Synergistic effects of mixed oxide fluxes and nitrogen-enriched shielding gas on microstructure and corrosion in super duplex stainless steel

This investigation evaluates the synergistic effects of CeO₂ flux and its mixtures with TiO₂ or NiO in activated tungsten inert gas (A-TIG) welding of UNS S32750 super duplex stainless steel. It examines impacts on weld morphology, microstructure, and corrosion under Ar or Ar+5%N₂ shielding. Microstructural analysis used optical microscopy, field-emission scanning electron microscopy, and feritscope measurements. Corrosion tests included cyclic potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis in 3.5 wt% NaCl solution. Results showed that TiO₂+CeO₂ flux with Ar+5%N₂ shielding achieved full 6 mm penetration. It increased the depth-to-width ratio by 321% compared to conventional TIG. A-TIG with CeO₂, TiO₂+CeO₂, or NiO+CeO₂ fluxes under nitrogen-enriched shielding restored austenite content. It refined grains to 3.8–4.9 µm and formed spherical CeAlO₃/Ce₂O₂S inclusions. These changes suppressed harmful phases. A-TIG welds (especially CeO₂-fluxed) exhibited superior pitting resistance (E pit = 1061 mV vs. SCE, I corr = 0.116 µA/cm²) versus conventional TIG (E pit = 919 mV vs. SCE, I corr = 2.531 µA/cm²). Polarization resistance reached 1.26 MΩ·cm², matching the base metal. Mott-Schottky results confirmed robust, low-defect passive films in CeO₂ welds. Donor and acceptor densities (N D = 7.2 × 10²⁰ cm⁻³, N A = 6.77 × 10²⁰ cm⁻³) approached base metal values. This work is one of the first systematic study of different mixed oxide fluxes' interactions with Ar+5%N₂ shielding in UNS S32750 A-TIG welding. It addresses key gaps in understanding their contributions to improved weld quality and chloride corrosion resistance.