EP0922123A1

EP0922123A1 - Process and aqueous solution for phosphatising metallic surfaces

Info

Publication number: EP0922123A1
Application number: EP97943803A
Authority: EP
Inventors: Thomas Kolberg; Peter Schubach
Original assignee: Metallgesellschaft AG
Current assignee: chemetal GmbH
Priority date: 1996-08-28
Filing date: 1997-08-11
Publication date: 1999-06-16
Anticipated expiration: 2017-08-11
Also published as: JP3940174B2; JP2000516999A; IN192301B; AU720551B2; KR20000035825A; ES2150791T3; HUP9903091A1; SK283857B6; GR3034297T3; HU228330B1; CA2264568A1; SI0922123T1; CZ68099A3; SK23299A3; DE19634685A1; DE59702088D1; PL192285B1; CN1231705A; PT922123E; AU4551697A

Abstract

An aqueous, phosphate-containing solution is disclosed for generating phosphate layers on metallic surfaces made of iron, steel, zinc, zinc alloys, aluminium or aluminium alloys. The solution contains 0.3 to 5 g Zn2+/l and 0.1 to 3 g nitroguanidine/l, the S-value being comprised between 0.03 and 0.3 and the weight ratio Zn : P¿2?O5 being from 1: 5 to 1: 30. Also disclosed is a phosphatising process in which the metallic surfaces are cleaned, then treated with the aqueous phosphatising solution for 5 seconds to 10 minutes at a temperature from 15 to 70 °C and finally rinsed with water.

Description

Adequate solution and process for phosphating metallic surfaces

description

The invention relates to an aqueous, phosphate-containing solution for producing phosphate layers on metallic surfaces made of iron, steel, zinc, zinc alloys, aluminum or aluminum alloys. The invention further relates to a method for phosphating using an aqueous phosphating solution.

DE-PS 750 957 discloses a method for improving the corrosion resistance of metals, in particular iron and steel, by treatment in a solution which forms phosphate coatings, in which the solution contains an accelerating agent and in which nitromethane, nitrobenzene is used as the accelerating agent , Picric acid, a nitraniline, a nitrophenol, a nitrobenzoic acid, a nitroresorcinol, nitrourea, a nitrourethane or nitroguanidine is used. The optimal concentration for the individual accelerators is different, but it is generally between 0.01 and 0.4% by weight in the phosphating solutions. The optimal concentration for the accelerator nitroguanidine should be 0.2% by weight. DE-OS 38 00 835 discloses a process for phosphating metal surfaces, in particular surfaces made of iron, steel, zinc and their alloys and aluminum as a pretreatment for cold forming, in which the surface is activated without activation in the temperature range from 30 to 70 ° C in contact with an aqueous solution containing 10 to 40 g Ca ^{2 *} / 1, 20 to 40 g Zn ^{2 *} / 1, 10 to 100 g PO ^' / l and as an accelerator 10 to 100 g N0 ₃ ^" / 1 and / or 0.1 to 2.0 g of organic nitro compounds per liter, the solution having a pH in the range from 2.0 to 3.8 and a ratio of free acid to total acid of 1: 4 to 1: 100. A nitrobenzenesulfonate and / or nitroguanidine can be used as the accelerator, and the phosphate layers produced by the known method have layer weights of 3 to 9 g / m ² .

Although it is known per se that nitroguanidine can be used as an accelerator in the phosphating of metallic surfaces, the practical use of this accelerator encounters difficulties because the results of the phosphating are very often unsatisfactory. This is obviously due to the fact that the action of the accelerator nitroguanidine is very strongly dependent on the inorganic constituents of the phosphating solution and the concentration of the inorganic constituents in the phosphating solution, so that the phosphate layers produced using nitroguanidine only have good performance properties if they are successful to provide a phosphating solution in which the individual components are coordinated with one another in such a way that when the nitroguanidine is used as an accelerator, phosphate layers of good, consistent quality can be produced even in continuous operation. In addition, the interactions between the nitroguanidine and the other constituents of the phosphating solution are not predicted by theoretical considerations or simple experiments or can be determined, but can only be determined by extensive experimental work on different phosphating systems. The often unsatisfactory results are also due to the poor solubility in water or the uneven distribution of the nitroguanidine.

The invention is therefore based on the object of providing an aqueous solution for phosphating metallic surfaces which contains nitroguanidine as an accelerator and the remaining constituents of which are coordinated with one another in such a way that the phosphate layers formed during the phosphating process are finely crystalline, have a low layer weight and good paint adhesion enable and ensure good corrosion protection. Furthermore, the invention is based on the object of providing a method for phosphating which uses the phosphating solution according to the invention, the method being intended to operate at the lowest possible temperatures, can be used for phosphating different metallic surfaces and has to operate using simple technical means and in a reliable manner .

The object on which the invention is based is achieved by the creation of an aqueous, phosphate-containing solution for producing phosphate layers on metallic surfaces made of iron, steel, zinc, zinc alloys, aluminum or aluminum alloy which contain 0.3 to 5 g of Zn ² l, and 0, Contains 1 to 3 g of nitroguanidine / 1, the S value being 0.03 to 0.3 and the weight ratio Zn: P ₂ 0 ₅ = 1: 5 to 1:30, and producing the fine-crystalline phosphate layers in which the crystallites have a maximum edge length of <15 μm. Surprisingly, it has been shown that the phosphating solution according to the invention can be used to produce very fine-crystalline phosphate layers which provide excellent paint adhesion and good corrosion protection. The crystallites have a platelet-like, cuboid or cube-like shape and always have a maximum edge length <15 μm, which is usually even <10 μm. Furthermore, the phosphating solution according to the invention is very suitable for phosphating cavities. The phosphate layers deposited on the metallic objects from the phosphating solution according to the invention have a layer weight of 1.5 to 4.5 g / m ² , preferably of 1.5 to 3 g / m ² , whereby the paint adhesion is favored in an advantageous manner. With a zinc content> 5 g / 1, the corrosion protection properties and paint adhesion deteriorate significantly.

The Zn: P ₂ 0 ₅ ratio relates to the total P ₂ 0 _5. The determination of the total P ₂ 0 ₅ is based on the titration of the phosphoric acid and / or the primary phosphates from the equivalence point of the primary phosphate to the equivalence point of the secondary Phosphate. The S value indicates the ratio of free acid, calculated as free P ₂ 0 ₅ , to total P ₂ 0 ₅ . The definitions and determination methods for the total P ₂ 0 ₅ and the free P ₂ 0 ₅ are explained in detail in the publication by W. Rausch "Die Phosphatierung von Metallen", 1988, pages 299 to 304.

According to the invention it is particularly advantageous when the aqueous, phosphate-containing solution from 0.3 to 3 g Zn ^{2/1 *} ^and contains ° _/ l ^b i ^s 3 g nitroguanidine / 1, wherein the S-value amounting to 0.03 to 0 3 and the weight ratio Zn: P ₂ 0 _s = 1: 5 to 1:30. With this solution according to the invention, which is suitable for carrying out the low zinc phosphating because of its zinc content of 0.3 to 3 g / l, particularly good work results have been achieved overall.

According to the invention it is provided that the aqueous solution contains 0.5 to 20 g N0 ₃ ^" / 1. The nitrate content according to the invention advantageously favors the maintenance of the optimal layer weight of 1.5 to 4.5 g / m ^2. The nitrate will the Phosphating solution in the form of alkali nitrates and / or by the cations present in the system, e.g. B. added as zinc nitrate, and / or as HN0 ₃ . Since the nitrate-free aqueous solution also delivers good phosphating results, the known acceleration effect of the nitrate is in most cases of minor importance in the present case.

According to the invention it is further provided that the phosphating solution 0.01 to 3 g Mn ^{2 *} / 1 and / or 0.01 to 3 g Ni ^{2 *} / 1 and / or 1 to 100 mg Cu ^{2 *} / 1 and / or Contains 10 to 300 mg Co ² '/ l. These metal ions are built into the phosphate layer and improve paint adhesion and corrosion protection.

In a further embodiment of the invention, it is provided that the aqueous phosphating solution contains 0.01 to 3 g F ^" / 3. And / or 0.05 to 3.5 g / 1 complex fluorides, preferably (SiF ₆ ) ^2" or (BF ^~ The fluoride is added to the phosphating solution when metallic surfaces consisting of aluminum or aluminum alloys are to be phosphated, and the complex fluorides are added to the phosphating solution, in particular for stabilization, as a result of which the phosphating baths have a longer service life.

The object on which the invention is based is further achieved by the creation of a method for phosphating, in which the metallic surfaces are cleaned, then treated with the aqueous, phosphate-containing phosphating solution for a period of 5 seconds to 10 minutes at a temperature of 15 to 70 ° C. and finally rinsed with water. This process can be carried out using simple technical means and is extremely reliable. The phosphate layers produced by the process have a consistently good quality, which does not decrease even with a longer operating time of the phosphating bath. The minimum phosphating time is at Process according to the invention less than in known low-zinc processes which work with the usual accelerators. The minimum phosphating time is the time in which the surface is 100% covered with a phosphate layer.

According to the invention it is provided that the treatment of the metallic surfaces with the phosphating solution is carried out by spraying, dipping, splash-dipping or rolling. These working techniques open up a very broad and diverse range of applications for the method according to the invention. According to the invention, it has proven to be particularly advantageous if the phosphating solution used for spraying has a weight ratio Zn: P ₂ 0 ₅ = 1:10 to 1:30 and if the phosphating solution used for dipping has a weight ratio Zn: P ₂ 0 ₅ = 1: 5 to 1:18.

After the invention, it is often advantageous to treat the metallic surfaces with an activating agent that contains a titanium-containing phosphate after cleaning. This supports the formation of a closed, fine-crystalline zinc phosphate layer.

Finally, it is provided according to the invention that the metallic surfaces are aftertreated with a passivating agent after the rinsing process following the phosphating. The passivating agents used can be both Cr-containing and Cr-free.

When cleaning the metallic surfaces according to the method according to the invention, both mechanical impurities and adhering greases are removed from the surface to be phosphated. The cleaning of the metallic surfaces belongs to the known state of the art and can advantageously be carried out with an aqueous alkaline cleaner. It is useful if the

Layered silicates have no adverse effect on the formation of the phosphate layers. In addition to their actual beneficial effect, they also improve the sedimentation of the phosphate sludge and increase its solids content.

The subject matter of the invention is explained in more detail below on the basis of exemplary embodiments.

Examples 1 and 2 were carried out using the following process steps:

a) The surfaces of sheet metal objects were cleaned with a weakly alkaline cleaner (2% aqueous solution) for 5 minutes at 60 ° C and degreased in particular.

b) This was followed by rinsing with tap water for 0.5 minutes at room temperature.

c) This was followed by activation with an activating agent (3 g / 1 H ₂ O), which contained a titanium phosphate, for 0.5 minutes at room temperature.

d) It was then phosphated by dipping at about 55 ° C. for 3 minutes.

e) Finally, it was rinsed with tap water for 0.5 minutes at room temperature.

f) The phosphated surfaces were dried with compressed air.

The composition of the aqueous solutions used for phosphating and the properties of the phosphate layers are shown in Table 1. In accordance with the exemplary embodiments 1 and 2, comparative tests were carried out using known methods

Phosphatization solutions carried out, but which contained a different accelerator (comparative tests A and B). In addition, a comparison test was carried out with a phosphating solution which was not in accordance with the Zn: P ₂ 0 ₅ ratio and which contained nitroguanidine as accelerator (comparison test C). In comparison tests A, B, C, process steps a) to f) were carried out. The composition of the phosphating solutions used for the comparative tests and the properties of the phosphate layers are shown in Table 2.

The comparison of the exemplary embodiments 1 and 2 with the comparative experiments A, B and C shows that good results are achieved with the phosphating solution according to the invention compared to the known and proven phosphating solutions, although the nitroguanidine has significantly better utility properties than the accelerator N0 ₂ ^" . The comparative experiment C shows that good and practical phosphating results can only be achieved by using the parameters according to the invention.

Exemplary embodiments 3 and 4 were carried out using the following process conditions, in particular the suitability of the invention for phosphating cavities to be tested: steel sheets were treated in a box which simulated a cavity in accordance with the process steps a) to e), which also at Examples 1 and 2 were used. The phosphated steel sheets were dried in the cavity (box) at room temperature without compressed air. The composition of the aqueous used to phosphate a cavity Solutions and the properties of the phosphate layers are shown in Table 3.

The phosphate layers of exemplary embodiments 3 and 4 had approximately the same properties as the phosphate layers of exemplary embodiments 1 and 2 with regard to layer weight, crystallite edge length and minimum phosphating time.

Comparative tests D and E were carried out in accordance with exemplary embodiments 3 and 4, the individual method steps being identical. The phosphating solutions used in comparative experiments D and E are known per se and contain hydroxylamine as an accelerator. The composition of the solutions used to carry out comparative experiments D and E and the properties of the phosphate layers are given in Table 4.

A comparison of the exemplary embodiments 3 and 4 with the comparative experiments D and E shows that with the invention a very good phosphating of cavities can be achieved, because according to the invention complete, closed phosphate layers are produced and no rust formation occurs. The term "rust formation" means that a rust layer forms on the metallic surface, which does not have a complete, closed phosphate layer, during drying, which is very disadvantageous. In some cases, the formation of flash rust does not occur, although there is no complete, closed phosphate layer, which may be due to passivation of the metallic surface by the phosphating solution.

Paint adhesion test values were determined to test the corrosion properties of and paint adhesion on various metallic substrates phosphated according to the invention. Table 5 shows the paint adhesion and the corrosion protection test values which were determined for different metal sheets (substrates), the individual substrates being phosphated by dipping according to examples 5, 6 and 7 with solutions according to the invention and according to comparative tests F and G with known solutions have been. The individual substrates were dipped in accordance with the process steps a) to f) mentioned above. The composition of the phosphating solutions used for Examples 5, 6 and 7 is given in Table 7. There you will also find the compositions of the known phosphating solutions that were used to carry out comparative tests F and G. After phosphating the substrates by dipping, an electro-dipping lacquer, a filler and a topcoat were applied. The test was then carried out by outdoor exposure, assessed after 6 months, by a salt spray test and by stone chips after a 12-round climate change test. Table 5 shows the infiltration of the paint layer, measured in mm, determined in the individual tests, the paint flaking being stated in percent for the stone chip test.

Table 6 shows the paint adhesion and corrosion protection test values for various substrates that were phosphated by spraying. The spray phosphating of the substrates was carried out according to the invention using the following process steps: g) The surfaces of the substrates were cleaned with a weakly alkaline cleaner (2% aqueous solution) for 5 minutes at 60 ° C. and in particular degreased.

h) This was followed by a rinse with tap water for 0.5 minutes at room temperature. i) It was then phosphated by spraying at 55 ° C. for 2 minutes.

k) It was then rinsed with a chromium-free rinse aid containing (ZrF ₆ ) ^{2 "} at room temperature for 1 minute in order to passivate the phosphated substrates.

1) Finally, it was rinsed with deionized water for 1 minute at room temperature.

m) The phosphated substrates were dried in the oven at 80 ° C. for 10 minutes.

The composition of the aqueous phosphating solutions according to the invention, which were used to carry out Examples 8, 9 and 10, are given in Table 8. The composition of the known phosphating solution which was used to carry out comparative test H is likewise found in Table 8. An electrocoating lacquer, a filler and a topcoat were then applied to the substrates phosphated by spraying. The phosphated and coated substrates were then subjected to a test by outdoor exposure for 6 months, by a salt spray test, by a cross cut and by a 12-round climate change test followed by stone chips. Table 6 shows the test values determined for the individual substrates, an evaluation grade being given for the cross-cut and for the outdoor exposure, the salt spray test and the climate change test the infiltration of the paint layer, measured in mm, is given. For stone chips, the flaking of the paint is given in percent.

The corrosion protection that is achieved by the phosphating according to the invention is comparable to the corrosion protection that is achieved through the use of proven, known ones Phosphating processes occur that work with the accelerator nitrite. The phosphating according to the invention, on the other hand, avoids the use of the accelerator nitrite, the use of which is increasingly being rejected, since nitrite produces reaction products during the phosphating which damage the environment and are sometimes toxic to humans. The paint adhesion and corrosion protection effect achieved by the phosphating according to the invention can be rated as very good to good.

Table 1

Table 2

Table 3

Table 4

Table 5 Paint adhesion test values, immersion application

Substrate examples comparative experiments

5 6 7 F G

Outdoor exposure 6 months, mm infiltration, measured on one side by the scratch.

Steel <1 <1 1.5 <1 2.5

Electrolytically galvanized steel 1 1 1 1.5 2.5

Hot-dip galvanized steel 0 <1 1 0 <1

Steel with Fe-Zn layer <1 <1 <1 <1 <1

AlMgSi, unpolished 3 0 0 <1 to 3 ~

AlMgSi, ground 5 <1 0 4 -

Salt spray test, 1008 h, according to DIN 50021 SS, mm infiltration

Steel <1 <1 1.5 <1 1

12-round climate change test in accordance with VDA 621-415, infiltration in mm, measured on one side by the Ritz, and subsequent stone chipping in accordance with the specification of VW AG,% paint flaking, specified in ()

Steel <1 (0.5) <1 (0.5) 1.5 (0.5) <1 (1) 2 (1)

Electrolytically galvanized steel 6.5 (1.5) 7 (8.5) 7 (5) 5.5 (2) 8 (40)

Hot-dip galvanized steel 1.5 (0.5) 2 (7) 2 (2) 1 (0.5) 2.5 (15)

Steel with Fe-Zn layer 1 (0.5) 1 (0.5) 1 (0.5) 1 (0.5) 1 (0.5)

Table 6 Paint adhesion test values, spray application

Table 7

Table 8

Claims

claims

1. Aqueous, phosphate-containing solution for producing phosphate layers on metallic surfaces made of iron, steel, zinc, zinc alloys, aluminum or aluminum alloys, which contains 0.3 to 5 g of Zn ^{2 *} / 1 _/ and 0.1 to 3 g of nitroguanidine / 1 , the S value being 0.03 to 0.3 and the weight ratio Zn: P ₂ 0 _s = 1: 5 to 1:

30, and produces the fine crystalline phosphate layers in which the crystallites have a maximum edge length <15 microns.

2. An aqueous solution according to claim 1, which contains 0.3 to 3 g of Zn ² '/ 1 /.

3. Aqueous solution according to claims 1 to 2, which contains 0.5 to 20 g N0 ₃ ^" / 1.

4. Aqueous solution according to claims 1 to 3, the 0.01 to 3 g Mn ² 7l and / or 0.01 to 3 g Ni ^* / 1 and / or 1 to 100 mg Cu ^{2 *} / 1 and or 10 to Contains 300 mg Co ^{2 *} / l.

5. Aqueous solution according to claims 1 to 4, which contains 0.01 to 3 g T ^' / l and / or 0.05 to 3.5 g / 1 of at least one complex fluoride.

6. Aqueous solution according to claim 5, which contains as complex fluoride (SiF ₆ ) ^{2 "} or (BF ^~ .

7. Process for phosphating, in which the metallic surfaces are cleaned, then treated with the aqueous, phosphate-containing solution according to claims 1 to 6 for a period of 5 seconds to 10 minutes at a temperature of 15 to 70 ° C and finally rinsed with water become.

8. The method according to claim 7, wherein the treatment of the metallic surfaces with the phosphating solution is carried out by spraying, dipping, spray-dipping or rolling.

9. The method according to claim 8, wherein the phosphating solution used for spraying has a weight ratio of Zn: P ₂ O _s = 1:10 to 1:30.

10. The method according to claim 8, wherein the phosphating solution used for dipping has a weight ratio of Zn: P ₂ 0 ₅ = 1: 5 to 1:18.

11. The method according to claims 7 to 10, wherein the metallic surfaces are treated after cleaning with an activating agent containing a titanium-containing phosphate.

12. The method according to claims 7 to 11, wherein the metallic surfaces are aftertreated with a passivating agent after the rinsing process following the phosphating.

13. The method according to claim 7, wherein the nitroguanidine is introduced into the aqueous solution in the form of a stable, aqueous suspension.

14. The method according to claim 13, wherein the stable, aqueous suspension contains a layered silicate as stabilizer.

15. The method according to claim 14, in which the layer silicates [Mg ₆ (Si ₇₄ A1 ₀ O ₂₀ (OH) ₄ ] Na ₀ “xH ₂ 0 and

[(Mg _{s 4} Li _{0 6} ) Si ₈ O ₂₀ (0H ₃ F) ₄ ] Na ₀₀ xH ₂ 0 in an amount of 10 to 30 g / 1 nitroguanidine suspension.

16. Use of the aqueous, phosphate-containing solution according to claims 1 to 6 and the method for phosphating according to claims 7 to 15 for the treatment of workpieces before painting.

17. Use according to claim 16 for the treatment of workpieces before electrocoating.