When these four acid-resistant materials—tantalum, zirconium, titanium, and Hastelloy—are placed in the same pot of strong acid, which one will be corroded first?

Thread Content

Corrosion rate data is the first hurdle in material selection. Based on the ASTM G31 immersion corrosion testing standard and publicly available data from material suppliers, this issue provides a systematic comparison of the actual performance of tantalum, zirconium, titanium, and nickel-based alloys in the three major industrial acids: hydrochloric acid, sulfuric acid, and nitric acid. Corrosion rate: Why this figure is more important than the material’s name. Engineers make a common mistake when selecting materials: they remember the list of media for which the material is suitable, but forget the actual corrosion rate in those media. A material that \"can be used in hydrochloric acid\" and \"corrodes at 0.05 mm per year in hydrochloric acid\" versus \"corrodes at 2 mm per year\" represent three completely different engineering realities. The industry-standard ratings for corrosion rates (referencing the NACE and ASM Handbook) are as follows: Hydrochloric acid environment: The largest differences are observed here. Hydrochloric acid (HCl) is one of the most important inorganic acids in the chemical industry, and it is also one of the most aggressive corrosive agents when it comes to the selection of metal materials. Hydrochloric acid is a reducing acid, and its corrosion mechanism is completely different from that of oxidizing acids (such as nitric acid), which directly leads to significant differences in the behavior of various materials. Titanium alloy (Grade 2) is well-known in the industry for its resistance to seawater and nitric acid, but it has a little-known weakness: it offers almost no resistance to hydrochloric acid. The ASM Handbook on Corrosion (Volume 13) states clearly: \"Titanium is readily attacked by reducing acids such as hydrochloric acid.\" Even with a 10% hydrochloric acid solution at room temperature, the corrosion rate of Titanium Grade 2 exceeds the level considered acceptable for engineering purposes. Source: ASM Handbook, Vol. 13: Corrosion. ASM International. Corrosion rates of various materials at 20% HCl and 60°C (mm/year) — data from ASTM G31 immersion testing. Source: Haynes International’s technical report “Hastelloy C-276 Corrosion-Resistant Alloy” ； ATI Metals titanium alloy corrosion resistance data ； Allegheny Technologies zinc alloy data ； ASM Handbook Vol. 13. Interpretation: Both zirconium and tantalum perform well in hydrochloric acid – what is the difference? Zirconium Zr-702 indeed exhibits excellent performance in hydrochloric acid at moderate temperatures (≤100°C), with a corrosion rate of less than 0.1 mm/year. However, zirconium has a significant weakness: in hydrochloric acid solutions containing oxidizing ions such as Fe³+ and Cu²+, the corrosion rate of zirconium increases sharply. Industrial hydrochloric acid (especially steel pickling waste liquid) often contains such metal ions, in which case the actual performance of zirconium is significantly worse than the laboratory data indicates. Tantalum exhibits broader stability in such mixed media. Sulfuric acid environment: concentration and temperature. Sulfuric acid is the most widely produced industrial acid in the world, and it is also the corrosion medium with the most complex requirements regarding material selection – the corrosion mechanisms vary significantly depending on the concentration; the behavior of the same material can differ greatly when exposed to 20% sulfuric acid versus 80% sulfuric acid. Source: Haynes International Technical Data Manual ； B. Vyas, “Corrosion Behavior of Zirconium,” Materials & Corrosion ； ATI Technical Data ； ASM Handbook Vol.13. The “stall zone” of zinc sulfate is a key risk point. Zirconium Zr-702 performs well in dilute sulfuric acid, but its corrosion rate increases nonlinearly as the concentration and temperature rise – under conditions of 50% H₂SO4 at 100°C, zirconium begins to corrode significantly and is no longer suitable for long-term reliable use. Tantalum maintains its corrosion-resistant properties across the entire concentration-temperature range up to 100°C. Fluoride-containing media (HF): Not suitable for tantalum either; both are eroded by HF. Limits of applicability for tantalum: Hydrofluoric acid (HF) and fluoride-ion-containing media: Tantalum is rapidly dissolved in these substances, so their use is prohibited at any concentration, including mixed acids containing trace amounts of HF. Hot, concentrated strong alkalis (NaOH/KOH >10%, >60°C): Alkaline solutions erode the Ta₂O₅ protective layer. Fuming sulfuric acid (containing free SO₃): A specific risk assessment is required. Next issue preview: Part 2 – Comparison of hydrogen embrittlement and stress corrosion cracking risks. Statement: This article was first published on [HanTantalum Instrumentation Tubes and Valves]