Corrosion of metal materials by microorganisms has long been known as microbiologically influenced corrosion （MIC）. However, little is known about causal microorganisms and their corrosion mechanisms. Poor knowledge about iron-corrosive microorganisms is delaying the development of rapid and accurate diagnostic method against MIC. In this paper, I introduced general knowledge about microorganisms and reviewed about latest knowledge of iron-corrosive microorganisms and MIC investigation. Furthermore, I propose the ideal scheme and future insight of genetic analysis for MIC investigation.

In this study, we investigated the corrosion behavior of steel in a crevice by immersion tests in NaCl solution to propose a model of lap-joint corrosion of steel. The crevice was prepared by placing a crevice former above a steel plate. The crevice geometry was controlled by the radius of the crevice former and the distance between the crevice former and the steel. The results of the immersion test showed that corrosion morphology and corrosion weight loss of the steel depended on the crevice geometry. The corrosion potential of the steel during the immersion slightly depended on the crevice geometry and was in the diffusion-limiting region of oxygen reduction reaction. These results revealed that the corrosion of the steel proceeds by the formation of differential aeration inside and outside of the crevice, where the steel surface inside the crevice acts as an anode and the remaining steel surface outside the crevice acts as a cathode. Furthermore, it was shown that the corrosion weight loss evaluated by FEM analysis of dissolved oxygen concentration was consistent with the trend obtained experimentally in this study.

It is well known that the types of automotive corrosion can be divided into perforation, and cosmetic corrosion. Although a lot of studies concerning the mechanism of perforation corrosion has been conducted so far, there are very few studies for the mechanism of cosmetic corrosion. In our previous work, the authors have found out that the initial corrosion behavior consists of 3 steps by in-situ observations under a cyclic corrosion test. In the present work, in-situ observation under the exposure test at Okinawa was performed for the painted sample with scratch in order to examine the cosmetic corrosion behavior under the actual environment. It was found that the corrosion started at the timing of salt depositing around the scribed part from the air containing salts. After that, the initial corrosion behavior under the exposure test consisted of 3 steps, as same as the one under a cyclic corrosion test. In the first step, black rust of Fe₃O₄ generated at the scribe part, and in the second step, under-film corrosion progressed from the scribed part at which the rust generated. In the third step, the tip of the under-film corrosion displayed swelling behavior. The each step behavior was also discussed by combining in-situ observation results with the analysis of environmental factors such as relative humidity, or with the EPMA analysis.