Purpose The secondary phase decreased the corrosion resistance because of the segregation of Cr and Mo. Therefore, this paper aims to study the precipitation condition and the effect of secondary phase with volume fraction on corrosion behavior. Design/methodology/approach Secondary phase precipitated approximately from 375°C to 975°C because of saturated Cr and Mo at grain boundary by growth of austenite. Therefore, heat treatment from 800°C to 1,300°C was applied to start the precipitation of the secondary phase. Findings The secondary phase is precipitated at 1,020°C because of segregation by heterogeneous austenite. The growth of austenite at 1,000°C needs the time to saturate the Cr and Mo at grain boundary. When the volume fraction of austenite is 56 per cent (14 min at 1,000°C), the secondary phase is precipitated with grain boundary of austenite. The secondary phase increased the current density (corrosion rate) and decreased the passivation. That is checked to the critical pitting temperature (CPT) curves. The 1 per cent volume fraction of secondary phase decreased CPT to 60°C from 71°C. Research limitations/implications The precipitation of secondary phase not wants anyone. Casted super-duplex stainless steel (SDSS) of big size precipitates the secondary phase. This study worked the precipitation condition and the suppression conditions of secondary phase. Social implications Manufacturers need precipitation condition to make high-performance SDSS. Originality/value The corrosion resistance of SDSS is hard the optimization because SDSS is dual-phase stainless steel. The precipitation of the secondary phase must be controlled to optimize of the corrosion resistance of SDSS. Anyone not studied the precipitation condition of secondary phase and the effect of secondary phase with volume fraction on corrosion behavior of SDSS.

Volume fraction, Pitting resistance equivalent (PRE) as well as morphology of austenite on Duplex stainless steel can be changed through heat treatment by controlling annealing temperature and cooling rate. All specimens were evaluated for corrosion resistance according to shape of austenite phase after optimizing corrosion resistance as 5:5 volume fraction. The corrosion behavior was measured by the potentiodynamic polarization test and the pitting resistance was measured by Critical pitting temperature CPT. Austenite morphology increase corrosion current density on uniform corrosion without corrosion potential changing. The CPT of the specimen with the smallest grain boundary length of 2.3 μm/ μm 2 has the highest CPT as 76.3°C. Scanning Electron Microscopy showed that the pitting occurred in the grain boundary and propagated. It is confirmed that the reduction of the grain boundary length due to the control of the austenite morphology improve the pitting corrosion resistance of the UNS S 32750.