The best answer I can give comes straight out of the book "Materials Selector for Hazardous Chemicals: Sulfuric Acid," 2nd edition, published by MTI (Publication MS-1) in 2005. "In the production of aluminum sulfate from bauxite ores, the boiling dilute sulfuric acid is corrosive to most common alloys other than lead if the ore contains substantial amounts of iron. Alloy 400 (N04400) will resist low-iron ores during the digestion process. When alum solutions (e.g., 15–25%) of pH 2–3 are prepared, type 316L (S31603) is preferred for dissolving tanks, because of the presence of ferric ions, which render alloy 400 (N04400) and related alloys susceptible to attack. Higher nickel-rich, chromium-bearing alloys (e.g., CN7M, N08007) are used for pumps and valves. The wrought materials, alloy 20Cb-3 (N08020) or alloy 825 (N08825), are also used where type 316L (S31603) is borderline. In some cases, the high-chromium alloy 28 (N08028) has proven superior to type 316L (S31603) when oxidizing contaminants were present in smaller concentrations than usual."