EP2014793B1 - Anti-corrosion treatment for conversion coatings - Google Patents

Abstract

Formation of a corrosion inhibiting coating involves contacting the surface to be treated with an aqueous solution (I) containing (a) trivalent chromium ions and (b) phosphate compound(s), at a weight ratio of (a) to (b) (calculated as orthophosphate) of 1:1.5-3.

Description

Field of the invention

The invention relates to the corrosion protection of metallic materials, in particular of those which are provided with a conversion layer.

Background of the invention

To protect metallic material surfaces from corrosive environmental influences, different methods are available in the prior art. The coating of the metallic workpiece to be protected with a coating of another metal is a widely used and established method in the art. The coating metal can behave in the corrosive medium either electrochemically nobler or less noble than the material base metal. If the coating metal behaves less noble, it acts in the corrosive medium compared to the base metal as a sacrificial anode (cathodic corrosion protection). Although this protective function associated with the formation of corrosion products of the coating metal is thus desired, the corrosion products of the coating often lead to undesirable decorative and often also to functional impairments of the workpiece. In order to reduce or prevent the corrosion of the coating metal as far as possible, special attention is given to cathodically protective base coating metals, such as, for example, Zinc or aluminum and their alloys often used so-called conversion coatings. These are reaction products of the non-noble coating metal which are insoluble in aqueous media over a wide pH range with the treatment solution. Examples of these so-called conversion layers are so-called phosphating and chromating.

In the case of phosphating, the layer to be protected is immersed in an acidic solution containing phosphate ions (see, for example, US Pat WO 00/47799 ). The acidic medium leads to the partial dissolution of zinc from the coating. The liberated Zn ²⁺ cations form a sparingly soluble zinc phosphate layer on the surface with the phosphate ions of the treatment solution. Since zinc phosphate layers themselves form only a comparatively bad corrosion protection, but an excellent primer for For this applied paints and paints, their main application is in the function of a primer for coatings and paints.

In the case of chromating, the surface to be treated is immersed in an acidic solution containing chromium (VI) ions (see EP 0 553 164 A1 ). For example, if it is a zinc surface, some of the zinc will dissolve. Under the reducing conditions prevailing here, chromium (VI) is reduced to chromium (III), which in the surface film which is more alkaline by the evolution of hydrogen, inter alia as chromium (III) hydroxide or as sparingly soluble μ-oxo or μ-hydroxo bridged chromium (III) Complex is deposited. In parallel, sparingly soluble zinc chromate (VI) is formed. Overall, a tightly closed, very well protects against corrosion attack by electrolytes conversion coating on the zinc surface.

However, chromium (VI) compounds are acutely toxic and highly carcinogenic, requiring replacement of the methods associated with these compounds.

As a substitute for chromating processes with hexavalent chromium compounds, a multitude of processes have now been established that use different complexes of trivalent chromium compounds (see DE 196 38 176 A1 ). Since the corrosion protection thus achieved is usually inferior to that of the process using hexavalent chromium, an organic seal, which is usually deposited from aqueous polymer dispersions, is often additionally applied to the surface of the workpiece. In particular, when using so-called black passivations, ie processes which form black layers on zinc-containing surfaces with the aid of trivalent chromium compounds, a post-treatment of this first conversion layer to increase its corrosion protection in the prior art (see WO 02/07902 A2 ) essential. A disadvantage of this additional process step using polymer dispersions is the occurrence of drainage drops in the coating of workpieces produced on the frame and / or the bonding of coated bulk material; In addition, problems such as dimensional accuracy of threads and the like are associated with the layer thickness of these organic sealants. If such sealings have a high degree of corrosion protection, the adhesion to the coated substrate is generally very good. This also means a very good adhesion to machine parts of the coating equipment, what their Cleaning difficult. Goods with coating defects, which should again go through the entire coating process, also has to be stripped of costs again, which usually requires an additional process step.

In addition, surfaces obtained by treatment with the known polymer dispersions, the state of which is determined essentially by the characteristics of the dispersed polymer, make it difficult to adjust friction coefficients μ _ges . > 0.25 (DIN 946).

The JP 2005 023372 A describes a passivating post-treatment for zinc surfaces provided with a conversion coating. In a preferred embodiment, a zinc surface was provided with a black passivation based on Cr (III) and Co. The passivation was post-treated with a solution containing 10 g / l Cr (III), 30 g / l phosphate, 10 g / l Zn and 25 g / l citric acid. The pH of the bath was 4.0; the treatment was carried out at 50 ° C; the treatment time was 3 to 30 seconds; the zinc was added as a mixture of zinc oxide and zinc carbonate. The cited document also describes the addition of water-soluble polymers such as polyacrylates to the treatment solution.

Description of the invention

The object of the invention is to provide processes for increasing the corrosion protection of metallic, in particular zinc-containing, surfaces provided with conversion layers. The aim is to preserve or improve the decorative and functional properties of the surfaces. In addition, the above-mentioned problems in the use of chromium (VI) -containing compounds or post-treatments with polymer dispersions should be avoided.

To achieve this object, the invention provides a method for producing a corrosion-protective coating layer, wherein a metallic, provided with a conversion layer surface is brought into contact with an aqueous treatment solution containing chromium (III) ions and at least one phosphate compound, wherein the ratio of the molar concentration (ie the concentration in mol / l) of chromium (III) ions to the molar concentration of at least one phosphate compound (based on orthophosphate calculated) ([chromium (III) ions]: [phosphate compound]) between 1: 1 , 5 and 1: 3 and wherein the at least one phosphate compound is selected from the group consisting of the esters of ortho-phosphoric acid, polyphosphoric acids and meta-phosphoric acid with organic radicals having up to 12 carbon atoms and mixtures of these compounds.

Phosphate compounds are derived from phosphorus in the oxidation state + V derived oxo compounds and their esters with organic radicals having up to 12 carbon atoms and the salts of mono- and diesters. Suitable phosphate compounds are alkyl phosphates with alkyl groups of up to 12 carbon atoms.

Examples of suitable phosphate compounds are Phosphoric acid methyl esters (mono-, di- and triesters), phosphoric acid ethyl esters (mono-, di- and triesters), phosphoric acid n- propyl esters (mono-, di- and triesters), phosphoric acid isopropyl ester (mono-, di- and triesters), phosphoric acid n-butyl ester (mono-, di- and triester), phosphoric acid 2-butyl ester (mono-, di- and triester), phosphoric acid tert -butyl ester (mono-, di- and triester), the salts of said mono- and diesters and mixtures of these compounds. The term "salts" includes not only the salts of completely deprotonated acids, but salts in all possible protonation stages, eg hydrogen phosphates and dihydrogen phosphates.

The treatment solution preferably contains between 0.2 g / l and 20 g / l of chromium (III) ions, more preferably between 0.5 g / l and 15 g / l of chromium (III) ions and more preferably between 1 g / 1 and 10 g / l chromium (III) ions.

The ratio of the molar concentration of chromium (III) ions to the molar concentration of the at least one phosphate compound (calculated on the basis of orthophosphate) is between 1: 1.5 and 1: 3, preferably between 1: 1.7 and 1: 2.5.

Chromium (III) may be added to the treating solution either in the form of inorganic chromium (III) salts, e.g. basic chromium (III) sulfate, chromium (III) hydroxide, chromium (III) dihydrogen phosphate, chromium (III) chloride, chromium (III) nitrate, potassium chromium (III) sulfate or chromium (III) salts organic Acids such as Chromium (III) methanesulfonate, chromium (III) citrate can be added or produced by reduction of suitable chromium (VI) compounds in the presence of suitable reducing agents. Suitable chromium (VI) compounds are e.g. Chromium (VI) oxide, chromates, such as potassium or sodium chromate, dichromates, e.g. Potassium or sodium dichromate. Suitable reducing agents for in situ generation of chromium (III) ions are e.g. Sulfites, e.g. Sodium sulfite, sulfur dioxide, phosphites, e.g. Sodium hypophosphite, phosphorous acid, hydrogen peroxide, methanol.

The treatment solution preferably has a pH between pH 2.5 and pH 7, preferably between pH 3 and pH 6, and particularly preferably between pH 3.5 and pH 5.

The treatment solution may additionally (optionally) contain one or more complexing agents. Suitable complexing agents are, in particular, organic chelate ligands.

Examples of suitable complexing agents are polycarboxylic acids, hydroxycarboxylic acids, hydroxypolycarboxylic acids, aminocarboxylic acids or hydroxyphosphonic acids. Examples of suitable carboxylic acids are citric, tartaric, malic, lactic, gluconic, glucuronic, ascorbic, isocitric, gallic, glycolic, 3-hydroxypropionic, 4-hydroxybutyric, salicylic, nicotinic, alanine, glycine, asparagine, aspartic, cysteine, glutamic, glutamine , Lysine. Suitable hydroxyphosphonic acids are, for example, Dequest 2010 ^™ (from Solutia Inc.); suitable as aminophosphonic acids, for example Dequest 2000 ^™ (from Solutia Inc.).

Generally, to enhance corrosion protection, the treatment solution will contain at least one metal or metalloid, e.g. Sc, Y, Ti, Zr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Si, P. These elements may be added in the form of their salts or in the form of complex anions or the corresponding acids of these anions such as hexafluoroboric acid, hexafluorosilicic acid, hexafluorotitanic acid or hexafluorozirconic acid, tetrafluoroboric acid or hexafluorophosphoric acid or their salts.

Particular preference is given to adding zinc which may be added in the form of zinc (II) salts, for example zinc sulfate, zinc chloride, zinc phosphate, zinc oxide or zinc hydroxide. Preferably, the treatment solution is added between 0.5 g / l and 25 g / l, more preferably between 1 g / l and 15 g / l Zn ²⁺ . The list of zinc compounds merely gives examples of compounds suitable according to the invention, but does not limit the amount of suitable zinc compounds to the substances mentioned.

The treatment solution may additionally (optionally) contain one or more water-soluble or water-dispersible polymers selected from the group consisting of polyethylene glycols, polyvinylpyrrolidones, polyvinyl alcohols to improve the film formation on the surface to be treated and to increase the hydrophobicity of the surface. Polyitaconic acids, polyacrylates and copolymers of the respective underlying monomers.

The concentration of the at least one polymer is preferably in the range between 50 mg / l and 20 g / l.

By adding the above-mentioned polymers to the treatment solution, the layer properties of the deposited corrosion protection layer are significantly improved.

The treatment solution may additionally (optionally) contain one or more wetting agents. As a result, a more uniform layer structure and a better drainage behavior is achieved, especially on complex parts or on heavier wettable surfaces. Especially advantageous is the use of fluoroaliphatic polymeric esters such as Fluorad FC-4432 ^™ (from 3M).

The surfaces treated according to the invention are metallic, preferably zinc-containing, surfaces provided with a chromium (III) -containing conversion layer.

By the method according to the invention a layer is deposited on the treated surface comprising chromium, phosphate (s) and optionally a metal, e.g. Zinc, and optionally one or more polymeric components. The inventive method leads to untreated, that is freshly deposited, not provided with a conversion layer zinc or zinc alloy surfaces not significantly contributing to the corrosion protection layers.

The contacting of the treatment solution with the surface to be treated can be carried out in the inventive method according to known methods, in particular by immersion.

The temperature of the treatment solution is preferably between 10 ° C and 90 ° C, more preferably between 20 ° C and 80 ° C, more preferably between 40 ° C and 60 ° C.

The duration of the contacting is preferably between 0.5 s and 180 s, more preferably between 5 s and 60 s, most preferably between 10 s and 30 s.

The treatment solution can be prepared prior to carrying out the method according to the invention by dilution of a correspondingly higher concentrated concentrate solution.

The objects treated according to the invention are no longer rinsed after being brought into contact, but dried.

Examples

The invention will be explained in more detail by way of examples; However, these are not in the claimed range according to claim 1.

example 1

A treatment solution according to the invention was prepared which contained the following constituents: 7 g / l Cr ³⁺ from chromium (III) hydroxide 28 g / l PO ₄ ^3- from ortho-phosphoric acid 9 g / l Zn ²⁺ from zinc oxide 18 g / l citric acid

The pH of the solution was adjusted to pH 3.9 with 20% sodium hydroxide solution.

A total of twelve test parts made of steel were coated in a weakly acidic process (Protolux 3000 ^™ from Atotech Deutschland GmbH) with an 8-10 μm thick zinc coating and rinsed with demineralized water.

Three of the sample parts (group A, for comparison) were dried only at 70 ° C for 20 min in a convection oven.

Three further sample parts (group B, for comparison) were immersed without further treatment for 20 s in the inventive treatment solution heated to 60 ° C. The parts were then dried without rinsing at 70 ° C for 20 min in a convection oven.

Three further sample parts (Group C, for comparison) were treated with a trivalent chromium ion-containing solution (Corrotriblue ^™ from Atotech Deutschland GmbH) to produce a chromium-containing conversion layer on the surface, rinsed with demineralized water and quenched at 70 ° C for 20 min min dried in a convection oven.

Three further sample parts (group D, according to the invention) were treated with a trivalent chromium ion-containing solution (Corrotriblue ^™ from Atotech Deutschland GmbH) to produce a chromium-containing conversion layer on the surface, rinsed with demineralized water and placed in the 20 sec immersed according to the invention, heated to 60 ° C treatment solution. The parts were then dried without rinsing at 70 ° C for 20 min in a convection oven.

The sample parts of groups A to D (in each case three parts) were then examined for their corrosion properties in the neutral salt spray test according to DIN 50021 SS. The times until the occurrence of zinc corrosion were: Group A: 3 h Group B: 3 h Group C: 24 hours Group D: 72 h

Example 2

A treatment solution according to the invention having the same composition as in Example 1 was prepared; the pH of the solution was adjusted to pH 3.9 with 20% sodium hydroxide solution.

The test part used was a galvanized steel part, which was treated with a black passivation solution containing essentially Cr ³⁺ , NO ₃ ^- , F ^- , and Fe ²⁺ (Tridur Zn H1 ^™ from Atotech Deutschland GmbH) with a black Conversion layer has been provided. The sample part thus treated was rinsed after the black passivation and immersed without intermediate drying step for 20 s in the inventive, heated to 60 ° C treatment solution. The part was then dried without rinsing at 60-80 ° C for 5 minutes in a convection oven.

The part thus treated had a black, slightly iridescent surface. There were clearly visible trace traces. The neutral salt spray test according to DIN 50021 SS showed no white corrosion for up to 48 h.

Examples 3 to 6

Examples 3 to 6 were carried out as Example 2 except that the composition of the treatment solution was varied as shown in Table 1. (In addition, the drying time in Examples 4 to 6 was 15 minutes each.) The appearance of the surface of the obtained sample parts and the corrosion properties are also shown in Table 1 (together with the data of Example 2). <b> Table 1. </ b> Composition of the treatment solution and properties of the obtained sample parts in Examples 2 to 6 Ex.2 EX3 EX4 Bsp.5 Bsp.6 Cr ³⁺ (g / l) 7 7 7 7 7 PO ₃ ^4- (g / l) 28 28 28 28 23 Zn ²⁺ (g / l) 9 9 9 9 9 Citric acid (g / l) 18 18 18 18 18 Polyvinylpyrrolidone [1] (g / l) 1 ionic surfactant [2] (ml / l) 1 Polyvinyl alcohol [3] (g / l) 1 1 1 Fluoro-surfactant [4] (g / l) 1 1 Isopropyl phosphate [5] (g / l) 8.5 Appearance black, slightly iridescent, noticeable traces of wear homogeneous black, not iridescent homogeneous black, not iridescent homogeneous, shiny black, not iridescent homogeneous, shiny black, not iridescent corrosion 48 h 72 h 120 h 168 h 120 h [1] Polyvinylpyrrolidone used was: Sokalan HP 59 ^™ from BASF
As ionic surfactant was used: Lutensit TC-APS 35 ^™ from BTC
[3] The polyvinyl alcohol used was: Mowiol 5-88 ^™ from Kuraray Specialties Europe GmbH
[4] Fluorotenside used was: Fluorad FC-4432 ^™ from 3M
[5] The isopropyl phosphate used was: a mixture of the mono- and diesters from Merck

Under "Appearance", the appearance of the surface of the sample part after the treatment with the treatment solution of the invention and drying is given.

The term "corrosion" indicates the time until which no white corrosion (<1%) was observed in the neutral salt spray test according to DIN 50021 SS.

Claims (19)

1. A process for preparing an anti-corrosion coating layer, where a metallic surface to be treated which is provided with a conversion layer is contacted with an aqueous treatment solution containing chromium (III) ions and at least one phosphate compound, where the ratio of the molar concentration of chromium (III) ions to the molar concentration of the at least one phosphate compound (calculated on the basis of orthophosphate) is between 1 : 1.5 and 1 : 3, where the at least one phosphate compound is selected from the group consisting of the esters of orthophosphoric acid, poly-phosphoric acids and meta-phosphoric acid with organic residues comprising up to 12 carbon atoms and mixtures of these compounds.
2. A process according to claim 1, where the concentration of the chromium (III) ions is in the range between 0.2 g/l and 20 g/l.
3. A process according to claim 1 or 2 where the treatment solution additionally contains one or more polymers soluble in water or dispersible in water, selected from the group consisting of polyethylene glycols, polyvinyl pyrrolidones, polyvinyl alcohols, polyitaconic acids, polyacrylates and copolymers of the monomers forming the basis of each.
4. A process according to any of claims 1 to 3 where the treatment solution additionally contains one or more complexing agents, selected from the group consisting of polycarboxylic acids, hydroxy carboxylic acids, hydroxy polycarboxylic acids, amino carboxylic acids or hydroxy phosphonic acids and amino phosphonic acids.
5. A process according to claim 4 where the complexing agent is selected from the group consisting of citric acid, tartaric acid, malonic acid, lactic acid, gluconic acid, glucoronic acid, ascorbic acid, isocitric acid, gallic acid, glycolic acid, 3-hydroxy propionic acid, 4-hydroxy butyric acid, salicylic acid, nicotinic acid, alanine, glycine, asparagine, asparaginic acid, cysteine, glutamic acid, glutamine and lysine.
6. A process according any of the claims 1 to 5, where the treatment solution additionally contains one or more metals or metalloids.
7. A process according to claim 6, where the metal or metalloid is selected from the group consisting of Sc, Y, Ti, Zr, Mo, W, Mn, Fe, Co, Ni, Zn, B, Al, Si and P.
8. A process according to claim 6, where the metal is zinc and the concentration of the zinc is in the range between 0.5 g/l and 25 g/l.
9. A process according to any of the claims 6 to 8, where the metal or metalloid has been added to the treatment solution in the form of one of its salts or in the form of a complex anion or the respective acids of these anions such as hexafluoro boric acid, hexafluoro silicic acid, hexafluoro titanic acid, or hexafluoro zirconic acid, tetrafluoro boric acid or hexafluoro phosphoric acid or salts thereof.
10. A process according to any of the claims 1 to 9, where the pH value of the treatment solution is between pH 2.5 and pH 7.
11. A process according to any of the claims 1 to 9, where the pH value of the treatment solution is between pH 3.5 and pH 5.
12. A process to any of the claims 1 to 9, where the pH value of the treatment solution is between pH 3.8 and pH 4.5.
13. A process according to any of the claims 1 to 12, where the temperature of the treatment solution is between 10°C and 90°C.
14. A process according to any of the claims 1 to 12, where the temperature of the treatment solution is between 20°C and 80°C.
15. A process according to any of the claims 1 to 12, where the temperature of the treatment solution is between 40°C and 60°C.
16. A process according to claims 1 to 15, where the treatment solution has been prepared by diluting a concentrate solution having a correspondingly higher concentration.
17. A process according to any of the claims 1 to 16, where the duration of contacting is between 0.5 seconds and 180 seconds.
18. A process according to any of the claims 1 to 16, where the duration of contacting is between 5 seconds and 60 seconds.
19. A process according to any of the claims 1 to 16, where the duration of contacting is between 10 seconds and 30 seconds.