Electrochemical methods were employed as nondestructive means of evaluation of thermal aging embrittlement of cast duplex stainless steels. Potentiodynamic polarization experiments for duplex stainless steels of CF8 grade and Fe30Cr5Ni alloy steels aged at 370℃ and 400℃ for up to 7,000h were conducted in four different electrolytic solutions to detect degradation caused by spinodal decomposition in the ferrite phase. Microhardness measurements were also made to study correlation with results from electrochemical experiments.
In polarization experiments conducted in 5wt% $CH_3COOH$ solution, the anodic peak current density $i_{peak}$ increased, depending on aging time. Preferential dissolution of either the ferrite or the austenite phase, however, could not be determined by scanning electron microscope of the specimens after polarization experiments to the potential $E_{peak}$ at which the peak current density $i_{peak}$ occurred.
Changes of the peak current density $i_{peak}$ corresponding to the formation of Cr-depleted zone due to carbide precipitation at α/γ boundaries, could be observed by polarization experiments conducted in the mixture of 0.5M $H_2SO_4$ and 0.01M KSCN.
In polarization experiments for duplex stainless steel specimens conducted in the mixture of 0.05M $H_2SO_4$ and 0.01M KSCN and the mixture of $H_2SO_4$ and 0.005M KSCN, two current density peak, $i_{peak1}$ and $i_{peak2}$, appeared. $i_{peak1}$ was attributed to preferential dissolution of the ferrite phase and decreased by aging, while $i_{peak2}$ was caused by preferential dissolution of the austenite phase and was regardless of aging conditions. Scanning electron microscope and atomic force microscope observations indicated that the decrease of ferrite dissolution in aged specimens was the cause for the decrease of $i_{peak1}$. This appeared to be attributed to spinodal decomposition occurring in the ferrite phase that is the main cause of thermal aging embrittlement, from the consideration of results on polarization experiments for reannealed duplex stainless steel and aged Fe30Cr5Ni alloy specimens. A correlation was established between the increase of ferrite hardness and $i_{peak1}$ obtained with use of the mixture of 0.05M $H_2SO_4$ and 0.01M KSCN or the mixture of $H_2SO_4$ and 0.005M of pH=1.3, and this suggests that thermal aging embrittlement of duplex stainless steels of CF8 grade could be evaluated nondestructively by the present potentiodynamic polarization method.
