The volume fraction of ferrite and austenite phase and corrosion resistance of 26.2Cr-6.99Ni-2.37Mo-2.88W-0.35N duplex stainless steel have been studied by the use of optical microscopy, Feritscope, EBSD, SEM and Anodic polarization test after solution treatment at every 20 C at 1050 C$1200 C for 30 min. As the temperature of solution heat treatment increased, the content of ferrite that could be transformed to intermetallics such as and phase was higher, and besides, grain size increased and the number of grains decreased due to the growth of the phase and thus grain boundary and phase boundary that can be served as precipitation site of intermetallics were reduced, therefore the precipitation of intermetallics was suppressed.When PRE (Pitting Resistance Equivalent) values for the ferrite and austenite phases after solution heat treatment were calculated by weight percents of alloy elements using SEM-EDS, PRE value gap between the two phases was the smallest when heat treated at 1090 C. Because the increase over 1090 C of heat treatment temperature caused larger difference in PRE values between two phases, corrosion resistance between the two phases was out of balance and thus corrosion resistance was reduced when solution heat treated at the temperature over 1090 C.

Selective dissolution of hyper duplex stainless steel was studied by potentiodynamic and potentiostatic test in various concentrations of H 2 SO 4 /HCl solutions at various temperatures. There were two peaks in the active-to-passive transition region in potentiodynamic test in 2 M H 2 SO 4 + 0.5 M HCl solution at 60 C. In potentiostatic tests, the curve at À340 mV showed stable current density. As the potential increased, the current density increased and at above À310 mV potential, there was a much longer initial period of nonsteady current value. As the potential reached at À280 mV, the current density started to be stabilized and the current density was completely stabilized at À250 mV. It was found that a preferential dissolution of ferrite phase occurred at À330 mV and with the increase of potential, austenite phase was corroded at a high rate. On the other hand, both two phases were passivated at the potential above À270 mV, so that selective dissolution was absent.