I took a quick look at Schweitzer's Corrosion Resistance Tables (5th edition) and type 304 stainless steel is not recommended for a 10% ammonium persulfate solution. The same source rates 316 stainless steel as "Good" (< 20 mpy) for 10% ammonium persulfate up to 380 °F. If you cannot tolerate this level of corrosion, your idea of moving to a plastic is a good one. Schweitzer rates many plastics as resistant even at elevated temperatures. For example, polypropylene is resistant up to 220 °F. Deionized water and sodium bicarbonate are not a concern with any of these materials.