Aqueous solution and method for phosphatizing metallic surfaces

Details Aqueous solution and method for phosphatizing metallic surfaces Aqueous solution and method for phosphatizing metallic surfaces

2001-01-09

2002-12-24

King, Roy (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

Processes of coating utilizing a reactive composition which...

Reexamination Certificate

active

06497771

ABSTRACT:

The invention relates to an aqueous solution containing phosphate for producing phosphate layers on metal surfaces of iron, steel, zinc, zinc alloys, aluminium or aluminium alloys. The invention relates, furthermore, to a method for phosphatizing metal surfaces with the use of an aqueous phosphatization solution.
German Patent Application 196 34685.1 of the 28th August 1996 proposes an aqueous phosphate-containing solution for producing phosphate layers on metal surfaces of iron, steel, zinc, zinc alloys, aluminium or aluminium alloys, which solution contains 0.3 to 5 g Zn
2+
/l and 0. 1 to 0. 3 g nitroguanidine/l, with the S-value amounting to 0.03 to 0.3 and the ratio of the weight of Zn
2+
to P
2
O
5
=1:5 to 1:30, and which solution produces crystalline phosphate layers in which the crystallites have a maximum edge length <15 &mgr;m. Furthermore, a method for phosphatization, in which the metal surfaces are cleaned, are subsequently treated with the afore-mentioned aqueous phosphate-containing solution for a period of 5 seconds to 10 minutes at a temperature of 15 to 70° C. and are finally rinsed with water, is proposed in said German patent application.
The underlying object of the invention is to improve the aqueous phosphate-containing solution and the method for phosphatization that have been proposed in German Patent Application 196 34 685.1 in such a way that the maximum edge length of the crystallites present in the phosphate layers that are produced is significantly <15 &mgr;m, that the phosphate layers that are produced have a layer weight of 2 to 4 g/l, and that the phosphate layers that are produced are constant or uniform with regard to the layer weight and the edge length of the crystallites even when the phosphatizing bath is operated for a comparatively long time.
The underlying object of the invention is achieved by providing an aqueous phosphate-containing solution for producing phosphate layers on metal surfaces of iron, steel, zinc, zinc alloys, aluminium or aluminium alloys, which solution contains 0.3 to 5 g Zn
2+
/, 0.1 to 2 g nitroguanidine/l and 0.05 to 0.5 g hydroxylamine/l, with the S-value amounting to 0.03 to 0.3 and the ratio of the weight of Zn
2+
to P
2
O
5
=1:5 to 1.30. The nub of the invention thus lies in the fact that the solution, proposed in the aforementioned German patent application contains, in addition to the accelerator nitroguanidine also the accelerator hydroxylamine in a small concentration, with the nitroguanidine concentration of the solution in accordance with the invention having been significantly reduced in relation to the nitroguanidine concentration of the solution proposed in said German patent application.
According to the invention a solution that is particularly advantageous and preferred is one which contains 0.1 to 1.5 g nitroguanidine/l and also 0.1 to 0.4 g hydroxylamine/l. When these preferred features in accordance with the invention are applied, optimum phosphatization results are obtained.
Although, on the one hand, a solution for phosphatizing iron surfaces which has a zinc concentration of 0.2 to 2 g/l and contains as the accelerator hydroxylamine, hydroxylamine salts or hydroxylamine complexes that give the solution a hydroxylamine concentration of 0.5 to 50 g/l, preferably 1 to 10 g/l, is known from the printed specification EP-B 0 315 059 and although, on the other hand, a solution for producing copper-containing phosphate layers on metal surfaces of steel, zinc-coated steel, steel coated with zinc alloy, aluminium and its alloys, which solution 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, contains as the accelerator hydroxylamine, hydroxylamine salts and hydroxylamine complexes that give the solution a hydroxylamine concentration of 0.5 to 5 g/l, and can additionally contain an organic nitro-compound as an oxidizing agent is known from the printed specification EP-B 0 633 950, it was extremely surprising for the person skilled in the art that already with comparatively small concentrations of nitroguanidine and hydroxylamine it was possible to produce phosphate layers that have an optimum layer weight of 2 to 4 g/m
2
and whose layer weight is very uniform even during continuous operation and whose crystallites in each case have a maximum edge length <15 &mgr;m, with, nevertheless, the edge length as a rule being significantly <10 &mgr;m. These surprisingly advantageous effects of the solution in accordance with the invention are linked with a further advantageous effect that lies in the fact that, on account of the comparatively small concentration of the accelerators, relatively small quantities of accelerator from the solution in accordance with the invention are entrained out of the phosphatization bath into the subsequent treatment stages and ultimately into the waste water. The solution in accordance with the invention thus guarantees that the two phosphatization accelerators are supplied in an almost quantitative manner.
The solution in accordance with the invention is also neither disclosed nor made obvious to the person skilled in the art by the prior art mentioned above, since, in comparison with the solution proposed in German patent application 196 34 685.1 the solution in accordance with the invention only uses the comparatively low nitroguanidine concentrations as well as additionally hydroxylamine and, in comparison with the solutions which are known from the two European patent specifications that have been mentioned, the solution in accordance with the invention uses hydroxylamine concentrations which are lower than the hydroxylamine concentrations disclosed in the prior art that has been mentioned added to this is the fact that in the two European patent specifications that have been mentioned the use of nitroguanidine as an accelerator is not disclosed and the two European patent specifications mentioned call upon the person skilled in the art to use high hydroxylamine concentrations, since in accordance with printed specification EP-B 0 315 059 a hydroxylamine concentration of 1 to 10 g/l is claimed as being preferred and in accordance with example 1 of the printed specification EP-B 0 633 950 operations are carried out with a hydroxylamine concentration of 1.7 g/l the merit of the present invention therefore lies in having recognized that it is possible to deposit on various metal surfaces qualitatively very high-grade phosphate layers from a solution which has a very low hydroxylamine content and a comparatively small nitroguanidine content, in which case, with regard to the hydroxylamine content the invention does not follow the course marked out by the prior art, namely the use of quite high hydroxylamine concentrations.
In a further development of the invention it is provided that the solution contain 0.3 to 3 g Zn
2+
/l. The solution is therefore preferably suitable for use within the scope of low-zinc technology. Furthermore, in a further development of the invention it is provided that the solution additionally contain 0.5 to 20 g NO
3
−
/l, that the solution additionally contain 0.01 to 3 g Mn
2−
/l and/or 0.01 to 3 g N
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 in the absence of nickel is responsible for the fact that qualitatively high-grade phosphate layers are produced. During the phosphatization of surfaces containing aluminium it has proved to be particularly useful in accordance with 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. In accordance with the invention the solution contains (SiF
6
)
2−
or (BF
4
)
−
as the complex fluoride.
The nitrate content in accordance with the invention advantageously promotes the observance of a constant layer weight. The nitrate is added to the phosphatization solution in the form of alkali nitrates and/or by

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