It is a great honor for me to write this paper commemorating receipt of the Award of Japan Association of Corrosion Engineering for 2000. Several corrosion resistant metals and alloys have been developed in Corporate Research and Development Laboratories of Surnitomo Metal Industries Limited, and then Venture Business Laboratory, Hiroshima University since I began to work as researcher in 1962: sulfur dew point corrosion resistant low alloy for oil-fired boilers, seawater resistant duplex stainless steel, stress corrosion cracking resistant 316 stainless steel for boiling water reactors (BWR), nickel base thermally treated alloys 600 and 690 for pressurized water reactors (PWR), 310Nb stainless steel, and zirconium for reprocessing plants dealing with spent nuclear fuels. It was also identified that α-(Fe1-x, Crx) OOH was the main constituent of protective rust layers formed on the weathering steel exposed to atmosphere for a quarter century. These alloys were developed in the sequence, firstly clarification of corrosion mechanism for alloys, secondly development of an accelerated corrosion testing method, thirdly alloy designing, and finally field testing for evaluation of the alloys in practical corrosive conditions. Measurements of corrosion potential were very useful in the above mentioned procedures. The author appreciate heartily to the persons concerned with the developments of these alloys which have been successfully used in various industries up to now since each of them was put into the first market use.
Steam oxidation of heat-resistant steels is expected to be one of the major concerns for degradation of ultra supercritical (USC) boilers for an advanced power generation plant. This paper focuses on steam oxidation kinetics of newly developed heat resistant ferritic steels, which contain either 2.2%, 9%, or 11% Cr and about 2%W (HCM 2S, NF 616, HCM 12A). Oxidation rates of those steels were measured in a pressurized superheated steam as functions of temperature and pressure. Oxidation rate always showed parabolic nature. Effects of chemical composition and pressure showed interesting contrast depending on testing temperature ranges. In low temperature ranges, 570°C-600°C, clear negative effects of steam pressure and no compositional effect were found on the oxidation rate. In contrast, steam pressure effect was minor and chemical composition (Cr content) became dominant in higher temperature range, 620°C-700°C. Structure and chemical composition of oxide scales were analyzed using an analytical electron microscope and X-ray diffraction technique. Although oxide scales were always formed in a duplex layered structure for all the steels, oxide type, structure, and chemical composition of the oxide scales varied depending on material and testing condition. Formation of a Cr-enriched thin layer in the inner scales and oriented growth of the inner scale oxides were correlated with the oxidation kinetics.
Environmentally assisted crack initiation behavior is greatly affected by applied stress and environmental factors, such as water temperature, contained impurities and so on. On the other hand, crack initiation behavior also influences crack propagation. A typical example of this influence can be observed as the interference effects of multiple cracks, such as the coalescence of approaching crack tips or the arrest phenomena in the relaxation zone of an adjacent crack. To understand these effects of crack initiation on crack propagation behavior is very important to predict the lifetime of components, in which quite a few cracks tend to occur. This study aimed at revealing the crack initiation behavior and the influence of this behavior on propagation. At first, to evaluate the effect of applied stress on crack initiation behavior, sensitized stainless steel was subjected to a four-point bending test in a high temperature water environment at the constant potentials of ECP +50mV and ECP +150mV. Secondly, a crack initiation and growth simulation model was developed, in which the interference effect of multiple cracks is evaluated by the finite element method, based on the experimental results. Using this model, the relationship between crack initiation and propagation was studied. From the model, it was revealed that the increasing number of the cracks accelerates crack propagation and reduces life.
Ductile cast iron is used in various fields including piping materials. Recently, improved corrosion resistance of the material is desired for usage in more corrosive environments. In order to fabricate economical and corrosion-resistant material, a molten ductile cast iron was poured into a mold, which was pre-painted with polymer containing Ni and stainless steel powder mixed with lower-melting Ni base alloy powder. Liquid sintering and inter-diffusion between ductile cast iron and alloy powder due to the molten heat in casting resulted in formation of anti-corroded layer. The obtained coated layer showed good corrosion resistance in an aqueous salt spray test for 75 days. In addition, a numerical simulation of the solidification processes was successfully applied to estimate an optimum forming condition of coated layer.