long series of tests on the electrolytic pickling of stainless steel cold-rolled strips, a spent electrolyte solution containing metal contents, as is common in large-scale plants, was used. The Cr 6+ concentration was 4.8 g / l. (The iron (Fe 3+ ) and nickel (Ni 2+ ) are in the form of suspended hydroxide sludge and the chromium is in the form of chromate (Cr 6+ ), ie chemically dissolved.)

Es wurde nun eine zweite Lösung bestehend aus einer Mischung von Schwefelsäure, Flußsäure und FeSO4 hergestellt, so daß die nachfolgende chemische Analyse folgende Werte ergab:

  • freie H2SO4 =400g/l
  • freie HF = 50 g/l
  • Fe2+ = 40 g/l
    • A second solution was then prepared consisting of a mixture of sulfuric acid, hydrofluoric acid and FeSO 4 , so that the following chemical analysis gave the following values:
  • free H 2 SO 4 = 400g / l
  • free HF = 50 g / l
  • Fe 2+ = 40 g / l

    • Nun wurden Mischungen aus diesen beiden Lösungen hergestellt, so daß das Redoxpotential der neu hergestellten Mischung bei 400 -500 mV liegt.Now mixtures of these two solutions were prepared so that the redox potential of the newly prepared mixture is 400-500 mV.

    Die darauf folgenden Beizversuche mit verzundertem Edelstahlwarmband ergaben zwar eine etwas längere Beizzeit als in konventioneller Mischsäure, aber dennoch zufriedenstellende Ergebnisse. Dabei wurden beispielsweise gestrahlte AISI 304-Probebleche in ca. 70 sec zunderfrei gebeizt. Nach mehreren Probebeizungen wurde die Beizflüssigkeit chemisch analysiert und es konnte kein sechswertiges Chrom(Cr6+) nachgewiesen werden. Erwartungsgemäß hat das vorliegende zweiwertige Eisen (Fe2+) es zu dreiwertigen Chrom (Cr3+) reduziert. Es konnte in der durchgeführten Beizserie kein Einfluß von Natriumionen auf das Beizergebnis beobachtet werden und es wurde auch bestätigt, daß ein Zusatz von Inhibitoren von Vorteil ist, um den Korrosionsangriff der Schwefelsäure auf das Grundmaterial zu unterdrücken.Although the subsequent pickling tests with scaled-on stainless steel hot-rolled strip resulted in a slightly longer pickling time than in conventional mixed acid, satisfactory results were nevertheless obtained. For example, blasted AISI 304 sample sheets were pickled without scale in approx. 70 sec. After several sample treatments, the pickling liquor was chemically analyzed and no hexavalent chromium (Cr 6+ ) could be detected. As expected, the present divalent iron (Fe 2+ ) reduced it to trivalent chromium (Cr 3+ ). No influence of sodium ions on the pickling result could be observed in the pickling series carried out, and it was also confirmed that addition of inhibitors is advantageous for suppressing the corrosion attack of the sulfuric acid on the base material.

    Der Vorteil dieses "nitratfreien Beizverfahrens" bei dem keine nitrosen Gase entstehen ist, daß das dreiwertige Eisenion (Fe3+) durch eine verbrauchte Elektrolytlösung hergestellt wird - d.h. es ergibt sich ein zweifacher Kostenvorteil. Es muß nicht wie bei den bisherigen Verfahren Wasserstoffperoxid (H2O2) als Oxidationsmittel für die Fe3+ Herstellung und kein Reduktionsmittel (Na2S2O5) für die Chromatentgiftung zugekauft werden. Die verbrauchte Elektrolytlösung enthält das dreiwertige Eisen in Form vom suspendiertem Hydroxidschlamm ( Fe(OH)3) und sie enthält das sechswertige Chrom (Cr6+), welches unmittelbar mit dem durch den Beizvorgang entstehenden zweiwertigen Eisen (Fe2+) reagiert und dabei Fe3+ und Cr3+ entstehen.The advantage of this "nitrate-free pickling process" in which no nitrous gases arise is that the trivalent iron ion (Fe 3+ ) is produced by a spent electrolyte solution - ie there is a twofold cost advantage. It must not be purchased as in the previous methods hydrogen peroxide (H 2 O 2 ) as the oxidant for the Fe 3+ production and no reducing agent (Na 2 S 2 O 5 ) for the Chromatentgiftung. The spent electrolyte solution contains the trivalent iron in the form of suspended hydroxide sludge (Fe (OH) 3 ) and contains the hexavalent chromium (Cr 6+ ), which reacts directly with the bivalent iron (Fe 2+ ) produced by the pickling process, thereby causing Fe 3+ and Cr 3+ arise.

    Im Verlauf der Beiztests wurde ein Nachlassen der Beizwirkung mit gleichzeitigem Absinken des Redoxpotentials beobachtet. Es ist daher sinnvoll, die verbrauchte Elektrolytlösung in Abhängigkeit vom gemessenen Redoxpotential fortlaufend in das Beizbad nachzudosieren.In the course of the pickling tests, a decrease in pickling effect was observed observed simultaneous decrease in the redox potential. It is therefore meaningful, the used electrolyte solution depending on the measured Redox potential continuously metered into the pickling bath.

    Versuch 2:Trial 2:

    Ein weiterer Versuch wurde mit Edelstahlkaltband durchgeführt. Es wurde der verbrauchten Elektrolytlösung ein zweiwertiges Eisensalz (FeSO4) und 96%-ige Schwefelsäure zugegeben, so daß die Konzentration der freien Schwefelsäure 100g/l und das Redoxpotential 440 mV betrug. Die so gewonnene Flüssigkeit wurde als Elektrolytlösung für elektrochemische Beizversuche eingesetzt. Bei diesen Beizversuchen konnte beobachtet werden, daß, verglichen mit elektrolytischem Beizen in reiner Schwefelsäure, der Korrosionsangriff auf das Grundmaterial geringer ist und der ursprüngliche Glanz der Blechproben nach dem Beizen erhalten bleibt. Gegenüber einer elektrolytischen Behandlung im neutralen Bereich, bei der ein Qualitätsanstieg (=Glanz) mit fortlaufender Behandlung gegeben ist, konnte jedoch ein wesentlich höherer Beizverlust erzielt werden. Another trial was carried out with stainless steel cold strip. A divalent iron salt (FeSO 4 ) and 96% sulfuric acid were added to the spent electrolytic solution so that the concentration of the free sulfuric acid was 100 g / L and the redox potential was 440 mV. The liquid thus obtained was used as an electrolyte solution for electrochemical pickling tests. In these pickling tests it could be observed that, compared to electrolytic pickling in pure sulfuric acid, the corrosion attack on the base material is lower and the original gloss of the sheet samples is retained after pickling. Compared to an electrolytic treatment in the neutral range, in which an increase in quality (= gloss) is given with continuous treatment, however, a significantly higher pickling loss could be achieved.

    Beispielsweise bei einer AISI 304 Edelstahlprobe und einer Behandlung (=Ladungsdichte) von 600 A*sec/dm2 konnte ca. 1.2g/m2 Beizverlust im neutralen und ca. 2.0 g/m2 mit der neu hergestellten sauren Elektrolytlösung ermittelt werden.For example, with an AISI 304 stainless steel sample and a treatment (= charge density) of 600 A * sec / dm 2 , it was possible to determine about 1.2 g / m 2 pickling loss in the neutral and about 2.0 g / m 2 with the newly prepared acidic electrolyte solution.

    Versuch 3:Trial 3:

    Ein weiterer Versuch wurde folgendermaßen durchgeführt:Another experiment was carried out as follows:

    Die elektrolytische Behandlung mit einer Ladungsdichte von 200 A*sec/dm2 wurde in der angesäuerten Elektrolytlösung durchgeführt, in der das Redox-Potential auf 440 mV eingestellt war, und anschließend wurde das Probeblech im neutralen Elektrolyten elektrochemisch weiterbehandelt. Dieses kombinierte Beizverfahren hat den Vorteil gegenüber einer herkömmlichen Neutralelektrolytbehandlung, daß mit der gleichen Ladungsdichte von z.B. 600 A*sec/dm2 ca. 20% mehr Beizverlust bei geringfügig geringerem Glanz erreicht werden konnten. Beispielsweise kann eine Großanlage, welche aus sechs elektrochemischen Beizzellen besteht, so optimiert werden, daß die ersten zwei Zellen mit saurem Elektrolyt bei genau eingestelltem Redoxpotential und die darauffolgenden vier mit Neutralelektrolyt betrieben werden können. Die daraus resultierenden Vorteile sind: höhere Beizverluste im elektrochemischen Beizteil, d.h. kürzere Nachbehandlung im darauf folgenden chemischen Mischsäure-(HF + HNO3) Beizteil. Das Endprodukt hat den gleichen Glanzgrad. Der wesentlichste Vorteil jedoch ist, daß die gesamte chromathaltige Neutralelektrolytlösung verwertet werden kann und somit die Kosten für die Cr6+ - Reduktion nicht mehr anfallen.The electrolytic treatment with a charge density of 200 A * sec / dm 2 was carried out in the acidified electrolytic solution in which the redox potential was set to 440 mV, and then the sample sheet was further electrochemically treated in the neutral electrolyte. This combined pickling method has the advantage over a conventional neutral electrolyte treatment that with the same charge density of, for example, 600 A * sec / dm 2, about 20% more pickling loss could be achieved with a slightly lower gloss. For example, a large-scale plant consisting of six electrochemical pickling cells can be optimized so that the first two cells can be operated with acidic electrolyte at exactly set redox potential and the subsequent four with neutral electrolyte. The resulting advantages are: higher pickling losses in the electrochemical pickling part, ie shorter after-treatment in the following mixed chemical acid (HF + HNO 3 ) pickling part. The final product has the same gloss level. The most important advantage, however, is that the entire chromate-containing neutral electrolyte solution can be recycled and thus no longer incur the costs for the Cr 6+ reduction.

    Ein weiterer Vorteil dieser Optimierung ist, daß die für das Beizen im neutralen pH-Bereich erforderliche Natriumsulfatlösung (Na2SO4) durch Neutralisation der sauren Elektrolytlösung (H2SO4, Na2SO4, Fe2+, Fe3+, Cr3+, Ni2+) mit Natriumhydroxid (NaOH) oder Natriumcarbonat (Na2CO3) und anschließender Abtrennung der Metallhydroxide (Fe(OH)2, Fe(OH)3, Cr(OH)3, Ni(OH)2) durch Filtration, rückgewonnen werden kann. Die Filtration wird hierbei vorteilhafterweise als Mikrofiltration durchgeführt. Die Versuche stellen nur Beispiele der Verfahrensführung dar, wie sie in den Ansprüchen definiert sind.A further advantage of this optimization is that the sodium sulfate solution (Na 2 SO 4 ) required for pickling in the neutral pH range is obtained by neutralization of the acidic electrolyte solution (H 2 SO 4 , Na 2 SO 4 , Fe 2+ , Fe 3+ , Cr 3+ , Ni 2+ ) with sodium hydroxide (NaOH) or sodium carbonate (Na 2 CO 3 ) and subsequent separation of the metal hydroxides (Fe (OH) 2 , Fe (OH) 3 , Cr (OH) 3 , Ni (OH) 2 ) by filtration, can be recovered. The filtration is advantageously carried out as microfiltration. The experiments are only examples of process control as defined in the claims.

    Claims (10)

    1. Process for chemical and/or electrochemical pickling of stainless steel in an acid liquid, but containing no nitric acid, characterised by used electrolyte solution from electrochemical, neutral salt pickling lines, preferably sodium sulphate pickling lines, being added.
    2. Process according to Claim 1, characterised by the redox potential in the pickling liquid being set to a given value by the amount of electrolyte solution added.
    3. Process according to Claim 2, characterised by the oxidation-reduction potential (redox potential) of the pickling liquid being measured and the amount of used electrolyte solution from electrochemical sodium sulphate pickling lines being added in the appropriate quantity.
    4. Process according to one of Claims 1 to 3, characterised by a bivalent and a trivalent iron salt, preferably iron sulphates, being added when the acid pickling liquid is prepared fresh.
    5. Process according to one of Claims 1 to 4, characterised by a different oxidising agent, preferably hydrogen peroxide, being added to the pickling liquid in addition to the used electrolyte.
    6. Process according to one of Claims 1 to 5, characterised by inhibitors being added.
    7. Process according to one of Claims 1 to 6, characterised by acid and neutral electrochemical pickling stages being combined with one another.
    8. Process according to Claim 7, characterised by the used electrolyte solution from the neutral electrolyte stage being added to the acid stage.
    9. Process according to one of Claims 7 or 8, characterised by the acid pickling liquid being neutralised using caustic soda (NaOH) or sodium carbonate (Na2CO3) so that the metal hydroxides generated can then be separated, preferably filtered off, for example using microfiltration, allowing the sodium sulphate (Na2SO4) produced to be recirculated as neutral electrolyte.
    10. Process according to one of Claims 1 to 6, characterised by the used electrolyte solution being taken from one or several cold-rolled strip pickling lines and added to the acid pickling liquid, free of nitric acid, of one or more hot-rolled strip pickling lines.
    EP99123620A 1998-12-22 1999-11-27 Process for pickling stainless steel Expired - Lifetime EP1013800B1 (en)

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    AT212998 1998-12-22
    AT0212998A AT406486B (en) 1998-12-22 1998-12-22 METHOD FOR STAINLESSING STAINLESS STEEL

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    CN1195898C (en) 2005-04-06
    ATA212998A (en) 1999-10-15
    US6398876B1 (en) 2002-06-04
    EP1013800A2 (en) 2000-06-28
    KR20000048256A (en) 2000-07-25
    KR100562094B1 (en) 2006-03-17
    ZA997788B (en) 2000-06-30
    ATE247182T1 (en) 2003-08-15
    DE59906582D1 (en) 2003-09-18
    EP1013800A3 (en) 2000-11-15
    AT406486B (en) 2000-05-25
    BR9905932A (en) 2000-09-12
    CN1261630A (en) 2000-08-02

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