Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1994-07-13
1995-10-03
Silverberg, Sam
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148262, C23C 2206
Patent
active
054548829
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method for surface treatment of aluminiferous metal-containing structures with acidic conversion treatment baths. The method is particularly useful for treating the surfaces of car bodies, household electrical appliances, and the like, with phosphate conversion coating baths, chromate treatment baths, and they like. Particularly, the present invention relates to a method for reproducibly controlling the chemical activity of fluorine in conversion treatment baths.
BACKGROUND OF THE INVENTION
The addition of fluorine to acidic conversion treatment baths for treating aluminiferous metal-containing structures is well known. One example of this methodology is taught in Japanese Patent Application Sho 63-157879 (157,879/1988).
RELATED ART
The fluoride ion concentration in an aqueous solution is generally measured directly using a fluorine ion meter. Japanese Patent Application Sho 63-17879 discloses a measurement method that relates to phosphate conversion treatment solutions. In the disclosed method, the fluoride ion concentration in a sample is measured after calibration of the fluorine ion electrode with fluorine reference solutions of known fluorine ion concentration. Using the fluoride ion concentration calculated based on the results of this measurement the phosphate conversion treatment is operated by adjusting the bath components to maintain the fluoride ion concentration within a specified range.
A chromate conversion treatment wherein the fluoride ion concentration is maintained within a prescribed range is disclosed in Japanese Patent Application Hei 3-48271 (48,271/1991) .
Although not intended for controlling the activity of conversion treatment baths, a method is disclosed in Z. Anal. Chem. 245 67 (1969) for the measurement of fluorine concentration through a potentiometric titration which uses a fluorine ion electrode as indicator electrode, and an aluminum nitrate solution, lanthanum nitrate solution, or the like as titrant. The method measures the total fluorine content in aqueous solutions, and requires that the sample be pre-treated by adjustment of the pH to a range of 4 to 7.
Several problems are associated with direct concentration measurement of the fluoride ion concentration in acidic conversion treatment baths using a fluorine ion meter. The fluorine ion electrode ultimately degrades with time during the measurement process, which necessitates frequent calibration with, reference solutions. Moreover, due to a progressive deterioration of the fluorine ion electrode, the current state of the art of measurement of fluoride ion concentration in acidic aqueous solutions requires frequent performance of complex operations such as washing the electrode, electrode calibration and the like.
However, even when the fluoride ion concentration is accurately measured and maintained the conversion film thickness and the coating add-on per unit area is subject to substantial variations when conversion treatment is operated continuously while holding the fluoride ion concentration as well as other managed parameters (e.g., pH, total acidity) at their respectively prescribed values. Since the film weight is an indication of the conversion treatment performance, in conversion treatments such as zinc phosphate conversion treatment, the film weight value can be taken as an index of the conversion treatment performance. When conversion treatment is implemented based on the fluoride ion measurement instability is observed not only in the corrosion resistance and paint adherence of the treated substrate, but also in the corrosion resistance and adherence of the paint film.
Methods based on potentiometric titration as disclosed in Z Anal. Chem. 245 67 (1969), offer the advantages of no troublesome fluorine ion electrode calibration and low measurement error due to electrode deterioration. However, due to the preliminary adjustment of the sample pH to the range of 4 to 7, the fluoride ion concentration measured by this method is the total fluorine concentra
Asai Yasuo
Ishi Hitoshi
Matsubara Yuzuru
Henkel Corporation
Jaeschke Wayne C.
Silverberg Sam
Szoke Ernest G.
Wisdom, Jr. Norvell E.
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