Chemistry: electrical and wave energy – Processes and products
Patent
1986-07-11
1989-08-22
Niebling, John F.
Chemistry: electrical and wave energy
Processes and products
204 28, 204 34, 204 561, C25D 904, C25D 1134, C25D 1138
Patent
active
048592870
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a method for producing a colored stainless steel stock having improved abrasion resistance and minimized color shading and finding a major application as building material.
BACKGROUND OF THE INVENTION
Since colored stainless steel plates are mainly used as building material, they are required to have permissible wide color variation, color consistency or no color shading, and high abrasion resistance in addition to the corrosion resistance inherent to stainless steel.
To meet such requirements, there were proposed prior art techniques as shown below.
(1) Prior art known methods for imparting a wide variety of color tones to stainless steel stock are so called INCO methods primarily based on the use of a mixed solution of sulfuric acid plus chromic acid (see Japanese Patent Publication Nos. 52-32621, 52-25817, and 53-31817). These methods include two steps, "coloring" and "film hardening" steps, which are separately carried out with individual solution compositions, temperatures, and treating conditions. Most products are batchwise manufactured plates.
(2) When stainless steel is dipped in an aqueous solution comprising chromic acid and sulfuric acid, there forms a porous colored film of chromium oxides on the surface. This oxide film, however, is liable to abrasion because of porosity. Known methods for hardening such a colored film to overcome this problem are by effecting electrolysis in an aqueous solution containing chromic acid and a much lower concentration of sulfuric acid than in the coloring solution while setting the stainless steel plate colored by the aforementioned method as a cathode, thereby electrodepositing metallic chromium on the surface, as disclosed in Japanese Patent Publication Nos. 53-31817and 56-24040.
(3) Also disclosed is a method for continuously coloring stainless steel hoops (Japanese Patent Publication No. 60-22065). This method is to produce colored stainless steel strips by a dual step process based on the INCO method using dual solutions, "coloring" and "film hardening" tanks. Control of color tone is accomplished by measuring the potential between the steel strip and a counter electrode, platinum plate at a plurality of positions on the path of the strip in the "coloring" tank during the "coloring" step to compute a potential difference from a reference.
(4) Since the use of such sulfuric acid plus chromic acid solution leads to a great expenditure in the solution treatment required in view of pollution control, another coloring method is known involving dipping in sulfuric acid plus permanganate salt as a hexavalent chromium-free coloring solution (Japanese Patent Publication No. 51-40861). In this method, a dipping solution is prepared by adding a permanganate salt to aqueous sulfuric acid and allowing reaction to proceed until oxygen gas ceases to evolve, and stainless steel is dipped in the solution at a temperature in the range from 90.degree. to 110.degree. C., thereby forming a film colored in bronze, blackish brown or black color.
In addition to these solutions, a variety of coloring solutions have been developed. There is known a method for spontaneous coloring by dipping in a hot solution of sodium (or potassium) hydroxide plus potassium (or sodium) permanganate as one of such solutions (Japanese Patent Publication No. 54-30970).
However, the aforementioned prior art techniques have problems as described below.
The INCO method identified in (1) which consists of two steps, "coloring" and "film hardening" steps has the problems that water rinsing and drying operations must be inserted between the two "coloring" and "film hardening" steps in order to perform them in a continuous fashion; that because of a change of the originally imparted color during the "film hardening" step, the preceding "coloring" step requires a complicated adjustment to take into account the subsequent color change in order that the predetermined color be eventually obtained; and that dipping operations often used in the "coloring" treatment can
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F. A. Lowenheim, Electroplating, McGraw-Hill Book Co., New York, 1978, pp. 152-155.
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Kurahashi Hayao
Nakai Yoichi
Narutani Tetsu
Sone Yuji
Suzuki Shigeharu
Kawasaki Steel Corporation
Leader William T.
Niebling John F.
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