Zinc Treatment Effects on Corrosion Behavior of 304 Stainless Steel in High Temperature, Hydrogenated Water

Abstract

Trace levels of soluble zinc(II) ions (30 ppb) maintained in mildly alkaline, hydrogenated water at 260 C were found to lower the corrosion rate of austenitic stainless steel (UNS S30400) by about a factor of five, relative to a non-zinc baseline test after 10,000 hr. Characterizations of the corrosion oxide layer via grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy in combination with argon ion milling and target factor analysis, confirmed the presence of two spinel oxide phases and minor amounts of recrystallized nickel. Based on the distribution of the three oxidized alloying constituents (Fe, Cr, Ni) with respect to depth and oxidation state, it was concluded that: (a) corrosion occurs in a non-selective manner, but approximately 30% of the oxidized iron is released to the water, and (b) the two spinel oxides exist as a ferrite-based outer layer (Ni{sub 0.1}Zn{sub 0.6}Fe{sub 0.3})(Fe{sub 0.95}Cr{sub 0.05}){sub 2}O{sub 4} on top of a chromite-based inner layer (Ni{sub 0.1}Zn{sub 0.2}Fe{sub 0.7})(Fe{sub 0.4}Cr{sub 0.6}){sub 2}O{sub 4}. These results suggest that immiscibility in the Fe{sub 3}O{sub 4}-ZnFe{sub 2}O{sub 4} binary may play a role in controlling the zinc content of the outer layer. On the other hand, the lower corrosion rate caused by zinc additions is believed to be a consequence of corrosion oxide film stabilization due to the substitution reaction equilibrium: z Zn{sup 2+}(aq) + FeCr{sub 2}O{sub 4}(s) {approx} z Fe{sup 2+}(aq) + (Zn{sub z}Fe{sub 1-z})Cr{sub 2}O{sub 4}(s). The liquid-solid distribution coefficient for the reaction, defined by the ratio of total zinc to iron ion concentrations in solution divided by the Zn(II)/Fe(II) ratio in the solid, z/(1-z), was found to be 0.184. This interpretation is consistent with the benefits of zinc treatment being concentration dependent.