Process of phosphatizing metal surfaces

Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...

Utility Patent

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C148S262000, C427S353000

Utility Patent

active

06168674

ABSTRACT:

DESCRIPTION
This invention relates to a process of phosphatizing metal surfaes, which at least partly consist of iron and steel, in accordance with the low-zinc technology, and to the use of such process for the preparation of metal surfaces for electro-dipcoating, in particular for cathodic electro-dipcoating.
In the metal-processing industry the process of zinc phosphatizing is used on a large scale. As a pretreatment for lacquer coating, phosphatizing processes making use of the low-zinc technology are particularly advantageous. The phosphatizing solutions used here contain zinc in concentrations of only about 0.4 to 2 g/l and produce phosphate layers on the steel, which have a very good lacquer adhesion and a high resistance to subsurface corrosion of the lacquer.
As accelerators in low-zinc phosphatizing baths nitrite and chlorate as well as organic nitro compounds are particularly suited. These baths provide a high-quality, uniformly covering phosphate layers within a short period. It is also known to use peroxides as accelerators in low-zinc phosphatizing baths. For reasons of work-place hygiene and environmental protection these should be preferred over the use of the aforementioned accelerators, but they have a very high oxidizing effect as regards the oxidation of iron(II) to iron(III). The consequence is that even with a long treatment time comparatively thin phosphate layers with only a moderate protection against corrosion can be achieved.
To solve this problem, the EP-A-414296 proposes a process of phosphatizing iron and steel surfaces in accordance with the low-zinc technology by means of nitrite-free phosphatizing solutions containing zinc, phosphate and nitrate, where the weight ratio of free P2O
5
to total P
2
O
5
has been adjusted to a value in the range from 0.04 to 0.2. H
2
O
2
or alkali perborate should be added to the phosphatizing solution in such an amount that—in the incorporated condition—the maximum peroxide concentration is 17 mg/l (calc. as H
2
O
2
) and the maximum Fe(II) concentration is 60 mg/l (calc. as Fe).
The aforementioned process can, however, have the disadvantage that the phosphatizing speed is not sufficient for some technical applications. In practice, one therefore tends to increase the phosphatizing speed by adding chlorate. In doing so, a major advantage of the aforementioned process is, however, abandoned. In addition, there are obtained phosphate layers with a relatively low coating weight and a coarse-crystalline structure. Moreover, when zinc is present at the same time, specks are formed on zinc surfaces especially because of the nitrate content. When aluminum is present, crystalline phosphate layers cannot be formed on the aluminum surfaces.
It is the object of the invention to provide a process of phosphatizing metal surfaces at least partly consisting of iron or steel, which process leads to sufficiently thick and fine-crystalline phosphate layers, also leads to proper phosphate layers when zinc and/or aluminum surfaces are present at the same time, and does not have the disadvantage connected with the addition of chlorate.
This object is solved in that in accordance with the invention a process of the above-stated kind is used, where at a temperature of 30 to 65° C. and for a period of 1 to 8 min the metal surfaces are brought in contact with aqueous acid phosphatizing solutions, which contain
0.4 to 2.0 g/l Zn
7 to 25 g/l P
2
O
5
0.005 to 0.5 g/l peroxide (calc. as H
2
O
2
)
0.01 to 10 g/l formate (calc. as formate ion),
are free from chlorate and added nitrite, and in which the weight ratio of free P
2
O
5
to total P
2
O
5
has been adjusted to a value in the range from 0.03 to 0.20, and the content of free acid has been adjusted to a value in the range from 0.5 to 2.5.
Free from added nitrite means that no nitrite should be added to the phosphatizing solutions, but—when designing the process with addition of nitrate—there can at best be present minor contents due to a formation from nitrate.
For determining the free acid, the free P
2
O
5
and the total P
2
O
5
, reference is made to Rausch, “Die Phosphatierung von Metallen”, Leuze-Verlag/Saalgau, 1988, pages 300 to 304.
The process in accordance with the invention is determined in particular for the surface treatment of iron and steel. Together with iron and steel there can, however, also be treated zinc-plated steel, alloy zinc-plated steel, i.e. for instance steel coated with ZnAl, ZnFe and ZnNi, aluminized steel, aluminum, zinc and the alloys thereof.
It is known from WO 94/13856 that for phosphatizing metal surfaces, in particular zinc-plated or alloy zinc-plated steel strips, with treatment times of 2 to 20 sec., phosphatizing baths are used, which in addition to zinc, phosphate and certain contents of free acid and total acid contain water-soluble organic acids with a pK value for the first dissociation constant lying between the dissociation constants of the first and second stage of the phosphoric acid contained in the phosphatizing bath, where as an example for suitable organic acids formic acid is mentioned, and as an example for an additional oxidizing agent hydrogen peroxide or peroxide compounds are mentioned. Apart from the fact that in addition to H
2
O
2
or peroxide compounds various other oxidizing agents are referred to as suitable, it is emphasized as a particular advantage of the process that it produces bright metallic surfaces in the case of unilaterally zinc-plated substrates. Therefore, it had to be expected that phosphatizing solutions containing peroxide and formic acid and operating in accordance with the low-zinc technology would not be capable of producing proper, high-quality phosphate layers also on surfaces of iron and steel. It could in particular not be expected that the phosphatizing speed is increased considerably by also using formic acid.
The phosphatizing process in accordance with EP-A-361375 also provides for adding formic acid, possibly in combination with nitrate, chlorate, nitrite and nitrobenzene sulfonate to phosphatizing solutions, which preferably operate according to the low-zinc technology. The purpose of adding formic acid is to produce phosphate coatings with relatively high nickel contents when using nickel-containing phosphatizing solutions, even if the nickel concentration in the phosphatizing solution is comparatively low. Even from this prior art it could not be derived that the advantages obtained by means of the inventive process could be achieved.
In accordance with a preferred embodiment of the invention the phosphatizing solutions used in the inventive process may contain nitrate up to a concentration of 30 g/l.
As is usual in processes of the low-zinc technology, the weight ratio of Zn to P
2
O
5
in the phosphatizing solution preferably is (0.023 to 0.14):1.
When adjusting the kind and quantity of cations and anions of the phosphatizing solutions being used in the inventive process, it is regarded as a rule that for higher bath temperatures and/or zinc concentrations ratios in the upper range, and for lower bath temperatures and/or zinc concentrations ratios in the lower range should be selected.
In accordance with a preferred embodiment of the inventive process the metal surfaces are brought in contact with phosphatizing solutions which contain 0.01 to 0.1 g/l peroxide (calc. as H
2
O
2
) and 0.3 to 2.5 g/l formate (calc. as formate ion).
In accordance with a further advantageous embodiment of the invention the surfaces are brought in contact with phosphatizing solutions containing in addition up to 3 g/l each of manganese, magnesium, calcium, lithium, tungstate, vanadate, molybdate, possibly also nickel and/or cobalt or combinations thereof. From the point of view of work-place hygiene and environmental protection, the addition of nickel and/or cobalt should, however, be omitted. It is also expedient to add up to 0.030 g/l copper to the phosphatizing solutions, where the addition may be effected alone or in combination with the aforementioned cations.
If the phosphatizing solutions additionally co

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