Corrosion of Ceramics in Aqueous Hydrofluoric Acid

A variety of commercially available ceramic-based oxides, carbides, nitrides, and borides were evaluated for chemical attack in an azeotropic aqueous hydrofluoric acid (HF) test protocol at 90°C. Weight change measurements and microstructure analysis showed that HF corrosion in polycrystalline ceramics generally occurred at grain boundaries by the dissolution of grain boundary phases although the bulk single crystal may inherently resist attack. Virtually all commercially prepared polycrystalline oxide ceramics (i.e., Al₂O₃, TiO₂, ZrO₂) and nonoxide ceramics (i.e., Si₃N₄, AlN, BN) were extensively corroded while polycrystalline pure carbides (i.e., SiC, TiC, B₄C, WC) resisted corrosion. Equilibrium thermodynamic calculations show that these materials are soluble in HF; however, the kinetics of dissolution are slow enough in some cases to permit useful engineering lifetimes.