EP0633950B1 - Nickel-free phosphatization process - Google Patents

Abstract

The invention is a process for applying a nickel-free, copper-containing phosphate coating to a metal surface by contacting the metal surface with a phosphate solution containing 0.2 to 2.0 g/l zinc ions, 0.5 to 25 mg/l copper ions, and 5-30 g/l phosphate ions.

Description

The invention relates to a method for producing copper-containing, nickel-free phosphate layers on metal surfaces and to the use of the method as pretreatment of the metal surfaces before painting, in particular cataphoretic dip painting (KTL).

The quality of phosphate coatings before cataphoretic dip painting (KTL) depends on a variety of parameters. These include physical quantities such as the shape and size of the crystals, their mechanical stability and in particular the free metal surface after phosphating, the so-called pore surface. With regard to the chemical parameters, the alkali stability during the cataphoretic coating, the binding strength of the crystal water of the zinc phosphate crystals when baking the paints and the rehydration ability are of particular interest.

The layer weight can be controlled, in particular reduced, by using activating agents prior to phosphating. The polymeric titanium phosphates present in the activating agents form active centers on the metal surface from which crystal growth proceeds. The result is smaller and mechanically more stable crystals on the one hand, and on the other hand the pore area is reduced, making it more difficult to attack corrosive media if the paint is damaged.

In the prior art, it has proven to be advantageous to provide a separate treatment bath in order to optimally influence the layer quality of the subsequent phosphating. However, the duration of action (service life) of the activation baths is limited by the entry from the previous cleaning baths.

The water hardness ions in particular deactivate the polymeric titanium phosphates.

Therefore, ways were sought to achieve a dense, low-pore phosphate layer with a low mass per area by other means or in addition to activation directly in the phosphating bath. Extensive basic work was carried out for this. Part of this work was carried out at the Institute of Crystallography at the University of Cologne and led to the discovery of a new crystal phase Ba₃ (PO₄) ₂ · H _z O (Z. für Kristallographie 196, 312 - 313 (1991)).
Barium phosphate coatings do not contain zinc, but have a number of positive properties, such as high thermal stability. However, the achievable layer weights are not sufficient to achieve a high level of corrosion protection in combination with a cataphoretic dip coating. The barium phosphate coatings thus occupy an intermediate position between the "thin" iron phosphate coatings (0.3-0.5 g / m) and the "thicker" zinc phosphate coatings (2.0-3.5 g / m).

Aluminum ions reduce the phosphate layer weights even more, so that so-called "passivation phenomena" already occur from a concentration of 5 ppm Al³ + ions in the phosphating bath, i. H. disturbances in the formation of zinc phosphate coatings.

Additions of magnesium ions were also investigated. The application-technical effects of these ions were recognized early on (DE-A-39 20 296), which are based on several effects. The high crystal stability during heating is also important here. The release of crystal water, which weakens the crystal composite and thus the overall system, is shifted to higher temperatures with increasing magnesium incorporation. On the other hand, the crystals by adding Mg⁺ ions smaller, the phosphate layer thicker and the free metal surface after phosphating is minimized. The layer weight reduction by magnesium ions is so strong that other control parameters, which are usually also used to reduce the layer weight, such as very low zinc concentrations (0.6 g / l Zn⁺), high concentrations of accelerators such as sodium nitrite or meta-nitrobenzenesulfonate / Na salts do not have to be used to generate a mass per unit area in the range of 1.5 - 2.0 g / m.

The influence of Cu⁺ ions was also investigated. Additions of small amounts of copper ions to phosphating baths have been known for 40 years. In US-A-2 293 716, the smallest amounts of Cu⁺ ions are added as "accelerators" or to improve the whiteness of anodic electrocoating materials as "color neutralizers". It was observed that copper additives increase the layer weight, especially on steel.

From DE-A-40 13 483 a method for phosphating metal surfaces is known, in which one works with phosphating solutions which are essentially free of nickel. Zinc, manganese and low copper contents are mentioned as essential bath components. In addition, the concentration of Fe (II) is kept below a maximum value by oxygen and / or other oxidizing agents having the same effect. The method is used in particular for the pretreatment of metal surfaces for subsequent painting, in particular electro-dip painting, and the phosphating of steel, galvanized steel, alloy-galvanized steel, aluminum and their alloys.

EP-A-0 186 823 discloses strongly acidic phosphating solutions with a pH of 1.8-2.5 which contain 7.5-75 g / l zinc ions, Contain 0.1 - 10 g / l hydroxylamine and optionally up to 20 g / l manganese ions and 5 - 75 g / l nitrate ions. The solutions tolerate an iron content of up to 25 g / l.

EP-A-0 315 059 discloses a process for zinc phosphating iron-containing surfaces. The desired morphology of the zinc phosphate crystals is set by using hydroxylammonium salts, hydroxylamine complexes and / or hydroxylamine. In addition to zinc, all of the examples contain nickel as another layer-forming cation. The toxicological disadvantages of nickel are known.

The object was therefore to provide a process for the production of nickel-free phosphate layers which, in the absence of nickel on metal surfaces such as cold rolled steel, electrolytically galvanized steel and aluminum, ensures very good paint adhesion and excellent corrosion protection.

The above-mentioned object is achieved with the aid of a specially selected phosphating solution which contains hydroxylamine salts, hydroxylamine complexes and / or hydroxylamine in an amount of 500 to 5000 ppm of hydroxylamine, based on the phosphating solution, as an active ingredient for modifying the crystal morphology ("accelerator"). With the help of such phosphating solutions, it is possible to produce copper-containing phosphate layers with a defined copper content and a defined edge length of the phosphate crystals.

The present invention relates, according to claim 1, to a method for producing copper-containing nickel-free phosphate layers with a copper content in the range from 0.1 to 5% by weight and an edge length of Phosphate crystals in the range of 0.5 to 10 µm on metal surfaces selected from steel, galvanized steel, alloy galvanized steel, aluminum and their alloys, by treating them in a spray, dip or spray / dip process with a phosphating solution which does the following Components contains: Zinc ions 0.2 to 2 g / l Copper ions 0.5 to 25 mg / l Phosphate ions 5 to 30 g / l (calculated as P₂O₅) and also hydroxylamine salts, hydroxylamine complexes and / or hydroxylamine in an amount of 500 to 5000 ppm hydroxylamine based on the phosphating solution. Further preferred embodiments of the method according to the invention are claimed in dependent claims 2 to 9. The use of the method is claimed in claim 10. It was found that these phosphating solutions, even in the absence of nickel on metal surfaces as mentioned above, guarantee very good paint adhesion and excellent corrosion protection without speck formation. The zinc phosphate layers produced in this way are made up of small (0.5 to 10 µm), compact, densely grown crystals.

In particular, when examining copper ion-containing phosphating baths, it was found that only very small amounts of copper ions are required in the solution in order to set the desired copper content of the phosphate layer in the range from 0.1 to 5% by weight.

In a further embodiment of the present invention, it is therefore preferred that the phosphating solution contains 5 to 20 ppm of copper ions when the metal surface is brought into contact with the phosphating solution by means of immersion processes. When using the spraying process, it is equally preferred that the phosphating solutions contain 1 to 10 ppm copper ions in order to incorporate correspondingly high copper contents in the conversion coating.

In order to ensure proper formation of the phosphate layer, it is known to adjust the pH of the phosphating solution to a value between 2.5 and 3.5. If necessary, further cations, for example alkali metal cations and / or alkaline earth metal cations with corresponding anions known in the prior art, are used to adjust the pH of the phosphating solution. The pH value can be corrected during the phosphating, for example, with basic additives or acids.

By adding manganese (II) ions, fine crystals are formed in particular when spraying on surface-refined materials, which hardly show the known needle structure, but have a much more compact granular morphology. The use of manganese ions in addition to zinc ions in low-zinc phosphating processes improves corrosion protection, especially when using surface-coated thin sheets. The incorporation of manganese into the zinc phosphate coatings leads to smaller and more compact crystals with increased stability to alkali. Accordingly, a particularly preferred embodiment of the present invention is that the phosphating solution contains 0.1 to 5 g / l, in particular 0.5 to 1.5 g / l, of manganese (II) ions.

The quality of the copper-containing nickel-free phosphate layers produced with the aid of the method according to the invention is not impaired if the phosphating solution contains alkaline earth metal cations of up to 2.5 g / l, in particular magnesium and / or calcium ions.

The method according to the invention can be applied in particular to steel, steel galvanized on one or two sides, steel galvanized on one or both sides, aluminum and its alloys. The term steel in the sense of the present invention includes, in addition to low-alloy steels, also soft, unalloyed steels and higher as well as high-strength steels. The essential content of the invention is that the aqueous, acid phosphating solutions are free of nickel. However, this means that under technical conditions a small amount of nickel ions can be contained in the phosphating baths. However, in accordance with the prior art DE-A-40 13 483, this amount should be less than 0.0002 to 0.01 g / l, in particular less than 0.0001 g / l.

When using the phosphating process on steel surfaces, iron in the form of iron (II) ions dissolves. By adding suitable oxidizing agents, iron (II) is converted into iron (III) and can thus be precipitated as iron phosphate sludge. Accordingly, it is typical in the sense of the present invention that the phosphating solution contains up to 50 ppm - briefly in the production process, but also up to 500 ppm - iron (II) ions.

A number of oxidizing agents are known in the prior art for limiting the iron (II) ion concentration. For example, the contact of the phosphating solution with oxygen, for example atmospheric oxygen and / or the addition of suitable oxidizing agents, serves to limit the iron (II) ion concentration.

Accordingly, it is preferred according to the invention that the phosphating solution contains oxidizing agents selected from peroxide compounds, chlorates, permanganates and organic nitro compounds.

For the purposes of the present invention, the oxidizing agents of the phosphating solutions are preferably selected from peroxide compounds, in particular hydrogen peroxide, perborate, percarbonate and Perphosphate, and organic nitro compounds, especially nitrobenzenesulfonate. The amounts of oxidizing agent to be used are known from the prior art. For example: Peroxide compound calculated as hydrogen peroxide: 0.005 to 0.1 g / l, nitrobenzenesulfonate: 0.005 to 1 g / l.

If the phosphating process is applied to galvanized steel, alloy galvanized steel, aluminum and its alloys, the presence of iron (II) ions is not harmful. Accordingly, the addition of oxidizing agents can be completely dispensed with when phosphating these materials when using the method according to the invention.

Furthermore, it has proven to be particularly advantageous to work with nitrate-free phosphating solutions in the phosphating of galvanized metal surfaces according to the invention.

Furthermore, a preferred embodiment of the present invention consists in using phosphating solutions which are essentially free of nitrite ions. A significant advantage of this process variant is that no toxic decomposition products of nitrites, for example nitrous gases which are hazardous to health, can arise.

The use of modifying compounds from the group of surfactants, hydroxycarboxylic acids, tartrate, citrate, hydrofluoric acid, alkali metal fluoride, boron trifluoride, silicon fluoride is known in principle from the prior art. While the addition of surfactants (for example 0.05 to 0.5 g / l) leads to an improvement in the phosphating of lightly greased metal surfaces, it is known that hydroxycarboxylic acids, in particular tartaric acid, citric acid and their salts, in a concentration range of 0.03 to 0.3 g / l contribute to a significant reduction in the weight of the phosphate layer. Fluoride ions promote the phosphating of metals that are more difficult to attack, thereby shortening the phosphating time and also increasing the area coverage of the phosphate layer. As is known, about 0.1 to 1 g / l of the fluorides are used. The controlled addition of fluorides also enables the formation of crystalline phosphate layers on aluminum and its alloys. Salts of boron tetrafluoride and silicon hexafluoride increase the aggressiveness of the phosphating baths, which is particularly noticeable in the treatment of hot-dip galvanized surfaces, which is why these complex fluorides can be used, for example, in amounts of 0.4 to 3 g / l.

Phosphating processes are usually used at bath temperatures between 40 and 60 ° C. These temperature ranges are used in spraying as well as in spray-immersion and immersion applications.

The metal surfaces to be phosphated are cleaned, rinsed and, if necessary, treated with activating agents, in particular based on titanium phosphates, according to processes known per se in the prior art.

The phosphating baths for carrying out the process according to the invention are generally prepared in the customary manner which is known per se to the person skilled in the art. The following compounds are suitable as starting products for the preparation of the phosphating bath: Zinc: in the form of zinc oxide, zinc carbonate or optionally zinc nitrate; Copper: in the form of acetate, sulfate or, where appropriate, nitrate; Manganese: in the form of carbonate, magnesium and calcium: in the form of carbonates; Phosphate: preferably in the form of phosphoric acid. The fluoride ions which may be used in the bath are preferably used in the form of alkali metal or ammonium fluoride, in particular sodium fluoride or in the form of the complex compounds mentioned above. The compounds mentioned above are dissolved in water in the concentration ranges essential for the invention; then, as has also been said above, the pH of the phosphating solutions is adjusted to the desired value.

For the purposes of the present invention, hydroxylamine can originate from any source. Accordingly, any compound which provides hydroxylamine or a derivative thereof, for example a hydroxylamine salt or a hydroxylamine complex, which is often in the form of hydrate, can be used according to the invention. Examples which can be used include hydroxylamine phosphate, optionally hydroxylamine nitrate, hydroxylamine sulfate (also called hydroxylammonium sulfate [(NH₂OH) ₂.H₂SO₄]) or a mixture thereof. Hydroxylamine sulfate and hydroxylamine phosphate are particularly preferred as the hydroxylamine source.

Examples Procedure :

• 1. Degrease with a commercially available alkaline cleaner (Ridoline ^R 1558)

  Approach:

  2%

  Temperature:

  55 ° C

  Time:

  4 min.

• 2. Rinse with process water

  Temperature:

  Room temperature

  Time:

  1 min.

• 3. Activation with an activating agent containing oligo / polymeric titanium phosphates (FIXODINE ^R 950)

  Approach:

  0.1% in deionized water

  Temperature:

  Room temperature

  Time:

  1 min.

• 4. Phosphating with the solution mentioned in the examples and comparative examples

  Approach:

  see examples and comparative examples

• 5. Rinse with process water

  Temperature:

  Room temperature

  Time:

  1 min.

• 6. Post-passivation with a standard post-passivation (DEOXYLYTE ^R 41)

  Approach:

  0.1 vol%;

  Temperature:

  40 ° C

  Time:

  1 min.

• 7. Rinse with deionized water

example 1

• Beschleuniger (Hydroxylammoniumsulfat) 1,7 g/l,
• Gesamtsäure 22,7 Punkte,
• Freie Säure 0,9 Punkte

Starting from an aqueous solution of a bath composition in step 4 of the above process with the following ion concentrations Zn 1.1 g / l, Mn 0.8 g / l, Cu 0.015 g / l, PO ₄ ^3- 17.5 g / l, NO ₃ ^- 2.0 g / l, SiF ₆ ^2- 0.95 g / l, F⁻ 0.2 g / l,

• Accelerator (hydroxylammonium sulfate) 1.7 g / l,
• Total acidity 22.7 points,
• Free acidity 0.9 points

Surfaces made of sheet steel (Sidca) (example la) and electrolytically galvanized thin sheet (ZE) (example 1b) were phosphated at a temperature in the range from 52 to 54 ° C. in the course of 3 min, the corrosion protection data shown in table 1 being found .

Comparative Example 1

• Beschleuniger (Hydroxylammoniumsulfat) 1,8 g/l,
• Gesamtsäure 21,8 Punkte,
• Freie Säure 0,9 Punkte

Starting from an aqueous solution of a bath composition in step 4 of the above process with the following ion concentrations Zn 1.0 g / l, Mn 1.4 g / l, PO ₄ ^3- 16.9 g / l, NO ₃ ^- 2.0 g / l, SiF ₆ ^2- 1.0 g / l, F⁻ 0.2 g / l,

• Accelerator (hydroxylammonium sulfate) 1.8 g / l,
• Total acidity 21.8 points,
• Free acidity 0.9 points

Surfaces of steel sheet (Sidca) (example la) and electrolytically galvanized thin sheet (ZE) (example 1b) were phosphated at a temperature in the range from 52 to 54 ° C. over a period of 3 minutes, the corrosion protection data shown in table 1 being found .

Comparative Example 2

wurden bei einer Temperatur im Bereich von 52 bis 54° C im Verlauf von 3 min Oberflächen aus Stahlblech (Sidca) (Beispiel 2a) und elektrolytisch verzinktem Feinblech (ZE) (Beispiel 2b) phosphatiert, wobei die in der Tabelle 1 wiedergegebenen Korrosionsschutzdaten gefunden wurden.Starting from an aqueous solution of a bath composition in step 4 of the above process with the following ion concentrations

Surfaces of steel sheet (Sidca) (example 2a) and electrolytically galvanized thin sheet (ZE) (example 2b) were phosphated at a temperature in the range from 52 to 54 ° C. over a period of 3 minutes, the corrosion protection data shown in table 1 being found .

Examples 2a and 2b as well Comparative Examples 3a and 3b

Starting from an aqueous solution of a bath composition in step 4 of the above process with the following ion concentrations Zn 1.0 g / l Mn 0.8 g / l Cu see table 2 NO ₃ ^- see table 2 PO ₄ ^3- 13.7 g / l SiF ₆ ^2- 0.95 g / l F⁻ 0.22 g / l Accelerator (hydroxylammonium sulfate) 2.0 g / l Total acidity 20.0 points Free acid 1.2 points

surfaces were phosphatized from electrolytically galvanized thin sheet at a temperature of 53 ° C. in the course of 1 min. The test panels were then given a test coat of KTL and white top coat and subjected to the alternating climate test according to VDA 621-415. The results after a test period of 5 cycles are also shown in Table 2.

Test methods

The corrosion protection effect of the phosphating according to the invention was determined in accordance with the standards of the German Association of the Automotive Industry (VDA 621-414 (outdoor weathering) and VDA 621-415 (alternating climate test)).

The testing of the corrosion protection of motor vehicle paints by exposure to the weather serves to determine the corrosion protection effect of motor vehicle paints under the influence of natural weathering in the overall structure, as in the example without light protection and additional stress by spraying with saline solution.

Test coats consisting of an automobile-typical structure made of KTL, filler, white top coat, each according to Ford specification, are provided with a straight scratch mark parallel to the long side, which runs through to the metal surface. The test coats are stored on suitable racks. They are sprayed liberally once a week with a dilute sodium chloride solution.

The test time in the present case was 6 months.

For the final assessment, the sample coats are rinsed with clear, flowing water, blown dry with compressed air if necessary and examined for visible changes. The under rusting visible from both sides of the scoring is determined. The width of the metal surface damaged by rust next to the scratch is generally easy to see on the paint surface. The average total width is used for evaluation the rust zone in mm. For this purpose, the width is measured in several places and the arithmetic mean is formed.

Testing the corrosion protection of automotive paintwork in the case of cyclically changing stresses serves to assess the corrosion protection of automotive paintwork using a time-consuming laboratory process, which causes corrosion processes and corrosion patterns that are well comparable with those that occur during driving. The short test simulates in particular the under rust caused by a paint injury, as well as the edge and edge rust in the case of special corrosion test sheets or components with known weak points in the paint and the surface rust.

Analogous to the examinations of outdoor weathering, here, too, test panels were provided with a straight scratch mark parallel to the long side, extending through to the metal surface.

The test panels were set up in the tester at an angle of 60 ° to 75 ° to the horizontal.

1 Tag =

24 h Salzsprühnebelprüfung SS DIN 50 021

4 Tage =

4 Zyklen Kondenswasser - Wechselklima KFW DIN 50 017 und

2 Tage =

48 h Raumtemperatur 18° bis 28° C nach DIN 50 014.

A test cycle takes 7 days and consists of

1 day =

24 h salt spray test SS DIN 50 021

4 days =

4 cycles of condensed water - alternating climate KFW DIN 50 017 and

2 days =

48 h room temperature 18 ° to 28 ° C according to DIN 50 014.

The test time is 10 cycles corresponding to 70 days.

At the end of the test, the test plates are rinsed with clear, flowing water, blown dry with compressed air if necessary and examined for visible changes.

The under rusting visible from both sides of the scoring is determined.

In general, the width of the metal surface damaged by rust next to the scratch is easily recognizable as a trace of bubbles or rust on the paint surface. In addition, with an inclined knife blade, e.g. B. with an eraser, carefully remove the rusted paint film to the still adherent zone.

For evaluation purposes, the average overall width of the rust area in mm is also measured here. For this purpose, the width is measured in several places and the arithmetic mean is formed. Table 1 Corrosion test results 3-layer paint system Outdoor weathering VDA 621-414 Alternating climate test VDA 621-415 6 months infiltration mm Infiltration mm Stone chip characteristic Ex. 1 (a) steel 0.4 0.6 0.4 0.6 0.6 0.5 1-2 1 1 (b) ZE 0 0 0 0.9 0.8 1.0 1 1 1 See Example 1 (a) steel 0.5 0.6 0.8 0.5 0.8 0.9 1 1-2 1 (b) ZE 1.0 0.8 0.7 2.8 3.3 2.5 6 6 6 Compare Ex. 2 (a) steel 0.3 0.3 0.3 0.3 0.6 0.8 1 1 1 (b) ZE 0 0.3 0 1.6 1.0 1.3 1 1 1 example Ion concentrations in the bathroom Paint infiltration Cu [ppm] NO₃- [g / l] [mm] 2a 3rd - 1.1 - 1.7 2 B 8th - 1.6 - 1.9 See 3a 3rd 2nd 2.6 - 4.6 See 3b 8th 2nd 2.8 - 2.5

These examples clearly show the positive influence of nitrate-free phosphating solutions in the phosphating of galvanized metal surfaces.

Claims (10)

1. A process for the production of copper-containing nickel-free phosphate coatings with a copper content of 0.1 to 5.0% by weight and an edge length of the phosphate crystals of 0.5 to 10 µm on metal surfaces selected from steel, galvanized steel, alloy-galvanized steel, aluminium and alloys thereof by treatment of the surfaces by spraying, dipping or spraying/dipping with a phosphating solution containing the following components: zinc ions 0.2 to 2 g/l copper ions 0.5 to 25 mg/l phosphate ions 5 to 30 g/l (expressed as P₂O₅) and hydroxylamine salts, hydroxylamine complexes and/or hydroxylamine in a quantity of 500 to 5,000 ppm of hydroxylamine, based on the phosphating solution.
2. A process as claimed in claim 1, characterized in that the phosphating solution contains up to 500 ppm of iron(II) ions and, more particularly, up to 50 ppm of iron(II) ions.
3. A process as claimed in claim 1 or 2, characterized in that the phosphating solution contains 5 to 20 ppm of copper ions when applied by dipping and 1 to 10 ppm of copper ions when applied by spraying.
4. A process as claimed in one or more of claims 1 to 3, characterized in that the phosphating solution additionally contains 0.1 to 5 g/l and, more particularly, 0.5 to 1.5 g/l of manganese(II) ions.
5. A process as claimed in one or more of claims 1 to 4, characterized in that the phosphating solution additionally contains alkaline earth metal cations, more particularly magnesium and/or calcium ions, in a quantity of up to 2.5 g/l.
6. A process as claimed in one or more of claims 1 to 5, characterized in that the hydroxylamine salt is selected from hydroxylammonium phosphate, hydroxylammonium nitrate, hydroxylammonium sulfate or mixtures thereof.
7. A process as claimed in one or more of claims 1 to 6, characterized in that the phosphating solution additionally contains an oxidizing agent selected from peroxide compounds, chlorates, permanganates and organic nitro compounds.
8. A process as claimed in one or more of claims 1 to 7, characterized in that a phosphating solution substantially free from nitrite ions is used.
9. A process as claimed in one or more of claims 1 to 8, characterized in that a phosphating solution substantially free from nitrate ions is used.
10. The use of the process claimed in one or more of claims 1 to 9 as a pretreatment of the metal surfaces before lacquering, more particularly before cataphoretic dip lacquering.