EP1060290B1 - Aqueous solution and method for phosphatizing metallic surfaces - Google Patents

Abstract

An aqueous solution contains phosphate for producing layers of phosphate on metal surfaces selected from the group consisting of iron, steel, zinc, zinc alloys, aluminum and aluminum alloys. The solution contains 0.3 to 5 g Zn2+/1, 0.1 to 2 g nitroguanidine/1 and 0.05 to 0.5 g hydroxylamine/l, with an S-value amounting to 0.03 to 0.3 and the ratio of the weight of Zn2+ to P2O5=1:5 to 1.30.

Description

The invention relates to an aqueous, phosphate-containing solution for the production of phosphate layers on metallic Surfaces made of iron, steel, zinc, zinc alloys, aluminum or aluminum alloys. The invention further relates to a Process for phosphating metallic surfaces under Use of an aqueous phosphating solution.

GB-A 510684 discloses a method for improving the corrosion resistance of metals, in particular iron and steel, by treatment in a zinc-containing solution which forms phosphate coatings, in which the solution contains an accelerating agent and in which hydroxylamine is used as an accelerating agent , Nitromethane, nitrobenzene, picric acid, a nitraniline, a nitrophenol, a nitrobenzoic acid, a nitroresorcinol, nitrourea, a nitrourethane or nitroguanidine is used. The optimum 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 and for the accelerator hydroxylamine the optimal concentration should be 0.3% by weight. GB-A 510684 makes no statements about the zinc content, the S value and the Zn-P ₂ O ₅ ratio of the phosphating solution, nor does it recommend the use of a mixture consisting of several accelerators.

WO-A95 / 07370 discloses a process for phosphating metal surfaces with aqueous, acidic phosphating solutions which contain zinc, manganese and phosphate ions and, as an accelerator, hydroxylamine or a hydroxylamine compound and / or m-nitrobenzenesulfonic acid or its water-soluble salts which the metal surface is brought into contact with a phosphating solution which is free from nickel, cobalt, copper, nitrite and oxo anions from halogens and which contains 0.3 to 2 g / l of Zn ²⁺ , 0.3 to 4 g / l of Mn ^{Contains 2+} , 5 to 40 g / l phosphate ions, 0.1 to 5 g / l hydroxylamine in free or bound form and / or 0.2 to 2 g / l m-nitrobenzenesulfonate and at most 0.5 g / l nitrate ions, wherein the Mn content is at least 50% of the Zn content.

In the German patent application 196 34685.1 dated 28.08.1996, an aqueous, phosphate-containing solution for the production of phosphate layers on metallic surfaces made of iron, steel, zinc, zinc alloys, aluminum or aluminum alloys was proposed, which contained 0.3 to 5 g of Zn ²⁺ / l and Contains 0.1 to 3 g nitroguanidine / l, the S value being 0.03 to 0.3 and the weight ratio Zn ²⁺ to P ₂ O ₅ = 1: 5 to 1:30, and which produces crystalline phosphate layers, in which the crystallites have a maximum edge length of <15 µm. In the German patent application, a process for phosphating was also proposed in which the metallic surfaces were cleaned, then treated with the aforementioned aqueous, phosphate-containing solution for a period of 5 seconds to 10 minutes at a temperature of 15 to 70 ° C. and finally with water be rinsed.

The invention is based on the object of phosphate-containing solution and the process for phosphating, which in German patent application 196 34 685.1 have been proposed to improve the maximum Edge length of those present in the phosphate layers produced Crystallite is significantly <15 µm that the generated Phosphate layers have a layer weight of 2 to 4 g / l and that the phosphate layers produced with regard to Layer weight and the edge length of the crystallites too during a longer operating time of the phosphating bath are constant or even.

The object on which the invention is based is achieved by creating an aqueous, phosphate-containing solution for producing phosphate layers on metallic surfaces made of iron, steel, zinc, zinc alloys, aluminum or aluminum alloys which contain 0.3 to 5 g of Zn ²⁺ / l. 0 , 1 to 2 g nitroguanidine / l and 0.05 to 0.5 g hydroxylamine / l, the S value being 0.03 to 0.3 and the weight ratio of Zn ²⁺ to P ₂ O ₅ = 1: 5 to 1:30. The essence of the invention is therefore that the solution proposed in the cited German patent application contains not only the accelerator nitroguanidine but also the accelerator hydroxylamine in a low concentration, the nitroguanidine concentration of the solution according to the invention compared to the nitroguanidine concentration of that proposed in the German patent application Solution was significantly lowered.

According to the invention, a solution is particularly advantageous and preferred, the 0.1 to 1.5 g nitroguanidine / l and 0.1 to 0.4 g contains hydroxylamine / l. When using this preferred Features according to the invention become optimal Phosphating results achieved.

Although EP-B 0 315 059, on the one hand, discloses a solution for phosphating iron surfaces, which has a zinc concentration of 0.2 to 2 g / l and which, as an accelerator, contains hydroxylamine, hydroxylamine salts or hydroxylamine complexes contains, which give the solution a hydroxylamine concentration of 0.5 to 50 g / l, preferably 1 to 10 g / l, and although on the other hand from the publication EP-B 0 633 950 a solution for producing copper-containing phosphate layers on metal surfaces steel, galvanized steel, alloy galvanized steel, aluminum and its alloys is known which has a zinc concentration of 0.2 to 2 g / l, a copper concentration of 0.5 to 25 mg / l, a P ₂ O ₅ -Concentration of 5 to 30 g / l, which contains hydroxylamine, hydroxylamine salts and hydroxylamine complexes as accelerators, which give the solution a hydroxylamine concentration of 0.5 to 5 g / l, and which additionally serves as an oxidizing agent ganic nitro compound, it was extremely surprising to the person skilled in the art that even with comparatively low concentrations of nitroguanidine and hydroxylamine, phosphate layers can be produced which have an optimal layer weight of 2 to 4 g / m ² , the layer weight of which is very uniform even in continuous operation and whose crystallites in each case have a maximum edge length of <15 µm, although the edge length is generally significantly <10 µm. These surprisingly advantageous effects of the solution according to the invention are associated with a further advantageous effect, which consists in that relatively small amounts of accelerator are carried off from the phosphating bath into the subsequent treatment stages and ultimately into the waste water due to the comparatively low concentration of the accelerators , The solution according to the invention thus ensures that both accelerators of the phosphating are supplied almost quantitatively.

The solution according to the invention is also not known to the person skilled in the art discloses the prior art cited above or suggested because compared to that in the German patent application 196 34 685.1 uses the proposed solution solution according to the invention only the lower nitroguanidine concentrations and additionally hydroxylamine and compared to Solutions resulting from the two Europeans cited Are known patents, uses the invention Solution hydroxylamine concentrations lower than that in The prior art cited disclosed hydroxylamine concentrations are, with the fact that in both European patents cited the use of Nitroguanidine as an accelerator is not disclosed and that both European patents cited the expert prompt to use high hydroxylamine concentrations, because according to EP-B 0 315 059 is a hydroxylamine concentration from 1 to 10 g / l as preferred and claimed in accordance with Example 1 of EP-B 0 633 950 a hydroxylamine sulfate concentration of 1.7 g / l worked, which corresponds to a hydroxylamine concentration of 0.68 g / l. It is thus the merit of the present invention recognized have that on different metallic surfaces very high quality phosphate layers from a solution can be deposited that are very low Hydroxylamine content and a comparatively low Has nitroguanidine content, the invention with respect the hydroxylamine content is not that of the prior art prescribed way, namely the application quite high Hydroxylamine concentrations.

In a further embodiment of the invention it is provided that the solution contains 0.3 to 3 g of Zn ²⁺ / l. The solution is therefore particularly suitable for use in the context of low-zinc technology. It is further provided in a further embodiment of the invention that the solution additionally contains 0.5 to 20 g / NO ₃ ^- / l, that the solution additionally contains 0.01 to 3 g Mn ²⁺ / l and / or 0.01 to 3 contains Ni ²⁺ / l and / or 1 to 100 mg Cu ²⁺ / l and / or 0.01 to 3 g Co ²⁺ / l. In particular, the copper content of 1 to 100 mg Cu ²⁺ / l is responsible for the fact that high-quality phosphate layers are produced in the absence of nickel. When phosphating aluminum-containing surfaces, it has proven particularly useful according to the invention if the solution contains 0.01 to 3 g F ^- / l and / or 0.05 to 3.5 g / l of at least one complex fluoride. According to the invention, the solution contains as complex fluoride (SiF ₆ ) ^2- or (BF ₄ ) ^- .

The nitrate content according to the invention advantageously favors maintaining a constant layer weight. The nitrate is added to the phosphating solution in the form of alkali metal nitrates and / or by the cations present in the system, for example as zinc nitrate and / or as HNO ₃ . Since the nitrate-free aqueous solution also gives good phosphating results, the acceleration effect of the nitrate, which is known per se, is in most cases of minor importance in the present case. The metal ions Mn ²⁺ , Ni ²⁺ , Cu ²⁺ and Co ²⁺ added to the phosphating solution are built into the phosphate layer and improve paint adhesion and corrosion protection. The free fluoride is added to the phosphating solution when metallic surfaces consisting of aluminum or aluminum alloys are phosphated. The complex fluorides are added to the phosphating solution, in particular to improve the phosphating result on galvanized surfaces.

The object underlying the invention is further achieved by the creation of a process for phosphating metallic Surfaces solved where the metallic surfaces cleaned, then 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 treated finally be rinsed with water. This procedure can be carried out with simple technical means and works extremely reliably. The one with the procedure generated phosphate layers have a consistently good Quality that lasts even longer Phosphating bath does not decrease. The minimum phosphating time is lower in the method according to the invention than in known ones Low zinc process with the usual accelerators work. The minimum phosphating time is the time in which the Surface is phosphated closed. In a surprising way It was found that the process parameters, the at the in the German patent application 196 34 685.1 proposed methods have proven advantageous can generally also be used in the method according to the invention.

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 of Zn ²⁺ to P ₂ O ₅ = 1: 5 to 1:30, the time required for spraying being 5 to 300 seconds, and if the phosphating solution used for diving has a weight ratio of Zn ²⁺ to P ₂ O ₅ = 1: 5 to 1:18, the time required for diving being 5 seconds to 10 minutes.

According to the invention it is advantageous in many cases if the metallic surfaces after cleaning with a Activating agents are treated, which is a titanium-containing Contains phosphate. This will form a closed, crystalline phosphate layer supports. It is also provided according to the invention that the metallic surfaces after the one after the phosphating Rinsing process can be treated with a passivating agent. The passivating agents used can contain Cr as well also be Cr-free.

In the case provided by the method according to the invention Cleaning the metallic surfaces will be both mechanical impurities as well as adhering fats from the surface to be phosphated removed. The cleaning of the metallic surfaces belong to the known state of the art and can be beneficial with an aqueous alkaline cleaner be performed. It is useful if the metallic Surfaces are rinsed with water after cleaning. The Rinsing the cleaned or the phosphated metallic Surfaces are made either with tap water or with desalinated water.

According to the invention, it is particularly advantageous if the nitroguanidine is introduced into the aqueous solution in the form of a stable, aqueous suspension. This can be done either in that the stable, aqueous suspension contains a layered silicate as stabilizer, the layered silicates [Mg ₆ (Si ₇ , ₄ Al _0.6 ) O ₂₀ (OH) ₄ ] Na _0.6 x XH ₂ O or [(Mg _5.4 Li _0.6 ) Si ₈ O ₂₀ (OH, F) ₄ ] Na _0.6 x XH ₂ O in an amount of 10 to 30 g / l nitroguanidine suspension, or it can be used take place that the stable, aqueous suspension contains a stabilizer consisting of a polymeric sugar and polyethylene glycol, the weight ratio of the polymeric sugar to polyethylene glycol being 1: 1 to 1: 3 and the stabilizer in an amount of 5 to 20 g / l Nitroguanidine suspension is used. With both stabilizers of the nitroguanidine suspension it is advantageously achieved that the suspension remains unchanged for several months and that the sludge separation in the phosphating bath is favored. The introduction of the nitroguanidine into the phosphating solution in the form of a stabilized suspension avoids the disadvantages which result from the fact that nitroguanidine is present as a powder and can only be distributed evenly in the phosphating solution in this form with difficulty. The suspensions produced according to the invention can be easily pumped and are stable over 12 months, which means that the nitroguanidine does not settle even after a long time. The suspensions are prepared by suspending the layered silicate or the organic stabilizer in deionized water and then stirring in the nitroguanidine.

At the pH in the phosphating solution of 2 to 4, the suspension is destroyed, and the nitroguanidine is released and dissolved in a fine distribution.

Finally, it is provided according to the invention that the solution according to the invention and the method according to the invention for the treatment of workpieces before painting, in particular before electrocoating.

A) Definitionen: Das Zn²⁺:P₂O₅ Verhältnis bezieht sich auf das Gesamt-P₂O₅. Die Bestimmung des Gesamt-P₂O₅ basiert auf der Titration der Phosphorsäure und/oder der primären Phosphate vom Äquivalenzpunkt des primären Phosphats bis zum Äquivalenzpunkt des sekundären Phosphats. Der S-Wert gibt das Verhältnis von freier Säure, berechnet als freies P₂O₅, zum Gesamt-P₂O₅ an. Die Definitionen und Bestimmungsmethoden für das Gesamt-P₂O₅ und das freie P₂O₅ sind in der Veröffentlichung von W. Rausch "Die Phosphatierung von Metallen", 1988, Seiten 289 bis 304, ausführlich erläutert.

B) Verfahrensparameter: Die nachfolgenden Vergleichs- und Ausführungsbeispiele wurden unter Anwendung folgender Verfahrensschritte durchgefürt:

a) Die Oberflächen von aus Stahlblech bestehenden metallischen Gegenständen wurden mit einem schwach alkalischen Reiniger (2%ige, wässrige Lösung) während 6 Minuten bei 60°C gereinigt und insbesondere entfettet.

b) Es folgte eine Spülung mit Leitungswasser während 0,5 Minuten bei Raumtemperatur.

c)Anschließend erfolgte eine Aktivierung mit einem flüssigen Aktivierungsmittel, das ein Titanphosphat enthielt, während 0,5 Minuten bei 50°C.

d)Danach wurde bei ca. 55°C während 3 Minuten durch Tauchen phosphatiert.

e) Schließlich wurde mit Leitungswasser während 0,5 Minuten bei Raumtemperatur gespült.

f)Die phosphatierten Oberflächen wurden im Ofen bei 80°C während 10 Minuten getrocknet.

C) Konzentrate zum Ansetzen der Phosphatierungslösung: Das Konzentrat I enthält, mit Ausnahme von Hydroxylamin und Cu²⁺, alle anorganischen Bestandteile der Phosphatierungslösung. Das Konzentrat II besteht aus einer stabilisierten Nitroguanidin-Suspension. Das Konzentrat III besteht aus einer wässrigen Lösung von Hydroxylamin-Salzen, Hydroxylamin-Komplexen oder Hydroxylamin. Wird eine Cu²⁺ - haltige Phosphatierungslösung benötigt, kommt als Konzentrat IV eine konzentrierte Cu²⁺ -Lösung zur Anwendung. Sollen metallische Oberflächen aus Aluminium oder Aluminiumlegierungen phosphatiert werden, kommt als Konzentrat V eine Lösung zur Anwendung, die freies Fluorid bildende Verbindungen enthält. Die erfindungsgemäße Phosphatierungslösung wird durch Mischen der jeweils erforderlichen Konzentrate I bis V unter gleichzeitiger Zugabe von Wasser hergestellt. Bei einem längeren Stillstand des Phosphatierbades kommt es häufig zu einer teilweisen Zersetzung des Hydroxylamins. Die daraus resultierenden Hydroxylamin-Verluste werden durch Zugabe des Konzentrats III zum Phosphatierbad ausgeglichen. Als Hydroxylamin-Quelle werden in bekannter Weise wässrige Lösungen von Hydroxylamin-Salzen, Hydroxylamin-Komplexen oder von Hydroxylamin verwendet.

D) Ausführungs- und Vergleichsbeispiele: Entsprechend den unter B) angegebenen Verfahrensparametern wurden zwei einseitig verzinkte Stahlbleche unterschiedlicher Qualität (Z1 und Z2) phosphatiert. Das Phosphatierbad hatte jeweils die in der Tabelle angegebene Zusammensetzung, wobei der Gesamt-P₂O₅-Gehalt bei allen Beispielen 12 g P₂O₅/l betrug und wobei die in der Tabelle benutzten Symbole folgende Bedeutung haben:

FS = Freie Säure

GS = Gesamtsäure

Zn = Zn²⁺, g/l

NG = Nitroguanidin, g/l

HA = Hydroxylamin, g/l

Cu = Cu²⁺, mg/l

Mn = Mn²⁺, g/l

The subject matter of the invention is explained in more detail below, also using exemplary embodiments.

A) Definitions: The Zn ²⁺ : P ₂ O ₅ ratio refers to the total P ₂ O ₅ . The determination of the total P ₂ O 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 O ₅ , to total P ₂ O ₅ . The definitions and determination methods for total P ₂ O ₅ and free P ₂ O ₅ are explained in detail in the publication by W. Rausch "Die Phosphatierung von Metallen", 1988, pages 289 to 304.

B) Process parameters: The following comparative and exemplary embodiments 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 6 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 a liquid activating agent which contained a titanium phosphate for 0.5 minutes at 50 ° C.

d) Then it was 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 in the oven at 80 ° C for 10 minutes.

C) Concentrates for preparing the phosphating solution: With the exception of hydroxylamine and Cu ²⁺ , concentrate I contains all the inorganic components of the phosphating solution. Concentrate II consists of a stabilized nitroguanidine suspension. Concentrate III consists of an aqueous solution of hydroxylamine salts, hydroxylamine complexes or hydroxylamine. If a phosphating solution containing Cu ^{2+ is} required, a concentrated Cu ²⁺ solution is used as concentrate IV. If metallic surfaces made of aluminum or aluminum alloys are to be phosphated, a solution is used as concentrate V which contains free fluoride-forming compounds. The phosphating solution according to the invention is prepared by mixing the concentrates I to V required in each case with the simultaneous addition of water. If the phosphating bath is not used for a long time, the hydroxylamine is often partially decomposed. The resulting hydroxylamine losses are compensated for by adding concentrate III to the phosphating bath. Aqueous solutions of hydroxylamine salts, hydroxylamine complexes or of hydroxylamine are used in a known manner as the hydroxylamine source.

D) Exemplary and Comparative Examples: According to the process parameters given under B), two steel sheets of different quality (Z1 and Z2) galvanized on one side were phosphated. The phosphating bath in each case had the composition given in the table, the total P ₂ O ₅ content in all examples being 12 g P ₂ O ₅ / l and the symbols used in the table having the following meanings:

FS = free acid

GS = total acid

Zn = Zn ²⁺ , g / l

NG = nitroguanidine, g / l

HA = hydroxylamine, g / l

Cu = Cu ²⁺ , mg / l

Mn = Mn ²⁺ , g / l

The phosphating according to Comparative Example 1 was carried out under Accelerators excluded. At the Comparative example 2 was only the accelerator Hydroxylamine present, while in Comparative Example 3 only the accelerator nitroguanidine was used. The embodiments 4 to 9 were in the presence of both Accelerator performed, the concentration of both Accelerator in the preferred according to the invention Area.

The table shows both the layer weights and the crystallite edge lengths which could be achieved when Examples 1 to 9 were carried out. These data show that in Comparative Example 1, which was carried out in the absence of the two accelerators according to the invention, a phosphate layer of insufficient quality resulted, since both the layer weight and the edge length of the crystallites of the phosphate layer are comparatively large. In comparative examples 2 and 3, tolerable layer weights and sufficiently small crystallite edge lengths were obtained, so that both phosphate layers can be regarded as quite useful. The exemplary embodiments 4 to 9 show that, according to the invention, both optimum layer weights and extraordinarily fine-crystalline phosphate layers could be produced. Exemplary embodiments 4 to 9 thus demonstrate that the invention can be used to produce very high quality phosphate layers, using very low concentrations of nitroguanidine and hydroxylamine in the phosphating bath. Of course, the phosphate layers produced in accordance with Examples 1 to 9 were closed. The edge lengths of the crystallites given in the table were determined on the basis of electron microscopic images of the individual phosphate layers.

Claims (20)

1. Aqueous solution containing phosphate for producing layers of phosphate on metal surfaces of iron, steel, zinc, zinc alloys, aluminium or aluminium alloys, which solution contains 0.3 to 5 g Zn²⁺/l, 0.1 to 2 g nitroguanidine/ and 0.05 to 0.5 g hydroxylamine/l, with the S-value amounting to 0.03 to 0.3 and the weight ratio of Zn²⁺ to P₂O₅ = 1:5 to 1:30 and with the S-value indicating the ratio of free acid, calculated as free P₂O₅, to total P₂O₅.
2. Aqueous solution according to claim 1, which solution contains 0.1 to 1.5 g nitroguanidine/l.
3. Aqueous solution according to claims 1 to 2, which solution contains 0.1 to 0.4 g hydroxylamine/l.
4. Aqueous solution according to claims 1 to 3, which solution contains 0.3 to 3 g Zn²⁺/l.
5. Aqueous solution according to claims 1 to 4, which solution contains 0.5 to 20 g NO₃ ^-/l.
6. Aqueous solution according to claims 1 to 5, which solution contains 0.01 to 3 g Mn²⁺/l and/or 0.01 to 3 g Ni²⁺/l and/or 1 to 100 mg Cu²⁺/l and/or 0.01 to 3 g Co²⁺/l.
7. Aqueous solution according to claims 1 to 6, which solution contains 0.01 to 3 g F^-/l and/or 0.05 to 3.5 g/l of at least one complex fluoride.
8. Aqueous solution according to claims 1 to 7, which solution contains (SiF₆)^2- or (BF₄)^- as the complex fluoride.
9. Method for phosphatizing metal surfaces, wherein the metal surfaces are cleaned, are subsequently treated with the aqueous, phosphate-containing solution according to claims 1 to 8 for a period of 5 seconds to 10 minutes at a temperature of 15 to 70°C and are finally rinsed with water.
10. Method according to claim 9, wherein the treatment of the metal surfaces with the phosphatization solution is effected by spraying, dipping, spray-dipping or roller-application.
11. Method according to claims 9 to 10, wherein the phosphatization solution used for spraying has a ratio of the weight of Zn²⁺ to P₂O₅ = 1:5 up to 1:30 and the period required for spraying amounts to 5 to 300 seconds.
12. Method according to claims 9 to 10, wherein the phosphatization solution used for dipping has a ratio of the weight of Zn²⁺ to P₂O₅ = 1:5 to 1:18 and the period required for dipping amounts to 5 seconds to 10 minutes.
13. Method according to claims 9 to 12, wherein, after having been cleaned, the metal surfaces are treated with an activating agent which contains a containing-titanium phosphate.
14. Method according to claims 9 to 13, wherein after the rinsing process that follows phosphatization, the metal surfaces are subsequently treated with a passivating agent.
15. Method according to claim 9, wherein the nitroguanidine is introduced into the aqueous solution in the form of a stable, aqueous suspension.
16. Method according to claim 15, wherein the stable, aqueous suspension contains a layer silicate as a stabilizer.
17. Method according to claim 16, wherein the layer silicates [Mg₆ (Si_7.4 Al_0.6) O₂₀ (OH) ₄] Na _0.6 x xH₂O or [ (Mg _5.4 Li _0.6) Si₈ O₂₀ (OH, F) ₄] Na _0.6 x XH₂O are used as the stabilizer in a quantity of 10 to 30 g/l nitroguanidine suspension.
18. Method according to claim 15, wherein the stable, aqueous suspension contains a stabilizer which consists of a polymeric sugar and polyethylene glycol, with the ratio of the weight of the polymeric sugar to polyethylene glycol amounting to 1:1 to 1:3 and with the stabilizer being used in a quantity of 5 to 20 g/l nitroguanidine suspension.
19. Use of the aqueous phosphate-containing solution according to claims 1 to 8 and of the method for phosphatization according to claims 9 to 18 for the treatment of work pieces before lacquering.
20. Use according to claim 19 for the treatment of work pieces before electro-dip lacquering.