Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1999-03-10
2001-07-17
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S262000, C148S279000
Reexamination Certificate
active
06261384
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an aqueous, phosphate-containing solution for producing phosphate coatings on metallic surfaces of iron, steel, zinc, zinc alloys, aluminum or aluminum alloys. This invention furthermore relates to a process for phosphatizing by using an aqueous phosphatizing solution.
BACKGROUND OF THE INVENTION
From the DE-PS 750 957 there is known a process for improving the corrosion resistance of metals, in particular of iron and steel, by treating them in a solution forming phosphate coatings, where the solution contains an accelerator, and where nitromethane, nitrobenzene, picric acid, a nitroaniline, a nitrophenol, a nitrobenzoic acid, a nitroresorcinol, nitrourea, a nitrourethane or nitroguanidine is used as accelerator. The optimum concentration for the individual accelerators is different, but in the phosphatizing solutions it is generally in the range between 0.01 and 0.4 wt-%. For the accelerator nitroguanidine the optimum concentration is said to be 0.2 wt-%. From the DE-OS 38 00 835 there is known a process of phosphatizing metal surfaces, in particular surfaces of iron, steel, zinc and the alloys thereof as well as aluminum, as a pretreatment for the cold working, where without activation at a temperature in the range from 30 to 70° C. the surface is brought in contact with an aqueous solution containing 10 to 40 g Ca
2+
/l, 20 to 40 g Zn
2+
/l, 10 to 100 g PO
4
3−
/l and as accelerator 10 to 100 g NO
3
−
/l and/or 0.1 to 2.0 g organic nitro compounds per liter, where the solution has a pH value in the range from 2.0 to 3.8 and a ratio of free acid to total acid of 1:4 to 1:100. As accelerator, an m-nitrobenzene-sulfonate and/or nitroguanidine may be used. The phosphate coatings produced in accordance with the known process have coating weights of 3 to 9 g/m
2
.
Although it is known per se that nitroguanidine can be used as accelerator when phosphatizing metallic surfaces, the practical use of this accelerator meets with some difficulties, as the phosphatizing results achieved are very frequently unsatisfactory. This is quite obviously due to the fact that the effect of the accelerator nitroguanidine very much depends on the inorganic components of the phosphatizing solution and the concentration of the inorganic components of the phosphatizing solution, so that the phosphate coatings produced by using nitroguanidine only have good functional properties when one succeeds in providing a phosphatizing solution in which the individual components are adjusted to each other such that when using nitroguanidine as accelerator, phosphate coatings of good, constant quality can be produced also in a continuous operation. Moreover, the interaction between the nitroguanidine and the remaining components of the phosphatizing solution cannot be predicted or determined by theoretical considerations or simple experiments, but must be determined by extensive experiments on different phosphatizing systems. The frequently unsatisfactory results are also due to the poor water solubility and the uneven distribution of the nitroguanidine.
OBJECT OF THE INVENTION
It is therefore the object underlying the invention to create an aqueous solution for phosphatizing metallic surfaces, which contains nitroguanidine as accelerator, and whose remaining components are adjusted to each other such that the phosphate coatings formed during phosphatizing are finely crystalline, have a low coating weight, provide for a good lacquer adhesion and ensure a good protection against corrosion. It is furthermore the object underlying the invention to create a process of phosphatizing which uses the phosphatizing solution in accordance with the invention, where the process should operate at temperatures as low as possible, may be used for phosphatizing different metallic surfaces, and should operate by using simple technical means as well as to be safe in operation.
SUMMARY OF THE INVENTION
The object underlying the invention is obtained by preparing an aqueous, phosphate-containing solution for producing phosphate coatings on metallic surfaces of iron, steel, zinc, zinc alloys, aluminum or aluminum alloys, which contains 0.3 to 5 g Zn
2+
/l, and 0.1 to 3 g nitroguanidine/l, where the acid value is 0.03 to 0.3 and the weight ratio of Zn:P
2
O
5
=1:5 to 1:30, and which produces finely crystalline phosphate coatings, in which the crystallites have a maximum edge length <15 &mgr;m. It has surprisingly turned out that by means of the phosphatizing solution in accordance with the invention very finely crystalline phosphate coatings can be produced, which effect a good lacquer adhesion and a good protection against corrosion. The crystallites have a laminated, cuboid or cubic shape and always have a maximum-edge length <15 &mgr;m, which in general is even <10 &mgr;m. Furthermore, the phosphatizing solution in accordance with the invention is very well suited for phosphatizing cavities. The phosphate coatings deposited on the metallic articles from the inventive phosphatizing solution have a coating weight of 1.5 to 4.5 g/m
2
, preferably of 1.5 to 3 g/m
2
, so that the lacquer adhesion is favorably influenced. With a zinc content >5 g/l the anticorrosive properties and the lacquer adhesion deteriorate significantly.
The ratio of Zn:P
2
O
5
is based on 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 acid value indicates the ratio of free acid, calculated as free P
2
O
5
, to the total P
2
O
5
. The definitions and determination methods for the total P
2
O
5
and the free P
2
O
5
are explained in detail in the publication by W. Rausch “Die Phosphatierung von Metallen”, 1988, pages 299 to 304.
In accordance with the invention it is particularly advantageous when the aqueous, phosphate-containing solution contains 0.3 to 3 g Zn
2+
/l and 0.1 to 3 g nitroguanidine/l, where the acid value is 0.03 to 0.3 and the weight ratio of Zn:P
2
O
5
=1:5 to 1:30. With this inventive solution, which due to its zinc content of 0.3 to 3 g/l is suited for performing the low-zinc phosphatizing, particularly good results were achieved on the whole.
In accordance with the invention the aqueous solution should contain 0.5 to 20 g NO
3
−
/l. The inventive nitrate content favorably influences the maintenance of the optimum coating weight of 1.5 to 4.5 g/m. The nitrate is added to the phosphatizing solution in the form of alkali nitrates and/or by means of the cations present in the system, e.g. as zinc nitrate, and/or as HNO
3
. Since the nitrate-free aqueous solution also provides good phosphatizing results, the known accelerating effect of the nitrate is in all probability of minor importance in the present case.
In accordance with the invention it is furthermore provided that the phosphatizing solution contains 0.01 to 3 g Mn
2+
/l and/or 0.01 to 3 g Ni
2+
/l and/or 1 to 100 mg Cu
2+
/l and/or 10 to 300 mg Co
2+
/l. These metal ions are incorporated in the phosphate coating and improve lacquer adhesion and protection against corrosion.
In accordance with a further aspect of the invention it is provided that the aqueous phosphatizing solution contains 0.01 to 3 g F
−
/l and/or 0.05 to 3.5 g/l complex fluorides, preferably (SiF
6
) or (BF
4
). The fluoride is added to the phosphatizing solution when metallic surfaces consisting of aluminum or aluminum alloys should be phosphatized. The complex fluorides are added to the phosphatizing solution in particular for stabilization, so that a longer dwell time of the phosphatizing baths is achieved.
The object underlying the invention is furthermore obtained by employing a process of phosphatizing, where the metallic surfaces are cleaned, are subsequently treated with the aqueous, phosphate-containing phosphatizing solution for a period of 5 seconds to 10 minutes at a temperat
Kolberg Thomas
Schubach Peter
Dubno Herbert
Metallgesellschaft Aktiengesellschaft
Myers Jonathan
Oltmans Andrew L.
Sheehan John
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