Europaisches Patentamt ® J) European Patent Office Office europeen des brevets @ Publication number: 0 294 558 B1 EUROPEAN PATENT SPECIFICATION (45) Date of publication of patent specification : © int. ci.5: C23C 8/14, C23C 8/80, 17.04.91 Bulletin 91/16 C23C 22/78, C23C 22/82, C25F 3/24 @ Application number: 88105718.6 (22) Date of filing : 28.08.85 @ Method for treating stainless steel surface by high temperature oxidation. (30) Priority: 29.08.84 JP 181524/84 (72) Inventor : Takahashi, Haruji 12.02.85 JP 26083/85 973-31 Kozukayama Tamon-cho Tarumi-ku Kobe 655 (JP) Inventor : Goto, Shigeo @ Date of publication of application : 3-1-25-302 Kimikage-cho 14.12.88 Bulletin 88/50 Kita-ku Kobe 651-11 (JP) Inventor : Shibata, Mitsuaki 3-10 1 Kitaochiai © Publication number of the earlier application in Suma-ku Kobe 654 (JP) accordance with Art. 76 EPC : 0173564 Inventor : Hata, Tomihira 3-1-25 Kunikane Kamisho-cho Kakogawa 675-12 (JP) (45) Publication of the grant of the patent : Inventor: Takata, Syuichi 17.04.91 Bulletin 91/16 2-196 Tomogaoka Suma-ku Kobe 654 (JP) @ Designated Contracting States : DE FR GB (74) Representative : Hayward, Denis Edward Peter Lloyd Wise, Tregear & Co. Norman House 105-109 Strand London WC2R 0AE (56) References cited : (GB) US-A- 4 078 949 (re) Proprietor: SHINKO PANTEC CO., LTD. 4-78 Wakinhama-cho 1-chome Chuo-ku Kobe (JP) CQ CO in u> o> CM Note : Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Q. Notice of opposition shall be filed in a written reasoned statement It shall not be deemed to have been UJ filed until the opposition fee has been paid (Art 99(1) European patent convention). Jouve, 18, rue Saint-Denis, 75001 PARIS EP 0 294 558 B1 Description This invention relates to a method for treating the surface of stainless steel by high temperature oxidation. Conventionally there has been a "metal coloring" method that allows an oxide film formed on the surface 5 of various metals, such as aluminum, titanium or stainless steel, etc., to develop color by utilizing the phenome- non of light interference. Since this method can produce various color tones by controlling the thickness of oxide film without destroying the native brightness of the base metal, the method has been widely used on ornamental or construction materials. The conventional methods for metal coloring comprise : 10 (I) Dipping metallic material in chemical reagents (II) Anodically oxidizing in chemical reagents (III) Oxidizing at elevated temperatures in an oxidizing atmosphere (refer to Japanese Laid Open Pat. Appl. Nos. 48-99047, 49-58035 & 52-134833) Regarding (I) above, since the color tone of an oxide film varies delicately depending on the composition 15 of the reagent and on the dipping time (the color changes with every second and every minute), the color development requires a fine control against degradation of reagents. As to (II) above, inhomogeneities in the electric current density, or generation of oxygen gas can cause an unevenness in the coloring. Therefore, the treated material is limited to metal having simple configurations such as plates or sheets. 20 Colored oxide films obtained by the methods (I) (II) are subject to corrosion or abrasion because of their high porosity, and the film requires a hardening treatment after each coloring. As to method (III) above, the method is widely used for coloring materials such as stainless steels or titanium alloys having high temperature strength, because the method is easy to practice and can give a solid colored oxide film. While this method can form a colored oxide film having a tone corresponding to the heating 25 temperature of the treated metal, it has a drawback in that it causes an unevenness or shading in color, resulting in a poor appearance, because the degree of oxidation differs depending on the location of the metallic surface. Therefore, the use of this method has been limited to the blackening treatment of heat exchanger tubes or to small parts in respect of which there is no concern for aesthetic appearance. In the food or pharmaceutical industry, stainless steel is often used for equipment or factory plant, such as 30 storage tanks, pipes orvalves. The corrosion resistance of stainless steel is maintained, in general, by a passive film of Fe-, Cr-, Ni-oxide. However, because the thickness of the coating is only several A or tens of A, the dis- solution of Fe-ions cannot be avoided. For example, in the brewing industry, sake, wine, beer, etc. contain various kinds of organic acids. In par- ticular, the inner or outer surfaces of storage tanks, ultrafiltration equipment and/or pipes are treated by buffing 35 or pickling to prevent the adherence of germs or sal tartar and to improve their cleanliness. For example, the surface of ultrafiltration equipment used in the manufacture of sake is treated with a No. 400 mirror finish, because of the dissolution of iron into sake and the sanitary standards to be maintained. However, when sake is stored for longer than 10 hours, iron can dissolve from the stainless steel surface into the sake, making the sake colored and lowering its commercial value from the viewpoint of its taste. Accordingly, nowadays materials *o for piping in such plants or for the modules of ultrafiltration equipment include plastic or plastics-lined materials which are immune to the dissolution of iron. In the pure chemical field, or a field that requires clean water such as a nuclear power station or the elec- tronics industry, there are many processes that need water or solutions free from dissolved Fe-ions. Corrosion-resistant stainless steel is expected to have increased corrosion resistance as a result of a col- ts oring process, but in practice such coloring can decrease the resistance, depending upon the treatment process (Refer to Table 4 herein.) Accordingly, the coloring process can leave some problems for uses where high cor- rosion-resistance is required. The reason for the deterioration of corrosion-resistance seems to be due to the fact that the oxide film for- med by the heat-treatment after mechanical abrasion is not so dense nor so uniform that the base-metal cannot » be subjected to crevice corrosion or pitting corrosion. One solution for this problem is to dip a stainless steel article having a colored oxide film formed by high temperature oxidation in a nitric acid solution to passivate the base metal at the defective location of the film. This process helps to preserve the corrosion resistance from deterioration to some extent, but it has the risk of causing dissolution of the colored oxide film resulting in a change of color tone. ►5 Moreover, in a field of use where extremely rigorous conditions exist, the amount of dissolution of Fe-ions following this prior art process still proves to be too great. Therefore, it is an object of the invention to achieve a drastic reduction in the dissolution from stainless steel of Fe-ions as compared to the prior art. The invention accordingly provides a method for treating a stainless steel surface to decrease Fe-ion dis- 2 EP 0 294 558 B1 solution therefrom dunng subsequent use, in which method a colored oxide film is formed on said surface by high temperature heat-treatment in an oxidising atmosphere, characterised in that before the high temperature heat-treatment the surface to be treated is subjected to a cleaning step that includes electrolytically polishing the surface, and after cooling following the high temperature heat-treatment step the surface is subjected to a 5 decolorizing step in which the colored oxide film is removed. A further procedure that may optionally be employed is as follows : after electrolytic polishing and before the high temperature heat-treatment the surface to be treated has applied to it a coating agent comprising inert micro particles having a high melting point such that they will not be changed chemically or be melted during the high temperature heat-treatment, and after cooling of the surface following the high temperature heat-treat- 10 ment the layer of coating agent is washed away. In the preferred method, first, the surface of a stainless steel article to be colored is electrolytically polished to improve the characteristics of the polished surface of the base metal suitably for the subsequent formation of the oxide film. Then as an optional step the surface is treated with a coating agent, and afterwards the article is subjected to a heat-treatment in an oxidizing atmosphere, the temperature and time of treatment correspond- 15 ing to the color tone to be colored. The coating, if employed, is then removed and the decolorizing step is carried out. This process is summarized in more detail as follows : (1) The surface of the stainless steel article to be colored is cleaned by a traditional process, for example, by pickling, buffing and degreasing, to remove oxides or impurities on the surface, and then polished com- pletely by electrolytic polishing. 20 (2) Before the heat-treatment is performed, the surface may be optionally treated with a coating agent con- sisting of high-melting-point microparticles. (a) The coating agent is composed of materials that do not melt even under the high temperature-heat- ing of this method. As a suitable coating agent powders of TiOz and Si02 are mixed in a ratio between 100 : 0 and 25 : 75 in weight and the mixture is pulverized with a crusher, such as a ball mill, etc., and 25 graded by a 150-mesh sieve to achieve a small particle size, and water can then be added to the small- sized microparticles to make a slip.