JP2001326054A - 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 of a mounting screw 7 formed on the outer peripheral surface of the metal shell 1, at least a part of its surface contains cationic components mainly composed of chromium and silicon. 90% by weight or more of the chromium component is coated with a silicon composite chromate film which is trivalent chromium.

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.

A 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] Chromate films, including yellow chromate films, 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.

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 the surface and has excellent corrosion resistance and heat resistance as compared with a conventional chromate film, and a method of manufacturing the same. Is 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 covered with a silicon composite chromate coating in which the cationic component is mainly chromium and silicon, and 90% by weight or more of the contained chromium component is trivalent chromium. It is characterized by having been done.

[0009] The "cationic component" is defined as
In a photoelectron spectrum obtained by analysis by linear photoelectron spectroscopy (XPS or ESCA), a component in which the binding energy peak of the component of interest has a chemical shift in the direction in which the ionic valence is positive has occurred. Say.

[0010] The gasket is formed by screwing the threaded portion of the metal shell into a threaded hole on the cylinder head side so that a flange-shaped gas seal portion formed on the proximal end side of the threaded portion and the peripheral edge of the opening of the threaded 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
A silicon composite chromate film containing a silicon component and a chromium component as a main component of a cationic component, and 90% by weight or more of the chromium component is trivalent chromium. That is, in a normal 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 10% 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, or even if it does, the amount can be greatly reduced compared to the treatment solution such as a yellow chromate film. Is also unlikely to occur.

The silicon composite chromate film used in the present invention is characterized in that it contains a silicon component as a cationic component. The inclusion of the silicon component in the composite chromate coating significantly improves the anticorrosion performance as compared with a normal trivalent chromium-based chromate coating, and provides a gasket with sufficient corrosion resistance. Become. Further, by combining the silicon component, the heat resistance of the coating film is dramatically improved, and the corrosion resistance of the gasket can be sufficiently maintained even in an environment where the temperature of the spark plug tends to increase.

The silicon component can be contained, for example, in a form bonded to oxygen, that is, in the form of silicon oxide such as silica. In this specification, if the peak of silicon having a valence of +4 or near and the peak of oxygen having a valence of −2 or close thereto are simultaneously detected in the XPS photoelectron spectrum, the silicon component is oxygen. It is considered to be contained in a form combined with

On the other hand, a chromate film is formed by a reaction between a base metal and a solution containing a chromate ion. It is said that the main mechanism is the formation and deposition of a complex in the form of a gel on the surface of the underlying metal by forming a complex. When a hydroxyl group is bonded to trivalent chromium, the valence of chromium apparently shifts to +4 due to the effect of protons contained in the hydroxyl group.
In the present specification, in the XPS photoelectron spectrum, if a peak component chemically shifted from the peak position of trivalent chromium to a position corresponding to a valence of approximately +4 is observed,
The chromium component is considered to be a component of the chromate film.

The chromate treatment is a kind of chemical conversion treatment in which a chromium component is replacedly deposited by 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 adopted, a chromate film can be formed even if the metal component is mainly a nickel-based plating layer made of nickel.

Next, the method of forming the formed chromate film into a silicon composite chromate film which is a feature of the present invention includes, for example, a method in which a water-soluble silica-based compound such as an alkali silicate such as water glass is used in a chromate treatment bath. There is a method in which a salt is blended and a silica component is incorporated in the formed chromate film in a mixed state. However, from the viewpoint of further improving the anticorrosion performance (particularly the durability against the salt spray test) and the heat resistance, the silicon composite chromate film preferably has the following structure.
In other words, the silicon composite chromate film is formed such that the main component of the cation component is a chromium component and 90% of the chromium component
The above is at least two of a trivalent chromium-based chromate layer composed of trivalent chromium and a silica-based layer mainly composed of silicon oxide and directly or indirectly covering the trivalent chromium-based chromate layer via another layer. Layers.

In the trivalent chromium-based chromate layer, it is sufficient that the total weight of the chromium component is 50% by weight or more based on the total weight of the cation component, and the remainder is composed of another cation component such as silicon or zinc. , Nickel or the like. On the other hand, the silica-based layer also needs to have the total weight of the silicon component to be 50% by weight or more based on the total weight of the cation component, and the remainder is the other cation component.
For example, it may be a component such as chromium, zinc, or nickel.

Such a film formation can be realized by, for example, the method for manufacturing a spark plug of the present invention including the following steps. Chromate treatment step: By immersing the gasket in a chromate treatment bath, a trivalent chromium-based chromate layer in which 90% by weight or more of the chromium component is made of trivalent chromium is formed on the surface of the gasket. Silica-based layer forming step: After applying a silicate solution in which an alkali silicate is dissolved in a predetermined solvent to a gasket on which the trivalent chromium-based chromate layer is formed, the solvent is evaporated. Forming a silica-based layer mainly composed of silicon oxide on the trivalent chromium-based chromate layer. In this case, the silica-based layer mainly includes a cation component mainly composed of an oxide composed of an alkali metal element and silicon.

As described above, by forming a trivalent chromium-based chromate layer on the surface of the gasket in advance and covering the surface with the silica-based layer, the anticorrosion performance of the coating can be further greatly improved. In addition, the heat resistance of the coating is, of course, the conventional trivalent chromium-based chromate coating,
Levels that surpass even yellow chromate coatings containing hexavalent chromium are achieved.

The silica-based layer may be formed by a vapor phase film forming method such as high frequency sputtering, reactive sputtering, ion plating or chemical vapor deposition (CVD). However, according to the above method by applying a silicate solution, the gasket after the chromate treatment is immersed in the silicate solution, or the silicate solution is applied by spraying or the like, and then the coating film is only dried. Can easily form a silica-based layer.

A trivalent chromium-silicon mixed layer in which a trivalent chromium component and a silicon component are mixed at an intermediate content ratio between the trivalent chromium-based chromate layer and the silica-based layer. May be formed. Thereby, the anticorrosion performance or heat resistance of the silicon composite chromate film may be further improved in some cases. For example, trivalent chromium-
The formation of the silicon mixed layer means that a kind of composition gradient structure is formed between the trivalent chromium-based chromate layer and the silica-based layer. The above-described improvement in the performance of the coating film can be achieved by such effects as an improvement in the adhesive force between the chromate layer and the silica-based layer at the time of heating, and a reduction in stress based on a difference in shrinkage between the chromate layer and the silica layer.

The reason why the anticorrosion performance of the yellow chromate film is good is that even when the film is destroyed in a corrosive environment, the network structure of trivalent chromium is restored by the action of hexavalent chromium contained therein. It is said that there is. However, in the trivalent chromium-based chromate layer, such a repair effect by hexavalent chromium cannot be expected, so if a defect such as a pinhole occurs in the coating, the influence of corrosion is directly applied to the base such as a zinc-based plating layer. It is thought that corrosion progressed rapidly. However, in the silicon composite chromate film having the above structure, the trivalent chromium-based chromate layer is overcoated with a silica-based layer, so that the influence of corrosion is less likely to reach the surface of the trivalent chromium-based chromate layer and thus the underlayer. It is assumed that the anticorrosion performance is improved.

On the other hand, it is considered that the reason why the conventional chromate film is inferior in heat resistance is that the chromate film shrinks due to heating and easily causes defects such as cracks. However, in the above configuration, even if the above-described defect occurs in the chromate layer, the surface is overcoated with a silica-based layer having good heat resistance, so that the anticorrosion performance is hardly deteriorated. Guessed.

In order to form a uniform silica-based layer, it is also important to improve the wettability between the silicate solution and the base trivalent chromium-based chromate layer. For example,
If defects such as pinholes and cracks (in this case, those that have inherited the defects of the base due to scratches or foreign matter attachment) are formed in the trivalent chromium-based chromate layer, silicic acid with poor wettability When a salt solution is used, bubbles and the like tend to remain in the defect. In this case, it is also effective to mix an appropriate amount of a surfactant in the aqueous silicate solution.

On the other hand, there is also a method in which after the chromate treatment step is completed, the gasket surface is immersed in a silicate solution in an undried or semi-dried state to perform a silica-based layer forming step. That is, after completion of the chromate treatment, a trivalent chromium-based chromate layer is formed on the undried or semi-dried gasket surface in a slightly water-containing state, and is compatible with the subsequently applied silicate aqueous solution. Is also good. As a result, even if a defect is formed in the trivalent chromium-based chromate layer, the silicate aqueous solution sufficiently penetrates into the defect, and it is difficult for bubbles and the like to remain, thereby improving the anticorrosion performance of the coating. can do.

Another effect is that the chromate treatment solution remaining on the surface of the formed trivalent chromium-based chromate layer is mixed with a part of the applied silicate aqueous solution,
There is an advantage that the trivalent chromium-silicon mixed layer is easily formed. The effect of forming the trivalent chromium-silicon mixed layer has already been described.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 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 into the metal shell 1 so that a tip end thereof protrudes,
A center electrode 3 provided inside the insulator 2 with the tip protruding, and a ground electrode 4 having one end coupled to the metal shell 1 and the other end facing the tip of the center electrode 3. Etc. 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 galvanized layer 41 for corrosion protection is formed on the entire outer surface of the base layer (for example, carbon steel) 40 of the metal shell 1.
(A zinc-based plating layer) is formed, and the outside thereof is covered with a silicon composite chromate film 42. Further, a galvanized layer 45 and a silicon composite chromate film 46 are similarly formed on the outer surface of the gasket 30. Both the zinc plating layer and the silicon composite chromate film are 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.
A film thickness exceeding the above is excessive specification from the viewpoint of ensuring corrosion resistance, and leads to an increase in cost.

On the other hand, in the silicon composite chromate film 42, as shown schematically in FIG. 2A, the main component of the cation component is a chromium component, and 90% by weight or more of the chromium component is made of trivalent chromium. It includes a trivalent chromium-based chromate layer (hereinafter referred to as a chromate (III) layer) 42a and a silica-based layer 42c mainly composed of a silicon oxide and covering the chromate (III) layer 42a. , 42c
A trivalent chromium-silicon mixed layer 42 in which a trivalent chromium component and a silicon component are mixed at an intermediate content ratio between the two layers.
b is formed. 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.

The total thickness of the silicon composite chromate film 42 is
For example, it is 0.8 to 1.5 μm. The total thickness is 0.8
If the thickness is less than μm, the effect of imparting corrosion resistance and heat resistance to the galvanized layer 41 may be insufficient. On the other hand,
Conversely, a film thickness exceeding 1.5 μm is an excessive specification from the viewpoint of ensuring corrosion resistance, and leads to an increase in cost. In addition, the silicon composite chromate film may be easily peeled off.

The thickness of the chromate (III) layer 42a is preferably 0.2 to 0.3 μm. When the thickness is less than 0.2 μm, the silicon composite chromate film 42
May be insufficient in anticorrosion performance. Also, 0.3μ
m, the hexavalent chromium tends to remain in the coating, and the chromate (II
I) The layer 42a may not be obtained. On the other hand, the thickness of the silica-based layer 42c is preferably 0.2 to 0.8 μm. When the thickness is less than 0.2 μm, the anticorrosive performance and heat resistance of the silicon composite chromate film 42 may be insufficient. Conversely, a film thickness exceeding 0.8 μm is an excessive specification, which not only leads to an increase in cost, but also in some cases facilitates peeling of the film.

Hereinafter, an example of a method for forming the silicon composite chromate film 42 will be described. First, 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 liquid 50 (FIG. 3). Thus, as shown in FIG. 3A, a chromate (III) layer 42a is formed on the surface of the galvanized layer 41 of the gasket.

Examples of usable chromate treatment liquids include those containing the following components (so-called,
Colorless or blue chromate treatment solution). Chromic anhydride: 0.1 to 2 g / l Sulfuric acid: 0.3 to 5 g / l Nitric acid: 0.5 to 10 g / l Phosphoric acid: added up to about 2 g / l as needed Hydrofluoric acid: as needed The amount of chromic anhydride, which is a hexavalent chromium source, is reduced to less than half that of 4 to 10 g / liter of a so-called yellow chromate treatment solution. I have. Sulfuric acid functions as a reaction accelerator, and nitric acid functions as an oxidizing agent for elution of a base metal. On the other hand, phosphoric acid has a role of improving the adhesion of the chromate film to the underlying metal, and hydrofluoric acid is taken in as an anion in the film, strengthening the bridge bonds in the polymer-like complex structure, and improving the film strength and thus the anticorrosion performance. Play a role to improve.

It is also possible to use the following solution which does not use a solute serving as a hexavalent chromium source (a so-called chromium (III) chromate treatment solution). Chromium potassium sulfate (so-called chrome alum): 2.5 to
3.5 g / l nitric acid: 3.5 to 4.5 g / l hydrofluoric acid: 1.5 to 2.5 g / l provided that the content ratio of trivalent chromium to the chromium component in the obtained chromate film is 90% by weight or more. The chromate treatment liquid is not limited to the above as long as it can be used.

It is considered that the reaction during the chromate treatment is generally as follows. That is, when the gasket on which the galvanized layer is formed is immersed in the liquid, chromium ions in the liquid are replaced by the dissolution of zinc, and the gasket precipitates as a gel-like film mainly composed of chromium (III) hydroxide. . Part of the dissolved zinc is taken into the coating in the form of, for example, zinc chromate. As a structure of the formed chromate film, for example, a structure as shown in FIG. That is, a polymeric complex of trivalent chromium is formed in a network by a hydroxyl group or an oxygen bridge, and a part of the network forms an anion such as chromate, dichromate, sulfuric acid, chloride, or fluoride. Is different depending on the composition of the chromate treatment liquid to be used). The chromate (III) layer 42a
The formation thickness can be adjusted by, for example, adjusting the immersion time of the gasket in the chromate treatment liquid and the liquid temperature.

Next, the gasket after the formation of the chromate (III) layer 42a is immersed in an undried or semi-dried state in an aqueous silicate solution 51 (silicate solution), and then dried. As a result, as shown in FIG.
A trivalent chromium-silicon mixed layer 42b and a silica-based layer 42c are formed.

As the silicate aqueous solution 51, a so-called aqueous solution of water glass can be used. Water glass
The general formula is represented by M ₂ O · nSiO ₂ (where M is an alkali metal element such as sodium and potassium), and silicon dioxide is precipitated by drying and hardened into a gel. Then, the obtained silica-based layer 42c is composed of an oxide in which the main component of the cation component is an alkali metal element and silicon (eg,
(A gel-like cured product mainly composed of an alkali silicate and silicon dioxide).

As the water glass, those having n of about 2 to 4 are used. When n is 2 or less, gelling hardly occurs, and the obtained silica-based layer 42c becomes water-soluble, so that a stable coating cannot be obtained. On the other hand, when n exceeds 4, hydrolysis of the alkali silicate in the solution 51 proceeds excessively, and silicon dioxide gel precipitates out.
Because of the precipitation, the application step cannot be performed stably. Note that n is more preferably in the range of 3 to 4. In this case, if the content of the alkali metal M in the form of M ₂ O in the entire silicon composite chromate film 42 is μ1, and the content of the silicon component in the same manner as SiO ₂ is μ2, the value of μ2 / μ1 is obtained. From 2 to 4,
It is desirable to adjust within the range of 3 to 4.

The aqueous silicate solution 51 is composed of chromate (III)
In order to form the silica-based layer 42c as uniformly as possible on the layer 42a, the concentration of the alkali silicate is set to 30 to 200 g /
It is desirable to adjust to liters. If the concentration is less than 30 g / liter, the formed thickness of the silica-based layer 42c becomes insufficient, and it may not be possible to ensure the corrosion prevention performance or the heat resistance performance of the silicon composite chromate film 42. On the other hand,
When the concentration exceeds 200 g / liter, the aqueous solution of silicic acid 5
1 has too high a viscosity, and it is difficult to form a uniform silica-based layer due to occurrence of coating unevenness or the like.

As shown in FIG. 3A, the chromate (III) layer 42a immediately after being pulled up from the chromate treatment liquid 50
The chromate treatment liquid 50 remains on the surface layer of the silicate solution. When this is immersed in the silicate aqueous solution 51, as shown in FIG. Thus, a mixed layer 42b 'is formed. When this is dried, as shown in FIG. 3C, the mixed layer 42 is interposed between the chromate (III) layer 42a and the silica-based layer 42c.
A trivalent chromium-silicon mixed layer 42b derived from b ′ is formed. The trivalent chromium-silicon mixed layer 42b is mixed with the chromate treatment liquid 50 and the silicate aqueous solution 51 (or the permeation of the silicate aqueous solution 51 into the chromate (III) layer 42a).
Therefore, the trivalent chromium component and the silicon component are mixed at an intermediate content ratio between the two layers 42a and 42c. This is because a trivalent chromium-silicon mixed layer 42b is provided between the chromate (III) layer 42a and the silica-based layer 42c.
, A kind of composition gradient structure is formed, and effects such as improvement in adhesion between the two layers 42a and 42c and reduction in stress based on the difference in heat shrinkage can be achieved.

After completion of the chromate treatment, the surface of the gasket in an undried or semi-dried state is treated with chromate (III).
Since the layer 42a is formed in a wet state, the compatibility with the silicate aqueous solution 51 is improved. Therefore, as shown in FIG. 4 (a), even if a defect def such as a pinhole is formed in the chromate (III) layer 42a, the silicate aqueous solution 51 easily penetrates sufficiently there and the formed silica System layer 42
It is possible to make it difficult for bubbles and the like to remain in c (FIG. 4).
(B)).

The gasket 30 thus treated
The silicon composite chromate film formed on the galvanized layer has significantly higher anticorrosion performance and heat resistance than the conventional trivalent chromium-based chromate film and furthermore the yellow chromate film. It becomes possible to sufficiently impart durability against corrosion.

After the chromate treatment, the surface of the gasket may be dried and immersed in a silicic acid aqueous solution 51. In this case, since the chromate (III) layer 42a is once dried, the mixed layer 42b 'of the chromate treatment liquid 50 and the silicate aqueous solution 51 is hardly formed. Therefore, as shown in FIG. 2B, the chromate (III) layer 42
A clear trivalent chromium-silicon mixed layer 42b may not be formed between a and the silica-based layer 42c.

Further, an appropriate amount of water glass may be blended in the chromate treatment solution, and a gasket having a zinc-based plating layer formed thereon may be immersed and dried. The silicon composite chromate coating obtained by such a method also has good corrosion resistance and corrosion resistance. In this case, FIG.
As shown in (c), the obtained silicon composite chromate film 42 is a polymer complex substrate 42d of trivalent chromium,
It is considered that the gel-like cured product 42e mainly composed of an alkali silicate and silicon dioxide has a dispersed structure.

[0049]

EXAMPLES The following experiments were performed to confirm the effects of the present invention. First, a gasket having a shape shown in FIG. 1 was manufactured using a carbon steel wire SWCH8A for cold heading specified in JIS G3539 as a raw material. The nominal diameter of the threaded portion 7 of the metal shell 1 is 14 mm, and the axial length is about 19 mm.
mm. Next, a zinc plating layer having a film thickness of about 6 μm was formed by subjecting this to electrolytic zinc plating using a known alkaline cyanide bath.

Next, 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 as a chromate treatment solution, and the solution temperature was maintained at 20 ° C. . On the other hand, Na ₂ O
A sodium silicate (water glass) having a composition of 3.5 SiO ₂ was dissolved in deionized water at a concentration of 100 g / liter to prepare a silicate aqueous solution. Then, the gasket after galvanization is immersed in the above chromate treatment solution for 15 seconds,
Next, this was immediately immersed in an aqueous silicate solution without drying without performing only liquid drainage, and further dried with warm air at 80 ° C. to form a silicon composite chromate film (test product: product of the present invention) ).

While this silicon composite chromate film was etched in the thickness direction, the XPS photoelectron spectrum was measured. FIG. 5 shows the measurement result on the metal shell side which has been similarly subjected to the chromate treatment. The same result is obtained for the gasket 30. From the spectral peak intensity of each component at each etching depth, approximately 0.4μ from the surface
Up to about m, a peak of chromium (2p _2/3 ) was hardly observed, indicating that the layer was a silica-based layer mainly composed of silicon oxide. In addition, as a result of more detailed examination by a fluorescent X-ray analysis method, the silica-based layer
Approximately 77% by weight in terms of O ₂ , and sodium was converted to Na ₂
It was found that the content was about 22% by weight in terms of O.

On the other hand, at a depth of 0.7 to 1 μm from the surface, although a slight silicon peak was observed, the main component of the cation component was chromium, followed by zinc. Further, as a result of further examining the peak of chromium (2p _2/3 ), 99% by weight or more of the chromium component was trivalent chromium. That is, it was found that the portion having a thickness of about 0.3 μm in the above depth range was a trivalent chromium-based chromate layer.

From the spectral peak intensity of each component of XPS, a portion containing a chromium component and a silicon component in an intermediate composition between the two layers is located at a thickness of about 0.3 μm located between the two layers. It turned out that it was a valent chromium-silicon mixed layer.

After immersion in the chromate treatment solution 50 for 15 seconds, the product was not immersed in the silicate aqueous solution 51 but dried as it was (test product: comparative example). What was immersed only in the silicate aqueous solution 51 and dried (test product: comparative example) was also prepared. When the test sample and the formed coating film were analyzed by XPS and X-ray fluorescence spectroscopy, the former was found to be a chromate film having a weight content ratio of trivalent chromium in the chromium component of about 99% or more and a thickness of about 0.5 μm. In the latter, 77% by weight of silicon in terms of SiO ₂ and sodium
It was found that the oxide-based coating contained 22% by weight in terms of a ₂ O.

On the other hand, as a yellow chromate treatment liquid, 7 g / l of chromic anhydride and 3 g of sulfuric acid with respect to deionized water were used.
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 in this for about 15 seconds, pulled up, and dried to produce a comparative example (test product). When the formed film was analyzed by XPS, about 30% by weight of the chromium component was hexavalent chromium, and the balance was trivalent chromium.
It was found that a 5 μm chromate film was formed.

For the above test items, JIS Z2
A salt spray test specified in 371 is performed, and white rust due to corrosion of the galvanized layer appears at about 20% or more of the entire surface, or red rust due to corrosion of the underlying iron layer is visually observed even at all. Endurance evaluation was performed based on the time up to. FIG. 6 shows the evaluation result on the metal shell side which has been similarly subjected to the chromate treatment. The same result is obtained for the gasket 30. That is, it can be seen that the test article of the present invention on which the silicon composite chromate film was formed exhibited overwhelmingly superior durability than the test articles of any of the comparative examples, including the test article subjected to the yellow chromate treatment. . Also, from the test specimens and the results, it can be seen that when the trivalent chromium-based chromate layer or the silica-based layer was formed alone, good durability was not obtained.

Next, each of the above-mentioned specimens was subjected to a heat treatment at 200 ° C. for 30 minutes in the air, and then a salt spray test was conducted in the same manner. FIG. 7 shows a test result on the metal shell side which has been similarly subjected to the chromate treatment. The same result is obtained with the gasket 30. It can be seen that the test article subjected to the yellow chromate treatment has a significantly reduced durability time due to the heat treatment, whereas the test article of the present invention exhibits extremely good durability.

[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 conceptual diagram showing the structure of a silicon composite chromate film together with some modifications.

FIG. 3 is a view for explaining a process of forming a silicon composite chromate film and an estimated structure of a chromate layer.

FIG. 4 is a diagram illustrating the effect of a method of forming a silica-based layer in an undried state after chromate treatment.

FIG. 5 is a diagram showing an analysis result by XPS of a silicon composite chromate film formed on a test sample of an example.

FIG. 6 is a graph showing the results of a salt spray test on the test articles of Examples (without heat treatment).

FIG. 7 is a graph (after heat treatment) showing the results of a salt spray test on the test sample of the example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 metal shell 2 insulator 3 center electrode 4 ground electrode 30 gasket 41,45 zinc plating layer 42,46 silicon composite chromate coating 42a trivalent chromium-based chromate layer 42b trivalent chromium-silicon mixed layer 42c silica-based layer 100 spark plug

Claims (6)

[Claims] 1. A spark plug which is used by being fitted into a base end portion of a mounting screw formed on an outer peripheral surface of a metal shell of a spark plug, wherein at least a part of the surface is mainly composed of chromium and silicon having a cation-based component. A gasket for a spark plug, characterized in that 90% by weight or more of the chromium component contained is covered with a silicon composite chromate film which is trivalent chromium. 2. The silicon composite chromate coating comprises a trivalent chromium-based chromate layer in which a cation component is mainly a chromium component and 90% by weight or more of the chromium component is trivalent chromium, and a silicon oxide. The gasket for a spark plug according to claim 1, further comprising at least two layers mainly composed of a silica-based layer and directly or indirectly covering the trivalent chromium-based chromate layer via another layer. 3. A trivalent chromium-silicon mixture in which a trivalent chromium component and a silicon component are mixed between the trivalent chromium-based chromate layer and the silica-based layer at an intermediate content ratio between the two layers. The gasket for a spark plug according to claim 3, wherein a layer is formed. 4. The gasket for a spark plug according to claim 2, wherein the silica-based layer mainly comprises a cation component mainly composed of an oxide composed of an alkali metal element and silicon. 5. A method of 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, wherein the gasket is placed in a chromate treatment bath. By immersion, 90% by weight of the chromium component was added to the surface of the gasket.
The above is a chromate treatment step of forming a trivalent chromium-based chromate layer composed of trivalent chromium, and a gasket on which the trivalent chromium-based chromate layer is formed. A step of forming a silica-based layer mainly composed of silicon oxide on the trivalent chromium-based chromate layer by evaporating the solvent after applying the acid salt solution; A method for producing a gasket for a spark plug, comprising: 6. After the chromate treatment step is completed, the silica-based layer forming step is performed by immersing the gasket surface in the silicate solution in an undried or semi-dried state. A method for producing a gasket for a spark plug according to claim 5.