Abstract: Hyperbaric dry GMAW is widely employed commercially in the fabrication and maintenance of submarine oil and gas pipelines. The corrosion behaviors of welded joint in seawater are therefore of utmost importance in security operation of the pipelines. The corrosion performance and mechanism of X65 pipeline steel welded joints by hyperbaric GMAW under different ambient pressures (0.1, 0.3, 0.5 MPa) were studied by polarization curves and immersion simulation experiments in artificial seawater. It was found that compared with the welding base metal and welded zone, the corrosion resistance of heat affected zone is the worst, and the ambient pressure has little effect on the corrosion resistance of the welded joint. Corrosion morphology and corrosion product shows that pitting corrosion is the main corrosion form for the welded joints, and serious local pitting corrosion occurs at the late immersion stage. The corrosion product is composed of FeOOH, Fe₂O₃ and Fe₃O₄. The corrosion product on the surface of the welded joints can mitigate the corrosion at the initial immersion stage. With prolonged immersion durations, the compactness of the corrosion products layer reduces, the metal under the layer is vulnerable to the corrosion by Cl⁻, and thus the local pitting corrosion process is promoted.