Effect of Hf on the Self-Healing Property of the Alumina Film on AFA Alloy in a Decoking Environment

Abstract

This study investigated the effect of Hf content on the self-healing behavior of Al₂O₃ films formed on alumina-forming austenitic (AFA) alloys under the decoking environment of petrochemical cracking process. AFA alloys with different addition of Hf (0–1 wt%) were subjected to pre-oxidation under low oxygen partial pressure to form the initial Al₂O₃ films. Following simulated mechanical damage, the samples underwent coking and decoking tests. The addition of Hf significantly changes the self-healing performance of the oxide film on the AFA alloy. The scratch regions of AFA alloys with 0, 0.05, and 0.25 wt% Hf are covered with undesirable Cr₂O₃ films. A dense and continuous Al₂O₃ self-healing film formed with 0.5 wt% Hf addition. Hf⁴⁺ segregates along Al₂O₃ grain boundaries, blocking cation diffusion channels, promoting formation of Al₂O₃ and refining grains. Nanoscale HfO₂ pegs at the film/substrate interface, enhancing film adhesion. This study provides key insights for designing self-healing, coke-resistant furnace tubes with long-term performance.