Electric lamp and discharge devices – Spark plugs
Reexamination Certificate
2001-02-26
2003-03-25
O'Shea, Sandra (Department: 2875)
Electric lamp and discharge devices
Spark plugs
C445S007000, C313S141000
Reexamination Certificate
active
06538365
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a metal member with chromate coat, a method for manufacturing the metal member with chromate coat, a spark plug, and a method for manufacturing the spark plug.
2. Description of the Related Art
The surface of a metal member is often plated for corrosion prevention. For example, in the iron-based material, the surface is often plated with zinc, and it is known that this zinc galvanization is subjected to sacrificial corrosion to present the excellent anti-corrosion. However, there is a drawback that zinc galvanization is consumed severely due to sacrificial corrosion, and changed into white color due to produced zinc oxide, impairing the appearance. Thus, the surface of a zinc plating layer is covered with a chromate coat to prevent the corrosion of the galvanizing layer.
Among the chromate coats, a yellow chromate coat is excellent in corrosion resistance, and is widely used in a broad area including can films, for example. Also, a spark plug used to ignite a gasoline engine in the internal combustion engine of, for example, the automobile, may be often applied with the yellow chromate coat to prevent the corrosion owing to the attack of acid components from inside the engine, or the influence of acid rain during transportation.
However, the yellow chromate coat contains a part of a chromium component in the form of hexavalent chromium, and is shunned increasingly from the respect of environmental problems. Also, there is another drawback that, because a treatment bath of the yellow chromate coat contains a relatively high density of hexavalent chromium, the waste disposal may be expensive. Further, there is another drawback that the yellow chromate coat is inferior in heat resistance, and if the temperature is elevated to about 100° C. or higher, the chromate coat is liable to degrade, and the corrosion resistance is rapidly decreased.
On the other hand, the trivalent chromium based chromate coat containing a small amount of hexavalent chromium is excellent in heat resistance, but was often insufficient in its sliding ability when trying to slide with the other member on the surface of a member formed with its chromate coat. For example, in a case that a plastic working which may accompany the sliding with metal such as caulking or deep drawing is applied on a metal member formed with the trivalent chromium based chromate coat, the deformation does not proceed uniformly, unless the sliding is excellently performed, leading to undesired shape or size, or causing a defect such as a crack on the chromate coat. Also, in the case where the chromate coat is formed on a threaded member such as a bolt or nut, it is difficult to rotate a screw smoothly, unless the sliding property is excellent, resulting in either a lower working efficiency or a damaged thread.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a metal member with chromate coat, a method for manufacturing the metal member with chromate coat, a spark plug and a method for manufacturing the spark plug, the chromate coat having a small content of hexavalent chromium, and being excellent in both the corrosion resistance and the sliding property.
In order to accomplish the above object, according to the present invention, there is provided a method for manufacturing a metal member with chromate coat including a chromate treatment step of forming a chromate coat having a film thickness of 0.2 to 0.5 &mgr;m and composed of trivalent chromium at 95 mass % or more of chromium component at least on an expected sliding surface by immersing the metal member, having the expected sliding surface that is expected to slide with another member, into a chromating bath, and a heat treatment step of heat treating its chromate coat at temperatures from 85 to 255° C.
In the invention, a chromate coat composed of trivalent chromium at 95 mass % or more of chromium component contained and having a film thickness of 0.2 to 0.5 &mgr;m (hereinafter referred to as a trivalent chromium based chromate coat) is formed on the surface of the metal member. That is, a normal yellow chromate coat has hexavalent chromium at about 25 to 35 mass % of chromium component, while the chromate coat of this invention contains as small as 5 mass % or less of hexavalent chromium relative to the chromium component, thereby being beneficial for the environment by reducing the hexavalent chromium. Also, a chromate treating solution for use has no content of hexavalent chromium component at all, or a considerably smaller content than the treating solution for the yellow chromate coat, as will be described later. Hence, it is not required to make the reduction of hexavalent chromium, which is needed in the waste disposal of hexavalent chromium, resulting in less drainage problems.
As a result of acute examination, the present inventors have found that if the trivalent chromium based chromate coat is heat-treated in a temperature range from 85 to 255° C., the sliding property of trivalent chromium based chromate coat formed on the expected sliding surface of the metal member can be enhanced, whereby this invention has been accomplished.
The trivalent chromium based chromate coat is formed in a wet process, and contains much moisture immediately after forming the film. And if such moisture remains in excessive quantity particularly on the surface of chromate coat, an undesired adsorptive force is developed with the sliding member due to this moisture, and may possibly degrade the sliding ability. Thus, this moisture is subjected to the heat treatment in the above-mentioned temperature range to dry the film adequately, so that the sliding property of the chromate coat can be improved.
By heating the film, the film shrinks and is hardened. Such shrinkage may possibly be caused due to the fact that the hydrated chromate constituting the film is dewatered by heating. Further, as a result of acute examination, it has been found that trivalent chromium based chromate coat shrinks to some extent when a heat history is applied, but is hardened and becomes finer without causing almost any cracks, whereby in sliding, the chromate coat is unlikely to peel from a substrate (e.g., a zinc plating layer), and expose the substrate to give rise to increased sliding friction. Also, the peeling dust that may possibly decrease the sliding ability is unlikely to occur. Consequently, the sliding property can be improved.
At the heat treatment temperatures less than 85° C., the sliding property can not be effectively improved. On the other hand, at the heat treatment temperatures above 255° C., the sliding property is rather decreased, and the film peeling or lower corrosion resistance may be incurred along with the sliding. This is because if the heat treatment temperature is too high, the chromate coat is excessively hardened and becomes fragile. The heat treatment process may be performed in the air (or atmosphere), or in an inert atmosphere such as a nitrogen gas or Ar gas, for example. The heat treatment temperature is preferably in a range from 90 to 250° C.
If the film thickness of the chromate coat is below 0.2 &mgr;m, the anti-corrosion performance or heat resistance can not be sufficiently secured. Also, if the film thickness is above 0.5 &mgr;m, the film is more likely to crack or fall in sliding. The film thickness of the chromate coat is desirably 0.3 to 0.5 &mgr;m. It is desired that the chromate coat does not contain substantially hexavalent chromium.
The metal member can be a metal shell of a spark plug caulked against the outside of an insulator. In this case, when the metal shell is caulked against the insulator, a chromate coat is formed on an outside face of the metal shell containing an expected sliding surface, which is defined as a surface of an expected caulking portion of the metal shell expected to contact and slide with a caulking mold, and after the heat treatment process, the metal shell can be caulked with the expected caulking portion directed toward
Matsutani Wataru
Nasu Hiroaki
Uemura Yoshito
Macchiarolo Peter
NGK Spark Plug Co. Ltd.
O'Shea Sandra
LandOfFree
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