Corrosion behavior of 9Cr F/M steals in supercritical water

A commercial grade W modified 9Cr steel specimen (T92) as well as two heats of 9Cr steel (T91) specimens were corrosion tested in non-deaerated and deaerated supercritical water under 25 MPa. Tests were done within the temperature range between 350°C and 627°C for up to 500 hr. After the corrosion tests the weight changes per unit area were estimated. The corrosion product was analyzed using X.R.D (X-ray Diffractometer), S.E.M (Scanning Electron Microscope) and T.E.M (Transmission Electron Microscope) equipped with E.D.S (Energy Dispersive Spectroscope). W modified 9Cr steel (T92) showed a similar corrosion resistance with the T91 steel specimens at 550°C and above. From the S.E.M micrographs the corrosion product layer was explicitly revealed to consist of three layers. Using X.R.D and T.E.M it was found that the outermost layer is a magnetite type Fe₃O₄ with a coarse columnar structure and it reaches a thickness of about 30 μm after an exposure in non-deaerated supercritical water at 627°C for 200 hr. The inner layer was revealed to be an agglomerate affine (about 10 nm in diameter) magnetite type (Fe,Cr)₃O₄ particles. An internally oxidized zone with about a 10 μm thickness was observed next to the inner oxide layer. The internal oxidation that preferentially attacked the prior austenite grain boundaries and the lath boundaries may be due to the faster boundary diffusion of the oxygen atoms than the volume diffusion. On the as-polished cross section of the specimens that had been corrosion tested in deaerated supercritical water a thin (a few hundreds nm) Mo-rich layer was also observed between the outermost and the inner oxide layers.