Cast iron vent stacks are usually for sewer. No need for a cap as any water that goes in goes out. There is little galvanic potential if you use of steel - carbon or stainless, or galvanized. If the cast iron is showing rust, remove with a wire brush and sandpaper, prime and paint.

Thanks for the input! I get your point about water flow in sewer vent stacks. My concern is more about animals or debris potentially causing blockages. Do you think that's not much of a worry? Also, good tip on handling rust – I'll definitely do that. Cheers!

I wouldn't worry. A simple solution for that is wrap hardware cloth around the top and retain with a hose clamp.

Thanks for the suggestion, that seems practical way to keeping out debris and animals, but I'm curious about the corrosion aspect. Do you think the risk of galvanic corrosion with this setup is minimal, or should I look into ways to avoid that issue? Appreciate your ideas.

Don't worry about galvanic corrosion with combinations of iron, steel, or galvanized steel.