US20070095435A1

US20070095435A1 - Colored conversion layers on metallic substrates

Info

Publication number: US20070095435A1
Application number: US11/593,690
Authority: US
Inventors: Olaf Lammerschop; Pavel Gentschev; Ulrich Jueptner
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 2004-05-07
Filing date: 2006-11-07
Publication date: 2007-05-03
Also published as: DE102004022565A1; CN1950543A; WO2005116294A1; EP1751327A1

Abstract

A chromium-free treatment solution for producing colored or fluorescing corrosion prevention layers on metal surfaces which contains phosphoric acid and/or at least one fluoro acid of one or more elements from the group consisting of B, Si, Ti, Zr and Hf or anions thereof and at least one organic polymer, the organic polymer being capable of binding to the metal surface through oxygen and/or nitrogen atoms, and having covalently bonded groups which appear colored to the human eye or which fluoresce discernibly to the human eye on exposure to UV light.

Description

CROSS-REFERENCE

This application is a continuation under 35 USC Sections 365 (c) and 120 of International Application No. PCT/EP2005/000769, filed Jan. 27, 2005 and published Dec. 8, 2005 as WO 2005/116294 which claims priority to DE 10 2004 022 565.6 filed May 7, 2004, each of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to the chemical surface treatment of zinc or galvanized steel, aluminium, magnesium or alloys thereof and, more particularly, to chromium-free conversion processes for such metal surfaces, i.e. chemical treatment processes which lead to the formation of a surface layer in which both cations of the treated metal surface and ions from the treatment solution are incorporated. The chromium-free coating is colored so that it is possible to tell by simple visual inspection whether an adequate conversion layer has been formed. The function of this conversion layer is to reduce the tendency of the metal surface towards corrosion and to establish good adhesion between the metal surface and an organic coating applied to the conversion layer, for example in the form of a paint or an adhesive.

DISCUSSION OF THE RELATED ART

Extensive prior art exists on the production of chromium-free conversion layers on the metal surfaces mentioned.
U.S. Pat. No. 5,129,967 discloses treatment baths for the no-rinse treatment (or “dried in place conversion coating”) of aluminium which contain
a) 10 to 16 g/l polyacrylic acid or homopolymers thereof,
b) 12 to 19 g/l hexafluorozirconic acid,
c) 0.17 to 0.3 g/l hydrofluoric acid and
d) up to 0.6 g/l hexafluorotitanic acid.
EP-B-8 942 discloses treatment solutions, preferably for aluminium cans, containing
a) 0.5 to 10 g/l polyacrylic acid or an ester thereof and
b) 0.2 to 8 g/l of at least one of the compounds H₂ZrF₆. H₂TiF₆and H₂SiF₆, the pH value of the solution being below 3.5.
Other polymers which may be used in similar treatment baths are disclosed in WO 02/20652.
U.S. Pat. No. 4,992,116 describes treatment baths for the conversion treatment of aluminium with pH values of about 2.5 to 5 which contain at least three components:

- a) phosphate ions in concentrations of 1.1×10⁻⁵to 5.3×10⁻³mol/l, corresponding to 1 to 500 mg/l,
- b) 1.1×10⁻⁵to 1.3×10⁻³mol/l of a fluoro acid of an element from the group consisting of Zr, Ti, Hf and Si (corresponding to 1.6 to 380 mg/l, depending on the element) and
- c) 0.26 to 20 g/l of a polyphenol compound obtainable by reaction of poly(vinylphenol) with aldehydes and organic amines.

WO 92/07973 teaches a chromium-free treatment process for aluminium which uses 0.01 to about 18% by weight of H₂ZrF₆and 0.01 to about 10% by weight of a 3-(N—C_1-4-alkyl-N-2-hydroxyethylaminomethyl)-4-hydroxystyrene polymer as essential components in an acidic aqueous solution. Optional components are 0.05 to 10% by weight of dispersed SiO₂, 0.06 to 0.6% by weight of a solubilizer for the polymer and surfactant. The polymer mentioned comes under the group of “reaction products of poly(vinylphenol) with aldehydes and organic amines containing hydroxyl groups” described in the following which suitable for use in accordance with the present invention.
WO 00/71626 discloses a chromium-free corrosion-inhibiting composition containing water and

- a.) 0.5 to 100 g/l hexafluoro anions of titanium(IV), silicon(IV) and/or zirconium(IV)
- b.) 0 to 100 g/l phosphoric acid
- c.) 0 to 100 g/l of one or more compounds of cobalt, nickel, vanadium, iron, manganese, molybdenum or tungsten
- d.) 0.5 to 30% by weight of at least one water-soluble or water-dispersible film-forming organic polymer or copolymer (based on active substance)
- e.) 0.1 to 10% by weight of an organophosphonic acid
- f.) optionally other auxiliaries and additives.

It is clear in many cases from the documents cited above that the conversion layers produced are colorless and transparent, so that the treated metal surfaces have a bright metallic appearance. At least it is not disclosed in those documents that colored layers would be formed. The lack of color in the prior art coatings is a drawback where, from many years' experience in the chromating of metal surfaces, the expert in this field is accustomed to obtaining a colored layer as the outcome of the conversion treatment. One is then immediately able to see whether the treatment has produced the desired result. In the production of colorless layers, however, this involves complicated surface analysis, for example determining the Ti or Zr content of the surface by X-ray fluorescence measurement. Accordingly, there is a need in practice for surface treatment processes which not only are comparable with conventional chromating layers in their properties in regard to corrosion prevention and paint adhesion, but are also visible to the human eye in the same way as chromating layers.
Proposals for solving this problem exist in the prior art. For example, WO 94/256450 describes a process for producing blue-colored conversion layers on zinc/aluminium alloys. In this process, the metal surfaces are contacted with a treatment solution which has a pH of 3.5 to 6 and which contains 0.2 to 3.0% by weight molybdenum and 0.1 to 2.0% by weight fluoride. The molybdenum may be used as molybdate, as phosphomolybdic acid, as molybdenum chloride and the like. The fluoride may be used in the form of hydrofluoric acid, simple fluorides and complex fluoro acids, such as fluorotitanic acid or fluorozirconic acid for example.
The teaching of WO 00/26437 goes the way of coloring the conversion layer with an organic dye (alizerin dye). The conversion layer itself is produced with a treatment solution containing complex fluorides, for example of titanium and zirconium, besides other inorganic oxides, hydroxides or carbonates or reaction products thereof with the fluoro acids. A poly-4-hydroxystyrene substituted by amino groups (polyvinyl phenol) may additionally be used as an organic polymer.
FR 2 461 764 proposes the chemical oxidation of the aluminium surface with organic nitro compounds in alkaline solution. After the oxidation step, the layers may be colored with an organic dye. A similar two-step process is proposed in WO 01/71060. This document describes a multilayer coating of a conversion layer (obtainable, for example, electrochemically and having pores) applied to the metal and a colored layer on the conversion layer. The second colored layer may be obtained by contact with a solution containing at least one alkoxysilane compound and a dye, followed by polymerization and/or crosslinking of the alkoxysilane compound.
The processes cited above for producing chromium-free colored conversion layers on metals, such as aluminium for example, may be divided into two groups. In the first group, transition metal compounds, such as molybdates or polymolybdates for example, or organic dyes are incorporated in the conversion layer. In the second group, a conversion layer is conventionally produced and the organic dyes are applied to the conversion layer already formed in a second step. There appears to be no example of a conversion treatment of metal surfaces carried out in the presence of an organic polymer which, on the one hand, improves the corrosion-inhibiting effect and paint adhesion of the conversion layer and which, on the other hand, carries color-bearing substituents, so that the presence of this polymer on the metal surface can be detected with the eye.
In the treatment of circulating waters, for example of industrial cooling systems, it is known that organic polymers can be added to prevent corrosion or deposits. In this case, the problem arises of monitoring the concentration of these polymers in the circuit water. To this end, EP 504 520 proposes covalently bonding dye residues to the organic polymers used for this purpose, so that the concentration of the polymers in the circuit water can readily be determined by an absorption or fluorescence measurement. This document discloses a number of organic dye residues and monomers which can be reacted with one another and polymerized to obtain polymers containing color-bearing groups. For example, these polymers may contain one or more of the following monomers: acrylic acid, acrylamide, sulfomethacrylamide, vinyl acetate, methacrylic acid or acrylonitrile.

BRIEF SUMMARY OF THE INVENTION

The problem addressed by the present invention was to provide compositions and processes for producing chromium-free conversion layers on metal surfaces, conversion layers appearing colored to the human eye or visibly fluorescing being produced in a single step. The dye used would not be independent of the actual active conversion components because, otherwise, it could not be guaranteed that the intensity of the coloring or fluorescence would correlate with the thickness of the conversion layer. In Applicants' invention, the coloring agent is covalently bonded to the polymer forming the conversion layer and hence the color of the coating is directly related to the amount of the coating, e.g. the thickness of the coating, on the substrate.
This problem has been solved by a chromium-free treatment solution for producing colored or fluorescing corrosion prevention layers on metal surfaces which contains:

- a) phosphoric acid and/or at least one fluoro acid of one or more elements from the group consisting of B, Si, Ti, Zr and Hf or anions thereof and
- b) at least one organic polymer, the organic polymer being capable of binding to the metal surface through oxygen and/or nitrogen atoms,
  characterized in that the organic polymer carries covalently bonded groups which appear colored to the human eye or which fluoresce discernibly to the human eye on exposure to UV light.

In one embodiment, the invention provides a treatment solution for producing colored corrosion prevention layers on metal surfaces which comprises:
a) phosphoric acid and/or at least one fluoro acid of one or more elements selected from the group consisting of B, Si, Ti, Zr and Hf or anions thereof; and
b) at least one organic polymer capable of binding to a metal surface through oxygen and/or nitrogen atoms and selected from the group consisting of amino resins, phenol/aldehyde resins, polymers containing carboxyl groups, polymeric alcohols, esterification products of polymeric alcohols with polymers containing carboxylic acid groups, polymers containing amino groups, homo- or copolymers of vinyl pyrrolidone, and copolymers of alkylene phosphonic or alkylene phosphinic acids and one or more unsaturated carboxylic acids, wherein the treatment solution is chromium-free and the organic polymer carries covalently bonded groups which appear colored to the human eye.
Desirably the treatment solution comprises 0.02 to 20 g/l phosphoric acid and/or at least one fluoro acid of one or more elements from the group consisting of B, Si, Ti, Zr and Hf or anions thereof and 0.1 to 200 g/l of the organic polymer.
In another embodiment the treatment solution comprises: a) phosphoric acid present in a concentration of at least 1 g/l; and b) organic polymer present in a concentration of at least 5 g/l and not more than 150 g/l.
Desirably the percentage by weight of covalently bonded groups which appear colored to the human eye amounts to 0.1 to 20% by weight, based on the total weight of the organic polymer. In one embodiment, the covalently bonded groups which appear colored to the human eye are selected from Toluidine Blue and Neutral Red.
In one embodiment, the organic polymer has an upper molecular weight limit of 20,000,000 Dalton and an average molecular weight of greater than 100,000 Dalton.
In a preferred embodiment the organic polymer is selected from the group consisting of polymers of acrylate and/or methacrylate monomers, maleic anhydride which may be completely or partly hydrolyzed, and acrylate or methacrylate monomers terminated by phosphate groups.
Desirably the treatment solution may contain d) at least one inorganic compound in particle form with a mean particle diameter in the range from 0.005 to 0.2 μm; the inorganic compound in particle form may be present in the aqueous bath solution in a concentration of 0.1 to 80 g/l.
In one embodiment of the treatment solution, the organic polymer is a copolymer of a) acrylic acid and/or methacrylic acid with b) acrylate and/or methacrylate monomers terminated by phosphate groups which contains covalently bonded molecules of Toluidine Blue or Neutral Red.
In a one embodiment, the treatment solution comprises at least 0.025 g/l and up to 10 g/l Ti and/or Zr and/or Si ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6. The treatment solution may comprise both fluoro acids of Ti and of Zr, wherein the molar ratio of Ti to Zr is in the range from 10:1 to 1:10.
In another aspect of the invention, it is an object to provide a process for producing colored or fluorescent corrosion prevention layers on metal surfaces, comprising contacting the metal surfaces for 0.5 to 10 minutes with the treatment solution as described and/or claimed herein which has a temperature of 20 to 80° C. In one embodiment, the metal surfaces treated are selected from surfaces of aluminium and aluminium alloys, magnesium and magnesium alloys, titanium and titanium alloys, zinc and zinc alloys, galvanized and alloy-galvanized steel.
In yet another aspect of the invention, it is an object to provide a metal part having at least one surface comprising a colored corrosion prevention layer produced according to the process described and/or claimed herein.
Another embodiment of the invention provides a treatment solution for producing colored corrosion prevention layers on metal surfaces which comprises:
a) 0.02 to 20 g/l phosphoric acid and/or at least one fluoro acid of one or more elements from the group consisting of B, Si, Ti, Zr and Hf or anions thereof and
b) 0.1 to 200 g/l of at least one organic polymer capable of binding to a metal surface through oxygen and/or nitrogen atoms and selected from the group consisting of maleic anhydride/methyl vinyl ether copolymer and acrylic acid and methacrylate monomer terminated by phosphate groups; wherein the treatment solution is chromium-free and the organic polymer carries 0.1 to 20% by weight, based on the total weight of the organic polymer, of covalently bonded groups which appear colored to the human eye.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a treatment solution for producing colored corrosion prevention layers on metal surfaces which comprises:
a) phosphoric acid and/or at least one fluoro acid of one or more elements selected from the group consisting of B, Si, Ti, Zr and Hf or anions thereof; and
b) at least one organic polymer capable of binding to a metal surface through oxygen and/or nitrogen atoms and selected from the group consisting of amino resins, phenol/aldehyde resins, polymers containing carboxyl groups, polymeric alcohols, esterification products of polymeric alcohols with polymers containing carboxylic acid groups, polymers containing amino groups, homo- or copolymers of vinyl pyrrolidone, and copolymers of alkylene phosphonic or alkylene phosphinic acids and one or more unsaturated carboxylic acids, wherein the treatment solution is chromium-free and the organic polymer carries covalently bonded groups which appear colored to the human eye.
The treatment solution is, by definition, free from chromium, i.e. does not contain any intentionally added chromium compounds. However, traces of chromium as impurities cannot be ruled out, for example as a result of being dissolved out from the tank material. In addition, it preferably does not contain any other heavy metal ions than those of component a). This reduces the demands on the treatment of the wastewaters accumulating.
In particular, the treatment solution is not intended to deposit a complete layer-forming phosphate coating. The solution does not contain more than 3 g/l phosphoric acid and more than 0.3 g/l zinc and/or manganese ions at one and the same time. The deposition of zinc and/or manganese phosphate crystals on the metal surface is avoided in this way. This would correspond to a more or less complete layer-forming phosphating which is unwanted in the context of the present invention.
A preferred treatment solution is characterized in that it contains

- a) 0.02 to 20 g/l phosphoric acid and/or at least one fluoro acid of one or more elements from the group consisting of B, Si, Ti, Zr and Hf or anions thereof and
- b) 0.1 to 200 g/l of the organic polymer.

If the treatment solution contains phosphoric acid, its minimum concentration is preferably at least 1 g/l and, more particularly, at least 5 g/l. An upper concentration limit of 15 g/l is generally sufficient.
If the treatment solution contains a fluoro acid of boron, for example tetrafluoroboric acid, as component a), its concentration is preferably selected so that the treatment solution contains at least 0.01 g/l, preferably at least 0.02 g/l and up to 5 g/l, preferably up to 1 g/l and more particularly up to 0.5 g/l boron (expressed as element). If it contains fluoro acids of hafnium, for example as hexafluoro acid, its concentration is preferably selected so that the treatment solution contains at least 0.02 g/l, preferably at least 0.5 g/l and up to 10 g/l, preferably up to 5 g/l and more particularly up to 1 g/l hafnium.
Particularly preferred treatment solutions contain fluoro acids of Si, Ti and/or Zr as component a). A treatment solution is preferably used which contains in all at least 0.01 g/l, preferably at least 0.025 g/l and up to 10 g/l, preferably up to 1 g/l and more particularly up to 0.5 g/l Ti and/or Zr and/or Si ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6. The Ti, Zr and/or Si ions mentioned may be completely used in the form of hexafluoro complexes such as, for example, the hexafluoro acids or their salts soluble in water in the concentration range mentioned, such as the sodium salts for example. In this case, the atomic ratio is 1:6. However, complex compounds where less than six fluoride ions are attached to the central elements Ti, Zr or Si may also be used. These may be spontaneously formed in the treatment solution if both hexafluoro complexes of at least one of the central elements Ti, Zr or Si and at least one other compound of one of these central elements are added to the treatment solution. Suitable other compounds are, for example, nitrates, carbonates, hydroxides and/or oxides of the same central element or of another of the three central elements mentioned. For example, the treatment solution may contain hexafluorozirconate ions and (preferably colloidal) silica (SiO₂) or reaction products thereof. Unreacted silica may be suspended in the treatment solution. A treatment solution such as this may also be obtained by using hydrofluoric acid or (optionally acidic) salts thereof together with compounds of Ti, Zr and/or Si which are capable of forming fluoro complexes therewith. Examples are the already mentioned nitrates, carbonates, hydroxides and/or oxides. A preferred embodiment is characterized in that, in all, such a quantity of Ti, Zr and/or Si as the central metal and such a quantity of fluoride are used that the atomic ratio of central metal to fluoride is 1:2 or lower, more particularly 1:3 or lower. The atomic ratio may even be lower than 1:6 if the treatment solution contains more fluoride, for example in the form of hydrofluoric acid or salts thereof, than is stoichiometrically necessary for forming the hexafluoro complexes of the central metals Ti, Zr and/or Si. For example, the atomic ratio may be as low as 1:12 or 1:18 or even lower if a corresponding excess of fluoride is used, i.e. two, three or even more times the quantity required for complete formation of the hexafluoro complexes.
Treatment solutions containing fluoro acids of Ti and/or Zr are particularly preferred. It can be particularly beneficial to the corrosion-inhibiting effect if the treatment solution contains both fluoro acids of Ti and of Zr. In this case, the molar ratio of Ti to Zr may be in the range from 10:1 to 1:10 and, more particularly, is in the range from 2:1 to 1:2.
The pH of the treatment solution should not be significantly below a value of 1 because lower pH values lead to an increasingly stronger corrosive attack on the metal surface. The pH is preferably no lower than 2 and, more particularly, no lower than 2.5. At pH values above 6, the conversion layer is no longer formed to the desired extent. The treatment is preferably carried out at pH values no higher than 4 and, more particularly, no higher than 3.5.
The essential components a) and optionally b) mentioned are all protolytes, i.e. molecules or ions which are capable of entering into an acid/base reaction in which protons are released or taken up. Accordingly, it is clear to the expert that these components enter into such reactions with one another and with the solvent, water, until the corresponding chemical equilibria are reached. In the pH range mentioned, all these protolytes can be expected to be present in a partly proteolyzed state, irrespective of whether they have been introduced into the treatment solution in the form of their acids or their salts. The pH values mentioned, which are in the acidic range, are preferably established by introducing the phosphoric acid and/or complex fluoro acids in the form of the free acids. There is therefore no need for an additional acid for establishing the acidic pH value. However, these complex fluoride ions or phosphoric acid could even be used in the form of their salts and the desired pH value could be established by addition of another acid, such as nitric acid for example. Alkalis, such as NaOH or ammonia for example, are suitable for raising the pH value if necessary.
The concentration of the organic polymer b) in the treatment solution is preferably in the range from 0.1 to 200 g/l. The particularly preferred minimum concentration is—with increasing preference—at least 1 g/l, at least 5 g/l, at least 10 g/l, at least 15 g/l. The upper concentration limit is—with increasing preference—150 g/l, 100 g/l, 70 g/l.
So far as the concentrations of the active components a) and b) are concerned, it may generally be said that the quality of the layer gradually diminishes if the concentration falls below the minimum value mentioned. Any increase in concentration beyond the maximum mentioned is not normally harmful, but does not afford any significant advantages and is therefore uneconomical.
In the organic polymer b), the percentage by weight of covalently bonded groups which appear colored to the human eye or which fluoresce visibly to the human eye on exposure to UV light is at least 0.1, preferably at least 0.5 and more particularly at least 1% by weight and up to 20, preferably up to 15 and more particularly up to 10% by weight, based on the total weight of the organic polymer.
The organic polymer b) may be selected from various groups. The organic polymer is preferably selected from epoxy resins, amino resins, phenol/aldehyde resins, polymers containing carboxylic acid groups, polymeric alcohols, esterification products of polymeric alcohols with polymers containing carboxylic acid groups, polymers containing amino groups, homo- or copolymers of vinyl pyrrolidone and from polymers containing phosphinic acid, phosphonic acid or phosphoric acid ester groups.
In the case of phenol/aldehyde resins, formaldehyde resins are preferred. Particularly suitable polymers containing amino groups are amino-substituted poly-4-vinylphenol compounds. Examples of such poly-4-vinylphenol compounds without covalently bonded color-bearing groups can be found in the WO 00/26437 cited above and the literature cited therein and, more particularly, in U.S. Pat. No. 5,281,282.
In addition, the organic polymers b) may be selected in regard to their polymeric backbone from:

- e) polyvinyl alcohol or water-soluble or water-dispersible partial esters thereof,
- f) polymers or copolymers of unsaturated mono- or dicarboxylic acid esters or amides thereof
- g) esters of the group e) and group f) polymers,
- h) polymers or copolymers of vinyl pyrrolidone,
- i) polymers of diglycidyl ether of bisphenol A,
- k) copolymers of alkylene phosphonic or alkylene phosphinic acids and one or more unsaturated carboxylic acids.

Referring to the group e) polymer, the expression “partial ester” means that the alcohol groups are only partly esterified, the ester being formed with non-polymeric carboxylic acids. In particular, the esterification may be carried out with monobasic C_1-4carboxylic acids.
The group f) polymers or copolymers may be selected, for example, from homo- or copolymers of acrylic acid and/or methacrylic acid of which the acid groups may be partly replaced by amide groups or esterified with alcohols, more particularly with simple C_1-4alcohols. Special examples are homopolymers or copolymers of or with methyl methacrylate, n-butyl acrylate, hydroxyethyl acrylate and glycerol propoxytriacrylate. These special examples are known, for example, from WO 95/14117. In addition, the group f) polymers may be selected from polymers containing maleic acid monomers. A special example of this is a maleic acid/methyl vinyl ether copolymer.
Group e) polymers generally contain free alcohol groups while group f) polymers generally contain free carboxylic acid groups. Accordingly, these two polymers may not only be used in admixture with one another, but also in a form in which at least partial esterification has taken place between the alcohol groups of polymer e) and the carboxylic acid groups of polymer f). This is explained in more detail in WO 94/12570. The teaching described therein may also be used for the purposes of the present invention.
In addition, the treatment solution may contain group h) polymers. Such polymers and their use in conversion treatment solutions are described in detail in DE-A-100 05 113 and in DE-A-101 31 723.
The additional polymers may also be selected from those of group i), as described in more detail in U.S. Pat. No. 5,356,490.
Particularly preferred polymers are organic polymers b) which are selected in regard to their polymeric backbone from polymers or copolymers of acrylate and/or methacrylate monomers, maleic anhydride which may be completely or partly hydrolyzed (more particularly methyl vinyl ether/maleic anhydride copolymers) and acrylate or methacrylate monomers terminated by phosphate groups. Special examples of this can be found in the experimental section of this application.
The covalently bonded groups in the organic polymer b), which appear colored to the human eye or which fluoresce discernibly to the human eye on exposure to UV light, are preferably selected from Toluidine Blue (CAS No. 92-31-9), whose chemical name is 3-amino-7-(dimethylamino)-2-methylphenothiazin-5-ium chloride, and from Neutral Red (CAS No. 553-24-2), whose chemical name is 3-amino-7-(dimethylamino)-2-methyl phenazine.
The average molecular weight of the organic polymers a) is preferably at least 10,000 Dalton. The upper molecular weight limit is not critical as long as the polymer is soluble or dispersible in the desired concentration range in the preferably acidic treatment solution. For example, the upper molecular weight limit can be 50,000,000, preferably 20,000,000 and more particularly 10,000,000 Dalton. An upper limit of 5,000,000 Dalton can also be sufficient. The average molecular weight is preferably above 50,000 Dalton and more particularly above 100,000 Dalton. The average molecular weights can be determined, for example, by gel permeation chromatography with polyethylene glycol as standard.
In a particularly preferred embodiment, the organic polymer b) is a copolymer of i) acrylic acid and/or methacrylic acid with ii) acrylate and/or methacrylate monomers terminated by phosphate groups, the copolymer containing covalently bonded molecules of Toluidine Blue or Neutral Red. The production of this polymer is described by way of example in Examples 3 and 10. These polymers are examples of polymers which contain both carboxylate and phosphonic acid groups or phosphoric acid ester groups. By virtue of their particularly high affinity for metal surfaces, these polymers with their various polar adhesion groups are particularly preferred. These particularly preferred copolymers can also be obtained, for example, by reaction of acrylic acid and/or methacrylic acid with other unsaturated phosphonic acids, phosphinic acids or phosphoric acid esters. For example, vinyl phosphonic acid may be used as an unsaturated monomer containing a phosphonic acid group.
Applicants were unable to find any prior-art document which described the particularly preferred copolymer of i) acrylic acid and/or methacrylic acid with ii) acrylate and/or methacrylate monomers terminated by phosphate groups which contains covalently bonded molecules of Toluidine Blue or Neutral Red. Accordingly, the present invention also relates to this copolymer itself. The foregoing observations again apply in regard to the preferred molecular weights. Similarly, the foregoing observations on the percentage by weight of dye groups, based on the total weight of the polymer, again apply. The ratio by weight between monomers i) and ii) is preferably in the range from 10:1 to 1:10, more preferably in the range from 5:1 to 1:5 and most preferably in the range from 3:1 to 1:3.
Another preferred polymer for the treatment solution according to the invention is a methyl vinyl ether/maleic anhydride copolymer to which molecules of Toludine Blue or Neutral Red are covalently bonded and of which the anhydride groups are at least partly hydrolyzed, preferably at least 90% hydrolyzed. The molar ratio of the monomeric methyl vinyl ether and maleic anhydride is preferably in the range from 10:1 to 1:10, more preferably in the range from 5:1 to 1:5 and most preferably in the range from 1:2 to 2:1. For example, the molar ratio can be substantially 1:1.
In addition, the aqueous treatment solution may contain as an additional component i) a total of 1 to 2,000 mg/l of one or more chelating complexing agents which do not come under the definition of the organic polymers of group b). The chelating complexing agent c) is preferably not polymeric and is preferably selected from molecules containing two or more phosphonic acid groups, more particularly from geminal diphosphonic acids and phosphonocarboxylic acids and anions thereof. (As explained above, the corresponding acid/base equilibrium between the acid form and the salt form of the complexing agent is established by itself in the treatment solution and in the concentrate, depending on the pH value, irrespective of the form in which it was introduced into the solution or the concentrate.)
Examples of such complexing agents are those which may also be used in accordance with DE-A-103 39 165 to which reference is hereby made.
The properties of the conversion layer obtained may be further improved where necessary if the treatment solution (irrespective of whether or not component c) is also present) contains an inorganic compound in particle form with a mean particle diameter, as measured with a scanning electron microscope, of 0.005 to 0.2 μm as an additional component d).
The inorganic compound in particle form is present in the aqueous bath solution in a concentration of 0.1 to 80 g/l, preferably in a concentration of 0.2 to 25 g/l, more preferably in a concentration of 0.5 to 10 g/l and most preferably in a concentration of 1 to 4 g/l.
The ratio of the contents of organic polymer b) to the contents of inorganic compounds in particle form in the aqueous bath solution may vary within wide limits and, in one particular embodiment, may be <3.8:1. This ratio is preferably in the range from 0.05:1 to 3.5:1 and more particularly in the range from 0.18:1 to 2.5:1.
In the process according to the invention, a fine-particle powder, a dispersion or a suspension, such as for example a carbonate, an oxide, a silicate or a sulphate, more particularly colloidal or amorphous particles, is added as the inorganic compound in particle form. Particles based on at least one compound of aluminium, barium, cerium, calcium, lanthanum, silicon, titanium, yttrium, zinc and/or zirconium and especially particles based on aluminium oxide, barium sulphate, cerium dioxide, rare earth mixed oxide, silicon dioxide, silicate, titanium oxide, yttrium oxide, zinc oxide and/or zirconium oxide are particularly preferred as the inorganic compound in particle form. The at least one inorganic compound in particle form is present in the form of particles with a mean particle size of preferably 6 nm to 150 nm, more preferably 7 to 120 nm, most preferably 8 to 90 nm and, in one even more preferred embodiment, 8 to 60 nm up to 25 nm. Relatively large particles preferably have a platelet-like or elongate shape.
In cases where metallic substrates coated in accordance with the invention and optionally provided with paint or paint-like coatings are to be welded, it can be of advantage to use particles of relatively high or high electrical conductivity as the particles of the compound in particle form, more particularly particles of oxides, phosphates, phosphides or sulphides of aluminium, iron or molybdenum, more particularly aluminium phosphide or iron oxide, iron phosphide, at least one molybdenum compound, such as molybdenum sulphide, graphite and/or carbon black. These particles may even have such an average particle size that they may project slightly further out from the layer produced in accordance with the invention
The present invention also relates to a process for producing colored or fluorescent corrosion prevention layers on metal surfaces, characterized in that the metal surfaces are contacted for 0.5 to 10 minutes with the treatment solution claimed in one or more of claims 1 to 8 which has a temperature of 20 to 80° C.
The metal surface may be contacted with the treatment solution by standard methods such as, for example, immersion, spraying, a combination of spraying and immersion, roller application, etc. After contact with the treatment solution, the metal surfaces are preferably rinsed with water, more particularly with deionized water, and then dried by standard methods. The contact time is preferably at least 1 minute, more particularly at least 2 minutes. A contact time of up to 7 minutes, for example up to 5 minutes, is generally sufficient.
A no-rinse treatment, i.e. application (by roller, spraying and squeezing) without rinsing, is also possible. The temperature of the treatment solution is preferably at least 30° C., for example 35° C. The temperature of the treatment solution generally need not exceed an upper limit of 60° C. and more particularly 50° C.
The metal surfaces which can be treated by the process according to the invention are preferably selected from surfaces of aluminium and aluminium alloys, magnesium and magnesium alloys, titanium and titanium alloys, zinc and zinc alloys, galvanized or alloy-galvanized steel. The metal surfaces may be surfaces of the above-mentioned metals or their alloys as such or even surfaces of a substrate such as, for example, steel coated with the above-mentioned metals or their alloys. Examples of the latter are electrolytically galvanized or hot-dip-galvanized steel, aluminized steel or coated steels, such as Galvalume® or Galfan®, which carry a coating of zinc/aluminium alloys.
Finally, the present invention relates to a metal part of which the surface has a colored or fluorescent corrosion prevention layer obtainable by the process according to the invention. The foregoing observations on the particularly preferred metals apply accordingly. The metal part may carry the colored or fluorescent corrosion prevention layer obtainable by the process according to the invention as a single coating or as an outermost coating. However, the process according to the invention generally serves as a pretreatment for another coating, for example a coating with a paint or even with an adhesive if the treated metal parts are to be bonded to one another or to other substrates. Accordingly, the metal part according to the invention may have a paint as the outermost layer or may be bonded to another metal part according to the invention or even to another substrate.
The treatment step according to the invention is generally part of a treatment sequence such as typically applied in the conversion treatment of the above-mentioned metal surfaces before subsequent coating or bonding. A corresponding process sequence generally begins with cleaning/degreasing of the metal surfaces, for which an alkaline cleaner, for example, can be used. This is followed by one or more rinsing steps with water. These in turn are followed by an acidic treatment step for removing surface oxides that were not removed in the alkaline cleaning step. This step is also known as “deoxidizing” or “scouring” and is applied in particular to surfaces of aluminium and its alloys. After an intermediate rinse with water and preferably an additional rinse with deionized water, the treatment step according to the invention is carried out with the treatment solution according to the invention. This may be followed by another rinse with water. However, the process may also be carried out as a no-rinse process, i.e. there is no need for rinsing with water after the treatment step according to the invention.
The outcome of this treatment sequence is a colored or fluorescent, corrosion-protected metal surface which shows good adhesion to a subsequently applied layer based on organic polymers, for example a paint or an adhesive. This surface generally contains 1 to 70 mg titanium and/or zirconium per m²and more particularly 3 to 30 mg/m²where the fluoro acids of these metals were used as component a). These values may be measured by standard surface analysis techniques, for example by X-ray fluorescence methods.
Accordingly, the process according to the invention provides metal surfaces characterized by good corrosion protection and good paint adhesion which the expert knows to be the outcome of the technically highly advantageous, but ecologically and physiologically unsafe chromating processes. By virtue of the coloring of the surface, the expert is able immediately to see whether an adequate conversion layer—to which he is accustomed from chromating—has been formed during the treatment. Accordingly, the process according to the invention has the technical advantage over the formation of colorless conversion layers that the outcome of the treatment is immediately visible without any need for special surface analysis.

EXAMPLES

Preparing the Composition

Example 1

Production of a Toluidine Blue Monomer

3 g Toluidine Blue (CAS No. 92-31-9) were dissolved in 50 ml water and mixed with 200 ml pyridine. 1.1 g methacrylic acid chloride were added dropwise to the solution while cooling with ice, followed by stirring for one hour and 3 hours at 40° C. The solvent was distilled off in vacuo. 3.8 g of the crude product were obtained.

Example 2

Production of a Neutral Red Monomer

3 g Neutral Red (CAS No. 553-24-2) were dissolved in 50 ml water and mixed with 200 ml pyridine. 1.1 g methacrylic acid chloride were added dropwise to the solution while cooling with ice, followed by stirring for one hour and 3 hours at 40° C. The solvent was distilled off in vacuo. 3.7 g of the crude product were obtained.

Example 3

Polymerization I

0.2 g Toluidine Blue monomer, 13.8 g acrylic acid and 6 g methacrylate monomer terminated by phosphate groups (Sipomer PAM 100, a product of Rhodia) were dissolved in 300 ml water and freed from oxygen by introduction of nitrogen for 30 mins. at 50° C. The polymerization was then started by addition of 0.1 g V-44 (initiator). The solution was stirred for 24 hours at 50° C. and polymerization was restarted with another 0.1 g V-44 and continued for another 24 hours. Polymerization was restarted by adding 0.1 g V-50 (initiator), increasing the reaction temperature to 80° C. and stirring for another 4 hours. The colored polymer solution was dialyzed for 48 hours (dialysis tube with a cutoff limit of 10,000 Dalton). After dialysis, 670 ml of a 2.4% by weight of covalently bonded groups, colored solution were obtained, corresponding to a yield of 80%.

Example 4

Polymerization II

0.2 g Toluidine Blue monomer, 13.8 g acrylic acid and 6 g vinyl phosphonic acid were dissolved in 300 ml water and freed from oxygen by introduction of nitrogen for 30 mins. at 50° C. The polymerization was then started by addition of 0.1 g V-44 (initiator). The solution was stirred for 24 hours at 50° C. and polymerization was restarted with another 0.1 g V-44 and continued for another 24 hours. Polymerization was restarted by adding 0.1 g V-50 (initiator), increasing the reaction temperature to 80° C. and stirring for another 4 hours. The colored polymer solution was dialyzed for 48 hours (dialysis tube with a cutoff limit of 10,000 Dalton). After dialysis, 470 ml of a 3% by weight of covalently bonded groups, colored solution were obtained, corresponding to a yield of 70%. An average molecular weight M_wof 45,000 was determined by gel permeation chromatography. The GPC column was calibrated with a polyacrylate standard.

Example 5

Reaction of Toluidine Blue with Methyl Vinyl Ether/Maleic Anhydride Copolymers

0.1 g Toluidine Blue were dissolved in 300 ml of a water/ice mixture. 10 g maleic anhydride/methyl vinyl ether copolymer (Gantrez AN-119, a product of GAF General Aniline Firm Corp.) (M_w=216,000 g/mol) were then added with vigorous stirring, followed by stirring for 24 hours. For complete hydrolysis of the anhydride groups, the solution was heated for 2 hours to 80° C. To remove the unreacted dye molecules, the solution was dialyzed for 48 hours (dialysis tube with a cutoff limit of 10,000 Dalton). After dialysis, 400 ml of a 0.31% by weight of covalently bonded groups, colored solution were obtained.

Example 6

Reaction of Toluidine Blue with Methyl Vinyl Ether/Maleic Anhydride Copolymers

0.1 g Toluidine Blue were dissolved in 380 ml of a water/ice mixture. 10 g maleic anhydride/methyl vinyl ether copolymer (Gantrez AN-139, a product of GAF General Aniline Firm Corp.) (M_w=1,080,000 g/mol) were then added with vigorous stirring, followed by stirring for 24 hours. For complete hydrolysis of the anhydride groups, the solution was heated for 2 hours to 80° C. To remove the unreacted dye molecules, the solution was dialyzed for 48 hours (dialysis tube with a cutoff limit of 10,000 Dalton). After dialysis, 400 ml of a 2.34% by weight of covalently bonded groups, colored solution were obtained.

Example 7

Reaction of Toluidine Blue with Methyl Vinyl Ether/Maleic Anhydride Copolymers

0.1 g Toluidine Blue were dissolved in 300 ml of a water/ice mixture. 10 g maleic anhydride/methyl vinyl ether copolymer (Gantrez AN-169, a product of GAF General Aniline Firm Corp.) (M_w=1,980,000 g/mol) were then added with vigorous stirring, followed by stirring for 24 hours. For complete hydrolysis of the anhydride groups, the solution was heated for 2 hours to 80° C. To remove the unreacted dye molecules, the solution was dialyzed for 48 hours (dialysis tube with a cutoff limit of 10,000 Dalton). After dialysis, 400 ml of a 2.4% by weight of covalently bonded groups, colored solution were obtained.

Example 8

Reaction of Toluidine Blue with Methyl Vinyl Ether/Maleic Anhydride Copolymers

0.4 g Toluidine Blue were dissolved in 300 ml of a water/ice mixture. 10 g maleic anhydride/methyl vinyl ether copolymer (Gantrez AN-139, a product of GAF General Aniline Firm Corp.) (M_w=1,080,000 g/mol) were then added with vigorous stirring, followed by stirring for 24 hours. For complete hydrolysis of the anhydride groups, the solution was heated for 2 hours to 80° C. To remove the unreacted dye molecules, the solution was dialyzed for 48 hours (dialysis tube with a cutoff limit of 10,000 Dalton). After dialysis, 400 ml of a 2.24% by weight of covalently bonded groups, colored solution were obtained.

Example 9

Reaction of Neutral Red with Methyl Vinyl Ether/Maleic Anhydride Copolymers

0.1 g Neutral Red were dissolved in 380 ml of a water/ice mixture. 10 g maleic anhydride/methyl vinyl ether copolymer (Gantrez AN-139, a product of GAF General Aniline Firm Corp.) (M_w=1,080,000 g/mol) were then added with vigorous stirring, followed by stirring for 24 hours. For complete hydrolysis of the anhydride groups, the solution was heated for 2 hours to 80° C. To remove the unreacted dye molecules, the solution was dialyzed for 48 hours (dialysis tube with a cutoff limit of 10,000 Dalton). After dialysis, 400 ml of a 2.1% by weight of covalently bonded groups, colored solution were obtained.

Example 10

Polymerization III

0.2 g Neutral Red monomer, 13.8 g acrylic acid and 6 g methacrylate monomer terminated by phosphate groups (Sipomer PAM 100, a product of Rhodia) were dissolved in 550 ml water and freed from oxygen by introduction of nitrogen for 30 mins. at 50° C. The polymerization was then started by addition of 0.1 g V-44 (initiator). The solution was stirred for 24 hours at 50° C. and polymerization was restarted with another 0.1 g V-44 and continued for another 24 hours. Polymerization was restarted by adding 0.1 g V-50 (initiator), increasing the reaction temperature to 80° C. and stirring for another 4 hours. The colored polymer solution was dialyzed for 48 hours (dialysis tube with a cutoff limit of 10,000 Dalton). After dialysis, 650 ml of a 2.39% by weight of covalently bonded groups, colored solution were obtained, corresponding to a yield of 77%.
Pretreatment of Aluminium Substrates with Chromium-Free Polymer Solution
Test plates of the aluminium alloy A199.5 were subjected to the following standard pretreatment before the treatment step according to the invention:

- Cleaning: spraying with an alkaline cleaner (Ridoline® C 72, 2%, a product of Henkel KGaA), 65° C., 1 minute
- Rinsing: tap water, room temperature, 1 minute
- Rinsing: deionized water, room temperature, 1 minute

This was followed by the treatment step according to the invention at pH 2.7, carried out by dipping as described below.
After the conversion treatment step, the plates were dried at 60° C. in a recirculating air oven and then coated with a commercially available powder coating. A standard paint adhesion test was then carried out. The test plates were provided with a cross-hatch cut and then stored for 2 hours in boiling deionized water and then for one hour at room temperature. An adhesive tape was then applied to the cross-hatched area and peeled off. The amount of paint removed from the test plate was evaluated and characterized by cross-hatch scores: Ch 0: no paint loss, Ch 5: extensive paint loss. In addition, paint creepage was tested by the salt spray test.
The compositions of the treatment solutions and the treatment results are set out in the following:

Example 11

Composition of a 100 G Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 6) 2.55 g

Water 97.35 g

Dipping of aluminium plates in pretreatment after
a) 2 mins.: plates assumed a light blue shimmer after drying
b) 5 mins.: plates assumed a clearly visible blue tone after drying

Example 12

Composition of a 100 g Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 6) 1 g

Water 98.9 g

Dipping of aluminium plates in pretreatment after
a) 2 mins.: plates assumed a very light blue shimmer after drying
b) 5 mins.: plates assumed a very light blue shimmer after drying

Example 13

Composition of a 100 g Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 6) 1.25 g

Water 98.65 g

Dipping of aluminium plates in pretreatment after
a) 2 mins.: plates assumed a light blue shimmer after drying
b) 5 mins.: plates assumed a light blue shimmer after drying

Example 14

Composition of a 100 g Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 7) 3.6 g

Water 96.3 g

Dipping of aluminium plates in pretreatment after
a) 2 mins.: plates assumed a dark blue tone after drying
b) 5 mins.: plates assumed a dark blue tone after drying

Example 15

Composition of a 100 g Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 7) 1.8 g

Water 98.1 g

Dipping of aluminium plates in pretreatment after
a) 2 mins.: plates assumed a clearly visible blue tone after drying
b) 5 mins.: plates assumed a clearly visible blue tone after drying

Example 16

Composition of a 100 g Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 7) 0.9 g

Water 99 g

Dipping of aluminium plates in pretreatment after
a) 2 mins.: plates did not assume a blue tone after drying
b) 5 mins.: plates assumed a very light blue shimmer after drying

Example 17

Composition of a 100 G Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 3) 3.53 g

Water 96.37 g

Dipping of Aluminium Plates in Pretreatment after
a) 2 mins.: plates assumed a clearly visible blue tone after drying
b) 5 mins.: plates assumed a clearly visible blue tone after drying

Example 18

Composition of a 100 G Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 3) 1.8 g

Water 98.1 g

Dipping of aluminium plates in pretreatment after
a) 2 mins.: plates assumed a very light blue shimmer after drying
b) 5 mins.: plates assumed a light blue shimmer after drying

Example 19

Composition of a 100 g Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 10) 3.53 9

Water 96.37 g

Dipping of aluminium plates in pretreatment after
a) 2 mins.: plates assumed a clearly visible red tone after drying
b) 5 mins.: plates assumed a clearly visible red tone after drying

Example 20

Composition of a 100 g Aqueous Treatment Solution:

H₂TiF₆ 0.1 g

Polymer (Example 10) 1.8 g

Water 98.1 g

Dipping of aluminium plates in pretreatment after
a) 2 mins.: plates assumed a very light pink shimmer after drying
b) 5 mins.: plates assumed a light pink shimmer after drying
Corrosion-Inhabiting Properties of Plates (Al 99.5 F 19) Pretreated and then Painted in Accordance with the Invention:
Cleaning/Rinsing with Water: as Above
The treatment solutions contained 1% by weight H₂TiF₆and the quantity of polymer shown in Table 1. Temperature: 35° C. Treatment time, pH of the treatment solution and tests: see Table 1; test results: Tables 2 and 3.
Drying: recirculating air drying cabinet, 60° C.
Paint: BASF standard, white
Powder coating, Ral 9010
Hardening: 20 mins., 360° C.
Paint thickness: see Tables 2 and 3

Corrosion Tests: See Tables

TABLE 1


Treatment parameters, tests carried out

				Identification of
Exam-				Plate Numbers on
ple			Appear-	which Listed Tests
No.	Polymer	pH, time	ance	were Performed

21	Example 10	pH 2.7	Pink	Salt spray test
	3% Polymer	2 mins.	uniform	(DIN 50021 SS):
				(Plate No.: 6, 7)
				Boiling test:
				(Plate No.: 8, 9)
22	Example 10	pH 3.3	Dark pink	Salt spray test
	3% Polymer	2 mins.	opalescent	(DIN 50021 SS):
			uniform	(Plate No.: 11, 12)
				Boiling test:
				(Plate No.: 13, 14)
23	Example 8	pH 2.7	Blue	Salt spray test
	3% polymer	3 mins.	Uneven	(DIN 50021 SS):
				(Plate No.: 21, 23)
				Boiling test:
				(Plate No.: 24, 25)
24	Example 10	pH 2.96	Light pink	Salt spray test
	1.5% polymer	2 mins.	uniform	(DIN 50021 SS):
				(Plate No.: 30, 33)
				Boiling test:
				(Plate No.: 34, 35)
25	Example 8	pH 3.3	Dark blue	Salt spray test
	3% polymer	3 mins.	uniform	(DIN 50021 SS):
				(Plate No.: 37, 38)
				Boiling test:
				(Plate No.: 39, 40)

Table 2 recites the plate by plate results of the testing from Table 1.

TABLE 2


Test results: salt spray mist test to DIN 50021 SS

Plate No.

	6	7	11	12	21	23	30	33	37	38

Paint	99	98	84	95	73	69	51	92	100	114
thickness (μm)
Blister count	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)
to DIN EN ISO
4628-2 after 21
days
Creepage at	18.4	19	9.6	14.4	0.8	1.0	2.2	1.6	1.2	1.0
cut to DIN
53167 after 21
days (mm)
Blister count	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)	0(SO)
to DIN EN ISO
4628-2 after 21
days
Creepage at			15.4	15.7	1.5	1.4	2.7	2.1	1.8	1.6
cut to DIN
53167 after 21
days (mm)

TABLE 3


Test results from selected plates (cf. Table 1):
boiling test in deionized water, 2 hours at 100° C.

Plate No.

	8	9	13	14	24	25	34	35	39	40

Paint thickness (μm)	114	90	77	70	87	60	68	119	108	91
Cross hatching to DIN EN ISO	0	2	0	0	0	0	0	0	0	0
2409 [GT] before boiling test
Cross hatching to DIN EN ISO	0	0	0-4	0-3	0	0	0	0	0	0
2409 [GT] 1 hour after boiling
test

Claims

1. A treatment solution for producing colored corrosion prevention layers on metal surfaces which comprises

a) phosphoric acid and/or at least one fluoro acid of one or more elements selected from the group consisting of B, Si, Ti, Zr and Hf or anions thereof; and

b) at least one organic polymer capable of binding to a metal surface through oxygen and/or nitrogen atoms and selected from the group consisting of amino resins, phenol/aldehyde resins, polymers containing carboxyl groups, polymeric alcohols, esterification products of polymeric alcohols with polymers containing carboxylic acid groups, polymers containing amino groups, homo- or copolymers of vinyl pyrrolidone, and copolymers of alkylene phosphonic or alkylene phosphinic acids and one or more unsaturated carboxylic acids,

wherein the treatment solution is chromium-free and the organic polymer carries covalently bonded groups which appear colored to the human eye.

2. The treatment solution as claimed in claim 1, comprising:

a) 0.02 to 20 g/l phosphoric acid and/or at least one fluoro acid of one or more elements from the group consisting of B, Si, Ti, Zr and Hf or anions thereof and

b) 0.1 to 200 g/l of the organic polymer.

3. The treatment solution as claimed in claim 1, wherein:

a) the phosphoric acid is present in a concentration of at least 1 g/l; and

b) the organic polymer is present in a concentration of at least 5 g/l and not more than 150 g/l.

4. The treatment solution as claimed in claim 1, wherein the organic polymer has an upper molecular weight limit of 20,000,000 Dalton and an average molecular weight of greater than 100,000 Dalton.

5. The treatment solution as claimed in claim 1, wherein the percentage by weight of covalently bonded groups which appear colored to the human eye amounts to 0.1 to 20% by weight, based on the total weight of the organic polymer.

6. The treatment solution as claimed in claim 1, wherein the organic polymer is selected from the group consisting of polymers of acrylate and/or methacrylate monomers, maleic anhydride which may be completely or partly hydrolyzed, and acrylate or methacrylate monomers terminated by phosphate groups.

7. The treatment solution as claimed in claim 1, wherein the covalently bonded groups which appear colored to the human eye are selected from Toluidine Blue and Neutral Red.

8. The treatment solution as claimed in claim 1, further comprising:

d) at least one inorganic compound in particle form with a mean particle diameter in the range from 0.005 to 0.2 μm.

9. The treatment solution as claimed in claim 1, wherein the inorganic compound in particle form is present in the aqueous bath solution in a concentration of 0.1 to 80 g/l.

10. The treatment solution as claimed in claim 1, wherein the organic polymer is a copolymer of a) acrylic acid and/or methacrylic acid with b) acrylate and/or methacrylate monomers terminated by phosphate groups which contains covalently bonded molecules of Toluidine Blue or Neutral Red.

11. The treatment solution as claimed in claim 1, comprising at least 0.025 g/l and up to 10 g/l Ti and/or Zr and/or Si ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6.

12. The treatment solution as claimed in claim 1, comprising both fluoro acids of Ti and of Zr, wherein the molar ratio of Ti to Zr is in the range from 10:1 to 1:10.

13. A process for producing colored or fluorescent corrosion prevention layers on metal surfaces, comprising contacting the metal surfaces for 0.5 to 10 minutes with the treatment solution claimed in claim 1 which has a temperature of 20 to 80° C.

14. The process as claimed in claim 9, wherein the metal surfaces are selected from surfaces of aluminium and aluminium alloys, magnesium and magnesium alloys, titanium and titanium alloys, zinc and zinc alloys, galvanized and alloy-galvanized steel.

15. A metal part comprising at least one surface comprising a colored corrosion prevention layer produced according to the process claimed in claim 9.

16. A treatment solution for producing colored corrosion prevention layers on metal surfaces which comprises

b) 0.1 to 200 g/l of at least one organic polymer capable of binding to a metal surface through oxygen and/or nitrogen atoms and selected from the group consisting of maleic anhydride/methyl vinyl ether copolymer and acrylic acid and methacrylate monomer terminated by phosphate groups;

wherein the treatment solution is chromium-free and the organic polymer carries 0.1 to 20% by weight, based on the total weight of the organic polymer, of covalently bonded groups which appear colored to the human eye.