EP0486576B1 - Process for producing manganese-containing zinc phosphate coatings on galvanized steel - Google Patents

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

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.

Processes for phosphating surfaces made of iron, steel, zinc and their alloys as well as aluminum have long been state of the art (Ullmann's Encyklopadie der Technische Chemie, 4th edition, volume 15, pages 686 and 687). The phosphating of the surfaces mentioned serves to increase the adhesive strength of paint layers and to improve corrosion protection.

From W.A. Roland and K.-H. Gottwald, "Metallfläche", 42nd year, 1988/6, manganese-modified zinc phosphate coatings are known as a basis of 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 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 stability to alkali. At the same time, the working range of phosphating baths is increased; Aluminum can also be phosphated in combination with steel and electrolytically or hot-dip galvanized steel to form a layer, whereby the generally achieved quality standard is guaranteed.

DE-A-32 45 411 discloses a process for the phosphating of 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 Zn ^{2 +} cations is about 1 to 2.5 g / I, while the free acid content in the range of 0.8 to 3 points and the total acid / free acid ratio in the range of 5 to 10 are held. The duration of the treatment should not be significantly longer than 5 s.

Preferably, nitrate-containing phosphating baths are used, the weight ratio of Zn ²⁺ / NO _a - in the range from 1: 1 to 1: 8 and the weight ratio of PO ₄ ³ - / NO ₃ - in the range from 1: 0.1 to 1 : 2.5 is held.

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

A high nickel phosphating process is known from E-B-0 112 826. A molar ratio of nickel to zinc in the range from 5.2: 1 to 16: 1 is assumed.

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

In Patent Abstracts of Japan, Vo. 13, No. 24 (C-561), (3372), dated January 19, 1989 - unit of JP-A-63-227786 - describes a process for the simultaneous degreasing and phosphating of galvanized steel sheet before cathodic electroplating thereof. Here, preformed articles made of galvanized steel sheet are treated with acid phosphating solutions which contain the following components: 0.3 to 1.0 g / I Zn ²⁺ , 0.4 to 3.5 g / I Ni ²⁺ , 0.1 up to 3.5 g / I Mn ²⁺ , 10 to 20 g / I P0 ₄ ^3- , 0.5 to 1.5 g / IF, 15 g / I N0 ₃ -, 0.7 to 6 g / I surfactant , 2 to 6 points accelerator, based on the free acid content of the N0 ₂ -.

EP-A-0 060 716 relates to a phosphating process for automobile bodies before a cathodic electro-coating thereof. The metal surfaces based on steel or zinc are treated with solutions containing 0.5 to 1.5 g / l zinc ions, 5 to 30 g / l phosphate ions, 0.6 to 3 g / l manganese ions and an accelerator. In addition, these solutions can contain 0.1 to 4 g / l of nickel ions. In particular, nitrite ions, m-nitrobenzenesulfonate ions and hydrogen peroxide can be used as accelerators, as well as nitrate and chlorine ions. The duration of treatment in the immersion process is at least 15 seconds, in particular 30 to 120 seconds. The layer weights of the resulting phosphating layers are in the range from 2.0 to 3.2 g / m ² .

EP-A-0 219 779 relates to a process for phosphating electrolytically galvanized metal products, preferably electrolytically galvanized steel strips. Here, phosphating solutions are used which contain 0.1 to 0.8 g / l zinc cations and 0.5 to 2.0 g / l manganese cations and some Free acid content in the range of 4 to 8 points and an acid ratio in the range of 2.5 to 5. The solutions may additionally contain cobalt, the cobalt content being 1 part cobalt per 100 to 150 parts (Zn ²⁺ + Mn ²⁺ ). Layer weights of the phosphating layers of less than 2 g / m ^{2 result} with treatment times that are not significantly more than 5 seconds. Nitrate serves as an accelerator.

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 layer 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 3 to 20 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 achieve corrosion protection and paint adhesion, as in the trication processes used in the automotive industry, also in the case of continuous strip phosphating. Of course, it was imperative that dense, closed layers of the phosphate overlay 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 electrolytically applied zinc alloy containing Fe) with a. The term “zinc alloys” is generally understood to mean those zinc alloys which contain at least 45% by weight of zinc.

The above-mentioned objects are achieved by a method for phosphating electrolytically and / or hot-dip galvanized steel strip with the formation of manganese and nickel-containing zinc phosphate layers, which have a mass per unit area of less than 2 g / m ² , in particular in the range from 0.7 to 1. 6 g / m ² , by brief treatment with acid phosphating solutions containing Zn ^{2 + -} , Mn ^{2 + -} , Ni ^{2 + -} , P043 + - and NO ₃ ^- -lons,
characterized in that

wobei
das Gewichtsverhältnis von Ni^2+-Kationen zu NO₃ ^--Anionen im Bereich von 1 : 10 bis 1 : 60 und
das Gewichtsverhältnis von Mn^2+-Kationen zu N0₃--Anionen im Bereich von 1 : 1 bis 1 : 40 eingestellt wird.the duration of the treatment is 3 to 20 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 constituents or correspond to the following parameters:

in which
the weight ratio of Ni ^{2 +} cations to NO ₃ ^- anions in the range from 1:10 to 1:60 and
the weight ratio of Mn ²⁺ cations to N0 ₃ anions is set in the range from 1: 1 to 1:40.

The above-mentioned content of PO ₄ ^3- anions also includes HPO ₄ ^2- and H ₂ P0 ₄ - anions and undissociated H ₃ PO ₄ - in the form of the stoichiometric equivalent of P043- Anions - with a.

Chr. Ries, "Monitoring of Phosphating Baths", Galvanotechnik, 59 (1968) No. 1, pages 37-39 (Eugen G. Leuze-Verlag, Saulgau) describes both the terms of the parameters mentioned here and their determination in detail . 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.

• Die Konzentration von Zn^21-Kationen wird in einem sehr niedrig begrenzten Bereich gehalten. Geringe Mengen an Zinkionen werden dem Behandlungsbad bereits zu Beginn zugesetzt, um die Einstellung des Kationengleichgewichts zu beschleunigen. Zink wird bekanntlich durch die sauren Phosphatierungslösungen aus dem verzinkten Band schnell herausgelöst. Wenn der Zinkgehalt der Phosphatierungslösung vor Beginn der Phosphatierung mehr als 0,75 g/I beträgt, kann die Haftung eines anschließend aufgebrachten Lackes deutlich verschlechtert werden. Unter bestimmten Anlagenbedingungen stellt sich im Betrieb aufgrund des üblichen Eintrags von Zn^21-Kationen durch das verzinkte Stahlband auch ein höherer Zinkgehalt im Phosphatierungsbad ein, der jedoch das Verfahren nicht beeinflußt. Hierbei wird erfahrungsgemäß, bedingt durch die Gleichgewichtseinstellung, ein Gehalt an Zn^21-Kationen im Bereich von 1,1 bis 3 g/I, vorzugsweise 1,1 bis 2,2 g/I erhalten.

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

• The concentration of Zn ²¹ cations is kept in a very low limited range. Small amounts of zinc ions are added to the treatment bath at the beginning in order to accelerate the establishment of the cation equilibrium. 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 / l, the adhesion of a varnish applied subsequently can deteriorate significantly. Under certain system conditions, a higher zinc content in the phosphating bath also occurs in operation due to the usual entry of Zn ²¹ 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 ²¹ cations in the range from 1.1 to 3 g / l, preferably 1.1 to 2.2 g / l, is obtained.

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.

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

An essential part 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 usually 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 3 to 20 s.

The main advantage of the present invention is that zinc phosphate coatings on galvanized steel strip can be produced according to the invention which have a bright surface appearance, although they contain nickel. At the same time, however, the nickel content could be significantly reduced compared to the prior art by substitution with manganese without loss of the corrosion protection value. 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.

A 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 just 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 the applicability for phosphating both electrolytically and hot-dip galvanized steel strip. In the case of electrolytically galvanized steel strip, the presence of fluoride anions is not necessary in practice, but the presence of fluoride does not interfere with the phosphating process. When using hot-dip galvanized steel strip, however, the use of fluoride anions may be necessary, especially when it comes to 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 / l, preferably 0.4 to 0.6 g / l. The corresponding amount of fluoride anions is the phosphating solutions in the form of hydrofluoric acid or in the form of sodium or Potassium salts added to 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 corrosion protection, this oil must be removed using suitable means and processes that are already known before phosphating. The water-wettable galvanized metal surface is then expediently subjected to an activating pretreatment known per se before the phosphating solution is applied. Suitable pretreatment processes are described in particular in DE-A-20 38 105 and DE-A-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 such proportions 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-A-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 a weight per unit area of the zinc phosphate layers of less than 2 g / m ² , which have a closed, finely crystalline structure and give the electrolytically and / or hot-dip galvanized steel strip a desired, uniform, light gray appearance. A steel strip phosphated in this way can also be processed without subsequent coating. The thin phosphate layers produced by the method according to the invention behave more favorably in many shaping processes than the phosphate layers of a higher mass per unit area produced with the previously usual methods. However, organic coatings applied subsequently also show significantly improved adhesion compared to the prior art, both during and after the shaping 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 nickel 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.

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 electrocoating.

Examples:

• 1. Reinigen und Entfetten:
  • Verwendung von tensidhaltigen alkalischen Reinigungsmitteln (wie RIDOLINE^R C 72) im Spritzen bei 50 bis 60 _° C und Behandlungszeiten von 5 bis 20 s.
• 2. Spülen
• 3. Aktivieren:
  • Verwendung von titansalzhaltigen Mitteln (wie FIXODINE^R 950) im Spritzen bei 20 bis 40 °C und Behandlungszeiten von 2 bis 4 s.
• 4. Phosphatieren:
  • Zusammensetzung siehe Tabelle 1.
• 5. Spülen
• 6. Nachpassivieren:
  • Verwendung von chromhaltigen oder chromfreien Nachpassivierungsmitteln (wie DEOXYLYTE^R 41B oder DEOXYLYTE^R 80) im Spritzen oder Tauchen bei 20 bis 50 °C und Behandlungszeiten von 2 bis 6 s.
• 7. Abquetschen:
  • Überstehende Flüssigkeit wird ohne Verdichtung der Schicht mittels Abquetschwalzen entfernt.
• 8. Trocknen
  • Das Band trocknet nach dem Abquetschen durch Eigenwärme.
    erfolgte die Oberflächenbehandlung von elektrolytisch verzinktem Stahl (Auflage beidseitig 7,5 um Zn) und schmelztauchverzinktem Stahl (Auflage beidseitig 10 um Zn).

Within the usual process sequence with the stages:

• 1.Cleaning and degreasing:
  • Use of alkaline cleaning agents containing surfactants (such as RIDOLINE ^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 FIXODINE ^R 950) in syringes 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 DEOXYLYTE ^R 41B or DEOXYLYTE ^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 mass per unit area of 0.6 to 1.6 g / m ² was produced for electrolytically galvanized steel (ZE) and a mass per unit area of the phosphate layer of 0.8 to 1.6 g / m ² for hot-dip galvanized steel (Z).

• Zn: als Oxid oder Nitrat; Mn: als Carbonat; Ni: als Nitrat oder Phosphat; F: als Flußsäure oder Natriumfluorid; P0₄: als H₃P0₄ oder Nickelphosphat; N0₃: als HN0₃ oder Nickelnitrat.
• Zur Herstellung der Phosphatierungslösungen wurden die vorstehend genannten Verbindungen - in den für die jeweiligen Ionen angegebenen Mengen - in Wasser gelöst.
  

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; P0 ₄ : as H ₃ P0 ₄ or nickel phosphate; N0 ₃ : as HN0 ₃ or nickel nitrate.
• To prepare the phosphating solutions, the compounds mentioned above were dissolved in water in the amounts specified for the respective ions.
  

In the examples, the substrate to be phosphated was selected to be electrolytically galvanized steel on both sides (7.5 / 7.5 μm zinc) for the test using the 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 process on electrolytically galvanized steel:

The sheets obtained with the aid of Example 1 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 50 021 SS, 1008 h.

The standard KET primer FT 85 7042, manufacturer BASF Lacke und Farben AG, was used as the coating for the test VW P1210, while for the salt spray test the polyester primer BASF Universal No. 21110, 4 μm, and Unitecta Polyester topcoat No. 509 293 5002, 16 µm, was used.

1.VW alternating climate test P 1210

2. Salt spray test

When determining the degree of blistering of paints in accordance with DIN 53 209, blistering that occurs in paints is defined by specifying the degree of blistering. The degree of bubbles according to this standard is a measure of the formation of bubbles on a coating according to the frequency of the bubbles per unit area and the size of the bubbles. 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 m0 mean no bubbles, while m5 defines a certain frequency of bubbles per unit area according to the degree of bubbles 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 g0 has the meaning of freedom from bubbles, while with g5 the size of the bubbles is shown according to the degree of bubbles in DIN 53 209.

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 is used 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 preferably infiltrates from there. This leads to a reduction in adhesion or to loss of adhesion and corrosion of the metallic surface.

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 individual coatings, starting from a defined point of injury (Ritz) or from existing weak points (e.g. pores, edges). 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.

• 4 h Salzsprüh-Test gemäß DIN 50 021,
• 4 h Ruhezeit bei Raumtemperatur und
• 16 h Schwitzwasser-Konstantklima gemäß DIN 50 017.

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

• 4 h salt spray test according to DIN 50 021,
• 4 h rest at room temperature and
• 16 h condensation constant climate according to DIN 50 017.

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 an invisible corrosion, while with a key figure 10 practically the entire surface is corroded.

In the T-Bend test, the sample is bent for 1 to 2 s with different bending radii parallel to the rolling direction by 180 _° , with the coating on the outside. The smallest bending radius, which allows the sample to bend without tearing, determines the adherence at a 180 _° bend. In the TO test, 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 (11)

1. A process for phosphating electrolytically and/or hot-dip galvanized steel strip to form manganese- and nickel-containing zinc phosphate coatings with a weight per unit area of less than 2 g/m² and more particularly in the range from 0.7 to 1.6 g/m² by brief treatment with acidic phosphating solutions containing Zn²⁺, Mn²⁺, Ni²⁺, P0₄ ³⁺ and NO₃- ions, characterized in that the duration of the treatment is 3 to 20 s, phosphating is carried out at a temperature in the range from 40 to 70 °C and - at least at the beginning of the treatment - the phosphating solutions contain the following constituents or satisfy the following parameters:

the ratio by weight of Ni²⁺ cations to NO₃- anions being adjusted to between 1:10 and 1:60 and the ratio by weight of Mn²⁺ cations to N0₃- anions being adjusted to between 1:1 and 1:40.

2. A process as claimed in claim 1, characterized in that the ratio by weight of Ni²⁺ cations to N0₃- anions is adjusted to between 1:20 and 1:60.

3. A process as claimed in at least one of claims 1 and 2, characterized in that the ratio by weight of Mn²⁺ cations to NO₃- anions is adjusted to between 1:6 and 1:20.

4. A process as claimed in at least one of claims 1 to 3, characterized in that the phosphating solutions have a content of F- anions of 0.1 to 1.0 g/I.

5. A process as claimed in at least one of claims 1 to 4, characterized in that the phosphating solutions have a content of F- anions of 0.4 to 0.6 g/I.

6. A process as claimed in at least one of claims 1 to 5, characterized in that phosphating is carried out at a temperature in the range from 55 to 65 °C.

7. A process as claimed in at least one of claims 1 to 6, characterized in that the weight per unit area of the zinc phosphate coatings where electrolytically galvanized steel strip is used is between 0.7 and 1.6 g_/m2.

8. A process as claimed in at least one of claims 1 to 6, characterized in that the weight per unit area of the zinc phosphate coatings where hot-dip galvanized steel strip is used is between 0.8 and 1.6 g/m^2.

9. A process as claimed in at least one of claims 1 to 8, characterized in that the electrolytically and/or hot-dip galvanized steel strip used has been subjected to an activating pretreatment known per se, more particularly with titanium-containing activating solutions.

10. A process as claimed in at least one of claims 1 to 9, characterized in that, where electrolytically galvanized steel strip is used, the acid ratio is adjusted to between 25:1 and 10:1.

11. A process as claimed in at least one of claims 1 to 9, characterized in that, where electrolytically galvanized steel strip is used, the acid ratio is adjusted to between 15:1 and 10:1.