Process of treating metallic surfaces

Details Process of treating metallic surfaces Process of treating metallic surfaces

1999-12-17

2002-08-20

Cameron, Erma (Department: 1762)

Coating processes

With post-treatment of coating or coating material

Solid treating member or material contacts coating

C427S388400, C427S409000, C427S421100, C427S435000

Reexamination Certificate

active

06436475

ABSTRACT:

This invention relates to a process of treating metallic surfaces which consist of zinc, magnesium or aluminium, or of the alloys of zinc, magnesium or aluminium, and to which lacquers, plastic coatings, paints, sealing compounds or adhesives are applied after the treatment.
It is known that the corrosion of polymer-coated metallic surfaces is due to electrochemical reactions at the metal/polymer phase boundary. In many coating techniques applied in the art, the metal surfaces are therefore coated with an inorganic conversion layer (e.g. zinc phosphate) before applying a lacquer or plastic coating. Due to the conversion treatment by means of a coat-forming phosphatizing or chromatizing process, the metallic surface is prepared for subsequently being coated with a lacquer or a plastic.
Even today, the conversion treatment of aluminium surfaces is effected by means of yellow chromatizing, where an acid chromate solution with a pH value of 1 to 2 is used, so that a protective layer is formed on the aluminium. The protective layer consists of an insoluble aluminium-chromium(III) mixed oxide and effects the high passivity of the surface against corrosion. The residual content of unused chromate ions left in the oxide layer in addition produces a self-healing effect in a damaged lacquer or plastic coating. However, the yellow chromatizing aluminium surfaces has the disadvantage that it only has insufficient adhesion-promoting properties with respect to a lacquer or plastic coating. Moreover, chromate ions are disadvantageously washed out of naturally weathered coatings.
As an alternative to chromatizing a process was developed which employs zirconium salts, fluorides, phosphates and organic polymers (e.g. polyacrylates and polyvinyl alcohols). By means of this process, coatings are formed on aluminium surfaces which provide the substrate with a certain protection against corrosion and a fairly good adhesion for lacquer and plastic coatings. However, the achieved protection against corrosion is not always satisfactory.
Moreover there was no lack of attempts at optimally fitting metallic surfaces of zinc, magnesium, aluminium and the alloys thereof for the application of lacquer and plastic coatings by treating such surfaces with organic substances.
From DE-A-3137525 there is known a process of inhibiting corrosion in an aqueous system, where the aqueous system contains at least one inorganic nitrite soluble in water, and at least one organic diphosphonic acid or at least one salt of diphosphonic acid. The diphosphonic acid is present in the aqueous system in a concentration of 0.1 to 20 ppm. The known aqueous system is used to avoid in particular corrosion problems in cooling systems. In the aqueous system, the hydroxyethylidene diphosphonic acid and the inorganic salts thereof are particularly preferred.
From EP-A-0 012 909 it is known that benzimidazolyl-2-alkane-phosphonic acids and the salts thereof have a pronounced corrosion-inhibiting effect and can be used as corrosion inhibitors. For inhibiting corrosion, they can be used singly, in combination with each other, or together with other known corrosion inhibitors. For inhibiting corrosion, the compounds are generally added to aqueous, aqueous-alcoholic, alcoholic and/or oil-containing media. They may for instance be used as corrosion inhibitors in heat-transfer media of cooling or heating circuits, in coolants, mineral oils or pickling inhibitors. By adding the compounds and/or the salts thereof to said media or circulating liquids the corrosion of metals, in particular of copper and its alloys, is prevented. The benzimidazolyl-2-alkane phosphonic acids contain a phosphonic acid group, a straight-chain or branched, saturated or unsaturated, bivalent, possibly substituted hydrocarbon residue with 1 to 15 carbon atoms, and a substituted benzimidazole residue, where the straight-chain or branched hydrocarbon residue and the benzimidazole residue are connected with each other via position 2 of the benzimidazole residue.
From U.S. Pat. No. 4,351,675 there is known an aqueous solution for the treatment of zinc, zinc alloys or cadmium, which contains nitric acid, an oxidizing agent (H
2
O
2
, nitrate, nitrite, chlorate) and a diphosphonic acid, where the two phosphonic acid groups are connected with each other via a carbon atom which in addition has a hydroxyl group and an alkyl residue with 1 to 4 carbon atoms.
From U.S. Pat. No. 5,059,258 there is finally known a process, where on an aluminium substrate with a pH value of 2 to 14 there is first of all produced a layer of aluminium hydroxide, and where then a further layer is deposited on the aluminium hydroxide layer by treatment with an organic phosphinic acid or an organic phosphonic acid. The organic residue of the phosphinic or phosphonic acid each contains 1 to 10 organic groups and 1 to 30 carbon atoms. The molecule of the organic phosphinic and phosphonic acids contains 1 to 10 phosphinic and phosphonic acid groups. For producing the hydroxide layer there is used an aqueous solution containing amines, amino alcohols, alkali hydroxides, alkaline earth hydroxides, alkali carbonates, alkali hydrogen carbonates or ammonia. For producing the second layer there is used a solution in which the phosphinic and phosphonic acids are present in a concentration of 0.001 mol/l up to the saturation concentration, and which as solvent contains water, an alcohol or an organic solvent. As organic groups, the phosphinic and phosphonic acids contain for instance aliphatic hydrocarbons, aromatic hydrocarbons, organic acids, aldehydes, ketones, amines, amides, thioamides, imides, lactams, anilines, piperidines, pyridines, carbohydrates, esters, lactones, ethers, alkenes, alcohols, nitrites, oximes, silicones, ureas, thioureas, perfluorated organic groups, silanes, and combinations of these groups. The second layer should act on the substrate in particular as a good adhesion promoter for lacquer and plastic coatings as well as for paints.
It is the object underlying the invention to create a process of treating metallic surfaces which consist of zinc, magnesium or aluminium, or of the alloys of zinc, magnesium or aluminium, in order to provide the metallic surfaces in particular with a good adhesion for lacquers, plastic coatings, paints, sealing compounds and adhesives, and to protect the metallic surfaces against corrosion.
The object underlying the invention is solved in that the metallic surfaces are treated at 10 to 100° C. by dipping, spraying or rolling with an aqueous solution which has a pH value of 2 to 13 and contains 10
−5
to 1 mol/l of one or more compounds of the type XYZ, where Y is an organic group with 2 to 50 carbon atoms and a straight-chain structure, where X is a COOH—, HSO
3
—, HSO
4
—, (OH)
2
PO—, (OH)
2
PO
2
—, (OH)(OR′)PO— or (OH)(OR′)PO
2
— group, where Z is an OH—, SH—, NH
2
—, NHR′—, CN—, CH═CH
2
—, OCN—, epoxide—, CH
2
═CR″—COO—, acrylamide—, COOH—, (OH)
2
PO—, (OH)
2
PO
2
—, (OH)(OR′)PO— or (OH)(OR′)PO
2
— group, where R′ is an alkyl group with 1 to 4 carbon atoms, where R″ is a hydrogen atom or an alkyl group with 1 to 4 carbon atoms, and where the groups X and Z are each bound to the group Y in their terminal position.
The effect of the inventive process is based on the ability of the Compounds XYZ to organize spontaneously and to form very thin, closed films on metallic surfaces, where there is in particular effected an orientation of the acid groups in the direction of the metallic surface, and between the metal hydroxide groups present on the metallic surface and the acid groups of the compound XYZ a chemical bond is formed. In accordance with the invention, the structure of the compounds XYZ was chosen such that there is obtained a reactive coupling of the thin film both to the metal surface and to the matrix of lacquers, plastic coatings, paints, sealing compounds and adhesives. The straight-chain organic group Y acts as “spacer” between the groups X and Z; it more or less provides the compound XY

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