Does molten potassium wet steel?

The previous commenters are correct--potassium is exceedingly soft and should not pose a significant mechanical hazard to steel. Molten alkali metals are often used as coolant in nuclear reactors, where they are contained within steel components without any mechanical problems. Chemical safety, of course, should be your major concern with this design, as potassium will react violently with oxygen or water.

As an aside, the eutectic NaK alloy (about 79% potassium, 21% sodium) is actually liquid at room temperature; so this may eliminate the solidification problem altogether, unless you're dealing with cryogenic temperatures.