Steel staircase - close the hole - is insulation necessary?

I want to fill the gap - see photo - with screed.

The staircase is made of steel. Do I need to insulate the steel here to prevent rust or something? Or can I just use screed directly?

Thank you very much

Our screed expert here is Maybe he knows how to properly fill something like that.

Interesting staircase construction ;-)

Our screed expert here is
Maybe he knows how to properly fill something like that.

Interesting stair construction ;-)

I have already closed it.
I wrapped the steel staircase at the very bottom with foil, and placed foil (the kind used for a foundation slab) underneath on the floor as well as on the edge in the gap. In theory, water can never come through there, but I put it in just to be safe. Should be fine.

- Thanks, I don’t know if that’s meant to be ironic :) - but the wood will still go on top.