Irradiation assisted stress corrosion cracking (IASCC) is a potential failure mode suffered by the core-components of austenitic stainless steels in the aged light-water reactor (LWR), which is the intergranular type cracking caused by synergistic effects of neutron/gamma radiation and chemical environment. Effects of radiation on the materials and high-temperature water are discussed in this paper to understand IASCC phenomenon from a mechanistic viewpoint. It is essential to elucidate the radiation-induced micro-compositional and microstructural changes in the alloy for mechanistic and predictive investigations of IASCC. Although grain boundary segregations of alloying and impurity elements are significant factors affecting IASCC, it has been considered that the radiation-induced microstructural and mechanical changes of materials play critical roles in IASCC. For mechanistic understanding of IASCC, further fundamental research works with experimental and theoretical approaches are needed. Efforts directed to the researches at the Japan Atomic Energy Research Institute are also described.
In order to elucidate the cause of extraordinary corrosion which often occurs on the inside wall surface of heat conduction tubes of boilers, corrosion tests were conducted using a jet-in-slit apparatus in various boiler feed water flowing under high pressure at elevated temperature. As a result, ditch corrosion was detected with a rate in excess of 1mm/y under some combined conditions of material, environment and fluid flow. In this paper, the experimental results obtained to date are summarized with the aim of proposing a mechanism of ditch corrosion occurrence. That is, the influence of fluid flow conditions as well as environmental conditions such as dissolved oxygen content (DO), pH and the temperature of the feed water on the occurrence of ditch corrosion were investigated. The observation of corrosion damaged surfaces and the depth distribution of the damage revealed that the ditch corrosion occurs only under specific conditions where the corrosion products formed on the metal surface are not a single species but consist of two different types of oxides. The process by which these oxides are formed appear to be influenced by how the ferrous ions are solubilized and how the hydroxide ions are located on the metal surface. In order to verify this interpretation, corrosion tests were carried out on carbon steel to successfully reproduce the ditch corrosion at ambient temperature, thus validating the interpretation.
Controlling the reduction of Fe3O4 film is an important factor to improve corrosion resistance of carbon steel for geological disposal of radioactive waste. In the present study, the effect of alloying elements on the reduction reaction of Fe3O4 film was examined using sputter-deposited complex oxide films and low alloy steels containing the alloying elements. Fe3O4 films containing a small amount of another oxides were cathodically polarized in the simulated solution of bentonite-contacting water. It showed that the addition of Cr2O3, TiO2, Al2O3 and MoO2 to Fe3O4 is effective on the suppression of the reductive reaction. Fe3O4 films formed on the steels containing small amount of Cr, Ti, Al and Mo by alkaline solution treatment at high temperature also showed a high resistance against the reductive dissolution. The steels containing these alloying elements showed excellent corrosion resistances in the simulated solution of bentonite-contacting water.
It has been widely recognized that buried steel pipelines can be protected effectively by the combined use of protective coatings and cathodic protection. Particular considerations are prerequisite to apply the protective coatings and cathodic protection for buried steel pipelines heated by internal fluid, such as, pipelines of district heating and cooling system, pipelines of gas and petroleum plant, and pipelines transferring crude oil. This paper describes the laboratory studies on the cathodic protection potential and protective current density for buried steel pipelines heated by internal fluids. The studies were performed with soils submerged by water in the temperature range from 40°C to 90°C. At 40°C of the steel surface, a criterion of cathodic protection potentials more negative than -0.85V (Cu/CuSO4) was proposed. In the temperature range from 60°C to 90°C of the steel surface, a criterion of cathodic protection potentials more negative than -0.95V (Cu/CuSO4) was proposed. In the temperature range from 40°C to 60°C, the laboratory study was not performed. However, in this temperature range, a criterion of cathodic protection potentials more negative than -0.95V (Cu/CuSO4) shall be adopted from the safety point of view.
The dissolution behavior of Mg-1mass%Mn alloy anode for cathodic protection in tap-water was investigated with an electron probe micro analyzer (EPMA), a scanning electron microscope with energy dispersive X-ray spectrometer (SEM-EDX), and a microscope. It was found that the local dissolution of the alloy anode is due to the formation of a local cell within the alloy, and metallic impurities on the alloy surface act as cathode sites of the local cell. When the alloy specimen polished by α-Al2O3 of the particle size in 0.5μm was used at current densities of 0.1mA/cm2 or less, the local dissolution of the alloy was suppressed at least for three months, and the current efficiency was about 80%, which is the twice of the usual values. Under this condition, the alloy was dissolved almost uniformly, and amorphous film was formed on the alloy. When the polished alloy specimen was used under the condition exceeding 0.3mA/cm2, the film was not formed, and the current efficiency was decreased.
The corrosion behavior of carbon steel in working water of wire cut discharge machines, whose conductivity is typically controlled to around 10μS·cm-1, has been investigated in terms of the water quality. Among several samples of the working water, those which contained higher level of sulfate ion (0.6g·m-3) caused corrosion of carbon steel even when the water was flowing. The effect of concentration of several kinds of anions (sulfate, nitrate, hydrogenphosphate, carbonate and chloride) on the corrosion of carbon steel was also studied to know which anion was most aggressive. The result showed that 0.7g·m-3 of sulfate and 3g·m-3 of chloride caused severe corrosion when the solutions were flowing at 2.36cm·s-1. The steel did not passivate in any solutions when they were static. On the assumption that nitrite ion is introduced into the working water by an anion exchange method, the critical concentration of nitrite ion for inhibition was determined as a function of concentrations of sulfate and chloride.