Abstract
Cr-segregation by spinodal decomposition and G-phase precipitation were observed in δ-ferrite of austenitic stainless steel welds thermally aged at 400 °C for up to 20,000 h. A reversion heat treatment (R-HT) at 550 °C for 1 h dissolved the Cr-segregation in the aged welds while some intermetallic precipitates were present. The double-loop electrochemical potentiokinetic reactivation (DL-EPR) analysis showed no significant differences among them. However, after selective etching of the austenite phase, the DL-EPR values of δ-ferrite phase steadily increased with aging time due to the growth of Cr-depleted regions by spinodal decomposition. The electrochemical behavior of δ-ferrite after R-HT condition was similar to that of unaged welds, indicating the intermetallic precipitates did not affect the corrosion resistance in this case. Overall, DL-EPR analysis of δ-ferrite phase provided better correlation with spinodal decomposition.
Highlights
Austenitic stainless steels are widely used as coolant pipes in light water reactors and pressurizer surgeline pipes in pressurized water reactors (PWR), which are typically joined by fusion welding methods
The thermal aging during operation of light water reactors has been known to be related to phase separation, where the δ-ferrite decomposes into nano-sized Fe-rich α and Cr- rich α’ phases[1,2,3,4,5]
A reversion heat treatment (R-HT) can be applied to recover the mechanical properties degraded due to phase separation by dissolving it, while the residual intermetallic precipitates are left in the δ-ferrite microstructure[9,14,15,16]
Summary
Austenitic stainless steels are widely used as coolant pipes in light water reactors and pressurizer surgeline pipes in pressurized water reactors (PWR), which are typically joined by fusion welding methods. The ferrite remains in the passivation regime at the point where austenite is electrochemically active at higher electrochemical potentials In such cases, the electrochemical parameters for studying the thermal aging were taken from the region where austenite is electrochemically active, compromising the electrochemical responses from the ferrite phase where the nano-scaled phase separation has occurred. Park et al.[21] used micro-capillary technique while Örnek et al.[34] used scanning Kelvin probe force microscopy to study the electrochemical behavior ferrite phase in aged duplex stainless steels. These techniques would not be applicable for conventional electrochemical analysis of ferrite phase in aged stainless steels
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