The stress corrosion cracking behavior of an Incoloy825/X65 bimetallic composite pipe welded joint in wet hydrogen sulfide (H₂S) environment was investigated by means of the creviced bent beam (CBB) test in this study. The microstructure, element distribution and crack propagation behavior of the welded joint were analyzed by optical microscope (OM), scanning electron microscope (SEM), electron dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The results showed that two types of cracks were observed in the Incoloy825/X65 bimetallic composite pipe welded joint in wet H₂S environment, they initiated from the notch and the intersection of the three zones (cladding Incoloy825, base X65 and weld), respectively, and propagated along the fusion boundary(FB) and the Type-II-like grain boundary. The mechanisms of the two types of cracks are due to the combination of anodic dissolution, stress and hydrogen. Near the FB, there are high angle grain boundaries, Type-I, Type-II and the Type-II-like grain boundaries, which have high SCC sensitivity. The element distribution in the intersection of the three zones and the crack tip is complex, with element diffusion, Cr loss and large residual strain. All these provide the conditions for cracks initiation and propagation.

Hydrogen barrier coatings with Al₂O₃ as the main component are a good choice for solving the hydrogen embrittlement problem during hydrogen transportation in long-distance pipelines. However, the difference in the coefficients of thermal expansion between the substrate and the Al₂O₃ coating limits its further utilisation and development. In this study, rare earth oxides were added to the molten aluminium solution, and a Fe-Al transition layer was introduced on the surface of X80 steel by hot-dip plating to solve the thermal mismatch. Here, the microstructure and bonding strength of the hot-dip aluminium layer were investigated. It is found that the hot-dip aluminium coating consists of the outermost aluminium-rich layer and the inner Fe-Al alloy layer, and the microstructure of the two will change with the change in dip plating parameters. The best overall performance of the hot-dip aluminium layer was obtained from X80 steel substrate at a dip plating temperature of 700 °C and a dip plating time of 5 min. This coating has a good interface, moderate thickness, and a bond strength of 49 N. This study provides a reference value for solving the thermal mismatch between the steel substrate and the Al₂O₃ hydrogen barrier coating generated by subsequent anodising.

To enhance the corrosion resistance of underground pipelines made of low carbon steel, nano-TiO₂-modified polyurea was applied to their surface. The anti-corrosion performance of these nano-TiO₂-modified coatings was tested by immersing them in a NACE (5 wt.% NaCl + 0.5 wt.% CH3COOH) solution under high temperatures and high CO₂ pressures. The mass variation, SEM morphology, and open-circuit potential were determined. EIS tests, neutral salt spray tests, and contact angle measurements were carried out to analyze the effect of nanoparticles on corrosion resistance. Within the same pressure range, the polyurea coating shows the highest corrosion resistance when 5% TiO₂ nanoparticles were added compared to that of polyurea coatings with 0%, 10%, and 15% TiO₂ added. Coatings with 5% TiO₂ nanoparticles showed rapid diffusion after being immersed for 96 h, indicating that the anti-corrosion performance of the coating weakened.