Improved method for nickel-free phosphating metal surfaces

Abstract

The invention relates to a method for phosphating a metal surface, in which a metal surface, optionally after cleaning and / or activating, is initially treated with an acidic, aqueous, essentially nickel-free phosphating composition which contains zinc ions, manganese ions, iron(111) ions and phosphate ions, and then it is optionally rinsed and / or dried. The invention also relates to a corresponding phosphating composition and a correspondingly and to a correspondingly phosphate coated metal surface.

Description

[0001] The present invention relates to an improved method for the essentially nickel-free phosphating of a metallic surface, a corresponding phosphating composition and a correspondingly phosphated metallic surface.

[0002] Phosphate coatings on metallic surfaces are known from the prior art. Such coatings serve to protect the metallic surfaces from corrosion and also act as an adhesion promoter for subsequent paint layers.

[0003] Such phosphate coatings are primarily used in the automotive industry and general industry.

[0004] The subsequent coating layers consist primarily of cathodically applied electrocoatings (e-coatings), in addition to powder coatings and wet coatings. Since an electric current must flow between the metallic surface and the treatment bath during e-coating, it is important to establish a defined electrical conductivity for the phosphate coating to ensure efficient and homogeneous deposition.

[0005] Therefore, phosphate coatings are typically applied using a nickel-containing phosphating solution. The nickel deposited in this process, either in its elemental form or as an alloying element (e.g., Zn / Ni), ensures suitable conductivity of the coating during the subsequent electrocoating process.

[0006] However, due to their high toxicity and environmental impact, nickel ions are no longer desirable as a component of treatment solutions and should therefore be avoided if possible or at least their content reduced.

[0007] The use of nickel-free or low-nickel phosphating solutions is known in principle. However, this is limited to certain substrates such as steel.

[0008] Furthermore, with the aforementioned low-nickel or nickel-free systems, poor corrosion and paint adhesion values ​​can result under given KTL deposition conditions due to a less than optimal substrate surface.

[0009] Another problem with nickel-free phosphating baths is ensuring sufficient stability of the bath against parameter changes or the throughput of metallic substrates: While the bath is initially free of sludge or any turbidity, it becomes turbid after the first throughput of sheets, and eventually large quantities of sludge form. The parameters are not stable.

[0010] EP 0 370 535 A1 discloses a process for applying phosphate coatings to surfaces made of aluminum and / or zinc or their alloys using phosphating solutions operating according to low-zinc technology. DE 195 11 573 A1 discloses a process for phosphating metal surfaces with aqueous, acidic zinc-containing phosphating solutions.

[0011] The object of the present invention was therefore to provide a method by which metallic surfaces can be phosphated in a substantially nickel-free manner, avoiding the aforementioned disadvantages of the prior art and, in particular, achieving higher bath stability.

[0012] This problem is solved by a method according to claim 1, a phosphating composition according to claim 12 and a phosphate-coated metallic surface according to claim 13.

[0013] In the inventive process for phosphating a metallic surface, a metallic surface, optionally after cleaning and / or activation, is treated with an acidic, aqueous, essentially nickel-free phosphating composition comprising zinc ions, manganese ions, iron(III) ions and phosphate ions, and then optionally rinsed and / or dried. Definitions:

[0014] On the one hand, an uncoated metallic surface, and on the other hand, a metallic surface that has already been converted to a metallic coating, can be treated with the method according to the invention. Therefore, when the term "metallic surface" is used below, it should always include a metallic surface that has already been converted to a metallic coating.

[0015] For the purposes of the present invention, an "aqueous composition" is defined as a composition which contains at least partially, and preferably predominantly, water as a solvent. It may also include dispersed, i.e., emulsified and / or suspended components in addition to dissolved components.

[0016] "Essentially nickel-free" in this case means that less than 0.3 g / l of nickel ions are present.

[0017] For the purposes of the present invention, "phosphate ions" also includes hydrogen phosphate, dihydrogen phosphate, and phosphoric acid. Furthermore, pyrophosphoric acid and polyphosphoric acid, as well as all their partially and completely deprotonated forms, are also included.

[0018] For the purposes of the present invention, "metal ion" means either a metal cation, a complex metal cation or a complex metal anion.

[0019] The metallic surface is preferably steel, hot-dip galvanized, electrolytically galvanized, aluminum, or alloys thereof, such as Zn / Fe or Zn / Mg. In the case of hot-dip galvanizing and electrolytic galvanizing, this is specifically applied to steel. In particular, the metallic surface is at least partially galvanized.

[0020] The method according to the invention is particularly suitable for multi-metal applications.

[0021] When coating a metallic surface that is not freshly hot-dip galvanized, it is advantageous to first clean the metallic surface in an aqueous cleaning solution, particularly to degrease it, before treatment with the phosphating solution. For this purpose, an acidic, neutral, alkaline, or strongly alkaline cleaning solution can be used, and optionally, an acidic or neutral pickling solution can also be added.

[0022] An alkaline or strongly alkaline cleaning composition has proven to be particularly advantageous.

[0023] The aqueous cleaning composition may contain at least one surfactant, and optionally a cleaning framework and / or other additives such as complexing agents. The use of an activating cleaner is also possible.

[0024] After cleaning / pickling, it is advantageous to rinse the metallic surface with water at least once, to which an additive dissolved in water, such as a nitrite or surfactant, may optionally be added.

[0025] Before treating the metallic surface with the phosphating composition, it is advantageous to treat the metallic surface with an activation composition. The activation composition serves to deposit a large number of very fine phosphate particles as seed crystals onto the metallic surface. In the subsequent process step, these help to form, upon contact with the phosphating composition—preferably without intermediate rinsing—a phosphate layer, particularly a crystalline one, with the highest possible number of densely arranged fine phosphate crystals, or a largely continuous phosphate layer.

[0026] Suitable activation compositions include, in particular, acidic or alkaline compositions based on titanium phosphate or zinc phosphate.

[0027] However, it can also be advantageous to add activating agents, especially titanium phosphate or zinc phosphate, to the cleaning composition, thus performing cleaning and activation in one step.

[0028] The acidic, aqueous, essentially nickel-free phosphating composition includes zinc ions, manganese ions, iron(III) ions and phosphate ions.

[0029] The iron(III) ion content ensures sufficient stability of the phosphate composition against parameter changes or the throughput of metallic substrates.

[0030] The iron(III) ion content in the phosphate composition is in the range of 1 to 200 mg / l, preferably 1 to 100 mg / l, more preferably 5 to 100 mg / l, particularly preferably 5 to 50 mg / l and most preferably 5 to 20 mg / l.

[0031] The iron(III) ions can be added to the phosphate composition, for example as nitrate, sulfate, citrate or tartrate.

[0032] However, the iron(III) ions are preferably not added as nitrate, since too much nitrate negatively affects the layer composition: The manganese content of the formed layer is lower.

[0033] It is particularly advantageous if the iron(III) ions are added to the phosphating composition before the adjustment of the free acid (FS; see the explanations below), because this reduces the precipitation of zinc salts and thus increases the bath stability.

[0034] The phosphatizing composition can be obtained from a concentrate by dilution with a suitable solvent, preferably water, by a factor between 1 and 100, preferably between 5 and 50, and, if necessary, by adding a pH-modifying substance.

[0035] The phosphate composition preferably comprises the following components in the following preferred and particularly preferred concentration ranges, wherein the content of zinc ions, manganese ions, iron(III) ions, and phosphate ions corresponds to the contents specified in claim 1: Zn 0.3 to 3.0 g / l 0.5 to 2.0 g / l Mn 0.3 to 2.0 g / l 0.5 to 1.5 g / l Fe(III) 1 to 200 mg / l 1 to 100 mg / l Phosphate (calculated as P2O5) 8 to 25 g / l 10 to 18 g / l free fluoride 30 to 250 mg / l 50 to 180 mg / l Complex fluoride (calculated e.g. as SiF 6 2-< and / or BF 4 -< ) 0 to 5 g / l 0.5 to 3 g / l

[0036] However, with regard to manganese ions, a concentration in the range of 0.3 to 2.5 g / l has already proven advantageous, and with regard to free fluoride, a concentration in the range of 10 to 250 mg / l.

[0037] The complex fluoride is preferably tetrafluoroborate (BF 4 -< ) and / or hexafluorosilicate (SiF 6 2-< ).

[0038] Especially when treating aluminium and / or galvanised material, a content of complex fluoride as well as simple fluoride, for example sodium fluoride, in the phosphating composition is advantageous.

[0039] Al³⁺ is a bath poison in phosphating systems and can be removed from the system by complexation with fluoride, e.g., as cryolite. Complex fluorides are added to the bath as a "fluoride buffer" because otherwise the fluoride content would drop rapidly and no coating would form. Fluoride thus supports the formation of the phosphate layer and indirectly leads to improved paint adhesion and corrosion protection. Complex fluoride also helps to prevent defects such as pitting on galvanized material.

[0040] Furthermore, the phosphatizing composition preferably contains at least one accelerator selected from the group consisting of the following compounds in the following preferred and particularly preferred concentration ranges: Nitroguanidine 0.2 to 3.0 g / l 0.2 to 1.55 g / l H₂O₂ 10 to 100 mg / l 15 to 50 mg / l Nitroguanidine / H2O2 0.2 to 2.0 g / l / 10 to 50 mg / l 0.2 to 1.5 g / l / 15 to 30 mg / l nitrite 30 to 300 mg / l 90 to 150 mg / l Hydroxylamin 0.1 to 5 g / l 0.4 to 3 g / l

[0041] However, with regard to nitroguanidine, a concentration in the range of 0.1 to 3.0 g / l has already proven advantageous, and with regard to H 2 O 2, a concentration in the range of 5 to 200 mg / l has proven advantageous.

[0042] The at least one accelerator is preferably H2O2.

[0043] The phosphate composition contains less than 1 g / l, preferably less than 0.5 g / l, more preferably less than 0.1 g / l and particularly preferably 0.05 to 0.1 g / l nitrate.

[0044] Particularly on galvanized surfaces, the nitrate in the phosphate composition further accelerates the coating formation reaction, leading to lower coating weights and, more importantly, reducing the incorporation of manganese into the crystal. However, if the manganese content of the phosphate coating is too low, its alkali resistance is compromised.

[0045] Alkali resistance plays a crucial role in subsequent cathodic electrocoating. During this process, water undergoes electrolytic splitting at the substrate surface, forming hydroxide ions. This causes the pH value at the substrate interface to rise. While this is necessary for the electrocoat to agglomerate and be deposited, the elevated pH value can also damage the crystalline phosphate layer.

[0046] The phosphatizing composition preferably has a temperature in the range of 30 to 55 °C.

[0047] Furthermore, the phosphate composition can be characterized by the following preferred and especially preferred parameter ranges: FS 0.3 to 2.0 0.7 to 1.6 FS (verd.) 0.5 to 8 1 to 6 GSF 12 to 28 22 to 26 GS 12 to 45 18 to 35 S-value 0.01 to 0.2 0.03 to 0.15 Temperature °C 30 to 50 °C 35 to 45 °C

[0048] However, with regard to the FS parameter, a value in the range of 0.2 to 2.5 has already proven advantageous, and with regard to the temperature, a value in the range of 30 to 55 °C.

[0049] Here, "FS" stands for free acid, "FS (diluted)" for free acid (diluted), "GSF" for total acidity according to Fischer, "GS" for total acidity and "S-value" for acid value.

[0050] These parameters are determined as follows: Free acid (FA):

[0051] To determine the free acid, 10 ml of the phosphate composition are pipetted into a suitable vessel, for example a 300 ml Erlenmeyer flask.

[0052] If the phosphatizing composition contains complex fluorides, 2-3 g of potassium chloride (KCl) are added to the sample. The sample is then titrated using a pH meter and an electrode with 0.1 M NaOH until a pH of 3.6 is reached. The amount of 0.1 M NaOH consumed in ml per 10 ml of the phosphatizing composition yields the value of the free acid (FA) in points. Free acid (diluted) (FS (diluted)):

[0053] To determine the free acid (dilute), 10 ml of the phosphatizing composition are pipetted into a suitable vessel, for example, a 300 ml Erlenmeyer flask. Then, 150 ml of deionized water are added. Using a pH meter and an electrode, the mixture is titrated with 0.1 M NaOH to a pH of 4.7. The amount of 0.1 M NaOH consumed in ml per 10 ml of the dilute phosphatizing composition yields the value of the free acid (dilute) (FA(dilute)) in points. The content of complex fluoride can be determined from the difference between this value and the free acid (FA). Multiplying this difference by a factor of 0.36 gives the content of complex fluoride as SiF₆²⁻ in g / l. Total acidity according to Fischer (GSF):

[0054] Following the determination of the free acid (diluted), the diluted phosphating solution is titrated to a pH of 8.9 using a pH meter and an electrode containing 0.1 M NaOH after the addition of potassium oxalate solution. The consumption of 0.1 M NaOH in ml per 10 ml of the diluted phosphating solution yields the total acidity according to Fischer (GSF) in points. Multiplying this value by 0.71 gives the total phosphate ion content, calculated as P₂O₅ (see W. Rausch: "Die Phosphatierung von Metallen" [The Phosphation of Metals]. Eugen G. Leuze-Verlag 2005, 3rd edition, pp. 332 ff). Total acidity (GS):

[0055] The total acidity (TA) is the sum of the divalent cations and free and bound phosphoric acids (the latter being phosphates). It is determined by measuring the consumption of 0.1 M NaOH using a pH meter and an electrode. For this purpose, 10 mL of the phosphatizing composition are pipetted into a suitable vessel, such as a 300 mL Erlenmeyer flask, and diluted with 25 mL of deionized water. The mixture is then titrated with 0.1 M NaOH to a pH of 9. The consumption in mL per 10 mL of the diluted phosphatizing composition corresponds to the score of the total acidity (TA). Acidity level (S-value):

[0056] The so-called acid value (S-value) represents the ratio FS : GSF and is obtained by dividing the value of the free acid (FS) by the value of the total acid according to Fischer (GSF).

[0057] Surprisingly, the further improvement in paint adhesion, especially on hot-dip galvanized surfaces, was achieved by adjusting the acid value to the range of 0.03 to 0.065, particularly in the range of 0.04 to 0.06.

[0058] Surprisingly, it has been found that, particularly in the case of steel or hot-dip galvanizing as a metallic surface, a temperature of the phosphating composition of less than 45 °C, preferably in the range between 35 and 45 °C, leads to further improved corrosion and paint adhesion values.

[0059] The phosphatizing composition is essentially nickel-free. Preferably, it contains less than 0.1 g / l and particularly preferably less than 0.01 g / l nickel ions.

[0060] Due to its iron(III) ion content, the essentially nickel-free phosphating composition exhibits a significantly lower amount of sludge even after repeated processing with metallic substrates. Its parameters remain stable.

[0061] The addition of iron(III) ions to the phosphating composition also contributes to ensuring that the electrochemical properties of essentially nickel-free phosphated metallic surfaces are comparable or nearly comparable to those treated with nickel-containing phosphating solutions.

[0062] The addition of iron(III) ions to the phosphating composition leads to a significant improvement in paint adhesion and corrosion protection results, especially on steel, galvanized steel and aluminium.

[0063] The corresponding SEM images show that the phosphate layers formed are more closed and finer crystalline through the use of Fe(III) (see each image). Figs. 1 to 9 ). If Fe(III) is not added, "pickling holes" are visible, which are due to a long pickling attack and incomplete layer formation.

[0064] According to a non-inventive embodiment, the phosphating composition is a conventional trication composition, i.e., it contains, in addition to zinc and manganese ions, at least 0.3 g / l, preferably at least 0.5 g / l, and particularly preferably at least 0.8 g / l nickel ions. As explained above, trication phosphating also surprisingly results in a significant increase in bath stability, and, on aluminum, an improvement in paint adhesion and corrosion protection.

[0065] The treatment of the metallic surface with the phosphating composition is preferably carried out for 30 to 480 seconds, particularly preferably for 60 to 300 seconds and most preferably for 90 to 240 seconds, preferably by immersion or spraying.

[0066] By treating the metallic surface with the phosphating composition, the following preferred and particularly preferred zinc phosphate layer weights are achieved on the metallic surface, depending on the treated surface (determined by XRF, i.e., X-ray fluorescence analysis): Treated surface Zinc phosphate coating weight (g / m²< ) Steel 0.5 to 6 1 to 5 Hot-dip galvanizing 1.0 to 6 1.5 to 5 electrolytic zinc plating 1.0 to 6 1.5 to 5 aluminum 0.5 to 6 1 to 5

[0067] Preferably, the metallic surface, which has already been treated with the phosphating composition (i.e., phosphate-coated), is optionally rinsed and / or dried, but is then not treated with an aqueous rinsing composition, in particular not with one that comprises at least one type of metal ions and / or at least one polymer.

[0068] According to a particularly preferred embodiment, the metallic surface, which has already been treated with the essentially nickel-free phosphating composition, i.e., phosphate-coated, is optionally rinsed and / or dried, but is then not treated with an aqueous rinsing composition, in particular not with one comprising at least one type of metal ions and / or at least one polymer.

[0069] It was surprisingly discovered that by adding iron(III) ions to the essentially nickel-free phosphating composition, good results regarding paint adhesion and an improvement in corrosion protection can be achieved even without the use of a rinsing solution.

[0070] The invention further relates to a phosphate-coated metallic surface which can be obtained using the method according to the invention.

[0071] A cathodic electrocoating lacquer can then be deposited on the phosphate-coated metallic surface, followed by the application of a paint system.

[0072] If necessary, the metallic surface is first rinsed, preferably with demineralized water, and then dried if necessary.

[0073] The present invention will now be explained by means of exemplary embodiments and comparative examples, which are not to be understood as limiting. Comparative examples 1 to 3

[0074] Test plates made of hot-dip galvanized steel (EA), electrolytically galvanized steel (G) or aluminium (AA6014S) were coated using a nickel-free phosphating solution containing 1.3 g / l Zn, 1 g / l Mn and 13 g / l PO 4 3-< (calculated as P 2 O 5 ), at a temperature of 45 °C. Examples 1 to 3

[0075] Test plates made of hot-dip galvanized steel (EA), electrolytically galvanized steel (G) or aluminium (AA6014S) were coated using a nickel-free phosphating solution containing 1.3 g / l Zn, 1 g / l Mn, 13 mg / l Fe(III) and 13 g / l PO 4 3-< (calculated as P 2 O 5 ), at a temperature of 45 °C. Comparative examples 4 to 6

[0076] Test plates made of hot-dip galvanized steel (EA), electrolytically galvanized steel (G) or aluminium (AA6014S) were coated using a phosphating solution containing 1.3 g / l Zn, 1 g / l Mn, 14 g / l PO 4 3-< (calculated as P 2 O 5 ), 3 g / l NO 3 -< and also 1 g / l nickel, at a temperature of 53 °C.

[0077] Test plates according to comparison examples 1 to 6 (VB1 to VB6) and examples 1 to 3 (B1 to B3) were examined with a scanning electron microscope (SEM) after phosphating.

[0078] The resulting recordings are in Figs. 1 to 9 reproduced. Fig. 1 VB1, Test disk: EA Fig. 2 B1, Test plate: EA Fig. 3 VB4, test disk: EA Fig. 4 VB2, test disk: G Fig. 5 : B2, Test plate: G Fig. 6 VB5, Test disk: G Fig. 7 VB3, Test disk: AA6014S Fig. 8 : B3, Test disk: AA6014S Fig. 9 VB6, Test disk: AA6014S

[0079] On EA and G, the phosphate layers are not closed and are uneven without the addition of Fe(III) (cf. Fig. 1 and 4 ).Due to severe pickling, circular holes (so-called pickling holes) have formed. This is because the layer formation is not fast enough and therefore pickling is continuous. No phosphate layer is detectable at all on AA6014S (cf. Fig. 7 ). The surfaces of the test plates are black due to the deposition of elemental zinc. The addition of Fe(III) makes the phosphate layers thinner (see [reference]). Fig. 2 , 5 and 8 )- comparable in each case to the layer obtained by nickel-containing phosphating (cf. Fig. 3 , 6 or 9).

[0080] Furthermore, all test panels were coated with a cathodic electrocoating after phosphating, followed by a standard automotive paint system (primer, basecoat, clearcoat), and then subjected to a cross-cut test according to DIN EN ISO 2409. Three panels each were tested before and after exposure to condensation water for 240 hours (DIN EN ISO 6270-2 CH). The corresponding results can be found in Table 1. A cross-cut score of 0 is the best, and a score of 5 is the worst. Scores of 0 and 1 are considered comparably good. Table 1 (Comparative) example Test plate Cross-cut before load (0-5) after load (0-5) VB1 EA 1 / 1 / 1 3 / 5 / 4 B1 1 / 1 / 1 1 / 1 / 2 VB4 0 / 0 / 1 1 / 1 / 1 VB2 G 1 / 1 / 1 5 / 5 / 5 B2 0 / 0 / 1 1 / 0 / 1 VB5 1 / 1 / 1 1 / 1 / 1 VB3 AA6014S 1 / 1 / 1 5 / 5 / 5 B3 1 / 1 / 1 1 / 1 / 1 VB6 0 / 0 / 0 0 / 0 / 0

[0081] Tab.Figure 1 shows the poor results of VB1, VB2 and VB3 (nickel-free, without Fe(III)) after loading, while B1, B2 and B3 (nickel-free, with Fe(III)) deliver good results - comparable to VB4, VB5 and VB6 (containing nickel).

[0082] Furthermore, the test plates of comparison examples 3 and 6 (VB3 and VB6) as well as example 3 (B3) were subjected to a filiform test (with HCl) according to DIN EN 3665 (1997 version). After 504 hours, the damage was determined analogously to the mean undercutting according to DIN EN ISO 4628-8 (2013 version) or LPV 4 (2012 version). Table 2 (Comparative) example Test plate Filiform medium max VB3 10 / 9 / 10 17 / 17 / 19 B3 AA6014S 2,5 / 2,5 / 2 5 / 8 / 6,5 VB6 0,5 / 0,8 / 0,8 2,5 / 3 / 3,5

[0083] Table 2 The significant reduction in filiform corrosion achieved by adding Fe(III) can be seen (B3 compared to VB3).

[0084] Furthermore, the test panels were subjected to a VDA test (VDA 621-415) according to comparison examples 1, 2, 4, and 5 (VB1, VB2, VB4, and VB5) as well as examples 1 and 2 (B1 and B2). The paint undercut (U) in mm was determined, and—in the case of B1, VB1, and VB4—the paint delamination after stone impact (DIN EN ISO 20567-1, Method C) was also determined. A result of 0 is the best, and a result of 5 the worst. A value up to 1.5 is considered good. The results are also available in Table 3 summarized.

[0085] The test plates of comparison examples 3 and 6 (VB3 and VB6) as well as example 3 (B3) were subjected to a 240-hour CASS test according to DIN EN ISO 9227. The results are presented in Table 4 summarized. Table 3 (Comparative) example Test plate VDA U in mm Stonefall VB1 EA 0,8 / 1,5 / 1,3 1,5 / 1 / 1,5 B1 0,3 / 0,5 / 0,8 1 / 1 / 0,5 VB4 0,3 / 0,3 / 0,3 0,5 / 0,5 / 0,5 VB2 G 3 / 2,5 / 2,3 nb B2 1,3 / 1,5 / 1,5 nb VB5 0,8 / 0,8 / 1 nb Table 4 (Comparative) example Test plate CASS VB3 3,5 / 4 / 3,5 B3 AA6014S 2 / 1,8 / 1,8 VB6 0,5 / 0,5 / 0,5

[0086] To investigate the influence of the addition of Fe(III) on the bath stability, a nickel-free phosphating bath without the addition of Fe(III) (VB7) and one with the addition of Fe(III) (B4) were prepared. Comparative example 7

[0087] The bath without added iron was initially sludge-free. The bath values ​​were: FS (KCl) = 1.3 and Zn content = 1.2 g / l.

[0088] After processing just a few sheets of different substrates, the bath became cloudy. Steel gradually rusted, and aluminum darkened. The appearance of the deposited phosphate layer became uneven.

[0089] The precipitation of zinc salts led to significant sludge formation shortly thereafter. The Zn content dropped to 1.0 g / l, necessitating the addition of zinc as zinc phosphate.

[0090] At the end of the experiment, some heavy encrustations were observed on the bathroom wall.

[0091] Furthermore, the layer weight of the deposited phosphate layers was determined by XRF. It was found that in a bath without the addition of Fe(III) the layer weights fluctuated considerably (see the following). Table 5, (where the numbering of the sheets corresponds to the treatment sequence): Table 5 Sheet metal sequence SG in g / m²< Sheet 1 2,4 Sheet 2 2,3 Sheet 3 1,9 Sheet 4 2 Sheet 5 2,1 Sheet 6 2 Sheet 7 1,9

[0092] It can be seen that the layer weight was initially relatively high, decreased with increasing sheet metal throughput, and then fluctuated. Example 4

[0093] Ten mg / l of Fe(III) were added to the other nickel-free bath. The FS (KCl) was then adjusted to approximately 1.3. The Zn content remained unchanged and stable at 1.3 g / l.

[0094] The latter remained unchanged and stable the following day. The same was true for the FS (KCl). Compared to the bath without the addition of Fe(III), significantly less sludge formed. The amount of sludge did not increase substantially with the throughput of sheets, while the FS (KCl) (1.3) and the Zn content (1.3 g / L) remained constant.

Claims

1. A method for phosphating of a metallic surface, wherein a metallic surface, optionally after cleaning and / or activation, is treated with an acidic, aqueous, substantially nickel-free phosphating composition which comprises zinc ions, manganese ions, iron(III) ions and phosphate ions, wherein the content of iron(III) ions in the phosphating composition is in the range from 1 to 200 mg / l, the phosphating composition comprises 0.3 to 3.0 g / l of zinc ions, 0.3 to 2.0 g / l of manganese ions and 8 to 25 g / l of phosphate ions (calculated as P2O5), and the phosphating composition comprises less than 1 g / l of nitrate, and is thereafter optionally rinsed and / or dried, wherein substantially nickel-free means that the phosphating composition comprises less than 0.3 g / l of nickel ions.

2. The method according to claim 1, wherein the metallic surface is at least partly galvanized.

3. The method according to claim 1 or 2, wherein the content of iron(III) ions in the phosphating composition is in the range from 5 to 100 mg / l and more preferably from 5 to 20 mg / l.

4. The method according to any of the preceding claims, wherein the phosphating composition comprises 30 to 250 mg / l of free fluoride.

5. The method according to any of the preceding claims, wherein the phosphating composition comprises 0.5 to 3 g / l of complex fluoride.

6. The method according to claim 5, wherein the complex fluoride is tetrafluoroborate (BF4-) and / or hexafluorosilicate (SiF62-).

7. The method according to any of the preceding claims, wherein the phosphating composition has a free acid in the range from 0.3 to 2.0, a Free acid (diluted) in the range from 0.5 to 8, a Total acid, Fischer in the range from 12 to 28, a Total acid in the range from 12 to 45 and an Acid value in the range from 0.01 to 0.2.

8. The method according to any of the preceding claims, wherein the phosphating composition comprises H2O2 as accelerator.

9. The method according to any of the preceding claims, wherein the phosphating composition comprises less than 0.1 g / l and preferably 0.05 to 0.1 g / l of nitrate.

10. The method according to any of the preceding claims, wherein the iron(III) ions have been added to the phosphating composition prior to the establishment of the Free acid.

11. The method according to any of the preceding claims, wherein the metallic surface already treated with the phosphating composition, i.e. already phosphate-coated, is optionally rinsed and / or dried, but not treated thereafter with an aqueous after-rinse composition, especially not with one that comprises at least one kind of metal ions and / or at least one polymer.

12. An acidic, aqueous, substantially nickel-free phosphating composition for phosphating of a metallic surface as defined in any of the preceding claims.

13. A phosphate-coated metallic surface which is obtainable by a method according to any of claims 1 to 11.

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