The pitting corrosion of three duplex steels (DSS), SAF2205, SAF2507, Z3CN20.09M thermally aged at 350 - 500 ℃ for 5500 h was investigated by the means of electrochemical techniques. The pitting potential (Ep) of thermally aged Z3CN20.09M specimens decreased with increasing of ageing temperature, however, the Ep of SAF2205 and SAF2507 thermally aged specimens decreased first and reached the lowest value at 450 ℃ and then increased slightly. The electrochemical impedance spectroscopy results are coincident with the potentiodynamic polarisation testing results. The variation in the charge transfer resistance of specimens coincided with the Ep values. There is little influence of low temperature (below 350 ℃) on resistance to pitting of SAF2205 and SAF2507 DSS. But, the pitting resistance of them degrades drastically ageing at higher temperature above 450 ℃. The deterioration in pitting resistance of thermally aged DSS specimens mainly attributes to the precipitation of Cr-rich α′ phases which affected by temperature markedly. At the range of 350 -500 ℃, the temperature the higher, the precipitation of Cr-rich α′ phases the more. The formation of G phases and the healing process during the long thermal ageing at 500 ℃ is the other reasons for the change in pitting resistance.
Keywords:
Duplex stainless steel; thermal ageing temperature; pitting corrosion; α′ phase; healing
