Factors affecting the performances of sprayed chromium carbide coatings for gas-cooled reactor heat exchangers

Abstract

Abstract This paper discusses some important factors to be considered for using sprayed coatings in gas-cooled reactor heat exchangers. These factors include (a) high temperature gaseous corrosion, (b) thermal stability of coatings, (c) metallurgical compatibility between the coating and the substrate and (d) effects of the coating on the mechanical properties of the substrate alloy. The wear properties of the coatings are also important but will not be discussed in this paper. The coatings evaluated were Cr3C2(NiCr) and Cr23C6(NiCr) applied by either the plasma arc or the detonation gun process. Two types of gaseous corrosion can cause coatings to spall from the substrate material in high temperature gas-cooled reactor (HTGR) helium environments. In the high temperature sections of heat exchangers, which are made of alloy 800H because of its excellent strength properties at elevated temperatures, preferential oxidation of the substrate alloy 800H at the coating-substrate interface can occur at temperatures above 649 °C (1200 °F). The lateral growth of interfacial oxides can eventually spall the coating from the substrate. In the low temperature section of the heat exchanger made of 2 1 4 Cr−1 Mo steel, the exposure of the coated steel to the HTGR helium in the temperature range 300–600 °C can lead to carbon deposition at the coating-substrate interface resulting in spallation of the coating. Furthermore, during high temperature exposure, structural changes of carbide phases in the coating, accompanied by volume changes, can further promote coating spallation. In HTGR helium environments, the structural change of chromium carbide is expected to occur in the following manner: Cr3C2→Cr7C3→Cr23C6 Accordingly, Cr23C6-based coatings are preferred to Cr3C2-based coatings for high temperature applications. A limited degree of interdiffusion was observed between the coating and the substrate after thermal aging. Interdiffusion occurred mainly between the Ni-Cr phase and the substrate. No Kirkendall-type voids were observed as a result of this interdiffusion.