Zinc treatment effects on corrosion behavior of Alloy 600 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 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: (Zn 0.55Ni 0.3Fe 0.15)(Fe 0.25Cr 0.75) 2O 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 2 + ( aq ) + ( Ni 0.7 Fe 0.3 ) ( Fe 0.3 Cr 0.7 ) 2 O 4 ( s ) ⇄ 0.40 Ni 2 + ( aq ) + 0.15 Fe 2 + ( aq ) + ( Zn 0.55 Ni 0.3 Fe 0.15 ) ( Fe 0.3 Cr 0.7 ) 2 O 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%).