It is essentially necessary to understand the effect of corrosion products on the corrosion rate of carbon steel in order to evaluate the lifetime of carbon steel overpack. Especially, effect of magnetite on the long term integrity of overpack is one of the important subjects to be solved, because some experimental results showed that the magnetite layer formed on a carbon steel overpack as a corrosion product would accelerates the corrosion rate of the overpack. Various studies have been conducted on the corrosion mechanism of carbon steel in the presence of magnetite, its effect on the overpack lifetime and the countermeasures against the corrosion acceleration. At present, however, the interpretations on the results of these studies are not always consistent each other. In this report, the current status of the studies on corrosion of carbon steel in the presence of magnetite was reviewed, and the unsolved problems and future research subjects were extracted and discussed.
Copper heat exchanger tubes in water heater have been similarly suffering from type I' pitting corrosion, which have been experienced in the pass-through soft copper plumbing tubes, under well water. Through the case examination, the result showed that type I' pitting corrosion only appears on water inlet pipes and coiled pipes under conditions of maintained cold water as well as residual carbon over 5mg/m2. To solve this problem, the performance of 3 type heat exchangers was investigated by field test under well water. The heat exchanger with copper tubes treated by tin coating after manufacturing showed the most excellent resistance against pitting corrosion due to the stable tin oxide formation on the tin coating surface, followed by the heat exchanger made of copper tube with residual free carbon below 4mg/m2 which also showed excellent pitting corrosion resistance. However the heat exchanger with manufactured Cu-0.24 mass%Sn alloy tube corroded in either case: with or without carbon film. Consequently, it can be stated that residual free carbon has certain relevance to copper, as so for Sn concentration to Cu-Sn alloy concerning the occurrence of pitting corrosion.
A canister for a concrete cask in the spent fuel facilities will be exposed at the wet environment with deposited sea salt particles after long term storage periods. High corrosion resistant stainless steels, dual phase stainless steel (material D) and austenitic super stainless steel (material A), were selected for candidate materials of the canister, for the wide and 20 to 50mm thick hot rolled plates are able to produce from those alloys by the recent hot rolling technology. Then constant load stress corrosion cracking tests for both materials were conducted by Mayuzumi et al. for up to 16000 hours in the hot wet environments to confirm no failure in this period, however several penetrations are observed on the both surfaces of specimens. In order to evaluate the corrosion resistance of those materials, the penetrations of above mentioned specimens which authors were given from them are examined to obtain the base data of stress corrosion crack initiation and propagation. The following conclusions were obtained. 1) The micro stress corrosion cracks are observed only at the 80°C test specimens, although only penetrations were observed on the specimens tested at 70°C and 80°C. Crack initiates at the stress of larger than 1.1σy after approximately 15000 hours of test for material D, and at the stress 1.5σy and after approximately 1000 hours for material A. 2) SCC propagation rates of both materials are very slow. 3) The critical value of SCC initiation (KI SCC) are about 7MPa√m for material D and 5MPa√m for material A, respectively, in this investigation.
We proposed a new efficient sampling method for evaluating atmospheric corrosivity influenced by airborne sea salt and/or pollutants. X-ray Photoelectron Spectroscopy (XPS) was employed to analyze chlorine and sulfur captured in the top most thin layer of corrosion films. These elements are the main components of airborne sea-salt-particles and/or pollutants, both of which promote corrosion. Copper and silver specimens with super-clean surfaces pretreated by Ar+-ion sputtering were exposed to three different atmospheres for a period of 1 hour, before and after which they were preserved in a vacuum carrying vessel. XPS was sensitive enough to detect minute amounts of the target elements, which well reflect the atmospheric corrosivity, at the early stages of corrosion. This demonstrates that this method is useful for quick evaluation of atmospheric corrosivity.