EP0295980B1 - Process for colouring metal surfaces of zinc or zinc alloys or of zinc plated surfaces - Google Patents

Description

The present invention relates to a process for coloring metallic surfaces of zinc, or of metals coated with zinc (galvanized, electro-galvanized parts, etc.). The colors obtained belong to the whole visible spectrum.

Zinc coloring processes have been known for a long time for obtaining several colors, of which black and brown are the most common. Most of the known techniques are today abandoned because they are unsuitable for profitable industrialization. In reality, according to C. BARNES, - The Coloration of Metals, Rev. Prog. Coloring, Vol. 14, p. 127 (1984) - the techniques which make it possible to color zinc concern galvanized or electro-galvanized objects. These coloring techniques are generally used to increase the corrosion resistance of zinc; they then appear rather as anti-corrosion protection techniques, the contribution of color not being initially the desired objective.

We can cite the variations made from the CRONACK-ANDERSON process, Proc. Am. Electroplater's Soc. Bow, 6 (1943) and ROPERT, Metal Finishing Journal, 14, N ° 165, p. 285 (1968) which, by chromating, make it possible to form on the surface of the zinc a layer which can take, as the case may be, the colors yellow, green, bronze or black.

It is also possible to form on the surface of the zinc a film of light blue color, resulting from optical interference, with a technique of chromation with less anticorrosive properties than those of the CRONACK process. This process is however only suitable for the identification by color of objects thus treated, since this film has a relatively short lifespan.

A well-known process which is currently used despite its high cost and its toxicity is that using black chromation with silver salts. We are starting to prefer processes, the most important of which is based on the action of trivalent chromium salts - BARNES, WARD and CARTER, Inst. Metal Finishing Annual Technical Conference (May 1982) - and which color the zinc with blue or yellow iridescence.

The gray color is obtained by phosphating.

Finally, zinc can be colored greenish black by anodization in a solution containing hydrofluoric, chromic and phosphoric acids, but this process is not used because of the high voltages necessary to form a conversion layer on the surface of the metal.

The method according to EP-A-0083263 consists in producing a black precipitate on the zinc face of a part by subjecting it to the action of an aqueous solution containing cupric ions, zinc, nickel and orthophosphoric ions. This document implements a process for phosphating zinc and carburizing cupric ions with zinc, leading to the formation of a black conversion layer on the surface of the zinc.

• ― protection contre la corrosion
• ― variété des couleurs
• ― facilité de mise en oeuvre
• ― pas de toxicité
• ― faible coût.

We can see that none of these surface treatments simultaneously offers the following advantages:

• - corrosion protection
• - variety of colors
• - ease of implementation
• - no toxicity
• - low cost.

The present invention achieves the above advantages.

In accordance with the invention, the process for coloring zinc-plated, zinc-plated and alloyed parts thereof, in which the part is subjected to the action of an acidic aqueous solution to make the surface of the part porous on a thickness which is a fraction of the thickness of zinc which it comprises is characterized in that a dye is adsorbed with an aqueous coloring solution by the porous layer, then in that the piece is adsorbed by the minus a sealing material.

Various other features of the invention will also emerge from the subclaims and from the detailed description which follows.

In what follows, it is indicated that the method relates to the coloring of zinc parts. It is specified that these are massive zinc parts, for example parts made from sheet metal or parts covered with zinc by any known process, for example by hot-dip galvanizing of parts of another metal, in particular of ferrous metal parts, the thickness of zinc resulting from hot galvanizing being of the order of 14 microns.

Electro-galvanized parts are included in the same way in the process of the invention, the zinc thickness of an electro-galvanized part being between 5 and 30 microns.

According to a preferred method of the invention, the zinc part previously shaped or made up of a sheet intended to be shaped later is subjected to the action of a bath consisting of a mixture of acids in solution in the water also containing halide ions, in particular fluoride ions.

It is therefore advantageous to use acids such as nitric acid and acetic acid. A porous layer slightly grayer in color than the initial zinc color forms on the surface, highlighting the moiré or so-called "flower" appearance that zinc usually presents. It is also advantageous to use at least one of the salts of these acids (eg KNO₃, KOOCCH₃, etc.).

For example, a satisfactory treatment is carried out by preparing, for the first step of the process, an aqueous solution containing 10% by volume of nitric acid, 5% by volume of acetic acid and 2% by weight of fluoride of sodium.

According to a development of the invention, when the zinc part has been subjected to the acid treatment described above and then rinsed with water, it is transferred, for the second stage of the process, to an aqueous coloring bath or else it is sprayed with a coloring solution.

The aqueous coloring solution preferably contains organic dyes such as azo or anthraquinone dyes or dyes based on phthalocyanine, these dyes being preferred because of their purity and also because they are very soluble in water. .

Although organic dyes are preferred, it is nevertheless possible to use certain inorganic dyes, in particular ferric sodium or ammonium oxalate.

After application of the dye, the zinc part is subjected, for the third step of the process, to the action of sealing agents adsorbed in the porous layer created and covering it.

These agents are preferably constituted by driers or by synthetic resins, in particular polyurethanes and / or melamines. It is also possible to use sealants based on silicones, in particular when the part must subsequently undergo forming operations, in particular by folding and / or elongation.

In order to avoid deterioration of the dye or dyes when they are used, it is advantageous for the parts leaving the acid bath to be rinsed with demineralized water, for example by spraying.

We now describe various examples of implementation of the invention. Note however that Example 1 does not illustrate all the features of the invention.

EXAMPLE 1

In this example, a sheet of pure zinc 6/10 mm thick was treated which was only dusted without being subjected either to degreasing or to chemical pickling.

• 10% en volume d'acide nitrique à 65%
• 5% en volume d'acide acétique à 85%
• 20 g/l de fluorure de sodium.

It was immersed for 3 minutes in an aqueous bath containing:

• 10% by volume of 65% nitric acid
• 5% by volume of 85% acetic acid
• 20 g / l of sodium fluoride.

Before immersion of the part, the bath was stirred and kept at a temperature of 20 ° C. During the immersion time, the agitation was interrupted. The treated sheet was then extracted and rinsed with demineralized water sprayed in the form of a mist.

This first phase of treatment highlighted the "flower" or moire of zinc, the appearance of which presented a slightly stronger gray than before the treatment.

After drying, melamine in solution in ethylene glycol was sprayed on the sheet with a pneumatic gun and the latter was finally kept in an oven at 140 ° C for 25 minutes.

The gray color obtained initially does not undergo any modification during subsequent aging tests, in particular in a humid atmosphere.

EXAMPLE 2

In this example, a hot-dip galvanized steel sheet was used, the galvanization having a thickness of 14 microns.

• 14% en poids d'acide nitrique à 65%,
• 5% en poids d'acide acétique à 85%,
• 20 g/l de fluorure de sodium.

The simply dusted sheet was immersed in a bath containing:

• 14% by weight of 65% nitric acid,
• 5% by weight of 85% acetic acid,
• 20 g / l of sodium fluoride.

Immersion lasted 1 minute in the bath brought to a temperature of 20 ° C.

At the end of the bath, the sheet was rinsed with demineralized water and then, before it dried, was then immersed for 5 minutes in a coloring bath consisting of an aqueous solution containing an azo dye Green Aluminum LWN at a concentration of 4g / l. The temperature of the coloring bath was 50 ° C.

After treatment, the sheet was rinsed with ordinary water and then dried in the open air.

Melamine in solution in ethylene glycol was then sprayed onto the sheet using an electrostatic gun and this was placed for 25 minutes in an oven brought to a temperature of 140 ° C. The polymer thus applied to the sheet was colorless.

The green color conferred on the sheet does not undergo variations after accelerated aging tests corresponding to a usage time of 2 years in a humid atmosphere.

EXAMPLE 3

The same operations as those described in Example 2 were carried out but the melamine sealing treatment was replaced by the application of a colorless polyurethane resin, the polymerization of which was initiated by baking at 80 ° C. for 30 minutes.

EXAMPLE 4

An electrogalvanized mild steel sheet was treated, the sheet having a thickness of 8/10 mm and the thickness of deposited zinc being 10 microns.

• 150 g/l de nitrate de potassium,
• 5% en poids d'acide acétique à 85%,
• 20 g/l de fluorure de sodium

As in the previous examples, the sheet was only dusted and then immersed in a bath containing:

• 150 g / l of potassium nitrate,
• 5% by weight of 85% acetic acid,
• 20 g / l sodium fluoride

The immersion lasted 3 minutes in the bath brought to a temperature of 20 ° C. The sheet was then rinsed by spraying demineralized water, which highlighted the "flower" of zinc appearing through a porous layer of slightly gray color.

After the acid bath treatment, the sheet was immersed in an aqueous bath containing a non-metallized anthraquinone dye supplied by the company Sandoz under the name Sanodal, the concentration of the dye in the bath brought to the temperature of 45 ° C. being 6 g. / l.

After drying, the colored sheet was covered with a colorless polyurethane varnish.

The invention is not limited to the embodiments described in detail since various modifications can be made thereto without departing from its scope. In particular, the polymerization of the synthetic resins constituting the sealing product can in a known manner be initiated and / or accelerated by the action of ultraviolet, infrared, high frequency or microwave radiation.

Claims (10)

1. Method for colouration of parts made of zinc, coated with zinc and made of an alloy of this metal, in which the part is subjected to action of an acid aqueous solution for causing surface of the part to be porous over a thickness which is a fraction of thickness of the zinc that it includes, characterized in that the porous layer is caused to adsorb a colouring agent of an aqueous colouring solution, then in that the part is caused to adsorb at least one sealing arterial.

2. Method according to claim 1, characterized in that the acid aqueous solution contains halogen ions (F⁻, Cl⁻, I⁻,...)

3. Method according to one of claims 1 or 2, characterized in that the acid aqueous solution has a temperature close to room temperature and contains nitric acid and acetic acid.

4. Method according to one of claims 1 to 3, characterized in that the acid aqueous solution has a temperature close to room temperature and contains the nitrate ion or another anion of a nitrogen oxyacid or a peroxyde, the fluorine ion or another halogenated ion alone or tied to oxygen, the acetate ion or another carboxylate ion.

5. Method according to one of claims 1 to 4, characterized in that the coloring aqueous solution contains at least one coloring agent and is at a temperature comprised between room temperature and 60°C.

6. Method according to one of claims 1 to 5, characterized in that after treatment in the acid bath, the part is subjected to flushing before adsorbing to colouring agent.

7. Method according to one of claims 1 to 6, characterized in that the coloring agents are organic coloring agents, more particularly metalliferous coloring agents and in particular azo, anthraquinonic or phtalocyanine based coloring agents.

8. Method according to one of claims 1 to 7, characterized in that the coloring agents are mineral coloring agents in particular the sodium or ammonium ferric oxalate.

9. Method according to one of claims 1 to 8, characterized in that the parts are subjected to action of the acid aqueous solution, then to the coloring aqueous solution, either by dipping the parts in the solutions, or by spraying the solutions on the parts.

10. Method according to one of claims 1 to 9, characterized in that the acid concentration of the aqueous acid solution, as well as the treatment duration, are chosen so as to provide a porous layer the thickness of which is in the order of one micron.