Relating microstructures of hierarchical tertiary phases with corrosion performance in a super duplex stainless steel

This study explored the hierarchy of aging-induced tertiary phase formation and its effect on corrosion performance in a super duplex stainless steel. Tertiary precipitation started with very fine but coherent chromium nitride (Cr2N) and sigma (σ). Subsequent growth, especially for Widmanstätten σ, led to a loss of coherency and a diminishing orientation relationship with the matrix. Towards the end of the aging, eutectoid transformation of ferrite into σ and secondary austenite was also observed. These hierarchical phase transformations represented evolving size and chemical composition. The size of the tertiary phase, in general, determined the ‘severity’ of the chromium-depleted zone in the matrix. This ‘severity’ was quantified extensively with analytical transmission electron microscopy and showed a monotonic drop with the size of both Cr2N (maximum ∼600 nm in size) and σ (till ∼2 μm in size). σ phase >2 μm, evolving from the eutectoid transformation of ferrite, had a negligible effect. The hierarchical size of the tertiary phase(s) and associated solute depleted region in the matrix thus determined the corrosion performance. In particular, and as revealed by our direct observations on localized corrosion, the coherent sub-micron Cr2N and σ were extremely detrimental than the larger tertiary phases.