JP3874938B2 - Spark plug and manufacturing method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a spark plug for an internal combustion engine and a manufacturing method thereof.
[0002]
[Prior art]
Spark plugs used to ignite internal combustion engines, such as gasoline engines for automobiles, etc., have an insulator outside the center electrode and a metal shell on the outside, forming a spark discharge gap with the center electrode. The ground electrode is attached to the metal shell. And it attaches and uses to the cylinder head of an engine with the attaching screw part formed in the outer peripheral surface of a metal shell.
[0003]
By the way, the metal shell is generally made of an iron-based material such as carbon steel, and the surface thereof is often galvanized for corrosion protection. Although the galvanized layer has an excellent anticorrosive effect against iron, as is well known, the galvanized layer on iron is easily consumed by sacrificial corrosion, and the appearance is changed to white due to the generated zinc oxide. There are also disadvantages that 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.
[0004]
By the way, a so-called yellow chromate film has been used as the chromate film applied to the metal shell of the spark plug. Since this yellow chromate film has good anticorrosion performance, it has been widely used in fields other than spark plugs, such as canned inner surface coating. However, in recent years when interest in environmental protection has been increasing on a global scale due to the fact that a part of the chromium component is contained in the form of hexavalent chromium, it has been gradually shunned. For example, in the automobile industry in which spark plugs are used in large quantities, in view of the environmental impact of waste spark plugs, the use of chromate coatings containing hexavalent chromium has been studied to be eliminated in the future. In addition, since the treatment bath for the yellow chromate film treatment contains a relatively high concentration of hexavalent chromium, there is a problem that the waste liquid treatment is very expensive.
[0005]
[Problems to be solved by the invention]
In response to this trend, the development of a chromate film that does not contain hexavalent chromium, that is, a film in which substantially all of the chromium component is contained in the form of trivalent chromium has been underway relatively early. The treatment bath generally has a low hexavalent chromium concentration, and a bath containing no hexavalent chromium has been developed, and the problem of waste liquid treatment has been reduced. However, the trivalent chromium-based chromate film has a major drawback in that the anticorrosion performance is inferior to that of the yellow chromate film, and has not been widely used as a coating film for the metal shell of the spark plug.
[0006]
In addition, the chromate film, including the yellow chromate film, has a common drawback that it has 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 is unlikely to become extremely hot. However, if the engine operation is continued under conditions where a large heat load is applied, or if the spark plug is mounted at a position that is relatively close to the exhaust manifold, the temperature of the metal shell sometimes becomes about 200-300 ° C. May rise to. Under such circumstances, there is a problem that the deterioration of the chromate film is likely to proceed and the anticorrosion performance is drastically lowered.
[0007]
An object of the present invention is to provide a spark plug in which the chromate film covering the surface of the metal shell has a low hexavalent chromium content and has excellent anticorrosion performance and heat resistance as compared with a conventional chromate film, and a method for producing the same. There is.
[0008]
[Means for solving the problems and actions / effects]
In order to solve the above problems, a spark plug of the present invention includes a center electrode, an insulator provided outside the center electrode, It was fixed to the insulator by a caulking portion that was provided outside the insulator and was formed by caulking its opening edge inward. A grounding electrode disposed opposite to the metal shell and the center electrode so as to form a spark discharge gap; and Prior to the caulking, the main component of the cation component is a chromium component, and 90% by weight or more of the chromium component is mainly composed of a trivalent chromium-based chromate layer and silicon oxide. And at least two layers including a silica-based layer having a film thickness of 0.2 μm or more and 0.8 μm or less that covers the trivalent chromium-based chromate layer directly or indirectly through another layer. With silicon composite chromate coating, Above The surface of the metal shell is Together with the caulking part It is characterized by being coated.
[0009]
The “cationic component” means that the ion valence is positive at the binding energy peak of the component of interest in the photoelectron spectrum obtained when the film is analyzed by X-ray photoelectron spectroscopy (XPS or ESCA). It is a component in which a chemical shift in the direction occurs.
[0010]
In the above configuration, a silicon composite chromate film containing a silicon component and a chromium component as main components of the cationic component is formed on the surface of the metal shell, and 90% by weight or more of the chromium component is trivalent. It is chrome. That is, in the normal yellow chromate film, about 25 to 35% by weight of the chromium component is hexavalent chromium, whereas in the film 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, it is possible to enhance the effect on environmental measures to reduce hexavalent chromium. Also, the chromate treatment liquid used does not contain any hexavalent chromium component, or even if it contains it, the amount can be greatly reduced compared to the treatment liquid such as yellow chromate coating, so there is a problem with the drainage treatment. Is less likely to occur.
[0011]
The silicon composite chromate film used in the present invention is characterized in that it contains a silicon component as a cationic component. By including a silicon component in the composite chromate film, the anti-corrosion performance is greatly improved as compared with a normal trivalent chromium-based chromate film, and the metal shell can be sufficiently provided with durability against corrosion. Become. Further, by combining the silicon component, the heat resistance of the coating is dramatically improved, and the corrosion resistance of the metal shell can be sufficiently maintained even in an environment where the temperature of the spark plug is likely to rise.
[0012]
Some spark plugs are provided with a ring-shaped gasket to be fitted to the base end portion of the mounting screw portion formed on the outer peripheral surface of the metal shell. This 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 by screwing the screw portion of the metal shell into the screw hole on the cylinder head side. Compressive deformation to serve as a seal between the screw hole and the gas seal portion. In this case, at least a part of the surface of the gasket can be covered with the silicon composite chromate film. Thereby, corrosion resistance and heat resistance can be sufficiently imparted to the gasket as well as the metal shell.
[0013]
The silicon component can be contained, for example, in a form combined with oxygen, that is, in the form of a silicon oxide such as silica. In the present specification, if the silicon peak having a valence of +4 or close to it and the peak of oxygen having a valence of -2 or close to it are detected simultaneously in the above-mentioned XPS photoelectron spectrum, the silicon component is oxygen. It is thought that it is contained in a combined form.
[0014]
On the other hand, the chromate film is formed by the reaction between the base metal and a solution containing chromate ions, but this film formation reaction involves the formation of a polymer complex by trivalent chromium by a bridge of hydroxyl and oxygen. By doing so, it is said that the mechanism of precipitating and depositing in a gel form on the surface of the underlying metal is the main component. When a hydroxyl group is bonded to trivalent chromium, the valence of chromium is apparently shifted to +4 due to the influence of protons contained in the hydroxyl group. In the present specification, if a peak component chemically shifted from a trivalent chromium peak position to a position corresponding to a valence of approximately +4 is observed in the XPS photoelectron spectrum, the chromium component is regarded as a component of the chromate film. I think that it has become.
[0015]
The chromate treatment is a kind of chemical conversion treatment in which the chromium component is replaced and deposited while the base metal is oxidized and eluted. Therefore, in the electroless chromate treatment in which power is not supplied from the outside, the base metal needs to be a metal that can be eluted in the chromate treatment bath. In spark plugs, the metal shell or gasket is generally composed of an iron-based material such as carbon steel, and a zinc-based plating layer consisting mainly of zinc is formed on the surface of the metal for corrosion protection. can do. In this sense, this zinc-based plating layer is convenient as a base metal for forming a chromate film. In this case, the eluted zinc component is often taken into the chromate film. The zinc-based plating layer can be formed by known electrolytic galvanization or hot dip galvanization.
[0016]
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.
[0017]
Next, in the method of forming the chromate film to be formed into a silicon composite chromate film that is a feature of the present invention, for example, a water-soluble silica-based compound, for example, an alkali silicate such as water glass is mixed in a chromate treatment bath. Then, there is a method of incorporating silica components in the formed chromate film. However, from the viewpoint of further improving the anticorrosion performance (particularly durability against the salt spray test) and heat resistance, the silicon composite chromate film should have the following structure. That is, the silicon composite chromate film is mainly composed of a trivalent chromium-based chromate layer in which the main component of the cation component is a chromium component and 90% or more of the chromium component is composed of trivalent chromium, and an oxide of silicon, At least two layers including a silica-based layer covering the trivalent chromium-based chromate layer directly or indirectly through another layer are included.
[0018]
In the trivalent chromium-based chromate layer, it is sufficient that the total weight of the chromium component is 50% by weight or more with respect to the total weight of the cation component, and the remainder is other cation components such as silicon, zinc, nickel, etc. It may be a component of. On the other hand, the silica-based layer is also required that the total weight of the silicon component is 50% by weight or more with respect to the total weight of the cation component, and the remainder is a component such as chromium, zinc, nickel or the like. It may be.
[0019]
Such film formation can be realized, for example, by the spark plug manufacturing method of the present invention including the following steps.
(1) Chromate treatment step: By immersing the metallic shell in a chromate treatment bath, a trivalent chromium-based chromate layer in which 90% by weight or more of the chromium component is composed of trivalent chromium is formed on the surface of the metallic shell.
(2) Silica-based layer forming step: After applying a silicate solution in which an alkali silicate is dissolved in a predetermined solvent to the metal shell on which the trivalent chromium-based chromate layer is formed, the solvent is added. By evaporating, a silica-based layer mainly composed of silicon oxide is formed on the trivalent chromium-based chromate layer.
In this case, the silica-based layer is mainly composed of an oxide mainly composed of an alkali metal element and silicon.
[0020]
Needless to say, the same method can be applied to the formation of a coating on the gasket by replacing the metal shell with the gasket in the above process.
[0021]
As described above, the anticorrosion performance of the coating can be further greatly improved by previously forming the trivalent chromium-based chromate layer on the surface of the metal shell and covering the surface with the silica-based layer. Further, the heat resistance of the coating can be achieved at a level that surpasses that of the conventional trivalent chromium-based chromate coating as well as the yellow chromate coating containing hexavalent chromium.
[0022]
In addition, for the formation of the silica-based layer, a vapor phase film forming method such as high-frequency sputtering, reactive sputtering, ion plating, or chemical vapor deposition (CVD) may be used. However, according to the above method by application of the silicate solution, the metal shell (or gasket) after the chromate treatment is immersed in the silicate solution, or after the silicate solution is applied by spraying or the like, A silica-based layer can be easily formed simply by drying the coating film.
[0023]
In addition, a trivalent chromium-silicon mixed layer is formed between the trivalent chromium-based chromate layer and the silica-based layer, in which the trivalent chromium component and the silicon component are mixed at an intermediate content ratio between the two layers. It may be. Thereby, the anticorrosion performance or heat resistance of the silicon composite chromate film may be further improved. For example, the formation of the trivalent chromium-silicon mixed layer as described above means that a kind of composition gradient structure is formed between the trivalent chromium-based chromate layer and the silica-based layer. Improved coating performance as described above has been achieved by improving the adhesion between the trivalent chromium-based chromate layer and the silica-based layer and reducing stress based on the difference in shrinkage between the chromate layer and the silica-based layer during heating. it can.
[0024]
It is said that the anti-corrosion performance of yellow chromate coatings, etc., is due to the fact that the trivalent chromium network is restored by the action of the hexavalent chromium contained even when the coating is destroyed in a corrosive environment. ing. However, in the trivalent chromium-based chromate layer, since such a repair effect due to hexavalent chromium cannot be expected, if defects such as pinholes occur in the coating, the influence of corrosion directly affects the base such as the zinc-based plating layer, It is thought that corrosion progresses rapidly. However, in the silicon composite chromate film having the above-described structure, the trivalent chromium-based chromate layer is overcoated with a silica-based layer, and the influence of corrosion is less likely to reach the surface of the trivalent chromium-based chromate layer and the underlying layer. It is estimated that the anticorrosion performance is improved.
[0025]
On the other hand, it is considered that the conventional chromate film is inferior in heat resistance because the chromate film is contracted by heating and defects such as cracks are likely to occur. However, in the above configuration, even if the above-mentioned defects occur 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.
[0026]
In order to form a uniform silica-based layer, it is also important to improve the wettability between the silicate solution and the underlying trivalent chromium-based chromate layer. For example, if a trivalent chromium-based chromate layer has defects such as pinholes or cracks (in this case, including those that have inherited defects in the ground due to scratches or adhesion of foreign matter), the wettability is poor. When a silicate solution is used, bubbles and the like are likely 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.
[0027]
On the other hand, after completion of the chromate treatment step, there is also a method of performing a silica-based layer forming step by immersing the surface of the metal shell in a silicate solution in an undried or semi-dried state. That is, after the chromate treatment is finished, a trivalent chromium-based chromate layer is formed on the surface of the metal shell in an undried or semi-dried state with a slight amount of moisture, The familiarity is also good. As a result, even if defects are formed in the trivalent chromium-based chromate layer, the aqueous solution of silicate sufficiently penetrates into the defects, and it is difficult for bubbles and the like to remain, resulting in good anticorrosion performance of the coating. can do.
[0028]
Further, as another effect, the chromate treatment liquid remaining on the surface layer portion of the formed trivalent chromium-based chromate layer is mixed with a part of the applied silicate aqueous solution, and the aforementioned trivalent chromium-silicon mixture is mixed. There is an advantage that a layer is easily formed. The effect of forming the trivalent chromium-silicon mixed layer has already been described.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, some embodiments of the present invention will be described with reference to the drawings.
A spark plug 100 with a resistor 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 the tip portion protrudes, and a tip portion protruded. In this state, a center electrode 3 provided inside the insulator 2, a ground electrode 4 and the like that are disposed so that one end is coupled to the metal shell 1 and the other end faces the tip of the center electrode 3. A spark discharge gap g is formed between the ground electrode 4 and the center electrode 3.
[0030]
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. A terminal fitting 13 is inserted and fixed on one end side of the through hole 6, and the center electrode 3 is inserted and fixed on the other end side. A resistor 15 is disposed 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 through the conductive glass seal layers 16 and 17, respectively.
[0031]
The metal shell 1 is formed in a cylindrical shape from a metal such as carbon steel, and constitutes a housing of the spark plug 100. On the outer peripheral surface thereof, there is a screw portion 7 for attaching the plug 100 to an engine block (not shown). Is formed. In addition, 1e is a tool engaging part which engages tools, such as a spanner and a wrench, when attaching the metal shell 1, and has a hexagonal axial cross-sectional shape. On the other hand, a ring-shaped wire packing 62 that engages with the rear peripheral edge of the flange-shaped protrusion 2e is disposed between the inner surface of the rear opening of the metal shell 1 and the outer surface of the insulator 2, and further 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 crimping portion 1d is formed by crimping the opening edge of the metal shell 1 toward the packing 60 inward. It is fixed with respect to the insulator 2.
[0032]
Further, a gasket 30 is fitted into the base end portion of the threaded portion 7 of the metal shell 1. The gasket 30 is a ring-shaped part formed by bending a metal plate material such as carbon steel. By screwing the screw portion 7 into the screw hole on the cylinder head side, the flange-shaped gas seal portion 1f on the metal shell 1 side. Between the screw hole and the periphery of the opening of the screw hole, it is deformed so as to be compressed and crushed in the axial direction, and serves to seal the gap between the screw hole and the screw part 7.
[0033]
Next, a zinc plating layer 41 (zinc-based plating layer) for corrosion prevention is formed on the entire outer surface of the base layer (for example, carbon steel) 40 of the metal shell 1, and the outer side thereof is covered with the silicon composite chromate film 42. Yes. Similarly, a galvanized layer 45 and a silicon composite chromate film 46 are also formed on the outer surface of the gasket 30. The galvanized layer and the silicon composite chromate film are both formed by the same method, and will be described below representatively on the metal shell 1 side.
[0034]
The galvanized layer 41 is formed by a known electrolytic galvanizing method, and has a thickness of about 3 to 10 μm, for example. If the thickness is less than 3 μm, sufficient corrosion resistance may not be ensured. Conversely, a film thickness exceeding 10 μm is excessive in terms of ensuring corrosion resistance, leading to an increase in cost.
[0035]
On the other hand, as schematically shown in FIG. 2 (a), the silicon composite chromate film 42 has a cation component mainly composed of a chromium component, and 90% by weight or more of the chromium component is composed of trivalent chromium. A chromate layer (hereinafter referred to as a chromate (III) layer) 42a and a silica-based layer 42c mainly composed of an oxide of silicon and covering the chromate (III) layer 42a. A trivalent chromium-silicon mixed layer 42b in which a trivalent chromium component and a silicon component are mixed at an intermediate content ratio between the two layers is formed therebetween. It should be noted that as much of the chromium component as possible should be a trivalent chromium component, and desirably substantially all of the chromium component should be a trivalent chromium component.
[0036]
The total thickness of the silicon composite chromate film 42 is, for example, 0.8 to 1.5 μm. When the total thickness is less than 0.8 μm, the anticorrosion performance and heat resistance imparting effect on the galvanized layer 41 may be insufficient. On the other hand, a film thickness exceeding 1.5 μm is excessive in terms of ensuring corrosion resistance, leading to an increase in cost. In addition, the silicon composite chromate film may be easily peeled off.
[0037]
The thickness of the chromate (III) layer 42a is preferably 0.2 to 0.3 μm. If the thickness is less than 0.2 μm, the anticorrosion performance of the silicon composite chromate film 42 may be insufficient. On the other hand, if the film thickness exceeds 0.3 μm, hexavalent chromium tends to remain in the film, and the chromate (III) layer 42a having the desired composition may not be obtained. On the other hand, the thickness of the silica-based layer 42c is preferably 0.2 to 0.8 μm. If the thickness is less than 0.2 μm, the anticorrosion performance and heat resistance of the silicon composite chromate film 42 may be insufficient. On the other hand, a film thickness exceeding 0.8 μm is an excessive specification, which not only leads to an increase in cost, but also the film may be easily peeled off.
[0038]
FIG. 3 schematically shows an example of a method for forming the silicon composite chromate film 42. First, a metal shell 1 ″ having a zinc plating layer having a predetermined thickness formed by a known electrolytic galvanizing method or the like is immersed in a chromate treatment solution 50. As a result, as shown in FIG. A chromate (III) layer 42 a is formed on the surface of the 1 ″ galvanized layer 41.
[0039]
Examples of usable chromate treatment liquid include those containing the following components (so-called colorless or blue chromate treatment liquid).
Chromic anhydride: 0.1-2 g / liter
Sulfuric acid: 0.3-5 g / liter
Nitric acid: 0.5 to 10 g / liter
Phosphoric acid: Add to about 2 g / liter if necessary
Hydrofluoric acid: Add to about 2 g / liter if necessary
In this treatment liquid, the amount of chromic anhydride used as a hexavalent chromium source is reduced to less than half that of the so-called yellow chromate treatment liquid of 4 to 10 g / liter. Sulfuric acid functions as a reaction accelerator, and nitric acid functions as an oxidizing agent for elution of the base metal. On the other hand, phosphoric acid has the role of improving the adhesion of the chromate film to the underlying metal, and hydrofluoric acid is incorporated into the film as an anion, strengthening the bridge bond in the polymer complex structure, and improving the film strength and thus the anticorrosion performance. Play a role to improve.
[0040]
In addition, it is also possible to use the following liquid that does not use a solute serving as a hexavalent chromium source (so-called chromium (III) chromate treatment liquid).
Potassium chromium sulfate (so-called chromium alum): 2.5 to 3.5 g / liter
Nitric acid: 3.5-4.5 g / liter
Hydrofluoric acid: 1.5-2.5 g / liter
However, the chromate treatment solution is not limited to the above as long as the content ratio of trivalent chromium to the chromium component in the chromate film can be 90% by weight or more.
[0041]
The reaction during the chromate treatment is considered to be as follows. That is, when the metal shell 1 "formed with a galvanized layer is immersed in the liquid, the gel-like film mainly composed of chromium (III) hydroxide is replaced by the dissolution of zinc by replacing the chromium ions in the liquid. A part of the dissolved zinc is taken into the film in the form of, for example, zinc chromate, etc. The structure of the formed chromate film is, for example, in the form as shown in FIG. That is, a polymer complex of trivalent chromium formed in a network by a hydroxyl group or oxygen bridge is formed, and a part of the network is made of chromate, dichromate, sulfuric acid, chloride, fluoride, etc. The anion (the kind of anion varies depending on the composition of the chromate treatment solution to be used) is substituted with one or more of the anions (of the chromate (III) layer 42a). Adjustment of NaruAtsu is can be performed, for example, by the immersion time and the adjustment of the liquid temperature to the chromating solution of the metal shell 1 ".
[0042]
Next, as shown in FIG. 3B, the metal shell 1 ′ after the formation of the chromate (III) layer 42a is in an undried or semi-dried state in an aqueous silicate solution 51 (silicate solution). Then, this is dried to form a trivalent chromium-silicon mixed layer 42b and a silica-based layer 42c, as shown in FIG. 4 (c).
[0043]
As the silicate aqueous solution 51, a so-called aqueous solution of water glass can be used. Water glass has the general formula M ₂ O · nSiO ₂ (Wherein M is an alkali metal element such as sodium or potassium), and silicon dioxide is deposited by drying and hardened into a gel. The obtained silica-based layer 42c is composed of an oxide (for example, a gel-like cured product mainly composed of alkali silicate and silicon dioxide) composed mainly of an alkali metal element and silicon.
[0044]
As water glass, n is about 2-4. When n is 2 or less, gelation hardly occurs and the obtained silica-based layer 42c becomes water-soluble, so that a stable film cannot be obtained. On the other hand, if n exceeds 4, hydrolysis of the alkali silicate proceeds excessively in the solution 51, and silicon dioxide gel precipitates and precipitates, so that the coating process cannot be carried out stably. Note that n is more preferably in the range of 3-4. In this case, M in the entire silicon composite chromate coating 42 ₂ The content of alkali metal M in terms of O is μ1, also SiO ₂ If the content of the silicon component converted into is μ 2, it is desirable to adjust the value of μ 2 / μ 1 in the range of 2 to 4, preferably 3 to 4.
[0045]
In order to form the silica-based layer 42c as uniformly as possible on the chromate (III) layer 42a, the silicate aqueous solution 51 is desirably adjusted to a concentration of alkali silicate of 30 to 200 g / liter. When the concentration is less than 30 g / liter, the formation thickness of the silica-based layer 42c becomes insufficient, and the anticorrosion performance or heat resistance performance of the silicon composite chromate film 42 may not be ensured. On the other hand, when the concentration exceeds 200 g / liter, the viscosity of the aqueous silicic acid solution 51 becomes too high, and it becomes difficult to form a uniform silica-based layer due to the occurrence of coating unevenness and the like.
[0046]
As shown in FIG. 4 (a), the chromate treatment liquid 50 remains on the surface portion of the chromate (III) layer 42a immediately after being pulled up from the chromate treatment liquid 50, and is immersed in the silicate aqueous solution 51. As shown in FIG. 5B, for example, a mixed layer 42b ′ is formed by mixing with a part of the applied aqueous silicate solution 51. If this is dried, as shown in FIG. 4 (c), a trivalent chromium-silicon mixed layer 42b derived from the mixed layer 42b ′ is formed between the chromate (III) layer 42a and the silica-based layer 42c. It is formed. The trivalent chromium-silicon mixed layer 42b is formed by mixing the chromate treatment solution 50 and the silicate aqueous solution 51 (or the penetration of the silicate aqueous solution 51 into the chromate (III) layer 42a). A trivalent chromium component and a silicon component are mixed at an intermediate content ratio between 42a and 42c. This means that a kind of composition gradient structure is formed between the chromate (III) layer 42a and the silica-based layer 42c via the trivalent chromium-silicon mixed layer 42b. Effects such as improved adhesion and stress reduction based on the difference in thermal shrinkage can be achieved.
[0047]
Note that the chromate (III) layer 42a is formed in a wet state on the surface of the metal shell 1 'in an undried or semi-dried state after the chromate treatment is completed, so that the familiarity with the silicate aqueous solution 51 is improved. . Accordingly, as shown in FIG. 5 (a), even if a defect def such as pinholes is formed in the chromate (III) layer 42a, the silicate aqueous solution 51 is sufficiently easily penetrated here, and the formed silica. It is possible to make it difficult for bubbles and the like to remain in the system layer 42c (FIG. 5B).
[0048]
The metal shell 1 or gasket 30 treated in this way has a silicon composite chromate film formed on the galvanized layer, which is significantly higher in corrosion protection than the conventional trivalent chromium-based chromate film and further the yellow chromate film. It has performance and heat resistance, and can sufficiently impart durability against corrosion to the galvanized layer.
[0049]
Note that after the chromate treatment, the surface of the metallic shell 1 ′ may be dried and immersed in the aqueous silicic acid solution 51. In this case, since the chromate (III) layer 42a is once dried, the mixed layer 42b ′ of the chromate treatment solution 50 and the silicate aqueous solution 51 is hardly formed. Therefore, as shown in FIG. 2B, a clear trivalent chromium-silicon mixed layer 42b may not be formed between the chromate (III) layer 42a and the silica-based layer 42c.
[0050]
Alternatively, an appropriate amount of water glass may be blended in the chromate treatment solution, and a metal shell formed with a zinc-based plating layer may be immersed in the chromate treatment solution and dried. The silicon composite chromate film obtained by such a method also has good anticorrosion performance and corrosion resistance. In this case, as shown in FIG. 2 (c), the obtained silicon composite chromate film 42 is a gel-like cured product mainly composed of alkali silicate and silicon dioxide in a polymer complex substrate 42d of trivalent chromium. It is considered that 42e has a dispersed structure.
[0051]
【Example】
In order to confirm the effect of the present invention, the following experiment was conducted. First, using the carbon steel wire SWCH8A for cold heading defined in JISG3539 as a material, the metal shell 1 having the shape shown in FIG. 1 was manufactured by cold forging. The nominal diameter of the threaded portion 7 of the metal shell 1 was 14 mm, and the axial length was about 19 mm. Next, an electrolytic galvanizing treatment using a known alkali cyanide bath was performed on the galvanized layer to give a galvanized layer having a thickness of about 6 μm.
[0052]
Next, as the chromate treatment solution 50 shown in FIG. 3, a solution prepared by dissolving 3 g / liter of potassium chromium sulfate, 4 g / liter of nitric acid, and 2 g / liter of hydrofluoric acid in deionized water was prepared, and the liquid temperature was 20 ° C. Retained. On the other hand, Na ₂ O ・ 3.5SiO ₂ A silicate aqueous solution 51 was prepared by dissolving sodium silicate (water glass) having the following composition in deionized water at a concentration of 100 g / liter. Then, the galvanized metal shell is immersed in the chromate treatment solution 15 for 15 seconds, and then is immediately immersed in the silicate aqueous solution 51 in a state where only the liquid is removed and drying is not performed. A silicon composite chromate film was formed by drying with warm air (test product (3): product of the present invention).
[0053]
The result of measuring the photoelectron spectrum of XPS while etching this silicon composite chromate film in the thickness direction is shown in FIG. From the spectral peak intensity of each component at each etching depth to about 0.4 μm from the surface, chromium (2p _2/3 ) Is hardly observed, indicating that the silica-based layer is mainly composed of silicon oxide. In addition, when examined in more detail by fluorescent X-ray analysis, the silica-based layer is composed of silicon as SiO ₂ About 77% by weight in terms of converted value, sodium as Na ₂ It was found that the content was about 22% by weight in terms of O.
[0054]
On the other hand, although a slight silicon peak was observed at a depth of 0.7 to 1 μm from the surface, the cation component was mainly chromium and then a large amount of zinc was detected. The chromium (2p _2/3 As a result of examining the peak of) in more detail, 99% by weight or more of the chromium component was trivalent chromium. That is, it was found that the approximately 0.3 μm portion in the depth range was a trivalent chromium-based chromate layer.
[0055]
Then, the portion having a thickness of about 0.3 μm located in the middle of the two layers is a trivalent chromium containing a chromium component and a silicon component in an intermediate composition of both layers from the spectral peak intensity of each component of XPS. It was found to be a silicon mixed layer.
[0056]
In addition, after being immersed in the chromate treatment liquid 50 for 15 seconds, not immersed in the silicate aqueous solution 51 but dried as it is (test product {circle over (2)}: comparative example), conversely, not immersed in the chromate treatment liquid 50, What was immersed and dried only in the silicate aqueous solution 51 (test product (4): comparative example) was also prepared. When the formed coatings of the test products (2) and (4) were analyzed by XPS and X-ray fluorescence analysis, the former had a weight content ratio of trivalent chromium in the chromium component of about 99% or more and a thickness of about 0. .5μm chromate film, the latter is silicon with SiO ₂ 77% by weight in terms of converted value, sodium as Na ₂ It was found that the oxide-based coating contained 22% by weight in terms of O.
[0057]
On the other hand, a yellow chromate treatment solution prepared by dissolving chromic anhydride at a rate of 7 g / liter, sulfuric acid 3 g / liter, and nitric acid 3 g / liter in deionized water was prepared and kept at a liquid temperature of 20 ° C. Then, a metal shell was dipped in this for about 15 seconds, pulled up, and dried to prepare a comparative example (test product (1)). When the formed film was analyzed by XPS, it was found that the chromate film had a thickness of about 0.5 μm, in which about 30% by weight of the chromium component was hexavalent chromium and the balance was trivalent chromium.
[0058]
The salt spray test specified in JIS Z2371 is performed on the test products of (1) to (4) above, until white rust derived from corrosion of the galvanized layer appears about 20% or more of the entire surface, or Durability evaluation was performed according to the time until red rust derived from corrosion of the iron layer was visually confirmed. The result is shown in FIG. That is, the test product (3), which is a product of the present invention on which a silicon composite chromate film is formed, has an overwhelmingly superior durability than the test products of any of the comparative examples, including the test product (1) treated with yellow chromate. You can see that Further, from the results of the test products (2) and (4), it is understood that when the trivalent chromium-based chromate layer or the silica-based layer is formed alone, good durability is not obtained.
[0059]
Next, FIG. 8 shows the result of the same salt spray test after heat-treating each test product (1) to (4) in the air at 200 ° C. for 30 minutes. The test product {circle around (1)} treated with yellow chromate has a significantly reduced durability due to heat treatment, while the test product {circle around (3)} of the present invention shows extremely good durability. I understand.
[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 an explanatory diagram of a chromate treatment step and a silica-based layer formation step.
FIG. 4 is a diagram for explaining a formation process of a silicon composite chromate film and an estimated structure of a chromate layer.
FIG. 5 is a diagram for explaining the effect of a method of forming a silica-based layer in an undried state after chromate treatment.
FIG. 6 is a graph showing the XPS analysis results of the silicon composite chromate film formed on the test product (3) of the example.
FIG. 7 is a graph showing the results of a salt spray test on a test product of an example (no heat treatment).
FIG. 8 is a graph showing the results of a salt spray test on a test product of an example (after heat treatment).
[Explanation of symbols]
1 metal shell
2 Insulator
3 Center electrode
4 Ground electrode
30 Gasket
41, 45 Galvanized 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)

A center electrode, an insulator provided on the outside of the center electrode, and fixed to the insulator by a crimping portion provided on the outside of the insulator and caulked on the opening edge of the center electrode . A main electrode and a ground electrode disposed opposite to the center electrode so as to form a spark discharge gap between the center electrode, and further formed prior to the crimping, Is a chromium component, and 90% by weight or more of the chromium component is composed mainly of a trivalent chromium-based chromate layer composed of trivalent chromium, and a silicon oxide, and the trivalent chromium-based chromate layer is used directly or otherwise. The surface of the metal shell is covered with the caulking portion by a silicon composite chromate film including at least two layers of a silica-based layer having a film thickness of 0.2 μm or more and 0.8 μm or less that is indirectly covered through the layer. Spark plug characterized by being.  A ring-shaped gasket to be fitted to a base end portion of a mounting screw portion formed on the outer peripheral surface of the metal shell, wherein at least a part of the surface of the gasket is covered with the silicon composite chromate film. The spark plug according to 1. Between the trivalent chromium-based chromate layer and the 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 two layers is formed. The spark plug according to claim 1 or 2 . The spark plug according to claim 2 or 3, wherein the silica-based layer is mainly composed of an oxide mainly composed of an alkali metal element and silicon . The spark plug manufacturing method according to any one of claims 1 to 4, wherein the metallic shell is immersed in a chromate treatment bath so that 90 weight of a chromium component is formed on the surface of the metallic shell. The alkali silicate is dissolved in a predetermined solvent in the chromate treatment process for forming a trivalent chromium-based chromate layer comprising at least 3% chromium, and the metal shell on which the trivalent chromium-based chromate layer is formed. A silica-based layer forming step of forming a silica-based layer mainly composed of silicon oxide on the trivalent chromium-based chromate layer by applying the silicate solution and then evaporating the solvent; A method for producing a spark plug, comprising: 6. The spark according to claim 5, wherein after the chromate treatment step, the silica-based layer forming step is performed by immersing the surface of the metal shell in the silicate solution in an undried or semi-dried state. Plug manufacturing method.

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