Inter-diffusion between thermally sprayed Cr3C2-NiCr coatings and an Alloy 625 substrate during long-term exposure at 500 °C, 700 °C and 900 °C

Long-term operation of Cr3C2-NiCr thermal spray coatings at elevated temperatures, including service in oxidative and corrosive environments, leads to significant changes in the coating composition and microstructure. A critical, but often overlooked, aspect of high temperature service conditions is interdiffusion between the coating and substrate. This leads to changes in the coating/substrate compositions and microstructures with significant consequences in terms of the functionality of the coating. The aim of this work was to characterise the magnitude and mechanism of interdiffusion between a Cr3C2-NiCr coating and a Ni-based Alloy 625 substrate at 500-900 degrees C for up to 30 days. Interdiffusion was not observed at 500 degrees C, but occurred to a limited extent at 700 degrees C, and very intensively at 900 degrees C. The precipitation of M23C6 and M6C grains within the alloy grain boundaries was proposed to occur due to carbon diffusion from the coating into the substrate. It is proposed, that this carbon was liberated from the Cr3C2 grains in the coating due to formation of (Cr,Ni)(7)C-3. Chromium diffusion from the substrate into the coating resulted in the formation of a continuous oxide layer at the coating-substrate interface and the formation of a continuous (Cr,Ni)(7)C-3 layer in the coating adjacent to the interfacial oxide. Loss of Cr from the substrate led to the formation of a barren, precipitate free zone adjacent to the interface. (C) 2018 Elsevier B.V. All rights reserved.