In-Situ Corrosion Monitoring of Iron and Zinc in Atmospheric Environments

Abstract

Traditionally, for evaluating corrosion resistance of materials used in atmospheric conditions, exposure tests in the actual environment and accelerated corrosion tests in the laboratory have been widely used. Till now, it has been very difficult to estimate in detail corrosion rate variations caused by the change of each environmental factor such as temperature, relative humidity, amount of airborne sea salt during the test periods. Also, it requires a long time period to evaluate the corrosivity in actual corrosive atmospheric environments. For these reasons, an in-situ corrosion monitoring technique is useful. Time variation of corrosion rate has been successfully monitored by using an electrical resistance corrosion sensor, which converts the increase of electrical resistance caused by metal consumption to corrosion loss. Additionally, a new accelerated corrosion test was authorized as ISO 16539 Method B on March 1, 2013. This test has good correlation with corrosion behavior in an actual corrosive atmospheric environment. In this study, the details of the corrosion rate of steel and zinc were evaluated by the accelerated corrosion test (ISO 16539 Method B) and atmospheric exposure test. In addition, the relation between corrosion rate and each environmental factor was analyzed. Two kinds of electrical resistance corrosion sensors, an Fe sensor (250 µm thick) and a Zn sensor (50 µm thick), were used. In the accelerated corrosion test (ISO16539-Method B), the corrosion depth as evaluated by the sensors displayed a good correlation with that of the exposure test samples. The variation in the corrosion rate could be evaluated by the sensors. And also, it was confirmed that the corrosion rate increased during the wet stage and decreased during the dry stage in both sensors. In exposure tests in Okinawa and Kawasaki, the variation of the corrosion rate due to the effects of changeable environmental conditions throughout the year could be evaluated by the sensors. Okinawa is seashore area at the s southern island in Japan, and Kawasaki is industrial area in Japan. The corrosion rate in Okinawa as evaluated by the Fe sensor was greater than that found in Kawasaki. However, the corrosion rate in Okinawa as evaluated by the Zn sensor was almost the same as that in Kawasaki. The corrosion sensors could clarify the difference in corrosion behavior between steel and zinc. Furthermore, the relationship between the corrosion rate and the environmental factors (temperature, relative humidity, amount of airborne sea salt, etc.) was analyzed. The corrosion rate of steel had a positive relation with temperature, relative humidity and airborne sea salt. The dependence of corrosion rate of zinc on environmental factors was also investigated. Comparison of corrosion behavior and its dependence of environmental factors between iron and zinc will be discussed.