The impacts produced by cavitation bubble collapses can be used for surface modification to improve fatigue strength in stead of shot peeing. The cavitation impacts also improve corrosion resistance. In case of cavitation around a high-speed submerged water jet, i.e., a cavitating jet, the cavitating region and cavitation intensity can be controlled by hydraulic parameters such as upstream-and downstream-pressure of a nozzle. It can be said that a cavitating jet is very useful for practical application. In the present paper, examples of the surface modification such as the improvement of the fatigue strength and corrosion resistance by using a cavitating jet were shown, including the explanation of a cavitating jet flow.
Corrosion damage in chemical plants equipment may occur not only in shut-down of plants, but in the worst case may lead to severe accidents which threaten the very existence of the plant. Corrosion in equipment was monitored continuously and causes of the corrosion was clarified, using the development electrochemical noise monitoring method, during a operation of chemical plants. This paper describes a method of plant management in which operating conditions are controlled in order to suppress corrosion, and also introduces principles of corrosion measurement. Topics related to corrosion management, and future performance in the field, are also explained.
Zinc injection into PWR primary coolant in order to reduce radiation buildup has been widely studied, and the reduction effect has been experimentally confirmed. However, some items, such as the optimal concentration of zinc required to reduce radiation buildup, the corrosion control effect of zinc injection, and the influence of zinc injection on the integrity of fuel cladding, have not been clarified yet. In particular, the corrosion suppression effect of zinc remains unconfirmed. Therefore, it is necessary to measure and calculate the solubilities of zinc and nickel spinel oxides, which are formed on the surface of Ni-based alloys in PWR primary systems. In this study, in order to assess the effectiveness of zinc injection in the reduction of radiation buildup and the corrosion control of Ni-based alloy, the potential-pH diagrams for Zn-Cr-H2O, Ni-Cr-H2O and Co-Cr-H2O systems at 573K were constructed and the solubilities of Zn-Cr, Ni-Cr, and Co-Cr spinel oxides were also calculated. It is concluded that under pH conditions for which NiCr2O4 is stable, zinc injection is effective in corrosion control as well as in reducing radiation buildup.
Intergranular corrosion (IGC) of the recrystallized Alloy 600 containing 0.06%C has been investigated. Cold rolled Alloy 600 was recrystallized by electric furnace heating or by YAG (Yttrium-Aluminum-Garnet) laser irradiation. The IGC resistance was evaluated by Streicher test (JIS G 0572). It was observed that the cold rolled Alloy 600 of 45 to 75% are occupied by the recrystallized grains after the electric furnace heating at 800 to 900°C. The reycrystallized Alloy 600 is excellent in IGC resistance, because of the granular Cr-rich carbides distribution in the recrystallized grains. It was shown that the surface recrystallized layer of about 300 to 800μm thickness can be easily formed on the cold rolled Alloy 600 by the 900 W YAG laser irradiation. The IGC resistance of the surface recrystallized Alloy 600 is also excellent, except the excessive laser irradiation.
One of major causes of microbially influenced corrosion of stainless steels in marine environment is formation of biofilms on the surfaces by microorganisms. It has been known that the corrosion potential of stainless steel exposed to natural seawater is ennobled and that a biofilm formed on a metallic surface is thickening and structurally heterogeneous with the passage of exposure time. In this paper, we intend to discuss the relation between structural characteristics of biofilms formed on SUS 316L in natural seawater and the exposure time. Test pieces of SUS 316L were exposed to natural seawater for 3 days-60 days. Open-circuit potentials of test pieces in natural seawater were measured and biofilms on test pieces were observed with Atomic Force Microscope (AFM) in natural seawater. The thicknesses of biofilms were measured by the in-situ observation with AFM. Then test pieces were dried and observed with an optical microscope, a laser microscope and AFM in the atmosphere. Open-circuit potentials were ennobled and the biofilms in seawater thickened as the exposure time passed. The biofilms were growing partially and the major species in the biofilms shifted from bacteria to diatoms with the passage of exposure time. It is concluded that thickening of biofilms on SUS 316 L causes ennoblement of potentials in natural seawater and that biofilms become more heterogeneous with diatoms on the surfaces.
Relation between corrosion potential and distribution of localized corrosion depths was studied by the statistical analysis. Corrosion potentials and depths of localized corrosion under tubercles were measured at 27 survey sites on the inner surface of two steel water channels in which river water flowed. The results showed that there was poor correlation between the maximum localized corrosion depth and corrosion potential with regard to respective survey sites. At five sites where corrosion potentials were almost at the same level, depth distributions differed from each other significantly. This difference can be explained as attributable to variation of exposure time of the steel surface to water after the rupture of a coating film. The above results suggest that the extreme value statistical analysis is not always applicable even when the corrosion potential of the respective survey sites is identical. However, P [mm·y-1] plotted against the corrosion potential [Ecorr/V(vs. Cu/CuSO4)] successfully gives the following relation: P≤0.51·Ecorr+0.34 where P is the localized corrosion depth simply divided by a length of time during which a structure has been in use. Further when whole data of localized corrosion are classified by corrosion potential, the mean of and the standard deviation of distribution of localized corrosion depths are expressed by the neat functions of corrosion potential. The analysis results indicate that the corrosion potential and the exposure time have contributed to a corrosion process independently.
Material factors which cause intergranular corrosion (etched grain boundaries) of Type 304 stainless steel in 10% oxalic acid etching and nitric acid-fluoric acid etching test methods are studied. It has been clarified that the material factor of grain boundary etching in 10% oxalic acid etching test is never the Cr depletion at grain boundaries, but the Cr carbide precipitated at GB. Also, it has been found that the Cr depletion at grain boundaries is the only cause for GB etching in nitric acid-fluoric acid etching test and Cr depleted zone is completely healed by the diffusion of Cr from the matrix in the heat treatment at 800°C for more than 250h.
In order to study the mechanism of fracture by observing the deformation behavior at the crack tip, we established slow speed SZW measurement method. This method was applied to clarify the fracture mechanism of duplex stainless steel, SUS 329J1, under catholic potential. The result shows that the deformation style changed by the influence of hydrogen when the deformation speed was slow. From these, this method was found to be useful for the study of the mechanism of hydrogen embrittlement.