EP0662164A1

EP0662164A1 - Process for phosphating galvanised steel surfaces.

Info

Publication number: EP0662164A1
Application number: EP93920771A
Authority: EP
Inventors: Horst-Dieter Speckmann; Reinhard Seidel; Gerard Veldman; Karl-Dieter Brands
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 1992-09-28
Filing date: 1993-09-20
Publication date: 1995-07-12
Anticipated expiration: 2013-09-20
Also published as: DE4232292A1; DE59302529D1; EP0662164B1; ATE137815T1; WO1994008074A1; US5503733A

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

"Process for phosphating galvanized steel surfaces"

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 ions contain at least two other 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 a long time that high proportions of heavy metal ions, in particular nickel ions, in phosphate layers lead to particularly good corrosion protection, for example compare WO-A-85/03 089. However, in this context is It is also known that high nickel contents in the phosphating solutions to be used are also required to achieve high nickel contents in phosphate layers. 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 consumed from the. Phosphating solutions are disposed of, since generally only about 2% of the nickel from the phosphating solutions is incorporated into the phosphate layers. For ecological and work-physiological reasons, efforts are therefore made to dispense with the nickel, which is classified as particularly toxicologically harmful, without having to accept disadvantages with regard to 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 / 1 zinc and 0.2 to 4.0 g / 1 manganese, also 1 to 30 mg / 1 Cu ( II) included.

The positive influence of copper on the formation of the phosphate layer has long been known in the prior art, for example compare 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. In this case, 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 use 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 previously unpublished German patent application P 41 11 186.9 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 - a comparable good corrosion protection is achieved by using manganese ions in the zinc-containing phosphating solutions. 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.

Accordingly, the present invention relates to a process 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, are also preferred Contain ions of at least two further divalent metals, which is characterized in that a) one works with phosphating solutions which contain the following components:

Zn ²⁺ cations in the range from 0.1 to 5 g / 1,

PO ^ 'anions in the range from 5 to 50 g / 1,

Nθ3 ~ anions in the range of 0.1 to 50 g / 1, as well

Mn - *** ⁺ cations in the range of 0.1 to 5 g / 1, and

Cu ²⁺ cations in the range from 0.001 to 1 g / 1, b) while observing the following conditions: pH 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 time in the range of 1 to 300 seconds, c) and wherein, during the phosphating, the workpieces are further treated cathodically with a direct current with a density in the range from 0.01 to 100 mA / cm ² .

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 incorporation rate of manganese in the phosphate layers can be significantly increased , so that even with comparatively lower concentrations of manganese cations in the phosphating solution, similarly high contents can be achieved in the phosphating layers as was previously only possible if the phosphating solutions had comparatively high concentrations of manganese cations. A further advantage of the present invention can be seen in the fact that the phosphate layers obtained with the aid of the method according to the invention have a corrosion protection comparable to that which 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 in corresponding zinc-containing phosphating solutions which contain manganese and copper cations together, as defined in detail above, leads to the desired goal.

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 is used, for example, in the form of metal sheets for body construction. The use of galvanized steel, especially electrolytically galvanized steel in strip form, has become very important in recent years. 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 being in particular zinc / nickel alloys, zinc / iron alloys (galvanized) and zinc / Aluminum alloys (Galfan, Galvalu e) play an important role.

The method according to the invention is preferably carried out in the so-called immersion method; in general, however, it is also possible to apply the phosphating solutions according to the invention to the substrate surfaces by spray dipping. 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 the counterelectrode, and also graphite electrodes, noble metal electrodes, for example made of platinum or gold, electrodes which only have a coating of the aforementioned noble metals or in which such noble metals are implied , or in principle all electrode materials known from the relevant state of the art as counterelectrode in question.

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 which 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 geometrical arrangement of the electrodes on the other hand, a different relationship between current and Tension can exist. 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 phosphate 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.

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 / 1,

PO ^ 'anions in the range from 10 to 20 g / 1,

Nθ3 "anions in the range from 1 to 30 g / 1 and

Mn ²⁺ cations in the range of 0.5 to 2 g / 1 and

Cu ²⁺ cations in the range of 0.01 to 0.5 g / 1. 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 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 time in the range from 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 ² .

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 is in no way disturbed.

In this sense, it is preferred according to the invention to work with phosphating solutions which additionally contain Mg ²⁺ cations in the range from 0.01 to 2 g / 1, preferably from 0.1 to 1 g / 1. 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 / 1, preferably from 0.2 to 2 g / 1. 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 the above-mentioned parameters is essential for the optimal implementation of the method according to the invention. This includes, among other things, the specified range of the pH to be maintained. 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 to adjust. 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 which is required for the 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 abovementioned compounds are - in the concentration ranges essential for the invention - dissolved in water; the pH of the phosphating solutions is then adjusted to the desired value, as has also been said above.

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 adequately 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 deionized 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-2038 105 or DE-A-2043085. Accordingly, the metal surfaces to be phosphated subsequently 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. Dinatriu 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.

The actual phosphating process is then carried out following 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. Such passivation is always useful 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 / 1. 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-4031 817. However, if the phosphated substrates are subsequently subjected to a mechanical deformation process and subsequently phosphated again, as is the case, for example, in car body construction, then passivation treatment should be avoided. The phosphate 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 in 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.

B e i s p i e l e

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.

The test sheets used for all examples and comparative examples were 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. 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 in accordance with the method 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 over a period of 5 seconds.

4) Phosphating by immersion 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 (Aqualux ^R K, ICI, Hilden). The dry film density was 18 t 2 μm.

The corrosion protection of the respective phosphating layers was then determined by determining the coating infiltration in accordance with 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 Na2SO4 solution with a current flow of 0.75 A and a polarization time of 40 hours. The underpainting of the paint 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 detachment of the phosphate layer 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

See ^~ Comparative Example; f. S. = free acid; GS = total acid

Table 2: Current density, duration of phosphating, content of Cu and Nn in the phosphating layers, layer weights and corrosion test results

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, the manganese concentrations in the phosphating baths are lower equally high levels 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

Patent claims

1. Process 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 ions of at least two other divalent metals contain, characterized in that a) working with phosphating solutions which contain the following components:

Zn ²⁺ cations in the range from 0.1 to 5 g / 1,

Pθ43 ~ anions in the range from 5 to 50 g / 1,

Nθ3 "anions in the range from 0.1 to 50 g / 1, and

Mn ²⁺ cations in the range of 0.1 to 5 g / 1, and

Cu ⁺ cations in the range from 0.001 to 1 g / 1, b) while maintaining the following conditions: pH 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 from 1 to 300 sec, c) and furthermore the workpieces are treated cathodically during the phosphating with a direct current with a density in the range from 0.01 to 100 mA / cm ² .

2. The method according to claim 1, characterized in that one works with phosphating solutions which contain the following components: Zn ²⁺ cations in the range from 0.5 to 2 g / 1,

Pθ4 ³ "anions in the range from 10 to 20 g / 1,

N03 ~ anions in the range of 1 to 30 g / 1, as well

Mn ²⁺ cations in the range of 0.5 to 2 g / 1 and

Cu ²⁺ cations in the range of 0.01 to 0.5 g / 1.

3. The method according to claim 1 or 2, characterized in that in the phosphating treatment of the workpieces the following conditions are observed: pH 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 from 2 to 30 sec.

4. The method according to one or more of claims 1 to 3, characterized in that the workpieces are treated cathodically during the phosphating with a direct current having a density in the range from 1 to 50 mA / cm ² .

5. The method according to one or more of claims 1 to 4, characterized in that one works with phosphating solutions which additionally Mg ⁺ cations in the range of 0.01 to 2 g / 1, preferably from 0.1 to 1 g / 1, included.

6. The method according to one or more of claims 1 to 5, characterized in that one works with phosphating solutions which additionally simple or complex fluoride anions in the range of 0.1 to 50 g / 1, preferably from 0.2 to 2 g / 1, included.

7. The method according to one or more of claims 1 to 6, characterized in that the workpieces to be phosphated are previously subjected to an activation pretreatment known per se, in particular with titanium-containing activation solutions.

8. The method according to one or more of claims 1 to 7 as a pretreatment for a subsequent painting or coating.