Electrochemical methods were employed as nondestructive means of diagnosis of thermal aging embritlement of cast duplex stainless steels. Anodic polarization characteristics of unaged, laboratory-aged, and plant-aged specimens were measured in three different electrolytic solutions to detect degradation primarily due to spinodal decomposition in the ferrite phase. Microhardness measurements were made to study correlation with results from electrochemical diagnosis of the degradation.
In polarization experiments conducted in 5 wt% $CH_3COOH$ 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 microscopy of the specimens after polarization experiments to the potential($E_{peak}$) at which the peak current density $i_{peak}$ occurred. The reproducibility of polarization curves with use of this solution was poor.
Changes of the peak current density($i_{peak}$) corresponding to the formation of Cr-depleted zone due to $M_{23}C_6$ carbide precipitation at $\alpha/\gamma$ boundaries, could be observed by polarization experiments conducted in the mixture of 0.5M $H_2SO_4$ and 0.01M KSCN.
In polarization experiments conducted in the mixture of 0.05M $H_2SO_4$ and 0.01M KSCN, decrease of the first peak current density ($i_{peak}$) was observed. This appeared to be attributed to protection of the ferrite phase in duplex stainless steel(CF-8 grade) due to formation of the Cr-rich region by spinodal decomposition. Reproducibility of polarization curves with use of this solution was satisfactory. The decrease in $i_{peak}$ was correlated with the increase of microhardness due to thermal aging embrittlement. This correlation could explain the results of polarization and microhardness experiments with duplex stainless steel(CF-8 grade) specimens obtained from a primary coolant pump cover of a BWR plant after 12 years service at 284℃.
