INVESTIGATION OF ALPHA-PRIME PRECIPITATION IN AGED DUPLEX STAINLESS-STEEL

A ferritic/austenitic duplex stainless steel was aged at 450-degrees-C for up to 5000 h. Electrochemical behavior of the aged steel was examined in 0.1 approximately 0.5M H2SO4 solution containing 0.01M KSCN by electrochemical potentiokinetic reactivation and scanning vibrating electrode techniques (SVET). An attempt was made to relate the electrochemical behavior to the Cr concentration in the Fe-rich region formed by aging, which was determined by a laser atom probe. In the early stage of aging, while neither reactivation nor change in activation behavior is observed, the passivation current increases as aging proceeds. This coincides with the development of Cr concentration fluctuation, possibly caused by spinodal decomposition of the ferrite. As the aging continues, the Cr-rich alpha'-phase precipitates and the Cr concentration in the Fe-rich region decreases. Then the reactivation behavior is observed. The reactivation ratio is obtained by deconvolution of the active dissolution peaks based on SVET measurement. Thus the obtained reactivation ratio is closely related to the Cr concentration in the Fe-rich region. Furthermore local variation in the electrochemical behavior is noticed even in the same ferrite grain by SVET. This suggests the dependence of progress of phase decomposition on the microscopic location in the grain, in addition to the nanometer scale fluctuation typical of spinodal decomposition.