JP2001326055A - Gasket for spark plug and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gasket for a spark plug in which a chromate film covering a surface has a low hexavalent chromium content and is superior in corrosion resistance and heat resistance as compared with a conventional chromate film. SOLUTION: The gasket 30 includes a spark plug 10.
In addition to being used by being fitted into the base end portion of a mounting screw 7 formed on the outer peripheral surface of the metal shell 1, at least a part of the surface has 95% by weight or more of the chromium component contained therein, trivalent chromium. And the film thickness is 0.2 to 0.5 μm
m of the chromate coating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a gasket for a spark plug and a method for manufacturing the same.

[0002]

2. Description of the Related Art A spark plug used for ignition of an internal combustion engine, for example, a gasoline engine for an automobile or the like, is provided with an insulator outside a center electrode and a metal shell outside the center electrode. It has a structure in which a ground electrode forming a spark discharge gap is attached to the metal shell.
The metal shell is mounted on the cylinder head of the engine by a mounting screw formed on the outer peripheral surface of the metal shell. In addition, a ring-shaped gasket is fitted to the base end of the mounting screw formed on the outer peripheral surface of the metal shell. In this gasket, the screw portion of the metal shell is screwed into the screw hole on the cylinder head side, so that the gasket is crushed between the flange-shaped gas seal portion formed on the base end side of the screw portion and the opening peripheral portion of the screw hole. And serves to seal between the screw hole and the gas seal portion.

[0003] The gasket is generally made of an iron-based material such as carbon steel, and its surface is often plated with zinc for corrosion protection. The galvanized layer has an excellent anticorrosion effect on iron, but as is well known, the galvanized layer on iron is easily consumed due to sacrificial corrosion, and the zinc oxide produced changes its appearance to white due to zinc oxide. Also have the disadvantage that they are easily damaged. Therefore, in many spark plugs, the surface of the galvanized layer is further covered with a chromate film to prevent corrosion of the plated layer.

Incidentally, a so-called yellow chromate film has been used as a chromate film applied to a gasket of a spark plug. Since this yellow chromate film has good anticorrosion performance, it has been widely used in fields other than spark plugs, such as, for example, inner can coating. However, the concern that some of the chromium components are contained in the form of hexavalent chromium has led to a growing concern on environmental protection in recent years that has been increasingly avoided. For example, in the automobile industry where a large amount of spark plugs are used, studies are being made to completely abolish the use of a chromate film containing hexavalent chromium in the future in consideration of the environmental impact of waste spark plugs. Further, since a treatment bath containing a relatively high concentration of hexavalent chromium is used as a treatment bath for the yellow chromate film treatment, there is a problem that a large cost is required for waste liquid treatment.

[0005]

In response to this trend, the development of chromate coatings that do not contain hexavalent chromium, ie, coatings in which substantially all of the chromium component is contained in the form of trivalent chromium, has been relatively developed. It has been advanced from an early stage.
The treatment bath generally has a low hexavalent chromium concentration, and some baths containing no hexavalent chromium have been developed, and the problem of waste liquid treatment has been reduced. However, the trivalent chromium-based chromate coating has a major drawback in that the anticorrosion performance is inferior to the yellow chromate coating, and has not been widely used as a coating for a spark plug.

[0006] Further, the chromate coatings used so far, including the yellow chromate coating, have a common drawback of poor heat resistance. In an automobile engine or the like, since the cylinder head to which the spark plug is attached is water-cooled, the spark plug rarely becomes extremely hot. However, if the engine continues to operate under the condition that the heat load is large, or if the spark plug is mounted at a position relatively close to the exhaust manifold, the temperature of the gasket sometimes rises to about 200 to 300 ° C. May rise. Under such circumstances, there is a problem that the chromate film is apt to deteriorate and the anticorrosion performance is rapidly reduced. Further, the conventional chromate film receives an attack of acidic components such as acid rain, carbon dioxide gas, nitrogen oxide or sulfur oxide contained in exhaust gas, and acid water generated from the engine in the case of a gas engine or the like. Then, there is a problem that the performance is further likely to deteriorate.

SUMMARY OF THE INVENTION An object of the present invention is to provide a gasket for a spark plug which has a low hexavalent chromium content in a chromate film covering a gasket surface and which is superior in anticorrosion performance and heat resistance as compared with a conventional chromate film, and a method of manufacturing the same. And to provide.

[0008]

In order to solve the above problems, a gasket for a spark plug according to the present invention is provided at a base end of a mounting screw portion formed on an outer peripheral surface of a metal shell of a spark plug. At least a part of the surface is coated with a chromate film having a chromium content of at least 95% by weight of trivalent chromium and a film thickness of 0.2 to 0.5 μm. It is characterized by having been done.

[0009] The gasket is formed by screwing a screw portion of the metal shell into a screw hole on the cylinder head side, so that a flange-shaped gas seal portion formed on the base end side of the screw portion and an opening peripheral portion of the screw hole. It compresses and deforms so as to be crushed, and serves to seal between the screw hole and the gas seal portion. In the above configuration, what is formed on the surface of the gasket is
95% by weight or more of the chromium component contained is trivalent chromium and is a chromate film having a thickness of 0.2 to 0.5 μm. That is, in the ordinary yellow chromate coating, hexavalent chromium accounts for about 25 to 35% by weight of the chromium component, whereas in the coating of the present invention, the content of hexavalent chromium with respect to the chromium component is as low as 5% by weight or less. Therefore, the effect on environmental measures for reducing hexavalent chromium can be enhanced. In addition, the chromate treatment solution used does not contain any hexavalent chromium component as described later, or even if it does, the amount can be significantly reduced as compared with a treatment solution such as a yellow chromate film. Processing problems are less likely to occur.

In the case of a conventional trivalent chromium-based chromate film such as a gloss chromate film commonly referred to as Unichrome or a blue chromate film, the inventors of the present invention have a maximum thickness of about 0.1 μm. And thin,
We thought that sufficient corrosion resistance and heat resistance could not be secured for the gasket in the main use environment of the spark plug, and as a result of intensive studies on the film thickness, we found a film thickness range particularly suitable for the spark plug, The present invention has been completed. That is, by ensuring that the thickness of the chromate film is 0.2 μm or more, the corrosion prevention performance of the chromate film mainly composed of trivalent chromium is significantly improved, and the gasket is sufficiently imparted with corrosion resistance. Will be able to In addition, even in an environment peculiar to a spark plug in which the temperature is likely to rise and an acid attack or the like is apt to occur, the corrosion resistance of the gasket can be sufficiently maintained.

The thickness of the chromate film is 0.2 μm.
If it is less than 1, the anticorrosion performance and heat resistance cannot be sufficiently secured. On the other hand, when the film thickness exceeds 0.5 μm, cracks are generated in the coating (for example, during processing in assembling or the like), or the coating is liable to fall off, leading to impairment of the anticorrosion performance. . The thickness of the chromate film is desirably 0.3 to 0.5 μm. In addition, it is desirable that the chromate film does not substantially contain hexavalent chromium.

The chromate treatment is a kind of chemical conversion treatment in which a chromium component is replacedly deposited while oxidizing and eluting a base metal. Therefore, in an electroless chromate treatment in which power is not supplied from the outside, the base metal needs to be a metal that can be eluted into the chromate treatment bath. In a spark plug, a gasket is generally made of an iron-based material such as carbon steel, and a zinc-based plating layer mainly made of zinc is formed on the surface of the gasket for corrosion protection. it can. This zinc-based plating layer is advantageous as a base metal for forming a chromate film in the above sense. In this case, the eluted zinc component is
Often incorporated into chromate coatings. The zinc-based plating layer can be formed by known electrolytic zinc plating or hot-dip galvanizing. On the other hand, if the electrolytic chromate treatment method is employed, a chromate film can be formed even if the metal component is mainly a nickel-based plating layer made of nickel.

A zinc plating layer is used as the base metal layer, and a chromate film having the above-mentioned thickness range is formed on the zinc plating layer, so that the plating corrosion resistance test method stipulated in JIS 8502 defines “5. Neutral salt spray test”. ”, It is possible to secure a durable time of at least 40 hours until white rust resulting from corrosion of the galvanized layer appears at about 20% or more of the entire surface. This is a sufficient level of corrosion resistance that the gasket of a spark plug should have.

Further, as a problem peculiar to the spark plug,
If the engine continues to operate under the condition of a large heat load, or if the spark plug is mounted at a position relatively close to the exhaust manifold, the temperature of the gasket sometimes rises to about 200 to 300 ° C. There are cases. However, by forming the underlayer metal layer as a galvanized layer and forming a chromate film having the above-described thickness, good durability performance can be obtained even in the following test assuming such a situation. That is, after heating in air at 200 ° C. for 0.5 hour, J
When performing “5. Neutral salt spray test method” in the plating corrosion resistance test method specified in ISH8502,
White rust caused by corrosion of the galvanized layer is approximately 2 on the entire surface.
The endurance time until 0% or more appears is secured for 40 hours or more.

Further, the gasket of the spark plug is easily attacked by acidic components such as carbon dioxide, nitrogen oxides and sulfur oxides contained in exhaust gas and the like. However, by forming the undercoat metal layer as a galvanized layer and forming a chromate film having the above-described thickness, good durability performance can be obtained even in the following test assuming such a situation. That is, when the “7. Cass test method” in the plating corrosion resistance test method specified in JIS H8502 was performed, the durability time until white rust resulting from corrosion of the galvanized layer appeared about 20% or more of the entire surface. , 2
Reserved for more than 0 hours.

Next, the method for producing a gasket for a spark plug according to the present invention is characterized in that the gasket is immersed in a chromate treatment bath in which a trivalent chromium salt and a complexing agent for trivalent chromium are blended. And a chromate film as described above.

By using a bath containing a trivalent chromium salt and a complexing agent for trivalent chromium as a chromate treatment bath, a dense and thick trivalent chromium-based chromate film, which is difficult with a general chromate treatment method, can be formed. Thus, a trivalent chromium-based chromate film having a thickness of 0.2 to 0.5 μm, which is the gist of the gasket for a spark plug of the present invention, can be easily formed. A method for forming such a chromate film is disclosed in German Published Patent Application DE.
Details are disclosed in 19638176A1. The outline will be described below.

As described above, in the process of forming the chromate film, the underlying metal (eg, zinc) is first oxidized and eluted in the treatment bath, and the eluted underlying metal component reacts with the solution containing chromate ions. It has been theorized that the main mechanism is that trivalent chromium forms a polymer complex by a hydroxyl group or an oxygen bridge and precipitates and deposits on the surface of the underlying metal in a gel state. In this case, in order for the chromate film to grow, the elution of the base metal and the reaction and deposition of the eluted base metal with the chromate ions in the bath must proceed in parallel. However, when the chromate film is deposited to some extent, the elution reaction of the underlying metal layer, which is a heterogeneous reaction via the interface with the bath solution, is prevented, and the film growth stagnates.

According to the disclosure of the above-mentioned German published patent application, in order to increase the thickness of the coating, dissolution of the base metal and
It is important that the rate of reverse dissolution of the deposited chromate film be as small as possible while increasing the rate of film deposition by the reaction between the dissolved base metal component and trivalent chromium. Then, in the above method, it is considered that by adding an appropriate complexing agent to the bath and complexing trivalent chromium, the precipitation of the film is promoted and the film can be made thicker.

Examples of complexing agents include various chelating agents (dicarboxylic acid, tricarboxylic acid, oxyacid, hydroxyl dicarboxylic acid or hydroxyl tricarboxylic acid, etc .: for example, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid) , Coric acid, aseleic acid, sebacic acid, maleic acid, phthalic acid, terephthalic acid, tartaric acid, citric acid,
It is effective to use malic acid, ascorbic acid, etc., but other complexing agents may be used. The complexing agents that can be used are as described in the aforementioned German patent publication.

In order to increase the film thickness, it is also effective to raise the temperature of the chromate treatment bath to about 20 to 80 ° C. If the bath temperature is lower than 20 ° C., the effect of increasing the coating thickness by increasing the temperature can hardly be expected, and if the bath temperature is 80 ° C. or higher, control of bath conditions becomes difficult due to severe evaporation of water from the bath. Further, the immersion time of the gasket as the object to be treated in the chromate bath is preferably set to 20 to 80 seconds. If the immersion time is less than 20 seconds, the thickness of the formed chromate film may not be sufficiently secured. On the other hand, the immersion time is 80
When the time exceeds seconds, the formed chromate film becomes too thick, cracks are generated in the film (for example, during processing in assembling or the like), or the film is liable to fall off.
Rather, the anticorrosion performance may be impaired.

On the other hand, in order to promote the dissolution of the underlying metal, the re-dissolution of the deposited film should not be severe,
It is effective to lower the pH of the chromate treatment solution. A desirable pH range is, for example, about 1.5 to 3. Further, in order to suppress the re-dissolution of the deposited film, it is effective to incorporate a hardly re-dissolved hydroxide such as nickel, cobalt or copper into the film. For this purpose, the above metal compound can be dissolved and blended in the chromate treatment bath.

Next, as a result of further studies by the present inventors, a predetermined amount of a sodium salt (for example, sodium nitrate, etc.) is adjusted so that the content of the sodium component in the chromate-treated film is 2 to 7% by weight. ) Was found to be more easily formed into a thick chromate film by adding it to the chromate treatment bath. Although the detailed mechanism is unknown, it is presumed that if sodium ions are taken into the chromate film, the chromate film is less likely to be redissolved in the treatment bath. If the content of the sodium component in the chromate-treated film is out of the range of 2 to 7% by weight, it may be difficult to secure a thick film of the chromate film to 0.2 μm or more. The content of the sodium component in the chromate-treated film is more desirably 2 to 6% by weight.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiments of the present invention will be described below with reference to the drawings. A spark plug 100 as an example of the present invention shown in FIG. 1 has a cylindrical metal shell 1, an insulator 2 fitted in the metal shell 1 so that the tip protrudes, and an insulator with the tip protruding. A center electrode 3 provided inside the body 2 and a ground electrode 4 having one end coupled to the metal shell 1 and the other end facing the center electrode 3 are provided. A spark discharge gap g is formed between the ground electrode 4 and the center electrode 3.

The insulator 2 is made of, for example, a ceramic sintered body such as alumina or aluminum nitride, and has a through hole 6 for fitting the center electrode 3 along its own axial direction. . The terminal fitting 13 is inserted and fixed to one end of the through hole 6, and the center electrode 3 is inserted and fixed to the other end of the through hole 6. A resistor 15 is arranged between the terminal fitting 13 and the center electrode 3 in the through hole 6. Both ends of the resistor 15 are electrically connected to the center electrode 3 and the terminal fitting 13 via conductive glass seal layers 16 and 17, respectively.

The metal shell 1 is formed of a metal such as carbon steel into a cylindrical shape, and forms a housing of the spark plug 100 and has a plug 100 on its outer peripheral surface.
Is formed on the engine block (not shown). Reference numeral 1e denotes a tool engagement portion that engages a tool such as a wrench or a wrench when the metal shell 1 is attached, and has a hexagonal axial cross-sectional shape. On the other hand, between the inner surface of the rear opening of the metal shell 1 and the outer surface of the insulator 2, a ring-shaped wire packing 62 that engages with the rear peripheral edge of the flange-shaped protrusion 2e is arranged. On the rear side, a ring-shaped packing 60 is arranged via a filling layer 61 such as talc. Then, the insulator 2 is pushed forward toward the metal shell 1 and, in this state, the opening edge of the metal shell 1 is swaged inward toward the packing 60 to form a swaged portion 1d. It is fixed to the insulator 2.

A gasket 30 is fitted into the base end of the screw 7 of the metal shell 1. The gasket 30 is a ring-shaped part obtained by bending a metal plate material such as carbon steel, and the screw portion 7 is screwed into a screw hole on the cylinder head side to form a flange-shaped gas seal portion 1f on the metal shell 1 side. Between the screw hole and the opening peripheral portion of the screw hole, it is compressed in the axial direction and deformed so as to be crushed, and serves to seal a gap between the screw hole and the screw portion 7.

Next, a zinc plating layer 41 for corrosion protection is formed on the entire outer surface of the underlayer (for example, carbon steel) 40 of the metal shell 1.
(A zinc-based plating layer) is formed, and further outside thereof is covered with a chromate film 42. Gasket 30
Similarly, a galvanized layer 45 and a chromate film 46 are also formed on the outer surface. These galvanized layers and chromate films are both formed by the same method.

The galvanized layer 41 is formed by a known electrolytic galvanizing method.
It is about 10 μm. If the thickness is less than 3 μm, it may not be possible to secure sufficient corrosion resistance.
If the film thickness exceeds the above range, the specification is excessive in terms of securing the corrosion resistance, and the plating time is prolonged and the production efficiency is reduced, leading to an increase in cost.

On the other hand, in the chromate film 42, 95% by weight or more of the chromium component contained is trivalent chromium, and the thickness thereof is 0.2 to 0.5 μm. The chromium component preferably has a trivalent chromium component as much as possible, more preferably substantially all of the chromium component is a trivalent chromium component.

Hereinafter, an example of a method for forming the chromate film 42 will be described. That is, a gasket on which a galvanized layer having a predetermined thickness is formed by a known electrolytic galvanizing method or the like is immersed in a chromate treatment bath 50 (FIG. 2). The use of a chromate treatment bath has already been described. Thereby, as shown in FIG. 1, a chromate film 42 is formed on the surface of the galvanized layer 41 of the gasket. Although the gasket may be simply immersed in the chromate treatment bath, a known barrel treatment method or the like may be employed.

After being washed and dried, the metal shell 1 after the chromate treatment is incorporated into the spark plug 100 shown in FIG. The metal shell 1 or the gasket 30 has a chromate coating formed on its galvanized layer, which has significantly higher anticorrosion performance and heat resistance than a conventional trivalent chromium-based chromate coating and furthermore a yellow chromate coating. It becomes possible to sufficiently impart corrosion resistance to the plating layer. Hereinafter, results of an experiment performed to confirm the effect will be described.

[0033]

EXAMPLES (Example 1) A gasket having the shape shown in FIG. 1 was manufactured using a carbon steel wire SWCH8A for cold heading specified in JIS G3539 as a raw material. In addition, the nominal size of the screw portion 7 of the metal shell 1 was 14 mm, and the axial length was about 19 mm. Next, by subjecting this to electrolytic zinc plating using a known alkaline cyanide bath,
A galvanized layer having a thickness of about 6 μm was applied.

Next, the chromate treatment bath was washed with deionized water.
Per liter of chromium (III) chloride (CrCl₃
・ 6H₂O) and 50 g of cobalt (II) nitrate (Co (NO
₃) ₂) And sodium nitrate (NaNO₃) To 10
0 g, malonic acid 31.2 g
Take a bath, maintain the liquid temperature at 60 ° C with a heater,
Adjust pH of bath to 2.0 by adding aqueous sodium hydroxide solution
did. Then, place the gasket after galvanizing
Immersion for 60 seconds in water treatment solution 50, then after washing and drying,
Dry with 80 ° C hot air to form a chromate film
(Test article: Example).

On the other hand, as a yellow chromate treatment bath, 7 g / liter of chromic anhydride and 3 g of sulfuric acid were used with respect to deionized water.
Per liter and nitric acid at a rate of 3 g / liter were prepared and maintained at a liquid temperature of 20 ° C. Then, a gasket was immersed therein for about 15 seconds, pulled up, and dried to produce a product (test product: Comparative Example). Further, as a luster chromate treatment bath, a solution prepared by dissolving 3 g / l of potassium chromium sulfate, 4 g / l of nitric acid, and 2 g / l of hydrofluoric acid in deionized water was prepared and kept at a liquid temperature of 20 ° C. Then, a gasket was immersed therein for about 15 seconds, pulled up, and dried to produce a product (test product: Comparative Example). In each of the above test products, the chromate film thickness was set to SE
M was 0.33 μm, 0.31 μm, 0.07 μm
m. The cross-sectional SEM image used for the film thickness measurement is shown in FIG. 10 (the results are shown for the metal shell similarly subjected to the chromate treatment, but the same results are obtained for the gasket 30). (A) is the test sample, (b)
Is an SEM image of the test article, and (c) is an SEM image of the test article.
In order to facilitate observation of the chromate film, an Au thin film is formed on the surface of the film by a sputtering method. In the SEM image, the chromate coating layer having a low conductivity is darker than the zinc plating layer having a high conductivity and the Au coating layer, so that the image of the chromate coating can be easily confirmed from the contrast. . In addition, in each SEM image,
A white line is displayed at a position corresponding to each boundary between the chromate coating layer confirmed from the contrast, the galvanized layer and the Au coating layer, and the film thickness is identified from the distance between the white lines.

On the other hand, the presence of chromium in each of the formed chromate films was examined by X-ray photoelectron spectroscopy (XPS). FIG. 2 shows the peak portion of chromium (2p _2/3 ) in the photoelectron spectrum of the test sample (the results are shown here for the metal shell similarly subjected to chromate treatment, but the same results are obtained for the gasket 30). Becomes). In the test sample (solid line), no peak appears at the position corresponding to hexavalent chromium, and it can be confirmed that almost all of the chromium component is trivalent chromium. On the other hand, in the test sample, the peak of trivalent chromium overlaps with the peak of hexavalent chromium, and a bump-like bulge appears on the shoulder on the high energy side of the peak.

FIG. 3 shows the shape of each peak assuming that the intensity of the photoelectron X-ray is I (vertical axis in the figure: cps) and the binding energy value is x (horizontal axis in the figure: eV). -(X-μ) ² / 2σ ² ｝ ‥‥ (1) (where μ represents the x-coordinate of the peak and σ represents the half-width of the peak curve), and the result of the peak separation analysis. (Here, the results are shown for the metal shell similarly subjected to the chromate treatment, but the same result is obtained for the gasket 30 as well). According to this, assuming that the peak height of trivalent chromium is I1 and the peak height of hexavalent chromium is I2, I2 / (I1 + I2) is about 0.2 (in addition,
From the viewpoint of hexavalent chromium reduction, it can be said that I2 / (I1 + I2) is desirably 0.05 or less.
A calibration curve was prepared using a standard sample of dichromium trioxide, and the weight content of hexavalent chromium in the total amount of chromium components was calculated based on the calibration curve. About 15% by weight was hexavalent chromium and the balance was trivalent chromium. It turned out to be chrome. In addition,
The same analysis was performed for the sample and the sample, but substantially all of the chromium component was trivalent chromium.

For the above test items, JIS85
Perform the “5. Neutral salt spray test method” in the corrosion resistance test method of plating specified in No. 02, and evaluate the durability by the time until white rust resulting from corrosion of the galvanized coating appears about 20% or more of the entire surface. went. In this specification, the metal shell is used as a sample as it is, and one surface of the tool engaging portion (hexagonal portion) is used as a sample surface. The results are shown in FIG. 4 (the results are shown for the metal shell similarly subjected to the chromate treatment, but the same results are obtained for the gasket 30 as well). That is, a gasket satisfying the requirements of the spark plug of the present invention has a durability time of 24 hours.
The result was very good at 0 hours. This is almost equivalent to the test sample using the yellow chromate treatment, and is about 12 times as large as the test sample using the conventional thin glossy chromate film.

Further, for a test specimen prepared in the same manner as above, which was prepared separately, the “7. Cas test method” in the corrosion resistance test method for plating specified in JIS H8502 was performed.
The evaluation was made based on the durability time until white rust resulting from corrosion of the galvanized coating appeared on about 20% or more of the entire surface. Further, a durability test using sulfuric acid and nitric acid was performed as follows. First, a sulfuric acid solution or a nitric acid solution having a pH of 2 is placed in a desiccator, and the test article is sealed in the desiccator so that the test article and the acid solution do not come into direct contact with each other but can come into contact with the solution vapor. Then, the desiccator was held in a thermostat at a temperature of 90 ° C., and evaluated according to the same criteria as in the above-mentioned Cass test. The above results are shown in FIG.
6 and FIG. 6 (the results are shown for the metal shell similarly subjected to the chromate treatment, but the same result is obtained for the gasket 30). In each of the tests, the test specimens using the yellow chromate treatment or the gloss chromate treatment showed only a short durability time, whereas the test specimens according to the examples of the present invention showed good durability results. It turns out that it shows.

Next, using the gasket 30 described above, FIG.
And a 6c, 2000c spark plug
c gasoline engine, engine speed 56
A mounting test was performed by operating continuously for 10 hours at 00 rpm with the throttle fully open. The temperature of the gasket during operation was about 200 ° C. FIG. 7 shows the results of a neutral salt spray test similar to the above for each test product after the mounting test (the results are shown here with a metallic shell similarly subjected to chromate treatment, but the gasket 30).
Has exactly the same result). While the test product using the yellow chromate treatment or using the gloss chromate treatment showed only a short durability time of about 20 hours, the test product according to the embodiment of the present invention has the durability even after mounting the engine. The time was 180 hours, which was a good result.

The specimen was heated to 200 ° C. in the air using a thermostat, kept for 30 minutes, and then subjected to the same neutral salt spray test. While the test article using the chromate treatment showed only a short durability time of about 20 hours, the test article according to the example of the present invention showed a good result of 200 hours.

(Example 2) A gasket similar to that of Example 1 was manufactured under the same conditions up to zinc plating. Then, the chromate treatment bath, per liter of deionized water, chromium chloride (III) (CrCl ₃ · 6H
₂ O) to 50 g, cobalt nitrate (II) (Co (N
₃ g of O ₃ ) ₂ ) and 5 g of sodium nitrate (NaNO ₃ )
A bath was prepared by dissolving 0 to 150 g and malonic acid at a ratio of 31.2 g. The bath was maintained at a liquid temperature of 60 ° C. by a heater, and the pH of the bath was adjusted to 2.0 by adding an aqueous solution of caustic soda. Then, the gasket after galvanization was immersed in the chromate treatment liquid 50 for 60 seconds, washed with water, dried, and dried with warm air at 80 ° C. to form chromate films of various thicknesses. Then, the thickness of the obtained chromate film was measured by the same SEM cross-sectional observation as in Example 1, and the Na content was determined by X-ray photoelectron spectroscopy (XP).
S). The above results are shown in FIG. 8 (the results are shown for the metal shell similarly subjected to the chromate treatment, but the same results are obtained for the gasket 30 as well). That is, the Na content in the coating is 2 to 7% by weight,
In particular, when the content is 2 to 6% by weight, a chromate film having a large film thickness can be obtained in a relatively short time.

(Example 3) A gasket similar to that of Example 1 was manufactured under the same conditions up to galvanizing. Then, the chromate treatment bath, per liter of deionized water, chromium chloride (III) (CrCl ₃ · 6H
₂ O) to 50 g, cobalt nitrate (II) (Co (N
₃ g of O ₃ ) ₂ ) and 5 g of sodium nitrate (NaNO ₃ )
A bath was prepared by dissolving at a rate of 0 to 150 g and malonic acid at 31.2 g. The temperature of the bath was maintained at 60 ° C. by a heater, and the pH of the bath was adjusted by adding an aqueous solution of caustic soda.
Adjusted to zero. Then, the gasket after galvanizing was immersed in the above-mentioned chromate treatment solution for 40 to 80 seconds, washed with water, dried, and dried with warm air at 80 ° C to form chromate films of various thicknesses. Each gasket 30 after the formation of the chromate film was evaluated by the same neutral salt spray test as in Example 1. The above results are shown in FIG. 9 (the results are shown for the metal shell similarly subjected to the chromate treatment, but the same results are obtained for the gasket 30). The coating thickness is 0.2-0.5 μm, especially 0.3-
It can be seen that good durability results were obtained when the thickness was 0.5 μm.

[Brief description of the drawings]

FIG. 1 is a longitudinal half sectional view showing a spark plug according to an embodiment of the present invention.

FIG. 2 is a view showing the result of X-ray photoelectron spectroscopy analysis of the chromate film of the test sample of Example 1 and the test product thereof (peak portion of chromium (2p _2/3 ) in the photoelectron spectrum).

FIG. 3 is a view showing a result of performing peak separation analysis on a chromium (2p _2/3 ) peak portion of a photoelectron spectrum of the same test sample.

FIG. 4 is a graph showing the results of a neutral salt spray test performed on each test sample of Example 1.

FIG. 5 is a graph showing the result of the same Cass test.

FIG. 6 is a graph showing the result of the same acid durability test.

FIG. 7 is a graph showing the results of a neutral salt spray test after mounting the engine.

FIG. 8 shows Na in the chromate film of the test sample of Example 2.
5 is a graph showing the relationship between the amount and the coating thickness.

FIG. 9 is a graph showing the relationship between the thickness of a chromate film and the durability of salt water spray of the test article of Example 3.

FIG. 10 is a cross-sectional SEM of each test sample used in Example 1.
image.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Metal shell 2 Insulator 3 Center electrode 4 Ground electrode 30 Gasket 50 Chromate treatment bath 41, 45 Zinc plating layer 42, 46 Chromate coating 100 Spark plug

Claims (11)

[Claims] 1. A spark plug which is used by being fitted into a base end portion of a mounting screw portion formed on an outer peripheral surface of a metal shell of a spark plug, wherein at least a part of the surface has 95% by weight of a chromium component contained therein. A gasket for a spark plug, characterized in that the above is trivalent chromium and is coated with a chromate film having a thickness of 0.2 to 0.5 μm. 2. The gasket for a spark plug according to claim 1, wherein said chromate film does not substantially contain a hexavalent chromium component. 3. The gasket for a spark plug according to claim 1, wherein the content of the sodium component in the chromate film is 2 to 7% by weight. 4. The gasket for a spark plug according to claim 1, wherein the gasket is formed with a zinc plating film as a base metal layer of the chromate film. 5. “5. Neutral salt spray test method” in the plating corrosion resistance test method specified in JIS H8502.
5. The gasket for a spark plug according to claim 4, wherein a durability time required for white rust resulting from corrosion of the galvanized coating film to appear by about 20% or more of the entire surface is 40 hours or more. 6. After heating at 200 ° C. for 30 minutes in the air, the corrosion of the galvanized film is observed when “5. Neutral salt spray test method” in the corrosion resistance test method for plating specified in JIS H8502 is performed. The gasket for a spark plug according to claim 4 or 5, wherein a durable time until white rust originating from the surface appears about 20% or more of the entire surface is 40 hours or more. 7. When the “7. Cass test method” in the corrosion resistance test method for plating specified in JIS H8502 is performed, the white rust resulting from the corrosion of the galvanized film appears until about 20% or more of the entire surface appears. Endurance time is 20
The gasket for a spark plug according to any one of claims 4 to 6, wherein the time is not less than an hour. 8. A method for manufacturing a gasket for a spark plug, which is used by being fitted into a base end portion of a mounting screw portion formed on an outer peripheral surface of a metal shell of a spark plug, comprising: a trivalent chromium salt; By immersion in a chromate treatment bath containing a complexing agent for chromium, 95% by weight or more of the chromium component contained on the surface of the gasket was trivalent chromium and the film thickness was 0.2%. ~ 0.5
A method for producing a gasket for a spark plug, comprising forming a μm chromate film. 9. The chromate treatment bath is at 20 to 80 ° C.
9. The method for producing a spark plug according to claim 8, wherein the spark plug is used in a state where the bath temperature is adjusted. 10. The method for producing a gasket for a spark plug according to claim 8, wherein the immersion time of the gasket in the chromate treatment bath is 20 to 80 seconds. 11. The chromate treatment bath according to claim 1, wherein the content of the sodium component in the obtained chromate film is 2-7.
The method for producing a gasket for a spark plug according to any one of claims 8 to 10, wherein a predetermined amount of a sodium salt is blended so as to be the weight%.