Abstract: A welding joint of 2507 super duplex stainless steel (SDSS) was obtained through the method of oscillating laser beam welding (OLBW). The weld joint exhibited good formability and a dual-phase microstructure, achieving an acceptable ferrite (α)-to-austenite (γ) ratio about 3:1. The weld zone (WZ) was predominantly composed of equiaxed α grains, with the γ phase primarily forming a network structure within the α grain boundaries, manifested as grain boundary γ (GBA), intragrain γ (IGA), and Widmanstätten γ (WA). These two phases formed a coherent structure, with their interfaces connected by misfit dislocations, and a small number of dislocations were present within the α phase. The 2.5 mm thick specimen exhibited an impact energy of 33.5 J at -46 °C. The yield strength, tensile strength, and elongation of the weldment reached 732.8 MPa, 874.6 MPa, and 30 %, respectively, with the overall mechanical properties surpassing those of conventional laser beam welding (LBW). Furthermore, the passive current density (i_P) of the weldment in seawater and acidified seawater environments measured only approximately 1.2 and 2.5 µA cm^-2, respectively. This superior passivation behavior could be attributed to two critical factors: the potential difference between dual phases being maintained below 60 mV, along with the formation of a protective Cr₂O₃ oxide film that effectively stabilized the passive state. The stress concentration and micro voids induced by plastic deformation significantly promoted pitting corrosion susceptibility of weldments in acidic seawater environments, with the i_P exceeding 3 µA cm^-2. Particularly for 20 % strain-deformed specimens, premature breakdown of passive films occurred under elevated potentials due to strain-accelerated localized dissolution mechanisms. The acidic environment of seawater and tensile strain exerted opposing effects on the potentiodynamic polarization behavior of welded joints. This paper provides a reference for the service behavior of 2507 SDSS weldments in harsh marine environments.

Key words: 2507 SDSS, Oscillating laser beam welding, Mechanical properties, Corrosion, Strain, Acidic seawater