The features of unusual wall thinning of carbon steel piping used in the transport of pure water at elevated temperatures are described. The case of Mihama nuclear power plant is used as a typical example, since the results of investigations conducted by the investigation committee of the Nuclear and Industrial Safety Agency are already available at their web site, and detailed information, such as data on residual wall thickness distribution and clear photographs of the thinned pipe wall surface etc. are readily available. Two models for illustrating the generation mechanism were examined : the FAC (flow accelerated corrosion) model and the macro-cell corrosion model. The latter explained the mechanism reasonably well, and methods for preventing wall thinning, based on this model, are given. Procedures for detecting the progress of wall thinning were also described.
This study was conducted in order to clarify the corrosion behavior of a carbon steel disposal container for low-level radioactive waste. The corrosion morphologies to be considered and the occurrence conditions of localized corrosion with the carbon steel as a candidate material were evaluated in aqueous solutions that simulate groundwater equilibrated with cement. As a result of the electrochemical evaluation for the possibility of occurrence of localized corrosion, the re-passivation potential for crevice corrosion was consistently more noble than the corrosion potential in DO<3×10−4 mass ppm, pH 11.5∼13.5. As a result, it was considered possible to make allowance for the localized corrosion in the solution environment despite Cl− content. Furthermore, when Cl− content was low concentration near 0 mass ppm, the occurrence of localized corrosion could be neglected despite DO and pH. On the other hand, the measurement of hydrogen generation equivalent corrosion rate on the assumption that general corrosion was dominant revealed that the corrosion rate was in 0.067∼0.1μm/y at pH 12.3. Assuming that the burial period of a carbon steel container would be about 10 million years, it was concluded that the corrosion wastage would be approximately 10 mm.
In order to shed light on a phenomenon (blackening) occurring on the surface of tanks of chemical tankers used to carry crude phosphoric acid, corrosion tests and surface analyses were conducted on various types of stainless steels in crude phosphoric acid. The tests and analyses revealed that when stainless steels were exposed to crude phosphoric acid, the surface of the metal blackened in the gas phase, while no corrosion or discoloration occurred in the liquid phase. The overall corrosion, resembling light surface roughening occurring directly below the part discolored in the gas phase, resulted in no problem with all stainless steels because the corrosion rate was lower than 0.1 mm/year. When the practical use of chemical tanks is considered, however, it is necessary to clean the tanks because blackening might possibly contaminate other chemicals through the mixing of impurities. Blackening results primarily from the fluoride gas liberated from the crude phosphoric acid that forms a black film as the product of corrosion by reacting with the metal surface. The black film is considered to comprise either or both of oxides or fluorides of such metals as iron, chromium and nickel. In order to prevent or inhibit the discoloration, it is necessary to remove or dilute the fluoride gas from the crude phosphoric acid. This can be effectively achieved by replacing the gas phase with inactive gases.
To investigate the corrosivity of the atmospheric environment where the electrical control unit was set up, the corrosion rates of silver and copper were monitored by the quartz crystal microbalance and temperature, relative humidity and the insulation resistance were monitored simultaneously with each equipment. In this environment, the corrosion rate of silver was larger than the corrosion rate of copper. From the classification of the severity of the installation environment based on JIS C0116 and ISA-S71.04, the corrosivity of the atmospheric environment was comparatively not severe. However the migration was generated even in the environments with the highest relative humidity of 62% and the insulation resistance higher than 1010Ω for the measurement period.
A corrosion test for carbon steel tube with obstruction that had narrow inner diameter was conducted. Water treatment conditions of thermal power plants were applied for the test to evaluate wall thinning rates. The wall thinning rate under an AVT (All Volatile Treatment) condition was larger than that under an OT (Oxygenated Treatment) condition. Wall thinning occurred at the front of narrow obstruction and the neighborhood of the front. Corrosion test results and hydrodynamic analyses using a turbulent flow model indicated that the values of turbulent energy and mass transfer coefficient at wall surface were associated with wall thinning rate and wall shear stress was not the dominant factor of wall thinning rate. The wall thinning rate derived from mass transfer coefficient was almost the same as that from the corrosion test. Therefore the dominant factor of the wall thinning rate was considered to be mass transfer rate of Fe ions dissolved from magnetite layer on the component surface.