High-Temperature Oxidation of Pyrolytic Graphite in Simulated Steam Ingress Conditions: Structure, Kinetics and Mechanism

Abstract

The reliability and robustness of high-density graphite-made components in pyrochemical reprocessing are vulnerable to oxidative degradation during accidental ingress of water or steam into the containment chamber of the pyro reprocessing facility. The present research aims to investigate the steam oxidation kinetics and associated oxidation mechanisms of chemical vapor-deposited pyrolytic graphite (PyG) in simulated steam environments using thermogravimetric analysis (TGA). PyG specimens synthesized at temperatures ranging from 1800 to 2400 °C were subjected to oxidation in a steam-rich H₂O–Ar atmosphere containing approximately 68 vol% H₂O and 32 vol% Ar, with Ar serving as the inert carrier and purge gas. Weight loss percentage and oxidation onset temperatures were derived by dynamic TGA from 200 to 1200 °C, and normalized linear oxidation rate constants were calculated by performing isotherm TGA experiments at temperatures of 1000, 1100, and 1200 °C. The PyG deposited at a higher pyrolysis temperature of 2400 °C, demonstrated the highest oxidation onset point of 1090 °C and the lowest weight-normalized linear rate constant of 2.498 × 10^− 4 and 0.267 × 10^− 2 mg min^− 1 mg^− 1 at the 1000 and 1200 °C isotherms, respectively. Apparent Arrhenius relationships were constructed from the isothermal rate constants, while SEM was used to characterize the associated steam-gasification morphology without assigning a definitive transition in the rate-controlling mechanism. The results provide intrinsic steam-gasification kinetics and mechanisms for free-standing CVD PyG under steam-rich conditions, creating a comparative database to guide future assessments of PyG-coated components for accident-tolerant nuclear applications.