Papers by Author: Yo-ichi Takeda

Abstract: The effects of cold working and heat treatment on caustic stress corrosion cracking (SCC) of mill annealed (MA) alloy 800M in boiling solution of 50%NaOH+0.3%SiO2+0.3%Na2S2O3 were investigated by means of microstructure examination, tensile test, X-ray stress analysis, SCC testing of C-rings, Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and metallography. The microstructure of alloy 800M under tested conditions was austenite. With a strain of 25% by cold working, the grains of alloy 800M became longer, yield strength (YS) and ultimate tensile strength (UTS) increased, elongation (δ ) decreased, residual stress and the susceptibility to SCC increased. With increasing temperature of heat treatment of alloy 800M with cold working, the grains became bigger , residual stress, YS and UTS decreased and δ increased, the susceptibility to SCC of alloy 800M decreased. In boiling caustic solution, SCC cracks on the surfaces of C-ring specimens polarized potentiostatically at –20mV/SCE initiated from pitting and propagated along grain boundaries. AES analysis indicated that the surface films on MA alloy 800M were enriched in nickel and depleted in iron and chromium.

Abstract: A possible approach to describe the role of the environment in the phenomena behind crack initiation and crack propagation in stress corrosion cracking (SCC) is to assume that the transport of species through the oxide film on the material surface is one of the rate-controlling factors. The transport rates of ionic and electronic defects through the oxide film are, in addition to the environment, also affected by the stress and strain applied to the bulk material. In this paper, the surface oxide film formed on AISI 316L steel in slow strain rate tests (SSRT) in simulated BWR condition has been analyzed by using Electron Spectroscopy for Chemical Analysis (ESCA). The obtained film composition and structure have been combined with in-situ contact electric resistance (CER) measurements in order to evaluate the changes in oxide film electric properties during straining in the above environment. The results show that oxide film resistance of the strained part exhibits a maximum at around 2% of strain, which seems to correlate with a maximum in the Cr(III) concentration in the inner layer of the oxide. The implications of these results to SCC are discussed based on Mixed-Conduction Model (MCM).

Abstract: The electronic properties of the interfacial oxide film formed on 304L stainless steel in high temperature water are investigated by contact electric resistance (CER) measurements. Tests are performed in pure water with a wide range of dissolved oxygen (DO) content at 200, 250, and 288°C. The electrochemical potential (ECP) moves in the noble direction and CER increases when increasing DO. Results show that DO content has a dominant effect on the electronic properties of oxide film. The change of oxide film properties can also be attributed to the variation of the electrochemical potential, which is directly affected by DO content. Critical potentials exist for the formation and reduction of oxide films in high temperature water. Multiple steps are found for the reduction of oxide films due to de-aeration in 200, 250, and 288°C water, implying the presence of multiple-layer interfacial oxide films. The film reduction process is relatively slower than the film formation process. Present results show that even in high purity water, a moderate change of DO content can result in different surface conditions. Dissolved hydrogen has a moderate effect on interfacial surface films in deaerated water. In-situ monitoring of the oxide film properties by CER technique provides information on the interfacial reactions that are related to the SCC behavior of materials in high temperature water environments.