In situ Raman spectroscopic investigation of chromium surfaces under hydrothermal conditions

Three chromium coupons were exposed to air-saturated water at pressures of ca. 25 MPa and temperatures up to 545°C in an optically accessible flow cell. In situ Raman spectra were collected at different temperatures as the coupons were heated and then cooled. Coupons were also characterized ex situ with Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, and X-ray diffraction. Coupons heated to either 374 or 505°C exhibited the same corrosion product, tentatively identified as α-CrOOH that originated as corrosion product released from the optical cell and/or flow system. A coupon heated to 545°C exhibited a variety of CrIII, mixed chromium valence CrIII/CrVI, and/or CrVI species during heating. During cooling, the same corrosion product was observed at all temperatures on this coupon and was tentatively identified as Cr2O3 with CrVI incorporated into the Cr2O3 CrIII–oxygen network. The difference in observed corrosion processes among the three coupons was attributed to a difference in water purity to which they were exposed.