Improvement of Corrosion Resistance of Stainless Steel Used in Nuclear Power Plant Cooling Water Components By Plasma Electrolysis

Abstract

In this study, Plasma Electrolytic Oxidation (PEO) technology was adapted to stainless steel for improving its corrosion resistance. Stainless steel is widely used in various fields due to its remarkable properties including high corrosion resistance, excellent weldability, good performance at all temperatures, etc. Although, in the case of nuclear power plants secondary cooling water components, which mainly utilize stainless steel, aggressive environments are situated created. Plenty of corrosive and abrasive species from the coolants attack the component materials to initiate a wide range of corrosion problems such as general corrosion, pitting corrosion, crevice corrosion, and Stress Corrosion Cracking (SCC), etc. In order to protect the substrate from those corrosive environments, a pre-oxidized layer was deposited through an electrochemical process. The technology used in this study, namely PEO, is an advanced surface engineering method that can produce a protective film having an excellent hardness, heat resistance, and adhesiveness with a compact layer. PEO, which is a developed version of the anodization method, utilizes relatively high voltage with a pulse to enhance the properties of the ceramic layer on the surface by heating and quenching repetitive processes. PEO on stainless steel was conducted by cathodic process and plasma discharges were generated through vapor gas envelope insulating film. After the experiment, a variety of characterizations were conducted. Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were firstly utilized to check the surface morphologies and compositions. Then using hot mounting and polishing, the cross-section of the sample was examined to check the oxide layer thickness and chemical composition by depth. By using an X-ray Diffractometer (XRD), the exact crystalline structure of the fabricated film was characterized. The following characterizations were mainly focused on electrochemical corrosion resistance tests. A typical three-electrode cell system was used to conduct the Potentiodynamic Polarization (PDP) technique for evaluating the improvement of general corrosion resistance. With corrosion potential and corrosion current density parameters from the PDP technique, pitting potential was also evaluated to check the pitting corrosion resistance. Lastly, Electrochemical Impedance Spectroscopy (EIS) method was conducted to figure out the corrosion resistance mechanism of the fabricated protective film and support the numerical data from the perspective of corrosion resistance improvement.PEO is a simple, fast, and relatively safe surface engineering method that can be applicated to various metal substrates. It is expected to improve diverse properties of the metal materials including corrosion resistance. It can be said that this technology can open a new era in material engineering in a wide range of industrial fields.