Research on the corrosion resistance of porous coatings for heat transfer enhancement on the head of nuclear reactor pressure vessels

The preparation of a porous coating on the outer surface of nuclear power reactor pressure vessel’s lower head is considered an effective measure to enhance heat transfer and ensure safety under the condition of In-Vessel Retention (IVR) during severe accidents. However, oxidation and corrosion of the porous coating are inevitable, as it will be exposed to the marine atmosphere during the vessel’s construction and operation phases. Therefore, it is necessary to strengthen its corrosion resistance under the premise of high critical heat flux (CHF). In this paper, based on the successful development of high CHF porous coatings, a composite coating consisting of a Ni-Cr bottom layer and a 316L stainless steel porous coating was designed and prepared by flame spraying on SA-508 Gr.3 steel. The corrosion resistance of the composite coating was evaluated by Neutral Salt Spray Test (NSS) and the corrosion mechanism of the coating was analyzed. The results showed that the Ni-Cr bottom coating exhibited no obvious corrosion after 240 h of the NSS test, but the corrosion resistance of the porous coating was decreased due to surface oxidation, chromium deficiency in the particle melting zone and the presence of ferrite. By incorporating corrosion-resistant alloying elements into 316L powders, the corrosion resistance of the coating can be significantly improved while preserving its porosity. The investigation of porous composite coatings for reactor pressure vessels is a critical endeavor with the potential to significantly enhance the performance, safety, and longevity of nuclear reactors.