Assuming you are talking about sodium sesquisulfate, which is an equimolar mixture of sodium sulfate and sodium bisulfate, 316L stainless steel would appear to be a good choice. For a simple mixture such as this, one can get an idea of its compatibility by looking up compatibility data for the individual components. Schweitzer's Corrosion Resistance Tables rates 316L stainless steel as excellent for sodium sulfate up to 204 °C and excellent for sodium bisulfate up to 71 °C. To confirm this, I suggest you talk to a manufacturer of sodium sesquisulfate. Manufacturers of specific chemicals are an excellent source of compatibility information since they deal with the chemicals they make on a routine basis.