On the viewpoint of practical service condition for various environmental and energy machineries such as gas turbine, jet engine and boiler, a significance of considering the complicated failure factors of the hot-section components was demonstrated in order to assure the durability and reliability of them. In particular, some examples were presented about the aggressive environmental effect on the creep and/or fatigue strength properties associated with the characteristic natures of failure manners on the basis of failure analysis. Material system design concept including the coating system for high-temperature components were also proposed together with the microstructural control toward to further progress in high-efficiency of advanced machineries, according to the concept of the learning a lesson from the past and failure.
A port structure is no longer thought to be everlasting but is maintainable with an increasing number of options such as reinforcement and corrosion protection. In Chapter 2, the PW method was applied to the LCM of the port structures that have deteriorated. The construction, repair, shutdown and demolition costs were used for calculating the PW in the Kamsar port. In Chapter 3, how to calculate the shutdown cost both from the exported ore cost and the demurrage was shown in detail, and the effect of the maintenance cost/total cost ratio to the PW value was investigated. Because the benefits to the citizenry at planning phase are difficult to estimate with much accuracy, the B/C ratio method is applied to a public sector project instead of the PW method. However, if all the cash flows at construction phase which consist of construction, maintenance and demolition are quantified, the PW method can be applied to the LCM of both private and public sector projects. In Chapter 4, an example of the B/C ratio method’s application and the applicability of the PW method to the LCM of port structures were shown.
This study focused on a breakdown of passive film which is followed by rust staining, and the objective of this study was to clarify the effect of stability of passive film on the resistance of rust staining of stainless steels. Atomospheric exposure test was carried out for 12 months. In order to compare the stability of passive film, measurements of potential-decay curves, and potentiostatic polarization tests were performed in acidic aqueous chloride solution. As a result, rust area of austenitic stainless steel was higher than that of ferritic stainless steel. This order didn't follow the orders of pitting potentials and densities of inclusions on surface between specimens. On the contrary, the order of the resistance of rust staining of stainless steels followed the order of the stability of passive film. One of the reasons why the resistance of rust staining of austenitic stainless steel was worse than that of ferritic stainless steel was seemed that chloride more easily broke passive film on the surface of austenitic stainless and formed micro pits which become initiations of rust staining and increase density of stains.
To evaluate the corrosion risk of offshore steel structures at submerged zone caused by stainless steel sheathing and cathodic protection failures, it is essential to grasp the degree of the carbon steel-stainless steel galvanic corrosion. The objective in this study was to clarify the effect of area ratio of stainless steel coupled to carbon steel on corrosion behavior in sea water. As a result, stainless steels promote galvanic corrosion, but the degree of accelerated corrosion was much smaller than that estimated from the area ratio. Calcareous deposits had formed on the surface of stainless steels during galvanic tests in seawater. The calcareous deposits appeared to diminish the diffusion of the dissolved oxygen and diminish the cathodic reaction of oxygen reduction on the stainless steel surface.
This study was carried out to examine the effect of silver surface temperature on sulfide formation behavior in sulfur vapor at constant temperature. QCM was used to measure mass change by silver sulfide formation. In a closed glass container, flowers of sulfur, S8 were placed as a sulfur source and QCM prepared by vacuum deposition of silver was exposed. The temperature of the testing environment, T1 was controlled by an oven and the silver surface temperature, T2 was varied by an aluminum foil heater. The saturated sulfur vapor pressure p was calculated from T1 using Antoine’s equation. The results showed that in the case of T1=T2, the corrosion rate increased linearly with increasing p. In the case of T1＜T2, at p=8.3 and 24 mPa, the corrosion rate increased with T2, while at p=2.6 mPa, the corrosion rate remained almost constant with T2. This result indicates that in this environment, natural convection generated by heating the silver surface caused the corrosion rate increased by facilitating the supply of sulfur molecules to the silver surface. The degree of natural convection (T1－T2)/T1 and p determine the corrosion rate increase.
The objective in this study was to clarify the change of pH with time inside crevice in potentiostatic crevice corrosion test．Sensing plate to discoloration by the change of pH was made. The crevice corrosion of a duplex stainless steel in potentiostatic crevice corrosion test using a sensing plate was in situ observation. The change of pH with time inside crevice was measured by discoloration of the coating agent of the sensing plate. Crevice corrosion of duplex stainless steel occurred at the point where pH is lowered locally. And, it was found that pH of crevice corrosion tip was below the depassivation pH of both phases.
Electrochemical measurements of SUS316L and BNi‐5 were carried out to investigate the corrosion resistance of the portion brazed with nickel brazing filler metals in the heat exchanger made of austenitic stainless steel. Corrosion potentials and galvanic currents of SUS316L and BNi‐5 were measured in 0.06 mol/L sodium chloride and sodium sulfate solution at room temperature and at 80℃. The corrosion potential of BNi‐5 in NaCl solution at 80℃ is lower than that in NaCl solution at room temperature, while the corrosion potential of SUS316L in NaCl solution at 80℃ is almost the same as that in NaCl solution at room temperature. The corrosion potential of SUS316L was nobler than that of BNi‐5 in NaCl solution. The potential difference between SUS316L and BNi‐5 in NaCl solution at 80℃ was up to about 0.4V. The galvanic current between SUS316L and BNi‐5 in NaCl solution at room temperature was almost 0 μA/cm2, while the galvanic current at 80℃ was up to about 0.2μA/cm2. We found that the galvanic corrosion on couple between BNi‐5 and SUS316L may be induced in the solution containing chloride ion at high temperature.
Diffusion coefficients of D2O in the films on Fe oxidized at high temperature in air have been determined in order to estimate corrosion rate of carbon steel for the overpack in ground water with quite less dissolved oxygen. Fe plate specimens were heated at 573, 723 and 873 K in air to form oxide films on them. The films on the specimens were characterized with X-ray diffraction method and scanning electron microscopy. After that, the specimen was contacted with D2O for several periods to 5184 ks, followed by being subjected to thermal desorption gas spectroscopy to obtain an amount of D2O absorbing into the film. As a result, single-layered film of Fe3O4 was formed at 573 and 723 K, and double-layered film of Fe2O3 and Fe3O4 was formed at 873 K. It was found from the D2O absorption test that an amount of D2O in the film correlated linearly with a square root of the absorption period, and that the amount was steady for a long period. From the experimental evidence and Fick’s second law, apparent diffusion coefficients of D2O were estimated as 9.5×10－13 cm2.s－1 for the Fe3O4 film, and 5.4×10－13 cm2.s－1 to 2.1×10－12 cm2.s－1 for Fe2O3 film.