In Situ Raman Spectroscopic Investigation of Stainless Steel Hydrothermal Corrosion

Abstract

In situ Raman spectroscopy was used to investigate corrosion of Type 304L stainless steel ([SS] UNS S30403) in air-saturated water at a pressure of 25.2 MPa and temperatures up to 496°C in an optically accessible flow cell. The exposed Type 304L SS coupon also was characterized ex situ with Raman spectroscopy and scanning electron microscopy. After exposure, nickel(II) iron(III) oxide (NiFe2O4) and alpha chromium(III) oxide hydroxide (α-CrOOH) phases were detected on the coupon surface. The NiFe2O4 phase was first identified after the coupon was heated to 247°C and was observed under all subsequent conditions. Upon heating from 247°C to 326°C, the crystallinity and possibly the thickness of the NiFe2O4 phase increased. The α-CrOOH phase was first identified after the coupon was heated to 326°C and was observed under all subsequent conditions. At least a portion of the α-CrOOH phase probably originated as a corrosion product released from the optical cell and/or flow system. In situ Raman spectra indicated that the NiFe2O4 phase was present in greater amounts and/or was more crystalline during cooling, while there was probably less of the α-CrOOH phase present in the Raman scattering volume during cooling than during heating.