Electrochemical measurements based on potentiostatic method are reviewed in this manuscript. In addition to chrono amperometric method, the potentiostatic investigations for analyzing the corrosion behaviors of passivation, localized corrosion, repassivation process on passive metals are introduced. The polarization resistance measurement which can determine the corrosion rate without breaking the specimen is also introduced.
Scanning vibrating electrode technique （SVET） system is applied to measure corrosion current density distributions around metal electrode. In this report, the basic characteristics of SVET and application examples of it are explained. Because the identification sensitivity and the resolution of the system are affected by the measurement conditions including height of sensor probe from the specimen surface, h, vibration frequency, f, and conductivity of the test solution, κ, those conditions must be arranged in considerations of the target for measurement, corrosion rate, locations of anodes/cathodes, and so on. However, recently, image processing techniques and detection techniques of the microsignals are being developed, the SVET system will be established as an useful tool for corrosion measurement.
The demand for micro-electrochemical measurements has been increased in fields of corrosion and protection. Here, features and principle of scanning electrochemical microscopy （SECM）, one of scanning probe techniques, are briefly described and a part of applications of SECM in the field of corrosion and protection are reviewed.
Soil corrosion of metal is a complicated phenomenon because various environmental factors are related with each other, and the effect of these environmental factors on corrosion rate of metal is still unclear. In this study, the effects of soil particle size and water content on the corrosion rate were investigated. The AC impedance method was used for measuring the corrosion rates of carbon steel buried in soil whose grains were coarse and fine, respectively. The corrosion rate of carbon steel in the fine-grain soil showed a maximum value at lower water content than in the coarse-grain soil. This can be explained by supposing a balance between water-dissolved oxygen diffusion distance and wet area of the carbon steel surface. In the fine-grain soil, the wet area and thin moisture layer on the carbon steel might be preserved even at the lower water content.
Thermal sprayed aluminum coated steels were exposed to subtropical climate at the University of the Ryukyus, Okinawa. They were found to exhibit good corrosion resistance even after 25 years. The results of the present study indicate that the thickness of the thermal sprayed aluminum film need not necessarily decrease, rather it may increase with the duration of exposure, but the porosity clearly increased. Furthermore, a film of alumina was formed on the rust where damaged steel was exposed to the atmospheric environment. Therefore, the thermal sprayed aluminum film is believed to provide 1) environment interception and 2) electrolytic protection.
In seawater, the corrosion rate of iron increases as the flow velocity increases. If a flow velocity distribution exists, a macro-cell is formed due to the differential flow velocity, by which corrosion in low flow velocity areas is promoted, while it is suppressed in high flow velocity areas. As a result, the corrosion rate of the former may be higher than that of the latter. This phenomenon is called “differential flow velocity cell corrosion”, and is regarded as a form of “differential aeration cell corrosion”（“differential oxygen concentration cell corrosion”）. The author considered the mechanism of “differential flow velocity cell corrosion” by examining the flow velocity dependency of the internal anodic polarization curve. A corrosion test on cast iron was conducted in seawater by changing the flow velocity and potential, to obtain an internal anodic polarization curve based on the corrosion rate. It was confirmed that the internal anodic polarization curve moved to the noble side （low current density side） as the flow velocity increased. Consequently, we experimentally verified a hypothesis developed to explain the mechanism of “differential aeration cell corrosion” and “differential flow velocity cell corrosion” of cast iron and steel.