Abstract

A quantitative characterization of the micro structural changes in gas (GTA) surface melted duplex stainless steel (DSS) when subjected to annealing at 900°C for times up to 120 minutes was carried out in order to correlate the structural parameters with pitting corrosion resistance. The kinetics of δ ferrite transformation and coarsening of the initial lathy ferrite structure were monitored by estimating the volume fraction of phases, Vv, surface area of the interphase boundary in a unit volume, S_v, and the ratio of these two parameters, S_v/V_v The corrosion testing was performed in 0.5 M NaCl solution at room temperature using potentiodynamic and potentiostatic polarization techniques. It has been shown that the applied heat treatment dramatically changed the corrosion behaviour of resolidified DSS. Due to the simultaneous formation of γ2 and σ-phase, the pitting potential value, E_p, decreased from 570 mV (SCE) for as-resolidified material to 150, 125 and 30 mV (SCE) after annealing for 5, 15 and 30 min, respectively. The high density of fine precipitates caused an increase in the amount of δ/secondary phase boundaries providing more sites for pit nucleation. As a result, numerous, small and regular pits were formed at the interphase boundaries. For longer exposure times (60 and 120 min), the δ transformation was almost complete. The slight increase in the E_p value and changed initial form of attack can be attributed to the different partitioning of alloying elements, due to the extensive σ-phase precipitation within ferrite matrix.

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