1. Field of the Invention
The present invention relates to a glow plug for previously heating a diesel engine or the like, a spark plug for an internal combustion engine, and a manufacturing method therefor.
2. Description of the Related Art
In general, a glow plug has a structure that a resistance heater is disposed in a main metal shell having the outer surface on which a joining thread portion has been formed such that a leading end heating portion of the resistance heater projects over either end surface of the main metal shell. The thread portion is used to join the glow plug to an engine head.
A spark plug for igniting a gasoline engine for an automobile or the like incorporates an insulating member disposed on the outside of a central electrode and a main metal shell disposed on the outside of the insulating member. Moreover, a ground electrode for forming a spark discharge gap from the central electrode is joined to the main metal shell. A joining thread portion provided for the outer surface of the main metal shell is used to joint the spark plug to the cylinder head of the engine.
The main metal shell is usually made of an iron material, such as carbon steel, and structured to have the surface applied with zinc plating to prevent corrosion. Although the zinc-plated layer has an excellent anti-corrosion effect for iron, the zinc-plated layer formed on iron can easily be consumed owing to sacrificial corrosion as known. What is worse, the zinc-plated layer is decolored to white owing to zinc oxide, causing the quality of the appearance to deteriorate. Therefore, a major portion of the glow plugs and the spark plug is structured such that the surface of the zinc-plated layer is coated with a chromate film to prevent corrosion of the plated layer.
The chromate film to be formed on the main metal shell of the glow plug and the spark plug has been a so-called yellow chromate film. Since the yellow chromate film exhibits excellent anti-corrosion performance, the yellow chromate film is widely employed in a variety of fields including coating of the inner surface of a can as well as the glow plug and the spark plug. Since a portion of contained chrome components is hexavalent chrome, use of the yellow chromate film has gradually been inhibited in recent years owing to global focusing on the environmental protection. For example, discontinuance of the chromate film containing hexavalent chrome in the future has been considered in, for example, the automobile industrial field in which glow plugs and spark plugs are used in a large quantity. Since a processing bath for forming the yellow chromate film contains hexavalent chrome at a relatively high concentration, there arises a problem in that an excessively large cost is required to dispose waste water.
Therefore, chromate films of a type which does not contain hexavalent chrome, that is, films of a type that the substantially overall portion of chrome components is contained as trivalent chrome have been researched and developed at a relatively earlier time. Thus, processing baths containing hexavalent chrome at a relatively low concentration or baths containing no hexavalent chrome have been developed. Therefore, the problem of disposal of waste waster has been overcome. However, the chromate film employing the trivalent chrome suffers from unsatisfactory anti-corrosion performance as compared with the yellow chromate film. Therefore, wide use of the yellow chromate film as a film with which the main metal shell of the glow plug and the spark plug is coated has not been realized.
Further, the conventional chromate films including the yellow chromate films suffer from a common problem of unsatisfactory heat resistance. Since, for example, the engine of an automobile incorporates a cylinder head to which the spark plug is joined is cooled with water, the temperature of the spark plug is not raised excessively. When the operation of the engine is continued under a condition that a great load of heat is exerted or when the spark plug is joined relatively adjacent to the exhaust manifold, the temperature of the main metal shell is sometimes raised to about 200xc2x0 C. to 300xc2x0 C. In the foregoing case, the chromate film easily deteriorates. Thus, there arises a problem in that the anti-corrosion performance rapidly deteriorates. Moreover, the conventional chromate film suffers from deterioration in the performance owing to attack of an acid component, such as carbon dioxide, a nitrogen oxide or a sulfur oxide, contained in acid rain and exhaust gas and, in a case of a gas engine, acid water produced by the engine.
It is an object of the present invention is to provide a glow plug and a spark plug having a chromate film which covers the surface of its main metal shell and which contains hexavalent chrome in a small quantity and exhibiting excellent anti-corrosion performance and heat resistance as compared with those of a conventional chromate film and a manufacturing method therefor.
To solve the foregoing problems, according to one aspect of the present invention, there is provided a glow plug comprising: a resistance heater disposed in a main metal shell such that the leading end of the resistance heater projects over either end surface of the main metal shell, wherein the surface of the main metal shell is coated with a chromate film containing trivalent chrome by 95 wt % or more of contained chrome components and having a thickness of 0.2 xcexcm to 0.5 xcexcm.
Further, according to another aspect of the present invention, there is provided a spark plug comprising: a central electrode; an insulating member disposed on the outside of the central electrode; a main metal shell disposed on the outside of the insulating member; and a ground electrode disposed opposite to the central electrode such that a spark discharge gap is formed, wherein the surface of the main metal shell is coated with a chromate film containing trivalent chrome by 95 wt % or more of contained chrome components and having a thickness of 0.2 xcexcm to 0.5 xcexcm.
The foregoing structure are arranged such that the surface of the main metal shell is coated with a chromate film containing trivalent chrome by 95 wt % or more of contained chrome components and having a thickness of 0.2 xcexcm to 0.5 xcexcm. That is, a usual yellow chromate film contains hexavalent chrome by about 25 wt % to 35 wt % of the chromate components. On the other hand, the film according to the present invention contains hexavalent chrome in a small quantity of 5 wt % or less of the chrome components. Therefore, and effect required of the environmental protection can be improved. The employed chromate processing solution does not contain any hexavalent chrome or contains the same in a small quantity as compared with a processing solution for forming the yellow chromate film. Hence it follows that a problem of disposal of waste water does not easily occur.
The inventors of the present invention has considered that, for example, a glossy chromate film, called a uni-chrome film, and a conventional trivalent chrome film, such as a blue chromate film, having a small thickness of 0.1 xcexcm cannot realize satisfactory anti-corrosion characteristic and heat resistance with respect to the main metal shell in a major environment for the glow plug and the spark plug for use. Therefore, investigations of the thickness have been performed energetically. As a result, a preferred thickness range for the glow plug and the spark plug has been detected and, therefore the present invention has been achieved. That is, when the thickness of the chromate film is made to be 0.2 xcexcm or greater, the anti-corrosion performance of the chromate film mainly composed of trivalent chrome can considerably be improved. Therefore, the durability against corrosion of the main metal shell can sufficiently be improved. In an environment peculiar for the glow plug and the spark plug in which the temperature can easily be raised and attack of acids caused from exhaust gas components (CO2 and NOX) cannot be prevented, the anti-corrosion performance of the main metal shell can satisfactorily be maintained.
A main portion of the glow plugs has a structure that an energizing terminal shaft for energizing the resistance heater is disposed such that the rear end of the energizing terminal shaft projects over another end surface of the main metal shell. Moreover, a nut for securing a power supply cable to the energizing terminal shaft is engaged to a male thread portion formed in the rear end portion of the energizing terminal shaft. In the foregoing case, at least a portion of the surface of the nut is coated with the chromate film. Therefore, satisfactory anti-corrosion performance and heat resistance can be imparted to the nut as well as the main metal shell.
A portion of spark plugs incorporates an annular gasket which must be fitted to the base of a joining thread portion provided for the outer surface of the main metal shell. When the thread portion of the main metal shell is screwed in a thread hole of the cylinder head, the gasket is compressed and deformed as if it is crushed between a flange-shape gas sealing portion provided for the base of the thread portion and the periphery of the opening of the thread hole to seal a space between the thread hole and the gas sealing portion. In the foregoing case, at least a portion of the surface of the gasket can be coated with the foregoing chromate film. Therefore, satisfactory anti-corrosion performance and heat resistance can be imparted to the gasket as well as the main metal shell.
When the thickness of the chromate film is smaller than 0.2 xcexcm, satisfactory anti-corrosion performance and heat resistance cannot be realized. When the thickness is larger than 0.5 xcexcm, a crack of the film occurs and/or separation of the film easily takes place. Thus, the anti-corrosion performance undesirably deteriorates. It is preferable that the thickness of the chromate film is 0.3 xcexcm to 0.5 xcexcm. It is preferable that the chromate film does not substantially contain hexavalent chrome.
The chromate process is one of conversion treatment processes with which substitution and deposition of the chrome components are performed while base metal is being oxidized and eluted. Therefore, an electroless chromate process in which no electric power is supplied from outside must use metal which can be eluted into the chromate processing bath as the base metal. In general, the main metal shell, the nut or the gasket of the glow plug and/or spark plug is constituted by an iron material, such as carbon steel. Thus, a zinc type plated layer, the main metal component of which is zinc, may be formed on the surface of the main metal shell, the nut or the gasket to prevent corrosion. The zinc-plated layer serves as a preferred base metal for forming the chromate film. In the foregoing case, the eluted zinc components are usually taken in the chromate film. Note that the zinc-plated layer can be formed by performing known electrolytic zinc plating or molten zinc plating. When electrolytic chromate processing method is employed, the chromate film can be formed even in a case of a nickel-plated layer, the main metal component of which is nickel.
When the base metal layer is the zinc-plated layer and the chromate film satisfying the above-mentioned thickness range is formed on the base metal layer, time for which white rust appears by about 20% or more of the overall surface caused from corrosion of the zinc-plated film can be made to be 40 hours or longer after chapter five xe2x80x9cneutral salt water spray testxe2x80x9d of anti-corrosion test of plating conforming to JIS H8502 has been performed. The foregoing anti-corrosion performance level required of the main metal shell of the glow plug and the spark plug is a satisfactory level.
When the base metal layer is constituted by the zinc-plated layer and the chromate film having the above-mentioned thickness is formed, satisfactory durability can be realized even in the following test on the assumption of an environment of use in which the temperature of the glow plug and the spark plug is raised. That is, when heating at 200xc2x0 C. in the atmosphere for 30 minutes is performed and chapter five xe2x80x9cneutral salt water spray testxe2x80x9d of anti-corrosion test of plating conforming to JIS H8502 is performed, time for which white rust appears by about 20% or more of the overall surface caused from corrosion of the zinc-plated film can be made to be 40 hours or longer.
Also in the following test on the assumption that the environment of use in which the glow plug and the spark plug is attacked with acids, satisfactory durability can be realized. That is, time for which white rust appears by about 20% or more of the overall surface caused from corrosion of the zinc-plated film can be made to be 20 hours or longer after chapter seven xe2x80x9cCASS testxe2x80x9d of anti-corrosion test of plating conforming to JIS H8502 has been performed.
In a method of manufacturing a glow plug and a spark plug according to the present invention, the main metal shell (or the nut, or gasket) is immersed in a chromate processing bath containing trivalent chrome salt and a complexing agent for the trivalent chrome mixed therein so that the foregoing chromate film is formed on the main metal shell (or the nut, or gasket).
The chromate processing bath contains the trivalent chrome salt and the complexing agent for the trivalent chrome. Therefore, a close and thick trivalent-chrome type chromate film, which cannot be formed by a usual chromate processing method, can be formed. Thus, the trivalent-chrome type chromate film having a thickness of 0.2 xcexcm to 0.5 xcexcm which is the essential portion of the glow plug and the spark plug according to the present invention can easily be formed. A method of the above-mentioned chromate film has been disclosed in Germany Patent Laid-Open No. DE19638176A1. Then, the method will now be described.
As described above, there is an established theory that the chromate film is formed such that the base metal (for example, zinc) is first oxidized and eluted in the processing bath. The eluted base metal member components and solution containing chromate ions react with one another so that trivalent chrome forms polymer-like complexes owing to hydroxyl groups or oxygen bridges so that the complexes in the form of gels are precipitated and deposited on the surface of the base metal member. In the foregoing case, the chromate film can be grown only when elution of the base metal member and reactions and deposition of the chromate ions contained in the bath take place simultaneously. When the chromate film has been deposited to have a somewhat large thickness, the elution reaction of the base metal member, which is disproportionation through the interface from the solution, is inhibited. Hence it follows that the growth of the film is inhibited.
According to the above-mentioned laid-open Germany patent, it is important for enlarging the thickness of the formed film to minimize the rate at which the deposited chromate film is inversely dissolved while the rate at which the base metal member is dissolved and that at which the film is deposited owing to the reactions between the dissolved base metal member components and trivalent chrome are being raised. It can be considered that the foregoing method enables the thickness of the film to be enlarged because the deposition of the film can be enhanced by adding an appropriate complexing agent into the bath to complex the trivalent chrome.
An effective complexing agent is any one of a variety of chelating agents (dicarboxylic acid, tricarboxylic acid, oxyacid hydroxyl-group dicarboxylic acid or hydroxyl-group tricarboxylic acid: for example, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, cork acid, selenious acid, sebacic acid, meleic acid, phthalic acid, terephthalic acid, tartaric acid, citric acid, malic acid or ascorbic acid. Another complexing agent may be employed. Complexing agents which can be employed are as disclosed in the foregoing laid-open German Patent.
To enlarge the thickness of the film, it is also effective to raise the temperature of the chromate processing bath to about 20xc2x0 C. to about 80xc2x0 C. When the temperature of the bath is lower than 20xc2x0 C., the effect of enlarging the thickness of the film owing to raising of the temperature cannot substantially be obtained. When the temperature is 80xc2x0 C. or higher, vaporization of water from the bath takes place excessively. Thus, the conditions of the bath cannot easily be controlled. It is preferable that duration of immersion of the member which must be processed in the chromate bath (the main metal shell and the nut) is 20 seconds to 80 seconds. When the immersion is performed for 20 seconds or shorter, a required thickness of the chromate film cannot sometimes be realized. When the duration of immersion is longer than 80 seconds, the formed chromate film is excessively thickened. Thus, a crack of the film occurs (when, for example, a joining process is performed) or separation of the film easily occurs. Therefore, the anti-corrosion performance sometimes undesirably deteriorates.
To enhance dissolution of the base metal member, it is effective to lower the pH of the chromate processing solution in a range in which re-dissolution of the film formed owing to deposition takes place excessively. A preferred range of the pH is, for example, about 1.5 to about 3. To prevent re-dissolution of the film formed owing to deposition, it is effective that the film contains a hydroxide, such as nickel, cobalt or copper, which cannot easily be re-dissolved. To achieve this, a compound of the foregoing metal may be dissolved and mixed in the chromate processing bath.
Results of repeated investigations performed by the inventors will now be described. When sodium salt (for example, sodium nitrate) in a predetermined quantity is mixed in the chromate processing bath in such a manner that the content of sodium component in the chromate film is 2 wt % to 7 wt %, a close chromate film having a large thickness can be formed. Although the detailed mechanism cannot be detected, it can be considered that containing of sodium ions in the chromate film prevents re-dissolution of the chromate film in the processing bath. When the content of the sodium component in the chromate film does not satisfy the range from 2 wt % to 7 wt %, the thickness of the chromate film cannot sometimes easily be made to be 0.2 xcexcm or larger. It is preferable that the content of the sodium components in the chromate film is 2 wt % to 6 wt %.
When a film is provided for the nut or the gasket, the foregoing process may be performed by substituting the nut or the gasket for the main metal shell. Thus, the same method may, of course, be employed.