Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1993-04-07
1994-11-22
Silverberg, Sam
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148267, C23C 2237
Patent
active
053665678
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a chromate treatment method which can produce a strongly corrosion-resistant, alkali resistant, and weld-tolerant chromate film, with excellent paint film adherence and corrosion resistance after painting, on the surface of electrogalvanized steel, zinc alloy electroplated steel, galvannealed hot dip galvanized steel, or any other type of iron or steel with a surface coating that is predominantly zinc, all of these various types of coated steel being encompassed within the term "zinc coated steel" as used herein. The method according to the invention is particularly adapted to coating sheet stock.
BACKGROUND ART
While older chromate treatment baths consisted simply of aqueous solutions of chromic acid or dichromic acid, in recent years various improved methods have been proposed in which the chromate treatment bath lays down a film which is only slightly soluble in acid or alkaline treatment liquid compositions which may follow chromate film formation. Examples of this relatively recent art will be considered below.
The teaching of Japanese Patent Application Laid Open [Kokai or Unexamined] Number 50-158,535 [158,535/75] concerns a method for the formation of a slightly soluble chromate film on the surface of zinc coated steel sheet. A chromate bath is disclosed which is based on chromic anhydride (CrO.sub.3)+phosphoric acid (H.sub.3 PO.sub.4)+water soluble or water dispersible polymeric compound. At least 70% of the hexavalent chromium ion in this treatment bath is reduced by a reductant such as ethylene glycol or the like. However, since the chromate films formed according to the examples of this invention contain polymer, they suffer from a poor weldability although they are excellent with regard to lack of solubility, corrosion resistance, and adhesion to paint and corrosion resistance after painting (the last two characteristics being sometimes briefly denoted hereinafter as "coatability").
The chromate bath disclosed in Japanese Patent Publication Number 61-58522 [58,522/86] is a chromic acid (CrO.sub.3) +chromic acid reduction product+silica sol system. The major disadvantage with the method according to this invention is the tendency for the chromium, chiefly the hexavalent chromium, in the chromate film to elute during the alkaline rinse which is carried out after chromating but before the treated steel sheet carrying the chromate film is painted. This results in a decline in the film's corrosion resistance.
Japanese Patent Application Laid Open Numbers 58-22383 [22,383/83] and 62-83478 [83,478/87] disclose the use silane coupling agent in order to reduce the hexavalent chromium ion in the chromate treatment bath. Each of the films formed by the methods according to these inventions provides an excellent paint-film adherence. However, the chromate film produced by the method of the first invention has a poor alkali resistance. The alkali resistance is similarly unsatisfactory in the case of the method according to the second invention.
DESCRIPTION OF THE INVENTION
Problem to Be Solved by the Invention
The present invention seeks to solve the various problems associated with the prior art by introducing a method for the chromate treatment of zinc coated steel sheet which produces a strongly corrosion resistant, alkali resistant, and weld tolerant chromate film which also has good coatability.
SUMMARY OF THE INVENTION
The present invention comprises a method for the chromate treatment of zinc coated steel that comprises and is characterized by steps of:
(1) providing an aqueous liquid chromate containing composition made by substeps (1.1)-(1.2) or by substeps (1.1')-(1.3'), where substeps (1.1)-(1.2) are: or more preferably consists essentially of, or still more preferably consists of, water and: to 50.0 grams per liter (hereinafter "g/L") of dissolved hexavalent chromium; of trivalent chromium: and phosphate ions; and, optionally, hexavalent chromium originally present to trivalent chromium, trivalent chromium to hexavalent chromium in t
REFERENCES:
patent: 4671825 (1987-06-01), Ishii et al.
patent: 5091023 (1992-02-01), Saeki
patent: 5141575 (1992-08-01), Yoshitake
patent: 5230750 (1993-07-01), Shindou
Chromate Treatment of Metal-Coated Steel sheets; Chemical Abstracts, vol. 99, No. 6, p. 214.
Chromating Bath for Galvanized Steel Sheets; Chemical Abstracts, vol. 109, No. 12, p. 235.
Aoki Takayuki
Koga Mitsuyuki
Ogino Takao
Suda Arata
Henkel Corporation
Jaeschke Wayne C.
Silverberg Sam
Szoke Ernest G.
Wisdom, Jr. Norvell E.
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