Method for coating a metal surface with an ultra-fine layer

Abstract

The present invention relates to a method for continuously coating a substrate in motion such as a metal strip made of steel, the coating formed being an ultra-fine film of a thickness between 10 and 100 nm, deposited on the substrate:from a solution containing nanoparticles of oxides,in conditions of controlled pH,said substrate being at a temperature higher than 120° C.,the total duration of the deposition being less than 5 seconds and preferably less than 1 second,characterised in that at least one chemical additive, called a “refiner”, is incorporated into said solution, said refiner having, mutatis mutandis, the effect of restricting the formation of said coating.

Description

FIELD OF THE INVENTION[0001]The present invention relates to the improvement of the method described in international patent application WO-A-03 / 048403 by the use of chemical additives affecting the deposition reaction of an ultra-fine layer of oxide nanoparticles. The addition of such compounds allows to obtain layers of a thickness that is even less than in the above-mentioned patent application, i.e. of a thickness of typically less than 100 nm.TECHNOLOGICAL BACKGROUND AND THE STATE OF THE ART[0002]The method described in application WO-A-03 / 048403 A1 is part of a global project intended to reduce the production costs of pre-painted metal strips. In this context, the metallurgists hope to incorporate the lacquering process at the end of the galvanising line.[0003]The main difficulty to obtain this result has been to find a conversion treatment for the strip that is fast enough to be put between the galvanising and the painting treatment. The above-mentioned method has also been c...

AI Technical Summary

Benefits of technology

The present invention provides a method for coating a metal with an ultra-fine protective film of oxide, preferably silicon, titanium, zirconium, cerium, or antimony. The method allows for maximum flexibility in the entry temperature of the strip into the bath and guarantees reproducibility of the deposition in terms of thickness. The solution used for coating contains nanoparticles of oxides, and a refiner is added to prevent agglomeration in the solution. The refiner can be selected from a group of compounds such as catechin and its derivatives, hydrofluoric and boric acids, and ammonium hydroxide and amines. The pH of the solution is adjusted to allow for pickling of surface oxides and to make the particles as reactive as possible without destabilizing the solution. The method can be carried out by immersion, spraying, or roller application. The substrate can be a bare metal or a metal coated with another metal. The solution can be kept at a temperature of less than 100°C, and the substrate temperature should be higher than the melting point of the coating metal.

Problems solved by technology

The main difficulty to obtain this result has been to find a conversion treatment for the strip that is fast enough to be put between the galvanising and the painting treatment.

Thus, with the help of some modifications, the investment in the new equipment is limited.

Indeed, for a solution with a low concentration, the nanoparticles in suspension are some distance from each other and thus have little tendency to correctly agglomerate when the solvent evaporates.

However, one problem caused by the use of solutions with medium and high concentration is the formation of localised excessive thicknesses that form a network of very friable “ribs” on the surface of the oxide deposit, as shown in FIG. 1.

2.b) and it is detrimental to the subsequent adhesion of paint.

In fact, despite the presence of this equipment, it has been noted that the bath is adversely affected.

Another disadvantage of an “immersion deposit” treatment such as this in comparison with a cold method is that it is, in addition to being affected by a change in the temperature of the substrate, sensitive to a variation in the thickness of the strip.

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