Ferritic/Martensitic Steels and High Nickel Alloys in Supercritical Water and their Corrosion Products

J. Jang (Sp), C.H. Han, B.H. Lee, S.S. Hwang, Y.S. Yi, Korea Atomic Energy Research Institute, Daejeon (Korea, Republic) 
Three ferritic/martensitic (F/M) steels (9Cr1MoVNb, 9Cr0.5Mo2WVNb, 12Cr0.5Mo2WCuVNb) and three high nickel alloys (alloy 625, 690, 800H) were exposured in supercritical water with deaerated and non-deaerated conditions for up to 500 hours. The corrosion behavior was estimated by weight change measurement and the corrosion products were analyzed using a glancing angle X.R.D (X-ray Diffractometer). Cross sections of the tested samples were observed with S.E.M (Scanning Electron Microscope) and T.E.M (Transmission Electron Microscope), and the oxide layers were identified by S.A.D (Selected Area Diffraction) and E.D.S (Energy Dispersive Spectroscopy).
In general corrosion rate of F/M steels revealed to depend significantly on Cr content, but the extent was reduced at lower temperatures and the reduced D.O (Dissolved Oxygen). Three distinctive corrosion layers were observed on F/M steel samples. The outermost one was identified to be magnetite type Fe3O4 by T.E.M as well as by the glancing angle X.R.D method, and the inner layer was a magnetite type M3O4 partitioned with Cr atoms. At the innermost, or next to the matrix, an internal oxidation zone was observed along the prior austenite and the lath boundaries. In the matrix adjacent to the internal oxidation zone Cr atoms were depleted significantly and a carbide-free zone was observable. In contrast to F/M steels a dense chromia (Cr2O3) phase was formed on high nickel alloy samples with much thinner dimensions. On 30% Cr high nickel alloy samples (alloy 690) the oxide layer was found to compose with mostly Cr2O3, but on 20% Cr high nickel alloys (alloy 625 and 800H) the oxide layer was found to be a mixture of Cr2O3 and (Fe,Cr)3O4.

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