The Role of Displacement Damage in Irradiation Affected Corrosion of 316L Stainless Steel

Abstract

The objective of this research is to determine the effects of radiation-induced displacement damage on the corrosion of 316L stainless steel in high temperature water. The benchmark experimental approach was to transmit 5.4 MeV protons through 316L stainless steel and into an autoclave containing 320 °C water. The displacement damage rate is set to a nominal 7 × 10–7 dpa/s, and the resulting radiolysis-inducing dose rate is 650–700 kGy/s. To separate the effects of water radiolysis from displacement damage, a specialized sample geometry and steam environment were employed. 316L stainless steel bars were placed in the autoclave with a face parallel to the proton beam to receive exposure to similar levels of water radiolysis with negligible displacement damage. Steam at 480 °C with a low partial pressure of water yields a low water radiolysis with less than 1 kGy/s while keeping the 7 × 10–7 dpa/s damage rate. Irradiation-corrosion exposures of 316L were performed in both steam and water for 24 h and 72 h, and the different irradiated conditions were compared to oxide films from non-irradiated regions of the same sample. Oxide films were characterized by Raman spectroscopy, scanning electron microscopy, scanning transmission electron microscopy, and energy dispersive X-ray spectroscopy. The thermodynamics of the corrosion system in both water and steam was modelled to compare with experimental results and provide a basis for corrosion kinetics models. A new concept of excess volume was devised to explain the stability of inner oxide porosity in different environments. Porosity measured experimentally and excess volume were found to correlate well with corrosion rates observed; however, solid state diffusion through the oxide layer predicts a reverse trend between corrosion rate and oxygen activity from non-irradiated observations. The rate-limiting step for most corrosion in this work was found to be oxygen transport to the metal/oxide interface via pores. Radiolysis had the effect of increasing the system oxygen activity, reducing the corrosion rate, and causing inner oxide dissolution. The increased oxygen activity reduces excess volume which results in lower corrosion rates from decreased porosity despite the active dissolution of the protective oxide layer. In steam, radiolysis was found to have no effect on the corrosion rate because the quantity was reduced as designed. Displacement damage effects were found to be dependent on the environment. In radiolyzed water, displacement damage had no significant effect on the corrosion rate because stable pore growth was limited by the reduced excess volume. Because solid state diffusion was not rate-limiting, radiation enhanced diffusion had no measurable effect. In steam, however, displacement damage caused a large increase in the corrosion rate and a corresponding increase in inner oxide porosity. Therefore, the role of displacement damage is to increase the size of pores in the inner oxide layer, but only where pore formation is thermodynamically favored. Porous inner oxides are more permeable to oxygen which increases the corrosion rate through elevated oxygen penetration to the metal.