Initial stage of corrosion formation for X70 pipeline external surface in acidic soil (peat) environment

Organic peat with a pH range from 4.5 to 5.5 was utilised as an alternate corrosive medium in this investigation to produce a naturally acidic environment that may promote surface deterioration on the X70 pipeline exterior surface. The physical and chemical properties of the corrosion product during the initial stage of corrosion formation have been analysed using several analytical techniques, including Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX), X-ray Diffraction analysis (XRD), and Raman spectroscopy. As the external surface is exposed to the acidic soil environment, the oxide layer is formed and initiated by intergranular corrosion on the elongated grain boundaries, leading to various shapes and sizes of corrosion products. After 60 days of exposure, XRD revealed that the corrosion products formed were consists of amorphous oxides and low-intense crystalline phases, namely lepidocrocite, γ-FeOOH (2θ≈26.85°), goethite, α-FeOOH (2θ≈21.75°), hematite, Fe2O3 (2θ≈35°) and magnetite, Fe3O4 (2θ≈38°). The morphology structure and composition analysis by SEM-EDX indicated that the corrosion products compositions are mainly contributed by Fe (55%) and O (25.99%), with some trace of C, Ca and Si compounds. Finally, the molecular structure for the micro-oxide coating around the external surface of this X70 pipeline is mainly dominated by species of lepidocrocite (γ-FeOOH), goethite (α-FeOOH), hematite (α-Fe2O3), and magnetite (Fe3O4). Overall, it can be concluded that a significant oxide layer formation by four types of iron oxides species [lepidocrocite (γ-FeOOH), goethite (α-FeOOH), hematite (α-Fe2O3), and magnetite (Fe3O4)] is observed in this work. This study also indicates that the corrosion product of the external surface of the X70 pipeline could be formed via chemisorption and physisorption process by the interaction between the acidic soil particles and the metal surface.