EP0401616A1 - Process for applying manganese containing phosphate coatings on metallic surfaces - Google Patents

Abstract

Process for producing manganese-containing phosphate coatings on metal surfaces, in particular steel, galvanized steel, aluminium and/or aluminium alloy or aluminium-steel composite surfaces, by spraying or spray dipping with an aqueous solution containing: 0,8 to 1,4 g/l of Zn<2+>, 0,6 to 2,0 g/l of Mn<2+>, 0,3 to 1,4 g/l of Ni<2+>, 10 to 25 g/l of PO4<3->, 2 to 10 g/l of NO3<->, 0,2 to 1,0 g/l of F<-> and as an accelerator for the phosphate precipitation 0,04 to 0,12 g/l of NO2<->, 0,6 to 2,0 g/l of ClO3<-> and/or 0,2 to 1,0 g/l of sodium-3-nitrobenzolsulphonate. The free acid content is adjusted to 1.4 to 1.8 and the solution has a total acid content of 18 to 30.

Description

The present invention relates to a method for producing manganese-containing phosphate coatings on surfaces made of steel, galvanized steel, aluminum and / or their alloys by spraying or dipping.

In order to improve corrosion protection and paint adhesion, consumer goods such as automobile bodies, automotive accessories and spare parts, agricultural equipment, refrigerators and all kinds of small parts are phosphated using the so-called low-zinc process and then usually cataphoretically dip-coated. Such manganese-modified zinc phosphate coatings as a primer for modern painting systems are, for example, from WA Roland and K.-H. Gottwald, metal surface, 1988/6 known. It is found here that the use of manganese ions in addition to zinc and nickel ions in low-zinc phosphating processes has been shown to improve corrosion protection, in particular when surface-coated thin sheets are used. The incorporation of manganese into the zinc phosphate coatings leads to smaller and more compact crystals with higher alkali stability. At the same time, the working range of phosphating baths is increased; Aluminum can also be phosphated in combination with steel and galvanized steel to form a layer, whereby the generally achieved quality standard is guaranteed. Composite parts of this type, which consist of a wide variety of materials, for example aluminum and steel, have recently been used to an increasing extent in automobile construction.

From EP-A 0 261 704 a method for producing phosphate coatings on such surfaces is known, wherein by spraying or spray-dipping to form uniform phosphate layers with a high degree of coverage, a phosphating solution is used which, in addition to zinc and phosphate, has at least one accelerator and one which must be precisely observed Must have the amount of fluoride ions. Furthermore, according to EP-A 0 261 704, the free acid content (FS) is to be adjusted according to a formula mentioned there.

In contrast, it is an object of the present invention to provide an improved method for producing phosphate coatings on surfaces made of steel, galvanized steel, aluminum and / or their alloys, which leads to even more uniform phosphate coatings, and in addition, a change in the phosphating technology should not be necessary.

The above object has been achieved by a process for producing manganese-containing phosphate coatings on surfaces made of steel, galvanized steel, aluminum and / or their alloys or aluminum-steel composites by spraying or spray-dipping with an aqueous solution
0.8 to 1.4 g / l Zn²⁺,
0.6 to 2.0 g / l Mn²⁺,
0.3 to 1.4 g / l Ni²⁺,
10 to 25 g / l PO₄³⁻,
2 to 10 g / l NO₃⁻,
0.2 to 1.0 g / l F⁻ and
as an accelerator of phosphate separation
0.04 to 0.12 g / l NO₂⁻,
0.6 to 2.0 g / l ClO₃⁻ and / or
0.2 to 1.0 g / l sodium 3-nitrobenzenesulfonate,
the free acid content being adjusted to 1.4 to 1.8 points and having a total acid content of 18 to 30.

As is known, the free acid content in conventional low zinc processes is approximately 0.6 to 0.9 points. If the values are also increased, uneven, unclosed phosphate layers form on the metal surfaces treated in this way; The formation of flash rust is also observed on steel. In principle, the addition of manganese in the fluoride-containing phosphating baths makes it possible to increase the free acid content without the disadvantages mentioned.

The use of manganese in the phosphate layer has various advantages for the above-mentioned substrate surfaces.

With the help of the present invention, it was found that, in particular when using the method according to the invention, a relatively small amount of fluoride can be used on aluminum surfaces in order to achieve optically uniform phosphate layers.

In the surface treatment of steel, the proven low-zinc technology can be retained to form phosphophyllite or manganese-containing phosphophyllite, and at the same time the zinc content can be increased without loss of quality.

On galvanized, alloy-galvanized steel sheets and on aluminum, the use of manganese with a simultaneously increased zinc content due to the manganese incorporation in the layer with subsequent coating by means of cataphoretic electrocoating leads to improved results. This simultaneous interaction of the Levels of zinc and manganese in the phosphating solution were surprisingly found using the present invention.

The aluminum materials to be treated by the method according to the invention include the pure metal and its alloys. Pure aluminum, AlMg and AlMgSi kneading materials should therefore be mentioned as examples. A detailed description of these materials can be found in the aluminum paperback, 14th edition, Aluminum-Verlag, Düsseldorf, 1983. The steels to be treated with the aid of the method according to the invention are, in particular, consumer goods such as automobile bodies, automobile accessories and spare parts, agricultural equipment, refrigerators and other types of small parts which are usually used in the form of sheet metal. The term "galvanized steel" encompasses galvanizing by electrodeposition and hot dip application and thus refers to zinc and known zinc alloys.

When the method according to the invention is carried out in spray-immersion mode, the spraying time must be designed in such a way that a largely closed phosphate layer is formed.

Modern full immersion systems are characterized by a large number of steps connected in series. The term "full immersion system" results from the phosphating by immersion application. In other process steps, spraying processes are also carried out, with spraying out of the immersion bath after the body has emerged. A separate activation step is important for optimal phosphating. To pre-clean and activate the layer to be phosphated, it is usually cleaned, rinsed and then activated in several work steps before phosphating. For example, aqueous suspensions containing titanium phosphate can be used here.

Compliance with the concentration ranges according to the main claim is an essential component for the production of high quality, i.e. uniform phosphate coatings. If the concentrations are undershot, the layers become uneven. In particular, their suitability for subsequent electrodeposition painting is decreasing.

The fluoride concentrations mentioned according to the invention are measured with a special ion-sensitive electrode in a buffered solution at pH 5.3. Therefore, these values are in no way comparable to the values mentioned in the prior art, in which the concentration of fluoride was measured directly in the phosphating solution.

In a preferred embodiment of the present invention, the process is characterized by the use of an aqueous solution containing 0.8 to 1.0 g / l Zn²⁺,
0.8 to 1.2 g / l Mn²⁺,
0.3 to 0.8 g / l Ni²⁺,
14 to 20 g / l PO₄³⁻,
3 to 6 g / l NO₃⁻ and
0.3 to 0.6 g / l F⁻.

The surfaces coated with the aid of the abovementioned process can then be used in known processes for electrocoating. Accordingly, a further embodiment of the present invention consists in the application of the method for the preparation of the surfaces for electrocoating.

The invention is illustrated by the following examples.

Example 1: Within the usual process sequence with the stages:

Cleaning, rinsing, (activating) phosphating, rinsing, re-passivation, VE rinsing, the conversion treatment was carried out under the following bath conditions in accordance with the three compositions A, B and C: Application type Bath parameters Syringes (A) Syringes (comparison) (B) Spray / Dive (C) "Free Acid" (points) 1.5 1.5 1.5 "Total acidity" 26.5 26.3 22.2 Zn²⁺ (g / l) 0.9 0.9 1.0 Mn²⁺ (g / l) 0.9 - 0.6 Ni²⁺ (g / l) 0.7 0.7 0.7 PO₄³⁻ (g / l) 20.5 20.2 17.1 ClO₃⁻ (g / l) 0.95 0.95 - F⁻ (ppm) * 380 380 400 Accelerator ** (ml gas) points 1.0 1.1 1.7 Temperature ( ^o C) 58 58 58 Time (s) 120 120 180 * measured with an ion sensitive electrode in buffered solution at pH 5.3 ** = sodium nitrite - For steel, the "low zinc" technology can be retained to form phosphophyllite or manganese-containing phosphophyllite and the zinc content can be increased without loss of quality.
- On galvanized, alloy-galvanized steel sheets and on aluminum, the manganese incorporation in the layer with subsequent coating with a KET primer results in a significant improvement in the corrosion protection results (see example 2).

The determination of the mass per unit area after application of process variant A on the individual substrates according to DIN 50942 resulted in the following measured values (mean values): Steel St 1405 2.0 g / m² Steel, electrolytically galvanized 2.4 g / m² Aluminum (AlMg 0.4 Si 1.2) 2.8 g / m² Aluminum (AlMg 4.5 Mn) 2.7 g / m² The manganese content of the phosphate layer was determined quantitatively by means of atomic absorption spectrometry (AAS): Steel St 1405 6.4% Steel, electrolytically galvanized 4.7% Aluminum (AlMg 0.4 Si 1.2) 6.2% No new specific band was found in the manganese-containing phosphate layers by X-ray diffraction.

Example 2:

The VDA alternating climate test (VDA test specification 621 415) was used to investigate the corrosion resistance of the phosphating coatings mentioned below with various substrates. Following the coating, a standard electrophoresis dip coating (KET primer FT 85 7042 from BASF Farben und Lacke) was used.

The test time of the VDA alternating climate test is 5 to 10 rounds. During this time, the treated substrate is exposed to a condensation water change climate in accordance with DIN 50017 KFW. Furthermore, the substrate is stored for a certain time at room temperature (18 to 28 ^o C) according to DIN 50014 within the test period. Furthermore, a salt spray test in accordance with DIN 50021 is carried out as part of this alternating climate test.

After the test cycles were completed, the following data were determined. A Phosphating solution, containing manganese Substrate Infiltration at the cut 10 rounds 20 rounds Steel St 1405 0.6-0.8mm - Steel, electrolytically galvanized 2.0 - 2.5 mm - Aluminum (AlMg 0.4 Si 1.2) - 0.2 mm Aluminum "without fluoride - up to 10 mm B phosphating solution, manganese-free (comparison) Substrate Infiltration at the cut 10 rounds 20 rounds Steel St 1405 0.9 - 1.2 mm - Steel, electrolytically galvanized 3.4 - 4.0 mm - Aluminum (AlMg 0.4 Si 1.2) - 0.8 - 1.0 mm Aluminum "without fluoride - up to 10 mm

Claims (4)

1. A method for producing manganese-containing phosphate coatings on surfaces made of steel, galvanized steel, aluminum and / or their alloys or aluminum-steel composites by spraying or spray-dipping with an aqueous solution containing
0.8 to 1.4 g / l Zn²⁺,
0.6 to 2.0 g / l Mn²⁺,
0.3 to 1.4 g / l Ni²⁺,
10 to 25 g / l PO₄³⁻,
2 to 10 g / l NO₃⁻,
0.2 to 1.0 g / l F⁻ and
as an accelerator of phosphate separation
0.04 to 0.12 g / l NO₂⁻,
0.6 to 2.0 g / l ClO₃⁻ and / or
0.2 to 1.0 g / l sodium 3-nitrobenzenesulfonate,
wherein the free acid content is adjusted to 1.4 to 1.8 points and the solution has a total acid content of 18 to 30. 2. The method according to claim, characterized in that one containing an aqueous solution
0.8 to 1.0 g / l Zn²⁺,
0.8 to 1.2 g / l Mn²⁺,
0.3 to 0.8 g / l Ni²⁺,
14 to 20 g / l PO₄³⁻,
3 to 6 g / l NO₃⁻ and
0.3 to 0.6 g / l F⁻
starts. 3. Process according to claims 1 to 2, characterized in that the phosphate coatings are produced on aluminum-steel composite materials. 4. Application of the method according to claims 1 to 3 for the preparation of the surfaces for electrocoating.