EP0154384A2 - Process for pretreating zinc surfaces before lacquering - Google Patents

Abstract

1. Process for preparing metal surfaces of zinc or zinc alloys for a succeeding application of paint, lacquer etc. by means of an alkaline cleaning and passivating by chemical conversion coating characterized in that the metal surfaces a) are cleaned and degreased with a solution being free of silicate and containing at least one cleaning agent of alkaline reaction, at least one surfactant, cations of at least one metal different from alkali metals ions and at least one complexing agent in a quantity sufficient to stabilize the metal cations b) are rinsed with water c) are contacted with a solution containing alkali, cations of at least one metal different from alkali metal ions and at least one complexing agent d) are rinsed with water and e) are subsequently treated with an acid passivating solution.

Description

The invention relates to a method for preparing metal surfaces made of zinc or zinc alloys for the subsequent application of paint, lacquer and the like. By means of alkaline cleaning and passivation through chemical conversion coatings.

It has long been known to produce a conversion coating on surfaces of zinc and zinc alloys as well as galvanized steel before the application of paint and the like. In particular, it is known from FR-PS 1 411 272 to use aqueous solutions to form conversion coatings which contain alkali, in general sodium hydroxide, cations of at least one metal other than alkali ions, preferably iron, cobalt and / or nickel, and at least one complexing agent contain. The amount of the complexing agent should be sufficient to keep the aforementioned cations in solution in the alkaline medium from which they would precipitate as metal hydroxide without the complexing agent. The most frequently used complexing agents for this are sodium gluconate and sodium glucoheptonate. The solutions for producing the chemical conversion coating are usually applied by spraying at a temperature between 40 and 70 ° C. for a period of about 3 to 15 seconds.

The industrial use of such aqueous alkaline Solutions for forming conversion coatings are associated with significant progress in terms of paint adhesion and corrosion resistance after painting, especially for strip and sheet metal. Industrial experience has also confirmed that it is interesting to subject the metal surface to a subsequent treatment with an aqueous, acidic solution in accordance with FR-PS 1 487 183 after the formation of a conversion coating in an alkaline medium and water rinsing. Such solutions contain 0.001 to 0.25% by weight of chromium (III), 0.01 to 0.06% by weight of fluoride and 0.02 to 0.1% by weight of zinc. They are used in spraying or dipping for 2 to 10 seconds at a temperature between 40 and 70 ° C. The surfaces are then dried immediately so that they can then be painted or painted.

The industrial use of these processes has shown the importance of the composition of the products used in the step of cleaning and degreasing the metal surface in the alkaline medium, which in turn before the steps of producing the chemical conversion coating and the after-treatment - as described above - must be carried out.

It is known from FR-PS 1 496 683 that an essential condition placed on the stage of alkaline degreasing is not to inhibit the formation of the conversion coating and therefore to work with silicate-free cleaning solutions. With regard to the requirements of the industry in the sense of a constant improvement in the quality of the products, however, even with a method of working in accordance with FR-PS 1 496 683, paint adhesion and corrosion protection are achieved, which are suitable for certain types of metal surfaces and in particular for hot-dip galvanized steel Acceptable limit.

The study of the various problems observed has also highlighted the adverse impact of certain additives to hot dip galvanizing baths on the metal surface / treatment solution interface at the time of surface preparation, chemical conversion coating formation, and post-treatment. This is particularly the case for certain metals, such as aluminum, tin and lead.

It is known that the accumulation of such contaminating metals on the surface of zinc or of galvanized parts interferes with the pickling reaction accompanying the treatment in the alkaline medium. An attempt has been made to remedy this by increasing the concentration of alkali and thus increasing the surface pickling attack on the zinc surface. However, the result was only an exacerbation of the deficiencies already identified, in particular with regard to the liability of the paint and the like.

The object of the present invention is to provide a method which avoids the disadvantages of the known, in particular the aforementioned methods, and which is associated with an improvement in the adhesion of paint, paint and the like and the corrosion resistance.

• a) mit einer silikatfreien Lösung reinigt und entfettet, die wenistens ein alkalisch reagierendes Reinigungsmittel, wenigstens ein Tensid, Kationen wenigstens eines von Alkaliionen verschiedenen Metalles und wenigstens einen Komplexbildner in einer Menge, die ausreicht, die Metallkationen in Lösung zu stabilisieren, enthält,
• b) mit Wasser spült,
• c) mit einer Lösung in Berührung bringt, die Alkali, Kationen von wenigstens einem von Alkaliionen verschiedenen Metall und mindestens einen Komplexbildner enthält,
• d) mit Wasser spült und
• e) anschließend mit einer sauren Passivierungslösung behandelt.

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 surface

• a) cleans and degreases with a silicate-free solution which contains at least one alkaline-reacting cleaning agent, at least one surfactant, cations of at least one metal other than alkali ions and at least one complexing agent in an amount sufficient to stabilize the metal cations in solution,
• b) rinsed with water,
• c) in contact with a solution which contains alkali, cations of at least one metal other than alkali ions and at least one complexing agent,
• d) rinsing with water and
• e) then treated with an acidic passivation solution.

The type and amount of the alkaline-reacting cleaning agent are preferably chosen such that the pH of the solution is 11.5 to 12.5.

The alkaline cleaning agents can be hydroxides, phosphates, carbonates and borates of alkali metals with the exception of silicate. These components are usually used in a concentration of 3 to 150 g / l, preferably 7 to 9 g / 1.

The surfactants can in principle be anion-active, non-ionic, cation-active or amphoteric. They must be compatible in the alkaline medium and under the conditions of use of the solution. Your choice essentially depends on the pH of the cleaning and degreasing agent, the application conditions, i.e. Dip or spray application, and economic considerations. Non-ionic surfactants are preferred for solutions used in the spraying process. Their concentration should be between 0.1 and 5 g / 1, preferably about 1 g / 1. Such surfactants can be used alone or in a mixture.

The metal ions other than alkali ions can be selected from those mentioned in French Pat. No. 1,411,272. These are in particular aluminum, antimony, Silver, cadmium, cerium, chrome, cobalt, tin, iron (II or III-valued), magnesium, manganese, molybdenum, nickel, titanium and tungsten. Cobalt, iron and nickel ions are preferred, especially nickel.

The anion assigned to the cation for introduction should be compatible with the solution and should not have any adverse effect. For this reason, nitrate is preferably used to introduce the metal ions other than alkali ions.

The concentration of the metal cations other than alkali ions in the solution depends on the concentration of the alkaline cleaning agent. The higher it is, the higher the concentration of cations should be. The concentration of cations other than alkali ions can be between 50 and 5000 mg / l, preferably between 100 and 1000 mg / l. With a concentration of the alkaline detergent of approx. 10 g / 1 and a pH of the order of 12, the preferred range of metal cations is 500 to 700 mg / 1.

At high concentrations of cations other than alkali ions, the improvement can practically not be greater than that which is usually obtained with the preferred concentration of 500 to 700 mg / l. Due to the high pH, the conditions of use and the high content of cations other than alkali ions, the solution is not entirely sludge-free, which is not harmful, however, if the amount of sludge is small, i.e. is in the order of 2 to 3% by volume, based on the entire solution.

The complexing agents used in the cleaning and degreasing solution can be the same as those described in FR-PS 1 411 272. In particular, it can be condensed phosphates, dicarboxylic acids, hydroxycarboxylic acids, Act benzol carboxylic acids, hydroxy aldehydes and amines. The best results in terms of degreasing the metal surface and complexing the metal ions can be achieved with sodium gluconate, sodium glucoheptonate, sorbitol and sodium ethylenediaminotetraacetate.

The concentration of complexing agent should be sufficient to stabilize the metals other than alkali ions, taking into account their concentration and the pH of the solution in the solution. The concentration can range between 0.1 and 30 g / l. It is preferably 0.5 to 1 g / l.

The aqueous alkaline solution for cleaning and degreasing the metal surface can be obtained by adding a solid concentrate which contains the alkaline-reacting cleaning agent or agents, the surfactant (s), the metal ions other than alkali ions and the coplexing agent or agents.

However, the solution can also be obtained if solid or liquid concentrates are used, one of which contains the alkaline detergent (s), the total amount or only part of the necessary surfactant (s) and the total amount or only part of the necessary complexing agent contains and the other of which contains the metal cations other than alkali ions, the total amount or only part of the necessary surfactant (s) and the total amount or only part of the necessary complexing agent.

In general, the cleaning and degreasing solutions described above can be sprayed or dipped at a temperature in the range from 20 to 90 ° C., preferably from 45 to 65 °, for a period of from about 3 to 180 seconds, preferably from 10 to 30 seconds. be applied.

Then the cleaned and degreased metal surface surface rinsed with water.

The metal surface is then brought into contact with a solution which contains alkali, cations of at least one metal other than alkali ions and at least one complexing agent. This can be, for example, a solution as described in FR-PS 1 411 272.

It is then rinsed again with water and treated with an acidic passivation solution. The passivation solution can contain, for example, chromium III and / or chromium IV and have the nature described in French Pat. No. 1,487,183.

Other types of passivation treatments can also be used to improve the adhesion of paint, varnish and the like, and to improve corrosion protection. For example, a conversion coating can be created by applying a dispersion of chromic acid and / or chromium compound with colloidal silicon dioxide.

The passivation solution applied for the passivation treatment is preferably dried. The metal surface treated in this way is then provided with paint, lacquer and the like.

The method according to the invention is suitable for the surface treatment of objects which consist of zinc or zinc alloys or a coating of zinc or zinc alloys, e.g. galvanized steel. It is particularly suitable for the treatment of sheet metal or strip.

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

example 1

Hot-dip galvanized steel sheets were used to carry out the tests.

The following solutions were used for cleaning and degreasing:

These solutions were sprayed with the metal surface under a pressure of 1.5. 10 ⁵ Pa brought into contact at 50 ° C for 10 sec.

• 26,6 g/1 Na₂CO₃
• 0,3 g/1 CO²⁺
• 0,3 g/1 Fe³⁺
• 1,8 g/1 Natriumglucoheptonat

in Kontakt gebracht. Dies geschah ebenfalls im Spritzen mit einem Druck von 1,5 . 10⁵ Pa während 10 sec, jedoch mit 65°C.The sheets were then rinsed with cold running water and containing a solution

• 26.6 g / 1 Na ₂ CO ₃
• 0.3 g / 1 CO ²⁺
• 0.3 g / 1 Fe ³⁺
• 1.8 g / 1 sodium glucoheptonate

brought into contact. This was also done by spraying at a pressure of 1.5. 10 ⁵ Pa for 10 sec, but at 65 ° C.

• 0,2 g/1 Fluorid
• 0,3 g/_{1 Zn} ²⁺
• 0,7 g/1 Cr³⁺
• 1,6 g/1 _Cr ⁶⁺

enthielt und im Spritzen während 5 sec bei ca. 20°C appliziert wurde. Danach wurden die Bleche durch Abquetschrollen von überschüssiger Behandlungslösung befreit und im Ofen getrocknet.After rinsing again with cold running water, a passivation treatment was carried out with a solution that

• 0.2 g / 1 fluoride
• 0.3 g / _{1 Zn} ²⁺
• 0.7 g / 1 Cr ³⁺
• 1.6 g / 1 _Cr ⁶⁺

contained and was applied in spraying for 5 sec at about 20 ° C. The metal sheets were then squeezed off to remove excess treatment solution and dried in the oven.

Finally, the sheets were coated with a coating of alkyd resin or acrylate resin with a thickness of approx. 30 microns. After drying, the sheets rested for 48 hours.

First, it was found that the sheets that had been treated in degreasing bath 2 looked different after the treatment in the chromic acid solution. They were dark gray and matt, whereas the other sheets had a practically colorless coating.

The influence of the nature of the cleaning and degreasing solution was then determined by bending around a conical mandrel. The sheets treated with solutions 3 to 5 (according to the invention) are initially superior to those which had been treated with solution 1 (according to the prior art), but also compared to the sheets treated with solution 2. Solution 2 had the same qualitative nature as solution 1 and differed only in the double concentration of the components in order to intensify the pickling effect. In this respect, solution 2 also agreed with solutions 3 to 5, which in turn differed in the content of metal cations other than alkali ions.

As a result, cracking in the lacquer and lacquer detachment increased from Examples 3, 4, 5 to 1, followed by 2.

The tests to determine the salt spray resistance according to AFNOR X 41 002 have shown after 1000 hours a clear advantage of the sheets treated with solutions 3, 4 and 5 compared to the sheets treated with solution 1 and even compared to the sheets treated with solution 2.

Example 2

Hot-dip galvanized steel sheets were treated according to the procedure in Example 1, but solutions of the following nature were used for cleaning and degreasing:

The varnishes applied by the roller application process were of the following type: polyvinyl chloride, polyester, silicone-modified polyester, polyvinylidene and polyacrylate.

There were conducted bending tests and salt spray tests and experiments to explore the surface by means of spectroscopy by _L umineszenzentladung. The bending tests have confirmed that the paint adhesion increases significantly when the Nickel concentration in the cleaning and degreasing solution increased. For example, when using solution 8, a value is obtained which is very interesting both from a technical and an economic point of view.

The results of spectroscopic analyzes showed that when using the cleaning and degreasing solution according to Examples 6, 7 and 8, a very uniform statistical distribution of the cobalt and iron in the conversion coating on the metal surface is obtained and that the surface concentration of aluminum and lead is reduced. Aluminum and lead are known to be interfering elements that impair the quality of the treated and painted metal product.

Claims (10)

1. A process for preparing metal surfaces made of zinc or zinc alloys for the subsequent application of paint, varnish and the like. By means of alkaline cleaning and passivation by chemical conversion coatings, characterized in that the metal surface a) cleans and degreases with a silicate-free solution which contains at least one alkaline-reacting cleaning agent, at least one surfactant, cations of at least one metal other than alkali ions and at least one complexing agent in an amount sufficient to stabilize the metal cations in solution, b) rinsed with water, c) in contact with a solution which contains alkali, cations of at least one metal other than alkali ions and at least one complexing agent, d) rinsing with water and e) then treated with an acidic passivation solution. 2. The method according to claim 1, characterized in that the metal surface is cleaned and degreased with a solution a) which has a pH of 11.5 to 12.5. 3. The method according to claim 1 or 2, characterized in that the metal surface is cleaned and degreased with a solution a) which contains cobalt, iron and / or nickel ions as metal cations other than alkali ions. 4. The method according to claim 1, 2 or 3, characterized in that the metal surface is cleaned and degreased with a solution a), the cations of at least one metal other than alkali ions in amounts of 50 to 5000 mg / l, preferably 100 to 1000 mg / l. 5. The method according to one or more of claims 1 to 4, characterized in that the metal surface is cleaned and degreased with a solution a) which contains sodium gluconate, sodium glucoheptonate, sorbitol and / or ethylenediaminetetraacetate as complexing agents. 6. The method according to one or more of claims 1 to 5, characterized in that the metal surface is cleaned and degreased with a solution a) containing the complexing agent in an amount of 0.1 to 30 g / l, preferably 0.5 to 1 g / l, contains. 7. The method according to one or more of claims 1 to 6, characterized in that the metal surface is cleaned and degreased with a solution a) which contains a nonionic surfactant. 8. The method according to one or more of claims 1 to 7, characterized in that the metal surface with a solution a) at a temperature of 20 to 90 ° C, preferably 45 to 65 ° C, for a period of 3 to 180 sec, preferably 10 to 30 sec, cleaned and degreased in immersion and / or spraying. 9. The method according to one or more of claims 1 to 8, characterized in that the metal surface is treated in step e) with a solution which contains a chromium compound, in particular chromate, and colloidal silica. 10. The method according to one or more of claims 1 to 9, characterized in that the solution applied in the treatment in step e) is dried.