Influence of irradiation on stainless steel corrosion in PWR primary conditions

Some internal components of pressurized water reactor (PWR) are made on stainless steel. They are exposed to: a corrosive environment which is the primary medium that is to say water at high temperature (290°C-330°C) and high pressure (155 bars) with boric acid (H3BO3) , lithium hydroxide (LiOH) and dissolved hydrogen, a mechanical stress, an irradiation process due to neutron flux which can cause up to 1 displacement per atom (dpa) by year. In these conditions, these materials can be damaged by a process of IASCC (Irradiation Assisted Stress Corrosion Cracking). This degradation process has been observed since the eighties with the formation of cracks on baffle bolts under operating conditions. Oxide film rupture is the first step in this corrosion process and thereby the properties of the oxide formed on stainless steels are key parameters relative to SCC initiation. The morphology and the structure of the oxide formed on stainless steel in PWR primary medium have been detailed by several authors [1, 2, 3]. The oxide layer in divided in two parts: an internal and continuous layer rich in chromium and an external discontinuous layer composed of crystallites rich in iron. Moreover a nickel enrichment has been observed in the alloy under the oxide close to the alloy/oxide interface. In this alloy layer with a higher composition in nickel, a trapping of hydrogen has also been observed [4]. The aim of this work is to study the influence of defects created in the alloy by irradiation on the oxide properties (thickness, morphology, composition) and on oxygen transport in this oxide scale during exposition of stainless steel in PWR primary medium. In order to simulate the defects due to irradiation, ionic implantation have been performed. So samples of austenitic stainless steel (316L) have been implanted with Xenon ions at energy of 240 keV. Transmission Electron Microscopy (TEM) observations indicate an affected depth of about 80 nm with the formation of small cavities. The implanted samples have been oxidized at 325°C in hydrogenated primary water which simulates the primary medium in different devices, static autoclave and corrosion loop during different durations. The oxide film properties have been investigated by a set of techniques (SEM, TEM, SIMS) .As shown on Fig. 1, which presents implanted sample before and after 600 hours of oxidation, it has been observed a correlation between the affected area created by implantation and the morphology of the oxide formed after corrosion test. The composition of the internal layer is also linked to the defects created by implantation.

Some internal components of pressurized water reactor (PWR) are made on stainless steel.They are exposed to: -a corrosive environment which is the primary medium that is to say water at high temperature (290°C-330°C) and high pressure (155 bars) with boric acid (H 3 BO 3 ) , lithium hydroxide (LiOH) and dissolved hydrogen, -a mechanical stress, -an irradiation process due to neutron flux which can cause up to 1 displacement per atom (dpa) by year.In these conditions, these materials can be damaged by a process of IASCC (Irradiation Assisted Stress Corrosion Cracking).This degradation process has been observed since the eighties with the formation of cracks on baffle bolts under operating conditions.Oxide film rupture is the first step in this corrosion process and thereby the properties of the oxide formed on stainless steels are key parameters relative to SCC initiation.The morphology and the structure of the oxide formed on stainless steel in PWR primary medium have been detailed by several authors [1,2,3].The oxide layer in divided in two parts: an internal and continuous layer rich in chromium and an external discontinuous layer composed of crystallites rich in iron.Moreover a nickel enrichment has been observed in the alloy under the oxide close to the alloy/oxide interface.In this alloy layer with a higher composition in nickel, a trapping of hydrogen has also been observed [4].The aim of this work is to study the influence of defects created in the alloy by irradiation on the oxide properties (thickness, morphology, composition) and on oxygen transport in this oxide scale during exposition of stainless steel in PWR primary medium.In order to simulate the defects due to irradiation, ionic implantation have been performed.So samples of austenitic stainless steel (316L) have been implanted with Xenon ions at energy of 240 keV.Transmission Electron Microscopy (TEM) observations indicate an affected depth of about 80 nm with the formation of small cavities.The implanted samples have been oxidized at 325°C in hydrogenated primary water which simulates the primary medium in different devices, static autoclave and corrosion loop during different durations.The oxide film properties have been investigated by a set of techniques (SEM, TEM, SIMS) .As shown on Fig. 1, which presents implanted sample before and after 600 hours of oxidation, it has been observed a correlation between the affected area created by implantation and the morphology of the oxide formed after corrosion test.The composition of the internal layer is also linked to the defects created by implantation.Moreover the study of oxidation kinetic evaluated by ion beam analysis (NRA and RBS) have shown that the growth rate of the internal oxide layer follows a logarithmic-type law: after a 24 h exposure, the internal oxide reaches a thickness equivalent to the implanted affected depth which does not change much for longer exposition duration confirming the influence of the defect on the oxidation process.
Oxygen transport in the oxide layer has also been studied by two-stages corrosion experiments, a first stage in natural primary water (during 600h) followed by a second stage with water enriched in 18 O (during 16h).Comparison between TEM observation after implantation, EDX analysis after corrosion and SIMS analysis after these two corrosion sequences (Fig. 2) enable to link the distribution of defect created by implantation with the oxygen transport through the internal oxide layer formed during exposition in primary medium at 325°C.As a conclusion, this study revealed an effect of the defect distribution (generated by prior Xeimplantation) on the oxide formed on of 316L in primary medium, on the oxidation kinetic and also on the oxygen transport properties in the oxide layer.
Influence of irradiation on stainless steel corrosion in PWR primary conditions

Chemical composition
Austenitic stainless steel 316L (composition in wt.%)  Oxide layer formed on stainless steel in PWR primary medium 500 nm SS 316L oxidized 600h in primary medium

Fig. 1 :
Fig. 1: TEM images on implanted 316L samples before (a) and after (b) 600 h of oxidation

Fig 2 :
Fig 2: Superposition of TEM image showing the microstructure samples after implantation, concentration profiles of 18 O obtained by SIMS after two corrosion stages and O, Fe, Cr and Ni EDX concentration profiles obtained after 600 h of exposition to primary medium.

[ 1 ]𝐶 1 SS
T. Terachi, et al., Journal of Nuclear Science and Technology 45 (2008) 975-984.[2] S. Lozano-Perez, et al., Acta Materialia 57 (2009) 5361-5381.[3] S. Perrin, et al., Oxidation of Metals 80, (2013) 623-633 Crystallites of external layer rich in Fe (spinel) Nickel enrichment at the oxide/alloy interface Well agree with other studies Hydrogen enrichment at the alloy/oxide interface CEA -DEN 2 nd Int.MINOS Workshop -November 4-6, 2015, CEA -INSTN Cadarache, France 14 et al, Corrosion Science, 85 (2014) 251-257 Diffusion profile of 18 O in the internal oxide layer (anionic growth)  Calculation of diffusion coefficient : D o = (9 ± 1).10 -17 cm 2 .s-H.Lefaix-Jeuland, et al, Corrosion Science, 53 (2011) 3914-3922 [Per13] S. Perrin, et al., Oxidation of Metals 80, (2013) 623-633 [Dum14] M. Dumerval, thesis, (2014)  Link between chromium content in the oxide layer and the diffusion coefficient of oxygen Comparison with others values of O diffusion coefficient 325°C CEA -DEN 2 nd Int.MINOS Workshop -November 4-6, 2015, CEA -INSTN Cadarache, France 16 Oxide layer formed on implanted sample Internal continuous layer and external layer with cristallites  Both spinel structure  Internal layer divided in two area with different chromium composition 600h in primary medium 50 nm Xe-implanted sample before oxidation Xe-implanted sample after oxidation Oxide layer formed on implanted sample 600h in primary medium  The implanted affected depth corresponds to the thickness of the oxide formed after 600h of oxidation (about 90 nm).oxide thickness by NRA and RBS  After 24 h of exposure, the thickness of the internal layer is estimated at about 90 nm.The growth rate of the internal oxide layer is fast between 0 and 24 h and then strongly decreases after this first period → logarithmic-type kinetics law logarithmic-type kinetics law Good agreement with TEM observation Diffusion in the internal oxide layer Two oxidation sequences : 600 h H 2 O + 16 h with H 2 18 O  18 O diffusion regime does not correspond to a single diffusion profile  Different from profile obtained on non implanted sample  Two different profiles seem to be present Internal oxide layer