Effect of Si-Addition in Stainless Steel on Surface Oxidation in High Temperature Water Environments
Authors: Ma Jiarong , Jia Yibo , Ling Sichun , Xiong Qi , Lu Zhanpeng 
Published: February 2020

Being used as the internal components of pressurized water reactor (PWR), austenitic stainless steels (SSs) are susceptible to irradiation-assisted stress corrosion cracking (IASCC) that is a critical issue for the long term safe operation. Radiation induced segregation (RIS) at grain boundaries in austenitic SSs, such as the depletion of Cr and enrichment of Ni and Si, is thought to contribute to IASCC. Studying the oxidation behavior of the model alloys simulating grain boundary RIS in high temperature water could provide evidence for the effect of RIS on IASCC. In this study, the model alloys 9Cr-25Ni-1.5Mo-5Si and 9Cr-25Ni-1.5Mo-3Si were used to simulate the depletion of Cr and enrichment of Ni and Si at grain boundaries of irradiated 316L SS. The oxide film properties of 316L SS and model alloys in of high temperature water environments were investigated. After the immersion in high-temperature water environments, the surface morphologies of the oxide films formed on the immersed specimens were examined using JEOL JSM-7500F field emission scanning electron microscope (FESEM). The oxide films were analyzed using INVIA laser Raman spectrometer with laser wavelength of 514.5 nm. The compositions of oxide particles were analyzed by the energy-dispersive spectrometry (EDS) attached to the FESEM.
Fig.1 shows the SEM morphologies of the 316L SS, 9Cr-25Ni-1.5Mo-3Si and 9Cr-25Ni-1.5Mo-5Si alloy surfaces after immersion in pure water at 290 oC (pH at 290 oC of about 5.6) for 120h. The surfaces of all the test specimens were covered with polyhedral particles of different sizes. Larger size outer layer oxides are formed on 9Cr-25Ni-1.5Mo-3Si and 9Cr-25Ni-1.5Mo-5Si specimen surface. On the surface of 316L SS, there were only sparsely distributed large-size particles. Si addition in the alloy matrix favors the formation of the oxide layer in this environment.
Fig. 2 shows the SEM morphologies of 316L SS, 9Cr-25Ni-1.5Mo-5Si and 9Cr-25Ni-1.5Mo-3Si alloys surfaces after the immersion in high temperature water of pH 9.5 (pH adjusted with monoethanolamine) at 290 oC for 24h, 48h and 120h. Compact and homogeneous oxide particles were formed on the surfaces of 316L SS and model alloys. The size of the oxide particles on model alloys was smaller than that on 316L SS. The oxide particle size of 9Cr-25Ni-1.5Mo-5Si and 9Cr-25Ni-1.5Mo-3Si was similar. The morphologies of particles became more stereoscopic with increasing immersion time. The outer layer oxide particles formed on the 316L SS and model alloys decreased with increasing solution pH value. The pH value has a significant influence on the oxidation of 316L SS and model alloys. These results show that the effect of Si in the alloy on oxide film properties is strongly dependent on the water chemistry. Si in the alloy facilitated the formation of large-sized outer layer in 290 oC water of pH 5.6, which refined the outer layer oxide particles in 290 oC water of pH 9.5. High solubility of Si-bearing oxide in high temperature water can modify the oxide film on the surface. These results show that Si in the alloy would play a role in IASCC through the modification of oxide film.

Figure 1