This invention relates to the art of metal treatment. More specifically, it relates to the art of treating a zinc or zinc alloy surface to improve corrosion resistance and paint receptivity.
Conventionally, zinc is employed to provide corrosion resistance to steel via electro or hot-dip galvanizing. However, a zinc surface tends to produce white rust when it is in an atmosphere with high humidity. Therefore, the appearance deteriorates, and the ability to protect the base material from rust decreases.
As a conventional means of protection against white rust, a chromate treating method is practiced. In this method, chromic acid, dichromic acid and the salts of these acids are used as the main treating agents. However, the toxic effects of hexavalent chromium and the problems of environmental pollution have made elimination of chromate usage desirable. The standards for operating and safety control, such as use of the salts and treatment of waste water had become more strict, and a treating method that can take place of the chromate method is desirable.
Conventionally, the method of preventing corrosion or improving the paint receptivity by treating the metal surface, with tannic acid is well known. (For example, Patent Bulletin 1976-2902), and many theories on rust proof by tannin are presented. (For example, Mihara, Kazuyuki, Journal of Association of Coloring Material. Vol. 37. No. 2, P., 62, 1964; E. Knowles and T. White. Journal of the Oil Color Chemistry Association. Vol. 41, P. 10, 1956.) However, when hot-dip galvanized panels are treated in tannic acid, the appearance is adversely affected because the metal luster is decreased and corrosion resistance is sub-standard. When the panel is treated in a solution of pH 6.5-9.0, the change in surface luster is relatively small, but there is little improvement in corrosion resistance because the reaction of tannin in such solution is weak. Generally, when the pH of the treating solution is in a range higher than 7.0, the tannin or tannic acid in the treating solution will decompose.
It is known that when zinc or zinc alloy is treated to improve corrosion resistance by the chromate method, processed by pressing or bending, and painted, the chromate film on the surface will provide good corrosion resistance, but the adhesion of paint to the surface and/or the scratch resistance are worse than those obtainable by phosphate treatment. Oftentimes, the zinc surface may be chromate treated to provide short-term bare corrosion resistance during storage or shipping. If the treated part is ultimately to be painted, it may be necessary to first remove the chromate film before phosphate treating.
When there is a chromate film on a zinc or zinc alloy surface, the action of the phosphoric acid treatment is inhibited so that little phosphate film forms. Therefore, when the phosphoric acid method is to be used to form a film, it is necessary to first remove the chromate film by either a physical method or chemical method. Buff-polishing is one of the physical methods employed, and treating the material in an alkaline solution of potassium permanganate is one chemical method. By either method, complete removal of the chromate film is difficult, and there remains the problem of pollution caused by the removed chromates.