Iron phosphating using substituted monocarboxylic acids

Details Iron phosphating using substituted monocarboxylic acids Iron phosphating using substituted monocarboxylic acids

1997-09-09

1999-07-06

Sheehan, John

Metal treatment

Process of modifying or maintaining internal physical...

Processes of coating utilizing a reactive composition which...

148247, 148253, C23C 2207

Patent

active

059193182

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a new phosphating solution for the so-called "non-coating" phosphating of reactive metal surfaces, more particularly surfaces of steel, aluminum, zinc or alloys of which the main component is at least one of the metals iron, aluminum or zinc. In "non-coating" phosphating, the metal surfaces are treated with acidic solutions (pH range 3.5 to 6) of phosphates which results in the formation on the metal surface of a coating of phosphates and/or oxides of which the cations emanate from the metal surface and not from other components of the phosphating bath. This distinguishes "non-coating" iron phosphating from "coating-forming" zinc phosphating in which the cations of the phosphating bath are incorporated in the phosphate coating. Processes for iron phosphating are known from the prior art. They are used, for example, as a pretreatment before painting in cases where the surfaces in question are not expected to be exposed to significant corrosive influences.


STATEMENT OF RELATED ART

To meet corrosion control requirements, it is desirable that the iron phosphate coatings have a weight per unit area (coating weight) of more than about 0.2 g/m.sup.2. In principle, the corrosion-inhibiting effect increases with increasing coating weight. However, with relatively high coating weights, for example above about 0.8 g/m.sup.2, the coatings are in danger of becoming powdery and not adhering firmly to the metal surface. This leads to unacceptably poor paint adhesion. Accordingly, efforts have been made to produce iron phosphate coatings which, on the one hand, have a high coating weight, for example of about 0.5 to about 1 g/m.sup.2, the coatings at the same time being intended to form firmly adhering coatings.
It is known that coating formation is influenced to a considerable extent by the presence of so-called "accelerators". Accelerators are inorganic or organic substances with an oxidizing effect and, occasionally, with a reducing effect. Inorganic accelerators are, for example, nitrates, chlorates, bromates, molybdates and tungstates. Known organic accelerators are aromatic nitro compounds such as, for example, nitrobenzene sulfonic acid, more particularly m-nitrobenzene sulfonic acid ("NBA"). One example of an inorganic substance with more of a reducing effect and good accelerator properties is hydroxylamine and its salts. Phosphating baths containing such accelerator systems are known, for example, from U.S. Pat. No. 5,137,589 and from WO 93/09266. According to the second of these documents, particularly good coatings are obtained where oxidizing and reducing accelerators are combined with one another, in the present case for example hydroxylamine with organic nitro compounds, with molybdates or tungstates.
Relatively thin coatings (0.2 to 0.5 g/m.sup.2), generally with a bluish iridescence, are obtained when a molybdate accelerator is used. With organic accelerators, it is possible to obtain thicker coatings up to 1 g/m.sup.2 which generally afford significantly better protection against corrosion in the form of creeping rust. Phosphate coatings with a weight of more than 0.5 g/m.sup.2 are produced by thick-coating iron phosphating while phosphate coatings with a weight of less than 0.5 g/m.sup.2 are produced by thin-coating iron phosphating.
It is also known that the formation of iron phosphate coatings is favorably influenced by the presence in the phosphating solution of chelating complexing agents for iron. According to U.S. Pat. No. 5,137,589, gluconic acid is particularly suitable for this purpose. In addition, CA 874,944 recommends the use of ethylenediamine tetraacetic acid, nitrilotriacetic acid, diethylenetriamine pentaacetic acid, citric acid, tartaric acid and glucoheptonic acid. One feature common to the complexing agents mentioned is that they represent chelating carboxylic acids containing at least 4 carbon atoms and at least 3 substituents selected from carboxyl and hydroxy groups.
One of the requirements modern iron phosphating baths are expect

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