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Yamamoto, Masahiro; Irisawa, Eriko; Igarashi, Takahiro; Komatsu, Atsushi; Kato, Chiaki; Ueno, Fumiyoshi
Proceedings of Annual Congress of the European Federation of Corrosion (EUROCORR 2019) (Internet), 5 Pages, 2019/09
Intergranular corrosion phenomena were analysed using modified reprocessing solution. The data indicated that corrosion rates increased with time at the initial stage, and these stayed at constant value. Intergranular corrosion propagated at grain boundary in the initial stage and then attacked whole grain boundary causing drop out of grains. Corrosion rates of steady state were sum of intergranular corrosion amounts and weight losses of dropped grains. Surface appearances and cross sections of corroded samples were analyzed. The results indicated that the initial stage of intergranular corrosion was characterized by the ratio of corrosion rates between grain boundary and matrix. These ratios differed from individual grain boundaries. Total corrosion rates were affected by the distribution of these ratios. These data were based on the numerical modelling of intergranular corrosion using cellular automata. And also, calculated results were compared with these analytical data.
Otani, Kyohei; Tsukada, Takashi; Terakado, Hiroshi*; Ebata, Koei*; Ueno, Fumiyoshi
no journal, ,
Inner components of the primary containment vessels (PCVs) in Fukushima Daiichi Nuclear Power Station was observed by remote-controlled robots and the observation showed that carbon steels of the PCVs wall above the contaminated water level was exposed to an air-solution alternating condition. Previous studies have reported that the corrosion rate of carbon steel is accelerated in case of the steel with thin water film exposed in air under the dry/wet condition. This suggests that the corrosion rate of carbon steel will be accelerated in the air-solution alternating condition. However, the corrosion rate of the steel on the corrosion tests which simulated the air-solution alternating condition has not been clarified. In the present study, a novel corrosion test of carbon steel which simulated the air-solution alternating condition was carried out. In order to simulate the alternating condition in the novel corrosion test, the carbon steel was alternately exposed to air and solution by rotating in a water tank which was not completely filled with solution. The surface observations showed that the rust layer formed on the steel after the tests in the alternating condition was thicker than that of the steel rotated always in solution. The mass loss measurements showed that the corrosion rate of carbon steel in the alternating condition was more than three times larger than that of the steel rotated always in solution. A thin water film was confirmed on the steel when the specimen exposed to the air during the tests. It has been reported that the mass transfer of dissolved oxygen to the carbon steel surface is accelerated in case of the carbon steel covered by a thin water film and corrosion of carbon steel is accelerated by the acceleration of oxygen reduction reaction (cathodic reaction). This suggests that the accelerated corrosion of the steel in the alternating condition would be caused by the thin water film on the steel during the tests.
Igarashi, Takahiro; Irisawa, Eriko; Komatsu, Atsushi; Yamamoto, Masahiro; Kato, Chiaki; Ueno, Fumiyoshi
no journal, ,
Nuclear fuel reprocessing plants contain high-temperature condensed nitric acid solution for dissolution of spent nuclear fuel, and austenitic stainless steels are used as a main material for spent nuclear reprocessing systems. In such a highly oxidizing environment, stainless steel shows the morphology of an intergranular corrosion surface. To keep maintain the safety of the system, it is important to understand the intergranular corrosion behavior. According to our experiments, grain dropping occurred with intergranular corrosion progress and that lead accelerating the corrosion rate. To predict corrosion rate due to intergranular corrosion, it is necessary to consider not only dissolution of the metal surface but also grain dropping. In our previous study we developed computational intergranular corrosion propagation model by cellular automata method. In the model, simulation space is parted into three kinds of non-cubic cells: interior of grain (matrix) cell, grain boundary cell, and solution cell. The only matrix and grain boundary cell contacted with solution cell corrode and change into solution cells with local matrix and grain boundary dissolution rates. The corrosion rates were determined by dissolution rate ratio between grain boundary and matrix, and reaction rate theory with our experimental data. By setting appropriate dissolution rate ratio from experiments into the model, change of corrosion rate was estimated by the simulation and it was good agreement with that of real corrosion test.
Aoki, So; Engelberg, D.*
no journal, ,
The occurrence of localised corrosion (e.g. crevice, pitting) is still a major concern for the application of lean (grade 2101) and standard (grade 2205) duplex stainless steels in seawater environments. This presentation will address recent progress in providing a mechanistic understanding of the effect of microstructure that can lead to the nucleation of localized corrosion, and typical examples of ex- and in-situ corrosion experiments under electro-chemical control are provided. Both duplex stainless steel grades are assessed for their susceptibility to seawater crevice corrosion, with the effect of crevices nucleating around weld microstructure and heat affected zones (HAZ) discussed.
Aoki, So; Engelberg, D.*
no journal, ,
This presentation will address recent progress in providing a mechanistic understanding of differences in crevice corrosion initiation and propagation behaviour of duplex stainless steels when exposed via potentio-static and galvano-static polarization by means of time-lapse, in-situ observations. In the potentiostatic test, the crevice corrosion initiated underneath the edge of the washer, and then grew towards the edge of the crevice concentrically with time. It suggested for crevice corrosion to initiate at the site of a very tight crevice gap, with propagation in a stable manner towards the crevice mouth, where the crevice gap is wider. In the galvanostatic test, the crevice corrosion initiated all over at the circumference of the washer. Based on these results, the differences in crevice corrosion initiation and propagation behaviour between potentiostatic and galvanostatic conditions will be discussed.