Oxidation kinetics of recently developed ferritic heat-resistant steels, HCM12A, NF616, and HCM2S, were investigated in a superheated steam to evaluate the effects of chemical composition of the steels, testing temperature (560–700°C), steam pressure (1–10 MPa), and degrees of microstructural evolution by aging on oxidation. The contribution of alloyed Cr to oxidation resistance was pronounced above 600°C, while no material dependency was found at 600°C or lower. The apparent activation energy of the oxidation rate clearly changed at around 600°C for NF616 and HCM12A. In contrast, HCM2S showed single activation energy over the range of temperatures. Although temperature and chemical composition were the major factors, steam pressure also showed a clear negative effect on the oxidation rate in the lower temperature range, 570–600°C.

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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.

Abstract-The atomic arrangement and segregation behavior of Ti, Cr, and Y in Σ3 (111) grain boundary of vanadium was performed to illustrate by first-principle calculation. The analyses on the binding energies and geometric positions show that Cr atom is more stable than Ti or Y atom both in the bulk and grain boundary, and Y atom would push away the surrounding V atoms. The computations of segregation energies show that Cr/Y atom segregates at the grain boundary, while Ti atom prefers to stay in bulk V rather than in grain boundary. Furthermore, it is found that Ti/Y atom has a strong driving force to surface segregation. The grain boundary energies calculations and the work of separation and the electron properties show that the segregated Cr can strengthen the grain boundary, whereas segregated Y has the strong tendency to decrease crystallite size.

Water droplets formed by the deliquescence of pre-deposited NaCl on 316L stainless steel were investigated. Different total weights of NaCl particles between 0.0005 g and 0.01 g were deposited on 316L stainless steel surface, which were exposed to an atmospheric condition at 80oC and relative humidity of 80% until NaCl droplets were stabilized. The volume of NaCl droplet was linearly proportional to the total weights of pre-deposited NaCl and consequently the chloride concentration in droplets, ranging from 3 to 6 M, did not depend on the NaCl weights. After exposed to the NaCl droplets at 80oC and relative humidity of 80% over 5, 10, and 15 days, all 316L stainless steel samples suffered from pitting. The Cl- ions in the surface films of samples were measured by ToF SIMS. The pit densities on samples were well correlated with the Cl- intensities in the surface films. These results suggest that the pit initiation under NaCl droplets by deliquescence is caused by the Cl- ions either incorporated into surface films or penetrated through them.