An electrochemical method was employed as a nondestructive method of evaluation of thermal aging embrittlement of cast duplex stainless steels. Polarization characteristics of unaged and laboratory aged specimens were measured in three different electrolytic solutions to detect degradation primarily due to spinodal decomposition in the ferrite phase. Microhardness measurements made to study correlation with electrochemical parameters. For the case of CF8 specimens, a correlation was established with using the mixture solution of $H_2SO_4$ and 0.005M KSCN of pH=1.3. For the case of CF8M specimens, polarization curves similar to these for CF8 were obtained with using the mixture solution of $H_2SO_4$ and 0.005-0.01M KSCN of pH=0.95. The difference in peak current densities for aged CF8M specimens was slight and correlation between the difference and changed microhardness was poor, however, this seems to be attributed to the effect of Mo and low ferrite content really 6.6(%) in CF8M specimens. In polarization experiments conducted with using the pH 1.1 nitric acid for CF8M specimens, the increase of peak current density was observed in the cathodic polarization region. Selective dissolution of ferrite phase was observed in SEM micrographs of the specimens after the potentiostatic test for 30 minute at the peak potential and the potentiodynamic test up to the peak potential. The increase of peak current density may be mainly attributed to Cr depleted region developed by spinodal decomposition in ferrite phase. The increase of peak current density was correlated with the increase of microhardness due to thermal aging embrittlement. This indicates the possibility of diagnosis of thermal aging embrittlement of CF8M by use of this correlation.
