Effect of Phosphate Addition to Electrolytes on Corrosion Behavior of Stainless Steels in Seawater Electrolysis

Water electrolysis is an effective method for producing hydrogen, which is a valuable alternative to fossil fuels. However, the presence of chloride ions can cause significant structural degradation, and electrolysis systems compatible with seawater must be developed. In this study, phosphate was added to an electrolyte simulating seawater electrolysis, and the effects on the corrosion resistance of ferritic (Type 430) and austenitic (Type 304 and 316) stainless steels were investigated using electrochemical techniques. The phosphate-induced changes in the passive films on the steels were examined using X-ray photoelectron spectroscopy (XPS). The phosphate in the electrolyte enhanced the pitting corrosion resistance of all the stainless steels. However, excessive phosphate concentrations promoted the partial dissolution of the passive film, particularly for the 430 steel. XPS analysis showed that phosphorus was incorporated into the passive film as phosphate for every type of steel in this study, which likely enhanced the pitting corrosion resistance. Cyclic polarization measurements of the 430 steel indicated that the pH-buffering action of the phosphate in the electrolyte suppressed pitting propagation. These findings provide fundamental insights into the role of phosphate additives in stabilizing stainless steels, which may contribute to the improved safety of seawater electrolysis systems.