hot water tank corrosion
Yep, that's electrolysis alright. Edwardt is right, you need a Dieletric Union to stop it &amp; it should've been standard practice by whomever chose to do garbage work &amp; guarantee a new water heater very prematurely.

That T &amp; P valve on top might be shot too, which shouldn't be there anyway &amp; was only used as an elbow. If it's from the same guy or firm that did the insulation, then find a new guy or firm...they're absolutely horrible &amp; not even up to hack level work.