In situ Raman spectroscopic investigation of aqueous iron corrosion at elevated temperatures and pressures

In situ Raman spectroscopy was employed to investigate iron corrosion in ai r saturated water at a pressure of 25.1 MPa and temperatures from 21 to 537 degrees C. Upon heating, various combinations of Fe3O4, alpha-Fe2O3, gamma -FeOOH, and gamma-Fe2O3 were observed depending on the location on or tempe rature of the iron coupon. In some cases, different species were observed a t the same temperature at different locations on the surface. This was attr ibuted to oxygen concentration gradients in the solution caused by recircul ation zones in the cell. The surface of the corrosion coupon changed little after it was heated to 537 degrees C. This was attributed to the formation of a relatively thick, protective oxide scale after exposure to supercriti cal water. The ex situ Raman spectra were very similar to the irt situ spec tra obtained during cooling but different from those obtained during heatin g. This indicates that the corrosion layer present during cooling is simila r to that observed ex situ but different from that observed during heating. Ex :situ characterization of the coupon identified a two layered structure : an inner corrosion layer consisting of Fe3O4 and alpha-Fe2O3 and an outer layer consisting of gamma-Fe2O3 and alpha-Fe2O3 (C) 2000 The Electrochemic al Society. S0013-4651(99)11-105-4. All rights reserved.