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 [S.E. Ziemniak, M. Hanson, Corros. Sci. 44 (2002) 2209] after 10,000 h. 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 spinel binaries—Fe(Fe 1− m Cr m ) 2O 4 and (Fe 1− n Zn n )Fe 2O 4—play a significant role in determining the composition and structure of the corrosion layer(s). Although compositions of the inner and outer corrosion oxide layers represent solvus phases in the Fe 3O 4–FeCr 2O 4 binary, zinc(II) ion incorporation into both phases leads to further phase separation in the outer (ferrite) layer. Recrystallization of the low zinc content ferrite solvus phase is seen to produce an extremely fine grain size (∼20 nm), which is comparable in size to grains in the inner layer and which is known to impart resistance to corrosion. Zinc(II) ion incorporation into the inner layer creates additional corrosion oxide film stabilization by further reducing the unit cell dimension via the substitution reaction 0.2 Zn 2 + ( aq ) + Fe ( Fe 0.35 Cr 0.65 ) 2 O 4 ( s ) ⇄ 0.2 Fe 2 + ( aq ) + ( Zn 0.2 Fe 0.8 ) ( Fe 0.35 Cr 0.65 ) 2 O 4 ( s ) The equilibrium constant for the substitution reaction is similar in magnitude to an estimate based on available free energies for FeCr 2O 4, ZnCr 2O 4, Fe 2+(aq) and Zn 2+(aq). This interpretation is consistent with the benefits of zinc treatment being concentration dependent.