316L/304L stainless steel should work fine for sodium sulfate. The issue may be the "5% other chemicals". The overall suitability of any material of construction depends on all components that are present and not just the major one. It sounds like you are mixing dry materials and the humidity in the air is causing your issues. If this is true, a sacrificial anode would not work since your mixture is not likely to be conductive and the corrosion you are seeing is probably occurring in many small localized spots. Impressed current systems are tricky to design properly and a poor design can actually make corrosion worse. Your best bet is probably a different material of construction than you are currently using.