I have access to metal cones (pontoons) that once floated a barge. They were sealed and filled with styrofoam when used under the barge. I am wondering about using them with no foam as I feel that would add weight and wouldn't be necessary. I know that plastic drums are used successfully, but I have these metal ones available to me free. They are ready to weld to a dock frame. If I completely seal them with nothing more than just the trapped air in them, will they float and be able to hold up a dock? I could sure use some help with this.

They would float the dock if sealed with nothing but air inside.

However, the foam would ensure the dock continued to float if/when the pontoons get holes (rust/ice damage/drive a nail through from the top/etc.). We had a discussion of foam-filling containers used to float docks and the biggest drawback seemed to be the cost of the foam - if it was already in place I'd leave it there and count myself ahead. And not worry about my welds being hermetically perfect.

Agree with Theo.