EP0866888B1 - Method of phosphating metal surfaces - Google Patents

Description

The invention relates to a method for phosphating consisting at least partially of iron or steel Metal surfaces according to the low zinc technology as well as its Application to prepare the metal surfaces for the Electrocoating, especially for the cathodic Electro dip painting.

In the metalworking industry, this becomes very large Zinc phosphating process applied. As a pre-treatment for the painting bring phosphating processes by the Use low zinc technology, special advantages. The Phosphating solutions used here contain zinc in Concentrations of only about 0.4 to 2 g / l and produce Steel phosphate layers that have a very good paint adhesion and high Resistance to paint infiltration Offer corrosion stress.

As accelerators in low-zinc phosphating baths especially nitrite and chlorate and organic Proven nitro compounds. These baths deliver in a short time high quality, uniformly covering phosphate layers. It is also known to use peroxides in low-zinc phosphating baths used as an accelerator. These would be for the sake of Workplace hygiene and environmental protection using the Although the aforementioned accelerators are preferable, theirs is Oxidation effect with regard to the oxidation of iron (II) Iron (III) very strong. This means that even with long Treatment time using comparatively thin phosphate layers only moderate corrosion protection can be achieved.

To solve this problem, EP-A-414296 proposes a method for phosphating iron and steel surfaces according to the low zinc technology with nitrite-free, zinc, phosphate and nitrate-containing phosphating solutions in which the weight ratio of free P ₂ O ₅ increases Total P ₂ O _{5 is set} to a value in the range of 0.04 to 0.2. H ₂ O ₂ or alkali perborate should be added to the phosphating solution in such an amount that - in the incorporated state - the maximum peroxide concentration 17 mg / l (calculated as H ₂ O ₂ ) or the maximum Fe (II) concentration 60 mg / l (calculated as Fe).

In the above method, however, the disadvantage occur that the phosphating rate for some technical applications are not sufficient. In practice it is the phosphating rate was therefore adopted by adding chlorate. However, this becomes a essential advantage of the aforementioned method again given up. Also, phosphate layers with a relatively low Obtained layer weight and coarse crystalline structure. In addition comes that in the presence of zinc in particular because of the nitrate content on zinc surfaces be formed. In the presence of aluminum you can also no crystalline phosphate layers on the aluminum surfaces be generated.

The object of the invention is to provide a method for Phosphating of at least partially iron or steel to provide existing metal surfaces that too leads to sufficiently thick and fine crystalline phosphate layers, in the event of the simultaneous presence of zinc and / or Aluminum surfaces thereon too Leads phosphate layers and that with the addition of chlorate associated disadvantage does not have.

The object is achieved in that, according to the invention, a method of the type mentioned at the outset is used, in which the metal surfaces are kept at 30 to 65 ° C. for a period of 1 to 8 minutes. in contact with aqueous acid phosphating solutions, the 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 (calculated as H ₂ O ₂ ) 0.01 to 10 g / l formate (calculated as formation) Contain that are free of chlorate and added nitrite, in which the weight ratio of free P ₂ O ₅ to total P ₂ O ₅ to a value in the range of 0.03 to 0.20 and the content of free acid to a value is set in the range from 0.5 to 2.5.

Free of added nitrite means that the No nitrite should be added to phosphating solutions, but - if the process is designed with nitrate addition, if necessary low levels due to nitrate formation can.

For the determination of free acid, free P ₂ O ₅ and total P ₂ O ₅ , reference is made to Rausch, Die Phosphatierung von Metallen, Leuze-Verlag / Saalgau 1988, pages 300 to 304.

The inventive method is particularly for Surface treatment of iron and steel determined. Together with iron and steel, however, galvanized steel, alloy galvanized steel, i.e. e.g. with ZnAl, ZnFe and ZnNi coated steel, aluminized steel, aluminum, zinc and their alloys are treated.

It is known from WO 94/13856 to use phosphating baths for phosphating metal surfaces, in particular galvanized or alloy-galvanized steel strips, with treatment times of 2 to 20 seconds. In addition to zinc, phosphate and certain contents of free acid and total acid, they are water-soluble, organic acids contain a pK value between the dissociation constants of the first and the second stage of the phosphoric acid contained in the phosphating bath for the first dissociation constant, formic acid being mentioned as an example of suitable organic acids and hydrogen peroxide or peroxide compounds as an example of an additional oxidizing agent. In addition to the fact that in addition to H ₂ O ₂ or peroxide compounds, various other oxidizing agents are designated as suitable, it is emphasized as a particular advantage of the method that it leaves metal bright steel sides on substrates galvanized on one side. Because of this, it was to be expected that phosphating solutions which contain peroxide and formic acid and which work according to the low-zinc technology would not be able to produce flawless, high-quality phosphate layers on surfaces of iron and steel. In particular, it was not to be assumed that the use of formic acid would significantly increase the rate of phosphating.

The phosphating process according to EP-A-361375 also provides Phosphating solutions, which preferably correspond to the Low zinc technology work, formic acid, if necessary in combination with nitrate, chlorate, nitrite and Nitrobenzenesulfonate to add. Purpose of the addition Formic acid is when using nickel-containing Phosphating solutions Phosphate coatings with relatively high Generate nickel levels even if the nickel concentration is comparatively low in the phosphating solution. Also from this prior art it could not be deduced that the with advantages obtained by the method according to the invention can be achieved are.

According to a preferred embodiment of the invention, the used in the inventive method Phosphating solutions nitrate up to a concentration of 30 g / l included.

As is common in low zinc technology processes, the weight ratio of Zn to P ₂ O ₅ in the phosphating solution is preferably (0.023 to 0.14): 1.

When coordinating the type and amount of cations and anions of those used in the method according to the invention Phosphating solutions apply as a rule that for higher Bath temperatures and / or zinc concentration ratio pay in above range and for lower bath temperatures and / or zinc concentration ratio numbers in the lower range are to be chosen.

A preferred embodiment of the method according to the invention provides for the metal surfaces to be brought into contact with phosphating solutions which contain 0.01 to 0.1 g / l peroxide (calculated as H ₂ O ₂ ) and 0.3 to 2.5 g / l Formate (calc. As formation) included.

According to a further advantageous embodiment of the invention the surfaces are brought in with phosphating solutions Touching up to 3 g / l manganese, magnesium, Calcium, lithium, tungstate, vanadate, molybdate, if necessary also nickel and / or cobalt or combinations thereof contain. From the perspective of workplace hygiene and the Environmental protection should, however, add nickel and / or cobalt. It is also appropriate to Add phosphating solutions up to 0.030 g / l copper the addition alone or in combination with the aforementioned Cations can occur.

If the phosphating solutions additionally manganese and / or Magnesium and / or calcium optionally also nickel and / or Contain cobalt, the weight ratio of Mn: Zn, of Mg: Zn and Ca: Zn, possibly of (Ni + Co) at most 2: 1.

A further advantageous embodiment of the invention consists in bringing the metal surfaces into contact with phosphating solutions which contain fluoborate in amounts up to 3 g / l (calc. As BF ₄ ) and / or fluosilicate in amounts up to 3 g / l (calc. as SiF ₆ ) and / or simple fluoride in amounts up to 1.5 g / l (calculated as F). The anions fluoborate, fluosilicate 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 is essential for the crystalline phosphating of aluminum and its alloys.

The inventive method is in the temperature range of 30 up to 65 ° C. This is sufficient below 30 ° C Phosphating speed for modern series production in generally no longer while at higher temperatures Disadvantages, e.g. occur due to increased system incrustation can.

The method according to the invention can be used in spraying, dipping, Spray-diving or flooding can be carried out. If that The process is used as a spray process, the Zinc concentration 0.4 to 1.2 g / l. In case of Application in the spray-immersion or immersion process is one Concentration of zinc in the range of 1.0 to 2.0 g / l advantageous.

It is expedient to introduce the formations into the phosphating solution as alkali formate, ammonium formate or free formic acid. Zinc carbonate, zinc oxide and / or carbonates of the other optionally added cations are expediently used to adjust the content of the phosphating solution with regard to the free acid and the ratio of free P ₂ O ₅ to total P ₂ O ₅ , which is essential to the invention.

When carrying out the phosphating process according to the invention it is advisable to add water to the phosphating solutions withdraw and add rinse water to the following Level or the subsequent rinse levels. Of the Dehydration takes place, for example, by evaporation, Reverse osmosis and / or electrodialysis. Especially with the Successful use of hydrogen peroxide as a peroxide component to operate the method according to the invention in such a way that no waste water contaminated with phosphate from the Flushing process after phosphating. The expediently designed as a rinse bath cascade work in the last rinsing bath with salt-free or low-salt Water that opposes the flow of workpieces from the rinsing stage is fed to the phosphating bath at the rinsing stage. in the Phosphating bath it complements the above-mentioned dehydration the phosphating solution. That e.g. through reverse osmosis and Electrodialysis can remove water extracted from the phosphating bath the rinse levels are returned.

The pretreatment of the metals before the actual one Phosphating takes place in a conventional manner. For example, degreasing using aqueous, alkaline cleaners, which suitably contain surfactant, be made. If present, there is scale or rust by pickling treatment, e.g. with sulfuric acid, phosphoric acid or hydrochloric acid.

Although not absolutely necessary, the workpieces can be Phosphating to form fine crystalline phosphate coatings in a manner known per se, e.g. with a titanium phosphate containing activation bath, be rinsed.

After the phosphating treatment is usually done with water rinsed. For the purpose of improving corrosion protection e.g. with chromic acid-containing or chromic acid-free Rinse solutions are treated. It is particularly advantageous it, however, if according to another advantageous Embodiment of the invention instead of the aforementioned Rinse one with demineralized water that with Mineral acid to a pH in the range of 3.6 to 5.0 is set.

The generated by the inventive method Phosphate coatings are in all areas where Use phosphate coatings. However, it is of particular advantage in the phosphating of Metal surfaces for subsequent painting, in particular the subsequent electrocoating. Here is particular the process of preparing for cathodic Electrocoating of particular importance.

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

example 1

Lösung A:

1,0 g/l Zn    0,7 Punkte Freie Säure
1,0 g/l Mn    23 Punkte Gesamtsäure
13,0 g/l P2O5
0,05 g/l H2O2
1,0 g/l Formiat
3,0 g/l NO₃
Alkali zur Einstellung der Freien Säure

Lösung B:

Lösung A, jedoch ohne Formiat

The following phosphating solutions were prepared for use in the spray process:

Solution A:

1.0 g / l Zn 0.7 points free acid
1.0 g / l Mn 23 points total acidity
13.0 g / l P2O5
0.05 g / l H2O2
1.0 g / l formate
3.0 g / l NO ₃
Alkali to adjust the free acidity

Solution B:

Solution A, but without formate

In solutions A and B, degreased steel sheets were treated with an activating alkaline cleaner by spraying at 52 ° C. for 2 min. The layer weight, the crystal size, the minimum phosphating time and, after painting with a cathodic electrocoat, filler and topcoat, the adhesion and resistance to infiltration at a subsequently applied scratch were measured. The following values were measured: Solution A Solution B Layer weight 2.2 g / m ² 2.2 g / m ² Crystal size 12 um 35 µm Minimum phosphating time 1.2 min 1.4 min Lacquer adhesion (cross-cut grade) 0 0-1 Infiltration in the outdoor weathering test, 12 months (mm) 1.5 1.5

Example 2

Lösung C:

1,8 g/l Zn    1,6 Punkte Freie Säure
1,0 g/l Mn    25 Punkte Gesamtsäure
13,0 g/l P₂O₅
0,05 g/l H₂O₂
1,0 g/l Formiat
3,0 g/l NO₃
Alkali zur Einstellung der Freien Säure

Lösung D:

wie Lösung C, jedoch mit 2,5 g/l ClO₃ anstelle von Formiat

Lösung E:

wie Lösung C, jedoch ohne Formiat

The following compositions were selected for the phosphating solutions for use in the dipping process.

Solution C:

1.8 g / l Zn 1.6 points free acid
1.0 g / l Mn 25 points total acidity
13.0 g / l P ₂ O ₅
0.05 g / l H ₂ O ₂
1.0 g / l formate
3.0 g / l NO ₃
Alkali to adjust the free acidity

Solution D:

as solution C, but with 2.5 g / l ClO ₃ instead of formate

Solution E:

like solution C, but without formate

Steel sheets degreased with an alkaline cleaner were activated in a solution containing colloidal titanium phosphate and phosphated at 55 ° C. in solutions C to E for 3 minutes while immersing. The layer weight, the crystal size, the minimum phosphating time and - after coating with cathodic electrocoat, filler and topcoat - the adhesion and infiltration resistance were measured. The following results were obtained. Solution C. Solution D Solution E Layer weight (g / m ² ) 2.5 1.6 1.4 Crystal size (um) 10th 22 35 Minimum phosphating time (min) 2.0 2.0 3.0 Adhesion (cross-cut grade) 0 0 - 1 1 Infiltration in the outdoor weathering test, 12 months (mm) 1.5 1.5 2.2

Claims (9)

1. A process for phosphating metal surfaces consisting at least in part of iron or steel using low-zinc technology, in which the metal surfaces are brought into contact at 30 to 65°C for 1 to 8 min. with aqueous acidic phosphating solutions which contain 0.4 to 2.0 g/l Zn 7 to 25 g/l P₂O₅ 0.005 to 0.5 g/l peroxide (calculated as H₂O₂) 0.01 to 10 g/l formate (calculated as formate ion) which are free of chlorate and added nitrite, in which the weight ratio of free P₂O₅ to total P₂O₅ is set to a value in the range from 0.03 to 0.20 and the content of free acid is set to a value in the range from 0.5 to 2.5.
2. A process according to Claim 1, characterised in that the metal surfaces are brought into contact with phosphating solutions which additionally contain up to 30 g/l nitrate.
3. A process according to Claim 1 or 2, characterised in that the metal surfaces are brought into contact with phosphating solutions which contain 0.010 to 0.1 g/l peroxide (calculated as H₂O₂) and 0.3 to 2.5 g/l formate (calculated as formate ion).
4. A process according to Claim 1, 2 or 3, characterised in that the metal surfaces are brought into contact with phosphating solutions which additionally contain manganese, magnesium, calcium, lithium, tungstate, vanadate, molybdate or combinations thereof, optionally also nickel and/or cobalt in quantities of up to 3 g/l each.
5. A process according to Claim 4, characterised in that the metal surfaces are brought into contact with phosphating solutions in which the weight ratios Mn : Zn, Mg : Zn, Ca : Zn and optionally (Ni and/or Co) : Zn are each at most 2 : 1.
6. A process according to one or more of Claims 1 to 5, characterised in that the metal surfaces are brought into contact with phosphating solutions which additionally contain up to 0.030 g/l Cu.
7. A process according to one or more of Claims 1 to 6, characterised in that the metal surfaces are brought into contact with phosphating solutions which additionally contain complex and/or simple fluorides.
8. A process according to one or more of Claims 1 to 7, characterised in that the phosphated metal surfaces are post-rinsed with demineralised water, which has been set with mineral acid to a pH value in the range of 3.6 to 5.0.
9. The application of the process according to one or more of Claims 1 to 8 for the preparation of metal surfaces for subsequent electro-dip painting, in particular cathodic electro-dip painting.