What is the mechanism by which hydrofluoric acid corrodes metals?

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The core mechanism by which hydrofluoric acid corrodes metals can be divided into two parts, and it differs significantly from the corrosion mechanism of ordinary strong acids: the basic displacement effect of hydrogen ions. Hydrofluoric acid ionizes to release H⁺ in aqueous solution, which then undergoes a displacement reaction with reactive metals such as iron, resulting in the formation of metal fluorides and hydrogen gas. For example, the reaction between iron and hydrofluoric acid is: Fe + 2 HF = FeF₂ + H₂↑. This is a fundamental corrosion process common to all acids. The core destructive effect of fluoride ions: Fluoride ions have an extremely small radius and high electronegativity, allowing them to adsorb preferentially on metal surfaces. They directly destroy the protective passivation film on metals such as stainless steel (e.g., Cr2O3), converting it into soluble metal-fluoride complexes. At the same time, they prevent the reformation of this passivation film on the metal surface, leaving the metal substrate continuously exposed to corrosive environments. This ultimately leads to rapid localized damage such as pitting and intergranular corrosion.