EP0414296B1 - Process for obtaining phosphate coatings on metal surfaces - Google Patents

Abstract

In a process for phosphating iron and steel surfaces according to the low-zinc technology, a nitrite-free aqueous acidic phosphating solution of 30 to 65 DEG C is used which contains 0.4 to 1.7 g/l of Zn 7 to 25 g/l of P2O5 2 to 30 g/l of NO3 and in which the weight ratio of free P2O5 to total P2O5 is adjusted to a value in the range from 0.04 to 0.20. H2O2 or alkali metal perborate is added to the phosphating solution in an amount such that - in the incorporated state - its maximum peroxide concentration is 17 mg/l, preferably 8 mg/l (calculated as H2O2), or its maximum Fe(II) concentration is 60 mg/l, preferably 30 mg/l (calculated as Fe). It is particularly advantageous to control the addition of H2O2 and/or alkali metal perborate according to the electrochemical potential determined with a redox electrode. The phosphating solution may additionally contain Mn, Ni, Co, Mg and/or Ca or fluoroborate, fluorosilicate and/or fluoride. The process can be used in particular for the preparation of metal surfaces for coating. <IMAGE>

Description

The invention relates to a method for phosphating iron and steel surfaces according to the low-zinc technology with nitrite-free, zinc, phosphate and nitrate-containing phosphating solutions and its application to the preparation of iron and steel surfaces for painting.

The zinc phosphating process is widely used in the metalworking industry. The phosphate layers produced on the treated metal surfaces with this process are used in particular to facilitate sliding and non-cutting cold forming, as well as for corrosion protection and as a paint primer. A phosphating process which belongs to the genus of normal zinc processes and works on the iron side with iron II concentrations of 0.1 to 7 g / l is dealt with in EP-A-287133.

As a pretreatment for painting, phosphating processes that make use of low-zinc technology bring special advantages. The bath solutions used here contain zinc in concentrations of only about 0.4 to 1.7 g / l and produce phosphate layers on steel with a high proportion of phosphophyllite, which offers better paint adhesion and greater resistance to paint infiltration when exposed to corrosion than Hopeit from phosphating baths with a higher zinc content.

In particular, nitrite and chlorate and organic nitro compound have proven themselves as accelerators in low-zinc phosphating baths. One containing nitrite and possibly also nitrate and / or chlorate Phosphating bath is described in FR-A-2449135. These baths deliver high-quality, uniformly covering phosphate layers in a short time. It is also known to use peroxides as accelerators in low-zinc phosphating baths. These would be preferable to the use of the aforementioned accelerators for reasons of workplace hygiene and environmental protection, but their acceleration effect is sufficient among those previously used Treatment conditions did not rule out. Another disadvantage of the per compounds is that comparatively thin phosphate layers with only moderate corrosion protection can be achieved even with a long treatment time.

The object of the invention is to provide a method according to the preamble of claim 1, which does not have the known, in particular the aforementioned disadvantages.

The object is achieved by the method of the type mentioned according to the invention is designed such that the weight ratio of free P₂O₅ to total P₂O₅ is set to a value in the range of 0.04 to 0.20 in the acidic phosphate solution, and that the acidic phosphating solution H₂O₂ or alkali perborate is added in such an amount that - in the incorporated state - its maximum peroxide concentration 17 mg / l (calc. as H₂O₂) or its maximum Fe (II) concentration 60 mg / l (calc. as Fe).

The method according to the invention is intended for the surface treatment of iron and steel. However, together with iron and steel, low-alloy steel, galvanized steel, alloy-galvanized steel, ie with ZnAl, ZnFe and ZnNi coated steel, aluminized steel, aluminum and its alloys are treated.

The phosphating is carried out in the temperature range from 30 to 65 ° C. Below 30 ° C the phosphating speed is no longer sufficient for modern series production, while at temperatures higher than 65 ° C disadvantages, e.g. due to increased system incrustation.

As is customary in methods of the so-called low zinc technology, the weight ratio of Zn to P₂O₅ in the phosphating solution is preferably (0.075 to 0.015): 1.

The content of peroxide or Fe (II) present in the phosphating solution is determined in a conventional manner, for example by titration with potassium permanganate. According to an advantageous embodiment of the invention, the surfaces are brought into contact with a phosphating solution in which the addition of H₂O₂ and / or alkali perborate is controlled in accordance with the electrochemical potential determined with a redox electrode. For example, a platinum electrode and a suitable reference electrode, e.g. a calomel or silver / silver chloride electrode. With the aid of such an electrode arrangement, the phosphating solution can be monitored continuously and the peroxide added in such a way that both the stationary Fe (II) ion concentration and the stationary concentration of the hydrogen peroxide remain within the aforementioned limit values.

The coordination of the type and amount of cations and anions in the phosphating solution used in the process according to the invention is carried out in such a way that the ratio of free P₂O₅ to total P₂O₅ is between 0.04 and 0.20. As a rule, for higher bath temperatures and / or Zinc concentrations ratios in the above range and for lower bath temperatures and / or zinc concentrations ratios in the lower range should be selected.

A preferred embodiment of the method according to the invention provides for the surfaces to be brought into contact with a phosphating solution to which H₂O₂ and / or alkali perborate has been added in such an amount that their maximum peroxide concentration is 8 mg / l or their maximum Fe (II) concentration Is 30 mg / l.

According to a further advantageous embodiment of the invention, the surfaces are brought into contact with a phosphating solution which additionally contains manganese in an amount of up to 3 g / l, nickel and / or cobalt in an amount of up to 3 g / l, magnesium in an amount of contains up to 3 g / l and / or calcium in amounts up to 3 g / l. By using manganese and / or magnesium and / or calcium, phosphate coatings are obtained which, in addition to zinc and possibly iron (II), also contain these cations. Mixed phosphates of this type are notable for increased alkali resistance and are particularly suitable as a primer for paints. Nickel and / or cobalt are preferably used to increase the aggressiveness of the phosphating solution on steel and - if zinc surfaces are also treated - to improve the phosphating of zinc surfaces. A possible addition of small amounts of copper increases the accelerating effect of the phosphating solution. Alkali and / or ammonium primarily serve to set the desired acid ratio.

Another advantageous embodiment of the invention is to bring the surfaces into contact with a phosphating solution, the fluoborate in amounts up to 3 g / l (calculated as BF₄) and / or silicofluoride in amounts up to 3 g / l (calculated as SiF₆) and / or fluoride in quantities up to 1.5 g / l (calculated as F). The anions fluoborate, silicofluoride and / or fluoride generally increase the rate of phosphating and are also particularly advantageous when the treatment of aluminum-containing zinc surfaces is intended. The presence of free fluoride (F⁻) is essential for the crystalline phosphating of aluminum and its alloys.

Chloride and sulfate can be used to adjust the electroneutrality of the phosphating solution and in special cases to increase the aggressiveness. Any possible use of, for example, polyhydroxycarboxylic acids, such as tartaric acid and / or citric acid, gives the possibility of influencing the thickness or weight per unit area of the phosphate coatings produced.

If the phosphating solution also contains manganese and / or nickel and / or cobalt and / or magnesium, the weight ratio of Mn: Zn, of (Ni and / or Co): Zn, of Mg: Zn and / or Ca: Zn should be a maximum Be 2: 1.

According to a further advantageous embodiment of the invention, the surfaces are brought into contact with a phosphating solution, the free P₂O₅ or the ratio of free P₂O₅ to total P₂O₅ is set during the work by adding manganese carbonate, zinc carbonate and / or zinc oxide. It is advisable to use the components mentioned in the form of an aqueous dispersion.

The process according to the invention can be carried out in spraying, dipping, spray-dipping or flooding.

A further advantageous embodiment of the invention provides for the metal surfaces to be brought into contact with a phosphating solution, from which water is removed and compensated for by adding rinsing water to the subsequent rinsing step or rinsing steps. The water removal from the phosphating bath takes place, for example, by evaporation, reverse osmosis and / or electrodialysis. In particular when using H₂O₂ as a peroxide component, it is possible to operate the method according to the invention in such a way that no waste water contaminated with phosphate is obtained from the rinsing process after the phosphating. The rinsing stages, which are expediently designed as a rinsing bath cascade, work in the last rinsing bath with salt-free or low-salt water, which is supplied to the phosphating bath in the opposite direction from the rinsing stage to rinsing stage. In the phosphating bath, it complements the above-mentioned dehydration of the phosphating solution. The water extracted from the phosphating bath, for example by reverse osmosis or electrodialysis, can be returned to the rinsing stages.

A further advantageous embodiment of the method according to the invention provides for the surfaces to be brought into contact with a phosphating solution which, by adding phosphate, has a ratio of free P₂O₅ to total P₂O₅ when adding (- 0.50 to + 0.20) : 1 is added. In the aforementioned definition of the ratio of free P₂O₅ to total P₂O₅ the minus sign means that there is no free P₂O₅, but rather part of the phosphate is in the secondary phosphate stage. The value minus 0.19 means, for example, that 19% of the total P₂O₅ are present as secondary phosphate.

According to another definition, the phosphate components in the supplement are in a range which is defined by 50% secondary and 50% primary phosphate (calc. As P₂O₅) on the one hand and limited by 80% primary phosphate and 20% free phosphoric acid (calculated as P₂O₅) on the other.

Since liquid supplement concentrates in the range from free P₂O₅ to total P₂O₅ are not stable, the addition in the method according to the invention is generally carried out with at least two separate concentrates.

With the aid of the method according to the invention, in particular in the preferred embodiment with addition of the coating-forming phosphating solution, it is possible over a long time to produce phosphate coatings which are uniform and closed, and not only on iron and steel, but also on accompanying surfaces, namely galvanized, alloy galvanized as well as aluminized steel and aluminum to produce flawless coatings.

The method according to the invention is of particular advantage in the preparation of surfaces for painting, in particular for electrocoating. Preparation for cataphoretic electrocoating is particularly important.

The invention is illustrated by the following examples, for example and in more detail.

example 1

wurde die H₂O₂-Konzentration durch Zugaben von H₂O₂ zwischen 10 und 70 mg/l H₂O₂ und bei Abwesenheit von H₂O₂ die Eisen(II)-Konzentration durch Einarbeiten mit Stahlblech zwischen 10 und 90 mg/l Fe(II) variiert.In a phosphating solution intended for spray treatment

the H₂O₂ concentration was varied by adding H₂O₂ between 10 and 70 mg / l H₂O₂ and, in the absence of H₂O₂, the iron (II) concentration by incorporating steel sheet between 10 and 90 mg / l Fe (II).

In these baths, steel sheets degreased with organic solvents were treated by spraying at 58 ° C. Fig.1 shows the phosphate layer weight obtained with a 3 min spraying time. Fig. 2 contains the minimum phosphating times found in these tests, ie the treatment times that were necessary to deposit phosphate layers with uniform coverage on the metal sheets. Both figures illustrate the advantageous result that is achieved with the method according to the invention.

Example 2

In a phosphating apparatus with a capacity of 5 l, alternately previously degreased steel (80%) and electrolytically galvanized steel (20%) sheets were phosphated with a phosphating solution of the following composition:

The temperature of the solution was 55 to 60 ° C. The treatment was carried out by spraying for 3 minutes. The throughput was 3 m² / l bath volume with a throughput density of 0.1 m² / h. The bath composition was maintained over the total throughput by adding zinc carbonate and a correspondingly supplemented solution.

From the supplement concentrate
23.4% P₂O₅
1.89% Na
1.74% Mn
1.34% Ni
3.39% Zn
0.01% Fe (III)
3.09% NO₃
19 g / m² surface were required to add a constant point. To adjust the ratio of free P₂O₅ to total P₂O₅ 1.8 g / m² of basic zinc carbonate (53.5% Zn) were also added to the bath. This addition corresponds to a ratio of free P₂O₅ to total P₂O₅ of (- 0.18): 1.

Hydrogen peroxide was dosed according to the measured electrochemical potential so that both the stationary Fe (II) ion concentration and the H₂O₂ concentration in the bath was a maximum of 10 mg / l. The phosphate coatings obtained were consistently uniform and closed and had layer weights of 2.0 ± 0.2 g / m² for steel and 2.5 ± 0.2 g / m² for electrolytically galvanized steel.

Example 3

enthielt.In a phosphating apparatus with a capacity of 5 l, alternately previously degreased sheets of steel (60%), electrolytically galvanized steel (30%) and aluminum (10%) of the quality AlMgSi and AlMg3 were treated with a phosphating solution

contained.

In compliance with the above conditions and stationary concentrations for Fe (II) and H₂O₂ of maximum 6 mg / l, uniform and closed coatings of the layer weights were consistently on all three materials
Steel: 2.1 ± 0.2 g / m²
E-zinc: 2.6 ± 0.2 g / m²
AlMgSi: 2.9 ± 0.3 g / m²
AlMg3: 3.1 ± 0.3 g / m²
receive.

Claims (9)

1. A method for phosphating iron and steel surfaces using low-zinc technology with nitrite-free phosphating solutions which contain zinc, phosphate and nitrate, in which the surfaces are brought into contact at 30 to 65°C with an aqueous acidic phosphating solution which contains
      0.4 to 1.7 g/l Zn
      7 to 25 g/l P₂O₅
      2 to 30 g/l NO₃,
   characterised in that the weight ratio of free P₂O₅ to total P₂O₅ in the acidic phosphating solution is set to a value in the range of 0.04 to 0.20, and that H₂O₂ or alkali perborate is added to the acidic phosphating solution in a quantity such that - in the worked-in state - the maximum peroxide concentration thereof is 17 mg/l (calculated as H₂O₂) or the maximum Fe(II) concentration thereof is 60 mg/l (calculated as Fe).
2. A method according to Claim 1, characterised in that the surfaces are brought into contact with a phosphating solution in which the addition of H₂O₂ and/or alkali perborate is controlled in dependence on the electrochemical potential which is determined with a redox electrode.
3. A method according to Claim 1 or 2, characterised in that the surfaces are brought into contact with a phosphating solution to which H₂O₂ and/or alkali perborate is added in a quantity such that the maximum peroxide concentration thereof is 8 mg/l or the maximum Fe(II) concentration thereof is 30 mg/l.
4. A method according to Claim 1, 2 or 3, characterised in that the surfaces are brought into contact with a phosphating solution which contains manganese in quantities of up to 3 g/l, nickel and/or cobalt in quantities of up to 3 g/l, magnesium in quantities of up to 3 g/l and/or calcium in quantities of up to 3 g/l.
5. A method according to one or more of Claims 1 to 4, characterised in that the surfaces are brought into contact with a phosphating solution which contains fluoborate in quantities of up to 3 g/l (calculated as BF₄) and/or silicofluoride in quantities of up to 3 g/l (calculated as SiF₆) and/or fluoride in quantities of up to 1.5 g/l (calculated as F).
6. A method according to one or more of Claims 1 to 5, characterised in that the surfaces are brought into contact with a phosphating solution, the free P₂O₅ of which is set in the working bath by the addition of manganese carbonate, zinc carbonate and/or zinc oxide.
7. A method according to one or more of Claims 1 to 6, characterised in that the surfaces are brought into contact with a phosphating solution from which water is removed and is compensated by the addition of rinsing water from the subsequent rinsing stage or stages.
8. A method according to one or more of Claims 1 to 7, characterised in that the surfaces are brought into contact with a phosphating solution which is replenished by the addition of phosphate with a ratio of free P₂O₅ to total P₂O₅ upon replenishment of (- 0.50 to + 0.20):1.
9. The application of the method according to one or more of Claims 1 to 8 to the preparation of surfaces of iron and steel for painting, in particular electro-dip coating, preferably cataphoretic electro-dip coating.

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