Before you get too carried away with the need to replace your
mast step, put a little carbide ball, or other grinding or
sanding bit, into your cordless drill, and polish through the surface
corrosion, just far enough to see if the aluminum is still shiny underneath.
Aluminum that has been compromised looks dull on the inside. I have heard it described as 'cottage cheese'
Polish up the mast step in several places, or put a nice shine and wax on the whole thing with little buffer wheels on a dremel tool. If it takes a nice shine, be happy.
If the aluminum will take a shine, clean the whole thing with white vinegar, rinse with baking soda in
water, hose the whole thing down with fresh
water, and go sailing.
If it stays dull under the surface, go to plan B: Make an overbuilt mast step out of carbon fiber, or even glass cloth in
epoxy resin. You can build up a whole inch of glass for the step, and eliminate the conductivity of carbon. No need for a lightweight structure in this application.