What unique anti-corrosion features do tantalum and niobium share?

Tantalum and niobium spontaneously form dense, stable oxide films on their surface once exposed to air or liquid media, which effectively isolate the substrate from corrosive substances. They remain intact in hydrofluoric acid, sulfuric acid, hydrochloric acid and mixed acid solutions under normal and medium-high temperature conditions, outperforming stainless steel and most nickel-based corrosion-resistant alloys. Only concentrated hydrofluoric acid or alkaline molten salts can slowly erode them. This outstanding chemical inertness makes them irreplaceable lining materials for high-corrosion chemical reactors and heat exchangers.