I have ra. across the top of 1 foot is a very prominent ridge of bone which my doc says is where cartilage has eroded. will my foot fuse naturally?

No: The problem is that it may partially fuse, which makes attempted motion even more painful.

Probable not: You will probable progress to a partial fusion. I have seen RA patients that have a complete fusion.Check with your dr.

Possible: But not necessarily. Would have to know specifically what bone, see x-rays etc to give thorough answer. If there is motion at that area the motion can negate the fusion.