The laboratory simulation tests which could be reproduced the corrosion reactions propagating in the actual environments were utilized to analyze the mechanism of corrosion phenomena. In this report, some results are introduced in the cases of maritime structures and nuclear facilities.
As for the maritime structures, corrosion maximizing at the portion just under the low water level was focused. The results obtained by the simulation test of marine environments show that this portion acts as anode accompanied by the tidal area as the cathode in the high tide and also acts as anode accompanied by the deeper immersion area as the cathode at the low tide. This portion stayed as anode site for a long time and corrosion amounts are larger than the others. It is estimated that continuous dissolution of this portion changes the circumstantial condition to anode site preferentially. This type of corrosion phenomenon might be called “Tokeguse”; keeping an anode site longer than expected.
As for the nuclear facilities, experimental apparatus was originally designed to obtain the data in high radioactive condition simulating actual plants. One is a result showing the effect of Np ion to the corrosion of stainless steel in nuclear fuel reprocessing plant.
Corrosion mechanism was revealed that Np6+ ion is reduced to Np5+ ion by a corrosion reaction of stainless steel and then re-oxidized to Np6+ ion in the bulk solution. And repetition of this cycle accelerated corrosion of stainless steel by a little amounts of Np addition in nitric acid solution. Another result is introduced that an effect of H2O2 created by radiolysis of cooling water at high radioactive environment in light water reactor.
The reactive paint which can control rust structure during corrosion process is introduced with discussing the structure and anti-corrosion properties of rust layer formed on a mild steel. It was shown that the reactive paint leads to preferential formation of α-FeOOH structure in the rust layer. The change in rust structure results in suppression of cathodic reaction and prevents corrosives from passing through the rust layer. The reactive paint can be employed for rusted steels as well as for steels with clean surface.
Corrosion of carbon steel tubes used under boiler water circumstances is possibly accelerated by unexpected flow conditions. Mechanisms of flow accelerated corrosion (FAC) are ordinary explained by the dissolution of oxide films formed on carbon steel surfaces, mass transport of metal ions, dissolved oxygen and water chemicals. These mechanisms are, however, not precisely proved or reproduced in an experimental level and actual service. In this study, corrosion or erosion-corrosion tests for carbon steel and low alloy steel were conducted in a batch test container at pH 9.0 and 9.2, temperatures of 393 and 413 K under flow conditions by rotating a gear type rotor. In order to clarify the corrosion mechanisms, total corrosion loss of carbon steel was separated as iron mass in oxide films and dissolved iron into the solution. The steel surfaces, and the debris and particles suspended in the solutions were observed and analyzed by scanning electron microscopy (SEM, EDX). As results, it was supposed that the partial dissolution of iron from steel surface and suspended debris occurred as followed by the mechanical removal of oxide films formed on the steel surface, from the observations of exposed steel surfaces and cracks on the oxide film surfaces. The dissolution rate of iron depended on the properties of oxide films and also on flow conditions.
Materials with petrolatum lining that had been exposed to the actual environment for 10-30 years were collected, and a component analysis was conducted through FT-IR measurement and molecular weight distribution measurement and defect rate measurement to compare them with unused materials. These results clearly showed that petrolatum materials are decomposed into low molecules due to oxidization with the exposure time increases. Steel corrosion causes partial defects in anticorrosion materials, and particularly notable at splash zone. Degradation mechanism was confirmed through the defect rate measurement, and AC impedance measurement with simulated materials. The possibility of proposing degradation level curve created by utilizing the absorbance ratio obtained from the FT-IR analysis was found.