Zinc treatment effects on corrosion behavior of Alloy 600 in high temperature, hydrogenated water

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 [S.E. Ziemniak, M. Hanson, Corros. Sci., in press, doi:10.1016/j.corsci.2005.01.006]. 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 μm−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 versus 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: (Zn0.55Ni0.3Fe0.15)(Fe0.25Cr0.75)2O4. 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 for0.55Zn2+(aq)+(Ni0.7Fe0.3)(Fe0.3Cr0.7)2O4(s)⇄0.40Ni2+(aq)+0.15Fe2+(aq)+(Zn0.55Ni0.3Fe0.15)(Fe0.3Cr0.7)2O4(s)0.55Zn2+(aq)+(Ni0.7Fe0.3)(Fe0.3Cr0.7)2O4(s)⇄0.40Ni2+(aq)+0.15Fe2+(aq)+(Zn0.55Ni0.3Fe0.15)(Fe0.3Cr0.7)2O4(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%).