Welcome Brian!
I’m assuming your question is regarding the difference between cast iron and cast steel? There are many different types of cast iron, and many different types of cast steel. A lot of alloying elements are common to both. I would think (and I am guessing here) that the metal used in boat keels might be highly variable because whatever scrap is available and on hand at the foundry at the time (within certain limitations) might be used.
If it was me, and I was limited to a small rusty window in the middle of the filler, like you are, I would use an angle grinder with a sharp, coarse wheel for ferrous materials to remove some of the surface rust down to bare metal.
Watch the sparks while you’re grinding. Cast irons will have a duller, redder spark than cast steels, which will be brighter, more yellow, and longer. A high carbon content in either will give you bursting yellow sparks, like a hand-held sparkler on the 4th of July.
Then look closely at the surface of the exposed metal. Cast iron will be greyer, duller, with a coarser, more porous grain structure, and maybe “dusty” from the graphite present. Steel will be brighter, more silvery and shiny, and less porous.
If you have a cold chisel, (or use a sharpened flat blade screwdriver) try to scrape a bit, maybe a short shallow chip from the surface. Cast iron is brittle, so you will get crumbling chips, none long. Steel will hold together as a longer, more continuous chip.
Lastly, and most destructively to the surface, if you still aren’t sure, take a sharp ¼” or so drill bit and drill a shallow hole. Cast iron will produce crumbling dusty particles, steel will give you shiny stringy chips. By the way, when drilling or otherwise machining cast iron, it can be done dry, without lubricant because of the lubricity of the graphite present. Drilling steel, on the other hand, is best done with a lubricant/coolant, such as oil.
It will be helpful from the outset if you have a sample of each metal that is known, (a chunk of cast steel, a chunk of cast iron) so you have something to reference. I assume the type of primer you use for repair and finishing might depend on which of the two metals the keel is made from?
Hope this helps, regards – Brian.
