Spent nuclear fuel contains not only nuclides such as uranium or plutonium, but many kinds of elements as fission products produced from nuclides in a nuclear reactor. In a reprocessing plant, solutions dissolved spent nuclear fuel with nitric acid would be treated by chemical equipment mainly made of stainless steel. In this paper, corrosion behaviors of two kinds of austenitic stainless steels, which are Types 304 ULC and Types 310Nb, in nitric acid solutions containing simulated fission products were studied. Corrosion behaviors of these metals were evaluated through examining electrochemical characteristics and corrosion rates which were obtained by weight loss measurements. From the results of the corrosion test, it was found that the corrosion rates of stainless steels are enhanced by co-existant ruthenium ion in nitric acid solutions, whereas immune from other elements. Corrosion potential measured only in ruthenium added solution shifts to the noble region where high corrosion rates are exhibited. It is thought that such a potential shift to noble region is caused by enhancement of cathodic reactions on metal surface due to co-existance of ruthenium in nitric acid solutions.
Bromide stress corrosion cracking (Br- SCC) of stainless steels has not been clear, owing to many unknown factors, however troubles caused by it have been reported in MSF desalination plant. In this study, environmental factors, materials and surface treatments of noble metals plating have been investigated in order to protect Br- SCC in bromide containing high temperature water. SCC tests have been conducted by using slit-welded type specimens. The results are summaried as follows: (1) Concentration of disolved oxygen must be kept below 0.1ppm for protection of Br- SCC. (2) Transgranular SCC due to pitting occurs in SUS 304 and SUS 316 stainless steels above 50ppm Br- concentration. (3) The number of SCC is more major in Cl- environment than in Br- one above 0.5ppm disolved oxygen concentration. (4) Occurrence percentage of growing SCC is much higher in Br- environment than in Cl- one in all disolved oxygen concentration. This fact shows that Br- SCC, especially in chemical plants, grows so large that contents leak. (5) Hastelloy C 276 is most completely resistant materials in heavily severe Br- environment. (6) Only Rh in noble matal platings is the most effective to resistantly protect Br- SCC. But the reason has not been clear in this study.
AC impedance method has been applied to the study of degradation of coated SUS 304 stainless steels with a macroscopic line defect. The 304 stainless steels having different surface treatments, i.e. 304 stainless steels of bright annealed (BA), hair line finished (HL) and chromate coated (TR), were coated with epoxy type paint and employed as specimens. A corrosion test was conducted by 5 days exposure in 0.5kmol/m3 NaCl solution containing 5vol% H2O2. After the corrosion test, the impedance was measured in NaCl solution and the delamination area due to the corrosion was estimated by the double layer capacitance. Also, the impedance was measured for specimens covered the macroscopic defective part with epoxy resin after the corrosion test and the degradation of coatings was evaluated by a breakpoint frequency method. The results from both the evaluation methods well agreed with the result of a conventional delamination test with adhesive tape. It was found that the corrosion resistance increased in the order of 304HL<304BA<304TR.
One point of technological importance in the use of stainless steels for natural water environments such as sea and fresh waters lies with their liability to stress-corrosion cracking. The stress-corrosion crack initiation in these environments is almost always via localized corrosion such as pitting or crevice-corrosion. Although the critical potentials for initiation of pitting and corrosion-crevice have been clearly defined and the experimental method of determination has been well standardized, neither definition nor standard method has been established for stress-corrosion cracking. In this paper, the critical conditions for initiation of the kind of stress-corrosion crack that originates from a corrosion-crevice have been discussed with intergranular stress-corrosion cracking occuring in the sensitied Type 304 stainless steel/neutral chloride solution environment system as an example. Following conclusions were drawn: (1) The repassivation potential for stress-corrosion cracks, ER, SCC, can be determined in the cyclic polarization tests using a specimen provided with an artificial crevice and applying a static load on it, (2) the ER, SCC thus determined was about 100mV lower than the repassivation potential for corrosion-crevice, ER, CREV, determind for the same specimen but with no load applied, (3) effects of applied stress, degree of sensitization, test temperature, and NaCl concentration on ER, SCC were documented, and (4) the ER, SCC agreed with the critical potential for initiation of stress-corrosion cracking, VC, SCC, determined in the potentiostatic holding test.
The rate of uniform corrosion, the likelihood of pitting and crevice corrosion for aluminum, Al, in white wine were investigated in this paper. In commercial wines, the K2S2O5 is commonly added to induce free-SO2 which reduces the dissolved oxygen and acts as a fungicide in the wine. The spontaneous electrode potential of Al, ESP, in a deaerated wine containing less than 1ppm of free-SO2, at 25°C was measured to be about -600mV vs. SCE. Since the K2S2O5 is an oxidizing agent for the Al, the ESP was ennobled with the addition of K2S2O5 reaching -520mV with a concentration of 1000ppm K2S2O5. The uniform corrosion rate was measured to be 13μm/y at 25°C and this rate was found to depend on the temperature with an activation energy of 42kJ/mol. However, this rate was found to have no dependency on the electrode potential within the ESP range. The critical pitting potential, VC, PIT, measured in the wine containing 25ppm of Cl- was -370mV which was more noble than the ESP, and the pitting corrosion can not occur. The repassivation potential for the growing crevice corrosion, ER, CREV, was measured to be -530mV. It was also found that this potential does not depend on either the temperature or the concentration of K2S2O5. It was observed that the ESP became more noble than the ER, CREV with the addition of 800ppm K2S2O5 at 25°C. At a lower temperature, however, the ESP became more noble than the ER, CREV when a lesser amount of K2S2O5 were added. An effective method to decrease the rate of uniform corrosion is to reduce the holding temperature; however, to effectively prevent the crevice corrosion at these lower temperatures, the K2S2O5 concentration must be kept below 100ppm.
The surface textures of corroded materials gave us important information to understand corrosion mechanisms. Various methods to measure these corrosion shapes were already reported. These methods were briefly explained in this article and the characteristics of these were summarized. Especially, a stereo image processing technique which can derive three dimensional feature of a corroded specimen is explained in detail. And the future trends of the analysis of corroded shapes are introduced briefly.