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To investigate the corrosion behavior and mechanisms of X70 steel in sands, polarization curves (PCs) and electrochemical impedance spectroscopy (EIS) were carried out using an electrochemical test set-up. X70 steel represents a common and standard API designation for pipelines with 70 ksi or 485 MPa strength. For acceleration of the process at different charging times, a direct current (DC) with 6 V voltage was applied. The results show that DC accelerates the corrosion process of X70 steel in sands and that the corrosion rate increases with charging time of the DC. Corrosion potential changes the process of positive-negative-positive charging with increasing process time, indicating that pore water, oxygen substances and corrosion ions of sands all affect the corrosion reaction. Based on the experimental results from the equivalent circuit of EIS, the corrosion process can be divided into the following three phases: developing and formatting of corrosion product film, pitting corrosion, and progressing pitting period. The low-frequency inductive slope on the EIS graph appears during the period of corrosion pitting as resulting from the corrosion product film. The film slows down the corrosion process and reduces the corrosion rate temporarily. With the increasing charging time, the corrosion product film may be damaged and the corrosion rate will thus increase significantly. The testing results of energy dispersive spectrometry (EDS) and scanning electron microscopy (SEM) indicate iron hydroxides as the original corrosion products. With increasing charging time, the original corrosion products may be oxidized to form stable and protective film products such as Fe 2 O 3  and Fe 3 O 4 . Lastly, during the pitting progressing period, oxidized film products will be damaged and the corrosion rate increases with increasing charging time.

Corrosion behavior of X70 steel in sands at different charging times