Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1996-08-02
1998-12-08
Silverberg, Sam
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148274, C23C 2253
Patent
active
058463429
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to an optimized surface treatment agent that imparts both an excellent corrosion resistance and an excellent paint adherence to the surface of steel, especially steel sheet, plated with zinc or zinc-containing alloy (hereinafter abbreviated as zinciferous-plated steel). The invention will be described below primarily with respect to use on steel sheet, but it is to be understood that any other shape of steel substrate may also be treated according to the invention.
BACKGROUND ART
Zinciferous-plated steel sheet utilizes the principle of sacrificial zinc corrosion, which is a general method for preventing the corrosion of iron steel. Zinciferous-plated steel sheet is used in a broad range of applications, extending over the automotive, building material, and household electrical appliance sectors. However, the corrosion product generated when zinc corrodes in the atmosphere produces so-called white rust on the steel sheet, which leads to a deterioration in appearance. Moreover, this white rust also impairs paint adherence to the plated steel sheet.
These problems are generally solved by subjecting the surface of zinciferous-plated steel sheet to a chromate treatment with a treatment bath whose main components are chromic acid, dichromic acid, or a salt thereof. This treatment produces a highly corrosion-resistant and strongly paint-adherent chromate film.
Unfortunately, the hexavalent chromium present in the chromate treatment baths used on zinciferous-plated steel sheet has a direct negative effect on the human body. This, and the recent increased demand for environmental protection, has created a desire to avoid the use of chromate treatments. In addition, the use of chromate treatments requires the implementation of special waste water treatment as stipulated in the Water Pollution Prevention Law, which drives up costs as a whole. Finally, when discarded or abandoned, chromate-treated zinciferous-plated steel sheet is classified as a chromium-containing waste and therefore cannot be recycled.
One well-known non-chromate surface treatment method uses surface treatment agents based on tannic acid (tannic acid contains polyphenolic carboxylic acid). When zinciferous-plated steel sheet is treated with an aqueous solution of tannic acid, the zinc tannate produced by the reaction between tannic acid and zinc forms a protective coating. It is thought that the corrosion resistance of the zinciferous-plated steel sheet is improved through the action of this protective coating as a barrier to the infiltration of corrosive substances. tannic acid-based surface treatment method comprising the treatment of zinciferous-plated steel sheet with an aqueous solution that contains at least tannic acid and silica sol. This method forms an ultrathin surface film that has a rust-inhibiting activity. The problem with this method is that this film still cannot deliver a satisfactory corrosion resistance. teaches a method in which zinc-containing metal articles are first treated with a strongly alkaline (pH.gtoreq.12.5) aqueous solution and are thereafter treated with an acidic aqueous solution whose main component is tannic acid. While the coating produced by this method has a relatively good corrosion resistance, this method requires a strongly alkaline treatment and a water rinse prior to the tannic acid treatment. This results in a poor productivity and poor economics.
Another surface treatment method is disclosed in Japanese Patent of zinciferous-plated steel sheet is treated with an aqueous solution containing organosilane coupling agent and one or both of water glass and sodium silicate. The surface film yielded by this method exhibits a good primary adherence, but a poor secondary adherence and a poor corrosion resistance.
Thus, no chromium-free surface treatment agent introduced to date is able to impart both an excellent corrosion resistance and an excellent paint adherence to zinciferous-plated steel sheet.
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
Aoyama Masayuki
Ogino Takao
Henkel Corporation
Jaeschke Wayne C.
Silverberg Sam
Szoke Ernest G.
Wisdom, Jr. Norvell E.
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