In Situ Raman Spectroscopic Investigation of Nickel Hydrothermal Corrosion

A nickel coupon was exposed to air-saturated water at a pressure of 25.4 MPa and temperatures ranging from 21°C to 460°C in an optically accessible flow cell. In situ Raman spectra were collected at a number of temperatures as the coupon was heated and then cooled. The coupon also was characterized ex situ with Raman spectroscopy, scanning electron microscopy, energy dispersive x-ray spectrometry, and x-ray diffraction. Corrosion species were observed in situ at temperatures >249°C during heating and under all conditions during cooling. The species were identified as nickel monoxide (NiO) and alpha chromium (III) oxide hydroxide (α-CrOOH). The α-CrOOH originated as a corrosion product released from the optical cell and/or flow system. The NiO Raman features were more intense during cooling than at the corresponding temperatures during heating, indicating that NiO was present in greater amounts and/or was more crystalline during cooling.