EP0454211A1 - 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 an activation agent based on titanium IV phosphate for the activation of metal surfaces prior to zinc phosphating and its use for the preparation of activation baths.

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, relief 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, fluctuations in the application-technical effects are obtained 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 to activate cation-exchanging zeolites with a primary particle diameter of <3 μm 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 the addition of poly (aldehyde carboxylic acids) 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 µm), 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 additions 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 since they bring heavy metals into solution or keep them in solution.

The object of the invention is an activation agent based on titanium IV phosphate for the activation of To provide metal surfaces before zinc phosphating, which does not have the disadvantages of the known activating agents, is easy to manufacture and, when used in the preparation of activating baths, leads to stable activating 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 formulating the activating agent of the type mentioned at the outset in accordance with the invention in such a way that it contains one or more copper compounds and has a weight ratio of Ti: Cu = 1: 100 to 60: 1.

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.

Another advantageous embodiment of the invention consists in formulating an activating agent that contains an additional content of at least one of the components has condensed phosphate, silicate, complexing agent, water-soluble organic polymer, thickener and surfactant. The additional additives to the activating agent mentioned here provide a number of additional advantageous properties when used as an activating bath. 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.

According to a further advantageous embodiment of the invention, the activating agent contains 0.1 to 4% by weight of titanium phosphate (calculated as Ti).

The use of the activating agent for the preparation of aqueous activating baths for the activation of iron, steel, galvanized steel, galvanized alloy steel, aluminized steel and aluminum before the zinc phosphating takes place in such a way that activating baths with contents
0.001 to 0.060 g / l Ti
0.020 to 1.2 g / l o-phosphate (calculated as P₂O₅) and
0.001 to 0.1 g / l Cu
with a pH of 7 to 11, preferably 7.5 to 10, result.

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

• 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

resultiert.According to a further advantageous embodiment of the invention, an activating agent with an additional content of at least one of the components condensed phosphate, silicate, complexing agent, water-soluble organic polymer, thickening agent or surfactant is used in such a way that an activating bath with regard to the aforementioned components with contents of

• 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

results.

The activating agents can also be incorporated into ready-to-use, aqueous alkaline cleaning baths or else into aqueous-liquid or solid concentrates which are diluted with water to produce the cleaning baths. Because of its easy dilutability, the production of a liquid cleaner / activating agent is particularly advantageous.

For the preparation of the alkaline cleaners in solid or aqueous-liquid form, for example, one or more compounds from the group of carbonates, silicates, phosphates, borates, hydroxides, hydroxycarboxylic acids and organic polymers such as sodium bicarbonate (NaHCO₃), sodium carbonate (Na₂CO₃), sodium metasilicate anhydrous (Na₂SiO₃), sodium disilicate (Na₂Si₂O₅), sodium silicate, disodium phosphate (Na₂HPO₄), sodium tripolyphosphate (Na₅P₃O₁₂B), sodium oxoateon, sodium borate Na sodium borate Bor , the trisodium salt of nitrilotriacetic acid, phenolsulfonic acid or naphthalenesulfonic acid-formaldehyde condensation products or - because of the better solubility in water - the corresponding potassium compounds.

Alkaline cleaners or the alkaline cleaner concentrates usually contain surfactants. Particularly suitable surfactants are anionic or nonionic surfactants, such as Sodium alkylbenzenesulfonates, sodium alkylsulfonates, alkylphenol polyethylene glycol ethers, alkylphenol polyethylene glycol polypropylene glycol ethers, alkyl polyethylene glycol ethers, alkylamine-polyethylene glycol abducts or block copolymers of ethylene oxide and propylene oxide. In the case of the aqueous-liquid cleaner concentrate, the surfactant content is about 0.5 to 10%, preferably 0.5 to 4%.

In order to prevent sedimentation of possibly insoluble, coarsely dispersed portions of the activating agent and possibly floating out of salted-out surfactants in an aqueous-liquid activating cleaner concentrate, thickeners, preferably polymers of natural origin, should be added. Suitable polymers are, for example, polypeptides such as gelatin, or polysaccharides such as starch, xanthan or dextrins. When producing such concentrates, the polymer is expediently first completely dissolved in water and then the various detergent components. Then, if necessary, the surfactants are dissolved or finely dispersed with vigorous stirring, and the activating agent is added last. At A suitable composition of such a liquid, activating alkaline cleaner concentrate between 0 and 35 ° C is stable for several months and pumpable.

For reasons of the solubility of the individual components, the method of manufacture and the costs for packaging and transportation, it is particularly favorable to produce a concentrate with a water content of 50-90%, preferably 60-75%.

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 Vorspülung

1 g/l Aktivierungsmittel 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°;
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

strongly alkaline immersion cleaner 20 g / l; 10 min; 70 ° C

2. Rinse

cold water 30 sec

3. Clean

mildly alkaline immersion cleaner 13 g / l; 5 min; 60 ° C;

4. Rinse

cold water 30 sec.

5. Activating pre-rinse

Immerse 1 g / l activating agent 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 °;
Phosphating time 3 and 6 minutes of immersion

7. Rinse

cold water 30 sec

8. Drying

with warm air

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 according to the invention, 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 produced 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 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 phosphating baths in days are summarized in the table below.

From the table above it follows in particular that the activating agent according to the invention according to Examples 1, 2, 3 and 4 leads to a high, almost complete coverage of the metal surface after only 3 minutes of phosphating time 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 show that the addition of maleic anhydride copolymer or of surfactant, if the short minimum phosphating time (cf. activating agent 1) is observed, leads to a considerable increase in the service life of the activating bath. The investigations with the scanning electron microscope showed that the phosphate layers obtained using the activating agents according to the invention were of fine-grained nature.

Claims (7)

Activating agent based on titanium IV phosphate for the activation of metal surfaces prior to zinc phosphating, characterized in that it contains one or more copper compounds and has a weight ratio of Ti: Cu = 1: 100 to 60: 1. Activating agent according to claim 1, characterized in that it contains copper compounds introduced via copper hydroxide, copper oxide hydrate, copper tartrate, copper nitrate and / or copper phosphate. Activating agent according to claim 1 or 2, characterized in that it additionally contains at least one of the components of condensed phosphate, silicate, complexing agent, water-soluble organic polymer, thickener and surfactant. Activating agent according to Claim 1, 2 or 3, characterized in that it contains 0.1 to 4% by weight of titanium phosphate (calculated as Ti). Use of the activating agent according to claim 1, 2 or 4 for the preparation of aqueous activating baths for activating iron, steel, galvanized steel, galvanized alloy steel, aluminized steel and aluminum before the zinc phosphating with contents

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 so much alkali that it has a pH of 7 to 11, preferably 7.5 to 10. Use of the activating agent according to claim 3 for the preparation of activating baths according to claim 5 with an additional content of at least one of the components - condensed phosphate (calc. as P₂O₅)
in amounts up to 1.2 g / l - Silicate (calc. As SiO₂)
in quantities 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 - Surfactant in amounts up to 0.3 g / l. Use of the activating agent according to claim 1, 2, 3 or 4 with the proviso that it is incorporated into a ready-to-use alkaline cleaner or into a liquid or solid cleaner concentrate.