EP0459549B1 - Production of conversion coatings on zinc or zinc alloy surfaces - Google Patents

Description

The invention relates to a method for producing conversion coatings on surfaces of zinc or zinc alloys, in which in a first step the surfaces are brought into contact with a solution which contains at least two different polyvalent metal ions and complexing agents in such an amount that the polyvalent metal ions are kept in solution, and has a pH ≧ 11, and in which rinsing is carried out in a subsequent step with a rinsing solution, and its use as a pretreatment for subsequent painting, film coating or adhesive coating.

Such a method for producing a conversion layer is known in particular as a pretreatment before painting and film coating from DE-C-1 521 854.

For the formation of layers on the zinc or zinc alloy surface, aqueous alkaline solutions are used which act as so-called non-alkali metal ions of one or more of the metals silver, magnesium, cadmium, aluminum, tin, titanium, antimony, molybdenum, chromium, cerium, tungsten, manganese, cobalt , Iron and nickel included. Particularly suitable solutions are those which contain ions of iron or cobalt with another of the metals mentioned as non-alkali metal ions. In addition, the solutions contain sufficient organic complexing agents to keep the non-alkali metal ions in solution. The conversion coatings produced by means of these ions increase the corrosion resistance and improve the adhesion of subsequently applied organic coatings.

Corrosion resistance and adhesion are further improved if the surfaces are rinsed with an acidic, hexavalent chromium and possibly additionally trivalent chromium solution after the conversion layer has been produced. (DE-C-1 521 854)

Although this known method shows good results in terms of corrosion protection and paint adhesion, the use of trivalent and in particular hexavalent chromium ions in the passivating rinse solution is very disadvantageous because of the toxicity and the need for special disposal of the hexavalent chromium (chromate detoxification).

It is also known to treat metal surfaces, in particular made of steel, aluminum or zinc, and alloys thereof, which may have a conversion coating, with an aqueous solution which contains reactive organosilane and additional titanium and / or zirconium compound (EP-A-153 973). Concrete coatings are zinc or iron phosphate layers or chromate layers, in the illustration of the prior art u. a. also called metal oxide coatings.

According to EP-A-161 667, metal surfaces made of iron or steel are to be treated with solutions containing aluminum ions, up to 200 ppm fluoride ions and preferably additionally up to 1000 ppm in order to avoid rust spots during drying and until they are finally protected by painting or another type of surface treatment Ions contain at least one of the metals titanium, zirconium and hafnium.

The object of the invention is to provide a method for producing conversion coatings on surfaces made of zinc or zinc alloys, which avoids the disadvantages of the method mentioned and in particular does not or only to a very small extent burden the environment and is at least the same in terms of corrosion protection and paint adhesion behaves well.

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 rinsing is carried out with a rinsing solution which contains aluminum, zirconium and fluoride with a molar ratio of Al: Zr: F of (0.15 to 2.0): 1: (5 to 16) and whose pH is set to 2 to 5.

Rinse solutions containing aluminum fluorozirconate with a molar ratio of Al: Zr: F of (0.15 to 0.67): 1: (5 to 7) and a total concentration of Al + Zr + F of 0.1 to 2 g are admittedly included / l have, described in EP-A1-410 497. However, they are used for the passivative rinsing of phosphate layers applied to metal surfaces.

The method according to the invention is suitable for all surfaces which contain zinc or zinc alloys, such as, for example, materials made of solid zinc or solid zinc alloys, but also for those whose surface is electrolytically galvanized or alloy-galvanized by deposition from the gas phase or in the hot-dip process has been. Aluminum, silicon, lead, iron, nickel, cobalt and manganese are particularly suitable alloying partners for zinc. In the case of flat workpieces, both one-sided and two-sided galvanizing or alloy galvanizing can be used.

The surfaces should be bare and as free of grease as possible. If necessary, they are cleaned alkaline, neutral or acidic before the conversion treatment and then expediently rinsed with water.

If the zinc or zinc alloy surface is only slightly greased or soiled, the surface may not need to be cleaned and degreased beforehand. Instead, in these cases, by adding surfactants to the treatment in the first stage, cleaning and degreasing are carried out using the same treatment solution that is used to produce the conversion coating. The main advantage of this embodiment is that the entire pretreatment of the surface can be carried out in fewer steps, since the separate cleaning and the associated water rinsing are eliminated.

The application of the alkaline solution used in the first stage can e.g. in spraying, dipping or flooding.

Particularly suitable alkaline solutions have been found to be those containing iron (III) ions and additionally cobalt and / or nickel and / or chromium (III) and / or aluminum ions, the total content of which was between 0.3 and 3 g / l, preferably between 0.4 and 1.2 g / l. The polyvalent metal ions can be used as salts of inorganic acids, for example nitric acid, or as salts of organic acids, for example formic acid, in particular also Acetic acid can be used. Salts of organic acids, which can simultaneously serve as complexing agents, are also suitable. Amphoteric metals, such as aluminum, can also be dissolved in the form of the hydroxyl complex without an additional anion or complexing agent.

As a result of a pickling process during the treatment in the first stage, further polyvalent cations, which are present in the surface to be treated and are not present in the freshly prepared bath solution, can get into the treatment solution from the zinc or zinc alloy surface. This is e.g. around zinc, aluminum and lead on hot-dip galvanized surfaces. The total concentration of these cations can increase up to a few g / l. The formation of the conversion coating is generally not disturbed by this.

In particular, organic chelating agents of various types can be used as complexing agents: for example dicarboxylic acids (malonic acid, fumaric acid etc.); Amino acids (eg glycine); Hydroxycarboxylic acids (e.g. citric acid, gluconic acid, lactic acid); 1,3-diketones (e.g. acetylacetone); aliphatic polyalcohols (e.g. sorbitol, 1,2-ethanediol); aromatic carboxylic acids (eg salicylic acid, phthalic acid); Aminocarboxylic acid (e.g. ethylenediaminetetraacetic acid). Other complexing agents, such as methanephosphonic acid diethanolamide, can also be used. The solution must contain at least such an amount of complexing agent that the multivalent metal ions present can be bound in a completely complex manner. Thus, if the polyvalent metal ion content in the solution increases, the complexing agent content must also be increased. Since increasing amounts of certain complexing agents, which are acidic in nature, can reduce the alkalinity of the solution, complexing agents are preferably used in Form of neutral salts, especially the alkali metal salts used. It has been found that excess amounts of complexing agents are of no benefit.

Particularly favorable results are achieved if salts of gluconic acid, but especially hexahydroxyheptanoic acid, are used as complexing agents. The content of complexing agent in the solution should be between 0.05 and 10 g / l, in most applications between 1.5 and 5.5 g / l (based on the sodium salt of hexahydroxyheptanoic acid).

The aqueous solution must have a pH ≧ 11. The best results are achieved in the pH range from 12.2 to 13.3. The adjustment of the pH value can e.g. by triethanolamine, alkali metal hydroxides, alkali metal carbonates, alkali metal phosphates, alkali metal polyphosphates, alkali metal pyrophosphates, alkali metal borates, alkali metal silicates or mixtures thereof. However, the most advantageous are alkali metal hydroxides, especially sodium hydroxide.

The temperature of the solution of the first stage can in principle be between 20 ° C and 90 ° C. The preferred temperature range is 45 to 65 ° C.

The treatment time is usually 2 to 60 seconds, preferably 5 to 30 seconds. It depends, among other things, on the application technology. For example, the duration of treatment in spraying is shorter than in diving under otherwise the same circumstances.

In general, solutions with a lower metal ion concentration require higher temperatures and longer treatment times than those with a higher metal ion concentration.

After generating a conversion coating, excess treatment solution should be removed as far as possible from the zinc or zinc alloy surface. This can e.g. by draining, squeezing, blowing off or rinsing with water or with an aqueous solution which has been acidified, for example with an inorganic or organic acid (hydrofluoric acid, boric acid, nitric acid, formic acid, acetic acid etc.).

The application of the rinse solution can e.g. by dipping, flooding, spraying or rolling.

A preferred embodiment of the invention consists in rinsing with a solution which contains aluminum, zirconium and fluoride in a total concentration Al + Zr + F between 0.1 and 8 g / l, preferably between 0.2 and 5 g / l. The molar ratios Al: Zr: F should advantageously be set to (0.15 to 0.67): 1: (5 to 7).

The rinsing solutions used in the process according to the invention contain, inter alia, acidic aluminum fluorozirconates and, in the case of an excess of aluminum, other salts of aluminum (for example fluorides Tetrafluoroborates, nitrates). They can be produced, for example, by first dissolving metallic zirconium or zirconium carbonate in aqueous hydrofluoric acid, forming complex fluorozirconic acid. Then metallic aluminum or aluminum hydroxide or an aluminum salt, for example nitrate, fluoride, tetrafluoroborate, formate, acetate, preferably in dissolved form, is added and, if necessary, dissolved. A possible slight clouding of the solution does not impair the effectiveness. Although the described manufacturing route is preferred, the solutions can also be prepared in other ways.

The pH of the solution is preferably adjusted with cations of volatile bases. These include in particular ammonium, ethanolammonium and di- and tri-ethanolammonium. When setting in particular higher pH values in the specified pH range and at higher concentrations in the specified range of the total concentration Al + Zr + F, the solution may become cloudy, which has no negative effect on the effectiveness of the process.

According to a further advantageous embodiment of the invention, rinsing is carried out with an aqueous solution which additionally contains at least one of the anions benzoate, caprylate, ethylhexoate, salicylate in a total concentration of preferably 0.05 to 0.5 g / l. This particularly increases the protection against bright corrosion. The anions can be introduced via the corresponding acids or their salts.

The duration of the application of the rinse solution is generally between about 1 and 120 seconds, according to a further expedient embodiment of the invention between 1 and 30 seconds. The application temperature can be between 20 ° C and about 80 ° C. Temperatures between 20 and 50 ° C are preferred.

Demineralized or low-salt water is preferably used to prepare the rinse bath. Water with a high salt content is less suitable for bath preparation.

After the passivating rinse, the surface can e.g. air-dried or oven-dried, if necessary rinsed with deionized water beforehand. An advantageous embodiment of the invention provides that the surface is accelerated after the passivative rinsing, e.g. dry with hot air or infrared radiation.

The method according to the invention primarily serves as preparation of the zinc or zinc alloy surfaces before painting, film coating or the application of adhesives. It increases the adhesion of the organic films to the metallic substrate, improves their resistance to blistering when exposed to corrosion and inhibits the corrosion penetration from damage points in the film.

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

Example:

Cleaned and degreased sheets of hot-dip galvanized steel were immersed in a solution for 30 seconds to produce a conversion coating, the temperature of which was 55 ° C. and had the following composition:

The sheets were then rinsed with water and rinsed passively. For this purpose, the sheets were immersed in the rinse solution for 5 seconds and then excess solution was removed by squeezing. After a drying time of 0.5 min in a forced air oven at 75 ° C, the pretreated sheets were coated with an epoxy primer and an acrylate top coat. The layer thickness of the total varnish was approx. 25 µm.

The treated sheets were then subjected to the following tests:

The paint adhesion was determined in the T-Bend test, the sheets being bent through 180 ° and the various radii of curvature being stated as n times the sheet thickness (n = 0.1.2 ...) (Tn). The percentage of the chipped paint surface of the total curved surface is given as the tested size in%.

On further treated sheet metal, cracks were made with a sheet metal needle down to the metal surface and a cutting edge with sheet metal scissors. The sheets were then subjected to the salt spray test according to DIN 50021 SS for 1008 hours. The paint infiltration (mm) that starts from the scratch or the cut edge is specified as the tested size.

The rinsing solutions used were diluted 1.6 g / l (rinsing solution A) or 20 g / l (rinsing solution B) of an aqueous concentrate with 0.855% by weight Al and 8.62% by weight Zr and 10 , 7 wt .-% F using deionized water. The pH in both solutions had been adjusted to approximately 3.6 with ammonia.

A Cr (VI) / Cr (III) -containing rinsing solution (rinsing solution C) with a pH of approx. 3.3 was used for comparison purposes.

The composition of the rinse solutions was:

Rinse A:

Al

0.014 g / l

Zr

0.14 g / l

F

0.17 g / l

NH₄

0.016 g / l

Rinse solution B:

Al

0.17 g / l

Zr

1.72 g / l

F

2.14 g / l

NH₄

0.40 g / l

Rinse solution C: (BE-A-684954)

Cr⁶⁺

2.0 g / l

Cr³⁺

0.8 g / l

F

0.2 g / l

Zn

0.3 g / l

The test results are given in the tables below.

A comparison of the table values shows that the method according to the invention in any case provided at least as good or better values as the comparison method also tested with a rinsing solution based on Cr (VI) / Cr (III).

Claims (12)

1. A method for producing conversion coatings on surfaces of zinc or zinc alloys, in which, in a first stage, the surfaces are brought into contact with a solution which has a content of at least two different polyvalent metal ions and of complexing agent in such a quantity that the polyvalent metal ions are kept in solution, and has a pH value of ≧ 11, and in which in a following stage rinsing with a post-rinsing solution is effected, characterised in that rinsing is effected with a post-rinsing solution which has a content of aluminium, zirconium and fluoride having a molar ratio of Al : Zr : F of (0.15 to 2.0) : 1 : (5 to 16) and the pH value of which is set to 2 to 5.
2. A method according to Claim 1, characterised in that rinsing is effected with a post-rinsing solution which has a total concentration of Al + Zr + F of between 0.1 and 8.0 g/l.
3. A method according to Claim 1 or 2, characterised in that rinsing is effected with a post-rinsing solution which has a total concentration of Al + Zr + F of between 0.2 and 5.0 g/l.
4. A method according to Claim 1, 2 or 3, characterised in that rinsing is effected with a post-rinsing solution in which the molar ratios Al : Z : F are set to (0.15 to 0.67) : 1 : (5 to 7).
5. A method according to one or more of Claims 1 to 4, characterised in that rinsing is effected with a post-rinsing solution, the pH value of which is set with cations of volatile bases.
6. A method according to Claim 5, characterised in that rinsing is effected with a post-rinsing solution, the pH value of which is set with ammonium, ethanolammonium, di-ethanolammonium or tri-ethanolammonium compounds.
7. A method according to one or more of Claims 1 to 6, characterised in that rinsing is effected with a post-rinsing solution which additionally contains at least one of the anions benzoate, caprylate, ethyl hexoate or salicylate in a total concentration of 0.05 to 0.5 g/l.
8. A method according to one or more of Claims 1 to 7, characterised in that rinsing is effected with the post-rinsing solution for 1 to 30 seconds.
9. A method according to one or more of Claims 1 to 8, characterised in that rinsing is effected with a post-rinsing solution which has a temperature of 20 to 80°C.
10. A method according to one or more of Claims 1 to 9, characterised in that rinsing is effected with a post-rinsing solution which has a temperature of 20 to 50°C.
11. A method according to one or more of Claims 1 to 10, characterised in that the surface is dried after the post-rinsing operation.
12. The application of the method according to one or more of Claims 1 to 11 as a preliminary treatment for a subsequent painting, film-coating or adhesive-coating operation.