Electric lamp and discharge devices – Spark plugs
Reexamination Certificate
2000-08-25
2004-07-27
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Spark plugs
C313S140000
Reexamination Certificate
active
06768249
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spark plug for use on internal-combustion engines and a method for producing it.
2. Description of the Related Art
The spark plug used to ignite internal-combustion engines such as automotive gasoline engines comprises a central electrode, an insulator provided outside the central electrode, a main metal shell provided outside the insulator, and a ground electrode fitted on the main metal shell in a face-to-face relationship to define a spark discharge gap. A mounting threaded section is formed on the peripheral surface of the main metal shell and by means of this thread section the spark plug is mounted on the cylinder head of the engine for subsequent service.
The main metal shell is typically made of ferrous materials such as carbon steels and its surface is in most cases galvanized for protection against corrosion. The zinc plate layer exhibits good anti-corrosive action on iron. However, as is well known, the zinc plate layer on the iron substrate will easily be consumed by sacrificial corrosion, with the additional disadvantage that its appearance is prone to be damaged by the resulting zinc oxide which causes white discoloration. To deal with this problem, most spark plugs have the surface of the zinc plate layer further coated with a chromate coat so that it is protected against corrosion.
Spark plugs have heretofore been coated with the so-called “gold chromate coat” on the main metal shell. Because of its high performance in corrosion protection, the gold chromate coat is extensively used in non-spark plug applications such as coatings on the inner surfaces of food reserving cans. However, since part of the chromium component is contained in the hexavalent form, the use of the gold chromate coat is being gradually discouraged by the increasing global concern for environmental protection. In the automotive industry which is a massive user of spark plugs, a total ban in the future on the use of chromate coatings containing hexavalent chromium is being reviewed considering the possible environmental impact of waste spark plugs. As a further problem, the treating baths for depositing the gold chromate coatings contain relatively high concentrations of hexavalent chromium and huge cost is necessary to treat the waste effluents.
Under these circumstances, the development of chromate coatings free from hexavalent chromium, or those in which substantially all chromium component is in the trivalent form, has been underway for comparatively many years. The treating baths proposed to date are mostly low in the concentration of hexavalent chromium and some of them are entirely free from this form of chromium, thus contributing to alleviating the problem of effluent treatment. However, a major defect of the trivalent chromium based chromate coatings is that they are inferior to the gold chromate coatings in the ability to prevent corrosion and this is why they have not found extensive use as coatings to cover the main metal shell of spark plugs.
In addition, the chromate coatings including the gold chromate coat share the common problem of being low in heat resistance. In automotive engines, the cylinder head on which the spark plug is mounted is water cooled and the spark plug seldom becomes extremely hot. However, if the engine is continuously run under high thermal load conditions or if the mounting position of the spark plug is fairly close to the exhaust manifold, the temperature of the main metal shell sometimes increases to about 200 to 300° C. In this situation, the chromate coat undergoes accelerated deterioration and its performance in corrosion protection may suddenly drop.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a spark plug having the surface of the main metal shell coated with a chromate coat that is reduced in the content of hexavalent chromium and which yet exhibits better anti-corrosion performance and higher heat resistance than the conventional chromate coatings.
It is another object of the invention is to provide a method for producing the spark plug.
[Means for Attaining the Objects, Mode of its Action and Resulting Advantages]
According to a first aspect of the present invention, a spark plug comprises a central electrode, an insulator provided outside said central electrode, a main metal shell provided outside said insulator and a ground electrode provided to oppose to said central electrode to define a spark discharge gap, wherein the surface of said main metal shell is coated with a complex chromate coat containing a chromium component and a phosphorus component as cationic components, at least 90 wt % of the chromium component being trivalent chromium and the phosphorus component being present in an amount of 1 to 15 wt % as calculated for PO
4
.
The “cationic components” as used herein is a term related to a photoelectron spectrum for a coating analyzed by X-ray photoelectron spectroscopy (XPS or ESCA) and means any component of interest (element or atom) that has a chemical shift in the peak of its binding energy toward a positive ionic valence.
In the above spark plug, the surface of the main metal shell is coated with a complex chromate coat that contains a chromium component and a phosphorus component as cationic components, at least 90 wt % of the chromium component being trivalent chromium and the phosphorus component being present in an amount of 1 to 15 wt % as calculated for PO
4
. In the ordinary gold chromate coat, about 25 to 35 wt % of the chromium component is hexavalent chromium. However, in the coating of the invention, the content of hexavalent chromium is less than 10 wt % of the chromium component, which is small enough to be of benefit to environmental protection by reducing the emission of hexavalent chromium. The treating bath used to deposit the chromate coating of the invention contains no hexavalent chromium at all or contains only a small amount of it as compared with the treating baths for the gold chromate coat and other conventional chromate coatings. As a result, the problems with effluent treatment are substantially reduced.
The complex chromate coat used in the spark plug is characterized by containing a phosphorus component as a cationic component. The complex chromate coat containing a phosphorus component is markedly improved over the ordinary trivalent chromium based chromate coat in terms of the ability to prevent corrosion, giving the main metal shell of the spark plug adequate durability against corrosion.
If the content of the phosphorus component in the complex chromate coat is less than 1 wt %, the desired performance in corrosion prevention is not attained. Incorporating more than 15 wt % of the phosphorus component is very difficult, since there is a limit on the concentration of the phosphorus component in the treating bath to be used. The content of the phosphorus component in the complex chromate coat is more desirably in the range of 5 to 10 wt %. For the purpose of enhancing the corrosion preventing performance of the complex chromate coat, it is desired that the phosphorus component be mainly contained in the form of phosphate ion (PO
4
3−
).
The complex chromate coat may contain a phosphorus component dispersing chromate layer that has the phosphorus component dispersed in a trivalent chromium based compound in an amount of 2 to 15 wt % as calculated for PO
4
. The phosphorus component dispersing chromate layer can be easily formed by immersing the main metal shell of a spark plug into a chromating bath containing phosphoric acid or a phosphate. The dispersion of the phosphorus component in the trivalent chromium based compound contributes to a further improvement in the corrosion preventing performance of the complex chromate coat. In this case, the phosphorus component originates from the phosphoric acid or phosphate contained in the chromating bath.
The phosphorus component dispersing chromate layer may independently constitute the whole of the complex chroma
Matsutani Wataru
Nasu Hiroaki
Berck Ken
NGK Spark Plug Co. Ltd.
Patel Ashok
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