Influence of irradiation on stainless steel corrosion in PWR primary conditions

Abstract

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.