EP0155547B1 - Process for the zinc-calcium phosphatizing of metal surfaces at a low treating temperature - Google Patents

Abstract

The invention relates to a process for phosphatizing metal surfaces with an acidic phosphatizing solution containing zinc ions, calcium ions, phosphate ions and accelerators and, optionally, other additives, characterized in that, after degreasing and without activation, the metal surfaces are brought into contact at 30 DEG to 65 DEG C. with solutions which contain Zn2+, Ca2+, PO43- and, as accelerators, nitrate and/or nitrite and/or chlorate ions and/or an organic nitro compound and/or H2O2 or an inorganic peroxide and which have a pH-value of from 2.2 to 3.8, a ratio of free acid to toal acid of from 1:10 to 1:60 and a ratio by weight of (Ca2++Zn2+) to PO43- of from 1:>8 to 1:40, and to the use of this process for pretreating the metal surfaces for painting by electrodeposition, particularly by cathodic electrodeposition.

Description

The invention relates to a method for phosphating metal surfaces, in particular surfaces made of iron, steel, zinc and / or aluminum, for pretreating the metal surfaces for electrocoating, in particular for cathodic electrocoating, using a zinc, calcium, phosphate and accelerator, and, if appropriate phosphating solution containing further additives.

The protection of metallic surfaces, in particular the protection of iron and steel surfaces, by phosphate-containing coatings has been known for a long time. The so-called "non-layer-forming phosphating", i.e. the use of alkali and / or ammonium orthophosphate solutions for the production of iron phosphate layers, in which the iron ion comes from the metallic surface to be coated, and the so-called "layer-forming phosphating", in which on metal surfaces using zinc or zinc / calcium / calcium phosphate Solutions of zinc phosphate layers or zinc calcium phosphate layers are formed.

Such phosphate layers not only improve the corrosion protection of the metal surfaces, but also increase the adhesion for paints to be applied to the surface. In certain cases, they can also help to improve the properties of metal sheets during cold forming and when using deep-drawing processes.

Zinc-calcium-phosphate solutions are increasingly being used especially for the phosphating of metal surfaces that are later to be coated with electrocoating materials. Experiences about the composition of the bathroom solutions on the one hand and basic knowledge about the structure of phosphating layers on the other (A. Neuhaus and M. Gebhardt, Werkstoffe und Korrosion, 567 (1966)) have conveyed the teaching that the construction of uniform and closed phosphating layers is not only of depends on the composition of the bath solutions, but also on the pretreatment of the raw sheets, the activation before the phosphating step, the choice of a suitable accelerator and other process parameters.

DE-A-1 521 818 discloses, inter alia, aqueous phosphating solutions which contain 0.01 to 1.5% calcium, 0.3 to 6% phosphate, 0.02 to 0.5% zinc as essential components. , 0.05 to 3% nitrate, 0.015 to 0.4% nickel and 0.001 to 0.02% nitrite ions, have an overall acidity in the range of 5 to 50 points and allow the phosphating of galvanized ferrous metal surfaces.

The following phosphating solutions are disclosed as examples:

Solution A: 0.11% Ca, 0.078% Zn, 0.995% P0 ₄ , 0.048% Ni, 0.0396% N0 ₃ , 0.002% N0 ₂ , total acid points: 13.6;

Solution B: 0.098% Ca, 0.043% Zn, 1.04% P0 ₄ , 0.07% Ni, 0.486% N0 ₃ , 0.002% NO ₂ , total acid points: 14.8.

The solutions used in the examples contain high levels of zinc and calcium ions with a low phosphate ion content; the weight ratio of the sum of zinc and calcium ions to phosphate ions is in the range from 1: 3.5 to 1: 7.4. The temperature of the aqueous phosphating solutions can vary within wide limits, for example from about room temperature to about 116 ° C. In general, the best results are obtained when the aqueous phosphating solutions are used at a temperature of at least about 38 ° C, generally within the range of about 49 to about 93 ° C.

• Bei Gesamteintauchzeiten von 1 bis 20 Sekunden liegen die Temperaturen zwischen 66 und 116°C.

A disadvantage of the disclosed phosphating solutions, however, is that the temperature of the phosphating step must be relatively high in order to be able to achieve an economically justifiable, short application time for the solutions:

• With total immersion times of 1 to 20 seconds, the temperatures are between 66 and 116 ° C.

DE-A-3 118 375 relates to a process for phosphating metals and its use for pretreatment for electrocoating. Here, the metal surfaces, in particular those made of iron, steel and zinc, are brought into contact with acidic, aqueous, oxidizing agent-containing zinc phosphate solutions which contain 0.4 to 1.5 g / l Zn, 0 to 1.3 g / l Ni and 10 to 26 g / l contain P ₂ 0 ₅ , the weight ratio of Zn to P ₂ 0 ₅ to a value of (0.012 to 0.12): 1 and of Ni to Zn to a value of (0 to 1.5): 1 is set. The temperature of the solutions is preferably in the range from 30 to 65 ° C. In particular, solutions are used which are 2 to 25 g / l N0 ₃ , 1 to 6 g / l CI0 ₃ , 0.1 to 2 g / l organic nitro compound, 0.05 to 0.5 g / l N0 ₂ and / or contain 0.02 to 0.1 g / l peroxide (calculated as H ₂ 0 ₂ ).

The solutions can also contain other cations, for example calcium, copper, manganese, cobalt, magnesium; however, their content is usually not more than 0.5 g / l. In order to produce particularly thin, fine crystalline layers, the use of activating agents, e.g. B. on titanium phosphate basis, in the pre-rinse bath or in the last cleaning stage.

EP-A-45 110 describes a process for the production of phosphate coatings on iron or steel surfaces with an aqueous acidic zinc phosphate solution and its use for the preparation of metal surfaces for cold working. The metal surfaces are treated with solutions containing at least 0.3 percent by weight Zn, at least 0.3 percent by weight P0 ₄ and at least 0.75 percent by weight N0 ₃ or an equivalent iron (11) non-oxidizing accelerator, the weight ratio ZB: P0 _4th is greater than 0.8, the ratio of total acid to free acid is at least 5 and in which is adjusted by suitable measurement of C10 ₃ or an equivalent iron (11) to iron (111) oxidizing accelerator, an iron (11) content of 0.04 to 1 percent by weight. Such solutions can additionally contain calcium in amounts of 0.1 to 0.88 percent by weight; the weight ratio Zn: Ca should be 1.5 to 4. The temperature of the solutions is in the range from 35 to 98 ° C. The previously cleaned metal surfaces can be subjected to an activation treatment, for example by hot water rinsing or with a titanium orthophosphate dispersion.

DE-A-2 540 685 also describes a process for the production of phosphate coatings on metal surfaces, in particular iron and steel, which has proven itself as a preparation for non-cutting cold forming. Here, aqueous acidic zinc phosphate solutions are used which contain chlorate and nitrate and have a weight ratio P ₂ 0 ₅ : Zn = 1: (0.8 to 4). Solutions which have the following composition are preferred: at least 6 g / l zinc, at least 5 g / IP ₂ 0 ₅ , at least 1 g / l C10 ₃ and at least 8 g / l NO ₃ ; Total acid number: 20 to 80, weight ratio P ₂ O ₅ : Zn: Cl0 _{3 =} 1: (0.8 to 4) :( 0.8 to 6) :( 0.03 to 2), weight ratio of free P ₂ 0 ₅ : total P ₂ 0 ₅ = (0.2 to 0.6): 1.

Such solutions can additionally contain at least 0.4 g / l Ca, the weight ratio Ca: Zn = (0.04 to 3): 1. The metal surfaces are treated with such solutions at temperatures between 35 and 98 ° C for about 5 to 15 minutes in immersion or flooding. Before the phosphating treatment, the metal surfaces can be activated by pre-rinsing with hot water or with titanium orthophosphate dispersions.

A similar process is the subject of DE-A-2 540 684. The aqueous, acidic phosphating solutions described here contain zinc, phosphate and nitrate and autocatalytically form nitrite. In this case, an N0 ₂ content of at least 0.03 g / l is to be maintained in the phosphating bath, but an increase to more than 0.2 g / l is to be prevented by adding a nitrite-destroying substance - for example urea, amidosulfonic acid, chlorate, peroxide . Preferred solutions contain: at least 6 g / l zinc, at least 5 g / IP ₂ 0 ₅ , at least 10 g / 1 N0 ₃ and 0.03 to 0.2 g / I N0 ₂ ; Total Acid Score: 20 to 80; Weight ratio P ₂ O ₅ : Zn: NO ₃ = 1: (0.8 to 4) :( 1.8 to 6); Weight ratio of free P ₂ 0 ₅ : total P ₂ 0 ₅ = (0.2 to 0.6): 1. These solutions can also contain at least 0.4 g / l of Ca, but not more than 3 parts by weight of Ca per 1 part by weight of Zn. The application temperatures here are in the range from 60 to 90.degree.

Aqueous solutions for phosphating processes which contain zinc, calcium, phosphate and optionally also nickel ions and additionally H ₂ 0 ₂ as accelerators are described in BE-A-811 220. However, the application temperatures of such solutions are also relatively high. In relation to the phosphate content, the content of zinc and calcium ions is very high.

Methods for phosphating metal surfaces by means of acidic, oxidizing agent-containing zinc phosphate solutions, in which a comparatively low zinc content contrasts with a significantly higher phosphate content and which can contain further divalent metal ions, for example also Ca ^{2 +} ions, are described in DE-B-2 232 067 and DE-A-3 118 375. With the method according to DE-B-2 232 067, high-quality phosphate layers can be formed with fresh application solutions; After passing through a higher number of sheets, however, the corrosion protection becomes qualitatively poorer due to irregular phosphating layers, in some cases no more useful protective layers are formed.

Another disadvantage of most of the processes described so far is that the quality of the heavy metal phosphate layers formed in the phosphating step is highly dependent on the degreasing pretreatment of the metal surfaces and on the activation. The activation step in particular is of great importance insofar as it forms the basis for the adhesion of the later phosphate layers and thus significantly influences the quality of the phosphate coatings which form. The desired formation of thin, fine-grained, crystalline phosphate coatings can only be achieved after sufficient activation by suitable activating agents, e.g. Activation solutions containing phosphate are possible. This results in particular in the difficulty of avoiding speck formation, which adversely affects the quality of the phosphating layer.

Surprisingly, it has now been found that thin, fine-grained, crystalline zinc-calcium-phosphate layers of high homogeneity can be obtained even at a low treatment temperature if metal surfaces are treated with acidic aqueous solutions, the zinc, calcium and phosphate ions, and one or more accelerators contain, whereby a narrow range of the pH value, a certain acid ratio, as well as a predetermined weight ratio of the sum of calcium and zinc ions to phosphate ions are to be observed.

• a) Gehalt an Ca²⁺: mehr als 0,5 bis 1,5 g/l, b) Gehalt an Zn²⁺: 0,5 bis 1,5 g/l,
• c) Gehalt an PO₄ ^3-: 10 bis 50 g/l,
• d) Gehalt an Beschleuniger
• NO₃ ^-: 0,5 bis 30 g/I und/oder
• N0₂-: 0,01 bis 0,6 g/I und/oder
• ClO₃ ^-: 0,2 bis 10 g/I und/oder
• organische Nitroverbindung: 0,1 bis 2 g/I und/ oder
• anorganisches Peroxid bzw. H₂0₂: 0,01 bis 0,5 g/I
• e) Gewichtsverhältnis Zn²⁺/Ca²⁺ = 1: (0,5 bis 1,5),
• f) Gewichtsverhältnis (Ca²⁺+Zn²⁺)/PO₄ ^3- = 1: ( > 8 bis 40),
• g) pH-Wert: 2,2 bis 3,8,
• h)Verhältnis freie Säure/Gesamtsäure = 1:(10 bis 60)

The invention therefore relates to a method for phosphating metal surfaces, in particular surfaces made of iron, steel, zinc and / or aluminum, for pretreating the same for a subsequent electrocoating, in particular a cathodic electrocoating, using an acidic, zinc, calcium, phosphate and accelerator and optionally further additives, such as nickel or fluoride, containing phosphating solution, the surfaces being brought into contact with solutions after the degreasing cleaning thereof without an activation step in the temperature range from 30 to 65 ° C., for which the following conditions apply:

• a) Ca ²⁺ content: more than 0.5 to 1.5 g / l, b) Zn ²⁺ content: 0.5 to 1.5 g / l,
• c) PO ₄ ^3- content: 10 to 50 g / l,
• d) Accelerator content
• NO ₃ ^- : 0.5 to 30 g / l and / or
• N0 ₂ -: 0.01 to 0.6 g / l and / or
• ClO ₃ ^- : 0.2 to 10 g / l and / or
• organic nitro compound: 0.1 to 2 g / l and / or
• inorganic peroxide or H ₂ 0 ₂ : 0.01 to 0.5 g / l
• e) weight ratio Zn ²⁺ / Ca ²⁺ = 1: (0.5 to 1.5),
• f) weight ratio (Ca ²⁺ + Zn ²⁺ ) / PO ₄ ^3- = 1: (> 8 to 40),
• g) pH: 2.2 to 3.8,
• h) Free acid / total acid ratio = 1: (10 to 60)

The method according to the invention is particularly suitable for the phosphating of metal surfaces made of iron, steel and zinc. However, surfaces made of aluminum with zinc-calcium layers are also to be coated with the method according to the invention.

As could be found, the treated metal surfaces are coated with a layer of dizinc calcium phosphate dihydrate (Scholzit). Although zinc and calcium ions are incorporated into the phosphating layer in a molar ratio of 2: 1, it is essential for the composition of the bath solutions according to the process according to the invention that they contain zinc and calcium ions in a weight ratio of 1: 0.5 to 1 : 1.5, preferably in a ratio of 1: 1.

For this purpose, suitable water-soluble zinc or calcium salts or solutions are added to the solutions, so that the Zn ²⁺ content is 0.5 to 1.5 g / l phosphating solution and the Ca ²⁺ content is more than 0. 5 to 1.5 g / l phosphating solution. In particular, ZnO, phosphoric acid and Ca (NO ₃ ) ₂ .4H ₂ 0 are used as starting compounds.

The proportion of P043 ions in the phosphating solutions according to the invention which can be adjusted by means of phosphoric acid is considerably higher: it is in the range from 10 to 50 g / l.

The stated amounts of the active ions forming the main constituents of the phosphating solution according to the invention are therefore characterized in that the weight ratio of the sum of calcium and zinc to phosphate is always in the range from 1: greater than 8 to 1:40. This ensures that homogeneous Scholzit layers are formed on all treated metal surfaces; the formation of e.g. B. tertiary zinc phosphate tetrahydrate (Hopeit) or dizine iron phosphate tetrahydrate (phosphophyllite), which together with scholzite would lead to a less homogeneous and poorly adhering protective layer, is not observed.

Another essential process parameter is the fact that the molar ratio of free acid to total acid (acid ratio) has to be set to values from 1:10 to 1:60. This means in particular that a relatively low value for the concentration of free acid is particularly important for the formation of good scholzite layers.

As accelerating oxidizing agents, the phosphating bath solutions for the process according to the invention are nitrate ions in an amount of 0.5 to 30 g / l, nitrite ions in an amount of 0.01 to 0.6 g / l, chlorate Ions in an amount of 0.2 to 10 g / l, organic nitro compounds in an amount of 0.1 to 2 g / l and / or inorganic peroxides or H ₂ 0 ₂ in an amount of 0.01 to 0.5 g / l bath solution added.

If aluminum surfaces are to be phosphated by the process according to the invention, simple and / or complex fluorides can be added to the bath solutions in an amount of 0.01 to 2 g / l, including the smallest amounts of aluminum which enter the bath from the metal surface and its effectiveness could be compromised by complexing with fluoride ions.

The phosphating solutions with which metal surfaces are phosphated according to the method according to the invention can also contain further metal ions, e.g. Ni2 + .Your content is in the range of 0.01 to 1.5 g / i bath solution.

The treatment of the iron, steel, zinc and / or aluminum surfaces according to the present method can be carried out by spraying, dipping or flooding; however, they are also combined processes, e.g. Spray diving applicable with equally good success. The times during which the phosphating solutions are in contact with the metal surfaces are between 60 and 240 seconds. B. for spraying between 60 to 180 sec and for diving 90 to 240 sec. However, much shorter treatment times are possible.

According to the invention, the metal surfaces are treated with the phosphating solutions in the temperature range from 30 to 65 ° C. Temperatures between 48 and 57 ° C are preferably used.

One of the main advantages of the method according to the invention is that the formation of the scholzite layers on the metal surfaces takes place completely independently of the type of cleaning which preceded the phosphating step. When using the method according to the invention there is complete freedom with regard to the choice of degreasing and cleaning agents.

Another advantage can be seen in the fact that particularly thin, fine-grained crystalline phosphate layers are obtained without the use of activating agents, as was customary in previously known processes. This not only saves at least one process step prior to phosphating, but also the necessary raw materials, such as Titanium phosphates, which are used as activating agents.

According to the method of the present invention, excellent protective layers with a layer thickness between 0.5 and 5 μm are obtained, which are used as the basis for electrocoating materials are particularly suitable for cathodic electrocoat materials, as are increasingly used in the automotive industry. However, the phosphating layers obtained according to the invention are also suitable as the basis for other organic surface protective layers.

The invention is illustrated by the examples below.

The phosphating solutions according to the invention were prepared by processes known per se by combining the desired components, in particular concentrates, salts and solutions containing zinc oxide, phosphoric acid and calcium nitrate tetrahydrate and diluting them with water to the concentrations according to the invention.

example 1

• ^{1,0 gl-1} C_a2+
• 1,2 gl^-1 _Zn2+
• 29,5 gl^-1 PO₄ ^3-
• 1,0 gl^-1 ClO₃ ^-
• 3,2 gl^-1 N03
• 0,1 gl^-1 N0₂ ^-

A phosphating solution was prepared that

• ^{1.0 gl-1} C _{a2 +}
• 1.2 gl ^-1 _Zn 2+
• 29.5 gl ^-1 PO ₄ ^3-
• 1.0 gl ^-1 ClO ₃ ^-
• 3.2 gl ^-1 N03
• 0.1 gl ^-1 N0 ₂ ^-

contained.

• pH-Wert: ca. 3,1
• Säureverhältnis: ca. 1:19
• (Ca²⁺ + Zn²⁺):P0₄ ^3- = 1:13,4

The phosphating solution had the following characteristics:

• pH: approx.3.1
• Acid ratio: approx. 1:19
• (Ca ²⁺ + Zn ²⁺ ): P0 ₄ ^3- = 1: 13.4

Steel sheets which had previously been immersed in an alkaline cleaning solution at 50 ° C. for 3 minutes and rinsed with water were treated with the aforementioned phosphating solution for 4 minutes at 55 ° C. in immersion. It was then rinsed with water and with distilled water and dried.

The phosphate layers produced were finely crystalline and closed.

The sheets were then coated with a cathodic electrocoating material and dried for 20 minutes by heating at 185 ° C. The dry film thickness of the lacquer was 18 µm.

The sheets were then provided with a single cut in accordance with DIN 53167 and subjected to the salt spray test in accordance with DIN 50 021 for a period of 480 h. The evaluation according to DIN 53 167 showed an infiltration of <0.1 mm.

The example illustrates that the process according to the invention produces good phosphate layers.

Example 2

• 0,6 gl^-1 _Ca ²⁺
• 0,7 gl^-1 _Zn ²⁺
• 22,6 gl^-1 PO₄ ^3-
• 0,3 gl^-1 _Ni ²⁺
• 2,7 gl^-1 N0₃ ^-
• 0,5 gl^-1 F^-
• 0,1 gl^-1 N0₂ ^-

A phosphating solution was prepared that

• 0.6 gl ^-1 _Ca ²⁺
• 0.7 gl ^-1 _Zn ²⁺
• 22.6 gl ^-1 PO ₄ ^3-
• 0.3 gl ^-1 _Ni ²⁺
• 2.7 gl ^-1 N0 ₃ ^-
• 0.5 gl ^-1 F ^-
• 0.1 gl ^-1 N0 ₂ ^-

contained.

• pH-Wert: ca. 3,3
• Säureverhältnis: ca. 1:39
• (Ca²⁺+Zn²⁺):PO₄ ^3- = 1:17,4

The phosphating solution had the following characteristics:

• pH: approx.3.3
• Acid ratio: approx. 1:39
• (Ca ²⁺ + Zn ²⁺ ): PO ₄ ^3- = 1: 17.4

Steel sheets which had previously been spray-cleaned with an alkaline cleaning solution at 45 ° C. for 60 seconds were sprayed with the aforementioned phosphating solution for 90 seconds at 48 ° C. It was then rinsed with water and distilled water and dried by compressed air.

The phosphate layers produced were finely crystalline and closed.

The sheets were then coated with a cathodic electrocoating material and dried for 20 minutes by heating at 185 ° C. The dry film thickness of the lacquer was 18 µm.

The sheets were then provided with a single cut in accordance with DIN 53 167 and subjected to the salt spray test in accordance with DIN 50 021 for a period of 480 h. The evaluation according to DIN 53 167 showed an infiltration of <0.1 mm.

The example illustrates that the process according to the invention produces good phosphate layers.

Example 3

• 1,3 gl^-1 _Ca ²⁺
• 1,3 gl^-1 _Zn ²⁺
• 21,2 gl^-1 PO₄ ^3-
• 1,0 gl^-1 Ni²⁺
• 2,2 gl^-1 CI0₃ ^-
• 0,6 gl^-1 Natriumnitrobenzolsulfonat

A phosphating solution was prepared that

• 1.3 gl ^-1 _Ca ²⁺
• 1.3 gl ^-1 _Zn ²⁺
• 21.2 gl ^-1 PO ₄ ^3-
• 1.0 gl ^-1 Ni ²⁺
• 2.2 gl ^-1 CI0 ₃ ^-
• 0.6 gl ^-1 sodium nitrobenzenesulfonate

contained.

• pH-Wert: ca. 2,9
• Säureverhältnis: ca. 1:18,6
• (Ca²⁺+Zn²⁺):P0₄ ^3- = 1:8,2

The phosphating solution had the following characteristics:

• pH: approx.2.9
• Acid ratio: approx. 1: 18.6
• (Ca ²⁺ + Zn ²⁺ ): P0 ₄ ^3- = 1: 8.2

Electrolytically galvanized steel sheets, which had been previously cleaned with an alkaline cleaning solution at 50 ° C. for 3 minutes by immersion treatment and rinsed with water, were treated with the aforementioned phosphating solution for 3 minutes at 57 ° C. while immersing. It was then rinsed with water and distilled water and dried by compressed air.

The phosphate layers produced were finely crystalline and closed.

The sheets were then coated with a cathodic electrocoating material and dried for 20 minutes by heating at 185 ° C. The dry film thickness of the lacquer was 18 µm.

The sheets were then provided with a single cut according to DIN 53 167 and the salt spray test according to DIN 50 021 during one Subjected to a duration of 480 h. The evaluation according to DIN 53167 showed an infiltration of <0.1 mm.

The example illustrates that the process according to the invention produces good phosphate layers.

Example 4

• 1,0 gl^-1 _Ca ²⁺
• 1,4 gl^-1 _Zn ²⁺
• 26,6 gl^-1 PO₄ ^3-
• 3,2 gl^-1 N0₃ ^-
• 0,1 gl^-1 N0₂ ^-

A phosphating solution was prepared that

• 1.0 gl ^-1 _Ca ²⁺
• 1.4 gl ^-1 _Zn ²⁺
• 26.6 gl ^-1 PO ₄ ^3-
• 3.2 gl ^-1 N0 ₃ ^-
• 0.1 gl ^-1 N0 ₂ ^-

contained.

• pH-Wert: ca. 3,6
• Säureverhältnis: ca. 1:48
• (Ca²⁺ +Zn²⁺):PO₄ ^3- = 1:11,1.

The phosphating solution had the following characteristics:

• pH: approx.3.6
• Acid ratio: approx. 1:48
• (Ca ²⁺ + Zn ²⁺ ): PO ₄ ^3- = 1: 11.1.

Steel sheets which had previously been spray-treated with an alkaline cleaning solution at 45 ° C. for 60 seconds were washed with the aforementioned phosphating solution 120sec. sprayed at 35 ° C. It was then rinsed with water and distilled water and dried by compressed air.

The phosphate layers produced were finely crystalline and closed.

The sheets were then coated with a cathodic electrocoating material and dried for 20 minutes by heating at 185 ° C. The dry film thickness of the lacquer was 18 µm.

The sheets were then provided with a single cut in accordance with DIN 53 167 and subjected to the salt spray test in accordance with DIN 50 021 for a period of 480 h. The evaluation according to DIN 53 167 showed an infiltration of 0.2 mm.

The example illustrates that the process according to the invention produces good phosphate layers.

Comparative example

According to DE-A-1 521 818, p. 12, solution B was prepared.

Steel sheets which had previously been cleaned with an alkaline cleaning solution at 72 ° C. for 30 seconds were sprayed with the aforementioned phosphating solution for 60 seconds at 66 ° C. It was then rinsed with water and distilled water and dried by compressed air. The phosphate layers produced were roughly crystalline and not completely closed.

The sheets were then coated with a cathodic electrocoating material and dried by heating at 185 ° C. for 20 minutes. The dry film thickness of the lacquer was 18 μm.

The sheets were then provided with a single cut according to DIN 53 167 and were happy to do the salt spray test. Subject to DIN 50 021 for a period of 480 h. The evaluation according to DIN 53 167 showed an infiltration of 4 to 6 mm.

The comparative example illustrates that, in contrast to the examples according to the invention, significantly reduced corrosion protection is achieved.

Claims (6)

1. A process for phosphating metal surfaces, particularly surfaces of iron, steel, zinc and/or aluminium, for the pretreatment thereof for subsequent painting by electrodeposition, particularly cathodic electrodeposition, using an acidic phosphating solution containing zinc, calcium, phosphate and accelerators and, optionally, further additions, such as nickel or fluorides, the surfaces - after degreasing and without activation - being brought into contact at 30 to 65°C with solutions satisfying the following requirements:

a) content of Ca²⁺: more than 0.5 to 1.5 g/l,

b) content of Zn²⁺: 0.5 to 1.5 g/l,

c) content of P0₄ ³-: 10 to 50 g/l,

d) content of accelerator

NO₃ ^-: 0.5 to 30 g/I and/or

NO₂ ^-: 0.01 to 0.6 g/I and/or

ClO₃ ^-: 0.2 to 10 g/I and/or organic nitro compound: 0.1 to 2 g/I and/or inorganic peroxide or H₂0₂: 0.01 to 0.5 g/I,

e) ratio by weight Zn²⁺:Ca²⁺ = 1:(0.5 to 1.5),

f) ratio by weight (Ca²⁺ +Zn²⁺):P0₄ ^3- = 1:(> 8 to 40),

g) pH value: 2.2 to 3.8,

h) ratio of free acid to total acid = 1: (10 to 60)

2. A process as claimed in claim 1, characterized in that the phosphating solutions have a ratio by weight of Zn²⁺ to Ca2+ of 1:1.

3. A process as claimed in claim 1 or 2, characterized in that the phosphating solutions have an Ni²⁺ content of from 0.01 to 1.5 g/I.

4. A process as claimed in claim 1 or 2, characterized in that the phosphating solutions have a content of simple and/or complex fluorides of from 0.01 to 2 g/l.

5. Process as claimed in claims 1 to 4, characterized in that the metal surfaces are brought into contact with the phosphating solutions at a temperature in the range from 48 to 57°C.

6. A process as claimed in claims 1 to 5, characterized in that the metal surfaces are brought into contact with the phosphating solutions by immersion, spraying, flooding or by a combination thereof.