of white crusts on the part that has been in the ground. They don't
need to be disinfected, since they won't be housing any plants. So
just scrubbing might do it?

PS: have you considered coating them with paraffin (the wax, not British for
gasoline) once they're clean. If they are not going to house plants, then if
you stop the wicking, you stop the crusts growing. Or you could raise them
on glazed pot feet or the like.