Electrochemical measurements such as polarization and impedance tests are used to evaluate the corrosion resistance of metallic biomaterials. The effect of in vivo biochemical and mechanical corrosion factors on the corrosion behavior of metallic biomaterials and the interaction between metal surface and cells are also examined with the electrochemical techniques. The findings will contribute to improve the corrosion resistance and biocompatibility of conventional and new metallic biomaterials including biodegradable metallic materials. The electrochemical techniques which are used to evaluate and to understand the effect of typical in vivo corrosion factors are introduced.
This article describes the current status of corrosion monitoring methods and examples of the measurement under deep underground environments for carbon steel overpacks for high-level radioactive waste disposal. Based on the studies on corrosion monitoring using AC Impedance technique, some of the typical measurement systems such as the electrodes arrangement are introduced. In-situ corrosion monitoring in engineering scale test is also being attempted using a deep underground research laboratory, and the measurement method and results are presented in this article.
A microelectrochemical system equipped with a confocal laser scanning microscopy was applied to the real-time observation of pit initiation in the pearlite structure of carbon steel. By using this system, high-resolution images of pit initiation process could be obtained. It was revealed that the initiation site of pitting was in a ferrite lamella, and the pit propagated along the lamellar structure. The cementite remained in the very early stage of pitting.
We considered the corrosion of a steel base at the through coating defective of thermal-sprayed aluminum（TAS） would be suppressed by adding Si to an Al spray material. The steel plate specimens that were thermally spray coated with Al-5Si or Al-12Si and were machined a disk-shaped artificial defect down to the base steel were prepared. For the sake of comparison, the specimens coated with the two types of Al with different purities or Al-5Mg were also prepared in a same manner. After subjecting to outdoor exposure in Okinawa, the charge transfer resistance at an interface between the rust layer and the base steel in the defective part was investigated by EIS. The charge transfer resistance at the interface for the specimen coated with the Al-5Si or the Al-12Si was higher than that coated with the Al or the Al-5Mg. It was estimated that the highly protective rust layers were formed by the action of Si and Al leaching from the coatings by atmospheric corrosion. The thermal spray coating of the Al-5Si and the Al-12Si was superior than that of the Al and the Al-5Mg in the respect of the tensile adhesive strength to the base steel.
We studied a highly accelerated corrosion test method for in-vehicle electronic modules. In the module, a circuit board on which the silver wiring was formed was stored in a casing sealed with silicone. SO2, H2S, S8 gas tests were conducted under the condition of 3300 ppm/90℃/80％RH in SO2 test, 60 ppm/90℃/80％RH in H2S test, and 4 ppm/90℃/＜10％RH in S8 test. As a result of examining the corrosion rate of the silver wiring, it was found that the S8 gas test has the highest rate of corrosion acceleration. High corrosion rate is presumed to be due to high permeability of S8 gas through silicone, in addition to its high corrosion ability to silver. Based on this result, we propose S8 gas test as a sulfur corrosion accelerated test method.
Since there were not so many reports concerning to the corrosion of painted steel in the indoor environment, we conducted long-term exposure test for Pb free cation electrodeposited painted steel sheet, which was widely used for steel industrialized housing, in the indoor environment. Further, observation and evaluation of blister width, red rust width, ratio of red rust area and the maximum corrosion depth were carried out. From results of evaluation and analysis after exposure test, better correlation between the corrosion on and under the paint film became clear, and the possibility of estimation for corrosion expansion and depth under the paint film by observing appearance of corrosion on the paint film was indicated. On the other hand, among the environmental factors, annual average relative humidity and amount of deposited sea salt are considered to influence in the corrosion behavior under the paint film.