Stock material or miscellaneous articles – Hollow or container type article – Glass – ceramic – or sintered – fused – fired – or calcined metal...
Reexamination Certificate
1999-04-06
2001-05-08
Speer, Timothy M. (Department: 1775)
Stock material or miscellaneous articles
Hollow or container type article
Glass, ceramic, or sintered, fused, fired, or calcined metal...
C428S469000, C428S472000, C428S472200
Reexamination Certificate
active
06228445
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an austenitic stainless steel article, particularly in the form of a tubing, having a passivated surface layer.
2. Description of the Prior Art
In the manufacture of austenitic stainless steel articles, and particularly tubing of austenitic stainless steel, it is desirable that the surface thereof be passivated so that during use the surface will not oxidize or otherwise react with environments to which it is subjected during use. Particularly, in the case of austenitic stainless steel tubing, specifically AISI type 316 stainless steel tubing as used in the pharmaceutical industry, during use the interior surface develops a reaction product in the form of an oxide exhibiting a reddish color. This phenomenon is typically termed “rouging.” This reaction product may constitute a source of contamination for product passing through the tubing during use thereof in various industrial applications.
OBJECTS OF THE INVENTION
It is accordingly a primary object of the present invention to provide an austenitic stainless steel article, particularly a tubing, having a passivated surface layer that will not develop rouging during exposure to oxidizing environments during use.
SUMMARY OF THE INVENTION
In accordance with the invention, a stainless steel article, which may be in the form of a tubing, has a passivated surface layer of Cr
2
O
3
and Fe
2
O
3
with a metal component of Cr with a valence of zero and Fe with a valence of zero. The ratio of the oxide component to the metal component is in excess of 8 to 1.
Preferably, the stainless steel is an austenitic stainless steel.
Preferably, the stainless steel is AISI type 316 austenitic stainless steel.
Preferably, the outside surface of the passivated surface layer will have a total Cr to Fe ratio of at least 1 to 1.
The passivated surface layer may at a depth therein of a maximum oxygen concentration have a total Cr to Fe ratio of at least 1.5 to 1. The passivated surface layer preferably constitutes an electropolished surface but may also be a mechanically polished surface, produced for example by swirl or belt polishing.
The reference to “total Cr to Fe ratio” includes the Fe and Cr present in the oxide component.
The term “electropolished” means a metallic bright surface created through a combination of electrical action and an acid solution, one component of which is phosphoric acid, the other usually sulfuric acid.
All compositions are in weight percent unless otherwise indicated.
REFERENCES:
patent: 5259935 (1993-11-01), Davidson et al.
Crucible Materials Corp.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Speer Timothy M.
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