EP2319957A1 - Black passivation of zinc and zinc-iron layers - Google Patents

Abstract

Acidic, aqueous passivation solution comprises chromium(III) ions, fluoride ions and nitrate ions, dithiodiglycolic acid, thioglycolic acid or a salt of these acids. Independent claims are included for: (1) producing black passivation layers on zinc- or zinc iron alloy layers, comprising contacting a workpiece, which is coated with the zinc- or zinc iron alloy layers, with the acidic, aqueous passivation solution; and (2) the black passivation layer, obtained by the above process.

Description

Technical area

The invention relates to an acidic, aqueous reaction solution for producing a black passivation layer on zinc and zinc iron layers, to a method in which this reaction solution is used, and to the black passivation layer obtainable by this method.

State of the art

The galvanic coating of components made of iron or steel with zinc or zinc alloys is carried out on a large scale and has a high economic importance. The deposited layers protect the base material against corrosion and thus lead to a high increase in the value of the components. The corrosion protection of iron or steel by zinc or zinc alloys is based on the anodic protection mechanism. Zinc or zinc alloys are less electrochemical than the base material. The corrosion attack therefore begins on the coating and the base material remains protected against corrosion as long as a closed zinc or zinc alloy surface is present. The applied less noble zinc or zinc alloy layers are therefore also referred to as "sacrificial layers".

When using a zinc alloy, eg zinc-nickel, care must be taken that the electrochemical potential the alloy still remains non-noble in relation to the base material. A zinc nickel layer with> 16% nickel is the nobler component compared to iron or steel and therefore can no longer provide anodic corrosion protection. The deposition of such a layer on iron or steel would therefore lead to preferred base material corrosion inhomogeneities of the coating (pitting corrosion).

In order to delay the corrosion attack on the sacrificial layers of zinc or a zinc alloy even further and thus improve the overall corrosion protection, it is known in the art to apply an additional conversion layer to the zinc or zinc alloy layers. Here, it has been customary in the past to produce such conversion layers by treating the components in acidic, chromate-containing solutions. A disadvantage of these layers is that also hexavalent chromium compounds are incorporated therein. These are partially water-soluble and can be leached out of the layers. Because of the high toxicity of chromium (VI) compounds, such layers pose a hazard to humans and the environment. The hexavalent chromium compound processes are therefore increasingly being replaced by non-toxic chromium (III) compound based processes.

The color of the chromating layers can be varied by composition of the solutions and by the working parameters in the treatment. You can create blue, yellow or olive colored surfaces. In this order, the layer thickness of the conversion layers and thus the corrosion protection increases.

Furthermore, it is also possible to produce black chromating layers. The black chromations for pure zinc coatings were often added silver salts in very low concentrations. By reaction with the zinc surface, the Silver ions reduced to metallic silver. This resulting in very finely divided form silver is incorporated into the forming chromate layer and acts there as a black pigment. Black chromations can produce deep black, uniform surfaces that can be used for decorative purposes. However, the incorporation of small amounts of silver in the layer can have a detrimental effect on the corrosion protection, since silver is a very noble metal and therefore corrosion can lead to local element formation and thereby deterioration of the corrosion protection.

For zinc alloy layers such as zinc-iron, zinc-cobalt or zinc-nickel, the alloying partners iron, cobalt or nickel can be converted into their oxides by suitable composition of the chromating solutions. These then act as black pigments in the chromating layer. Addition of silver salts to produce black pigments is not required in these cases.

However, in the field of methods for forming passivation layers from solutions based on trivalent chromium compounds, there is a need for further methods for the black passivation of zinc layers.

• Hyposchwefelige Säure (H₂SO₂)
• Schwefelige Säure (H₂SO₃)
• Schwefelwasserstoffsäure (H₂S)
• Thioschwefelsäure (H₂S₂O₃) und
• Dischweflige Säure (H₂S₂O₅)

A method for black passivation of zinc or zinc alloy layers is in WO 2005/068684 A1 disclosed. It describes how to add inorganic and / or organic sulfur compounds to passivating solutions based on trivalent chromium compounds. Suitable sulfur compounds are specifically named:

• Hyposulphurous acid (H ₂ SO ₂ )
• Sulfurous acid (H ₂ SO ₃ )
• Hydrosilicic acid (H ₂ S)
• Thiosulfuric acid (H ₂ S ₂ O ₃ ) and
• Dischweflige acid (H ₂ S ₂ O ₅ )

All of these compounds mentioned are those which are not stable in the acidic range around pH 2, which is usually used. In the acidic area, toxic gases are released in some cases (SO ₂ , H ₂ S). On the one hand, the decomposition of the sulfur compound leads to unstable processes and, on the other hand, the release of the toxic gases poses a threat to the operating personnel of such plants.

Of course, the problem of the decomposition of these sulfur compounds also remains, if one does not use the free acids, but their salts. The acids underlying the salts are weaker acids than e.g. Nitric acid or sulfuric acid used to adjust the pH of passivation solutions. The dissociation of the corresponding salts is thereby shifted to the side of the free acids, which then in turn uses the decomposition of the compounds.

WO 2005/068684 A1 The formation of the black passivation layer leads to the formation of iron-sulfur pigments. Although the method should also be suitable for the black passivation of pure zinc coatings, this hardly seems possible because they contain no iron and thus no iron-sulfur pigments can be formed.

A black corrosion resistant coating on zinc alloys and a method for its production is disclosed in US Pat EP 1 409 157 B1 described. The layers are generated in a two-step process. First, a passivation layer is formed in a passivation solution based on trivalent chromium compounds. This layer does not have a black appearance yet. In a second process step is then in a sealing solution, the Contains black pigments, submerged. Although it is possible to achieve layers with the desired black appearance, such a two-step process is undesirable in practice. The application of a seal based on organic compounds is not permitted in all applications. It is therefore more advantageous if the black optic can already be achieved by the passivation layer and not only by the additional step of sealing.

EP 1 484 432 A1 claims a treatment solution and method for depositing black passivation layers on zinc or zinc alloy layers. A disadvantage of this method is the relatively complicated composition of the treatment solution. The deposition of black passivation layers requires a very close ratio of nitrate to chromium. The ratio NO ^- ₃ / Cr ³⁺ should be less than 0.5 / 1. The chromium concentration is preferably 0.5 to 10 g / l, corresponding to 0.001 to 0.19 mol / l. This results in a resulting nitrate concentration of not more than 0.0005 to 0.095 mol / l or 0.03 to 5.9 g / l. The analysis of such low nitrate concentrations is very difficult with the analytical methods that are commonly available in a galvanic plant operation laboratory. Furthermore, the formation of the passivation layer continuously consumes chromium, which shifts the ratio NO ^- _{3 /} Cr ³⁺ . The operation of the solution, which requires a tight NO ^- ₃ / Cr ³⁺ ratio, thereby becomes unstable. The solution also contains complexing agents from the group of monocarboxylic acids, dicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids or aminocarboxylic acids. These form partially stable complexes with the metal ions contained in the solution. These cause difficulties in wastewater treatment and are therefore undesirable.

In EP 1 484 432 A1 Furthermore, a method for depositing black conversion layers obtained from a solution consisting of nitrate ions and trivalent chromium ions in a molar ratio NO ^- ₃ / Cr ³⁺ of less than 0.5 / 1 is described. In addition, the solution still contains phosphate ions. In the layer formation reaction, pH shifts occur at the substrate surface. As a result, insoluble phosphorus compound from the metal ions present in the solution (eg Zn ²⁺ , Cr ³⁺ , Ni ²⁺ or CO ²⁺ ) are formed. A disadvantage of such phosphate-containing solutions that they have only a short life. After only a short period of use, brownish to greenish iridescent layers are formed; the desired deep black hue is difficult to achieve.

The problem of insufficient blackening is said to be due to a method according to the EP 1 995 348 A1 be solved. There, a solution is claimed which contains zinc ions, trivalent chromium ions, complexing agents which form water-soluble complexes with the trivalent chromium ions, sulfur compounds and phosphite ions. The individual components must be in a certain relationship to each other. Deviations from this lead either to insufficient blackening of the layers or to a low corrosion resistance. During a passivation solution, zinc accumulates during operation, while chromium is consumed by the formation of the conversion layer. As a result, the ratio of zinc to chromium in the solution is constantly changing. The monitoring of such a passivation solution is therefore very expensive.

There is therefore a need for stable passivation solutions which enable the deposition of deep black passivation layers on zinc or zinc alloy layers with good corrosion resistance in a simple and reliable continuous operation operation.

Summary of the invention

Surprisingly, it has been found that the addition of dithiodiglycolic acid or thioglycolic acid or a salt of this acid to an acidic, aqueous passivating solution containing Cr (III) ions, fluoride ions and nitrate ions results in deep black conversion layers on zinc and zinc iron alloy layers which are already without subsequent sealing achieve excellent resistance in the salt spray test according to DIN EN ISO 9227. Subsequent sealing achieves a durability of> 240 hours.

Therefore, the present invention relates to the above-described acidic aqueous passivating solution containing Cr (III) ions, fluoride ions, nitrate ions and dithiodiglycolic acid, thioglycolic acid or a salt of these acids, and a process for producing black passivation layers on zinc or zinc iron alloy layers a workpiece, which is provided with a zinc or Zinkeisenlegierungsschicht, is brought into contact with the passivation solution according to the invention. Moreover, the present invention relates to a black passivation layer obtainable by the method according to this invention.

Detailed description of the invention

There are currently no standardized requirements for black passivation on zinc. In practice, however, at least the values which have already been achieved in the prior art for black passivation layers on zinc-iron alloys should be achieved. For such layers, the requirements according to DIN 50 979 apply when using additional seals for Cestellware at least 168 hours resistance in salt spray test according to DIN EN ISO 9227. With the method according to the invention this requirement is already reached by pure zinc coatings. The reaction solution according to the invention is suitable for the formation of black conversion layers on both pure zinc layers and zinc iron layers. However, the use of pure zinc coatings is preferred because there is a much greater advantage if pure zinc coatings can be directly black passivated.

According to the invention, the passivation solution contains a dithiodiglycolic acid (HOOC-CH ₂ -SS-CH ₂ -COOH) or a thioglycolic acid (HS-CH ₂ -COOH) or a salt of these acids. Any salts of dithiodiglycolic acid and thioglycolic acid can be used. Examples of these are the sodium or disodium salt, the potassium or dipotassium salt and the ammonium or diammonium salt. Particularly preferred is the use of the diammonium salt of dithiodiglycolic acid.

The passivation solution according to the invention contains dithiodiglycolic acid, thioglycolic acid or a salt of these acids in a concentration, based on the free acid, of preferably 2-10 g / l passivating solution, more preferably 4-6 g / l passivating solution.

For the Cr (III) ions, all soluble chromium (III) compounds can be used. Examples which may be mentioned are chromium (III) chloride, CrCl ₃ .H ₂ O, basic chromium (III) sulfate, CrOHSO ₄ , potassium chromium sulfate, KCr (SO ₄ ) ₂ .6H ₂ O and chromium nitrate, Cr (NO ₃ ). ₃ x 9H ₂ O. Preferred is chromium nitrate, Cr (NO ₃ ) ₃ .9H ₂ O.

The Cr (III) ions are preferably present in the passivating solution in an amount of 1-10 g / l passivating solution, more preferably 4-5 g / l passivating solution.

Further, the passivating solution according to this invention contains fluoride ions. The fluoride ions are preferably present in the passivation solution in an amount of 0.3-3 g / l passivating solution, more preferably 1-1.5 g / l passivating solution.

As fluoride ions, all soluble fluoride compounds can be used. Preferred are sodium fluoride, NaF, potassium fluoride, KF, ammonium fluoride, NH ₄ F, sodium bifluoride, NaF · HF, potassium bifluoride, KF · HF, ammonium bifluoride, NH ₄ F · HF. Very particularly preferred is the use of sodium fluoride, NaF.

The passivation solution according to the present invention also contains nitrate ions. Preferably, the amount of nitrate ions contained in the passivating solution is 3 - 35 g / l passivating solution, more preferably 15 - 25 g / l. The nitrate ions can be added to the passivating solution in any form of soluble nitrate salt. It is also possible to use a combination of several different soluble nitrate salts. For example, the nitrate ions may be added together with the Cr (III) ions as the chromium nitrate, Cr (NO ₃ ) ₃ .9H ₂ O. Another suitable nitrate salt is, for example, sodium nitrate. When the nitrate ion addition is in the form of sodium nitrate, it should be considered in terms of the preferred total nitrate concentration in which form the addition of the chromium compound occurs.

Particularly preferred is a passivating solution containing Cr (III) ions in an amount of 1-10 g / l passivating solution, fluoride ions in an amount of 0.3-3 g / l passivating solution, nitrate ions in an amount of 3-35 g / Passivierungslösung and dithiodiglycolic acid or a salt thereof in an amount based on dithiodiglycolic acid, 2-10 g / l.

According to the present invention, an acidic, aqueous passivating solution containing the following constituents is very particularly preferred: Cr (III) ions in an amount of 4-5 g / l passivating solution, fluoride ions in an amount of 1-1.5 g / Passivation solution, nitrate ions in an amount of 15-25 g / l passivating solution and dithiodiglycolic acid or a salt thereof in an amount, based on dithiodiglycolic acid, of 4-6 g / l.

In addition, the passivation solution of the present invention may contain cobalt ions. The incorporation of cobalt ions in passivations based on trivalent chromium compounds is known and serves to further improve the corrosion resistance. The concentration of cobalt ions in the passivating solution is preferably 0.1-10 g / l passivating solution, more preferably 0.5-5 g / l. The addition of the cobalt ions can be carried out in any form of water-soluble cobalt salts. Preference is given to cobalt sulfate, cobalt chloride or cobalt nitrate.

The pH of the passivating solution of the present invention is suitably adjusted to a preferred range of 1.8 to 2.2, more preferably 1.9 to 2.1.

The present invention further provides a process for producing black passivation layers on zinc and zinc iron layers using the passivation solution described above. In the method of the present invention, for this purpose, a workpiece provided with a zinc or zinc iron layer is brought into contact with the passivating solution. The contacting may be spraying, swelling or dipping. Preferably, dipping is used in the art. The reaction temperature is preferably 10 to 70 ° C. Particularly preferred is a temperature range of 15-50 ° C, more preferably 20-50 ° C. The workpieces are with the Passivation for a period of preferably 60- 90 seconds, more preferably 75 seconds, brought into contact. That is, when the workpieces are dipped in the passivation solution, the dipping time is preferably 60-90 seconds. Particularly preferred is a dive time of 75 seconds. According to the present invention, the black passivation layer may be formed on a zinc or zinc iron alloy layer. However, preference is given to the application of the method according to the invention to pure zinc coatings. This eliminates the costly process control, which is due to the process of each electrolyte for the deposition of an alloy.

After the workpieces have been brought into contact with the acidic, aqueous passivating solution of the present invention and thus the black passivation layer has been produced, a sealing layer, a so-called "topcoat", can additionally be applied to the black passivation layer. Such a sealing layer is known and improves corrosion resistance. For this purpose, the workpiece is brought into contact with a sealing solution. It is an aqueous solution containing organic, water-soluble or dispersible polymers. For example, copolymers of acrylic acid or methacrylic acid with ethylene, commercially available under the name Lugalavan DC Fa. BASF, are used. Inorganic sealant solutions contain colloidal silicic acids. It is also possible to prepare mixtures of aqueous solutions with organic polymers and aqueous solutions with colloidal silicas and to use these as a sealing solution.

Finally, the present invention also provides a black passivation layer obtainable by the method described above. Preferably, the black passivation layer is formed on a pure zinc layer.

Examples example 1

First, a solution of Cr (III) and fluoride ions is prepared having the following composition: 233 g Chromium nitrate (Cr (NO ₃ ) x 9H ₂ O) 20 g Sodium fluoride (NaF) 250 g water

The mixture is heated to 65 ° C for 30 minutes, then cooled to room temperature. It is then made up to 1 liter with deionized water.

• 130 ml der oben erhaltenen Lösung aus Cr-nitrat und NaF 8 g Cobaltsulfat (CoSO₄ x 7H₂O)
• 10 g Natriumnitrat (NaNO₃)
• 12 g Diammoniumdithiodiglykolatlösung (48 Gew.%)

To prepare the passivation solution, the solution obtained above is reacted with the following constituents:

• 130 ml of the above-obtained solution of Cr nitrate and NaF 8 g of cobalt sulfate (CoSO ₄ .7H ₂ O)
• 10 g of sodium nitrate (NaNO ₃ )
• 12 g of diammonium dithiodiglycolate solution (48% by weight)

The volume was made up to approximately 1 liter with deionized water, then the pH was reduced with conc. Nitric acid (53 wt.%) Adjusted to 1.8 to 1.9 and made up to the final volume of 1 liter.

In an alkaline zinc bath (ZINCASLOT 50 from Dr.-Ing. Max Schlötter), steel sheets were galvanized after conventional pretreatment at a current density of 2 A / dm ^{2 over} a period of 30 minutes. The layer thickness was 8 μm. After deposition, the test sheet was rinsed in deionized water, immersed in dilute nitric acid solution (10 ml / l HNO ₃ 53% by weight) for 5 seconds and then in the above immersed in the following conditions: Temperature: 25 ° C PH value: 1.8 - 1.9 Dive time: 60 seconds

The test sheet was then rinsed with deionized water and dried in a drying oven at 80 ° C for 15 minutes. The appearance of the test sheet was then deep black with slightly brownish iridescent spots.

Example 2

Example 1 was repeated as described. After passivation and rinsing, it was immersed in a sealing solution (SLOTOFIN 70 from Dr.-Ing. Max Schlötter, aqueous dispersion of a copolymer of acrylic acid and ethylene).

Conditions:

300 ml / l sealing concentrate SLOTOFIN 71 Dive time: 30 seconds Drip: 60 seconds Temperature: 25 ° C

After sealing, the test sheet was dried without rinsing at 80 ° C for 15 minutes in a drying oven.

The sealed test sheet had a uniform deep black, semi-glossy appearance.

corrosion tests

Test panels, which were prepared according to Examples 1 and 2, were tested in a corrosion test in the neutral salt spray test according to DIN EN ISO 9227. Following DIN 50 979, 1 sample was tested without additional heat treatment and 1 sample was annealed at 120 ° C. for 24 hours. corrosion protection without heat treatment with heat treatment
(120 ° C / 24h) example 1 72 h 72 h Example 2 > 240 h > 240 h

Comparative Example 1

According to Example 1, a solution of chromium nitrate and sodium fluoride was first prepared. Subsequently, a solution having the following composition was prepared.

• 8 g Cobaltsulfat (CoSO₄ x 7H₂O)
• 10 g Natriumnitrat (NaNO₃)
• 12 g Thiodiglykolsäure

130 ml of the solution of Cr nitrate and NaF according to Example 1

• 8 g cobalt sulphate (CoSO ₄ .7H ₂ O)
• 10 g of sodium nitrate (NaNO ₃ )
• 12 g of thiodiglycolic acid

The volume was made up to approximately 1 liter with deionized water, then the pH was reduced with conc. Nitric acid (53 wt.%) Adjusted to 1.8 to 1.9 and made up to the final volume of 1 liter.

Steel plates galvanized in an alkaline zinc bath were treated in the above-indicated passivation solution as indicated in Example 1. It could not be achieved blackening.

Comparative Example 2

A passivation solution was prepared as in Comparative Example 1, but thiodiglycolic acid was replaced by 4 g of cystine. The passivation reaction did not result in blackening.

Claims (10)

Acid, aqueous passivation solution containing black passivation layers on zinc or zinc iron alloy layers Cr (III) ions, - Fluoride ions and - nitrate ions, characterized in that the passivating solution additionally contains dithiodiglycolic acid, thioglycolic acid or a salt of these acids. The passivating solution according to claim 1, wherein the dithiodiglycolic acid or a salt thereof is contained in an amount of 2 - 10 g / l of a passivating solution based on dithiodiglycolic acid. A passivating solution according to claim 1, wherein the thioglycolic acid or a salt thereof is contained in an amount of 2 - 10 g / l of a passivating solution based on thioglycolic acid. Passivation solution according to one or more of claims 1-3, wherein the passivation solution has a pH of 1.8 to 2.2. Passivation solution according to one of the preceding claims, comprising: Cr (III) ions in an amount of 1-10 g / l passivation solution, Fluoride ions in an amount of 0.3-3 g / l passivation solution, - nitrate ions in an amount of 3 - 35 g / l passivation solution and - Dithiodiglycolic acid or a salt thereof in an amount, based on dithiodiglycolic acid, 2-10 g / l. A method for producing black passivation layers on a zinc or zinc-iron alloy layer, characterized in that a workpiece coated with a zinc or zinc-iron alloy layer is brought into contact with the acidic, aqueous passivating solution according to one or more of claims 1-5. A method according to claim 6, wherein the workpiece is contacted with the passivating solution at a reaction temperature of 10 - 70 ° C. The method of claim 6 or 7, wherein the workpiece is contacted with the passivating solution for a period of 60-90 seconds. Method according to one or more of claims 6-8, wherein the black passivation layer is formed on a pure zinc layer. Black passivation layer obtainable by the process according to one or more of claims 6-9.