EP0454211B1 - Process for applying phosphate coatings on metal surfaces - Google Patents

Abstract

The activating agent which is based on titanium(IV) phosphate and intended for use in the activation of metal surfaces before a zinc phosphating treatment contains one or more copper compounds and has a Ti:Cu weight ratio of 1:100 to 60:1 and optionally contains in addition at least one of the components consisting of condensed phosphate, silicate, complexing agent, water-soluble organic polymer, thickening agent, and surfactant. It is used to prepare aqueous activating baths for activating iron, steel, galvanized steel, zinc alloy-plated steel, aluminum-plated steel and aluminum before a zinc phosphating treatment, which baths contain 0.001 to 0.060 g/l Ti, 0.020 to 1.2 g/l orthophosphate (calculated as P2O5), and 0.001 to 0.1 g/l Cu and so much alkali that the bath has a pH value of 7 to 11, preferably of 7.5 to 10.

Description

The invention relates to a method for applying phosphate coatings on metal surfaces by activation with an activating agent based on Ti IV phosphate and zinc phosphating.

By phosphating with aqueous solutions based on zinc phosphate, zinc phosphate layers can be produced on numerous metallic surfaces, such as iron, steel, galvanized alloy steel, aluminum or aluminized steel. The application of the phosphating solutions, which in addition to zinc and phosphoric acid can also contain other cations and anions, is carried out by spraying, dipping or spraying / dipping. The zinc phosphate layers obtained serve to protect against corrosion, paint adhesion, reduction of sliding resistance, ease of cold forming and electrical insulation.

In addition to phosphating itself, a phosphating process also includes various pre-treatment and post-treatment stages. It is essential to clean the metal surface, which is generally done with alkaline or acidic cleaners and removes oils, greases, oxides and adhering solid particles from the metal surface. If cleaning is carried out with mildly alkaline cleaners, it is in principle possible to combine cleaning with activation of the metal surface. As a rule, however, the activation follows the cleaning as a separate process step.

The task of activating the metal surface is to form a with the shortest possible phosphating times to ensure the finest possible crystalline zinc phosphate layer. A criterion for the effectiveness of an activating agent is therefore the minimum phosphating time. The suitability for forming even finely crystalline zinc phosphate layers can be determined on the basis of the layer weights or by scanning electron microscope images.

Activating agents based on titanium IV phosphate have proven particularly useful in practice. Titan IV phosphates form when aqueous titanium IV salt solutions are reacted with soluble phosphates or phosphoric acid. However, products with activating properties are only obtained under special manufacturing conditions, which are described, for example, in US Pat. Nos. 2,310,239 and 2,456,947 and provide precise information regarding the type and concentration of the raw materials, temperature and pH range during manufacture. However, even if the reaction conditions are kept constant, there are fluctuations in the application effect from batch to batch.

A disadvantage of using activation agents based on titanium IV phosphate is that the activation baths have to be prepared with deionized water. The reason for this is that the alkaline earth metal ions present in the tap water as hardening agents destabilize titanium IV phosphate in activation baths. These alkaline earth metal ions can also be introduced into the activation bath by rinsing water.

In order to eliminate the disadvantageous influence of alkaline earth metal ions and thus to avoid destabilization of the activation bath, DE-A-3731089 proposes that cation-exchanging zeolites with a primary particle diameter of <3 »m the activating one Mix in titanium phosphate. Another way to improve the activation baths is described in EP-B-180523. The addition of phosphonic acid acting as a complexing agent into the activation bath permits the use of industrial water for the bath preparation. In addition, the phosphonic acid is said to cause the zinc phosphate layer to become extremely fine crystalline. A serious disadvantage of individual phosphonic acids, however, is that they act as a phosphating bath poison even in concentrations of a few mg / l. An entry of phosphonic acid from the activation bath into the phosphating bath can render the phosphating bath unusable in a very short time.

DE-A-3814287 provides for poly (aldehyde carboxylic acids) to be added in substoichiometric amounts as complexing agents for titanium IV during the production of activating titanium IV phosphates. This mainly produces titanium phosphates with a small grain size (<200 »m), which should have an increased effectiveness of the activating agent. Here too, the introduction of the poly (aldehyde carboxylic acids) into the phosphating bath can have considerable disadvantages.

However, the above-mentioned improvement in the stability of the activation baths against water hardness by means of additives or the improvement in quality with regard to the service life of the activation baths and crystallinity of the zinc phosphate coating applied in the subsequent step by complexing agents also have considerable disadvantages. A disadvantage of complexing agents in particular is that they act as phosphating bath poisons and make wastewater treatment more difficult because they bring heavy metals into solution or keep them.

Activating agents based on Ti IV phosphate containing at least one of the divalent metals barium, strontium and calcium are also known (FR-A-2 114 874). The number of equivalents of titanium should not exceed the number of equivalents of divalent metals. For comparison purposes, the result of the activating cleaning prior to zinc phosphating is reported using cleaning liquids which have a pH greater than 12 and which contain, inter alia, an activating agent based on Ti IV phosphate modified with a copper compound. According to the presentation in FR-A-2 114 874, this pretreatment leads to thin, powdery, irregular phosphate layers with coarse crystals in the subsequent zinc phosphating.

The object of the invention is to provide a method for applying phosphate coatings on metal surfaces by activation with an activating agent based on Ti IV phosphate and zinc phosphating, which does not have the disadvantages of the known processes, provides for the use of an activating agent which is simple to produce and for stable activation baths with a long service life, which also guarantee the formation of fine crystalline zinc phosphate layers in a short time.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the metal surfaces are activated with an activation bath which
0.001 to 0.060 g / l Ti
0.020 to 1.2 g / l orthophosphate (calc. As P₂O₅) and
0.001 to 0.1 g / l Cu
and contains so much alkali that it has a pH of 7 to 11.

The copper content was adjusted by adding copper compounds. These compounds have the effect that the minimum phosphating time is reduced considerably. The addition of copper also means that the activation bath is stable over a wide temperature range and shows very good activating properties.

Essentially all compounds are suitable for introducing the copper into the activating agent. According to an advantageous development of the invention, the activating agent contains copper compounds introduced via copper hydroxide, copper oxide hydrate, copper tartrate, copper nitrate and / or copper phosphate. Copper sulfate or copper chloride can be used but are not preferred.

• kondens. Phosphat (ber. als P₂O₅) in Mengen bis 1,2 g/l
• Silikat (ber. als SiO₂) in Mengen bis 0,5 g/l
• Komplexbildner in Mengen bis 1,0 g/l
• wasserlösliches organisches Polymer in Mengen bis 0,1 g/l
• Verdickungsmittel in Mengen bis 0,1 g/l und
• Tensid in Mengen bis 0,3 g/l

aufweist. Durch die hier erwähnten weiteren Zusätze zum Aktivierungsbad enthält man eine Reihe zusätzlicher vorteilhafter Eigenschaften. Beispielsweise bewirkt der Zusatz von kondensiertem Phosphat zum Aktivierungsmittel, daß das daraus hergestellte Aktivierungsbad auf eingeschleppte Härtebildner weniger empfindlich reagiert. Wasserlösliches organisches Polymer stabilisiert das im Aktivierungsbad in kollodialer Form dispergierte Titan-IV-Phosphat und verlängert somit die Standzeit des Aktivierungsbades erheblich. Tenside setzen die Oberflächenspannung herab, so daß das aktivierend wirkende Titanphosphat besser auf der Metalloberfläche haftet.Another advantageous embodiment of the invention consists in using an activation bath which contains an additional content of at least one of the components

• condensation Phosphate (calc. As P₂O₅) in amounts up to 1.2 g / l
• Silicate (calc. As SiO₂) in amounts up to 0.5 g / l
• Complexing agent in amounts up to 1.0 g / l
• water-soluble organic polymer in amounts up to 0.1 g / l
• Thickeners in amounts up to 0.1 g / l and
• Surfactant in amounts up to 0.3 g / l

having. The additional additives to the activation bath mentioned here contain a number of additional advantageous properties. For example, the addition of condensed phosphate to the activating agent has the effect that the activating bath produced therefrom reacts less sensitively to introduced hardness formers. Water-soluble organic polymer stabilizes the titanium IV phosphate dispersed in the activation bath in colloidal form and thus extends the service life of the activation bath considerably. Surfactants reduce the surface tension, so that the activating titanium phosphate adheres better to the metal surface.

The application of the method according to the invention to the activation of iron, steel, galvanized steel, alloy-galvanized steel, aluminized steel and aluminum is particularly advantageous.

Copper concentrations above 0.1 g / l should be avoided since they can lead to a disruption in the phosphating in the subsequent step.

The invention is explained in more detail and by way of example using the following examples.

example 1

• 1. Reinigen   stark alkalischer Tauchreiniger 20 g/l; 10 min; 70°C
• 2. Spülen   kaltes Wasser 30 sec
• 3. Reinigen   mild alkalischer Tauchreiniger 13 g/l; 5 min; 60°C;
• 4. Spülen   kaltes Wasser 30 sec.
• 5. Aktivierende   1 g/l Aktivierungsmittel Vorspülung 30 sec tauchen, 22°C
• 6. Phosphatierung   1,2 g/l Zn; 12,0 g/l P₂O₅; 0,8 g/l Mn; 0,8 g/l Ni; 7 g/l NO₃ 4,07 g/l Na; 0,17 g/l NaNO₂; 50°C;
     Phosphatierzeit 3 und 6 Minuten Tauchen
• 7. Spülen   kaltes Wasser 30 sec
• 8. Trocknung   mit Warmluft

Steel sheets of quality St 1405 were treated according to the following procedure:

• 1. Clean strong alkaline immersion cleaner 20 g / l; 10 min; 70 ° C
• 2. Rinse cold water for 30 sec
• 3. Clean mildly alkaline dip cleaner 13 g / l; 5 min; 60 ° C;
• 4. Rinse cold water 30 sec.
• 5. Immerse activating 1 g / l activating agent pre-rinse for 30 sec, 22 ° C
• 6. phosphating 1.2 g / l Zn; 12.0 g / l P₂O₅; 0.8 g / l Mn; 0.8 g / l Ni; 7 g / l NO₃ 4.07 g / l Na; 0.17 g / l NaNO₂; 50 ° C;
  Phosphating time 3 and 6 minutes diving
• 7. Rinse cold water 30 sec
• 8. Warm air drying

A total of 6 activating agents were produced, which were then used in each case to form activation baths.

Activating agent 1

To produce the activating agent, 3.27 kg of solid sodium hydroxide were dissolved in 4.9 kg of water and - after cooling - with a solution of 0.54 kg of H₂TiF₆ (40% by weight), 0.97 kg of Ca (NO₃) ₂ x 4H₂O in 4.36 kg of water. The resulting slurry was then added - again after cooling - a solution of 4.91 kg H₃PO₄ (55 wt .-% P₂O₅) in 0.46 kg water such that the temperature did not exceed 45 ° C. After the addition of phosphoric acid had ended, the mixture was slowly heated to 70-90 ° C. This temperature was maintained for 30 minutes to mature the activator. Then an aqueous solution of 216.67 g of Cu (NO₃) ₂ x 3H₂O was homogeneously distributed in the slurry and the slurry was dried. All mixing processes and the ripening process were carried out with stirring.

Activating agent 1a

An activating agent was described as described in the activating agent 1 process, but without the addition of copper nitrate.

Activation agent 2

To produce the activating agent, 50 kg of titanyl sulfate, 375 kg of NaOH (solid), 580 kg of phosphoric acid (55% by weight P₂O₅), 159 kg of Na₂CO₃ (solid) and 170 kg of water were mixed with kneading and then mixed with copper phosphate in such an amount that a copper concentration of 2% by weight resulted.

Activating agent 2a

An activating agent was produced which corresponded to activating agent 2 in terms of composition, but did not contain any copper phosphate.

Activation agent 3

Activating agent 1 was used as activating agent 3, to which maleic anhydride copolymer was added in such a way that the resulting activating bath contained 10 mg / l.

Activation agent 4

Activating agent 1, which was mixed with surfactant in such quantities that the surfactant concentration of 0.3 g / l was formed when the activation bath was prepared, also served as the starting product.

After a phosphating time of 3 minutes or 6 minutes, the degree of layer coverage was determined in percent. This value indicates the proportion of the metal surface that is provided with a closed zinc phosphate layer. The assessment was made visually. Furthermore, the layer weight of the phosphate coating was determined gravimetrically and the minimum phosphating times determined. The The minimum phosphating time is the minimum time required to form a closed phosphate layer. The crystallinity of the phosphate layers was checked using scanning electron microscope images at a magnification of 2000 times.

The results obtained with regard to the degree of coverage of the metal surface after 3 or 6 minutes of phosphating time, minimum phosphating time, layer weight and service life of the activation baths in days are summarized in the table below.

From the table above it follows in particular that the activating agent according to Examples 1, 2, 3 and 4 leads to a high, almost complete coverage of the metal surface after just 3 minutes of phosphating or enables short treatment times in the subsequent phosphating step. The weight of the phosphate layer is well within the usual range.

The results for the activating agents 3 and 4 indicate that the addition of maleic anhydride copolymer or surfactant leads to a considerable increase in the service life of the activating bath if the short minimum phosphating time is observed (cf. activating agent 1). The investigations with the scanning electron microscope showed that the phosphate layers obtained using the activating agents 1, 2, 3 and 4 were of fine-grained nature.

Claims (4)

1. A method for applying phosphate coatings to metal surfaces by activating with an activating agent on the basis of titanium IV phosphate and zinc-phosphating, characterised in that the metal surfaces are activated with an activating bath which contains
   0.001 to 0.060 g/l Ti
   0.020 to 1.2 g/l orthophosphate (calculated as P₂O₅)
   0.001 to 0.1 g/l Cu
   and a quantity of alkali such that it has a pH value of 7 to 11.
2. A method according to Claim 1, characterised in that the metal surfaces are activated with an activating bath which contains copper compounds introduced via copper hydroxide, hydrated copper oxide, copper tartrate, copper nitrate and/or copper phosphate.
3. A method according to Claim 1 or 2, characterised in that the activating bath has an additional content of at least one of the constituents

   - condensed phosphate (calculated as P₂O₅) in quantities of up to 1.2 g/l

   - silicate (calculated as SiO₂) in quantities of up to 0.5 g/l

   - complexing agent in quantities of up to 1.0 g/l

   - water-soluble organic polymer in quantities of up to 0.1 g/l

   - thickening agent in quantities of up to 0.1 g/l

   - surfactant in quantities of up to 0.3 g/l.
4. The application of the method according to one or more of Claims 1 to 3 to the activation of iron, steel, galvanised steel, zinc alloy-plated steel, aluminised steel and aluminium.