Effect of Cr Content on Supercritical Water Corrosion of High Cr Alloys

Abstract

High Cr alloys were corrosion tested in supercritical water and the oxide scale was analyzed. Commercial grade two steel specimens; 9CrMoVNb steels, one 9CrMoVNbW steel, one 12Cr-MoVNbWCu steel and one 20Cr Fe-based O.D.S (Oxide Dispersion Strengthened) alloy specimen were investigated. Corrosion tests were conducted within non-deaerated pure supercritical water at 627, 550, and 500oC with 25 MPa. Corrosion rate was estimated by the weight change per unit surface area and the oxide layer was analyzed using a grazing incidence X.R.D (x-ray diffractometer), S.E.M (scanning electron microscope) and T.E.M (transmission electron microscope) equipped with an E.D.S (energy dispersive spectroscope). Corrosion rates of the 9Cr steel specimens were observed to follow the parabolic growth rate law, while those of the specimens with a 12 per cent or higher Cr content showed significantly lower rates. Oxide scale on the 9Cr steel specimen after a corrosion test in a supercritical water was found to consist of three distinctive layers. Through the cross-section T.E.M the outermost layer with about a 35 µm thickness after 200 hr at 627 oC was identified to be magnetite type Fe3O4, and about 25 µm thick intermediate layer was a Cr partitioned magnetite type (Fe,Cr)3O4. The outermost layer showed a coarse columnar structure, while the intermediate one revealed an agglomerate of tiny oxide particles (several tens nm in diameter). The innermost layer next to the matrix phase was found to be the internally oxidized zone. Oxygen atoms seemed to have attacked along the grain boundaries and the lath boundaries and formed oxide CrO3 along the boundaries. Also a Cr depleted zone, and consequently a carbide-free zone, was observed along the interface between the internal oxidation zone and the matrix phase.