Patent ID: 7109645

Claim:
A spark plug comprising: a metallic housing carrying thereon a metallic member; an insulator fixed in said metallic housing; a center electrode fixed in said insulator; a ground electrode opposed to said center electrode via a spark discharge gap; and a protective coat formed on a part of a surface of at least one of said metallic housing and said metallic member, said protective coat including a galvanized film formed on said surface of said at least one of said metallic housing and said metallic member and a chromate film successively formed on said galvanized film, wherein said chromate film is hexavalent chromium-free and contains trivalent chromium as a major component, wherein said chromate film has a film thickness not smaller than 0.05 μm and not greater than 0.18 μm, and said chromate film contains cobalt and chromium with a weight ratio Co/Cr not smaller than 0.05 and not greater than 0.4, wherein said weight ratio Co/Cr is a weight ratio of cobalt elements to chromium elements contained in said chromate film.