| 摘要 |
PROBLEM TO BE SOLVED: To provide a spark plug wherein the content of chromium (VI) of a chromate film covering the surface of a main metal fitting is small and which is excellent in corrosion-proofing and heat resistance as compared with a conventional chromate film and to provide its manufacturing method. SOLUTION: By immersing a main metal fitting 1" having a zinc plated layer formed in a chromate treatment bath 50, a chromium (III)-based chromate layer of which chromium constituent is mainly composed of chromium (III) is formed on its surface. A silica-based layer containing an oxide of silicon as its main constituent is formed on the chromium (III)-based chromate layer by immersing the main metal fitting 1' having the chromium (III)-based chromate layer formed in a silicate solution 51 in which alkali silicate is dissolved and drying it. A silicon composite chromate film wherein the chromium (III)-based chromate layer and the silica-based layer are compound is significantly improved in corrosion-proofing as compared with a conventional chromate film and also dramatically improved in heat resistance. As a result, the corrosion-proofing of the main metal fitting can sufficiently be kept even under an environment where the temperature of the spark plug is apt to rise.
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