Trace levels of soluble zinc(II) ions (30 ppb) maintained in mildly alkaline, hydrogenated water at 260 C were found to reduce the corrosion rate of Alloy 600 (UNS N06600) by about 40% relative to a non-zinc baseline test. Characterizations of the corrosion oxide layer via SEM/TEM and grazing incidence X-ray diffraction confirmed the presence of a chromite-rich oxide phase and recrystallized nickel. The oxide crystals had an approximate surface density of 3500 (micro)m(sup -2) and an average size of 11 (+-) 5 nm. Application of X-ray photoelectron spectroscopy with argon ion milling, followed by target factor analyses, permitted speciated composition vs. depth profiles to be obtained. Numerical integration of the profiles revealed that: (1) alloy oxidation occurred non-selectively and (2) zinc(II) ions were incorporated into the chromite-rich spinel: (Zn(sub 0.55)Ni(sub 0.3)Fe(sub 0.15))(Fe(sub 0.25)Cr(sub 0.75))(sub 2)O(sub 4). Spinel stoichiometry places the trivalent ion composition in the single phase oxide region, consistent with the absence of the usual outer, ferrite-rich solvus layer. By comparison with compositions of the chromite-rich spinel obtained in the non-zinc baseline test, it is hypothesized that zinc(II) ion incorporation was controlled by the equilibrium for 0.55 Zn(sup 2+)(aq) + (Ni(sub 0.7)Fe(sub 0.3))(Fe(sub 0.3)Cr(sub 0.7))(sub 2)O(sub 4)(s) (r-equilibrium) 0.40 Ni(sup 2+)(aq) + 0.15 Fe(sup 2+)(aq) + (Zn(sub 0.55)Ni(sub 0.3)Fe(sub 0.15))(Fe(sub 0.3)Cr(sub 0.7))(sub 2)O(sub 4)(s). It is estimated that only 8% of the Ni(II) ions generated during non-selective oxidation of the alloy were retained as Ni(II) in the corrosion layer; the remainder either recrystallized to Ni(0) (38%) or were released to the aqueous phase (54%).
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