Electrochemical Corrosion Behaviour of X70 Steel in Zinc (II)-Contaminated Sand Soil

As a common pipeline material, X70 steel has been extensively used in China (West-East Gas Pipeline Project), Mexico and other countries. The macro-/micromorphology and surface composition of X70 steel in sand soil at exposure time of 35 d have been characterized using Canon 6D SLR cameras, SEM and EDS. To further study the effect of the heavy metal Zn (II) on the corrosion performance of X70 steel in sand soil, electrochemical corrosion tests (exposure times of 7 d and 35 d), polarization curves and EIS measurements have been conducted utilizing an electrochemical workstation. Additionally, six sand soil samples with various contents of Zn (II) were prefabricated. The results demonstrated that the addition of Zn (II) in sand soil clearly accelerated the corrosion process and affected the corrosion performance of X70 steel in sand soil. In non-contaminated sand soil, the corrosion products of X70 steel were mainly composed of Fe2O3 and FeOOH etc. whereas the inner and outer layer iron-containing compounds constituted the corrosion products in Zn (II)-contaminated sand soil. Moreover, the addition of Zn (II) in sand soil increased the degree of coverage and diversification of the corrosion products. The corrosion behaviour of X70 steel in non/low-contaminated sand soil (C-zinc (II) <= 0.034%) exhibited localized corrosion, whereas in high-contaminated sand soil (C-zinc (II) = 0.068%-0.680%), uneven general corrosion was confirmed. Furthermore, in contaminated sand soil, the corrosion degree of X70 was the most severe at a Zn (II) content of 0.034%. A high content of Zn (II) (0.034%) accelerated the corrosion process. In addition, the Warburg impedance was observed in the EIS data of X70 steel in sand soil at exposure time of 35 d, and the reaction control step of the electrode reaction changed from activation control to diffusion control for X70 steel in Zn (II)contaminated sand soil.