EP0662164B1 - Process for phosphating galvanised steel surfaces - Google Patents

Abstract

PCT No. PCT/EP93/02538 Sec. 371 Date Mar. 28, 1995 Sec. 102(e) Date Mar. 28, 1995 PCT Filed Sep. 20, 1993 PCT Pub. No. WO94/08074 PCT Pub. Date Apr. 14, 1994.The invention relates to a process for phosphating galvanized steel surfaces, preferably electrolytically or hot-dip galvanized surfaces of steel strip, by treating them in a bath or spray with acidic aqueous phosphating solutions, wherein the workpieces are given a d.c. cathodic treatment at the same time. In the process, (a) the phosphating solutions contain Zn+2 cations in the range from 0.1 to 5 g/l, PO4-3 anions in the range from 5 to 50 g/l, NO3- anions in the range from 0.1 to 50 g/l, Mn+2 cations in the range from 0.1 to 5 g/l, and Cu+2 cations in the range from 0.001 to 1 g/l; (b) the following conditions are used: pH of the phosphating solution in the range from 1.5 to 4.5, temperature of the phosphating solution in the range from 10 DEG to 80 DEG C., treatment time in the range from 1 to 300 sec; and (c) the workpieces are also cathodically treated with a direct current with a density in the range from 0.01 to 100 mA/cm2 during phosphating.

Description

The present invention relates to a method for phosphating galvanized steel surfaces, preferably electrolytically or hot-dip galvanized steel strip surfaces, by treating them by immersion or spray-immersion with acidic, aqueous solutions which, in addition to zinc, phosphate and nitrate ions, also contain at least two ions contain further divalent metals, wherein the workpieces are simultaneously treated cathodically with a direct current.

It has been known to the person skilled in the art for some time that high proportions of heavy metal ions, in particular nickel ions, in phosphate layers lead to particularly good corrosion protection, compare, for example, WO-A-85/03 089. In this connection, however, it is also known that for Achieving high levels of nickel in phosphate layers also requires high levels of nickel in the phosphating solutions to be used. On the one hand, this means higher process costs due to the high nickel price. On the other hand, larger amounts of toxic nickel compounds have to be disposed of from the used phosphating solutions, since generally only about 2% of the nickel from the phosphating solutions is incorporated into the phosphate layers.

For ecological and occupational physiological reasons, efforts are therefore made to dispense with nickel, which is classified as particularly toxicologically harmful, without having to accept disadvantages in terms of corrosion protection and paint adhesion. EP-A-459 541 proposes the use of phosphating solutions which, in addition to 0.3 to 1.7 g / l zinc and 0.2 to 4.0 g / l manganese, also 1 to 30 mg / l Cu ( II) included.

The positive influence of copper on the formation of the phosphate layer has long been known in the prior art, compare, for example, W. Rausch, "The Phosphating of Metals", Eugen G. Leuze Verlag, 2nd Edition 1988, pages 20, 56, 79f and 107. The copper ions can be added to the phosphating bath itself or to an upstream activation bath, for example based on colloidal titanium polyphosphates. For example, EP-A-454 211 describes an activation bath containing titanium phosphate, to which copper is added in the range of 1 to 100 mg / l. But also for titanium-free activation baths, as described for example in EP-A-340 530, a favorable influence of a copper content in the quantity range between 1 and 100 mg / l can be expected.

Furthermore, the use of electric current in phosphating processes is known per se. For example, cathodic treatment accelerates the phosphating process (see MH Abbas, Finishing, October 1984, pages 30-31). Corrosion protection layers can be deposited on galvanized steel surfaces with the aid of acidic, aqueous solutions based on aluminum phosphate and / or magnesium phosphate or polycondensed phosphoric acid and simultaneous use of cathodic currents (compare JP-A-77/047 537, JP-A-75/161 429 and JP-A-89/219 193). JP-A-85/211 080 relates to a method for producing Corrosion protection layers on metal surfaces with the help of zinc phosphating solutions with temporary use of a cathodic current. Here, in particular, a corrosion-resistant protective layer is also produced on the edges of the metal surfaces to be treated. A similar process is described in EP-A-171 790. Here, the metal surfaces are treated with an acidic, aqueous solution which contains zinc, phosphate and chlorine ions following a conventional zinc phosphating, a direct current being simultaneously applied to the anodically connected metal surfaces.

According to JP-A-87/260 073, with the aid of acidic phosphating baths which contain phosphoric acid, manganese and copper ions, phosphate layers with high abrasion resistance can be produced with the simultaneous application of cathodic currents on iron material. The phosphating of surface-finished material and the simultaneous use of zinc ions are not mentioned here. Phosphating of electrolytically or hot-dip galvanized steel surfaces with the help of this process did not lead to the desired success.

The applicant's unpublished DE-A-4111186 also describes a process for phosphating metal surfaces, preferably electrolytically or hot-dip galvanized steel strip surfaces, using acidic, aqueous phosphating solutions, the workpieces being simultaneously treated cathodically with a direct current. The phosphating solutions used here contain zinc, nickel and / or cobalt cations as well as phosphate and nitrate anions.

In contrast, it is the object of the present invention to provide a method for phosphating galvanized steel surfaces To provide, in which - despite the deliberate avoidance of the use of the currently undesirable nickel ions - by using manganese ions in the zinc-containing phosphating solutions a comparable good protection against corrosion is achieved. In addition, it is the object of the present invention to significantly increase the incorporation rate of the manganese in the phosphate coatings formed, although only comparatively low concentrations of manganese cations are present in the phosphating solutions used.

• a) mit Phosphatierlösungen arbeitet, die die folgenden Komponenten enthalten:
  • Zn⁺-Kationen im Bereich von 0,1 bis 5 g/l,
  • PO₄ ^3--Anionen im Bereich von 5 bis 50 g/l,
  • NO₃ ^--Anionen im Bereich von 0,1 bis 50 g/l, sowie
  • Mn⁺-Kationen im Bereich von 0,1 bis 5 g/l, und
  • Cu⁺-Kationen im Bereich von 0,001 bis 1 g/l,
• b) wobei man die folgenden Bedingungen einhält:
  pH-Wert der Phosphatierlösungen im Bereich von 1,5 bis 4,5,
  Temperatur der Phosphatierlösungen im Bereich von 10 bis 80 °C,
  Behandlungsdauer im Bereich von 1 bis 300 Sekunden,
• c) und wobei man ferner während der Phosphatierung die Werkstücke kathodisch mit einem Gleichstrom einer Dichte im Bereich von 0,01 bis 100 mA/cm behandelt.

Accordingly, the present invention relates to a method for phosphating galvanized steel surfaces, preferably electrolytically or hot-dip galvanized steel strip surfaces, by treating them by immersion or spray-immersion with acidic, aqueous solutions which, in addition to zinc, phosphate and nitrate ions, also contain at least ions contain two further divalent metals, which is characterized in that

• a) works with phosphating solutions that contain the following components:
  • Zn⁺ cations in the range from 0.1 to 5 g / l,
  • PO ₄ ^3- anions in the range from 5 to 50 g / l,
  • NO ₃ ^- anions in the range of 0.1 to 50 g / l, as well
  • Mn⁺ cations in the range of 0.1 to 5 g / l, and
  • Cu⁺ cations in the range from 0.001 to 1 g / l,
• b) observing the following conditions:
  pH value of the phosphating solutions in the range from 1.5 to 4.5,
  Temperature of the phosphating solutions in the range from 10 to 80 ° C,
  Treatment duration in the range of 1 to 300 seconds,
• c) and wherein the workpieces are further treated cathodically during the phosphating with a direct current with a density in the range from 0.01 to 100 mA / cm.

Surprisingly, it was found that by applying a cathodic direct current to the workpiece during the phosphating and by the simultaneous presence of copper cations in the zinc-containing phosphating solutions, the rate of incorporation of manganese into the phosphate layers can be increased significantly, so that even in comparison Lower concentrations of manganese cations in the phosphating solution can achieve similarly high contents in the phosphating layers, as was previously only possible if the phosphating solutions had comparatively high concentrations of manganese cations. Another advantage of the present invention can be seen in the fact that the phosphating layers achieved with the aid of the method according to the invention have comparable corrosion protection as can otherwise be achieved with nickel-containing phosphating solutions.

For the purposes of the present invention, it is essential that all of the parameters listed above be observed when carrying out the phosphating process. In other words, the cathodic direct current treatment of the workpieces during the phosphating only leads to the desired goal in corresponding zinc-containing phosphating solutions which contain manganese and copper cations together, as defined in detail above.

If in connection with the present invention one speaks of galvanized metal surfaces, then material surfaces are made of electrolytically or hot-dip galvanized or else alloy galvanized steel, preferably electrolytically or hot-dip galvanized steel strip, understood. The term steel is understood to mean unalloyed to low-alloy steel, as used, for example, in the form of sheets for body construction. The use of galvanized steel, especially electrolytically galvanized steel in strip form, has become very important in recent years. Here, the term "galvanized steel" encompasses both galvanizing by electrolytic deposition and by hot-dip application and generally also relates to alloy-galvanized steel, the zinc alloys in particular being zinc / nickel alloys, zinc / iron alloys (galvanized) and zinc / aluminum -Alloys (Galfan, Galvalume) play an essential role.

The process according to the invention is preferably carried out in the so-called immersion process; in general, however, it is also possible to apply the phosphating solutions according to the invention to the substrate surfaces by spray immersion. The workpieces to be treated are connected cathodically for the phosphating treatment, an electrode made of stainless steel, for example, being used as the counter electrode. In general, a metal container of the phosphating bath can also serve as a counterelectrode, furthermore there are also graphite electrodes, noble metal electrodes, for example made of platinum or gold, electrodes which only have a coating of the noble metals mentioned or in which such noble metals are implied, or in principle all of them Electrode materials known from the relevant prior art can be used as counter electrodes.

For the purposes of the present invention, the term “direct current” is understood not only to mean “pure” direct currents, but rather also practically identical currents, for example those that can be generated by full wave rectification of a single phase alternating current or by rectification of a three phase alternating current. So-called pulsating direct currents and chopped direct currents can also be used for the purposes of the invention. Of importance in the sense of the invention is only the current density of the direct current, which should be in the range defined above. The specification of suitable voltage values for the direct current, which is to be used in the sense of the present invention, is deliberately dispensed with, since, taking into account the different conductivities of the phosphating baths on the one hand and the geometric arrangement of the electrodes on the other hand, there may be a different relationship between current and voltage. In addition, concentration gradients, which are determined by the current density and not by the bath voltage, are decisive for the formation mechanism of the phosphating layers. In individual cases, the person skilled in the art will select suitable voltage values for carrying out the method according to the invention on the basis of the values given for the current density.

• Zn⁺-Kationen im Bereich von 0,5 bis 2 g/l,
• PO₄ ^3--Anionen im Bereich von 10 bis 20 g/l,
• NO₃ ^--Anionen im Bereich von 1 bis 30 g/l sowie
• Mn⁺-Kationen im Bereich von 0,5 bis 2 g/l und
• Cu⁺-Kationen im Bereich von 0,01 bis 0,5 g/l.

According to a preferred embodiment of the present invention, phosphating solutions are used which contain the following components:

• Zn⁺ cations in the range from 0.5 to 2 g / l,
• PO ₄ ^3- anions in the range from 10 to 20 g / l,
• NO ₃ ^- anions in the range from 1 to 30 g / l as well
• Mn⁺ cations in the range of 0.5 to 2 g / l and
• Cu⁺ cations in the range of 0.01 to 0.5 g / l.

According to a further preferred embodiment of the method according to the invention, the following conditions are observed in the phosphating treatment of the workpieces:
pH value of the phosphating solutions in the range from 2 to 3,
Temperature of the phosphating solutions in the range from 40 to 70 ° C,
Treatment duration in the range of 2 to 30 seconds.

For the purposes of the present invention, it is further preferred that during the phosphating the workpieces are treated cathodically with a direct current with a density in the range from 1 to 50 mA / cm.

According to a further embodiment of the method according to the invention, the phosphating baths can additionally contain magnesium cations. The incorporation of these cations into the phosphating layer is not significantly promoted by the use of direct current according to the invention, but it is not disturbed in any way.

In this sense, it is preferred according to the invention that one works with phosphating solutions which additionally contain Mg⁺ cations in the range from 0.01 to 2 g / l, preferably from 0.1 to 1 g / l. The additional use of magnesium cations in the phosphating baths according to the invention results in an improvement in the corrosion resistance of the phosphating layers achieved thereby.

In the case of the phosphating of hot-dip galvanized or alloy-galvanized steel surfaces with the aid of the method according to the invention, the use of fluoride ions leads to a more uniform degree of coverage of the phosphating layers on such surfaces. In this sense, it is preferred according to the invention that one works with phosphating solutions which additionally contain simple or complex fluoride anions in the range from 0.1 to 50 g / l, preferably from 0.2 to 2 g / l. In the phosphating of electrolytically galvanized steel surfaces, the presence of fluoride anions is not necessary, but the presence of fluoride anions does not interfere with the phosphating process according to the invention in these cases either. According to the invention, the fluoride anions can also be used in the form of complex fluorine compounds, for example tetrafluoroborate or hexafluorosilicate.

As already stated, compliance with all of the parameters mentioned above is essential for the optimal implementation of the method according to the invention. This includes the specified range of the pH to be observed. If the pH of the phosphating bath is not in the specified range, it is necessary to adjust the phosphating bath to pH values in the specified range by adding acid, for example phosphoric acid, or by adding an alkali, for example sodium hydroxide solution. If values for the free acid or total acid content of the phosphating solutions are given in the examples below, these were determined in the manner described in the literature. The so-called free acid score is accordingly defined as the number ml of 0.1 N NaOH required to titrate 10 ml bath solution against dimethyl yellow, methyl orange or bromophenol blue. The total acid score is then the number ml of 0.1 N NaOH required for titration of 10 ml bath solution using phenolphthalein as an indicator until the first pink color. The phosphating solutions according to the invention generally have points of free acid in the range from 0.5 to 3 and of total acid in the range from 15 to 25.

The phosphating baths for carrying out the process according to the invention are generally prepared in the customary manner which is known per se to the person skilled in the art. The following compounds can be considered as starting products for the preparation of the phosphating bath: Zinc: in the form of zinc oxide or zinc nitrate; Manganese: in the form of manganese carbonate; Copper: in the form of copper nitrate; Magnesium: in the form of magnesium nitrate, magnesium oxide, magnesium hydroxide or magnesium hydroxycarbonate; Phosphate: preferably in the form of phosphoric acid; Nitrate: in the form of the salts mentioned above, optionally also in the form of the sodium salt. The fluoride ions which may be used in the bath are preferably used in the form of sodium fluoride or in the form of the complex compounds mentioned above. The compounds mentioned above are dissolved in water in the concentration ranges essential for the invention; then, as has also been said above, the pH of the phosphating solutions is adjusted to the desired value.

Before the actual phosphating treatment, the metal surface to be treated must be completely water wettable. For this purpose, it is generally necessary to clean and degrease the metal surfaces to be treated by methods known per se and sufficiently described in the prior art. For the purposes of the present invention, it is further preferred, after the cleaned and degreased workpieces have been rinsed with water, preferably with demineralized water, to subject the workpieces to be phosphated to an activation pretreatment known per se. In particular, titanium-containing activation solutions are used for this purpose, as are described, for example, in DE-A-20 38 105 or DE-A-20 43 085. Accordingly, the metal surfaces subsequently to be phosphated are treated with solutions which are essentially titanium salts and Sodium phosphate, optionally together with organic components, for example alkyl phosphonates or polycarboxylic acids. Soluble compounds of titanium, such as potassium titanium fluoride and in particular titanyl sulfate, are preferred 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.

The actual phosphating process then takes place after this activation treatment; the phosphated metal surfaces are then rinsed again with water, again preferably with demineralized water. In certain cases, it may be advantageous to passivate the phosphate layers produced in a subsequent process step. Passivation of this type is always sensible and advantageous if the metal surfaces phosphated by the process according to the invention are subsequently painted or coated in another way with organic materials. As is well known to the person skilled in the art, such a passivation treatment can be carried out, 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 g / l. An alternative to this is a passivation treatment with chromium-free products, as described, for example, in DE-A-31 46 265 or DE-A-40 31 817. If, however, the phosphated substrates are first subjected to a mechanical deformation process and subsequently phosphated again, as is the case, for example, in car body construction, then a passivation treatment should be avoided.

The phosphating 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, for example by spray painting or electrocoating, or for coating with organic foils.

The following examples describe the procedure according to the invention.

Examples

Table 1 below shows the compositions of the phosphating baths used, including the respective pH values and the values of the free acid and total acid content, for Examples 1 to 3 according to the invention and for Comparative Examples 1 to 3.

In Examples 1 to 3 according to the invention, a cathodic direct current with different current densities was applied to the test sheets - during the entire immersion treatment of the same in the respective phosphating baths; the respective current densities are given in Table 2. In all cases, a platinum electrode served as the counter electrode.

In contrast, in Comparative Examples 1 to 3, the phosphating was carried out without such a direct current treatment. The phosphating baths used for comparative examples 1 to 3 correspond, moreover, to those of examples 1 to 3 according to the invention.

• 1) Chemisches Reinigen und Entfetten unter Verwendung eines tensid- und phosphathaltigen alkalischen Reinigungsmittels (Ridoline^R C 1250 E, Fa. Henkel KGaA) in einer Konzentration von 2 Gew.-% in wäßriger Lösung, im Spritzverfahren bei ca. 60 °C im Verlauf von 3 Minuten.
• 2) Spülen mit vollentsalztem Wasser bei Raumtemperatur im Verlauf von 30 Sekunden.
• 3) Aktivieren unter Verwendung eines titansalzhaltigen, wäßrigen Aktivierungsmittels (Fixodine^R 6, Fa. Henkel KGaA) in einer Konzentration von 0,2 Gew.-%, im Tauchverfahren bei Raumtemperatur im Verlauf von 5 Sekunden.
• 4) Phosphatieren im Tauchverfahren in den jeweiligen Phosphatierbädern gemäß Tabelle 1 bei 60 °C, die jeweilige Dauer der Phosphatierung ist in Tabelle 2 angegeben.
• 5) Spülen mit vollentsalztem Wasser bei Raumtemperatur im Verlauf von 30 Sekunden.
• 6) Trocknen bei 80 °C Objekttemperatur im Verlauf von 10 Minuten.

For all examples and comparative examples, electrolytically galvanized steel sheets (dimensions: 10 cm x 20 cm x 0.7 cm; zinc coating on both sides in a thickness of 7.5 µm) from Thyssen AG, Duisburg, were used as test sheets. The test sheets used for the respective examples and comparative examples were - apart from the treatment with direct current discussed above - otherwise treated in the same way according to the process steps described below:

• 1) Chemical cleaning and degreasing using an alkaline cleaning agent containing tenside and phosphate (Ridoline ^R C 1250 E, from Henkel KGaA) in a concentration of 2% by weight in aqueous solution, by spraying at about 60 ° C. for 3 minutes.
• 2) Rinse with deionized water at room temperature for 30 seconds.
• 3) Activation using an aqueous activating agent containing titanium salt (Fixodine ^R 6, Henkel KGaA) in a concentration of 0.2% by weight, in the immersion process at room temperature for 5 seconds.
• 4) Phosphating in the immersion process in the respective phosphating baths according to Table 1 at 60 ° C, the respective duration of the phosphating is given in Table 2.
• 5) Rinse with deionized water at room temperature for 30 seconds.
• 6) Drying at 80 ° C object temperature in the course of 10 minutes.

After drying, the respective test sheets were coated with an epoxy-based cathodic electrocoat material (Aqualux ^R K, ICI, Hilden). The dry film density was 18 ± 2 µm.

The corrosion protection of the respective phosphating layers was then determined by determining the infiltration of paint according to a cathodic polarization test. For this purpose, the respective test sheets were provided with a single cut in accordance with DIN 53 167 and then immersed in a 10% by weight aqueous Na 2 SO 4 solution at a current flow of 0.75 A and a polarization time of 40 hours. The paint infiltration was evaluated in accordance with DIN 53 167 (see Table 2).

The layer weights of the phosphating layers were determined by differential weighing (weight of the test plate: with phosphate layer. /. Weight of the test plate after the phosphate layer had been detached using chromic acid). Furthermore, the phosphating layers on the respective test sheets were detached with chromic acid to determine their composition and analyzed by AAS spectroscopy.

The results obtained in the above tests are summarized in Table 2 below. Table 1 : Composition of the phosphating baths Example No. Zn⁺ Cu⁺ Mn⁺ NO ₃ ^- PO ₄ ^3- pH f. S. points GS points each in [g / l] 1 1.6 0.01 1.0 2.3 12.3 2.7 1.5 20th 2nd 1.6 0.1 1.0 2.5 12.3 2.7 1.5 21 3rd 1.6 0.2 1.0 2.7 12.3 2.7 1.5 23 See 1 1.6 0.01 1.0 2.3 12.3 2.7 1.5 20th See 2 1.6 0.1 1.0 2.5 12.3 2.7 1.5 21 See 3 1.6 0.2 1.0 2.7 12.3 2.7 1.5 23 See = comparative example; f. S. = free acid; GS = total acid Current density, duration of phosphating, content of Cu and Mn in the phosphating layers, layer weights and corrosion test results Example No. Current density [mA / cm] Time [s] Cu% by weight Mn% by weight Layer weight [g / m] Paint infiltration [mm] 1 5 5 traces 4.9 1.4 13 2nd 5 5 0.5 5.1 1.5 9 3rd 2nd 5 0.7 5.1 1.8 9 3rd 5 5 0.7 6.1 1.8 7.5 3rd 5 30th 0.6 7.1 2.7 6.0 3rd 10th 5 0.8 7.8 2.0 7.0 See 1 0 5 traces 3.8 0.7 > 20 See 2 0 5 0.5 4.2 1.2 > 20 See 3 0 5 0.7 4.8 1.8 17th See 3 0 30th 0.9 4.6 2.0 10th

A comparison of the values in Table 1 - with regard to the composition of the respective phosphating baths - with those of the values in Table 2 - with regard to the content of manganese in the phosphating layers - shows that due to the procedure according to the invention with lower manganese concentrations in the phosphating baths, comparatively high contents of these cations can be achieved in the phosphating layers formed. In comparable cases of the examples according to the invention with the comparative examples, this leads to significantly improved corrosion protection.

Claims (8)

1. A process for phosphating galvanized steel surfaces, preferably electrolytically galvanized or hot-dip-galvanized steel strip surfaces, by immersion or spray/immersion treatment thereof with acidic aqueous solutions which, in addition to zinc, phosphate and nitrate ions, also contain ions of at least two other divalent metals, characterized in that

   a) phosphating solutions containing the following components are used:

   Zn⁺ cations in quantities of 0.1 to 5 g/l,

   PO₄ ^3- anions in quantities of 5 to 50 g/l,

   NO₃₋ anions in quantities of 0.1 to 50 g/l and

   Mn⁺ cations in quantities of 0.1 to 5 g/l and

   Cu⁺ cations in quantities of 0.001 to 1 g/l,

   b) the following conditions are established:
   pH value of the phosphating solutions 1.5 to 4.5,
   temperature of the phosphating solutions 10 to 80°C,
   treatment time 1 to 300 seconds,

   c) the workpieces are cathodically treated during phosphating with a direct current having a density of 0.01 to 100 mA/cm.
2. A process as claimed in claim 1, characterized in that phosphating solutions containing the following components are used:

   Zn⁺ cations in quantities of 0.5 to 2 g/l,

   PO₄ ^3- anions in quantities of 10 to 20 g/l,

   NO₃₋ anions in quantities of 1 to 30 g/l and

   Mn⁺ cations in quantities of 0.5 to 2 g/l and

   Cu⁺ cations in quantities of 0.01 to 0.5 g/l.
3. A process as claimed in claim 1 or 2, characterized in that the following conditions are maintained during the phosphating treatment of the workpieces:
   pH value of the phosphating solutions 2 to 3
   temperature of the phosphating solutions 40 to 70°C
   treatment time 2 to 30 seconds.
4. A process as claimed in one or more of claims 1 to 3, characterized in that, during phosphating, the workpieces are cathodically treated with a direct current having a density of 1 to 50 mA/cm.
5. A process as claimed in one or more of claims 1 to 4, characterized in that the phosphating solutions used additionally contain Mg⁺ cations in quantities of 0.01 to 2 g/l and preferably in quantities of 0.1 to 1 g/l.
6. A process as claimed in one or more of claims 1 to 5, characterized in that the phosphating solutions used additionally contain simple or complex fluoride anions in quantities of 0.1 to 50 g/l and preferably in quantities of 0.2 to 2 g/l.
7. A process as claimed one or more of claims 1 to 6, characterized in that the workpieces to be phosphated are subjected beforehand to an activating pretreatment known per se, more particularly with titanium-containing activating solutions.
8. A process as claimed in one or more of claims 1 to 7 as a pretreatment for subsequent painting or coating.