Corrosion Behavior of Rusted Carbon Steel Coated with Reactive Paint Under Wet and Dry Cyclic Condition

Degradation of steel infrastructure built in 20th century is seriously progressing. This deterioration is mainly caused by corrosion of steel in atmospheric environments that contain O2 and H2O. To ensure long-term reliable service of steel infrastructures, some measures to retard the atmospheric corrosion of steel structures must be applied. Among several measures, paint coating is widely employed for corrosion protection of steel infrastructures. However, paint coating layers applied over steel infrastructures are subjected to degradation, and therefore need to be repaired by reapplication of the paint coating. Even after the reapplication, steel substrate under the coating is prone to corrode, accompanied with formation of rust layer which accelerates corrosion even under the coating since rust layer acts as an oxidant. Therefore, simple reapplication of a conventional paint coating over the rusted steel is insufficient. It is desired to reduce the corrosion acceleration effect of rust, and moreover to increase the protectiveness of rust layer underneath the coating. In the atmospheric corrosion environment, four types of crystalline rusts can be produced; alpha-FeOOH, beta-FeOOH, gamma-FeOOH, and Fe3O4. The composition of the rust layer is acknowledged to exert a significant influence on corrosion resistance of steels. Thus, it is an interesting challenge to enhance corrosion protectiveness of the rust layer by reapplication of a sophisticated paint coating to the rusted steel structure. In this study, corrosion protection performance of the reactive paint coating, which is designed so as to modify the protectiveness of rust layer, applied over a rusted carbon steel is discussed. It was found that the reactive paint coating worked well to reduce corrosion damage of the rusted carbon steel. After 4,320 h cyclic corrosion test according to JIS K5600-7-9, a large amount of rust flow and swelling of the paint coating layer around the crosscut damage made in advance in the coated surface were detected for the conventional heavy-duty coating specimen. In contrast, very small flow of rust or swelling around the crosscut damage was detectable for the reactive paint coating specimen, implying that corrosion propagation from the residual rust on the steel surface was effectively suppressed by the reactive paint. Electrochemical analyses revealed that the reactive paint coating applied over the rusted carbon steel greatly reduced corrosion current density of the steel, compared to the conventional heavy-duty coating. This low corrosion rate of the rusted steel covered with the reactive paint is found to be attributed to the modification of rust structure.