I’ve had a few things get pretty warm including when I used diesel as a solvent on a gummed up drive chain and when I sat some carburettor components in a jar of carby cleaner.
In your case I would speculate that it may be a combination of both a chemical reaction occurring and the ultrasonic.
The POR15 kits are expensive but they are bombproof and you can get a lot of uses out of a single kit. I did a couple of tractor tanks with one kit. There’s also another brand, cheaper, not as good, but probably fine for a mower. I can’t remember the name but it used to be stocked by a couple of auto parts shops.
I’ll have a look inside one of my imperial tanks and post back about the “pipe”.
