EP1970470B1 - Chrome(VI)-free black passivates for surfaces containing zinc - Google Patents

Abstract

Treatment solution contains first carboxylic acids with 1-8 carbon atoms and having no polar groups with the exception of the carboxyl group, second carboxylic acids with 1-8 carbon atoms having polar groups, 2-400 mmol/l chromium ions and 50-2000 mmol/l nitrate ions. The total concentration of carboxyl groups in the first carboxylic acids is 5-150 mmol/l. The total concentration of carboxyl groups in the second carboxylic acids is 5-150 mmol/l. The ratio of concentration of nitrate ions to chromium ions is at least 1. Independent claims are also included for the following: (1) Composition producing the above treatment solution by diluting with water; and (2) Method for the black passivation of zinc-containing surfaces using the treatment solution.

Description

FIELD OF THE INVENTION

The present invention relates to a treatment solution and a method for producing substantially chromium (VI) -free black conversion layers on zinc-containing alloy layers.

BACKGROUND OF THE INVENTION

The use of conversion layers to increase the protective effect of cathodic corrosion protection systems and as a primer for paints and paints is known for a long time. Especially on substrates containing zinc, cadmium and aluminum, in addition to phosphating methods, the method of chromating the surfaces has been established.

Here, the surface to be treated is exposed to a treatment solution whose essential component is chromium (VI) compounds. The generated conversion layer therefore also contains chromium (VI) ions. Chromating layers generally have good corrosion protection and good decorative properties. A disadvantage of the use of chromium (VI) -containing solutions or chromium (VI) -containing coatings are the toxicological properties of chromium (VI). The use of chromium (VI) -containing conversion layers is therefore, e.g. severely restricted by the EC directive 2000/53 / EC (EC end-of-life vehicle directive).

As an alternative to chromating solutions, chromium (III) -containing, acidic treatment solutions, commonly referred to as "passivations" or "passivation solutions", unlike chromations, have been proposed. These treatment solutions exist as in DE 196 15 664 A1 proposed essentially from a chromium (III) salt in mineral acid solution, a dicarboxylic acid or hydroxycarboxylic acid and a cobalt salt. Such processes known as "thick-film passivation" are used at elevated temperature, about 40-60 ° C., in order to achieve a passivation layer thickness on zinc surfaces which is adequate for corrosion protection. The need to use the process at room temperature elevated temperature results from the high inertness of the reaction for the chromium (III) ion as opposed to the chromium (VI) ion is characteristic. An essential extension of the reaction times as an alternative to increasing the temperature is not feasible for economic reasons in the rule.

As a black pigment which is easy to produce, the metal alloyed with the zinc is suitable for zinc alloy surfaces such as zinc-iron or zinc-nickel or zinc-cobalt. By treatment in an acidic solution, the less noble zinc is dissolved out of the layer and the alloying metal finely distributed on the surface enriched. As a result, the surface is dark or almost black tinted. Such a method is for example in DE 199 05 134 A1 described. Here, an additional oxidizing agent is used for the black passivation of zinc-nickel surfaces in order to promote the etching effect of the acid. The result is a black surface that does not provide significant corrosion protection.

Alternatively, according to US 5,415,702 Cr (VI) -free black conversion layers on zinc-nickel alloy layers by treatment with acid, chromium (III) -containing solutions which further contain oxygen acids of the phosphorus are prepared. In this process, homogeneous black conversion layers with good decorative properties are formed. In laboratory tests, however, we were unable to reconstruct the corrosion protection described there.

WO 03/054249 describes a similar conversion layer which is also made with a chromium (III) -containing, acidic treatment solution which still contains phosphate ions. This surface also has good decorative properties, but does not achieve adequate corrosion protection properties without further post-treatment steps such as sealing.

EP 1 484 432 A1 describes chromium (III) -containing black passivation solutions for zinc alloy surfaces containing chromium (III) ions and nitrate as well as carboxylic acids such as tartaric acid, maleic acid, oxalic acid, succinic acid, citric acid, malonic acid or adipic acid. To improve the corrosion protection, the surfaces treated with it must be subjected to a subsequent sealing. The treatment solutions are used at temperatures above normal room temperature.

US 2004/156999 also describes a method for black passivation of zinc alloy surfaces. The treatment solutions contain, in addition to chromium (III) ions and phosphorus-containing anions, nitrate and an organic carboxylic acid. Examples of the organic carboxylic acids include citric, tartaric, maleic, glyceric, lactic, glycolic, malonic, succinic, oxalic and glutaric acids.

With the described treatment solutions it was not possible for us to achieve the corrosion protection described there.

The US patent application US 2004/173289 A describes a rustproofing agent for galvanized steel sheet containing: (A) an aqueous chromium-containing solution containing an organic substance, and (B) an acidic metal salt such as a metal salt of nitric acid or phosphoric acid. The chromium in the aqueous chromium-containing solution of component (A) is exclusively trivalent chromium. The organic substance of component (A) is preferably at least one kind of oxyacid or an oxide thereof. The metal in the acidic metal salt of component (B) is an alkaline earth metal, cobalt, nickel, iron, zirconium, titanium or the like. The rust inhibitor is used to prevent blackening of the treated metal surface.

The US patent application US 2006/054248 A describes that a zinc surface on a metal substrate can be colored and made corrosion resistant by treating the surface with a solution of a trivalent chromium compound and at least one additional metal salt selected from the group consisting of ferrous salts, nickel salts and cobalt salts capable of coloring the surface together with the chromium compound in the presence of a phosphate at a pH of about 0.5 to 5. The dyed surface is then topcoated to achieve high corrosion resistance. Example 7 of the abovementioned application describes a treatment solution which contains, inter alia: 2 g / l of chromium (III) nitrate, 5 g / l of oxalic acid, 4 g / l of citric acid and 25 g / l of acetic acid. Thus, the concentration ratio mentioned in claim 1 is 0.8.

The European Patent Application EP 1 944 390 A , which belongs to the state of the art under Article 54 (3) EPC, describes a treatment solution for use in forming a trivalent chromium black conversion layer of uniformly stable black color, gloss and corrosion resistance, irrespective of the type of acidic, neutral or alkaline zinc bath and regardless of whether a nickel eutectoid is formed or not. Furthermore, a method of forming the trivalent chromium black conversion layer will be described. The treating solution comprises trivalent chromium ions, a chelating agent capable of forming a water-soluble complex with the trivalent chromium, at least one metal ion selected from the group consisting of cobalt ions, nickel ions and iron ions, and formic acid or a salt thereof as a buffer. In the examples of said application, treatment solutions are described in which the total concentration of carboxylate groups in dicarboxylic acids (oxalic acid and / or malonic acid) is in each case more than 150 mmol / l.

Accordingly, black-passivated zinc or zinc alloy surfaces can not yet be produced in a fully satisfactory manner using known processes. A disadvantage of the described method is in particular that it is not possible to produce a black zinc alloy surface, which provides a good base corrosion protection. As a result, aftertreatment steps are generally necessary in order to improve the anti-corrosive properties of the layer.

DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a treatment solution and a process for the chromium (VI) -free black passivation of zinc alloys which meet the decorative properties required by the results obtained with conventional chromium (VI) black chromates. suffice. In addition, the surface should simultaneously be given very good anti-corrosive properties.

• mindestens eine erste Carbonsäure mit 1 bis 8 Kohlenstoffatomen, die außer der Carboxylgruppe keine polaren Gruppen enthält und eine Monocarbonsäure ist,
• mindestens eine zweite Carbonsäure mit 1 bis 8 Kohlenstoffatomen, die mindestens eine weitere polare Gruppe enthält, die ausgewählt ist aus -OH, -SO₃H, -NH₂, -NHR, -NR₂, -NR₃ ⁺ und -COOH (wobei R für eine C₁-C₆-Alkylgruppe steht),
• 20 bis 400 mmol/l Cr³⁺ und
• 50 bis 2000 mmol/l NO₃ ^-,

und wobei

• die Gesamtkonzentration an Carboxylgruppen der ersten Carbonsäure(n) im Bereich von 5 bis 150 mmol/l, bevorzugt 10 bis 50 mmol/l liegt,
• die Gesamtkonzentration an Carboxylgruppen der zweiten Carbonsäure(n) im Bereich von 5 bis 150 mmol/l, bevorzugt 10 bis 75 mmol/l liegt,
• das Verhältnis der Konzentration (in mol/l) von NO₃ ^- zu Cr³⁺ ≥ 1 ist und
• folgende Bedingung erfüllt ist: $$0,05\le \frac{c\left(\mathrm{C}1\right)}{c\left(\mathrm{C}1\right)}*\frac{c\left({\mathrm{Cr}}^{3+}\right)}{c\left({\mathrm{NO}}_3^{-}\right)}\le 0,5$$
  

wobei,

c(C1)

die Gesamtkonzentration (in mol/l) an Carboxylgruppen der ersten Carbonsäure(n) ist,

c(C2)

die Gesamtkonzentration (in mol/l) an Carboxylgruppen der zweiten Carbonsäure(n) ist,

c(Cr³⁺)

die Konzentration (in mol/l) an Cr³⁺ ist, und

c(NO₃ ^-)

die Konzentration (in mol/l) an NO₃ ^- ist.

This object is achieved by a treatment solution for producing essentially chromium (VI) -free black conversion layers on zinc-containing alloy layers, wherein the solution contains:

• at least one first carboxylic acid having 1 to 8 carbon atoms which contains no polar groups other than the carboxyl group and is a monocarboxylic acid,
• at least one second carboxylic acid of 1 to 8 carbon atoms containing at least one further polar group selected from -OH, -SO ₃ H, -NH ₂ , -NHR, -NR ₂ , -NR ₃ ⁺ and -COOH (wherein R is a C ₁ -C ₆ -alkyl group),
• 20 to 400 mmol / l Cr ³⁺ and
• 50 to 2000 mmol / l NO ₃ ^- ,

and where

• the total concentration of carboxyl groups of the first carboxylic acid (s) is in the range of 5 to 150 mmol / l, preferably 10 to 50 mmol / l,
• the total concentration of carboxyl groups of the second carboxylic acid (s) is in the range of 5 to 150 mmol / l, preferably 10 to 75 mmol / l,
• the ratio of the concentration (in mol / l) of NO ₃ ^- to Cr ³⁺ ≥ 1 and
• the following condition is met: $$0.05\le \frac{c\left(\mathrm{C}1\right)}{c\left(\mathrm{C}1\right)}*\frac{c\left({\mathrm{Cr}}^{3+}\right)}{c\left({\mathrm{NO}}_3^{-}\right)}\le 0.5$$
  

in which,

c (C1)

the total concentration (in mol / l) of carboxyl groups of the first carboxylic acid (s) is

c (C2)

the total concentration (in mol / l) of carboxyl groups of the second carboxylic acid (s) is

c (Cr ³⁺ )

the concentration (in mol / l) of Cr ³⁺ is, and

c (NO ₃ ^- )

the concentration (in mol / l) of NO ₃ ^- is.

In addition, the invention provides a composition which yields such a treatment solution by dilution with water.

Furthermore, the invention provides a method for black passivation of zinc-containing surfaces, wherein the surface to be treated is immersed in such a treatment solution.

The invention is based on the empirically discovered finding that good aesthetic properties (appearance, uniformity and coloration) in combination with good anticorrosive properties can be achieved by using at least one first carboxylic acid as defined above together with at least one second carboxylic acid as defined above mentioned concentration conditions.

The treatment solution is an acidic, aqueous solution. Their pH is preferably in the range of 1.4 to 2.5, more preferably in the range of 1.5 to 2.0.

The first carboxylic acid is preferably an alkyl, aryl, alkenyl or alkynylcarboxylic acid. Apart from the carboxyl group it contains no polar, eg protic, groups. In particular, it contains none of the following groups: -OH, -SO ₃ H -NH ₂ , -NHR, -NR ₂ , -NR ₃ ⁺ (where R is for a C ₁ -C ₆ alkyl group is). However, the first carboxylic acid may contain the following groups: halogen, alkyl, aryl, vinyl, alkoxy and nitro groups.

Examples of acids which are suitable as the first carboxylic acid include formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, hexanecarboxylic acid, cyclopentanecarboxylic acid, acetylsalicylic acid, benzoic acid, nitrobenzoic acid, 3,5-dinitrobenzoic acid, sorbic acid, trifluoroacetic acid, 2-ethylhexanoic acid, Acrylic acid, chloroacetic acid, 2-chlorobenzoic acid, 2-chloro-4-nitrobenzoic acid, cyclopropanecarboxylic acid, methacrylic acid, 3-nitrobenzoic acid, 4-nitrobenzoic acid, phenoxyacetic acid, isovaleric acid, pivalic acid, 2-ethylbutyric acid, furan-2-carboxylic acid, bromoacetic acid, crotonic acid, 2- Chloropropionic acid, dichloroacetic acid, glyoxylic acid, 4-methoxybenzoic acid, 3,4-dimethoxybenzoic acid, levulinic acid, pentenoic acid, phenylacetic acid, tiglic acid, vinylacetic acid, heptanoic acid, propargylic acid, ethacrylic acid, cyclohexenoic acid, cyclohexanoic acid, cyclopentenic acid and 2-butynoic acid.

The first carboxylic acid is preferably acetic acid.

The second carboxylic acid carrying at least one further polar group is preferably a di- or tri-carboxylic acid. Also suitable are amino acids.

Examples of acids suitable as the second carboxylic acid include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, maleic acid, phthalic acid, terephthalic acid, tartaric acid, citric acid, malic acid, ascorbic acid, ethylenedinitrilotetraacetic acid, tetrahydrofuran-2- carboxylic acid, ethylenediaminetetraacetic acid, diethylenediaminepentaacetic acid, nitrilotriacetic acid, lactic acid, adipic acid, 4-aminohippuric acid, 4-aminobenzoic acid, 5-aminoisophthalic acid, L-aspartic acid, L-glutamine, L-glutamic acid, alanine, beta-alanine, L-arginine, L-asparagine, L-alanine, N, N-bis (2-hydroxyethyl) glycine, L-cysteine, L-cystine, glutathione, glycine, glycylglycine, L-histidine, L-hydroxyproline, L-isoleucine, L-leucine, L-lysine , L-methionine, L-ornithine, L-phenylalanine, L-proline, L-serine, L-tyrosine, L-tryptophan, L-threonine, L-valine, N- [tris (hydroxymethyl) -methyl] -glycine, L-citrulline, N-acetyl-L-cysteine, N- (2-acetamido) -im inodiacetic acid, 1,2-cyclohexylenedinitrilotetraacetic acid, D (+) - biotin, L-norleucine, 5-aminolevulinic acid, DL-methionine, 3-aminobenzoic acid, 6-aminohexanoic acid, acetylenedicarboxylic acid, pyridine-2,3-dicarboxylic acid, (-) Quinic acid, 4-amino-2-hydroxybenzoic acid, pyridine-2,6-dicarboxylic acid, pyridine-2-carboxylic acid, pyrazine-2,3-dicarboxylic acid, pyrazine-2-carboxylic acid, pyridine-4-carboxylic acid, 3,5-dihydroxybenzoic acid , 2,4-dihydroxybenzoic acid, sebacic acid, benzene-1,3,5-tricarboxylic acid, furan-2-carboxylic acid, methylenesuccinic acid, DL-mandelic acid, DL-alpha-aminophenylacetic acid, DL-tropic acid, 2,2'-thiodiacetic acid, 3, 3'-thiodipropionic acid, 3- (2-furyl) -acrylic acid, Piperidine-4-carboxylic acid, 4-guanidinobenzoic acid, L-homoserine, trans-propene-1,2,3-tricarboxylic acid, (R) - (-) - citramalic acid, (3-hydroxyphenyl) -acetic acid, 4-hydroxyquinoline-2 carboxylic acid, N-acetyl-L-glutamic acid, N-acetyl-DL-valine, 4-aminohippuric acid, 2,6-dihydroxybenzoic acid, 4- (dimethylamino) benzoic acid, glucuronic acid, citracaic acid, indole-3-carboxylic acid, indole-5 carboxylic acid, butane-1,2,3,4-tetracarboxylic acid, DL-leucine, 2,2-bis (hydroxymethyl) propionic acid, quinoline-2,4-dicarboxylic acid, 2-aminopyridine-3-carboxylic acid, 5-amino 2-hydroxybenzoic acid, anthranilic acid, benzene-1,2,4-tricarboxylic acid, 3,5-diaminobenzoic acid, 4,8-dihydroxyquinoline-2-carboxylic acid, 3,3-dimethylglutaric acid, trans, trans-2,4-haxadienoic acid, 3- Hydroxybutyric acid, o-hydroxyhippuric acid, (4-hydroxyphenyl) -acetic acid, imidazole-4-acrylic acid, indole-2-carboxylic acid, indole-3-propionic acid, mercaptosuccinic acid, 3-oxoglutaric acid, pyridine-2,4-dicarboxylic acid, pyridine-3, 5-dicarboxylic acid, 2-methylalanine, 2-sulf benzoic acid, pyridine-2,5-dicarboxylic acid, gluconic acid, 4-aminobenzoic acid, (-) - shikimic acid, quinaldic acid, 5-hydroxyisophthalic acid, pyrazole-3,5-dicarboxylic acids, pyridine-3,4-dicarboxylic acid, 1,2-diaminopropane tetraacetic acid, 2-pyridylacetic acid, D-norvaline, 2-methylglutaric acid, 2,3-dibromosuccinic acid, 3-methylglutaric acid, (2-hydroxyphenyl) acetic acid, 3,4-dihydroxybenzoic acid, diglycolic acid, propane-1,2,3-tricarboxylic acid, 2 , 3-dimethylaminopropionic acid, 2,5-dihydroxybenzoic acid, 2-hydroxyisobutyric acid, phenylsuccinic acid, N-phenylglycine, 1-aminocyclohexanecarboxylic acid, sarcosine, tropic acid, pyruvic acid, mucic acid.

The carboxylic acids can also be introduced into the treatment solution in the form of their salts.

For the preparation of the treatment solution according to the invention are also all compounds that can be used in aqueous solution as a source of the corresponding acids, ie their esters, acid amides, acid halides, acid nitriles and acid anhydrides.

The treating solution preferably additionally contains cobalt (II) ions in a concentration in the range of 0.1 g / L to 3 g / L, more preferably in the range of 0.2 g / L to 2 g / L, most preferably in Range from 0.5 g / l to 1 g / l.

The treatment solution according to the invention serves to passivate zinc alloys, such as e.g. Zinc-iron, zinc-nickel or zinc-cobalt alloys.

Zinc-iron alloys preferably contain 0.4 to 1 wt .-% iron, zinc-nickel alloys preferably contain 8 to 20 wt .-% nickel and zinc-cobalt alloys preferably contain 0.5 to 5 wt .-% Cobalt.

These alloys can be electrochemically deposited on a substrate or applied by other methods such as hot dip galvanizing or make up the material of the article to be treated.

Preferably, the ratio {c (C1) / c (C2)} * {c (Cr ³⁺ ) / c (NO ₃ ^- )} is in the range of 0.1 to 0.2.

In the method according to the invention for the black passivation of zinc-containing surfaces, the surface to be treated is immersed in a treatment solution as described above. The temperature of the treatment solution is preferably in the range of 20 ° C to 60 ° C, more preferably in the range of 20 ° C to 40 ° C, most preferably in the range of 20 ° C to 30 ° C. The treatment time in the treatment solution is preferably between 10 s and 180 s, more preferably between 30 s and 120 s, most preferably between 45 s and 90 s. In a preferred embodiment of the method, the passivation treatment is assisted by cathodic switching of the substrate in the passivation solution. The cathodic current density on the substrate is preferably between 0.05 A / dm ² and 10 A / dm ² , more preferably between 0.1 A / dm ² and 5 A / dm ² , most preferably between 0.1 A / dm ² and 3 A / dm ² .

Conventional, chromium (VI) -free passivating solutions for zinc-containing surfaces typically consist of a source of chromium (IIII) ions, one or more complexing agents such as fluoride and / or polybasic carboxylic acids, hydroxycarboxylic acids or aminocarboxylic acids. Chromium (III) is present in the electron configuration 3d ^{3 of} the valence electrons and is known almost exclusively in aqueous solutions as an octahedrally coordinated ion. In this configuration, the ion has high ligand field stabilization energy (LFSE). This leads to extremely low reaction rates on the chromium (III) ion, which is reflected for example in the need to produce Cr (III) complexes either with long reaction times or at elevated temperature. This point is usually taken into account in the preparation of passivation solutions by using hot water in the approach or heating of the reaction solution.

In contrast to the Cr (III) ion, the Cr (II) ion with the electron configuration 3d ^{4 has} a significantly lower kinetic inhibition and therefore significantly faster ligand exchange reactions. Water ligands on chromium (III) exchange several orders of magnitude more slowly than chromium (II). If the ion is in the high-spin configuration, the reactivity is also accelerated by the Jahn-Teller effect that occurs here. The high-spin arrangement of the electrons in the octahedral complex is observed with ligands that generate a comparatively weak ligand field, such as water, oxide. The low-spin case is only observed for ligands that are very strong Create ligand field. This includes, for example, the cyanide ion. Such ligands are not included in the treatment solutions according to the invention. Carboxylate ions, which are part of the treatment solutions according to the invention, belong to the former class, ie to ligands which generate a weak ligand field and thus form high-spin complexes.

The reduction of Cr ( III ) to chromium ( II ) (Cr ²⁺ → Cr ³⁺ + e ^- , E ₀ = -0.41 V compared to standard hydrogen electrode) is readily carried out in a sufficiently acidic solution on zinc surfaces. The formation of a multi-dimensional network of μ-hydroxy bridged chromium (III) ions, as is generally assumed for the structure of a chromium (III) -containing conversion layer, most probably takes place via the intermediate step of reducing Cr (III) to Cr ( II) with subsequent rapid ligand exchange. By dissolved atmospheric oxygen, Cr (II) is easily oxidized back to chromium (III) in the presence of moisture. The reduction of Cr (III) to Cr (II) can also be carried out electrochemically. That is, by cathodic connection of the part to be passivated in the reaction solution, the layer formation reaction can be supported or carried out entirely by electrochemical means. Optionally, this process, especially on black passivated zinc surfaces, leads to an improvement of the corrosion protection.

Especially on black surfaces, the construction of a dense, low-defect passivation layer is difficult. As a black pigment, finely divided alloy metal (e.g., cobalt, nickel or iron) enriched by etching the surface and dissolving away zinc is often generated. Alternatively, depending on the treatment solution, the oxides of these elements are produced.

On pure zinc surfaces, blackening is accomplished by depositing small amounts of these more precious metals in comparison to the zinc by immersing the zinc surface in an e.g. Solution containing iron, nickel, cobalt, silver or copper ions by cementation. Charge exchange forms a thin layer of finely divided black metals or, depending on the treatment solution of the metal oxides, also non-stoichiometric zinc oxides.

On the black surfaces thus generated, the formation of a passivation layer is difficult and results in a poor corrosion protection effect of the black passivation layer.

This problem is solved by the present invention such that the intermediately occurring Cr (11) ions with the aid of monocarboxylic acids, which contain no further polar groups except the carboxyl group, converted into a sparingly soluble form and so on be fixed to the surface. In this way, compared with previous methods, an increased concentration of only low-mobility chromium is achieved, which is available for the construction of the passive layer. If, on the other hand, mainly polybasic carboxylic acids such as oxalic acid, malonic acid, succinic acid or hydroxycarboxylic acids such as lactic acid or polyhydric hydroxycarboxylic acids such as citric acid or tartaric acid are used, the Cr (II) possibly coordinated by these acids or their anions is not converted into a slightly soluble form and can be so the surface no or no sufficient enrichment for the purpose.

The use of acetic acid or acetate ions to convert Cr (II) into a sparingly soluble form is used for the preparative preparation of chromium (II) acetate (see the following formula). The binuclear structure as found for the chromium (II) acetate is not a prerequisite for the mode of action described in this invention. Intermediate polynuclear complexes with more than one chromium ion or even mononuclear complexes can also occur.

Chromium (II) acetate forms red crystals that are oxidized to Cr (III) species in contact with atmospheric oxygen. Similarly, under the conditions prevailing in the passivation solution at the metal-solution interface, the surface-enriched chromium species may undergo partial or complete ligand exchange to form a three-dimensional network.

In addition to improved base corrosion protection, there is a further advantage of using monocarboxylic acids in their incorporation into the conversion layer. By coordination to chromium ions in the layer network, the surface becomes hydrophobic through the non-polar alkyl, aryl, alkenyl or alkynyl radicals and exhibits an improved affinity for nonpolar polymers, as used in customary polymer dispersions.

Compared to conventional conversion layers containing chromium (VI) and conversion layers produced from solutions containing pure di-, tri- or hydroxycarboxylic acid or aminocarboxylic acid-containing solutions, according to the invention an increased affinity for hydrophobic polymers is achieved. This is reflected in an improvement of the corrosion protection by application of polymer dispersions to the conversion layers produced according to the invention.

The sole use of monocarboxylic acids as chelating ligands usually leads to a non-homogeneous blackening of the layer as a result of the accelerated by the poor solubility of the intermediately formed chromium (II) complex layer growth, as it is increasingly isolated against the attack of the reaction solution. By choosing suitable combinations of monocarboxylic acids with at least one second carboxylic acid (eg a polycarboxylic acid or hydroxycarboxylic acid) and their concentrations, the concentrations of readily soluble chromium (II) intermediates and poorly soluble chromium (II) reaction products on the surface in the sense of good corrosion protection at the same time homogeneous and thus attractive coloring of the surface can be adjusted. Empirical results in terms of corrosion protection of the zinc-containing surface with respect to white corrosion and homogeneous, deep coloring of the surface favorable concentration ratios, if the composition of the reaction solution meets the above conditions.

The invention will be explained in more detail by way of examples.

EXAMPLES Comparative Examples 1 and 2

• Reaktionslösung 1:
  • 4,5 g/l Cr³⁺, zugegeben als Chrom(III)-nitrat-nonahydrat
  • 17 g/l Salpetersäure (65 %)
• Reaktionslösung 2:
  • 4,5 g/l Cr³⁺, zugegeben als Kalium-chrom(III)-sulfat
  • 17,1 g/l SO₄ ^2-, zugegeben als Kalium-chrom(III)-sulfat
  • 0,3 g/l CO²⁺, zugegeben als Kobalt(II)-sulfat-hexahydrat
  • 90 mg/l NO₃ ^-, zugegeben als Salpetersäure
  • 1 g/l Oxalsäure-Dihydrat
  • 1 g/l Essigsäure
  • 1 g/l Maleinsäure

There were prepared aqueous reaction solutions having the following composition:

• Reaction solution 1:
  • 4.5 g / l Cr ³⁺ added as chromium (III) nitrate nonahydrate
  • 17 g / l nitric acid (65%)
• Reaction solution 2:
  • 4.5 g / l Cr ³⁺ added as potassium chromium (III) sulfate
  • 17.1 g / L SO ₄ ^2- , added as potassium chromium (III) sulfate
  • 0.3 g / l CO ²⁺ added as cobalt (II) sulfate hexahydrate
  • 90 mg / l NO ₃ ^- , added as nitric acid
  • 1 g / l of oxalic acid dihydrate
  • 1 g / l of acetic acid
  • 1 g / l maleic acid

The pH of the solution was adjusted to pH 1.5 each with nitric acid or sodium hydroxide.

A steel component was coated in a zinc-nickel alkaline alloy electrolyte (trade name: Reflectalloy ZNA, manufactured by Atotech) with a 5 μm-thick layer of a nickel-nickel-containing zinc-nickel alloy. The steel member was then immersed in a nitric acid-water mixture (about 0.3% HNO ₃₎ at 20 ° C for 10 seconds to activate the surface, which was then rinsed with demineralized water and immediately poured into the reaction solution 1 above 2 and immersed at 25 ° C. for 60 s, then rinsed with demineralized water and dried, the surface of the part assuming a matte, dark to dark brown color in the salt spray test according to DIN 50021 SS <12 h white corrosion.

Exemplary embodiments 1 - 6

Aqueous reaction solutions having the compositions shown in Table 1 were prepared (the individual components were added in the same form as in Comparative Example 2). The pH of the solution was adjusted to the value shown in Table 1 with nitric acid or sodium hydroxide, respectively.

Steel members were electrolytically coated with the Zn-containing alloy indicated in Table 1 under "Substrate", thoroughly rinsed with demineralized water after the electrolytic coating, then activated in 0.3% nitric acid at 20-30 ° C for 10 seconds, and then again thoroughly rinsed. The parts were then immersed in the reaction solutions under the conditions given in Table 1 (temperature, exposure time). Thereafter, a seal with Corrosil 501 was applied, which consists of an aqueous polymer dispersion with silicate proportions. The results of the visual assessment (color) and the salt spray test according to DIN 50021 SS before and after applying the seal (duration until the occurrence of white corrosion) are also given in Table 1. <b> Table 1 </ b> example 1# 2 3 4 5 6 Cr ³⁺ 4.5 g / l 4, 5 g / l 4.5 g / l 4.5 g / l 4.5 g / l 4.5 g / l NO ₃ ^- 17 g / l 17 g / l 17g / l 17g / l 17g / l 17g / l CO ²⁺ 0.3 g / l 0, 3 g / l 0.3 g / l 0.3 g / l 0.3 g / l 0.6 g / l formic acid 0g / l 0g / l 0g / l 0g / l 0g / l 0.8 g / l acetic acid 3.5 g / l 1 g / l 1 g / l 0g / l 0g / l 0g / l propionic 0g / l 0g / l 0g / l 0g / l 1.3 g / l 0g / l benzoic acid 0g / l 0g / l 0g / l 2 g / l 0g / l 0g / l Oxalic acid dihydrate 0g / l 1 g / l 1 g / l 1 g / l 1 g / l 1 g / l maleic 0g / l 1 g / l 1.5 g / l 1 g / l 1 g / l 1 g / l pH 1.5 1.5 1.5 1.5 1.5 1.5 temperature 25 ° C 25 ° C 25 ° C 25 ° C 25 ° C 25 ° C exposure time 60 s 60 s 60 s 60 s 60 s 60 s Substrate (*) Zn / Ni Zn / Ni Zn / Ni Zn / Ni Zn / Ni Zn / Ni colour stained dark shiny, homogeneous black evenly black evenly black evenly black black, slightly dull DIN 50021 SS 48 h 72 h 72 h 72 h 72 h 48 h sealing Corrosil 501 Corrosil 501 Corrosil 501 Corrosil 501 Corrosil 501 Corrosil 501 DIN 50021 SS 144 h 240 h 240 h 240 h 192 h 144 h * Zn / Ni = Zn / Ni alloy with 8 - 15% nickel content in the alloy
# not according to the invention

Comparative Examples 3 and 4

Embodiment 3 was repeated except that the concentrations of acetic acid and oxalic acid were changed as shown in Table 2, respectively. The results of the evaluation of the coloring and the corrosion properties are also shown in Table 2. <b> Table 1 </ b> Comparative example 3 4 Cr ³⁺ 4.5 g / l 4.5 g / l NO ₃ ^- 17 g / l 17 g / l CO ²⁺ 0.3 g / l 0.3 g / l formic acid 0g / l 0g / l acetic acid 5 g / l 1 g / l propionic 0g / l 0g / l benzoic acid 0g / l 0g / l Oxalic acid dihydrate 1 g / l 9 g / l maleic 1.5 g / l 1.5 g / l pH 1.5 1.5 temperature 25 ° C 25 ° C exposure time 60 s 60 s Substrate (*) Zn / Ni Zn / Ni colour spotty, brown evenly black DIN 50021 SS 48 h 24 hours sealing Corrosil 501 Corrosil 501 DIN 50021 SS 120 h 72 h * Zn / Ni = Zn / Ni alloy with 8 - 15% nickel content in the alloy

Comparative Example 3 shows that if the concentration of carboxyl groups from monocarboxylic acids is too high, only poor coloration of the treated surface is achieved.

Comparative Example 4 shows that when the concentration of carboxyl groups of polycarboxylic acids is too high, only poor corrosion properties of the treated surface are obtained.

Claims (22)

1. A treatment solution for producing substantially chromium(VI)-free black conversion layers on zinc-containing alloy layers, the solution containing:

   - at least one first carboxylic acid having 1 to 8 carbon atoms, the acid containing no polar groups with exception of the carboxylic group and being a monocarboxylic acid,

   - at least one second carboxylic acid having 1 to 8 carbon atoms, comprising at least one further polar group that is selected from -OH, -SO₃H, -NH₂, -NHR, -NR₂, -NR₃ ⁺ and -COOH (wherein R is a C₁-C₆ alkyl group),

   - 20 to 400 mmol/l Cr³⁺ and

   - 50 to 2000 mmol/l NO₃ ^-,
   and wherein

   - the total concentration of carboxyl groups of the first carboxylic acid(s) is within a range of 5 to 150 mmol/l,

   - the total concentration of carboxyl groups of the second carboxylic acid(s) is within a range of 5 to 150 mmol/l,

   - the ratio of the concentration (in mol/l) of NO₃ ^- to Cr³⁺ is ≥ 1, and

   - the following prerequisite is met: $$\mathrm{0.05}\mathrm{\le }\frac{c\left(\mathrm{C}\mathrm{1}\right)}{c\left(\mathrm{C}\mathrm{2}\right)}\mathrm{*}\frac{c\left({\mathrm{Cr}}^{\mathrm{3}\mathrm{+}}\right)}{c\left({\mathrm{NO}}_{\mathrm{3}}^{\mathrm{-}}\right)}\mathrm{\le }\mathrm{0.5}$$

   

   
   wherein,

   c(Cl) is the total concentration (in mol/l) of carboxyl groups of the first carboxylic acid(s),

   c(C2) is the total concentration (in mol/l) of carboxyl groups of the second carboxylic acid(s),

   c(Cr³⁺) is the concentration (in mol/l) of Cr³⁺, and

   c(NO₃ ^-) is the concentration (in mol/l) of NO₃ ^-.
2. The treatment solution according to claim 1, wherein the pH value of the solution is within the range of 1.4 to 2.5.
3. The treatment solution according to claim 1, wherein the pH value of the solution is within the range of 1.5 to 2.0.
4. The treatment solution according to claim 1, wherein the first carboxylic acid is selected from the group consisting of formic acid, acetic acid, propionic acid, butyric acid, pentanoic acid, hexanoic acid, benzoic acid, heptanoic acid, propargylic acid, acrylic acid, methacrylic acid, ethacrylic acid, crotonic acid, cyclohexenoic acid, cyclohexanoic acid, cyclopentanoic acid, cyclopentenoic acid and 2-butynic acid as well as isomers thereof.
5. The treatment solution according to claim 1, wherein the second carboxylic acid is a dicarboxylic acid.
6. The treatment solution according to claim 1, wherein the second carboxylic acid is selected from the group consisting of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, cork acid, azelaic acid, sebacinic acid, maleic acid, phthalic acid, terephthalic acid, tartaric acid, citric acid, malic acid, ascorbic acid, ethylene dinitrilotetraacetic acid, tetrahydrofuran-2-carboxylic acid, ethylene diaminetetraacetic acid, diethylene diaminepentaacetic acid, nitrilotriacetic acid, lactic acid, adipic acid, 4-aminohippuric acid, 4-aminobenzoic acid, 5-aminoisophthalic acid, L-asparagic acid, L-glutamine, L-glutamic acid, alanine, beta-alanine, L-arginine, L-asparagine, L- alanine, N,N-bis(2-hydroxyethyl)-glycine, L-cysteine, L-cystine, glutathione, glycine, glycylglycine, L-histidine, L-hydroxyproline, L-isoleucine, L-leucine, L-lysine, L-methionine, L-ornithine, L-phenylalanine, L-proline, L-serine, L-tyrosine, L-tryptophane, L-threonine, L-valine, N-[tris(hydroxymethyl)-methyl]-glycine, L-citrulline, N-acetyl-L-cysteine, N-(2-acetamido)-iminodiacetic acid, 1,2-cyclohexenylene-dinitrilotetraacetic acid, D(+)-biotine, L-norleucine, 5-aminolevulinic acid, DL-methionine, 3-aminobenzoic acid, 6-aminohexanoic acid, acetylene dicarboxylic acid, pyridine-2,3-dicarboxylic acid, (-)-quinic acid, 4-amino-2-hydroxybenzoic acid, pyridine-2,6-dicarboxylic acid, pyridine-2-carboxylic acid, pyrazine-2,3-dicarboxylic acid, pyrazine-2-carboxylic acid, pyridine-4-carboxylic acid, 3,5-diyhdroxybenzoic acid, 2,4-dihydroxybenzoic acid, sebacinic acid, benzene-1,3,5-tricarboxylic acid, furan-2-carboxylic acid, methylene succinic acid, DL-mandelic acid, DL-alpha-aminophenylacetic acid, DL-tropic acid, 2,2'-thiodiacetic acid, 3,3'-thiodipropionic acid, 3-(2-furyl)-acrylic acid, piperidine-4-carboxylic acid, 4-guanidinobenzoic acid, L-homoserine, trans-propene-1,2,3-tricarboxylic acid, (R)-(-)-citramalic acid, (3-hydroxyphenyl)-acetic acid, 4-hydroxyquinoline-2-carboxylic acid, N-acetyl-L-glutamic acid, N-acetyl-DL-valine, 4-aminohippuric acid, 2,6-dihydroxybenzoic acid, 4-(dimethylamino)-benzoic acid, glucuronic acid, citrazinic acid, indole-3-carboxylic acid, indole-5-carboxylic acid, butane-1,2,3,4-tetracarboxylic acid, DL-leucine, 2,2-bis-(hydroxymethyl)-propionic acid, quinoline-2,4-dicarboxylic acid, 2-aminopyridine-3-carboxylic acid, 5-amino-2-hydroxybenzoic acid, anthranilic acid, benzene-1,2,4-tricarboxylic acid, 3,5-diaminobenzoic acid, 4,8-dihydroxyquinoline-2-carboxylic acid, 3,3-dimethylglutaric acid, trans,trans-2,4-hexadienoic acid, 3-hydroxybutyric acid, o-hydroxyhippuric acid, (4-hydroxyphenyl)-acetic acid, imidazole-4-acrylic acid, indole-2-carboxylic acid, indole-3-propionic acid, mercaptosuccinic acid, 3-oxoglutaric acid, pyridine-2,4-dicarboxylic acid, pyridine-3,5-dicarboxylic acid, 2-methylalanine, 2-sulfobenzoic acid, pyridine-2,5-dicarboxylic acid, gluconic acid, 4-aminobenzoic acid, (-)-shikimic acid, quinaldinic acid, 5-hydroxyisophthalic acid, pyrazole-3,5-dicarboxylic acids, pyridine-3,4-dicarboxylic acid, 1,2-diaminopropanetetraacetic acid, 2-pyridylacetic acid, D-norvaline, 2-methylglutaric acid, 2,3-dibromosuccinic acid, 3-methylglutaric acid, (2-hydroxyphenyl)-acetic acid, 3,4-dihydroxybenzoic acid, diglycolic acid, propane-1,2,3-tricarboxylic acid, 2,3-dimethylaminopropionic acid, 2,5-dihydroxybenzoic acid, 2-hydroxyisobutyric acid, phenylsuccinic acid, N-phenylglycine, 1-aminocylcohexanecarboxylic acid, sarcosine, tropic acid, pyromucic acid, mucic acid.
7. The treatment solution according to one of the preceding claims, wherein the solution additionally contains cobalt(II) ions in a concentration within the range of 0.1 g/l to 3 g/l.
8. The treatment solution according to claim 7, wherein the concentration of the cobalt (II) ions is within the range of 0.2 g/l to 2 g/l.
9. The treatment solution according to claim 7, wherein the concentration of the cobalt (II) ions is within the range of 0.5 g/l to 1 g/l.
10. A composition yielding, by diluting with water, a treatment solution according to one of claims 1 to 9.
11. The composition according to claim 10, wherein the composition contains a salt, an ester, an acid amide, an acid halide, an acid nitrile, and/or an acid anhydride of the carboxylic acid(s) which releases the carboxylic acid in the aqueous treatment solution.
12. A method of black passivation of zinc-containing surfaces wherein the surface to be treated is immersed in a treatment solution according to one of claims 1 to 9.
13. The method according to claim 12, wherein the temperature of the treatment solution is within a range of 20°C to 60°C.
14. The method according to claim 12, wherein the temperature of the treatment solution is within a range of 20°C to 40°C.
15. The method according to claim 12, wherein the temperature of the treatment solution is within a range of 20°C to 30°C.
16. The method according to one of claims 12 to 15, wherein the processing time in the treatment solution is between 10 s und 180 s.
17. The method according to one of claims 12 to 15, wherein the processing time in the treatment solution is between 30 s and 120 s.
18. The method according to one of claims 12 to 15, wherein the processing time in the treatment solution is between 45 s and 90 s.
19. The method according to one of claims 12 to 18, wherein the passivation treatment is facilitated by cathodic arrangement of the substrate in the passivation solution.
20. The method according to claim 19, wherein the cathodic current density on the substrate is between 0.05 A/dm² and 10 A/dm².
21. The method according to claim 19, wherein the cathodic current density on the substrate is between 0.1 A/dm² and 5 A/dm² _.
22. The method according to claim 19, wherein the cathodic current density on the substrate is between 0.1 A/dm² and 3 A/dm² _.