Corroded spark plug

[fjc2]

I have a 2014 Explorer Sport with a little over 29000 miles on it. I'm preparing to put a tune on it, so decided to swap the factory installed plugs with new ones that I carefully gapped.

In removing the plugs, the middle rear one came out looking quite corroded, while all the rest looked to be in great shape. Here's a picture:

What would cause this? Bad grommet on the top of the coil pack? I inspected and cleaned them all and didn't notice any tearing or such. Also didn't notice any water coming out when I shot some compressed air into that well prior to removing the plug.

Thoughts?

 

[ptf18]

Have you (or previous owner if....) let the vehicle sit for any left of time? If so, perhaps water pooled around the plug (driving thru rain??) and the engine never got hot enough to flash off the water.

I've seen this before.

 

[fjc2]

I'm the original owner. Explorer is garaged every night, but yes I drive through plenty of rain, slush, snow in an area that uses a lot of road salt (Michigan).