Microstructures, mechanical properties and corrosion behaviors of TIG-welded 9Ni steel joint in simulated marine environment

The microstructures, mechanical properties and corrosion behaviors of TIG-welded 9Ni steel joints with Ni-based filler were investigated in simulated marine environment. Results show that the welded joints primarily consist of three regions: the weld zone (WZ), the heat-affected zone (HAZ) and the base material (BM). The WZ region exhibits a cellular dendritic microstructure, the HAZ region features coarse-grained and fine-grained quenched martensitic structures, and the BM region comprises tempered martensite with a small amount of dispersed reversed austenite. The welded joints have excellent mechanical properties, with average yield strengths of 684.4 MPa, tensile strengths of 721.6 MPa and elongation of exceeding 36 %. Corrosion testing demonstrates that the WZ region is always free from corrosion, whereas more corrosion pits and γ-FeOOH corrosion products are formed on the BM region compared to the HAZ region, indicating that the HAZ region possesses superior corrosion resistance compared to the BM region. Deeper corrosion pits are developed near the WZ/HAZ interface because of their electrochemical potential differences. Over time, thick γ-FeOOH rust layer with thin Fe₃O₄ layer at the matrix/rust interface can be formed on the HAZ and BM regions. The predominant corrosion mechanism transitions from localized pitting corrosion to uniform corrosion in the later corrosion stage, resulting in the progressive formation of corrosion steps between the WZ and HAZ regions.