Effects of nitrogen element on microstructure and pitting corrosion resistance of duplex stainless steel welded joints

Abstract

The microstructure, element distribution and pitting corrosion resistance of Gas Tungsten arc welding (GTAW) duplex stainless steel welded joints under 100% argon (Ar) and 98% Ar + 2% nitrogen (N₂) shielding gas were studied by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and electrochemical corrosion methods. These results show that the addition of N₂ to the shielding gas promotes the formation of austenite (γ) phase in the weld joint. Also, the austenite phase distribution is dendritic and the ferrite (δ) and austenite phases in the heat-affected zone are no longer banded structures. Moreover, the austenite phase in the welded joint is enriched in nickel (Ni) and nitrogen (N) elements, while the ferrite phase is enriched in chromium (Cr) and molybdenum (Mo) elements. The pitting corrosion index (PREN) of the austenite/ferrite (δ/α) phase boundary is lower than that of δ and γ phases, which leads to their preferential corrosion. The appearance of chromium nitride (Cr₂N) decreases the corrosion resistance of the weld seam and the heat affected zone. N₂ in the 98% Ar + 2% N₂ shielding gas can restrain Cr₂N precipitation, increase the pitting corrosion index of secondary austenite structure (γ ₂), and improve the pitting corrosion resistance of welded joints.