Abstract:
A spark plug superior in salt corrosion resistance and stress corrosion cracking resistance is provided. The spark plug includes a metallic shell coated with a composite layer which includes a nickel plating layer and a chromate layer formed on the nickel plating layer. The spark plug is characterized in that the nickel plating layer has a thickness A which satisfies a relational expression 3 μm≦A≦15 μm and that the chromate layer has a thickness B which satisfies a relational expression 2 nm≦B≦45 nm.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a spark plug for an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     A spark plug for providing ignition in an internal combustion engine, such as a gasoline engine, has the following structure: an insulator is provided externally of a center electrode; a metallic shell (main metal fitting) is provided externally of the insulator; and a ground electrode which forms a spark discharge gap in cooperation with the center electrode is attached to the metallic shell. The metallic shell is generally formed from an iron-based material, such as carbon steel, and, in many cases, plating is performed on its surface for corrosion protection. A known technique for performing such plating forms a plating layer having a 2-layer structure consisting of an Ni plating layer and a chromate layer (Japanese Patent Application Laid-Open (kokai) No. 2002-184552, “Patent Document 1”). 
     Problems to be Solved by the Invention 
     According to the technique for forming a plating layer having 2-layer structure, a plating process is performed before a crimping process. In the crimping process, an insulator to which a center electrode is attached is inserted into a hollow portion of a hollow, cylindrical metallic shell; then, a portion of the metallic shell is crimped inward (toward the insulator), thereby fixing the metallic shell to the insulator. This crimping process has involved a problem in which an associated deformation of the metallic shell causes cracking or peeling of the plating layer, resulting in deterioration in salt corrosion resistance. Also, the crimping process has involved the following problem: because of residual stress in the metallic shell stemming from the crimping process or an increase in hardness the metallic shell associated with a microstructural change caused by heating in hot crimping, stress corrosion cracking arises in a portion which has high hardness and where a large residual stress exists. However, conventionally, sufficient measures have not been devised for attaining a spark plug superior in salt corrosion resistance and stress corrosion cracking resistance. 
     An object of the present invention is to provide a spark plug superior in salt corrosion resistance and stress corrosion cracking resistance. 
     SUMMARY OF THE INVENTION 
     Means for Solving the Problems 
     The present invention has been conceived to solve, at least partially, the above problems and can be embodied in the following modes or application examples. 
     [Application example 1] A spark plug comprising a metallic shell coated with a composite layer which includes a nickel plating layer and a chromate layer formed on the nickel plating layer, characterized in that the nickel plating layer has a thickness A which satisfies a relational expression 3 μm≦A≦15 μm and that the chromate layer has a thickness B which satisfies a relational expression 2 nm≦B≦45 nm. 
     [Application example 2] A spark plug described in application example 1, wherein the thickness B satisfies a relational expression 20 nm≦B≦45 nm. 
     [Application example 3] A spark plug described in application example 2, wherein the thickness A satisfies a relational expression 5 μm≦A≦15 μm. 
     [Application example 4] A metallic shell for a spark plug, coated with a composite layer which includes a nickel plating layer and a chromate layer formed on the nickel plating layer, characterized in that the nickel plating layer has a thickness A which satisfies a relational expression 3 μm≦A≦15 μm and that the chromate layer has a thickness B which satisfies a relational expression 2 nm≦B≦45 nm. 
     The present invention can be implemented in various forms. For example, the present invention can be implemented in a method of manufacturing a spark plug and a method of manufacturing a metallic shell. 
     Effects of the Invention 
     In the spark plug of application example 1, since the thickness A of the nickel plating layer of the metallic shell is not less than 3 μm, there can be restrained the formation of a plating-repellant portion (pinhole) which could otherwise result from a situation in which oil or the like that has adhered to the surface of the metallic shell before formation of the nickel plating layer remains incompletely removed due to insufficient cleaning, whereby salt corrosion resistance can be enhanced. Additionally, since the thickness A of the nickel plating layer is not greater than 15 μm, there can be restrained cracking of the nickel plating layer which could otherwise result from a large thickness, whereby plating peeling resistance can be enhanced. Therefore, salt corrosion resistance can be enhanced. Also, since a thickness range smaller than a relatively small thickness of 2 nm is excluded for the thickness B of the chromate layer, there can be restrained a fracture of the chromate layer which could otherwise result from residual stress associated with crimping. Additionally, since thickness range greater than a relatively large thickness of 45 nm is excluded for the thickness B of the chromate layer, there can be restrained the occurrence of cracking during working which could otherwise result from poor adhesion to the metallic shell (the nickel plating layer). Therefore, stress corrosion cracking resistance can be enhanced. Thus, a spark plug superior in salt corrosion resistance and stress corrosion cracking resistance can be provided. 
     Employment of the configuration of application example 2 can further enhance stress corrosion cracking resistance. 
     Employment of the configuration of application example 3 can further enhance plating peeling resistance and salt corrosion resistance. 
     In the metallic shell of application example 4, since the thickness A of the nickel plating layer is not less than 3 μm, there can be restrained the formation of a plating-repellant portion (pinhole) which could otherwise result from a situation in which oil or the like that has adhered to the surface of the metallic shell before formation of the nickel plating layer remains incompletely removed due to insufficient cleaning, whereby salt corrosion resistance can be enhanced. Additionally, since the thickness A of the nickel plating layer is not greater than 15 μm, there can be restrained cracking of the nickel plating layer which could otherwise result from a large thickness, whereby plating peeling resistance can be enhanced. Therefore, salt corrosion resistance can be enhanced. Also, since a thickness range smaller than a relatively small thickness of 2 nm is excluded for the thickness B of the chromate layer, there can be restrained a fracture of the chromate layer which could otherwise result from residual stress associated with crimping. Additionally, since a thickness range greater than a relatively large thickness of 45 nm is excluded for the thickness B of the chromate layer, there can be restrained the occurrence of cracking during working which could otherwise result from poor adhesion to the metallic shell (the nickel plating layer). Therefore, stress corrosion cracking resistance can be enhanced. Thus, by use of the metallic shell of application example 4, a spark plug superior in salt corrosion resistance and stress corrosion cracking resistance can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of essential members, showing the structure of a spark plug according to an embodiment of the present invention. 
         FIG. 2  is an explanatory view showing an example step of fixing a metallic shell  1  to an insulator  2  through crimping. 
         FIG. 3  is a flowchart showing the procedure of the plating process for the metallic shell. 
         FIGS. 4(   a ) and  4 ( b ) are explanatory views showing the results of tests for plating peeling resistance, salt corrosion resistance, and stress corrosion cracking resistance with respect to 49 samples S 1  to S 49  prepared under the above-mentioned processing conditions. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. Configuration of Spark Plug 
       FIG. 1  is a sectional view of essential members, showing the structure of a spark plug according to an embodiment of the present invention. A spark plug  100  includes a tubular metallic shell  1 ; a tubular insulator  2 , which is fitted into the metallic shell  1  in such a manner that its forward end portion projects from the metallic shell  1 ; a center electrode  3 , which is provided in the insulator  2  in such a state that its forward end portion projects from the insulator  2 ; and a ground electrode  4  whose one end is joined to the metallic shell  1  and whose other end faces the forward end of the center electrode  3 . A spark discharge gap g is formed between the ground electrode  4  and the center electrode  3 . 
     The insulator  2  is formed from, for example, a ceramic sintered body of alumina or aluminum nitride and has a through hole  6  formed therein in such a manner as to extend along the axial direction thereof, and adapted to allow the center electrode  3  to be fitted therein. A metal terminal  13  is fixedly inserted into the through hole  6  at a side toward one end of the through hole  6 , whereas the center electrode  3  is fixedly inserted into the through hole  6  at a side toward the other end of the through hole  6 . A resistor  15  is disposed, within the through hole  6 , between the metal terminal  13  and the center electrode  3 . Opposite end portions of the resistor  15  are electrically connected to the center electrode  3  and the metal terminal  13  via electrically conductive glass seal layers  16  and  17 , respectively. 
     The metallic shell  1  is formed into a hollow, cylindrical shape from a metal, such as carbon steel, and forms a housing of the spark plug  100 . The metallic shell  1  has a threaded portion  7  formed on its outer circumferential surface and adapted to mount the spark plug  100  to an unillustrated engine block. A hexagonal portion  1   e  is a tool engagement portion which allows a tool, such as a spanner or a wrench, to be engaged therewith in mounting the metallic shell  1  to the engine block, and has a hexagonal cross section. In a space between the outer surface of the insulator  2  and the inner surface of a rear (upper in the drawing) opening portion of the metallic shell  1 , a ring packing  62  is disposed on the rear periphery of a flange-like projection  2   e  of the insulator  2 , and a filler layer  61 , such as talc, and a ring packing  60  are disposed, in this order, rearward of the ring packing  62 . In assembling work, the insulator  2  is pressed forward (downward in the drawing) into the metallic shell  1 , and, in this condition, the rear opening end of the metallic shell  1  is crimped inward toward the ring packing  60  (and, in turn, toward the projection  2   e , which functions as a receiving portion for crimping), whereby a crimp portion  1   d  is formed, and thus the metallic shell  1  is fixed to the insulator  2 . 
     A gasket  30  is fitted to a proximal end of the threaded portion  7  of the metallic shell  1 . The gasket  30  is formed by bending a metal sheet of carbon steel or the like into the form of a ring. When the threaded portion  7  is screwed into a threaded hole of the cylinder head, the gasket  30  is compressed in the axial direction and deformed in a crushed manner between a flange-like gas seal portion  1   f  of the metallic shell  1  and a peripheral-portion-around-opening of the threaded hole, thereby sealing the gap between the threaded hole and the threaded portion  7 . 
       FIG. 2  is an explanatory view showing an example step of fixing the metallic shell  1  to the insulator  2  through crimping.  FIG. 2  omits the illustration of the ground electrode  4 . First, as shown in  FIG. 2(   b ), the insulator  2  whose through hole  6  accommodates the center electrode  3 , the electrically conductive glass seal layers  16  and  17 , the resistor  15 , and the metal terminal  13  is inserted into the metallic shell  1  shown in  FIG. 2(   a ) from an insertion opening portion  1   p  (where a prospective crimp portion  200  which will become the crimp portion  1   d  is formed) at the rear end of the metallic shell  1 , thereby establishing a state in which an engagement portion  2   h  of the insulator  2  and an engagement portion  1   c  of the metallic shell  1  are engaged together via a sheet packing  63 . 
     Then, as shown in  FIG. 2(   c ), the ring packing  62  is disposed inside the metallic shell  1  through the insertion opening portion  1   p ; subsequently, the filler layer  61  of talc or the like is formed; and, furthermore, the ring packing  60  is disposed. Then, by means of a crimping die  111 , the prospective crimp portion  200  is crimped to an end surface  2   n  of the projection  2   e , which functions as a receiving portion for crimping, via the ring packing  62 , the filler layer  61 , and the ring packing  60 , thereby forming the crimp portion  1   d  and fixing the metallic shell  1  to the insulator  2  through crimping as shown in  FIG. 2(   d ). At this time, in addition to the crimp portion  1   d , a groove portion  1   h  ( FIG. 1)  located between the hexagonal portion  1   e  and the gas seal portion  1   f  is also deformed under a compressive stress associated with crimping. The reason for this is that the crimp portion  1   d  and the groove portion  1   h  are thinnest portions in the metallic shell  1 . The groove portion  1   h  is also called the “thin-walled portion.” After the step of  FIG. 2   d ), the ground electrode  4  is bent toward the center electrode  3  so as to form the spark discharge gap g, thereby completing the spark plug  100  of  FIG. 1 . The crimping step described with reference to  FIG. 2  is of cold crimping; however, hot crimping can also be employed. 
     B. Plating Process 
     In manufacture of the spark plug  100 , before the above-mentioned crimping step, a plating process is performed on the metallic shell  1 .  FIG. 3  is a flowchart showing the procedure for the plating process for the metallic shell. In step T 100 , nickel strike plating is performed. Nickel strike plating is performed for cleaning the surface of the metallic shell formed from carbon steel and for improving adhesion between plating and a base metal. However, nickel strike plating may be omitted. Usually employed processing conditions can be employed for nickel strike plating. A specific example of preferable processing conditions is as follows. 
     &lt;Example of Processing Conditions of Nickel Strike Plating&gt; 
     Composition of plating bath 
     
         
         Nickel chloride: 150-600 g/L 
         35% hydrochloric acid: 50-300 ml/L 
         Solvent: Deionized water
 
Processing temperature (bath temperature): 25-40° C.
 
Cathode current density: 0.2-0.4 A/dm 2  
 
Processing time; 5-20 minutes
 
       
    
     In step T 110 , an electrolytic nickel plating process is performed. The electrolytic nickel plating process can be a barrel-type electrolytic nickel plating process which uses a rotary barrel, and may employ another plating method, such as a stationary plating method. Usually employed processing conditions can be employed for electrolytic nickel plating. A specific example of preferable processing conditions is as follows. 
     &lt;Example of Processing Conditions of Electrolytic Nickel Plating&gt; 
     Composition of plating bath 
     
         
         Nickel sulfate: 100-400 g/L 
         Nickel chloride: 20-60 g/L 
         Boric acid: 20-60 g/L 
         Solvent: Deionized water
 
Bath pH: 2.0-4.8
 
Processing temperature (bath temperature): 25-60° C.
 
Cathode current density: 0.2-0.4 A/dm 2  
 
Processing time: 24-192 minutes
 
       
    
     In step T 120 , an electrolytic chromating process is performed. The electrolytic chromating process can also use a rotary barrel and may employ another plating method, such as a stationary plating method. An example of preferable processing conditions of the electrolytic chromating process is as follows. 
     &lt;Example of Processing Conditions of Electrolytic Chromating Process&gt; 
     Composition of processing bath (chromating processing solution) 
     
         
         Sodium dichromate: 20-70 g/L 
         Solvent: Deionized water
 
Bath pH: 2-6
 
Processing temperature (bath temperature): 20-60° C.
 
Cathode current density: 0.01-0.50 A/dm 2  (preferably 0.02-0.45 A/dm 2 )
 
Processing time: 1-10 minutes
 
       
    
     A usable dichromate other than sodium dichromate is potassium dichromate. Another combination of processing conditions (amount of dichromate, cathode current density, processing time, etc.) different from the above may be employed according to a desired thickness of the chromate layer. 
     By performing the above plating processes, a film of 2-layer structure consisting of the nickel plating layer and the chromate layer is formed on the outer and inner surfaces of the metallic shell. Another protection film can be formed on the film of 2-layer structure. For example, there can be formed a film of seizure inhibitor which contains C (mineral oil or graphite) and one or more components selected from among Al, Ni, Zn, and Cu. Through formation of a seizure inhibitor film, when the engine head is heated to a high temperature, there can be restrained seizure between the spark plug and the engine head. Also, for example, there can be formed a film of rust prevention oil which contains at least one of C, Ba, Ca, and Na. After a multilayered protection film is formed as mentioned above, the metallic shell is fixed to the insulator, etc., by the crimping step, thereby completing the spark plug. 
     C. Example 
     C1. Processing Conditions 
     The metallic shells  1  were manufactured, by cold forging, from a carbon steel wire SWCH17K for cold forging specified in JIS G3539. The ground electrodes  4  were welded to the respective metallic shells  1 , followed by degreasing and water washing. Subsequently, a nickel strike plating process was performed under the following processing conditions by use of a rotary barrel. 
     &lt;Processing Conditions of Nickel Strike Plating&gt; 
     Composition of plating bath 
     
         
         Nickel chloride: 300 g/L 
         35% hydrochloric acid: 100 ml/L
 
Processing temperature (bath temperature): 30±5° C.
 
Cathode current density: 0.3 A/dm 2  
 
Processing time: 15 minutes
 
       
    
     Next, an electrolytic nickel plating process was performed under the following processing conditions by use of the rotary barrel, thereby forming nickel plating layers. The nickel (Ni) content (% by mass) of the nickel plating layers was 98% or higher. 
     &lt;Processing Conditions of Electrolytic Nickel Plating&gt; 
     Composition of plating bath 
     
         
         Nickel sulfate: 250 g/L 
         Nickel chloride: 50 g/L 
         Boric acid: 40 g/L
 
Bath pH: 4.0
 
Processing temperature (bath temperature): 55±5° C.
 
Cathode current density: 0.3 A/dm 2  
 
Processing time: 24-192 minutes
 
       
    
     In the present example, there were prepared seven types of samples which differed in the thickness of the nickel plating layer as effected through control of the thickness of the nickel plating layer by means of the processing time of plating. Specifically, there were prepared seven types of samples which differed in the thickness of the nickel plating layer as effected by means of the following seven types of processing time. “The thickness of the nickel plating layer” means the total thickness of the thickness of a layer formed by the above-mentioned nickel strike plating process and the thickness of a layer formed by the above-mentioned electrolytic nickel plating process. 
     Processing time: 24 minutes 
     
         
         Nickel plating layer thickness: 2 μm
 
Processing time: 36 minutes
 
         Nickel plating layer thickness: 3 μm
 
Processing time: 48 minutes
 
         Nickel plating layer thickness: 4 μm
 
Processing time: 60 minutes
 
         Nickel plating layer thickness: 5 μm
 
Processing time: 108 minutes
 
         Nickel plating layer thickness: 9 μm
 
Processing time; 180 minutes
 
         Nickel plating layer thickness: 15 μm
 
Processing time: 192 minutes
 
         Nickel plating layer thickness: 16 μm 
       
    
     The relationship between processing time and the thickness of the nickel plating layer was experimentally obtained beforehand. The thickness of the nickel plating layer was measured by use of a fluorescent X-ray film thickness meter under the following conditions: beam diameter of X ray: 0.2 mm; and radiation time: 10 seconds. 
     Next, an electrolytic chromating process was performed by use of a rotary barrel under the following processing conditions, thereby forming a chromate layer on the nickel plating layer. 
     &lt;Processing Conditions of Electrolytic Chromating Process&gt; 
     Composition of processing bath (chromating processing solution) 
     
         
         Sodium dichromate: 40 g/L 
         Solvent: Deionized water
 
Processing temperature (bath temperature): 35±5° C.
 
Cathode current density: 0.01 A/dm 2 -0.50 A/dm 2  
 
Processing time: 5 minutes
 
       
    
     In the present embodiment, there were prepared seven types of samples which differed in the thickness of the chromate layer as effected through control of the thickness of the chromate layer by means of the cathode current density. Specifically, there were prepared seven types of samples which differed in the thickness of the chromate layer as effected by means of the following seven types of cathode current density. 
     Cathode current density: 0.01 A/dm 2    
     
         
         Chromate layer thickness: 1 nm
 
Cathode current density: 0.02 A/dm 2  
 
         Chromate layer thickness: 2 nm
 
Cathode current density: 0.10 A/dm 2  
 
         Chromate layer thickness: 10 nm
 
Cathode current density: 0.20 A/dm 2  
 
         Chromate layer thickness: 20 nm
 
Cathode current density: 0.40 A/dm 2  
 
         Chromate layer thickness: 40 nm
 
Cathode current density: 0.45 A/dm 2  
 
         Chromate layer thickness: 45 nm
 
Cathode current density: 0.50 A/dm 2  
 
         Chromate layer thickness: 50 nm 
       
    
     The relationship between cathode current density and the thickness of the chromate layer was experimentally obtained beforehand. The thickness of the chromate layer was measured as follows. First, a small specimen was cut out from near the outer surface of each of the samples by use of a focused iron beam machining apparatus (FIB machining apparatus). Then, by use of a scanning transmission electron microscope (STEM), the small specimen was analyzed at an acceleration voltage of 200 kV, thereby obtaining a color map image of Cr elements with respect to the vicinity of the outer surface on a cross section (a section perpendicular to the center axis represented by the dot-dash line in  FIG. 1 ) of the metallic shell. From this color map image, the thickness of the chromate layer was measured. 
     There were prepared 49 (7 types×7 types) metallic shell samples (S 1  to S 49 ) which differed in the thickness of the nickel plating layer and in the thickness of the chromate layer as effected through processing under the above-mentioned conditions. The samples S 1  to S 49  were tested for evaluation of salt corrosion resistance, plating peeling resistance, and stress corrosion cracking resistance. 
     C2. Evaluation Test Conditions 
     &lt;Salt Corrosion Resistance Test&gt; 
     The neutral salt spray test specified in JIS H8502 was conducted for evaluation of salt corrosion resistance. In this test, after a 48-hour salt spray test, there was measured the percentage of a red-rusted area to the surface area of the metallic shell of a sample. The percentage of a red-rusted area was calculated as follows: a sample after the test was photographed; there were measured a red-rusted area Sa in the photograph and an area Sb of the metallic shell in the photograph; and the ratio Sa/Sb was calculated, thereby obtaining the percentage of the red-rusted area. 
     &lt;Plating Peeling Resistance Test&gt; 
     The evaluation test for plating peeling resistance was conducted as follows. After the metallic shells of the samples underwent a chromating process, the insulators, etc., were fixed by crimping. Subsequently, the crimp portions  1   d  were inspected for a state of plating to see if lifting or peeling of plating was present. 
     &lt;Stress Corrosion Cracking Resistance Test&gt; 
     In order to evaluate stress corrosion cracking resistance, the following accelerated corrosion test was conducted. Four holes each having a diameter of about 2 mm were cut in the groove portions  1   h  ( FIG. 1 ) of the samples (metallic shells); subsequently, the insulators, etc., were fixed by crimping. The holes were cut for allowing entry of a corrosive solution for test into the metallic shells. The test conditions of the accelerated corrosion test are as follows. 
     [Test Conditions of Accelerated Corrosion Test (Stress Corrosion Cracking Resistance Test)] 
     Composition of corrosive solution 
     
         
         Calcium nitrate tetrahydrate: 1,036 g 
         Ammonium nitrate: 36 g 
         Potassium permanganate: 12 g 
         Pure water: 116 g
 
pH: 3.5-4.5
 
Processing temperature: 30-40° C.
 
       
    
     The reason for adding potassium permanganate as an oxidizer into the corrosive solution is to accelerate the corrosion test. 
     After the 10-hour test under the above-mentioned test conditions, the samples were taken out from the corrosive solution. Then, the groove portions  1   h  of the samples were externally examined by use of a magnifier to see if cracking was generated in the groove portions  1   h . When the samples were found to be free from cracking, the corrosive solution was replaced with a new one; then, the samples underwent the accelerated corrosion test under the same conditions for another 10 hours. The test was repeated until the cumulative test time reached 80 hours. As a result of the crimping step, a large residual stress is generated in the groove portions  1   h . Therefore, by means of the accelerated corrosion test, the groove portions  1   h  can be evaluated for stress corrosion cracking resistance. 
     C3. Test Results 
       FIGS. 4(   a ) and  4 ( b ) are explanatory views showing the results of tests for plating peeling resistance, salt corrosion resistance, and stress corrosion cracking resistance with respect to 49 samples S 1  to S 49  prepared under the above-mentioned processing conditions. 
     As shown in  FIGS. 4(   a ) and  4 ( b ), regarding plating peeling resistance, substantially the same results were yielded in all thickness cases of the chromate layer. Specifically, in all thickness cases of the chromate layer, lifting or peeling of plating did not arise at a nickel plating layer thickness of 2 μm to 15 μm; however, lifting or peeling of plating arose at a nickel plating layer thickness of 16 μm (samples S 7 , S 14 , S 21 , S 28 , S 35 , S 42 , and S 49 ). Therefore, in view of plating peeling resistance, preferably, the nickel plating layer has a thickness of 2 μm to 15 μm. Conceivably, this is for the following reason: when the nickel plating layer has an excessively large thickness, the plating layer is apt to crack even under a small stress. 
     Regarding salt corrosion resistance, substantially the same results were yielded in all thickness cases of the chromate layer. Specifically, in all thickness cases of the chromate layer, the formation of red rust was restrained to 10% or less at a nickel plating layer thickness of 3 μm to 16 μm; however, the formation of red rust exceeded 10% at a nickel plating layer thickness of 2 μm (samples S 2 , S 8 , S 15 , S 22 , S 29 , S 36 , and S 43 ). Therefore, in view of salt corrosion resistance, preferably, the nickel plating layer has a thickness of 3 μm to 16 μm. Conceivably, this is for the following reason: when the nickel plating layer has an excessively small thickness, a plating-repellant portion (pinhole) is formed from a situation in which oil, stain, or the like that has adhered to the surface of the metallic shell remains incompletely removed due to insufficient cleaning; consequently, rust is formed at and propagates from such a portion. 
     Regarding stress corrosion cracking resistance, substantially the same results were yielded in all thickness cases of the nickel plating layer. Specifically, in all thickness cases of the nickel plating layer, cracking was not generated in the groove portion  1   h  at a chromate layer thickness of 2 nm to 45 nm at a cumulative test time of 20 hours or less; however, cracking was generated in the groove portion  1   h  at a chromate layer thickness of 1 nm (samples S 1  to S 7 ) and 50 nm (samples S 43  to S 49 ) at a cumulative test time of 20 hours or less. Therefore, in view of stress corrosion cracking resistance, preferably, the chromate layer has a thickness of 2 nm to 45 nm. More preferably, the chromate film has a thickness of 20 nm to 45 nm (samples S 22  to S 42 ), since cracking is not generated at a cumulative test time of 80 hours or less. 
     In the case where the chromate layer has a small thickness (1 nm), stress corrosion cracking resistance is poor, conceivably, for the following reason: since the chromate layer is excessively thin, the chromate layer is apt to be destroyed by residual stress. In the case where the chromate layer has a large thickness (50 nm), stress corrosion cracking resistance is poor, conceivably, for the following reason: since the chromate layer is thick, adhesion to the metallic shell deteriorates; consequently, cracking is apt to arise in the course of working, such as crimping. 
     According to comprehensive evaluation of the above test results regarding plating peeling resistance, salt corrosion resistance, and stress corrosion cracking resistance, most preferably, the nickel plating layer has a thickness of 5 μm to 15 μm, and the chromate layer has a thickness of 20 nm to 45 nm. The samples S 25  to S 27 , S 32  to S 34 , and S 39  to S 41  which satisfy these conditions have made the best marks in all the tests. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1 : metallic shell 
           1   c : engagement portion 
           1   d : crimp portion 
           1   e : hexagonal portion 
           1   f : gas seal portion (flange portion) 
           1   h : groove portion (thin-walled portion) 
           1   p : insertion opening portion 
           2 : insulator 
           2   e : projection 
           2   h : engagement portion 
           2   n : end surface 
           3 : center electrode 
           4 : ground electrode 
           6 : through hole 
           7 : threaded portion 
           13 : metal terminal 
           15 : resistor 
           16 ,  17 : electrically conductive glass seal layer 
           30 : gasket 
           60 : ring packing 
           61 : filler layer 
           62 : ring packing 
           63 : sheet packing 
           100 : spark plug 
           111 : die 
           200 : prospective crimp portion