Original Paper(Vol.62 No.8 pp.510-517)
Evaluation of Corrosion around Defect on Paint-Coated Steel

Takahiro NISHIDA, Nobuaki OTSUKI, Toshiki ANNAKA and Kenji WADA

**Abstract:**Corrosion of paint-coated steel exposed to marine environment is one of the serious problems and there are many researches related to such corrosion. Also it is recognized that the defect plays very important roles in such corrosion. Nevertheless, there are still few studies about the deterioration progress of paint-coated steel around defect. Therefore, the objective of present study is to clarify the corrosion progress of paint-coated steel around defect based on experimental and mathematical approach. In the experiments, accelerated test simulating the sub-tidal zone was conducted to observe the macrocell corrosion behavior as well as microcell corrosion. Here, temperature of the solution was changed as parameter in order to clarify the influence of temperature on corrosion rate. Also a mathematical model for calculation of the macrocell corrosion current around defect was proposed.
The conclusions derived from present study were as follows. (1) Anodic current of macrocell corrosion at defect parts and its cathodic current at paint-coated parts increased, as time passed. After much cathodic reaction of macrocell corrosion occurred on steel plate under paint-coating, the coating was blistered and/or ripped, and then microcell corrosion current increasingly occurred there. (2) The higher corrosion current observed with temperature elevation. Also it was estimated that the diffusion of oxygen or Fe^{2+} controlled the corrosion current after coatingfs degradation based on the activation energy obtained in present study. (3) A mathematical model for macrocell corrosion current was proposed and the trends of the model agreed with experimental data.
**Key Words:**Macrocell and microcell, Steel corrosion Paint, Defect, Acceleration test, Mathematical model