Hot Corrosion Behavior and Degradation of Mechanical Properties of Haynes 230 Weldments in a Mixed Molten Salt Environment

Abstract

The hot corrosion behavior of nickel-based alloy Haynes 230 welded joints exposed to a molten salt environment of 75 wt% Na₂SO₄ + 25 wt% NaCl under 850 °C is investigated, with analyses on microstructural evolution and local mechanical properties under different corrosion durations. The results indicate that surface corrosion intensifies with increasing corrosion time, characterised by pronounced delamination, exfoliation, cracks, and pores. The measured corrosion depth in the base metal (BM) increases from 20.6 μm at 1 h to 52.5 μm at 50 h, before decreasing to 33.8 μm at 150 h, while in the weld metal (WM) it increases from 16.5 μm to 35.0 μm and then decreases to 28.6 μm. The primary corrosion products include Fe₂O₃, Cr₂O_3, NiO, CrS, and NiCr₂O₄. The corrosion in the base metal propagates along equiaxed grain boundaries, while in the weld metal it extends in a columnar form along dendritic boundaries. The hot corrosion process is driven by a synergistic interaction of oxidation, sulphidation, and chlorination, exhibiting characteristics of intense corrosion evolution in the early stage and continuous corrosion evolution in the later stage. Prolonged exposure leads to the formation of a double-layer oxide scale with a certain thickness and protective function. Microhardness tests show that near-surface hardness dropped by over 20% after 150 h, from 186.9 to 143.4 HV in BM and from 192.6 to 125.1 HV in WM, while the internal matrix remained stable (187.1 HV for BM and 197.5 HV for WM), indicating a corrosion penetration depth of about 70 μm. Nanoscratch tests further reveal that corrosion reduced resistance to plastic deformation, with scratch depths increasing from about 2000 nm in uncorroded samples to 4400 nm after 150 h, accompanied by a transition of wear mechanism from cutting to ploughing.

Graphical abstract