Sulfuric acid is a tricky substance to deal with as its corrosive properties change dramatically with velocity, concentration, impurities, and temperature. Inconel 625 is one of the materials of construction listed for use with sulfuric acid in the NACE standard SP0294 "Design, Fabrication, and Inspection of Storage Tank Systems for Concentrated Fresh and Process Sulfuric Acid and Oleum at Ambient Temperatures".

According to this standard, corrosion-resistant alloy selection should be based on corrosion resistance to normal process conditions, possible excursion conditions, maximum temperature, risk tolerance, and economic considerations. Another useful standard for help in selecting sulfuric acid materials of construction is NACE RP0391 "Materials for the Handling and Storage of Commercial Concentrated (90 to 100%) Sulfuric Acid at Ambient Temperatures". I strongly suggest you consider obtaining copies of both of these standards before making a final decision on a material of construction and arrive at a final design for valves and flanges.