WO2010052123A1

WO2010052123A1 - Ionic liquid composition for the removal of oxide scale

Info

Publication number: WO2010052123A1
Application number: PCT/EP2009/063706
Authority: WO
Inventors: Ramon Bacardit; Paolo Giordani; Mauro Rigamonti; Dennis Bankmann; Maria Del Mar Combarros Gracia
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 2008-11-05
Filing date: 2009-10-20
Publication date: 2010-05-14
Anticipated expiration: 2011-05-05

Abstract

The present invention refers to a composition comprising an ionic liquid (IL) for the conditioning and/or pickling of oxide scale on metal parts as well as to a process for the removal of oxide scale on metal parts, wherein the metal part is brought into contact with a composition comprising an ionic liquid (IL1) composed of (a) at least one organic salt as a source of inorganic Lewis-basic anions, (b) one or more Lewis-acidic inorganic metal salts, wherein at least one Lewis-acidic inorganic metal salt according to component (b) comprises a metal cation with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself. Consequently, the present invention encompasses metal parts, especially sheets, coils, wires, rods and preformed parts that have been treated in a process according to this invention. Metal parts that are at least partially composed of steel and/or stainless steel are preferred in a conditioning and/or pickling process as described herein.

Description

Ionic Liquid Composition for the Removal of Oxide Scale

The present invention refers to a composition comprising an ionic liquid (IL) for the conditioning and/or pickling of oxide scale on metal parts as well as to a process for the removal of oxide scale on metal parts, wherein the metal part is brought into contact with a composition comprising an ionic liquid (IL1 ) composed of

(a) at least one organic salt as a source of inorganic Lewis-basic anions,

(b) one or more Lewis-acidic inorganic metal salts, wherein at least one Lewis-acidic inorganic metal salt according to component (b) comprises a metal cation with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself. According to the invention, metal parts that are at least partially composed of steel and/or stainless steel are preferred in a conditioning and/or pickling process as described herein. Furthermore, the present invention encompasses metal parts, especially sheets, coils, wires, rods and preformed parts that have been treated in a process for the removal of oxide scale according to this invention.

Thus, the present invention relates to the field of metal surface treatment, particularly the treatment of thermally created oxide layers, such as those occurring in hot processing of metals into strip and wire. These oxides must be removed from the finished metal product. According to industrial standards the finished metal product has to be metallic bright with almost no residues of oxides on the metal surface. Especially, the removal of thermal oxide scale on forgeable metals, e.g. various types of steel, is a major aspect of the underlying invention.

In prior art, methods are described to remove oxide scale from metal parts. These methods involve chemical or mechanical treatments. While chemical treatment is performed usually in a two-step process which consists of "conditioning" and "pickling" of the oxide scale, mechanical treatment encompasses shot-blasting, sanding or peeling (machining). The major drawback of the mechanically achieved removal of oxide scale consist in an appreciable loss in metallic material, increased surface roughness or patterning of the metal part, and dust and fine particulates during the process that also include heavy metal oxides.

Chemical oxide removal ("pickling") systems are typically aqueous and consist of a strong acid, an oxidizing agent and a complexing agent. A typical, simple yet hazardous system uses a mixture of concentrated nitric acid and concentrated hydrofluoric acid. Various combinations of acids and oxidizing agents have been used in the past. Current state-of-the-art systems rely on the presence of at least some amount of fluoride or hazardous hydrofluoric acid in the mixture.

On particular oxide scales, chemical pickling agents as outlined above do not deliver acceptable results or require extended treatment times. This may lead to etching of the base alloy in the regions of the treated specimen which first become free from oxide. Materials prone to this issue thus are typically given a "conditioning" of the oxide scale before the actual pickling step.

Oxide conditioning can be achieved thermally or by chemical modification of the oxide scale. Thermal treatment using high temperature gradients may cause cracking of closed oxide layers allowing the pickling agent to penetrate the scale more readily. Chemical modification of the scale involves oxidation or reduction of the part of the oxides present in the scale.

Oxidative treatment by means of a "conditioning step" is typically conducted in a bath of molten sodium nitrate with sodium hydroxide as a flux agent at around 500⁰C. Such systems are commercially available under the brand name Kolene^® (Shoemaker, R. H. (1982), "Metal purification by molten salt process", Metalurgia y Electricidad 46, 44-6, 49-50).

"Reductive conditioning" is also conducted in a molten salt bath, but employs sodium hydride as a reducing agent instead of the sodium nitrate oxidizing agent. Such a de-scaling process for metal parts is disclosed in GB1221694. Both molten salt technologies require furnaces and substantial safety measures due to the highly reactive and caustic nature of the chemicals used. Furthermore, these processes consume significant energy for heating the bath to flux. They also produce caustic sludges and quenching parts from molten salt baths creates toxic and caustic steam.

A further example of oxidative treatment uses permanganate in methanol. Methanol is a toxic, volatile and flammable organic solvent and permanganate is a strong oxidizing agent with the ability to produce toxic Cr(VI) species from chromium(lll) oxide present in oxide scale of stainless steel metal parts.

Thus the current oxide conditioning processes are hazardous by virtue of the chemicals employed and the dust and sludge produced and are energy-intensive and environmentally unfriendly. A significant fraction of potential users refrain from employing such processes for the aforementioned reasons, thereby also precluding use of chemical pickling agents and resorting to mechanical oxide descaling.

Nevertheless, comparatively benign compositions exist for pickling baths in a process for pickling martensitic or ferhtic stainless steel disclosed in the German Application DE10160318, wherein the stainless steel is placed in contact with a pickling solution which has a temperature in the range 15 to 29 ⁰C and contains 50 to 120 g/l of free sulfuric acid, 5 to 40 g/l of free HF and 5 to 40 g/l of Fe(III) ions. Such kinds of compositions are provided by the Cleanox^® (Henkel AG & Co. KGaA, Germany) range of products.

The present invention, by virtue of the compositions described, relates to the field of non-aqueous solvents containing organic salts. These are well known in the art and are usually referred to as molten salts, ionic liquids or deep eutectic solvents, depending on their precise nature. Reviews on ionic liquids can be found in the concurrent literature (Wasserscheid, Peter / Welton, Thomas (Eds.), "Ionic Liquids in Synthesis", 2007, Wiley-VCH, Weinheim; J. Dupont et al, Chem. Rev. 2002, 102, 3667)

The term deep eutectic solvent refers to a mixture of at least two components which do not form discrete new compounds, but show a very low temperature eutectic point in their phase diagram.

The international application WO 2002026701 teaches the use of deep eutectic solvents based on choline chloride as a medium for the dissolution of inorganic oxide powders. The eutectic nature per se of these solvents is of little consequence other than rendering the medium liquid and does not impart an ability to chemically alter metal oxides by change of oxidation state.

A marked solubility of particular transition metal oxides in certain ionic liquids is disclosed in the international application WO 2007147222. It must be noted however, that the claimed structures of the ionic liquids are restricted to those carrying protic groups such as alcohols and carboxylic acids and that expensive fluorinated anions are employed to render the media liquid. The formation of room temperature ionic liquids by combining organic halide salts with appropriate Lewis-acidic salts of main group or transition metals such as aluminum, iron, tin and zinc chlorides are established in prior art and disclosed in the international application WO2000056700.

The most investigated systems are based on 1 -ethyl-3-methylimidazolium chloride and AICI3 (Wasserscheid, Peter / Welton, Thomas (Eds.), "Ionic Liquids in Synthesis", 2007, Wiley-VCH, Weinheim; J. Dupont et al, Chem. Rev. 2002, 102, 3667). These however, hydrolyze in a strong exothermic reaction even when in contact with atmospheric moisture and are thus per se not practical for the pretreatment of metal parts in large open baths. Moreover, those ionic liquids and deep eutectic solvents that have been disclosed in the prior art have not been reported to assist in conditioning and/or pickling of oxide scale on metal parts. According to the state of the art many different ionic liquids can be prepared by combination of an organic salt with a Lewis-basic anion and a Lewis-acidic inorganic material, thus forming new ionic liquid compounds with complex anions (Wasserscheid, Peter / Welton, Thomas (Eds.), "Ionic Liquids in Synthesis", 2007, Wiley-VCH, Weinheim; J. Dupont et al, Chem. Rev. 2002, 102, 3667).

In WO 2000056700, ionic liquids based on choline chloride and inorganic Lewis- acidic metal salts wherein the molar ratio of the the Lewis-acidic metal salt to the quarternary ammonium compound is at least 1 : 1 are disclosed for the reprocessing of an oxidized transition metal catalyst in powdered form.

The reports described in the preceding paragraphs concern only the solubility of metal oxide powders that exhibit high surface areas and thus are relatively active compared to aged and mixed oxide scale on the surface of metal parts.

Until now, the disclosure of a simple liquid medium capable of efficiently interacting with oxide scales on actual metal parts and its application for conditioning and/or removal of thermal oxide scale that is especially useful for iron(ll), iron(lll) and chromium(lll) oxides is still missing in the prior art.

Therefore, the problem to be solved according to this invention consists in establishing a composition comprising an ionic liquid suitable for the conditioning of oxide scale on metal parts, especially of oxide scale on various types of steel, together with an aqueous process for the removal of such oxide scale. A further problem to be solved consists in establishing a composition comprising an ionic liquid that is also suitable for the removal of oxide scale on metal parts, especially of oxide scale on various types of steel.

The term "steel" according to this invention includes all forgeable iron-based materials, but also all highly alloyed metallic materials in which the element iron (Fe) is the major metallic elemental constituent. This definition also includes those iron-based materials that are generically referred to as stainless steels. A list of the technically most important stainless steels, together with the material Nos., identifications and alloy components, as well as the mechanical and chemical properties thereof are given in Ullmanns Encyklopadie der technischen Chemie, 4^th Edition, Vol. 22, pp. 106-112 and in German Industrial Standard DIN 17440, July 1985. Stainless steels are iron-based alloys containing at least 10% chromium. The formation of chromium oxide on the material surface imparts to the stainless steels the corrosion-resistant character thereof.

Stainless steels may be sub-divided into the following families: austenitic steels, ferhtic steels, martensitic steels, precipitation hardened steels and duplex steels. These groups differ in the physical and mechanical properties thereof, as well as in corrosion resistance, as a result of the various alloying constituents. Austenitic stainless steels are listed as stainless steels of the 200 and 300 Series. They are the most widely employed stainless steels and represent 65 to 85% of the stainless steel market. They are chemically characterised by a chromium content of > 17% and a nickel content of > 8%. They have a cubic face-centered structure and are outstandingly ductile and weldable. The most widely used of these steels is probably Type UNS S 30400 (Type 304), or "18/8". Modifications include S 32100 (stabilized with titanium) and S 34700 (stabilized with niobium). Alloys having higher contents of chromium, nickel or molybdenum are available and provide increased corrosion resistance. Examples are S 31600, S 31700, S 30900 and S 31000. The 200 Series of austenitic stainless steels has, on the other hand, a reduced nickel content and contains manganese instead.

The term "ionic liquid" corresponds to salts with a relatively low melting point and low viscosity. The common definition of an ionic liquid (IL), or a room temperature ionic liquid (RTIL), is that it is a liquid composed entirely of ions, which is fluid below 100 ⁰C, preferably already at 20 ⁰C. Ionic liquids are generally much denser (δ = 1 -1.6 g/cm³) and more viscous (η = 10-500 mPas) than conventional molecular solvents. Ionic liquids can be stable up to temperatures of 500 ⁰C. As a medium they can provide excellent solubility for numerous organic, inorganic and polymeric substances. Further notable characteristic of ionic liquids are their non- measurable vapor pressure and the non-flammable nature of most members of the family of ionic liquids.

The terms "conditioning" and "pickling" according to this invention constitute process steps in an overall process for the removal of oxide scale on metal parts. During the preceding conditioning step the compact oxide film covering the surface of the metal part has to be altered in contact with the composition comprising an ionic liquid by means of physical and chemical attack in a way that the subsequent pickling of the metal part is rendered more efficient yielding an almost complete removal of oxide scale. In principle a composition suitable for conditioning might as well be suitable for the pickling of oxide scale on metal parts. Consequently, the conditioning of the compact oxide film and the pickling of the oxide scale on the metal part might be accomplished in one process step provided that the composition comprising an ionic-liquid acts as a markedly effective pickling agent as well.

It has been found that conditioning and pickling can be performed by compositions comprising an ionic liquid (IL) composed of

(a) at least one organic salt as a source of Lewis-basic anions, wherein the organic salts are selected from halides and/or pseudohalides of mono- or polyfunctional ammonium, iminium, imidazolium, pyrazolinium, pyridinium, phosphonium and/or sulfonium compounds,

(b) one or more Lewis-acidic inorganic metal salts selected from halides and pseudohalides, wherein at least one Lewis-acidic inorganic metal salt according to component (b) comprises a metal cation with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself, wherein the conditioning and/or pickling composition comprises a molar ratio of metal cations of components (b) to Lewis-basic anions that originate from components (a) that is less than 1 : 1. The term "oxidation state" according to this invention is defined by the corresponding IUPAC Rule 1-5.5.2.1 ("Nomenclature of Inorganic Chemistry - Recommendations 1990", Blackwell: Oxford, 1990) and thereby defines the hypothetical charge an atom might be imagined to have when electrons are counted in accordance with the electronegativity of the respective elements that assemble the molecule or salt, while the element with the higher electronegativity gathers all the electrons shared with elements that are less electronegative. In this context the lowest accessible positive oxidation state means the lowest common positive oxidation state of a metal element that is listed in the "Periodic Table of the Elements" by Fluck, E. and Heumann, K. G. (Wiley-VCH, 4^th Edition, 2007).

According to this invention Lewis-basic anions of component (a) react with at least one of the Lewis-acidic inorganic metal salt of component (b) thereby forming an ionic liquid (IL) that comprises anionic metal complexes, while the overall molar concentration of Lewis-basic anions that possibly originate from component (a) organic salts exceeds the molar concentration of metal cations of component (b) Lewis-acidic inorganic metal salts. Therefore, an excess of organic salt as a source of Lewis-basic anions dissolved within the ionic liquid is established which strongly affects the conditioning and pickling properties of the composition according to this invention. Nevertheless, a certain amount of Lewis-acidic metal salts according to component (b) within the compositions is preferred. Thus, it was found that compositions with a molar ratio of metal cations of components (b) to Lewis-basic anions that originate from components (a) that is less than 1 : 100 yield from a technical point of view unacceptable minor results in conditioning and pickling of oxide scale on metal parts. Most preferably, the molar ratio of metal cations of components (b) to Lewis-basic anions that originate from components (a) is not less than 1 : 10 to deploy a considerable conditioning or removal of oxide scale.

In a preferred embodiment of the conditioning and/or pickling composition those organic salts as a source of Lewis-basic anions are selected that do not have hydrogen atoms being bonded to the cationoid heteroatoms selected from nitrogen, phosphorus and/or sulfur of the above-mentioned halides and/or pseudohalides of component (a). For these preferred organic salts according to component (a), thermal decomposition reactions of the ionic liquid at elevated temperatures can be avoided.

Lewis-basic anions of component (a) that contribute to the formation of the ionic liquid (IL) of the conditioning and/or pickling composition are according to the Lewis concept electron donors. Those electron donors are selected preferably from chloride, bromide, fluoride, dicyanamide and/or thiocyanate, most preferably chloride.

Lewis-acidic inorganic metal salts of component (b) that compose the ionic liquid (IL) of the conditioning and/or pickling composition are according to the Lewis concept electron acceptors.

In the context of the present invention, only those Lewis-acidic metal salts according to component (b) are of use where the metal can exist in two reasonably stable, distinct oxidations states greater than zero, thus salts like AICI3 and ZnC^ are found to be unsuitable. Metals with just one oxidation state greater than zero and which may be described as an oxidizing agent, will likely deposit in elemental form by electroless plating on suitable substrates, which contradicts the purpose of the invention to provide a clean, bare surface of the base alloy of the treated metal part. Finally, a preferred inorganic salt used in the design and construction of the preferred conditioning and/or pickling composition herein described will be incapable of oxidizing chromium(lll) to toxic chromium(VI) compounds.

Thus, those electron acceptors are preferred wherein the metal with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself is selected from those metal cations that have a standard reduction potential with respect to their metal cations in their lowest accessible positive oxidation state in the ionic liquid (IL) higher than at least one standard reduction potential of the major metallic elemental constituent of the metal part to be treated with respect to any oxidation state of its metal cations in an ionic liquid (IL2) that differs from the ionic liquid (IL) according to this invention only in that the component (b) Lewis-acidic metal salts comprise the metal cations in their lowest accessible positive oxidation state only.

The standard reduction potential E°(Meⁿ⁺/Me^m+) of a redox couple Meⁿ⁺(iL)/Me^m+(iL) in the ionic liquid (IL), wherein MeⁿV) originates from the at least one component (b) Lewis-acidic metal salt with an oxidation state of the metal cation higher than the lowest accessible oxidation state of the metal itself, is defined as the half cell potential of the following electrochemical reaction (1 ) versus the half cell potential of the standard hydrogen electrode (SHE), wherein all thermodynamic concentrations of the metal species equal to 1 :

Me^πV) ⁺ (n-m) e → Me^m V) (1 )

with n being an integer number of at least 1 and m being zero or an integer number of at least 1 , wherein n is greater than m.

Thus, the preferred electron acceptors according to the invention are based on metal cations of a Lewis-acidic metal salt of component (b) wherein the following condition is fulfilled:

E°(Meⁿ⁺ _(b, ID / Me^m+ _(b, ID) > E°(Me^x+ ₍oχ.de, IL₂₎ / Me°₍ox,de, IL₂₎) (2)

with Me(b): metal cations that originate from an Lewis-acidic inorganic metal salt according to component (b) with n and m as defined above;

IL: ionic liquid according to the composition of the invention

Me_(oXlde):nnajor elemental constituent of the metal to be treated, wherein the metal to be treated is covered by oxide scale, with x being a positive integer of at least 1 ; and

IL2: ionic liquid that differs from the ionic liquid (IL) according to this invention only in that the component (b) Lewis-acidic metal salts comprise the metal cations in their lowest accessible positive oxidation state only. For these preferred Lewis-acidic inorganic metal salts according to component (b) the oxidizing action is high enough to enable the release of metal ions from the metal part to be treated and thus to remove oxide scale.

In order to select at least one component (b) Lewis-acidic inorganic metal salt wherein the metal cation occupies an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself that possesses enough oxidizing action towards the metal part to be treated, the person skilled in the art may also determine the cell voltage of the following electrochemical measurement sequence (3):

Meⁿ⁺ ₍iL) (Pt) // Me^x+ ₍iL2) / Me⁰ (3)

with Meⁿ⁺: metal cation of the at least one component (b) Lewis-acidic inorganic metal salt with an oxidation state more positive than the lowest accessible oxidation state of the metal element itself; IL: ionic liquid according to the invention; Pt: inert platinum electrode;

Me^x+: metal cations of the major elemental constituent of the metal to be treated;

IL2: ionic liquid that differs from the ionic liquid (IL) according to this invention only in that the component (b) Lewis-acidic metal salts comprise the metal cations in their lowest accessible positive oxidation state only; and Me⁰: metal electrode of the major elemental constituent of the metal.

For a preferred embodiment of the conditioning and/or pickling composition comprising the ionic liquid (IL) the cell voltage of this electrochemical measurement sequence (3) imposes a cathodic current in the half cell Meⁿ⁺ ₍i_L)(Pt), more preferably a cathodic current and an absolute value of the cell voltage of more than 50 mV and less than 1500 mV, more preferably less than 1000 mV. Generally more preferred according to this invention are conditioning and/or pickling compositions comprising a ionic liquid, wherein the at least one component (b) Lewis-acidic inorganic metal salt with an oxidation state of the metal cation more positive than the lowest accessible positive oxidation state of the metal element itself is selected from bromides, chlorides, fluorides, dicyanamides and/or thiocyanates of iron, cobalt, nickel, tin, vanadium, titanium, lead, cerium and/or manganese.

For the various types of stainless steel it is further preferred that the oxidizing action of the component (b) Lewis-acidic metal salts does not produce chromium(VI) compounds that might be released from the metallic substrate and/or the chromium(lll) containing oxide scale on the metal part. Consequently, those compositions are preferred for the conditioning and/or pickling of oxide scale on stainless steel wherein the ionic liquid is composed of component (b) Lewis- acidic metal salts comprising a metal cation with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself that are selected from those metal cations that do not possess any standard reduction potential with respect to a lower positive oxidation state of this metal element in the ionic liquid (IL) that is higher than the standard reduction potential of the redox couple Cr^+I"/Cr^+Vl in an ionic liquid (IL2) that differs from the ionic liquid (IL) according to this invention only in that the component (b) Lewis-acidic metal salts comprise the metal cations in their lowest accessible positive oxidation state only.

Moreover, to prevent from metallization of the metal part to be treated, the conditioning and/or pickling composition according to this invention preferably comprises component (b) Lewis-acidic metal salts with an oxidation state of the metal cations more positive than the lowest accessible positive oxidation state of the metal element itself, wherein the metal cations are further selected from those metal cations that have at least one standard reduction potential with respect to a lower positive oxidation state of this metal element in the ionic liquid (IL) that is higher than the standard reduction potential of this metal element in this lower positive oxidation state with respect to its metallic state in an ionic liquid (IL3) that differs from the ionic liquid (IL) only in that the component (b) Lewis-acidic metal salts comprise the metal cations in the lower positive oxidation state only.

In a preferred embodiment of the invention and especially for the treatment of various types of steel the at least one component (b) Lewis-acidic metal salt with an oxidation state of the metal cation more positive than the lowest accessible positive oxidation state of the metal element itself is selected from iron(lll)halides, more preferably iron(lll)chloride and most preferably non-hydrated iron(lll)chloride.

As the organic cation is not a reactive part, any organic cation that does not carry interfering functional groups may be used in such a composition, if it allows for a suitably low melting point of the mixture of the components (a) and (b), i.e. below the decomposition temperature of the organic parts and below the desired processing temperature. In this context functional groups with protic reactivity such as alcohols and carboxylic acids are not preferred compounds as they inhibit the performance in respect to the present invention. Furthermore, an organic material that is oxidizable due to its structure or functionality, i.e. thiols, aldehydes, are less preferred, since they possibly decrease the oxidizing action of the at least one component (b) Lewis-acidic metal salt within the ionic liquid (IL) according to this invention comprising metal cations in a higher positive oxidation state than the lowest accessible positive oxidation state of the respective metal itself.

Although the chemical structure of the organic cations of components (a) is not strictly limited by the scope of this invention, optimum results in the conditioning and pickling of oxide scale on metal surfaces are obtained for compounds of component (a) wherein the organic salts that also act as a source of Lewis-basic anions, are constituted by organic cations with a monofunctional ammonium, iminium, imidazolium, pyrazolinium and/or pyridinium structure, most preferably with a monofunctional imidazolium structure according to the general formula (I):

wherein R¹ and R² are independently from each other selected from branched or linear alkyl or hydroxyalkyl groups with not more than 10 carbon atoms, preferably not more than 6 carbon atoms, or from alkoxylates with not more than 6, preferably not more than 4 repeating units, preferably propoxylates and ethoxylates; wherein R³, R⁴ and R⁵ are independently from each other selected from hydrogen atoms, branched or linear alkyl or hydroxyalkyl groups with not more than 10 carbon atoms, preferably not more than 6 carbon atoms, or wherein the residues R⁴ and R⁵ are part of the same anellated aromatic, heteroaromatic, aliphatic or heteroaliphatic ring system with not more than 7 ring members.

For these preferred organic cations based on imidazolium cations according to component (a) of the ionic liquid the conditioning of the oxide scale that in turn consists mainly in altering the strength and porosity of the compact oxide film on the metal part is more efficiently accomplished.

Considering the chemical accessibility of those imidazolium cations, those imidazolium cations according to the general structure (I) are even more favorable, wherein R³, R⁴ and R⁵ are hydrogen atoms and R¹ and R² are independently from each other selected from a linear or branched alkyl group with not more than 10 carbon atoms, more preferably less than 4 and most preferably not more than two carbon atoms.

Another aspect of this invention consist in a process for the removal of oxide scale on metal parts by means of conditioning and/or pickling, wherein the metal part is brought into contact with a composition comprising an ionic liquid (IL1 ) composed of

(a) at least one organic salt as a source of inorganic Lewis-basic anions,

(b) one or more Lewis-acidic inorganic metal salts, wherein at least one Lewis-acidic inorganic metal salt according to component (b) comprises a metal cation with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself.

The conditioning and/or pickling performance of the composition used in the process according to this invention can be further improved with an ionic liquid (IL1 ) composed of organic salts of halides and/or pseudohalides as a source of inorganic Lewis-basic anions according to component (a).

Furthermore, the organic cations that correspond to the inorganic Lewis-basic anions of component (a) are preferably selected from cations of mono- or polyfunctional ammonium, iminium, imidazolium, pyrazolinium, pyridinium, phosphonium and/or sulfonium compounds in order to constitute ionic liquids with a melting point of not more than 100 ⁰C when combined with Lewis-acidic inorganic metal salts according to component (b). Generally, monofunctional ammonium, iminium, imidazolium, pyrazolinium and/or pyridinium compounds are most preferred, especially monofunctional imidazolium compounds.

It was found that for reasons of stability of the ionic liquid composed of the components (a) and (b) at an elevated process temperature those components (a) are preferred that do not carry any hydrogen atoms bonded to respective heteroatoms of the organic cations selected from nitrogen, phosphorus and/or sulfur.

Other preferred embodiments of component (a) of the composition used in the process according to this invention correspond to the above-defined specified embodiments of the conditioning and/or pickling composition comprising the ionic liquid (IL). With regard to component (b) composing the ionic liquid (IL1 ) of the composition used in a conditioning and/or pickling process those Lewis-acidic metal salts with an oxidation state of the metal cation more positive than the lowest accessible positive oxidation state of the metal element itself are preferred, wherein the metal cations of the metal salt have a standard reduction potential with respect to their metal cations in their lowest accessible positive oxidation state in the ionic liquid (IL1 ) higher than at least one standard reduction potential of the metal to be treated with respect to any oxidation state of its metal cations in an ionic liquid (IL2) that differs from the ionic liquid (IL1 ) only in that the component (b) Lewis-acidic metal salts comprise the metal cations in their lowest accessible positive oxidation state only.

For the various types of stainless steel that is treated in a conditioning and/or pickling process according this invention it is further preferred that the oxidizing action of the component (b) Lewis-acidic metal salts does not produce chromium(VI) compounds that might be released from the metallic substrate and/or the chromium(lll) containing oxide scale on the metal part. Consequently, those compositions are preferred for the conditioning and/or pickling of oxide scale on stainless steel wherein the ionic liquid (IL1 ) is composed of component (b) Lewis-acidic metal salts comprising a metal cation with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself that are selected from those metal cations that do not possess any standard reduction potential with respect to a lower positive oxidation state of this metal element in the ionic liquid (IL1 ) that is higher than the standard reduction potential of the redox couple Cr^+I"/Cr^+Vl in an ionic liquid (IL2) that differs from the ionic liquid (IL1 ) according to this invention only in that the component (b) Lewis-acidic metal salts comprise the metal cations in their lowest accessible positive oxidation state only.

Moreover, to prevent from metallization of the metal part to be treated during the conditioning and/or pickling process, the ionic liquid (IL1 ) of the conditioning and/or pickling composition preferably contains component (b) Lewis-acidic metal salts with an oxidation state of the metal cations more positive than the lowest accessible positive oxidation state of the metal element itself, wherein the metal cations are further selected from those metal cations that have at least one standard reduction potential with respect to a lower positive oxidation state of this metal element in the ionic liquid (IL1 ) that is higher than the standard reduction potential of this metal element in this lower positive oxidation state with respect to its metallic state in an ionic liquid (IL3) that differs from the ionic liquid (IL1 ) only in that the component (b) Lewis-acidic metal salts comprise the metal cations in the lower positive oxidation state only.

In a conditioning and/or pickling process the use of compositions comprising the ionic liquid (IL1 ) is preferred, wherein the Lewis-acidic inorganic metal salts according to component (b) of the composition are selected from halides and/or pseudohalides such as chlorides, bromides, fluorides, dicyanamides and/or thiocyanates of iron, cobalt, nickel, tin, vanadium, titanium, lead, cerium and/or manganese. The most preferred Lewis-acidic metal salts according to component (b) for the conditioning and pickling of various types of steel and/or stainless steel are selected from iron(lll)halides while the equivalent fluorides preferably do not constitute the major species according to component (b) but might as well be present in a mixture of component (b) Lewis-acidic metal salts, more preferably non-hydrated iron(lll)halides and most preferred is non-hydrated iron(lll)chloride.

The conditioning and/or pickling process according to this invention yields optimum results for compositions comprising a ionic liquid (IL1 ), wherein the molar ratio of metal cations of components (b) to Lewis-basic anions that originate from components (a) is less than 1 : 1 , but not less than 1 : 100, preferably not less than 1 : 10. Those ionic liquid compositions (IL1 ) are preferred in a process for the removal of oxide scale as they contain a molar excess of Lewis-basic anions that originate from component (a) with respect to the Lewis-acidic metal salt according to component (b). On the other hand a certain minimum amount of Lewis-acidic metal salts according to component (b) is from a technical point of view preferred in order to provide a considerable conditioning and/or pickling of oxide scale.

In a one-step process for the removal of oxide scale that solely relies on a composition comprising a ionic liquid (IL1 ) and wherein no subsequent aqueous pickling step is applied to the metal part, those compositions are preferred wherein the molar ratio of metal cations of components (b) to Lewis-basic anions that originate from components (a) is not less than 1 : 10, more preferably not less 1 : 5, but less than 9 : 10, more preferably less than 4 : 5.

In a continuous conditioning and/or pickling process according to this invention the presence of water, e.g. gathered from the ambient atmosphere, can not be excluded. In could be shown that the compositions comprising an ionic liquid that were used in a conditioning and/or pickling process according to this invention tolerate small amounts of water and acids without any detrimental effect for the removal of oxide scale on the treated metal specimens. However, in a process for conditioning and/or pickling of metal parts covered with a compact oxide layer the use of compositions with less than 5 wt.-%, preferably less than 2 wt.-% acids and/or water is favored.

The composition used in the process according to this invention may additionally contain auxiliary components (c) selected from flux agents, chelating agents, surfactants or dispersing agents, fillers, thixotropic compounds and/or corrosion inhibitors.

Flux agents as auxiliary components (c) aid in lowering the melting point of the composition realizing in this way stable low temperature conditioning and pickling bathes that account for minimum energy consumption and prolonged bath lifetime. Chelating agents according to this invention do neither belong to the definition given for component (a) organic salts nor to the definition of component (b) Lewis- acidic metal salt according to the ionic liquid (IL1 ). A preferred flux agent in a process according to this invention is urea. Chelating agents as auxiliary components (c) aid in complexing metal cations in order to improve solubility of the metal ions dissolved during the conditioning and/or pickling treatment of the specimens. Chelating agents used in aqueous media are well known to the person skilled in the art and can as well be used according to this invention. Chelating agents according to this invention do neither belong to the definition given for component (a) organic salts nor to the definition of component (b) Lewis-acidic metal salts according to the ionic liquid (IL1 ). Thus, preferred chelating agents are selected from EDTA, rhodanite and/or amines.

Surfactants or dispersing agents as auxiliary components (c) may be added to aid in improving effectiveness of the conditioning and/or pickling process on oiled substrates as well as in dispersing solid oxide scale released from the specimens during the conditioning and/or pickling process. Especially useful are perfluorinated anionic surfactants or non-ionic surfactants belonging to alkoxylated alcohol derivatives. Non-ionic polymeric compounds such as polyvinylpyrrolidone are preferred as dispersing agents.

Fillers as well as thixotropic compounds as auxiliary components (c) impart heightened viscosity to the composition used in the process according to this invention and thereby allow the alignment of the viscosity of the mixture and the application of the composition as a paste or as a thin adherent film to the metal part to be treated.

Such fillers are well known to the person skilled in the art, but have to be selected from inorganic particulate matter that does not dissolve in the composition comprising the ionic liquid.

Thixotropic compounds as auxiliary components (c) of the composition used in a process according to this invention are preferably selected from particulate silica, particulate aluminum oxide or particulate matter with a non-spherical shape and a preferred particle size with regard to the longest dimension of no more than 50 μm, more preferably of no more than 10 μm, i.e. clay minerals and mica. Alternatively, organic polymeric substances that are non-functional ized and non-soluble in the ionic-liquid and thus form colloidal solutions of the polymeric material are preferred as thixotropic compounds.

Corrosion inhibitors are compounds that act to prevent corrosion, i.e. etching of the base alloy of the treated specimen during the conditioning or pickling process. They serve to amplify the reactivity difference towards the conditioning/pickling agent between base alloy and scale. Typical corrosion inhibitors are for example amino alcohols, phosphoric acid esters, thiourea derivatives.

In case that auxiliary components (c) are present in a composition used in a process for the conditioning and/or pickling of oxide scale according to this invention it is preferred that the composition contains (i) 50-95 wt. -% of the ionic liquid composed of the components (a) and (b) (ii) 0.1 -50 wt.-% of the auxiliary components (c) selected from flux agents, chelating agents, surfactants or dispersing agents, fillers, thixotropic compounds and/or corrosion inhibitors, wherein the overall amount of components (a) and (b) and the auxiliary components (c) adds up to at least 95 wt.-%. The residual amount of the composition that adds up to 100 wt.-% consists of acids and/or water.

Moreover, the auxiliary components (c) are preferably present in the composition in an amount of

(i) 0-50 wt.-%, preferably 1-30 wt.-% of flux agents, fillers and/or thixotropic compounds, (ii) 0-10 wt.-%, preferably 0.1 -5 wt.-% of chelating agents, surfactants or dispersing agents and/or corrosion inhibitors.

During the conditioning and/or pickling of the metal specimens in a continuous process according to this invention, the oxidizing action of the conditioning and/or pickling bath has to be monitored and kept at a certain level. In order to maintain the oxidizing action within the composition used in the conditioning and/or pickling process it is preferred to establish a molar fraction of Lewis-acidic inorganic metal salts according to component (b) with an oxidation state of the metal higher than the lowest accessible positive oxidation state of the metal element itself to the overall amount of components (b) of not less than 50 %, preferably not less than 80 % and more preferably not less than 90 %. This specific ratio can be monitored and if necessary aligned with the addition of oxidizing agents to the composition used in the conditioning and/or pickling process. In the case of use of iron(lll)chloride as the Lewis-acidic metal salt of component (b), the depletion state of a conditioning and/or bath may be determined by taking aliquots and titrating with standard methods for content in iron(ll) and iron(lll). Preferred oxidizing agents according to this invention that are used to align the composition of the bath are those ones that are incapable of oxidizing Cr(III) to Cr(VI), most preferably hydrogen peroxide and/or oxygen. Furthermore, electrolytic oxidation may be applied to return the Lewis-acidic metal salt of component (b) to its higher oxidation state. Electrolytic treatment consumes energy, but much less than required for the maintenance of conditioning baths in prior art using molten salt baths that have to be permanently heated in a furnace.

Generally, the composition used in the conditioning and/or pickling process according to this invention can be applied to the metal part or metal specimen with conventional industrial coating techniques such as immersing, spraying, wiping and squeegee and/or roller applications. The application by means of immersing the metal part in a bath of the composition comprising the ionic liquid (IL1 ) is preferred. Preferably, the bath is agitated, by mechanical stirring or injection of air or an inert gas, wherein the relative humidity is preferably less than 10 %, most preferably less than 5 %.

While for wiping and squeegee and/or roller applications the use of fillers and/or thixotropic compounds as auxiliary components (c) is preferred, those auxiliary components are preferably excluded in spray applications of the composition comprising the ionic liquid (IL1 ). On the other hand, flux agents may be especially useful in spray applications to prevent from crystallization of the ionic liquid. The temperature of the composition comprising an ionic liquid (IL1 ) used in a conditioning and/or pickling process according to this invention can be determined by several factors and will usually be chosen so that the conditioning and/or pickling composition is liquid and maintains a suitable viscosity. Increased working temperatures can be employed to speed up the conditioning and pickling process. The temperature may be chosen to exceed the boiling point of water under the prevailing conditions, typically around 100⁰C, to continuously drive off humidity from the conditioning and/or pickling composition in open vessels, thus maintaining a high level of dryness. Alternatively, vacuum may be applied to achieve drying at lower temperatures.

After a given treatment time, typically in the range from 1 to 20 minutes for oxide conditioning and at least 10 minutes for pickling, the treated metal specimens are removed and rinsed, preferably at high pressure, with tap or deionized water to remove residuals from conditioning and/or pickling composition and to wash off oxide scale flakes. The good water solubility of the conditioning and/or pickling composition comprising the ionic liquid (IL1 ) allows for efficient removal by water rinse.

A conditioning and pickling process according to this invention is preferably a two- step process that consists in contacting the metal part to be treated with a conditioning composition comprising an ionic liquid (IL1 ) and subsequently with or without intermediate rinsing step in treating the conditioned metal part with an aqueous pickling solution.

Such an aqueous pickling solution preferably contains iron(lll) ions, sulfuric acid and hydrofluoric acid. For such a pickling solution the following amounts are especially preferred: a) 50 to 120 g/l of free sulfuric acid, b) 5 to 40 g/l of free HF and c) 5 to 40 g/l of Fe(III) ions. Further similar embodiments of aqueous pickling solutions are given within the Patent Applications WO 1999/031296, US 6,210,558 which are herewith enclosed by reference.

Another preferred aqueous pickling solution for a two-step process may comprise: a) 2 to 100 g/l of one or more strong acids selected from sulfuric acid, phosphoric acid, and hydrochloric acid, or mixtures thereof b) of one or more oxidizing agents containing a peroxo-group, more preferably 1 to 30 g/l of hydrogen peroxide, c) 50 to 300 mmoles per liter of complex fluoride ions of elements of B, Si, Ti and/or Zr.

In a special embodiment the two-step process is conducted without an intermediate rinsing step. Especially for conditioning compositions comprising ionic liquids (IL1 ) composed of iron(lll) metal salts and for aqueous pickling compositions based on iron(lll) ions, the intermediate rinsing step in the two-step conditioning and pickling process may be omitted.

In another aspect of this invention a metal part is encompassed that has been treated according to a conditioning and/or pickling process as described herein. Typical metal parts that have been treated in a process according to this invention are preferably steel metal parts that due to their production process accrue with a compact thermal oxide layer such as sheets and coils after hot- and cold-rolling, drawn wires and wire rods. Furthermore, due to the relative mildness of the compositions comprising an ionic liquid (IL1 ), the conditioning or pickling process according to this invention may be applied to more complex structures, e.g. to oxide formed in welding seams, on larger or preformed parts. Most preferably the treated metal part is at least partially composed of stainless steel.

The conditioning and/or pickling composition used in a process according to the present invention exhibits the following advantages: 1. low hazard classifications such as irritant or harmful on constituents as compared to fluorous pickling systems, with hydrofluoric acid classified as corrosive and very toxic

2. non-flammability as opposed to solvent based systems and sodium hydride molten salt baths

3. liquidity at and below room temperature as compared to molten salt treatment baths

4. near zero vapor pressure the required constituents due to their ionic nature as opposed to solvent based systems

5. tolerance towards oiled specimens

The described invention therefore represents a significant improvement relative to existing conditioning baths in the prior art, which require capital investment of special furnaces, high energy requirements for temperature in excess of 500⁰C, handling of dangerous and strongly corrosive materials, processing of caustic dust, sterna and sludges, or employ flammable, volatile toxic solvents in the presence of strong oxidizing agents.

Moreover, a composition comprising an ionic liquid is disclosed that reveals adequate pickling properties and thus can be used in a one-step process for the removal of oxide scale on metal parts. In such a one-step process, pickling baths known in the prior art that are based on compositions that contain strong acids (e.g. hydrofluoric acid) can be replaced with the less hazardous compositions based on ionic liquids according to this invention.

Examples:

Preparation example P1 : lron(lll)chloride (anhydrous form) and 1 -ethyl-3-methylimidazolium chloride (Abbr.: emimCI; BASF AG, Germany) were weighed in a 2:3 w/w ratio and mixed in an open vessel at room temperature by mechanical agitation until a brown liquid was formed.

Other mixing ratios can be realized and are also found to be liquid at room temperature over a wide range of compositions.

Conditioning example E1 :

An AISI 304 non-annealed stainless steel wire piece was immersed in a stirred bath of an ionic liquid composition according to the preparation example P1 at 80⁰C for 15min. The rod was removed from the bath, rinsed with Dl water and subsequently treated in a Cleanox^® 352 bath (Henkel AG & Co. KGaA, Germany) for 10 min. The rod was rinsed with Dl water and blowdried. The rod was completely pickled (>90% of the surface is metallic bright).

In a comparative example C1 without oxide conditioning, visible oxide residues remained after 10 min of Cleanox^® treatment. The treatment was repeated for a total of 20 min treatment time, after which the rod was completely pickled.

The following Table 1 resumes the effect of conditioning with an ionic liquid composition according to the preparation example P1 on different types of steel compared to a one-step process in which the conditioning step is omitted and which solely relies on the aqueous pickling step accomplished with a Cleanox ^® 352 bath.

It is evident that the conditioning step using an ionic liquid is crucial for the removal of oxide scale on stainless steel. Especially for DIN1.4567 stainless steel (Examples E3, C3) a conditioning step in a process according to this invention exerts a huge impact on the efficiency of oxide scale removal.

Pickling Examples E5-E8 and Comparative Examples C5 and C6: An AISI 304 non-annealed stainless steel wire piece was immersed in a stirred bath according to P1 at 80⁰C for 4 h. The rod was removed, rinsed with Dl water and blow-dried. The weight loss was found to be 380 g/m² and the wire piece was metallic and shiny by visual inspection, and only a small number of oxide scale spots remained (>90 % of the surface is metallic bright). A time dependence of the one-step pickling process according to E6 is given in Table 2. For the Comparative Examples the organic salt according to component (a) was 1 - hydroxyethyl-3-methylimidazolium chloride (C5) and choline chloride (C6) while component (b) was still non-hydrated iron(lll)chloride and the molar ratio of metal cations of component (b) to chloride ions that possibly originate from the respective organic salt according to component (a) was kept according to example P1.

The pickling results clearly show the detrimental effect of functional groups with protic reactivity such as alcohols and carboxylic acids that are linked with the organic cation. Ionic liquids that are composed of these interfering functional groups are consequently less preferred as components (a) according to this invention as they inhibit the performance with respect to weight loss and pickling result significantly.

Conditioning and Pickling Examples P2-P20:

1-ethyl-3-methylimidazolium chloride (Abbr.: emimCI; BASF AG, Germany) and non-hydrated iron (III) chloride were weighed at the given ratios according to Table 3 and mixed mechanically at room temperature until a brown liquid was formed.

AISI 304 non-annealed stainless steel wire pieces have been immersed in the ionic liquid compositions P2-P20 according to Table 3. Subsequently, the wire pieces were removed from the respective ionic liquid bath, rinsed with Dl water and blow-dried. Table 3:

Dependence of the weight loss on the molar ratio of the Lewis-acidic metal salt FeCb to the organic salt emimCI during the conditioning and pickling step of the process for the removal of oxide scale on AISI 304 stainless steel wire

It becomes evident that for the conditioning as well as for the pickling of oxide scale on stainless steel a molar ratio [FeCIs] : [emimCI] of less than 1 : 1 has to be preferably chosen in order to yield acceptable results as indicated by a considerable weight loss during the respective process step provided that at least a certain amount of FeC^ as the Lewis-acidic inorganic metal salt is present (see Figures 1 and 2).

Replenishment example R1 :

A used bath according to Example P1 was subjected to air bubbling at 100 ⁰C for 2 hours. The amount of contained Fe²⁺ after treatment was found by redox titration to be reduced from 2.7 % Fe²⁺ (relative to Fe³⁺) to 1.5% Fe²⁺, thus proving the concept of re-establishing the oxidizing action of the ionic liquid composition by means of air injection. Replenishment example R2:

A used bath according to Example P1 of total weight of 230 g was treated with 12ml_ of 30% aqueous hydrogen peroxide solution at room temperature and then agitated mechanically while heating at 120⁰C to evaporate water from the mixture. The amount of contained Fe²⁺ after treatment was found by redox titration to be reduced from approx. 18 % Fe²⁺ (relative to Fe³⁺) to 10 % Fe²⁺, thus proving the concept of re-establishing the oxidizing action of the ionic liquid composition by means of adding an oxidizing agent.

Claims

Claims:

1. A process for the removal of oxide scale on metal parts, wherein the metal part is brought into contact with a composition comprising an ionic liquid (IL1 ) composed of

(a) at least one organic salt as a source of inorganic Lewis-basic anions,

2. The process according to claim 1 , characterized in that the Lewis-basic anions that originate from components (a) that compose the ionic liquid (IL1 ) of the composition are selected from halides and/or pseudohalides.

3. The process according to one or both of the preceding claims, characterized in that the cations that originate from components (a) that compose the ionic liquid (IL1 ) of the composition are selected from cations of mono- or polyfunctional ammonium, iminium, imidazolium, pyrazolinium, pyridinium, phosphonium and/or sulfonium compounds.

4. The process according to claim 3, wherein the cations that originate from components (a) do not carry any hydrogen atoms bonded to the respective heteroatoms selected from nitrogen, phosphorus and/or sulfur.

5. The process according to one or both of the claims 3 and 4, characterized in that the composition comprises organic cations that originate from components (a) according to the following general formula (I)

6. The process according to one or more of the preceding claims, wherein the metal cation of the at least one Lewis-acidic inorganic metal salt according to component (b) that comprises a metal cation with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself is selected from those metal cations that have a standard reduction potential with respect to their metal cations in their lowest accessible positive oxidation state in the ionic liquid (IL1 ) higher than at least one standard reduction potential of the metal to be treated with respect to any oxidation state of its metal cations in an ionic liquid (IL2) that differs from the ionic liquid (IL1 ) only in that the component (b) Lewis-acidic metal salts comprise the metal cations in their lowest accessible positive oxidation state only.

7. The process according to claim 6, wherein the metal cation of the at least one Lewis-acidic inorganic metal salt according to component (b) that comprises a metal cation with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself is further selected from those metal cations that have none standard reduction potential with respect to a lower positive oxidation state of this metal element in the ionic liquid (IL1 ) that is higher than the standard reduction potential of the redox couple Cr^+I"/Cr^+Vl in an ionic liquid (IL2) that differs from the ionic liquid (IL1 ) only in that the component (b) Lewis-acidic metal salts comprise the metal cations in their lowest accessible positive oxidation state only.

8. The process according to one or more of the preceding claims, characterized in that the Lewis-acidic inorganic metal salts according to component (b) of the composition are selected from halides and/or pseudohalides of iron, cobalt, nickel, tin, vanadium, titanium, lead, cerium and/or manganese.

9. The process according to one or more of the preceding claims, characterized in that the composition comprises a molar ratio of metal cations of components (b) to Lewis-basic anions that originate from components (a) that is less than 1 : 1 , but not less than 1 : 100, preferably not less than 1 : 10.

10. The process according to one or more of the preceding claims, characterized in that the composition comprises a molar fraction of Lewis-acidic inorganic metal salts according to component (b) with an oxidation state of the metal more positive than the lowest accessible positive oxidation state of the metal element itself to the overall amount of components (b) of not less than 50 %, preferably not less than 80 % and more preferably not less than 90 %.

11. The process according to one or more of the preceding claims, characterized in that the composition contains

(i) 50-95 wt. -% of the ionic liquid composed of the components (a) and (b) (ii) 0.1 -50 wt.-% of auxiliary components (c) selected from flux agents, chelating agents, surfactants or dispersing agents, fillers, and/or thixotropic compounds, wherein the overall amount of components (a) and (b) and the auxiliary components (c) adds up to at least 95 wt.-%.

12. The process according to one or more of the preceding claims, characterized in that the treated metal part is subsequently brought into contact with an aqueous pickling solution.

13. A metal part treated in a process according to one or more of the claims 1 to 12.

14. A conditioning and/or pickling composition comprising an ionic liquid (IL) composed of

(a) at least one organic salt as a source of inorganic Lewis-basic anions, wherein the organic salts are selected from halides and/or pseudohalides of mono- or polyfunctional ammonium, iminium, imidazolium, pyrazolinium, pyhdinium, phosphonium and/or sulfonium compounds,

(b) one or more Lewis-acidic inorganic metal salts selected from halides and pseudohalides, wherein at least one Lewis-acidic inorganic metal salt according to component (b) comprises a metal cation with an oxidation state more positive than the lowest accessible positive oxidation state of the metal element itself, characterized in that the composition comprises a molar ratio of metal cations of components (b) to Lewis-basic anions that originate from components (a) that is less than 1 : 1.

15. A composition according to claim 14, characterized in that the at least one organic salt according to component (a) does not have any hydrogen atoms being bonded to the heteroatoms of the organic cations selected from nitrogen, phosphorous and/or sulfur.