Effect of Water Chemistry on 304 SS Oxide in high Temperature Water

The effect of water chemistry on the 304 stainless steel (SS) oxide formed in high temperature water containing oxygen, hydrogen or noble metals was examined by the transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS).  The electrochemical corrosion potential (ECP) of 304 SS electrode was also measured to confirm the presence of noble metal on the oxide surface.             Under oxidizing water chemistry condition, the majority of outer oxide particles contain primarily Fe with very small amounts of Cr and Ni.  Most particles have the a-Fe₂O₃ crystal structure.   A thin inner oxide layer next to the metal has the spinel crystal structure corresponding to Fe₃O₄.  In hydrogen water, the majority of outer oxide particles have the spinel (or highly substituted Fe₃O₄ and ZnFe₂O₄) crystal structure and the thin inner oxide layer at the metal interface has the Cr-enriched spinel (FeCr₂O₄) crystal structure.  This is markedly different from the previous specimen in oxygen water.  The thin inner oxide layer has a much higher Cr concentration.  It was further observed that under excess hydrogen water chemistry condition with noble metals, the presence of noble metals on the oxide surface promotes the transformation of a-Fe₂O₃ to a spinel Fe₃O₄ type, a Zn-enriched outer oxide particle and a Cr-enriched inner oxide layer and, thereby, increases the iron concentration in water.