The corrosion behavior of an austenitic stainless steel (UNS S30400) has been characterized in a 10,000 hour test conducted in hydrogenated, ammoniated water at 260°C. The corrosion kinetics were observed to follow a parabolic rate dependency, the parabolic rate constant being determined by chemical descaling to be 1.16 mg dm-* h r -l h . X-ray photoelectron spectroscopy, in combination w i t h argon ion milling and target factor analysis, was appIied to provide an independent estimate of the rate constant that agreed with the gravimetric result. Based on the distribution of the three oxidized alloying constituents (Fe, Cr, Ni) with respect to depth and elemental state, it was found that: (a) corrosion occurs in a non-selective manner, and (b) the corrosion film consists of two spinel oxide layers -a ferrite-based outer layer ~io.zFeo.s)(Feo.ssCro.o5)204 on top of a chromite-based inner layer (Nio.zFeo.p)(Cro.7Fe~.~)~O4. These compositions agree closely with the solvi phases created by immiscibility in the Fe304-FeCr204 binary, implying that immiscibility plays an important role in the phase separation process.KEY WORDS : austenitic stainless steel, aqueous corrosion, hydrothermal reactions, X-ray photoelectron spectroscopy, high temperature water, Auger electron spectroscopy, type 304 stainless steel, corrosion oxide analysis, Argon ion sputter rate, spine1 oxides, target factor analysis.

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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 [3] 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, revealed that miscibility gaps in two