Oxidation mechanism of T91 steel in liquid lead-bismuth eutectic: with consideration of internal oxidation

Abstract

Clarification of the microscopic events that occur during oxidation is of great importance for understanding and consequently controlling the oxidation process. In this study the oxidation product formed on T91 ferritic/martensitic steel in oxygen saturated liquid lead-bismuth eutectic (LBE) at 823 K was characterized at the nanoscale using focused-ion beam and transmission electron microscope. An internal oxidation zone (IOZ) under the duplex oxide scale has been confirmed and characterized systematically. Through the microscopic characterization of the IOZ and the inner oxide layer, the micron-scale and nano-scale diffusion of Cr during the oxidation in LBE has been determined for the first time. The micron-scale diffusion of Cr ensures the continuous advancement of IOZ and inner oxide layer, and nano-scale diffusion of Cr gives rise to the typical appearance of the IOZ. Finally, a refined oxidation mechanism including the internal oxidation and the transformation of IOZ to inner oxide layer is proposed based on the discussion. The proposed oxidation mechanism succeeds in bridging the gap between the existing models and experimental observations.

Understanding the surface oxide evolution of T91 ferritic-martensitic steel in supercritical water through advanced characterization

“…The oxidation behavior of F-M steels at elevated temperatures has been widely studied, and the results obtained in SCW are very similar to that in high-temperature steam, supercritical CO 2 , and liquid lead-bismuth eutectic (LBE) [10][11][12]. The typical oxide shows a duplex structure consisting of a defective magnetite outer layer and a protective spinel inner layer [13,14].…”

“…Bischoff [33] observed a thin Cr 2 O 3 layer at the metal-oxide interface in 9Cr oxide dispersion strengthened (ODS) steel and HCM12A steel after 4 weeks exposure to SCW at 500 and 600 ºC, but this thin Cr 2 O 3 layer did not show up after 2 weeks exposure in the 9Cr ODS steel. In comparison, Ye [10] observed a Cr-depletion zone instead of a Cr-rich oxide layer at the metal-oxide interface of T91 in LBE, and attributed it to the micron-scale diffusion of Cr from the matrix into the internal oxide layer. The occurrence of the thin Cr-rich oxide layer seems to depend on the environmental parameters, as well as exposure times.…”

“…Under SCW conditions, the resistance to environmental degradation of structural materials depends critically on the formation and long-term stability of protective surface oxide in contact with corrosive environments [9]. Regardless of the various advantages, the common issue of F-M steels is the severe oxidation at elevated temperatures [10]. Therefore, the corrosion process and related oxidation mechanisms under SCW condition must be fully understood in order to better evaluate and control the in-service behavior of F-M steels.…”

“…In addition to the duplex oxide structure, an internal oxide layer beneath the inner oxide has been frequently observed with selective oxidation along the grain/lath boundaries and nanometric Cr-rich oxide precipitates in the grain interior [10,[22][23][24][25][26][27][28][29][30]. The formation mechanism of the internal oxide layer is still unclear, and more efforts have been made to reveal the evolution of the internal oxide layer in recent years.…”

“…Bischoff et al [33] attributed the formation of nanometric Cr-rich oxide precipitates to the prior oxidation of Cr when the oxygen diffused inside the grains because of the negligible oxygen solubility in iron and high oxygen affinity of Cr. While Ye [10] proposed "internal oxidation" to explain the formation of nanometric Cr-rich oxide precipitates, where the presence of oxygen induces the short-range diffusion of Cr at the nanoscale to promote the formation of nano-clusters enriched in Cr [34,35]. The internal oxide layer locates at the metal-inner oxide layer interface and is the origin of the inner oxide layer, which makes it more critical in understanding the oxidation mechanisms.…”