WO1993022474A1

WO1993022474A1 - Copper-containing, nickel-free phosphatizing process

Info

Publication number: WO1993022474A1
Application number: PCT/EP1993/001015
Authority: WO
Inventors: Wolf-Achim Roland; Karl-Heinz Gottwald; Matthias Hamacher; Jan-Willem Brouwer; Frank-Oliver Pilarek
Original assignee: Henkel Kommanditgesellschaft Auf Aktien
Priority date: 1992-05-06
Filing date: 1993-04-27
Publication date: 1993-11-11
Also published as: DE4214992A1

Abstract

A process is disclosed for producing copper-containing, nickel-free phosphate layers with a defined copper content in the phosphate coatings. No pinholing is observed on the metal surface, in particular on electrolytically zinc coated steel, when an appropriate ratio between copper and nitrate is set in the phosphatizing bath.

Description

Copper-containing, nickel-free phosphating process

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 coating (KTL) depends on a large number 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. Of particular interest in the chemical parameters are the alkali stability during the cataphoretic coating, the binding strength of the water of crystallization of the zinc phosphate crystals when the lacquers are stoved, and the rehydration capacity.

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

In the prior art, it has proven advantageous to provide treatment baths containing nickel in order to improve the layer quality of the subsequent phosphating, in particular in the case of surface-thickened material. Because of the toxic effect of nickel-containing dusts, which can be developed when grinding phosphated metal sheets, and the increasingly stringent requirements when discharging nickel-containing waste water into the sewage system, people are looking for nickel-free phosphating solutions.

Additions of magnesium ions were examined. The positive effects in terms of application technology of these ions, which are based on several effects, were recognized early on (DE-A-39 20 296). 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 addition of Mg ²⁺ ions makes the crystals smaller, the phosphate layer denser and the free metal surface after the 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 / 1 Zn ²⁺ ), high concentrations of accelerators such as sodium nitrite or meta-nitrobenzenesulfonate / Na salts cannot be used additionally in order not to press the area-related mass below 1.5-2.0 g / m ² .

In this way, nickel-free phosphating systems based on zinc, manganese and magnesium ions could be developed for steel surfaces that meet the specifications of the users. In the case of surface-finished material, however, the results with these magnesium-containing processes were not always satisfactory.

The influence of Cu ²⁺ ions was examined. DE 40 13 483 AI discloses a method for phosphating metal surfaces, in which one works with phosphating solutions which are generally are 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 means of oxygen and / or other oxidizing agents having the same effect. The method serves in particular for the phosphating of steel, galvanized steel, alloy-galvanized steel, aluminum and their alloys for the pretreatment for a subsequent painting, in particular an electrocoating. 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 ²⁺ 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. However, the occurrence of annoying "specks" is described for galvanized material. "Specks" are understood to mean small, white crystal efflorescences which, under the microscope, turn out to be pickling pits with zinc phosphate crystals piled up on the edge (cf. W. Rausch: "The Phosphating of Metals, 2nd Edition, 1988, Leuze-Verlag, Saulgau, p. 108) The formation of specks is particularly pronounced with longer treatment times, for example when the system is at a standstill.

The object was therefore to provide a process for the production of copper-containing 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, and in particular the formation avoided by specks on galvanized steel. The above-mentioned object is achieved with the aid of a specially selected phosphating solution, with which it is possible to produce copper-containing phosphate layers with a defined copper content while avoiding speck formation.

Surprisingly, the investigations had shown that the occurrence of the so-called "specks" depends both on the concentration of the copper and on the concentration of the nitrate in the phosphating bath. The product of the two concentrations (in mg / 1) must not exceed a limit value which is 50,000 in the immersion process and 15,000 in the spray process.

In a first embodiment, the present invention thus relates to a process for the production of copper-containing nickel-free phosphate layers with a copper content in the range of 0.5-5% by weight on metal surfaces containing a phosphate solution

Zinc 0.2 - 2 g / 1, copper 0.5 - 25 mg / 1,

Phosphate 5.0 - 30 g / 1 (calculated as P2θ5) _r nitrate 0.1 to 15 g / 1, the product of the proportions of Cu ions (in mg / 1) and nitrate ions (mg / 1) contained in the phosphating solution ) is less than 50,000 when using immersion processes and less than 15,000 when using spraying processes.

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 the formation of specks. The zinc phosphate ten are made up of small (0.5 - 10 μ), compact, densely grown crystals. In particular, it was found in the investigation of copper ion-containing phosphating baths that only very small amounts of copper (II) ions are required in the solution in order to set the desired copper content of the phosphate layer in the range of 0.5-5% by weight. It was found here that if the amount of copper of more than 5% by weight of the phosphate layer was exceeded, specks were found on electrolytically galvanized steel surfaces.

In a further embodiment of the present invention, it is therefore preferred that the phosphating solution contains 5-20 pp of copper (II) ions when the metal surface is brought into contact with the phosphating solution by means of immersion processes. When using the spraying method, it is equally preferred that the phosphating solutions contain 1-10 ppm of copper (II) ions in order to incorporate corresponding copper contents in the conversion coating without producing the disruptive specks on electrolytically galvanized steel surfaces.

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 3.0 and 3.8. 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. Corrections to the pH value during the phosphating can be carried out, for example, by basic additives or acids.

The method according to the invention can be used in particular on steel, galvanized steel, alloy galvanized steel, aluminum and the like 1

Alloys are applied. The term steel in the sense of the present invention encompasses not only low-alloy steels but also soft, unalloyed steels and higher as well as high-strength steels. It is essential content of the invention that the aqueous, acidic phosphating solutions are free of nickel. However, this means that under technical conditions a small amount of nickel (IΙ) ions can be contained in the phosphating baths. However, in accordance with the prior art DE 40 13 483 A1, this amount should be less than 0.0002-0.01 g / 1, in particular less than 0.0001 g / 1.

When the phosphating process is used on steel surfaces, iron dissolves in the form of iron (II) ions. By adding suitable oxidizing agents, iron (II) is converted into iron (III) and can thus be precipitated as iron phosphate sludge.

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. For the purposes of the present invention, the oxidizing agents of the phosphating solutions are selected from nitrite, chlorate, hydroxylain and its salts, bromate, permanganate, peroxide compounds (in particular hydrogen peroxide, perborate, percarbonate, perphosphate) and organic nitro compounds, in particular nitrobenzenesulfonate. The amounts of oxidizing agents to be used are known from the prior art. For example, sodium nitrite 0.1-0.5 g / 1, peroxide compound calculated as hydrogen peroxide: 0.005-0.1 g / 1, nitrobenzenesulfonate: 0.005-1 g / 1. These oxidizing agents known as "accelerators" also have the task of "depolarizing" the metal surface when the acid is attacked, ie To prevent the formation of elemental hydrogen by reducing the accelerators instead of the hydrogen ions.

Through the addition of 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, particularly 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 0.1-5 g / 1, in particular 0.5-1.5 g / 1, 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 up to 2.5 g / l, alkaline earth metal cations, in particular magnesium and / or calcium ions, for example from the process water.

The use of modifying compounds from the group of surfactants, hydroxycarboxylic acids, tartrate, citrate, hydrofluoric acid, alkali metal fluorides, boron tetrafluorides, silicon fluorides is known in principle from the prior art. While the addition of surfactants (for example 0.05-0.5 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 0.03-0.3 g / 1 contribute to a significant reduction in the weight of the phosphate layer. Fluoride ions promote phosphating metals which are more difficult to attack and thereby leads to a reduction in the phosphating time and, in addition, to an increase in the surface coverage of the phosphate layer. As is known, about 0.1-1 g / 1 of the fluorides mentioned are used. The controlled addition of the fluorides also makes it possible to form 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 or aluminum surfaces, which is why these complex fluorides can be used, for example, in amounts of 0.4-3 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 treatment of the surfaces is chosen to be as short as possible for economic reasons; for example, exposure times of 1 to 5 minutes are sufficient to produce a uniformly covering phosphate layer. In the case of galvanized steel, it is known that contact times of less than 1 minute are sufficient to produce uniformly covering phosphate layers. Accordingly, the method can also be used in fast-running conveyor systems. In such systems, however, downtimes occur relatively frequently, so that it happens that the metal surfaces remain in contact with the phosphating solution for a much longer time. With galvanized substrates, this can lead to the harmful speck formation mentioned above. However, these disadvantages can be avoided when using the method according to the invention. The metal surfaces to be phosphated are cleaned, rinsed and, if necessary, treated with activating agents, in particular based on titanium phosphates, according to methods known in the art prior to phosphating.

Examples

Procedure:

1. Degrease with a mildly alkaline cleaner (RIDOLINE ^R 1558)

Batch: 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 oligomeric / polymeric titanium phosphate (Fixodine R 950)

Approach: 0.1% in deionized water

Temperature: room temperature time: 1 min.

4. Phosphating with the solution mentioned in each of the examples and comparative examples

Approach: see examples

Temperature: 53 ° C

Time: according to the example ΛA

5. Rinse with process water

Temperature: room temperature

Time: 1 min.

6. Post-passivation with a commercially available chrome-containing post-passivation (DE0XYLYTE ^R 41)

Approach: 0.1 vol% temperature: 40 ° C time: 1 min.

7. Rinse with deionized water.

example 1

starting from an aqueous solution with a bath composition in step 4 of the above-mentioned process

Hydroxylammonium sulfate accelerator 1.7 g / 1 total acid 22.7 points,

Free acidity 0.9 points ML

a surface of sheet steel (Sidca) (example la) and electrolytically galvanized sheet (ZE) (example lb) was phosphated at a temperature in the range from 52 to 54 ° C. in the course of 3 min and the corrosion protection data shown in table 1 were obtained hold.

Comparative Example 1

starting from an aqueous solution with a Badzusa composition in step 4 of the above process

Zn (II) 1.0 g / 1,

Mn (II) 1.4 g / 1,

P0 ₄ 3- 16.9 g / 1,

N0 ₃ "2.0 g / 1,

SiF ₆ 2- 1.0 g / 1,

F "0.2 g / 1,

Accelerator (hydroxylammonium sulfate) 1.8 g / 1,

Total acidity 21.8 points,

Free acidity 0.9 points

were at a temperature in the range of 52 to 54 ° C in the course of 3 min a surface made of steel sheet (Sidca) (see example la) and electrolytically galvanized sheet (ZE) (see example lb) phosphated and the in Table 1 contain corrosion protection data.

Comparative Example 2

starting from an aqueous solution of a bath composition in step 4 of the above-mentioned process

were at a temperature in the range of 52 to 54 ° C in the course of 3 min a surface made of steel sheet (Sidca) (see example 2a) and electrolytically galvanized sheet (ZE) (see example 2b) and that in the table 1 received corrosion protection data.

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)).

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 with additional stress by spraying with salt solution.

Test coats consisting of an automobile-typical structure made of KTL, filler, top coat white, each with Ford specification, are provided with a straight scratch mark, parallel to the long side, which is controlled through to the metal surface. The trial dashes 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 surface of the paint. For evaluation, the average total width of the rust zone is measured in mm. For this purpose, the width is measured at several places and the arithmetic mean is formed.

The test of the corrosion protection of motor vehicle paints in the case of cyclically changing loads serves to assess the corrosion protection of motor vehicle paints using a time-consuming laboratory method, which causes corrosion processes and corrosion patterns which are well comparable with those occurring during driving operation. 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, sample plates were provided with a straight scratch track running parallel to the long side down to the metal background.

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

A test cycle takes 7 days and consists of

1 day = 24 h salt spray test SS DIN 50021

4 days = 4 cycles of condensation water change climate KFW DIN 50017 and

2 days = 48 h room temperature 18 ° to 28 ° C according to DIN 50014.

The test time is 10 cycles corresponding to 70 days.

After completion of the test, the sample 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 rust surface damaged by rust along with the scratch is easily recognizable as a trace of bubbles or rust on the lacquer surface. In addition, with an obliquely held knife blade, e.g. B. with an eraser, carefully remove the undersized paint film to the still firmly adhering 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 at several places and the arithmetic mean is formed.

A b

Table 1:

Corrosion test results 3-layer coating system

Outdoor weathering, alternating climate test VDA 621-414 VDA 621-415

6 months infiltration infiltration rockfall mm m characteristic value

(1 = paint adhesion very good) (10 = paint adhesion very bad)

Example 1 (process according to the invention) 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

Compare Ex. 1 (nickel free)

y

Example 2

To investigate the influence of the Cu and nitrate concentration and the phosphating time on the formation of specks in diving with extended treatment times, steel sheet (Sidca) and electrolytically galvanized sheet (ZE) were phosphated with phosphating solutions of the following composition at a temperature of 53 ° C for 10 minutes:

Accelerator: sodium nitrite 0.15 g / 1

Free acidity at pH 3.6: 0.9 points

Total acidity: 21.5 points

The following results were obtained, which are shown in Table 2. Λ%

i. 0. = OK: normal phosphate layer formation St = specks / Sp = speckles / copper deposition = Cu small = (k) / large = (g) / + = some / ++ = many

Comparative Example 3

Table 3 shows a comparison test without nitrate with a treatment time of 4 min.

9

Example 3

To investigate the influence of the Cu and nitrate concentration on the formation of specks in spraying processes, electrolytically galvanized steel sheets (ZE) were treated with the following phosphating bath concentrations for 2 min at 50 - 55 ° C:

Zn ⁺ 0.8 g / 1 Mn ²⁺ 0.7 g / 1 P04 ³ ~ 16.5 g / 1

SiF ₆ ² "0.2 g / 1

F ~ (free fluoride) 0.1 g / 1

Cu ²⁺ 0 - 35 mg / 1

NO3- 2 - 5 g / 1

Accelerator: sodium nitrite 200 mg / 1 Free acid at pH 3.6: 0.6 total acid at pH 8.5: 22.0

The results listed in Tab. 4 were achieved:

Tab. 4

Phosphating results from example 3, visual assessment

Cu ²⁺ (mg / 1) 8 15 35

colour

Specks 0 0

Copper deposits 0 0 0 0-1 3

Color 1: very bright

2: light gray

3: medium gray

4: dark gray i / \

Speck a few a lot a lot

Copper deposition not a little clearly very strong

Note: With bath values of 35 mg / 1 copper ions and 5 g / 1 nitrites, the entire ZE surface shows speck formation and copper deposition.

Claims

1. Process for the production of copper-containing phosphate layers containing nickel and containing copper, in the range from 0.5 to 5.0% by weight, on metal surfaces containing a phosphating solution

Zinc 0.2-2 g / 1

Copper 0.5 - 25 mg / 1 phosphate 5.0 - 30 g / 1 (calculated as P2O5) nitrate 0.1 - 15 g / 1, whereby the product of the proportions of copper ions contained in the phosphating solution (in mg / 1) and nitrate ions (in mg / 1) is less than 50,000 when using immersion processes and less than 15,000 when using spray processes.

2. The method according to claim 1, characterized in that the phosphating solution contains 5 - 20 ppm copper (II) ions when using immersion processes and 1 - 10 ppm copper (II) ions when using spraying processes.

3. The method according to claim 1 or 2, characterized in that the phosphating solution contains 0.15 - 5 g / 1, in particular 0.5 - 1.5 g / 1 manganese (II) ions.

4. The method according to one or more of claims 1 to 3, characterized in that the phosphating solution contains alkaline earth metal ions, in particular magnesium and / or calcium ions in an amount of up to 2.5 g / 1.

5. The method according to one or more of claims 1 to 4, characterized in that the oxidizing agent of the Phosphatier¬ solution is selected from nitrite, chlorate, hydroxyla in and its salts, broat, peroxide compounds, organic nitro compounds, especially nitrobenzo1su1fonate.

6. The method according to one or more of claims 1 to 5, characterized in that the metal surface is selected from steel, galvanized steel, alloy galvanized steel, aluminum and its alloys.

7. The method according to one or more of claims 1 to 6, characterized in that the metal surface is brought into contact with the phosphating solution by spraying, dipping or spray-dipping.

8. The method according to one or more of claims 1 to 7, characterized in that the metal surface consists of galvanized or alloy galvanized steel.

9. Application of the method according to one or more of claims 1 to 8 as pretreatment of the metal surface before painting, in particular cataphoretic dip painting.