Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1994-05-19
1996-12-03
Silverberg, Sam
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148286, C23C 814, C23C 818, C22C 3844
Patent
active
055803989
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for forming a passive oxide film having chromium oxide as a chief component thereof, as well as to a stainless steel.
BACKGROUND OF THE INVENTION
Conventionally, two methods were known for the formation of a passive oxide film having chromium oxide as a chief component thereof on a stainless steel surface: the dry method, in which, after directly reacting stainless steel with oxygen gas, the oxidized steel was reduced with hydrogen gas and heat treated with an inert gas such as argon after reduction, and thereby, a passive film having chromium oxide as a chief component thereof was formed; and the wet method, in which the steel was etched using a chemical such as nitric acid or the like, and chromium oxide was obtained. A diagram of the processes of the dry method is shown in FIG. 5(b).
In FIG. 5(b), (1) indicates a baking process which removes moisture adhering to the stainless steel surface, and moisture released by the stainless steel surface. (2) indicates an oxidation process which is conducted in an oxygen atmosphere. The film obtained by this oxidation process is a passive oxide film having iron oxide as a chief component thereof. (3) indicates a reducing process in which the iron oxide is reduced in a hydrogen atmosphere in order to obtain chromium oxide. (4) indicates a heat treatment process in an inert gas atmosphere for the purpose of conversion to a film having chromium oxide as the chief component thereof. In this way, in accordance with the dry method, the formation of the chromium oxide is conducted by means of independent oxidation and reduction reactions, so that the period required for the processes is long.
FIG. 6 shows data relating to moisture released at normal temperatures from passive oxide films obtained by means of the wet method and the dry method, as measured by APIMS. As is clear from FIG. 6, in contrast to the passive oxide film formed in accordance with the dry method, which ceased giving off moisture after several minutes, the passive oxide film obtained in accordance with the wet method continued to give off moisture even after the passage of 100 minutes. In this way, the passive oxide film obtained in accordance with the wet method contained a large moisture component, so that if the moisture were not removed, such a passive oxide film could not be used in semiconductor production apparatuses, which must be free of outside gasses, and heat treatment such as baking or the like was necessary, so that in the same manner as with the dry method, considerable time was required.
The present invention has as an object thereof to provide a method of forming a passive oxide film having chromium oxide as a chief component thereof which is capable of easily forming a passive oxide film having chromium oxide as a chief component thereof, and to provide a stainless steel having a passive oxide film having chromium oxide as a chief component thereof.
SUMMARY OF THE INVENTION
A first essential feature of the present invention resides in a stainless steel having a crystal grain number of 6 or above and having formed on the surface thereof a passive oxide film having a thickness of 5 nm or above and in which the value of Cr/Fe (hereinbelow, this refers to an atomic ratio) at the outermost layer of the film is 1 or above.
A second essential feature of the present invention resides in a stainless steel having an amount of warp of 0.2% or more having formed on the surface thereof a passive oxide film having a thickness of 5 nm or above, and wherein the value of Cr/Fe at the outermost layer of the film is 1 or above.
A third essential feature of the present invention resides in a method of forming a passive oxide film having chromium oxide as a chief component thereof, characterized in that stainless steel is subjected to electrolytic polishing, then baking is conducted in an inert gas, and thereby, moisture is removed from the surface of the stainless steel, and then heat treatment is conducted at a temperature within a range of
REFERENCES:
patent: 4266987 (1981-05-01), Wang
patent: 5188714 (1993-02-01), Davidson
patent: 5226968 (1993-07-01), Ohmi
patent: 5259935 (1993-11-01), Davidson
Hei 198463 Aug. 10, 1989 Ohmi.
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