| 摘要 |
PROBLEM TO BE SOLVED: To provide a glow plug to reduce the hexavalent chromium content of a chromate film to cover the surface of a main body metal fitting and be more excellent in anticorrosive performance and heat resistance compared with a conventional chromate film. SOLUTION: By immersing a main body metal fitting 3'', on which a galvanized layer is formed, in a chromate treating bath 50, a trivalent chromium system chromate layer of which a chromium component consists mainly of trivalent chromium is formed on the surface thereof. A main body metal fitting 3' on which the trivalent chromium system chromate layer is formed is immersed in a silicate solution 51 in which an alkali silicate is dissolved and by drying the main body metal fitting, a silica system layer consisting mainly of an oxide of silicon is formed on the trivalent chromium system chromate layer. A silicon composite chromate film wherein the trivalent chromium system chromate layer and the silica layer are brought into a composite state is more remarkably improved in anticorrosive performance and further dramatically improved in heat resistance compared with a conventional chromate film. As a result, corrosion resistance of the main body metal is sufficiently maintained even under environment wherein a glow plug temperature is apt to increase.
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