JP2005285490A - Spark plug and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spark plug where the corrosion resistance and the adhesiveness of a nickel plating layer formed on the main fittings of the spark plug can be increased. <P>SOLUTION: A strike layer 71 is formed over the material of the main fittings 5 of the spark plug 100 and a complex metal layer 70 comprising a lower metal layer 73 and an upper metal layer 75 is formed thereon. Both the lower metal layer 73 and the upper metal layer 75 contain nickel as their principal component. The upper metal layer 75 that contains more sulfur than the lower metal layer 73 is more likely to be ionized as a whole than the lower metal layer 73, thus providing sacrificial corrosion to the lower metal layer 73. In this way the corrosion resistance of the main fittings 5 can be increased. Also, the lower metal layer 73 having a lower sulfur content than the upper metal layer 75 is softer than the upper metal layer 75 and can increase adhesiveness to the main fittings 5 by reducing internal stresses. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a spark plug for an internal combustion engine in which a plating layer is formed on the surface of a metal shell that holds an insulator in which a center electrode is inserted.

  Conventionally, spark plugs for ignition are used in internal combustion engines. A general spark plug includes a center electrode, an insulator that holds the center electrode at the tip end side of the shaft hole, a metal shell that holds the insulator, and a ground electrode that forms a spark discharge gap with the center electrode. . Then, a spark discharge is performed between the center electrode and the ground electrode, and the air-fuel mixture exposed between the two electrodes is ignited.

By the way, in such a spark plug, in order to improve the corrosion resistance of the metal shell, it has recently been proposed to form a nickel plating layer mainly composed of nickel on the surface of the metal shell (for example, Patent Document 1). reference.).
JP 2002-184552 A

  By the way, since nickel has a lower ionization tendency than iron, when a pinhole exists in such a nickel plating layer, the base material made of a steel material is first corroded. In order to prevent this, it is desirable to form the nickel plating layer sufficiently thick so that no pinhole is generated.

  However, when the nickel plating layer is formed thick, the adhesion between the nickel plating layer and the metal shell is reduced. For this reason, when an external force is applied to the metal shell by caulking the metal shell after forming the nickel plating layer when holding the insulator, or by incorporating it into an internal combustion engine, the internal stress of the nickel plating layer increases, peeling, There was a problem that problems such as dropout would occur. In particular, when a spark plug is used in an internal combustion engine using natural gas as a fuel, there is a problem that sufficient corrosion resistance cannot be obtained for the metal shell with the conventional nickel plating layer because the combustion chamber becomes an oxidizing atmosphere. .

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a spark plug that can improve the corrosion resistance and adhesion of a nickel plating layer formed on a metal shell of the spark plug.

  In order to achieve the above object, a spark plug according to a first aspect of the present invention includes a center electrode, an insulator having an axial hole penetrating in the axial direction, and holding the central electrode on a tip side of the axial hole. A spark plug comprising a metal shell surrounding the insulator in the radial direction and holding the insulator, the lower metal layer having nickel as a main component on the surface of the metal shell made of a steel material And an upper metal layer containing nickel as a main component and containing a larger amount of sulfur than the lower metal layer is formed on the surface of the lower metal layer, and includes the lower metal layer and the upper metal layer. The thickness of the composite metal layer is 4 μm or more and 35 μm or less, and the thickness of the lower metal layer is 30% or more and 80% or less of the thickness of the composite metal layer.

  The spark plug of the invention according to claim 2 has a center electrode, an axial hole penetrating in the axial direction, an insulator holding the central electrode on the tip side of the axial hole, and a diameter of the insulator A spark plug that includes a metal shell that surrounds a direction and holds the insulator, wherein a lower metal layer mainly composed of nickel is formed on a surface of the metal shell made of a steel material, and the lower metal layer On the surface of the intermediate metal layer containing nickel as a main component and containing a larger amount of sulfur than the lower metal layer, and on the surface of the intermediate metal layer, nickel as a main component and in a larger amount than the lower metal layer And an upper metal layer containing a smaller amount of sulfur than the middle metal layer, and the composite metal layer composed of the lower metal layer, the middle metal layer, and the upper metal layer has a thickness of 5 μm. Above 36μm And the thickness of the intermediate metal layer is 0.5 μm or more and 1.5 μm or less, and the thickness of both the lower metal layer and the upper metal layer is 100%. The thickness is 20% or more and 80% or less.

  Further, in the spark plug of the invention according to claim 3, in addition to the configuration of the invention of claim 1 or 2, 95% by weight or more of the chromium component contained on the surface of the upper metal layer is trivalent. A chromate film layer, which is chromium, is formed.

  In addition to the configuration of the invention according to any one of claims 1 to 3, the spark plug of the invention according to claim 4 includes nickel or copper between the lower metal layer and the surface of the metal shell. A strike layer having a main component and a thickness of 0.2 μm or more and 0.7 μm or less is formed.

  In the spark plug of the invention according to claim 1, on the surface of the metal shell of the spark plug made of a steel material, a lower metal layer mainly composed of nickel, and on the surface, nickel is a main component, and the lower metal layer And an upper metal layer containing a larger amount of sulfur than that. Since the upper metal layer containing a large amount of sulfur has a higher ionization tendency than the lower metal layer, it is corroded before the lower metal layer by so-called sacrificial corrosion. For this reason, even if the upper metal layer corrodes and the lower metal layer is partially exposed to the outside, the remaining upper metal layer corrodes preferentially than the exposed lower metal layer corrodes, The period until the lower metal layer is corroded and the surface of the metal shell is exposed to the outside becomes longer, and the corrosion resistance of the metal shell can be improved. Furthermore, since the metal layer has a two-layer structure, even when a pinhole occurs in one metal layer when forming the lower metal layer or the upper metal layer, the metal layer can be protected by the other metal layer, and the corrosion resistance is improved. be able to. On the other hand, since the lower metal layer has a lower sulfur content than the upper metal layer, the lower metal layer has higher ductility than the upper metal layer, and the adhesion to the surface of the metal shell and the upper metal layer is maintained. That is, it can suppress that both metal layers peel and peel from the surface of a metal shell. In the present invention, the “main component” indicates that the component occupies 70% by weight or more of the total components contained.

  And the thickness of the composite metal layer comprised by a lower layer metal layer and an upper layer metal layer was 4 micrometers or more and 35 micrometers or less. When the thickness of the composite metal layer is less than 4 μm, the thickness of each of the lower metal layer and the upper metal layer is reduced, so that it is not possible to secure a sufficient period until the surface of the metal shell is exposed. Protection cannot be performed for a long time and corrosion resistance may be inferior. However, in the spark plug of the invention according to claim 1, since the thickness of the composite metal layer is 4 μm or more, the corrosion resistance of the composite metal layer as a whole can be improved and the metal shell can be effectively protected.

  On the other hand, when the thickness of the composite metal layer is larger than 35 μm, the corrosion resistance is excellent, but the adhesion to the surface of the metal shell is lowered, and the composite metal layer may float, peel or fall off from the metal shell. In addition, the time required for plating is lengthened, and productivity is lowered. Furthermore, the amount of nickel used for plating increases and the production cost increases. However, in the spark plug of the invention according to claim 1, since the thickness of the composite metal layer is 35 μm or less, the adhesion between the composite metal layer and the metal shell is ensured, the productivity of the spark plug is increased, and the nickel It is possible to reduce the production cost by reducing the amount of use.

  Furthermore, the thickness of the lower metal layer was 30% or more and 80% or less of the thickness of the composite metal layer. When the thickness of the lower metal layer is less than 30% of the thickness of the composite metal layer, the thickness of the upper metal layer with respect to the composite metal layer is increased, the adhesion between the lower metal layer and the upper metal layer is reduced, and the upper metal layer May peel off or fall off. Moreover, since the lower metal layer with respect to a composite metal layer becomes thin and it becomes difficult to utilize the difference of the ionization tendency of both metal layers, there exists a possibility that improvement in corrosion resistance cannot be aimed at. On the other hand, when the thickness of the lower metal layer exceeds 80% of the thickness of the composite metal layer, the thickness of the lower metal layer with respect to the composite metal layer is increased, and the adhesion between the lower metal layer and the surface of the metal shell is reduced. The layer may peel off or fall off. Moreover, since the upper metal layer with respect to the composite metal layer becomes thin and it becomes difficult to utilize the difference in ionization tendency between the two metal layers, there is a possibility that the corrosion resistance cannot be improved. However, in the spark plug according to the first aspect of the present invention, since the lower metal layer is in the above range, the corrosion resistance of the metal shell can be improved and the metal layer can be prevented from peeling off and falling off.

  Moreover, in the spark plug of the invention according to claim 2, on the surface of the metal shell of the spark plug made of a steel material, a lower metal layer mainly composed of nickel, and on the surface thereof, the main component is nickel, and the lower layer An intermediate metal layer containing a larger amount of sulfur than the metal layer, and a nickel as a main component on the surface of the intermediate metal layer, a larger amount of sulfur than the lower metal layer, and a smaller amount than the intermediate metal layer And an upper metal layer containing sulfur. On the other hand, the middle layer metal layer containing the largest amount of sulfur among the above three metal layers has a higher ionization tendency than the lower layer metal layer and the upper layer metal layer. Therefore, the so-called sacrificial corrosion precedes the lower layer metal layer and the upper layer metal layer. Corrosive to. For this reason, when the upper metal layer is corroded and the intermediate metal layer is partially exposed to the outside, the remaining upper metal layer is prevented from corroding, and the exposed intermediate metal layer is further corroded. Then, the lower layer metal layer is partially exposed to the outside due to the corrosion of the intermediate layer metal layer, but the lower layer metal layer is prevented from corroding, and the corrosion of the intermediate layer metal layer further proceeds. That is, the corrosion proceeds in the layer direction of the intermediate metal layer. After the middle metal layer corrodes, the upper metal layer contains more sulfur than the lower metal layer, so that the upper metal layer corrodes earlier than the lower metal layer due to sacrificial corrosion. Therefore, since the period until the lower metal layer corrodes and the surface of the metallic shell is exposed to the outside becomes longer, the corrosion resistance of the metallic shell can be improved. The lower metal layer has a lower sulfur content than the middle metal layer and the upper metal layer, so it has higher ductility than the middle metal layer and the upper metal layer, and has good adhesion to the surface of the metal shell and the middle metal layer. Maintained. That is, it can suppress that three metal layers peel from a metal shell. Furthermore, since the metal layer has a three-layer structure, even when a pinhole occurs in any one of the metal layers, it can be protected by another metal layer when forming the lower metal layer, the middle metal layer, and the upper metal layer. And corrosion resistance can be improved.

  Furthermore, the thickness of the composite metal layer composed of the lower metal layer, the middle metal layer, and the upper metal layer was set to 5 μm or more and 36 μm or less. When the thickness of the composite metal layer is less than 5 μm, the thickness of each of the lower metal layer and the upper metal layer is reduced, so that it is not possible to secure a sufficient period until the surface of the metal shell is exposed. It cannot be protected for a long time and has poor corrosion resistance. However, in the spark plug of the invention according to claim 2, since the thickness of the composite metal layer is 5 μm or more, the corrosion resistance of the composite metal layer as a whole can be improved and the metal shell can be effectively protected.

  On the other hand, when the thickness of the composite metal layer is larger than 36 μm, the corrosion resistance is excellent, but the adhesion to the surface of the metal shell is lowered, and the composite metal layer is lifted, peeled off, or dropped from the metal shell. In addition, the time required for plating is lengthened and productivity is lowered. Furthermore, the amount of nickel used for plating increases and the production cost increases. However, in the spark plug of the invention according to claim 2, since the thickness of the composite metal layer is 36 μm or less, the adhesion between the composite metal layer and the metal shell is ensured, and the productivity of the spark plug is increased, Production costs can be reduced by reducing the amount of nickel used.

  In addition, the thickness of the middle metal layer at this time is 0.5 μm or more and 1.5 μm or less. When the thickness of the intermediate metal layer is less than 0.5 μm, the above effects cannot be sufficiently obtained, the metal shell cannot be protected for a long time, and the corrosion resistance may be inferior. However, in the spark plug of the invention according to claim 2, since the thickness of the intermediate metal layer is 0.5 μm or more, the metal shell can be effectively protected. On the other hand, when it is larger than 1.5 μm, the middle metal layer becomes thick, and the adhesion between the middle metal layer and the lower metal layer and the adhesion between the middle metal layer and the upper metal layer are lowered. However, in the spark plug of the invention according to claim 2, since the middle metal layer is 1.5 μm or less, the adhesion is effectively maintained.

  Furthermore, the thickness of the lower metal layer was set to 20% or more and 80% or less of the total thickness of the lower metal layer and the upper metal layer. When the thickness of the lower metal layer is less than 20% of the total thickness of the lower metal layer and the upper metal layer, the thickness of the upper metal layer with respect to the composite metal layer is increased, and the adhesion between the middle metal layer and the upper metal layer is increased. The upper metal layer may peel off or fall off. Moreover, since the lower metal layer with respect to the composite metal layer becomes thin and it becomes difficult to utilize the difference in ionization tendency with the middle metal layer, there is a possibility that the corrosion resistance cannot be improved. On the other hand, when the thickness of the lower metal layer exceeds 80% of the total thickness of the lower metal layer and the upper metal layer, the thickness of the lower metal layer with respect to the composite metal layer is increased, and the adhesion between the lower metal layer and the surface of the metal shell is increased. May decrease, and the composite metal layer may peel off or fall off. Moreover, since the upper metal layer with respect to the composite metal layer becomes thin and it becomes difficult to utilize the difference in ionization tendency of the middle metal layer, there is a possibility that the corrosion resistance cannot be improved. However, in the spark plug of the invention according to claim 2, since the lower metal layer is in the above range, the corrosion resistance of the metal shell can be improved, and peeling and dropping of the metal layer can be suppressed.

  In addition, in the spark plug of the invention according to claim 3, in addition to the effect of the invention according to claim 1 or 2, 95% by weight or more of the chromium component contained on the surface of the upper metal layer is trivalent chromium. A chromate coating layer was formed. Therefore, the composite metal layer can be protected by the chromate film layer, and the corrosion resistance of the metal shell can be improved.

  Moreover, in the spark plug of the invention according to claim 4, in addition to the effect of the invention according to any of claims 1 to 3, nickel or copper is a main component between the lower metal layer and the surface of the metal shell. A strike layer was formed. And the thickness of this strike layer was made into 0.2 micrometer or more and 0.7 micrometer or less. If the thickness of the strike layer is less than 0.2 μm, it is difficult to obtain the effect of the strike layer that further improves the adhesion. In addition, if the thickness of the strike layer is greater than 0.7 μm, it is difficult to buffer the internal pressure generated inside the strike layer, and the strike layer itself may break without resisting internal stress, There is a possibility of peeling from the surface. However, in the spark plug of the invention according to claim 4, the thickness of the strike layer formed on the surface of the metal shell is limited to the above range, thereby improving the adhesion between the surface of the metal shell and the lower metal layer. The composite metal layer can be further prevented from peeling off and falling off.

  Hereinafter, an embodiment of a spark plug embodying the present invention will be described with reference to the drawings. First, the structure of a spark plug 100 as an example of the spark plug in the first embodiment will be described with reference to FIG. FIG. 1 is a partial cross-sectional view of a spark plug 100. In addition, in the direction of the axis O shown in FIG. 1 (indicated by the one-dot chain line O in the figure), the side where the center electrode 2 is provided is the front end side of the spark plug 100 and the side where the terminal fitting 4 is provided is the rear end side. explain.

  As shown in FIG. 1, the spark plug 100 is roughly held by an insulator 1 constituting an insulator, a metal shell 5 that holds the insulator 1, and an insulator 1 that extends in the direction of the axis O. One end 62 is welded to the center electrode 2 and the front end surface 57 of the metal shell 5, and the other end 61 is opposed to the front end 22 of the center electrode 2, and the rear end of the insulator 1. It is comprised from the terminal metal fitting 4 provided in the part.

  First, the insulator 1 constituting the insulator of the spark plug 100 will be described. The insulator 1 has a cylindrical shape and is formed by firing alumina or the like as is well known. A long leg portion 13 that is exposed to the combustion chamber of the internal combustion engine is provided at the distal end portion (the end portion on the distal end side in the axis O direction) of the insulator 1. A shaft hole 12 is formed at the axial center of the insulator 1.

  The center electrode 2 is formed of a nickel-based alloy such as Inconel (trade name) 600 or 601, and has a metal core 23 made of copper or the like having excellent thermal conductivity. The center electrode 2 is electrically connected to the upper terminal fitting 4 via a seal body 14 and a resistor 3 provided inside the shaft hole 12. A high voltage cable (not shown) is connected to the terminal fitting 4 via a plug cap (not shown) so that a high voltage is applied from an external circuit.

  Next, the metal shell 5 will be described. The metal shell 5 is a cylindrical metal fitting for holding the insulator 1 and fixing the spark plug 100 to an internal combustion engine (not shown). The metal shell 5 is held so as to surround the insulator 1. The metal shell 5 is formed of a low carbon steel material, and includes a tool engaging portion 51 into which a spark plug wrench (not shown) is fitted, and a screw portion 52 to be screwed with an engine head provided on the internal combustion engine (not shown). ing.

  Further, the metal shell 5 has a caulking portion 53 on the rear end side of the tool engaging portion 51. By caulking the caulking portion 53, the insulator 1 is supported by the step portion 56 of the metal shell 5 via the plate packing 8, and the metal shell 5 and the insulator 1 are integrated. In order to complete sealing by caulking, annular ring members 6 and 7 are interposed between the metal shell 5 and the insulator 1, and talc (talc) 9 powder is interposed between the ring members 6 and 7. Filled. A flange 54 is formed at the center of the metal shell 5, and the gasket 10 (annular packing obtained by folding a thin plate) is inserted into the vicinity of the rear end side of the screw 52, that is, the seat surface 55 of the flange 54. Has been.

  Next, the ground electrode 60 will be described. The ground electrode 60 is made of a metal having high corrosion resistance. As an example, a nickel alloy such as Inconel (trade name) 600 or 601 is used. In the spark plug 100 according to the first embodiment, the two ground electrodes 60 are provided so as to face each other with the center electrode 2 interposed therebetween. The ground electrode 60 has a substantially rectangular cross section in the longitudinal direction, and one end 62 is joined to the front end surface 57 of the metal shell 5 by welding. The other end 61 of the ground electrode 60 is bent so as to face the tip 22 of the center electrode 2.

  In the spark plug 100 having such a configuration, in the first embodiment, a plurality of platings are applied on the surface of the metal shell 5 in order to improve the corrosion resistance of the metal shell 5. Specifically, as shown in FIG. 2, a strike layer 71 containing nickel as a main component is formed on the base of the metal shell 5. The strike layer 71 is a nickel strike layer containing 99.9% by weight of nickel, and is formed on the surface of the metal shell 5 with a thickness of 0.5 μm. On the strike layer 71, a lower metal layer 73 containing nickel as a main component is formed, and an upper metal layer 75 containing a larger amount of sulfur than the lower metal layer 73 is further formed thereon. The lower metal layer 73 contains 99.99% by weight of nickel and is formed on the surface of the strike layer 71 with a thickness of 6 μm. The upper metal layer 75 contains 99.94 wt% nickel and 0.05 wt% sulfur and is formed on the surface of the lower metal layer 73 with a thickness of 4 μm. Further, a chromate film layer 76 for protecting the upper metal layer 75 is formed on the composite metal layer 70 including the lower metal layer 73 and the upper metal layer 75. The chromate film layer 76 is formed on the surface of the upper metal layer 75 with 95% by weight or more of chromium contained being trivalent chromium. In the thickness direction of the metal layer to be formed, the base side of the metal shell 5 will be described as the lower side, and the opposite side will be described as the upper side.

  In this way, the composite metal layer 70 in which the lower metal layer 73 is formed on the lower side and the upper metal layer 75 is formed on the upper side is formed on the surface of the metal shell 5. The lower metal layer 73 and the upper metal layer 75 on the upper side both contain nickel as a main component. The upper metal layer 75 contains more sulfur than the lower metal layer 73, and the ionization tendency of the entire upper metal layer 75 is higher than that of the lower metal layer 73. That is, the upper metal layer 75 can perform sacrificial corrosion on the lower metal layer 73 between the upper metal layer 75 and the lower metal layer 73. For this reason, since the period until the surface of the metal shell 5 which is a lower layer than the composite metal layer 70 is exposed to the outside can be made longer, the corrosion resistance of the metal shell 5 is formed by forming the composite metal layer 70. Becomes better.

  Further, even if pinholes are generated in the lower metal layer 73 and the upper metal layer 75 when plating is performed, by forming the metal layer in two layers, the pinhole generation position in each layer is uniform. It has become difficult to do. This makes it difficult to form a pinhole that penetrates the composite metal layer 70 and exposes the base metal 5.

  Such a spark plug 100 is manufactured as follows. However, the manufacturing method of the metal shell 5 of the spark plug 100 will be mainly described, and the description of known parts will be omitted or simplified.

  First, the insulator 1 is formed by using alumina as a main raw material and firing it into a predetermined shape at a high temperature. Moreover, the metal shell 5 is formed by using a steel material and plastic processing into a predetermined shape. At this time, a screw portion 52 is formed on the outer peripheral surface of the front end portion of the metal shell 5. Next, a rod-shaped center electrode 2 and a ground electrode 60 made of a Ni heat-resistant alloy are produced. In addition, when forming the center electrode 2, the metal core 23 is inserted and formed. Then, the ground electrode 60 is electrically resistance-welded to the front end surface 57 of the metal shell 5.

  Then, the metal shell 5 (with the ground electrode 60 welded) is subjected to a strike plating process using a barrel device to form a strike layer 71 on the surface of the terminal metal fitting 4. The strike plating process is an electroplating process in which the metal shell 5 is immersed in a plating bath (so-called wood bath) and the current density is increased to perform the treatment in a short time (about 1 to 10 minutes). As this wood bath, for example, one containing nickel chloride at a rate of 150 g to 300 g / liter and hydrochloric acid at a rate of 100 g to 180 g / liter is used.

  Next, the metal shell 5 on which the strike layer 71 is formed is subjected to barrel plating treatment by a barrel device to form the composite metal layer 70 on the surface of the strike layer 71. Specifically, the metal shell 5 is immersed in a bathtub for forming the lower metal layer 73 to form the lower metal layer 73, then taken out once, and then immersed in a bathtub for forming the upper metal layer 75. The plating bath used to form the lower metal layer 73 is, for example, nickel sulfate (hexahydrate) 200 g to 380 g / liter, nickel chloride (hexahydrate) 20 g to 60 g / liter, boric acid. Of 20 g to 60 g / liter, a primary brightener of 0.7 cc to 1.0 cc / liter, and a secondary brightener of 0.7 cc to 1.0 cc / liter. The plating bath used to form the upper metal layer 75 is, for example, nickel sulfate (hexahydrate) 200 g to 380 g / liter, nickel chloride (hexahydrate) 30 g to 100 g / liter, boric acid. Of 20 g to 60 g / liter, a primary brightener of 3 cc to 7 cc / liter, and a secondary brightener of 7 cc to 12 cc / liter.

  Then, chromate treatment is performed on the metal shell 5 on which the strike layer 71 and the composite metal layer 70 are formed by a barrel device, and a chromate film layer 76 is formed on the surface of the composite metal layer 70. In addition, the chromate treatment bath used for chromate treatment uses a bath containing at least one of sodium dichromate and potassium dichromate.

  Then, after the chromate film layer 76 is formed on the metal shell 5, it is washed with water and dried, and the insulator 1 to which the center electrode 2, the terminal metal fitting 4 and the like are fixed is assembled by a known method. Then, the ground electrode 60 is bent toward the center electrode 2 so that the ground electrode 60 and the center electrode 2 face each other with a spark discharge gap therebetween, and the spark plug 100 as shown in FIG. 1 is completed.

  Next, the spark plug 200 of the second embodiment will be described. The spark plug 200 is different in the form of plating applied on the surface of the metal shell 5 of the spark plug 100 described above, and shows a cross-sectional view of the metal shell 5 in which the metal layer formed by plating is enlarged. 3 shows. Other than this metal layer, the configuration is the same as that of the spark plug 100 of the first embodiment, and the same parts are denoted by the same reference numerals. Therefore, the metal layer formed on the surface of the metal shell 5 will be described below.

  As shown in FIG. 3, in the spark plug 200 of the second embodiment, the strike layer 71 and the composite metal layer 70 are formed on the surface of the metal shell 5 as in the first embodiment. In the composite metal layer 70, an intermediate metal layer 74 is provided between the lower metal layer 73 and the upper metal layer 75. The intermediate metal layer 74 contains 99.8 wt% nickel and 0.19 wt% sulfur, and is formed on the surface of the lower metal layer 73 with a thickness of 1 μm. A chromate film layer 76 is formed on the composite metal layer 70.

  Thus, the middle metal layer 74 contains nickel as a main component, and contains more sulfur than the upper metal layer 75 having a higher sulfur content than the lower metal layer 73. Thereby, when the upper metal layer 75 is corroded and the intermediate metal layer 74 is partially exposed to the outside, the corrosion of the remaining upper metal layer 75 is suppressed and the corrosion of the exposed intermediate metal layer 74 proceeds. . Then, the lower metal layer 73 is partially exposed to the outside due to the corrosion of the intermediate metal layer 74, but the lower metal layer 73 is prevented from being corroded, and the corrosion of the intermediate metal layer 74 further proceeds. Therefore, a sufficient period can be secured until the lower metal layer 73 is corroded and the surface of the metal shell 5 is exposed to the outside. Therefore, the corrosion resistance of the metal shell 5 is increased.

  Furthermore, even if the middle metal layer 74 is corroded by sacrificial corrosion, the upper metal layer 75 is now sacrificial to the lower metal layer 73, and a sufficient period can be secured until the surface of the metal shell 5 is exposed to the outside. Corrosion resistance of the metal shell 5 is increased. Even if pinholes are generated in the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75, by forming the metal layer in three layers, the position of occurrence of pinholes in each layer can be determined. It is difficult to match. Thereby, the base of the metal shell 5 is hardly exposed through the composite metal layer 70.

  In the same way as in the first embodiment, the intermediate metal layer 74 is subjected to barrel plating with a barrel device on the metal shell 5 on which the strike layer 71 and the lower metal layer 73 are formed, and is formed on the surface of the lower metal layer 73. An intermediate metal layer 74 is formed. Specifically, the metal shell 5 is immersed in a bathtub for forming the middle metal layer 74 to form the middle metal layer 74, and then taken out once, and then immersed in a bathtub for forming the upper metal layer 75. The plating bath used to form the intermediate metal layer 74 is, for example, nickel sulfate (hexahydrate) 200 g to 380 g / liter, nickel chloride (hexahydrate) 50 g to 130 g / liter, boric acid. Is 20 g to 60 g / liter, and the primary brightener is contained at a rate of 10 cc to 25 cc / liter.

Next, in order to evaluate the adhesion and the corrosion resistance of the metal layer formed on the metal shell 5 as described above, six tests shown in Examples 1 to 6 were performed. In addition, about Examples 1-3, it tests with respect to what the composite metal layer 70 is formed with the lower layer metal layer 73 and the upper metal layer 75, and about Examples 4-6, the composite metal layer 70 is The test was performed on the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 formed. First, for the first to twentieth test samples, the metal shell 5 made of S20C was formed into a predetermined shape. A plating bath using nickel chloride: 200 g / liter, hydrochloric acid: 150 g / liter, solvent: deionized water, bath temperature: 35 ° C., bath pH: 0.5, cathode current density: 0.7 A / dm ² The strike plating process was performed under the conditions described above. And the strike layer 71 of the thickness of the 1st-20th test sample was formed by changing plating time. Next, the first to twentieth test samples with the strike layer 71 formed thereon were nickel sulfate (hexahydrate): 300 g / liter, nickel chloride (hexahydrate): 41 g / liter, boric acid: 43 g / Liter, primary brightener: 0.8 cc / liter, secondary brightener: 0.8 cc / liter, solvent: plating bath using deionized water, bath temperature: 58 ° C., bath pH: 4, cathode current density : Barrel plating was performed under the condition of 5 A / dm ² . And the lower layer metal layer 73 of the thickness of the 1st-20th test sample was formed by changing plating time. Further, the first to twentieth test samples on which the lower metal layer 73 was formed were nickel sulfate (hexahydrate): 300 g / liter, nickel chloride (hexahydrate): 65 g / liter, boric acid: 45 g / liter. Primary brightener: 5 cc / liter, secondary brightener: 10 cc / liter, solvent: plating bath using deionized water, bath temperature: 58 ° C., bath pH: 4, cathode current density: 7 A / dm ² Barrel plating treatment was performed under the conditions. And the upper metal layer 75 of the thickness of the 1st-20th test sample was formed by changing plating time. Further, as a chromate treatment bath, sodium dichromate: 34 g / liter, solvent: deionized water is used, and chromate treatment is performed under conditions of bath temperature: 30 ° C., bath pH: 3.5, chromate treatment time: 90 seconds. Then, a chromate film layer 76 was formed. And after forming the said 4 layers, after washing with water, it dried with 80 degreeC warm air. The thicknesses of the strike layer 71, the composite metal layer 70, and the chromate film layer 76 are measured using a fluorescent X-ray film thickness meter.

On the other hand, for the 21st to 41st test samples, the metal shell 5 is produced in the same manner as described above, but the intermediate metal layer 74 is formed between the formation of the lower metal layer 73 and the upper metal layer 75. Yes. This is because the 21st to 41st test samples in which the lower metal layer 75 was formed were nickel sulfate (hexahydrate): 300 g / liter, nickel chloride (hexahydrate): 85 g / liter, boric acid: 35 g / liter. Liter, primary brightener: 17 cc / liter, solvent: deionized water, bath temperature: 45 ° C., bath pH: 2.7, cathode current density: 3.5 A / dm ² immersed in a plating bath, barrel plating The processing time is 2 minutes, and the intermediate metal layer 74 is formed.

  And the test for evaluating adhesiveness was performed as follows. First, about the 1st-41st test samples produced as mentioned above, the collar part 54 of the metal shell 5 is pressed from a first direction orthogonal to the direction of the axis O, and the metal shell 5 is 1-2 mm. Add the deformation. Next, the flange portion 54 is pressed in the same manner from a second direction orthogonal to the direction of the axis O and orthogonal to the first direction, and the metal shell 5 is deformed by 1 to 2 mm. Then, the flange portion 54 is pressed again from the first direction to deform the metal shell 5 by 2 to 3 mm. After performing such deformation, the composite metal in the upper and lower portions (the front end side and the rear end side portion of the tool engagement portion 51 in the axis O direction) of the tool engagement portion 51 by visual observation or using a magnifier of about 10 times. The state of the layer 70 was observed.

  When either peeling or dropping occurred in the composite metal layer 70, it was evaluated as “x” that the adhesion was inferior. Moreover, when the crack and the swelling generate | occur | produced, it evaluated that adhesiveness was good as "(circle)". Furthermore, when nothing occurred in the composite metal layer 70, it was evaluated as “☆” and excellent adhesion.

  Moreover, the test for evaluating corrosion resistance was performed as follows. The neutral salt spray test based on JIS H8502 was performed for 48 hours with respect to the spark plug 100 manufactured using the metal shell 5 for the first to 41st test samples manufactured as described above. Then, the rusting state of the external appearance of the metal shell 5 was observed.

  When the ratio of the portion where red rust was generated was less than 5% with respect to the total surface area of the metal shell 5, it was evaluated as “☆” and excellent in corrosion resistance. Similarly, when the ratio of the portion where red rust occurred was 5% or more and less than 10%, it was evaluated as “◯” and there was no problem in corrosion resistance. When the portion where red rust occurred was 10% or more and less than 20%, it was evaluated as “Δ” that there was a problem in corrosion resistance. When the portion where red rust occurred was 20% or more, it was evaluated as “x” that the corrosion resistance was not good.

[Example 1]
First, the relationship between the thickness of the composite metal layer 70 and the corrosion resistance was tested. As shown in FIG. 4, the 1st-8th test sample from which the thickness of the composite metal layer 70 each differs was produced. A strike layer 71 having a thickness of 0.5 μm was formed on each of the first to eighth test samples. The thickness of the composite metal layer 70 of the first test sample is 1.5 μm (the thicknesses of the lower metal layer 73 and the upper metal layer 75 are 1 μm and 0.5 μm, respectively), and the composite metal of the second test sample. Plating was performed so that the thickness of the layer 70 was 3 μm (the thickness of the lower metal layer 73 and the upper metal layer 75 was 2 μm and 1 μm, respectively). Similarly, the composite metal layer 70 of the third test sample has a thickness of 4 μm (the lower metal layer 73 and the upper metal layer 75 have thicknesses of 2.5 μm and 1.5 μm, respectively), and the composite of the fourth test sample. The thickness of the metal layer 70 was 6 μm (the thickness of the lower metal layer 73 and the upper metal layer 75 was 4 μm and 2 μm, respectively). Furthermore, the thickness of the composite metal layer 70 of the fifth test sample is 12 μm (the thicknesses of the lower metal layer 73 and the upper metal layer 75 are 8 μm and 4 μm, respectively), and the thickness of the composite metal layer 70 of the sixth test sample. Was 18 μm (the thicknesses of the lower metal layer 73 and the upper metal layer 75 were 12 μm and 6 μm, respectively). The thickness of the composite metal layer 70 of the seventh test sample is 35 μm (the thicknesses of the lower metal layer 73 and the upper metal layer 75 are 22.5 μm and 12.5 μm, respectively), and the composite metal of the eighth test sample. The thickness of the layer 70 was 45 μm (the thicknesses of the lower metal layer 73 and the upper metal layer 75 were 30 μm and 15 μm, respectively).

  As a result of evaluating the corrosion resistance of the first to eighth test samples, the first test sample is “x”, the second test sample is “△”, the third to fifth test samples are “◯”, The sixth to eighth test samples were “☆”. On the other hand, as a result of evaluating the adhesiveness of the first to eighth test samples, the first to seventh test samples were “◯” and the eighth test sample was “x”. From this, it was found that the thickness of the composite metal layer 70 is preferably 35 μm or less.

[Example 2]
Next, the thickness of the lower metal layer 73 and the relationship between the thickness ratio of the upper metal layer 75 and the corrosion resistance were tested. As shown in FIG. 5, ninth to fifteenth test samples having different thickness ratios of the lower metal layer 73 and the upper metal layer 75 were produced. In the ninth to fifteenth test samples, the thickness of the strike layer 71 was 0.5 μm, and the thickness of the composite metal layer 70 (the lower metal layer 73 and the upper metal layer 75) was 10 μm. The thicknesses of the lower metal layer 73 and the upper metal layer 75 of the ninth test sample are 9 μm and 1 μm, respectively, and the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the tenth test sample are 8 μm and 2 μm, respectively. Was plated. Similarly, the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the eleventh test sample are 6 μm and 4 μm, respectively, and the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the twelfth test sample are 5 μm and 5 μm, respectively. . Further, the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the thirteenth test sample are 3 μm and 7 μm, respectively, and the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the fourteenth test sample are 1.5 μm and 8. The thickness was 5 μm. Furthermore, the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the fifteenth test sample were 0.5 μm and 9.5 μm, respectively.

  As a result of evaluating the corrosion resistance of the ninth to fifteenth test samples, the ninth and twelfth to fourteenth test samples are “Δ”, the fifteenth test sample is “x”, the tenth and eleventh test samples. Became "○". On the other hand, as a result of evaluating the adhesion of the ninth to fifteenth test samples, the ninth, fourteenth and fifteenth test samples are “x”, and the tenth, twelfth and thirteenth test samples are “◯”. The eleventh test sample was “☆”. From this, it was found that the thickness of the lower metal layer 73 is preferably 30% or more and 80% or less of the thickness of the composite metal layer 70.

[Example 3]
Subsequently, the relationship between the thickness of the strike layer 71 and the adhesiveness was tested. As shown in FIG. 6, in the sixteenth, seventeenth, eighteenth, nineteenth and twentieth test samples, the strike layer 71 has a thickness of 0.1, 0.2, 0.5, 0.7, It formed so that it might become 0.9 (micrometer). Then, the sixteenth to twentieth test samples were produced so that the thicknesses of the lower metal layer 73 and the upper metal layer 75 were fixed to 6 μm and 4 μm, respectively.

  As a result of evaluating the adhesion of the sixteenth to twentieth test samples, the sixteenth and twentieth test samples were “◯”, and the seventeenth to nineteenth test samples were “☆”. In addition, although the corrosion resistance of the sixteenth to twentieth test samples was also evaluated, all the test samples were evaluated as “◯”. Thus, it was found that the thickness of the strike layer 71 is preferably 0.2 μm or more and 0.7 μm or less.

[Example 4]
Next, the relationship between the thickness of the composite metal layer 70 and the corrosion resistance was tested. As shown in FIG. 7, the 21st-28th test samples from which the thickness of the composite metal layer 70 each differed were produced. Strike layers 71 having a thickness of 0.5 μm were formed on the 21st to 28th test samples, respectively. The thickness of the composite metal layer 70 of the twenty-first test sample was 2.5 μm (the thicknesses of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 were 1 μm, 1 μm, and 0.5 μm, respectively). The plating was performed so that the composite metal layer 70 of the 22 test samples had a thickness of 4 μm (the thickness of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 was 2 μm, 1 μm, and 1 μm, respectively). . Similarly, the thickness of the composite metal layer 70 of the 23rd test sample was 5 μm (the thicknesses of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 were 2.5 μm, 1 μm, and 1.5 μm, respectively). The thickness of the composite metal layer 70 of the 24th test sample was 7 μm (the thicknesses of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 were 4 μm, 1 μm, and 2 μm, respectively). Furthermore, the thickness of the composite metal layer 70 of the 25th test sample is 13 μm (the thicknesses of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 are 8 μm, 1 μm, and 4 μm, respectively), and the 26th test sample. The thickness of the composite metal layer 70 was 19 μm (the thicknesses of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 were 12 μm, 1 μm, and 6 μm, respectively). The thickness of the composite metal layer 70 of the 27th test sample was 36 μm (the thicknesses of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 were 22.5 μm, 1 μm, and 12.5 μm, respectively). The thickness of the composite metal layer 70 of 28 test samples was 46 μm (the thicknesses of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 were 30 μm, 1 μm, and 15 μm, respectively).

  As a result of evaluating the corrosion resistance of each of the 21st to 28th test samples, the 21st test sample is “×”, the 22nd test sample is “△”, the 23rd and 24th test samples are “◯”, The 25th to 28th test samples were “☆”. On the other hand, as a result of evaluating the adhesiveness of the 21st to 28th test samples, the 21st to 27th test samples were “◯”, and the 28th test sample was “X”. From this, it was found that the thickness of the composite metal layer 70 is preferably 36 μm or less.

[Example 5]
Next, the thickness of the lower metal layer 73 and the relationship between the thickness ratio of the upper metal layer 75 and the corrosion resistance were tested. As shown in FIG. 8, 29th to 36th test samples having different thickness ratios of the lower metal layer 73 and the upper metal layer 75 were produced. In the 29th to 36th test samples, the thickness of the strike layer 71 is 0.5 μm, and the thickness of the composite metal layer 70 (the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75) is 10 μm (of which the middle metal layer) 74 was 1 μm and had a constant thickness. The thicknesses of the lower metal layer 73 and the upper metal layer 75 of the 29th test sample are 8 μm and 1 μm, respectively, and the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the 30th test sample are 7 μm and 2 μm, respectively. Was plated. Similarly, the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the 31st test sample are 6 μm and 3 μm, respectively, and the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the 32nd test sample are 4 μm and 5 μm, respectively. . The thickness of the lower metal layer 73 and the upper metal layer 75 of the 33rd test sample was 3 μm and 6 μm, respectively, and the thickness of the lower metal layer 73 and the upper metal layer 75 of the 34th test sample were 2 μm and 7 μm, respectively. Further, the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the 35th test sample are 1 μm and 8 μm, respectively, and the thicknesses of the lower metal layer 73 and the upper metal layer 75 of the 36th test sample are 0.5 μm and 8. The thickness was 5 μm.

  As a result of evaluating the corrosion resistance of the twenty-ninth to thirty-sixth test samples, the thirty-sixth test sample is “x”, the thirty-ninth and thirty-fifth to thirty-fifth test samples are “Δ”, and the thirty-second test sample is “◯”. The 30th and 31st test samples were “☆”. On the other hand, as a result of evaluating the adhesiveness of the 29th to 36th test samples, the 29th, 35th and 36th test samples are “x”, and the 30th, 32nd, 33rd and 34th test samples are “○” and the 31st test sample were “☆”. From this, it was found that the thickness of the lower metal layer 73 is preferably 20% or more and 80% or less of the total thickness of the lower metal layer 73 and the upper metal layer 75.

[Example 6]
Subsequently, the relationship between the thickness of the strike layer 71 and the adhesiveness was tested. As shown in FIG. 9, in the 37th, 38th, 39th, 40th and 41st test samples, the strike layer 71 has a thickness of 0.1, 0.2, 0.5, 0.7, It formed so that it might become 0.9 (micrometer). Then, the 37th to 41st test samples were prepared so that the thicknesses of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 were fixed to 6 μm, 1 μm, and 3 μm, respectively.

  As a result of evaluating the adhesiveness of the 37th to 41st test samples, the 37th and 41st test samples were “◯”, and the 38th to 40th test samples were “☆”. In addition, although the corrosion resistance of the 37th to 41st test samples was also evaluated, all the test samples were evaluated as “☆”. Thus, it was found that the thickness of the strike layer 71 is preferably 0.2 μm or more and 0.7 μm or less.

  As described above, in the spark plug 100 according to the first embodiment, the strike layer 71 containing nickel as a main component is formed on the base material of the metal shell 5, and nickel is used as the main component on the strike layer 71. A lower metal layer 73 and an upper metal layer 75 containing nickel as a main component and containing more sulfur than the lower metal layer 73 were formed. By setting the thickness of the strike layer 71 to 0.2 μm or more and 0.7 μm or less, the adhesion between the base of the metal shell 5 and the composite metal layer 70 composed of the lower metal layer 73 and the upper metal layer 75 is enhanced. . However, even if the strike layer 71 is not formed, the lower metal layer 73 having a lower sulfur content than the upper metal layer 75 is softer than the upper metal layer 75, so that plating for preventing corrosion is formed from a single metal layer. Compared with the case where it does, the adhesiveness with respect to the base material of the metal shell 5 increases.

  The composite metal layer 70 is composed of a lower metal layer 73 and an upper metal layer 75. By forming multiple layers in this way, even if a pinhole is generated in one metal layer, a pin is formed in the other metal layer. A hole can be covered and the corrosion resistance as the whole metal layer can be improved.

  The thickness of the composite metal layer 70 was set to 4 μm or more and 35 μm or less. Thereby, adhesiveness can also be maintained, improving the corrosion resistance as the composite metal layer 70 whole. The thickness of the lower metal layer 73 was set to 30% or more and 80% or less of the thickness of the composite metal layer 70. Thereby, it is possible to improve the corrosion resistance by sacrificial corrosion of the upper metal layer 75 with respect to the lower metal layer 73 and to maintain the adhesion between the metal shell 5 and the upper metal layer 75 in the lower metal layer 73.

  In the spark plug 200 according to the second embodiment, a strike layer 71 containing nickel as a main component is formed on the base material of the metal shell 5, and a lower metal layer 73 mainly containing nickel is formed thereon, The middle layer metal layer 74 containing nickel as a main component and containing more sulfur than the lower layer metal layer 73; and the main component containing nickel and containing a larger amount of sulfur than the lower layer metal layer 73; The upper metal layer 75 to be contained was formed. By setting the thickness of the strike layer 71 to 0.2 μm or more and 0.7 μm or less, the base of the metal shell 5 and the composite metal layer 70 composed of the lower metal layer 73, the middle metal layer 74, and the upper metal layer 75 are adhered to each other. Increases sex. However, even if the strike layer 71 is not formed, the lower layer metal layer 73 having a lower sulfur content than the upper layer metal layer 75 and the middle layer metal layer 74 is softer than the upper layer metal layer 75 and the middle layer metal layer 74, thus preventing corrosion. Compared with the case where the plating for forming is made of a single metal layer, the adhesion of the metal shell 5 to the substrate is enhanced.

  In addition, the composite metal layer 70 includes a lower metal layer 73, an intermediate metal layer 74, and an upper metal layer 75. By forming multiple layers in this way, even if a pinhole occurs in any of the metal layers, The pinhole can be covered with another metal layer, and the corrosion resistance of the entire metal layer can be improved.

  The thickness of the composite metal layer 70 was set to 5 μm or more and 36 μm or less. Thereby, it can restrict | limit so that it may not become an excessive specification, improving the corrosion resistance as the composite metal layer 70 whole. The thickness of the lower metal layer 73 was set to 20% or more and 80% or less of the total thickness of the lower metal layer 73 and the upper metal layer 75. Accordingly, it is possible to improve the corrosion resistance of the upper metal layer 75 with respect to the lower metal layer 73 after the middle metal layer 74 is corroded by sacrificial corrosion and to maintain the adhesion of the metal shell 5 and the composite metal layer 70 with each other. it can.

  Needless to say, the present invention can be modified in various ways. For example, although strike plating has nickel as a main component, it may have copper as a main component. Further, rust preventive oil may be applied to the surface of the chromate film layer 76 of the present embodiment in order to prevent the chromate film layer 76 from directly contacting outside air and being oxidized.

  The present invention can be applied to a spark plug used in an internal combustion engine.

It is a fragmentary sectional view of spark plug 100 of a 1st embodiment. It is an expanded sectional view of the metal layer formed on the base material of the metal shell 5 of the first embodiment. It is an expanded sectional view of the metal layer formed on the base of the metal shell 5 of the second embodiment. It is a table | surface which shows the test result about the relationship between the thickness of the composite metal layer 70 of 1st Embodiment, and corrosion resistance. It is a table | surface which shows the test result about the relationship between the thickness of the lower metal layer 73 of 1st Embodiment, and the ratio of the thickness of the upper metal layer 75, and corrosion resistance. It is a table | surface which shows the test result about the relationship between the thickness of the strike layer 71 of 1st Embodiment, and adhesiveness. It is a table | surface which shows the test result about the relationship between the thickness of the composite metal layer 70 of 2nd Embodiment, and corrosion resistance. It is a table | surface which shows the test result about the relationship between the thickness of the lower metal layer 73 of 2nd Embodiment, and the ratio of the thickness of the upper metal layer 75, and corrosion resistance. It is a table | surface which shows the test result about the relationship between the thickness of the strike layer 71 of 2nd Embodiment, and adhesiveness.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Insulator 2 Center electrode 5 Metal shell 12 Shaft hole 70 Composite metal layer 71 Strike layer 73 Lower metal layer 74 Middle metal layer 75 Upper metal layer 76 Chromate film layer 100,200 Spark plug

Claims (4)

A center electrode, an axial hole penetrating in the axial direction, and an insulator that holds the center electrode on the tip end side of the axial hole, and a metal shell that surrounds the periphery of the insulator in the radial direction and holds the insulator A spark plug with
On the surface of the metal shell made of steel material, a lower metal layer mainly composed of nickel,
On the surface of the lower metal layer, an upper metal layer containing nickel as a main component and containing a larger amount of sulfur than the lower metal layer is formed.
The thickness of the composite metal layer composed of the lower metal layer and the upper metal layer is 4 μm or more and 35 μm or less, and the thickness of the lower metal layer is 30% or more and 80% of the thickness of the composite metal layer. A spark plug characterized by: A center electrode, an axial hole penetrating in the axial direction, and an insulator that holds the center electrode on the tip end side of the axial hole, and a metal shell that surrounds the periphery of the insulator in the radial direction and holds the insulator A spark plug with
On the surface of the metal shell made of steel material, a lower metal layer mainly composed of nickel,
On the surface of the lower metal layer, nickel as a main component, an intermediate metal layer containing a larger amount of sulfur than the lower metal layer,
On the surface of the middle metal layer, an upper metal layer mainly composed of nickel, larger in amount than the lower metal layer, and containing a smaller amount of sulfur than the middle metal layer, is formed.
The composite metal layer composed of the lower metal layer, the middle metal layer, and the upper metal layer has a thickness of 5 μm to 36 μm, and the middle metal layer has a thickness of 0.5 μm to 1.5 μm. The spark plug is characterized in that the thickness of the lower metal layer is 20% or more and 80% or less when the thickness of both the lower metal layer and the upper metal layer is 100%.   The spark plug according to claim 1 or 2, wherein a chromate film layer in which 95% by weight or more of the chromium component contained is trivalent chromium is formed on the surface of the upper metal layer. A strike layer having nickel or copper as a main component and having a thickness of 0.2 μm or more and 0.7 μm or less is formed between the lower metal layer and the surface of the metal shell. Item 4. The spark plug according to any one of Items 1 to 3.

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