Formation of Rust Scales on Mild Steel Pile in Brackish Water of Phu My Port - Vietnam

Corrosion test of mild steel (graded JIS-STK400) was investigated in brackish water environment of Phu My industrial port (Ba Ria - Vung Tau Province, Vietnam). Steel sample was exposed in 3 years. X-ray diffraction (XRD) results show that formed rust powder was composed of a-FeOOH, Fe3O4 and a- Fe2O3 in which the amount of first component is larger than second and third one. It demonstrates that natural corrosion products evolved from low to high oxida-tion state of iron by presence of dissolved oxygen in brackish environment. The mechanism of phase conversion is revealed by metallographic of rust layer cross section. a-Fe2O3 (white phase) was gathered at bank of cracking paths where was favouring diffusion of oxygen and water into inner layer. After contacting Fe3O4 (dark gray phase), corrosive agents converted it to a-FeOOH and a-Fe2O3. The phase conversion changes the weight density of materials and creates the porous tunnels in rust scales. The growth of micro tunnels to form the tunnel network allowed internal stress and cracks of rust scales evolved.