Do you have the MTI MS book on sulfuric?
It is about the most definitive source.
Gray CI is not a very good option. It is susceptible to cracking (oxidation of C flakes).
A ductile iron is a bit better. But they both suffer from velocity related effects.
In a pump you are better off with a high Si CI (14.5% F47003).
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P.E. Metallurgy, Plymouth Tube