Corrosion behavior of niobium, tantalum and their alloys in boiling sulfuric acid solutions

The corrosion resistance of niobium, tantalum and Nb-20, 40, 60 and 80 wt% Ta alloys in boiling 20, 40, 60 and 80 wt% H₂SO₄ solutions is studied using the mass-loss technique. The corrosion rates of all materials increase with acid concentration and diminish initially with time. After nearly 200 h, the corrosion rates stabilize due to superficial oxide formation. The corrosion resistance increases with tantalum content. A preferential dissolution of niobium from the Nb-Ta alloys is noted. Recrystallization heat treatments reduce slightly the corrosion rates. The anodized materials are more corrosion resistant than the untreated ones during the first 200 h, but behave similarly for longer exposure times.