WO1991002829A2

WO1991002829A2 - Process for producing manganese-containing zinc phosphate coatings on galvanized steel

Info

Publication number: WO1991002829A2
Application number: PCT/EP1990/001295
Authority: WO
Inventors: Jörg Riesop
Original assignee: Henkel Kommanditgesellschaft Auf Aktien
Priority date: 1989-08-17
Filing date: 1990-08-08
Publication date: 1991-03-07
Also published as: CA2065004A1; DE3927131A1; JPH04507436A; WO1991002829A3; CN1034681C; ES2071110T3; EP0486576A1; EP0486576B1; AU633135B2; CN1049531A; DE59008978D1; ATE121803T1; ZA906507B; AU6167590A

Abstract

In a process for phosphatizing electrolytically and/or hot-dip galvanized steel strip, the steel strip is briefly treated with acidic phosphatizing solutions which contain, in addition to zinc and phosphate ions, manganese and nickel cations and anions of oxygen-containing acids with an accelerator effect. The weight ratio of nickel cations to nitrate anions is ajusted to between 1:10 and 1:60 and the weight ratio of manganese cations nitrate anions is adjusted to between 1:1 and 1:40.

Description

Process for the production of zinc phosphate layers on galvanized steel

The present invention relates to a method for phosphating electrolytically and / or hot-dip galvanized steel strip with the formation of zinc phosphate layers containing manganese and nickel. These manganese and nickel-containing zinc phosphate layers are applied by spraying, splash-dipping and / or dipping with aqueous solutions.

Methods are for phosphating surfaces of iron, steel, zinc and their alloys and aluminum have long been state of the ^"technology (Ull ann's Encyclopedia of Industrial Chemistry, 4th edition, volume 15, pages 686 and 687) of said Ober¬. Phosphating surfaces serves to increase the adhesive strength of lacquer layers and to improve corrosion protection.

From WA Roland and K.-H. Gottwald, "Metallfläche", 42nd year, 1988/6, manganese-modified zinc phosphate coatings are known as a liability for modern coatings. Here it is stated that the use of manganese ions in addition to zinc and nickel ions in low-zinc phosphating processes has been shown to improve the corrosion protection, in particular when using surface-coated thin sheets. The incorporation of manganese into the zinc phosphate coatings leads to smaller and more compact crystals with increased alkali stability. At the same time, the working range of phosphating baths is increased; Aluminum, too, can be phosphated in combination with steel and electrolytically or hot-dip galvanized steel to form a layer, the generally achieved quality standard being guaranteed. DE 32 45 411 A1 discloses a method for phosphating electrolytically galvanized metal products, in particular electrolytically galvanized steel strips, by short-term treatment with acid phosphating solutions which, in addition to zinc and phosphate ions, can contain further metal cations and / or anions of oxygen-containing acids with accelerating action. In these processes, zinc phosphate layers with a mass per unit area of less than 2 g / m 2 are formed. It works with acid phosphating solutions, the content of Zn2 ⁺ cations is about 1 to 2.5 g / 1, while the free acid content in the range of 0.8 to 3 points and the total acid / free acid ratio in the range be kept from 5 to 10. The duration of the treatment should not be significantly longer than 5 s.

It is preferred to work with phosphate baths containing nitrate, the weight ratio of Zn2 ⁺ / Nθ3 ~ in the range from 1: 1 to 1: 8 and the weight ratio of P043- / N03 "in the range from 1: 0.1 to 1: 2.5 is held.

EP 0 106 459 A1 discloses a phosphating process in which zinc phosphate coatings containing manganese and nickel are formed. The presence of fluoride ions is regarded as essential, as is the upper limit of 10 g / l of nitrate anions.

A high nickel phosphating process is known from EP 0 112826 B1. A molar ratio of nickel to zinc in the range from 5.2: 1 to 16: I is assumed.

Furthermore, a phosphating process is known from EP 0 175 606, in which the use of iron-containing phosphating baths in particular is used. Furthermore, as Accelerators used a number of organic substances, while the presence of manganese is not required. In addition, the setting of certain ratios of zinc to nickel and zinc to iron is required.

The methods currently used in practice for the phosphating of electrolytically and / or hot-dip galvanized steel strip still have restrictions, the elimination of which is desirable. Thus, in order to ensure adequate protection against corrosion, it is considered necessary to form surface-related masses of the phosphate layers of less than 2 g / m ² . The result of a comparatively high mass per unit area is often unsatisfactory to poor adhesion of subsequent coatings,. especially when phosphated and coated material is deformed. The duration of the phosphating in the processes used in practice is usually above 2 s, in particular at belt speeds of about 60 to 120 m / min.

It is known that improved adhesion and corrosion protection values can be achieved by using nickel cations in the phosphating solutions. However, it has been found in the context of the present invention that an increase in the nickel concentration, which leads to an improvement in the corrosion protection value, simultaneously causes a darkening of the manganese and nickel-containing zinc phosphate layer.

The object of the present invention was to avoid darkening of the zinc phosphate layers on electrolytically and / or hot-dip galvanized steel strip at treatment times of 2 to 30 s while maintaining the corrosion protection values. At the same time, the nickel content of processes known from the literature should be greatly reduced by substitution with manganese in order to Corrosion protection and paint adhesion as in the trication processes used in the automotive industry can also be achieved in continuous strip phosphating. It was, of course, imperative that dense, closed layers of the phosphate layer be formed during the treatment times mentioned and that the deformation properties also be satisfactory. The invention deliberately intends to accept thin overlays of the phosphate layers without, however, having to give up the uniform covering of the galvanized steel strip with a finely crystalline, firmly adhering, self-contained zinc phosphate layer. According to the invention, the term “electrolytically and / or hot-dip galvanized steel strip” also includes, of course, generally known zinc alloys (for example “Neuralyt”, ZNE electrolytically applied zinc alloy containing 10 to 13% Ni or “galvannealed”, ZFE electrolytic) applied zinc alloy containing Fe). Here, the term "zinc alloys" generally means those zinc alloys which contain at least 45% by weight of zinc.

The above-mentioned objects are achieved by using a method for phosphating electrolytically and / or hot-dip galvanized steel strip with the formation of manganese and nickel-containing zinc phosphate layers by brief treatment with acidic phosphating solutions, the duration of the treatment being 2 up to 30 s, the phosphating is carried out in the temperature range from 40 to 70 ° C and the phosphating solutions - at least at the beginning of the treatment - contain the following components or correspond to the following parameters:

Content of Zn ²⁺ cations: 0.02 to 0.75 g / 1, content of Mn ²⁺ cations: 0.2 to 2.0 g / 1, content of Ni ²⁺ cations: 0.1 to 2.0 g / 1, P04 ³ "anion content: 10 to 20 g / 1,

Content of N03 ^" anions: 0.5 to 30 g / 1,

Free acid content: in the range from 1.6 to 3.0 points,

Total acidity: in the range of 12 to 40 points.

The weight ratio of Ni ⁺ cations to N03 ^" anions should be set in the range from 1:10 to 1:60 and the weight ratio of Mn ⁺ cations to Nθ3" anions in the range from 1: 1 to 1:40 become.

For the purposes of the invention, the above-mentioned content of Pθ43 ~ anions also includes HP04 "and H2 Ü4" anions present in the phosphating solutions as well as undissociated H3PO4 - in the form of the stoichiometric equivalent of Pθ43 anions.

Chr. Ries, "Monitoring of Phosphating Baths", Galvanotech¬ nik, 59 (1968) No. 1, pages 37-39 (Eugen G. Leuze-Verlag, Saulgau) contains both the terms of the parameters mentioned here and their determination in described individually. The free acid score is accordingly defined as the number of ml 0.1 N NaOH required to titrate 10 ml bath solution against dimethyl yellow, methyl orange or bromophenol blue. The total acid score is calculated as the number of ml of 0.1 N NaOH required to titrate 10 ml of bath solution using phenolphthalein as an indicator until the first pink color.

The combination of all the parameters mentioned is accordingly essential for the method according to the invention:

The concentration of Zn ²⁺ cations is kept in a very low range. Small amounts of zinc ions are the Treatment bath added at the beginning to accelerate the adjustment of the cation balance. As is well known, the acidic phosphating solutions quickly remove zinc from the galvanized strip. If the zinc content of the phosphating solution before the start of phosphating is more than 0.75 g / 1, the adhesion of a lacquer applied subsequently can be significantly impaired. Under certain system conditions, a higher zinc content in the phosphating bath occurs in operation due to the usual entry of Zn ²⁺ cations through the galvanized steel strip, but this does not influence the process. Experience has shown that, due to the establishment of equilibrium, a content of Zn ²⁺ cations in the range from 1.1 to 3 g / 1, preferably from 1.1 to 2.2 g / 1, is obtained.

If the content of manganese cations is less than 0.2 g / 1, the manganese content in the zinc phosphate layer becomes so low that the adhesion between the substrate and the coating after the cataphoresis is insufficient. On the other hand, if the manganese content is more than 2.0 g / 1, no further improved effects can be obtained for the subsequent coating. However, when the manganese concentration is increased, precipitates separate from the phosphating solution, so that it is impossible to provide a stable solution.

The simultaneous presence of nickel cations and manganese cations results in extremely good paint adhesion and extraordinarily good corrosion protection values for the zinc phosphate coating after the paint coating.

If the content of phosphate anions in the solution is less than 10 g / l, a defective zinc phosphate layer is formed. On the other hand, if the phosphate content is more than 20 g / 1, no additional advantageous effects can be achieved. The use of larger proportions of phosphate is therefore disadvantageous for economic reasons.

For the purposes of the invention, the phosphating solutions preferably contain no strong oxidizing agents, such as nitrites, chlorates or hydrogen peroxide.

An essential component of the present invention is the weight ratio of nickel cations to nitrate anions and the weight ratio of manganese cations to nitrate anions. As is known, the simultaneous use of nickel and manganese cations leads to improved corrosion protection values, but in the processes known from the literature to a darkening of the zinc phosphate layer. The coloring of this zinc phosphate layer does not play a major role in the automotive industry, but the color of the zinc phosphate layer is extremely important, for example, in the manufacture of household appliances due to the very thin layers of lacquer that are often applied in the following.

Another essential criterion of the present invention is the duration of the phosphating treatment. While times above 120 s are normally used for the phosphating in the automotive industry, a time below 1 min is in any case aimed for in the phosphating of galvanized steel strip. For the purposes of the present invention, the duration of the treatment will therefore be between 2 and 30 s. A duration of the treatment of 3 to 20 s is particularly preferred.

The main advantage of the present invention is that zinc phosphate coatings can be produced according to the invention on galvanized steel strip, which have a bright surface appearance. point, although they contain nickel. ^■ At the same time the Ge could halt of nickel compared to the prior art by substitution with manganese without loss of corrosion protection value are significantly reduced. This is of ecological as well as economic importance, as it is the first time that a manganese-containing trication process has been described for the band sector.

In a preferred embodiment of the present invention, the method for phosphating electrolytically and / or hot-dip galvanized steel strip is characterized in that the phosphating solutions contain - at least at the beginning of the treatment - the following constituents or correspond to the following parameters:

Zn ²⁺ cation content: 0.4 to 0.6 g / 1,

Content of Mn ²⁺ cations: 0.9 to 1.1 g / 1,

Content of Ni ²⁺ cations: 0.6 to 0.9 g / 1,

P04 ^{3_ anion} content: 12 to 16 g / 1,

Content of N03 "anions: 10 to 30 g / 1.

Another preferred embodiment of the present invention is that the weight ratio of nickel cations to nitrate anions is set in the range from 1:20 to 1:60. In the context of the present invention, it was found that an excessively large amount of nitrate has as negative an effect on the phosphating process as an excessively low nickel content. This has a negative influence on the corrosion protection values. In a further preferred embodiment of the present invention, the weight ratio of manganese cations to nitrate anions is set in the range from 1: 6 to 1:20. This can have a particularly positive influence on the wet paint adhesion. Of particular importance for the present process is that it can be replaced both for phosphating electrolytically and hot-dip galvanized steel strip. With electrolytically galvanized

Steel strip does not require the presence of fluoride anions in practice, but the presence of fluoride does not interfere with the phosphating process. When using hot-dip galvanized steel strip, however, it may be necessary to use fluoride anions, in particular when complexing aluminum cations. Accordingly, a further preferred embodiment of the present invention is characterized in that the phosphating solutions contain a fluoride anion content of 0.1 to 1.0 g / 1, preferably 0.4 to 0.6 g / 1. The corresponding. The amount of fluoride anions is added to the phosphating solutions in the form of hydrofluoric acid or in the form of the sodium or potassium salts of this acid. Instead of these compounds, complex fluoride compounds such as fluoroborates or fluorosilicates can also be used.

The phosphating itself takes place at moderately elevated temperatures in the range from about 40 to 70 ° C. The temperature range from 55 to 65 ° C. can be particularly suitable. Any technically useful way of applying the treatment solution is suitable. In particular, it is therefore possible to carry out the new method both by means of spraying technology and by immersion.

Before the phosphating solution is applied, the electrolytically and / or hot-dip galvanized surface must be completely water-wettable. This is usually the case in continuously operating conveyor systems. If the surface of the galvanized strip is oiled for storage and protection against corrosion, this oil must be removed before the phosphating with known known means and methods. The water wettable the zinc-coated metal surface is then expediently subjected to a known, activating pretreatment prior to the application of the phosphating solution. Suitable pretreatment processes are described in particular in DE-OS 20 38 105 and DE-OS 20 43 085. Accordingly, the metal surfaces to be phosphated subsequently are treated with solutions which contain, as activating agents, essentially titanium salts and sodium phosphate together with organic components such as, for example, alkyl phosphonates or polycarboxylic acids. Soluble compounds of titanium such as potassium titanium fluoride and in particular titanyl sulfate can preferably be used as the titanium component. Disodium orthophosphate is generally used as the sodium phosphate. Titanium-containing compounds and sodium phosphate are used in proportions such that the titanium content is at least 0.005% by weight, based on the weight of the titanium-containing compound and the sodium phosphate.

As described in the prior art - for example in DE-OS 32 45 411 - it can also be advantageous for the process according to the invention or the zinc phosphate layers produced thereafter to passivate the phosphate layers produced in a subsequent process step. Such passivation can take place, for example, with dilute chromic acid or mixtures of chromic and phosphoric acid. The concentration of chromic acid is generally between 0.01 and 1.0 g / l. Between the phosphating and the post-treatment step, rinsing with water is carried out.

The process according to the invention produces zinc phosphate coatings with an area-related mass of the zinc phosphate layers of less than 2 g / m ² , which have a closed, finely crystalline structure and which give the electrolytically and / or hot-dip galvanized steel strip a desired, uniform, light gray Give appearance. A steel strip phosphated in this way can also be processed without subsequent coating. The thin phosphate layers produced by the process according to the invention maintain themselves more favorably in many deformation processes than the phosphate layers of a higher mass per unit area produced with the previously customary processes. However, even subsequently applied organic coatings show significantly improved adhesion compared to the prior art, both during and after the deformation processes.

In a preferred embodiment of the present invention, surface-based masses of the zinc phosphate layer in the range from 0.7 to 1.6 g / m ² are produced when using electrolytically galvanized steel strip. When using hot-dip galvanized steel strip, the production of a mass per unit area of the zinc phosphate layer in the range from 0.8 to 1.6 g / m ² should be emphasized as particularly advantageous.

The method according to the invention allows the zinc and manganese-containing zinc phosphate layer to be applied by techniques known per se in the prior art, such as spraying, dipping and / or spray-dipping, in particular their combined methods.

In a preferred embodiment of the invention the acid ratio is determined when using electrolytically galvanized steel strip, i.e. the quotient from “total acid” to “free acid” is set in the range from 25: 1 to 10: 1, preferably in the range from 15: 1 to 10: 1.

A further embodiment of the present invention is characterized in that the content of N03 "anions in the phosphating solutions is 1.0 to 30 g / l. The surface layers produced with the aid of the method according to the invention can be used well in all fields in which phosphate coatings are used. A particularly advantageous application is the preparation of the metal surfaces for painting, in particular electro-dip painting.

Examples:

Within the usual process sequence with the stages:

1.Cleaning and degreasing:

Use of alkaline cleaning agents containing surfactants (such as RID0LINE ^R C 72) in spraying at 50 to 60 ° C. and treatment times of 5 to 20 s.

2. Rinse

3. Activate:

Use of agents containing titanium salt (such as FIX0DINE ^R 950) in spraying at 20 to 40 ° C and treatment times of 2 to 4 s.

4. Phosphating:

For composition, see table 1.

5. Rinse

6. Post-passivation:

Use of chrome-containing or chrome-free post-passivation agents (such as DE0XYLYTE ^R 41B or DE0XYLYTE ^R 80) in spraying or dipping at 20 to 50 ° C and treatment times of 2 to 6 s.

7. Squeeze:

Surplus liquid is removed without squeezing the layer using squeeze rollers. 8. Drying

The tape dries after being squeezed by its own heat.

the surface treatment was carried out on electrolytically galvanized steel (coating on both sides 7.5 μm Zn) and hot-dip galvanized steel (coating on both sides 10 μm Zn).

A surface-related mass of 0.6 to 1.6 g / m ² was obtained for electrolytically galvanized steel (ZE) and a surface-related mass of the phosphate layer of 0.8 to 1.6 g / m ² for hot-dip galvanized steel (Z) manufactured.

The ions listed in Table 1 below were introduced into the phosphating solutions in the form of the following compounds:

Zn: as oxide or nitrate; Mn: as carbonate; Ni: as nitrate or phosphate; F: as hydrofluoric acid or sodium fluoride; PO4: as H3PO4 or nickel phosphate; NO3: as HNO3 or nickel nitrate. To prepare the phosphating solutions, the compounds mentioned above were dissolved in water in the amounts specified for the respective ions.

Table 1

Composition of phosphating baths

Examples Comparative example

Bath parameters

1) FS = free acid

2) GS = total acid

In the examples, the substrate to be phosphated was selected to be galvanized steel on both sides (7.5 / 7.5 μm zinc) for testing by means of VW-P 1210 alternating climate test and hot-dip galvanized steel (10/10 μm zinc) for the salt spray test .

Typical layer analysis (determination quantitatively by atomic absorption spectroscopy, AAS) of the method on electrolytically galvanized steel:

Dimensions * 1.0 1.1 0.9 1.0 1.2 Surface light gray light gray light gray light gray medium gray

* Average mass per unit area in accordance with DIN 50942 in g / m ² :

The sheets obtained with the help of Examples 1, 3 and 4 and the comparative example were used to carry out corrosion tests with an alternating climate in accordance with VW standard P 1210 over a test period of 15 and 30 days, and corrosion tests in a salt spray test in accordance with DIN 50021 SS, 1008 h.

The standard KET primer FT 85 7042, manufacturer BASF Lacke und Farben AG, was used as the coating for the VW P1210 test, while the polyester primer BASF Universal No. 21110, 4 μm, and Unitecta Polyester topcoat No. 5092935002, were used for the salt spray test. 16 μm was used. 1.VW alternating climate test P 1210

15 days

Example 1 Example 3 Example 4 Comparative example

Area according to mO / gO mO / gO mO / gO mO / gO DIN 53209

Cut after

DIN 53 167 0.5 0.3 0.3 1.1 in mm

Rockfall K2 K2 - K5-6 according to VW norm

30 days

Example 1 Example 3 Example 4 Comparative example

Area according to mO / gO mO / gO mO / gO mO / gO DIN 53209

Cut according to DIN 53 167 1.1 0.6 0.8 1.7 in mm

Rockfall K3 K3 K4 K9 according to VW standard 2. Salt spray test

Example 1 Example 3

Area after

DIN 53209 mO / gO mO / gO

Cut after

DIN 53 167 2.2 0.0 in mm

T-bend test) 0 0

1) T-Bend-Test TO according to ECCA-T7 [1985]

When determining the degree of blistering of paints in accordance with DIN 53209, blistering that occurs in paints is defined by specifying the degree of blistering. The degree of blistering according to this standard is a measure of a blistering that has occurred on a coating according to the frequency of the blisters per unit area and size of the blisters. The degree of bubbles is indicated by a code letter and a code number for the frequency of the bubbles per unit area, as well as a code letter and a code number for the size of the bubbles.

The code letter and the code mO mean no bubbles, while m5 defines a certain frequency of bubbles per unit area according to the bubble degree images according to DIN 53 209.

The size of the bubbles is given the code letter g and the code number in the range from 0 to 5. Code letter and code number GO has the importance of freedom from bubbles, while g5 shows the size of the bubbles in accordance with the bubble degree images of DIN 53209.

By comparing the paint after the test with the bubbles degree images, the degree of bubbles is determined, the image of which is most similar to the appearance of the paint.

According to DIN 53 167, the salt spray test according to this standard serves to determine the behavior of paints, coatings and similar coatings when exposed to sprayed sodium chloride solution. If the coating has weak points, pores or injuries, the coating is preferably infiltrated from there. This leads to a reduction in adhesion or to loss of adhesion and corrosion of the metallic substrate.

The salt spray test is used so that such errors can be recognized and the infiltration can be determined.

Infiltration within the meaning of this standard is the penetration of sodium chloride solution into the interface between coating and substrate or into the interface between a defined point of injury (Ritz) or existing weak spots (e.g. pores, edges) individual coatings. The width of the zone with reduced or lost adhesion serves as a measure of the resistance of the coating on the respective substrate to the action of sprayed sodium chloride solution.

The VW standard P 1210 defines an alternating test which consists of a combination of different, standardized test methods. So in During the course of - in the present case - 15 and 30 days a test cycle was followed, which consists of

4 h salt spray test according to DIN 50021,

4 h rest time at room temperature and 16 h condensation constant climate according to DIN 50017.

At the beginning of the test, the test sheet is bombarded with a defined amount of steel shot with a certain grain size distribution. After the test period has expired, a key figure is assigned to the degree of corrosion. Corresponding to the key figures from 1 to 10, the key figure 1 denotes invisible corrosion, while with a key figure 10 the entire surface is practically corroded.

In the T-bend test, the sample is bent for 1 to 2 s with various bending radii parallel to the rolling direction by 180 °, the coating being on the outside. The smallest bending radius, which allows the sample to bend without tearing, determines the adhesive power at a 180 ° bend. In test T0, the sheet is bent evenly through 180 ° within 1 to 2 s without an intermediate layer. The sheet is examined immediately after bending with a magnifying glass that magnifies ten times. The test procedure is made more difficult by firmly pressing an adhesive film onto the edge and tearing it off quickly. The amount of lacquer torn off is then assessed.

Claims

Patent claims

1. Process for the phosphating of electrolytically and / or hot-dip galvanized steel strip with the formation of manganese and nickel-containing zinc phosphate layers by brief treatment with acid phosphating solutions, wherein

the duration of the treatment is 2 to 30 s, the phosphating is carried out in the temperature range from 40 to 70 ° C. and

the phosphating solutions - at least at the beginning of the treatment - contain the following components or correspond to the following parameters:

Zn ²⁺ cation content: 0.02 to 0.75 g / 1,

Mn ²⁺ cation content: 0.2 to 2.0 g / l,

Content of Ni ²⁺ cations: 0.1 to 2.0 g / 1,

P04 ³ anion content: 10 to 20 g / 1,

Content of Nθ3 "anions: 0.5 to 30 g / 1,

Free acid content: in the range from 1.6 to 3.0 points,

Total acidity: in the range of 12 to 40 points,

and where

the weight ratio of Ni ⁺ cations to Nθ3 "anions is set in the range from 1:10 to 1:60 and the weight ratio of Mn ²⁺ cations to N03" anions is set in the range from 1: 1 to 1:40.

2. The method according to claim 1, characterized in that the phosphating solutions contain the following constituents or correspond to the following parameters:

Zn ²⁺ cation content: 0.4 to 0.6 g / 1,

Content of Mn ²⁺ cations: 0.9 to 1.1 g / 1,

Content of Ni ²⁺ cations: 0.6 to 0.9 g / 1,

Content of Pθ4 ³ ~ anions: 12 to 16 g / 1,

Content of Nθ3 ~ anions: 10 to 30 g / 1.

3. The method according to at least one of claims 1 and 2, characterized in that the treatment time is 3 to 20 s.

4. The method according to at least one of claims 1 to 3, characterized in that the weight ratio of Ni ²⁺ cations to N03 "anions is set in the range from 1:20 to 1:60.

5. The method according to at least one of claims 1 to 4, characterized in that the weight ratio of Mn ²⁺ cations to Nθ3 "anions is set in the range from 1: 6 to 1:20.

6. The method according to at least one of claims 1 to 5, characterized in that the phosphating solutions have a content of F ~ anions of 0.1 to 1.0 g / 1.

7. The method according to at least one of claims 1 to 5, characterized in that the phosphating solutions have an F "anion content of 0.4 to 0.6 g / 1.

8. The method according to at least one of claims 1 to 7, characterized in that the phosphating is carried out in the temperature range from 55 to 65 ° C.

9. The method according to at least one of claims 1 to 8, characterized in that the area-related mass of the zinc phosphate layers is less than 2 g / m ² .

10. The method according to at least one of claims 1 to 8, characterized in that the weight per unit area of the zinc phosphate layers when using electrolytically galvanized steel strip is 0.7 to 1.6 g / m ² .

11. The method according to at least one of claims 1 to 8, characterized in that the weight per unit area of the zinc phosphate layers when using hot-dip galvanized steel strip is 0.8 to 1.6 g / m ² .

12. The method according to at least one of claims 1 to 11, characterized in that electrolytically and / or hot-dip galvanized steel strip is used which has previously been subjected to an activation pretreatment known per se, in particular with titanium-containing activation solutions is.

13. The method according to at least one of claims 1 to 12, da¬ characterized in that when using electrolytically galvanized steel strip, the acid ratio is set in the range from 25: 1 to 10: 1.

14. The method according to at least one of claims 1 to 12, characterized in that when using electrolytically galvanized steel strip, the acid ratio is set in the range from 15: 1 to 10: 1.

15. The method according to at least one of claims 1 to 14, da¬ characterized in that the content of Nθ3 "anions is 1.0 to 30 g / 1.