Five-Year Field Exposure for Visualized Corrosion of STK400 Graded Steel Pile in Brackish Environment of Phu My Industrial Port (Southern Vietnam)

A long-term field study on corrosion of STK400 steel was established at Phu My port. The large steel pile, which was equivalent to real structures, was placed in all environmental zones to evaluate the overall corrosive effects of the brackish water. After 5 years, the corrosion rate over the entire length of the steel pile was measured and the rust at the boundary altitude between the zones was characterized. The corrosion rate profile shows the polarization of the entire pile length into two large anodic areas at the highest water level and submerged zones, where had high corrosion rate and pitting corrosion form. X-ray diffraction and metallographic of rust layers showed that Fe₃O₄ phase increased with water depth, allowing to strong diffusion of Fe²⁺ ion from steel substrate into the environment. The change in organism populations along water altitude in the tidal and submerged zones made the rust layers more complicated. Such distribution of polarized electrodes and composition of rust were due to the renewal of the corrosive agent by tidal cycles and organism population in water. As a result, the steel substrate in the tidal and mud zones was protected as cathodic areas.