Theoretical passivation limits of binary aluminum-lithium alloys in oxygen and fluorine environments

Abstract

The theoretical passivation of binary aluminum-lithium alloys (0–100 wt% Li) was evaluated using a Pilling-Bedworth ratio analysis with a lithium diffusion model under various oxygen and fluorine environments near room temperature. Passivation was found to be dynamic, governed by atmospheric composition, surface reaction kinetics, alloy microstructure, and internal lithium diffusion. Water vapor reactions forming hydrates are particularly detrimental to passivation in both oxygen and fluorine environments. These theoretical calculations suggest that alloys with low lithium content (< 5 wt%) may remain passivated in oxygen-containing atmospheres for extended periods near room temperature. Experimental validation of these passivation models was conducted by reacting both passivated and un-passivated Al-Li alloy powders with water to measure hydrogen gas evolution. Results revealed a sharp increase in reactivity near 5 wt% Li, consistent with a percolation threshold of increased δ’- and δ-phase grain connectivity. Below this threshold, passivated alloys consumed < 10 % of their active metal content. Above it, passivation was largely ineffective, and nearly all the active metal content was consumed. These findings confirm that passivation strategies using ambient air are only effective below a critical lithium concentration and suggest fluorine-based or similar treatments may be required to protect high-Li-content alloys.