What causes a spark plug terminal to corrode through inside the insulator?

Encountered something new. Had an intermittent miss on my '99 Explorer so I changed the plugs that were in it when I got it. On one of the plugs when I pulled the boot the plug terminal came with it. Just pulled right out of the center of the ceramic insulator. As it happened when I then put a wrench on the plug to unscrew it, the insulator snapped off which gave me a good look at the stub of the terminal that was left in the plug. Both the end of the part of the terminal that pulled out and the remainder in the plug were well rusted at the junction where they came apart and there's blue/green residue. Looks like the terminal corroded through a while ago.

I'm sure this happens, I've just never encountered it before. What would cause this? That terminal seems to be pretty well protected from moisture since it's buried in the ceramic and tightly covered by a rubber boot. I would think the high heat would chase away moisture too.

wrote in
:

> Encountered something new. Had an intermittent miss on my '99 Explorer
> so I changed the plugs that were in it when I got it. On one of the
> plugs when I pulled the boot the plug terminal came with it. Just
> pulled right out of the center of the ceramic insulator. As it
> happened when I then put a wrench on the plug to unscrew it, the
> insulator snapped off which gave me a good look at the stub of the
> terminal that was left in the plug. Both the end of the part of the
> terminal that pulled out and the remainder in the plug were well
> rusted at the junction where they came apart and there's blue/green
> residue. Looks like the terminal corroded through a while ago.
>
> I'm sure this happens, I've just never encountered it before. What
> would cause this? That terminal seems to be pretty well protected from
> moisture since it's buried in the ceramic and tightly covered by a
> rubber boot. I would think the high heat would chase away moisture
> too.
>

its not uncomon, any moisture that gets in there is trapped. mostly its the
gap that it was sparking because the terminal wasnt on all the way. KB