Actually Chief, it was just an idea conjured up from my general knowledge, so I don't have first hand experience in a commercial
application. But a quick google
search produced this from wikipedia:
Ultrasonic cleaning - Wikipedia, the free encyclopedia
And, it turns out this methodology is used for many things, such as sterilization and cleaning of dental instruments, castings from foundaries, etc. Here is another link from an engineering forum, discussing cleaning of metal castings:
Manufacturing Plant engineering - About cleaning the casting parts with ultra-sound wave?
It seems you can get a metal probe that can be inserted into a tank, to provide the ultrasonic vibration. And, it appears there are water-based surfactants that could be used in lieu of solvent.
I guess when I am faced with cleaning a closed tank (haven't gotten to my old aluminum fuel tank
yet in my refit), I will pursue it further along to see if it is practical. In principle, it should work however.
Any other thoughts on whether this might work?