# Thermomechanical Treatment-Enabled Short-Circuit Diffusion Enhances Molten-Carbonate Corrosion Resistance of an Alumina-Forming Austenitic Alloy

## TL;DR

A thermomechanical treatment improves the corrosion resistance of an alumina-forming alloy in molten carbonates by enhancing aluminum diffusion.

## Contribution

A novel thermomechanical treatment strategy is introduced to enhance short-circuit diffusion and Al oxidation in molten-carbonate environments.

## Key findings

- Annealing at 800 °C after cold rolling significantly reduces corrosion rates compared to 1000 °C.
- Ultrafine grains and NiAl precipitates enable rapid formation of a protective LiAlO2/Al2O3 layer.
- High-temperature microstructures lead to non-protective oxide scales due to insufficient Al flux.

## Abstract

Developing stable alumina-based scales is critical for alumina-forming austenitic (AFA) alloys exposed to highly basic molten carbonates. However, the inherently sluggish diffusion of Al in austenite often limits the establishment of continuous protective layers. Herein, a thermomechanical treatment (TMT) strategy is proposed to enhance short-circuit diffusion pathways and promote selective Al oxidation in a Li–Na–K carbonate melt at 700 °C. After 90% cold rolling, annealing at 800 °C and 1000 °C generated two distinct microstructural states characterized by different grain boundary types, dislocation densities, and NiAl precipitate populations. The 800 °C-annealed alloy exhibits a significantly lower steady-state corrosion rate (~62 μm/yr) compared with the coarse-grained 1000 °C counterpart. EBSD and TEM analyses reveal that ultrafine grains, abundant low-angle boundaries, and finely dispersed NiAl precipitates provide efficient fast-diffusion channels and local Al reservoirs, enabling rapid formation of a continuous LiAlO2/Al2O3 inner layer. In contrast, insufficient Al flux in the 1000 °C microstructure results in extensive internal oxidation and growth of a thick, non-protective LiFeO2/NiO scale. These findings demonstrate that controlling the defect and grain-boundary structure via TMT is an effective route to overcome Al diffusion limitations and improve the molten-carbonate corrosion resistance of AFA alloys.