J-type spark plugs composed of Ni-base alloy electrodes with a pure Ir tip in the center electrode and a Pt-W alloy tip in the ground electrode were examined as-manufactured and after use in natural gas reciprocating engines by spectroscopic and metallurgical techniques. The spectroscopic examination indicated Ni emission from the Ni alloy electrodes in new plugs, but a strong Ca signal in engine used plugs. This was confirmed by metallurgical examination, which showed the presence of Ca containing glassy oxide phase(s) (with the electrode alloy components) in the used spark plug electrodes. Intergranular cracking was observed on the Ir and Pt-W alloy electrode insert tips. The interface between the Pt-W insert and the Ni alloy ground electrode also became a site for extensive cracking and oxidation during service. These oxidation/corrosion and metallurgical issues may represent a significant component of the wear mechanism of these plugs in natural gas engines.

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