Exploring Surface Modification of 316L Stainless Steel via Contact Glow Discharge Electrolysis for Nuclear Power Plant Applications

Abstract

In Light Water Nuclear Reactors (LWRs) zinc compounds are added to the coolant water because they are known to mitigate the undesirable radioactive build up on stainless steel surfaces by displacing cobalt radioisotopes in the oxide. This work investigated the possibility of modifying the surfaces of stainless steels, which are commonly used as structural materials in power plants, thus avoiding the need to modify the water chemistry in power plants. In this work, surface modification of stainless steel surfaces was explored by employing cathodic Contact Glow Discharge Electrolysis (CGDE). CGDE is a hybrid electrochemical technique that uses high potentials to create electronically excited species. When these excited species relax to their ground states, they release large amounts of energy which modifies conductive surfaces. The modified stainless steel cathode surfaces, and their cross sections, were characterised by SEM, EDS, GDOES and XRD. Results showed that it was possible to successfully modified the surface and to achieve surface zinc enrichment. Zinc enrichment of the stainless steel substrate was achieved up to 8 at% up to 4 μm in depth in localised areas of the surface. Zinc incorporation was observed to be the result of molten mixing of the stainless steel surface and zinc species from the electrolyte and this enrichment was not consistent with a diffusion mechanism. The impact of altering the electrolyte chemistry, the cathode geometry, and the electrolyte temperature on the electrochemical response of the investigated CGDE systems was assessed. Electrolytes with metallic additions produced thick and porous coatings with mixed chemistries on the stainless steel surfaces. The concentration of the electric field at edges led to these areas transitioning to CGDE behaviour before other sample regions. The modified stainless steels were also oxidised in a high temperature hydrogenated water environment, to evaluate their stability and performance in prototypical power plant coolant. The preliminary characterisation showed that the CGDE produced zinc coatings were not stable in this environment, resulting in a high release rate of zinc into the coolant water. The high zinc concentration in the coolant made multiple incorporation mechanisms into the oxides possible. However, the beneficial impact of zinc enrichment during oxide growth was inferred from the formation of a thinner oxide on the stainless steel unmodified control specimens that were oxidised in close proximity to zinc coated samples. Laser surface melting of zinc coated stainless steel specimens was also attempted, with the hope to increase the zinc inclusion depth. However, the laser power was excessively high and resulted in the removal of nearly all the zinc from the surface, probably by an evaporation mechanism, and lead to the production of a poor surface morphology. It was not possible to test whether these zinc enriched stainless steel surfaces are beneficial to retard the cobalt incorporation into its oxide. However, zinc metal and zinc oxide coatings may have other applications, for example as electrodes in solar cells.  

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Aleksey Yerokhin (Supervisor), Fabio Scenini (Supervisor) & Grace Burke (Supervisor)