Tantalum is the most corrosion-resistant metal in commercial use, and that is essentially the only reason it is ever specified. A dense, self-healing Ta₂O₅ film makes it inert to almost every mineral and organic acid at industrial concentrations and temperatures — its behaviour is compared to glass rather than to other metals — so it is bought for duties that have already defeated stainless steel, the nickel alloys and often titanium and zirconium too. The five grades here divide on two axes: the unalloyed pair R05200 and R05400 differ only in production route and oxygen ceiling; R05252 and R05255 add tungsten for strength; and R05240 dilutes with niobium to cut density and cost. Exotic Alloy manufactures tantalum fasteners from material certified to ASTM B365, B708 and B521, with full interstitial analysis on every batch.
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This is the single most important practical fact about tantalum in service. Tantalum absorbs atomic hydrogen readily and becomes brittle as it does so — embrittlement is significant by around 100 ppm hydrogen, and the damage is permanent. Critically, tantalum does not need to be corroding to pick hydrogen up; it only needs to be cathodic. Couple it electrically to a less noble metal in an acidic electrolyte — a steel flange, a stainless bolt, a graphite gasket — and the tantalum becomes the cathode of a galvanic cell, atomic hydrogen is evolved on its surface, and it diffuses straight in. The part then cracks or blisters with no measurable wall loss and no warning.
For a fastener this is a design rule. A tantalum bolt clamping a less noble flange in acid service is precisely the configuration that embrittles. At least one mitigation is mandatory: electrical isolation with non-conducting gaskets, sleeves and washers so no galvanic circuit exists; anodic protection by impressed current, holding the tantalum anodic so no hydrogen is evolved on it; or an oxidising addition to the process stream — nitric acid, ferric or cupric ions — to depolarise the cathode reaction. Graphite is a particular trap: noble, conductive and common as a gasket material. Tell us the mating material and the electrolyte and we will advise on the isolation detail.
Four absolute exclusions apply to every grade, and no alloying addition changes them. Hydrofluoric acid attacks tantalum at any concentration or temperature — HF is in fact the reagent used to pickle it, and nitric-plus-hydrofluoric dissolves it outright. Any fluoride-bearing acidic solution behaves the same way, which matters because fluoride is usually a trace contaminant rather than a deliberate ingredient. Free sulfur trioxide — oleum and hot fuming sulfuric acid — attacks it, and plain concentrated sulfuric becomes aggressive above roughly 175–200 °C. Strong alkalis attack it even at room temperature; tantalum is an acid-service metal and must not be used on the alkaline side.
Tantalum cannot be used hot in air — a separate limit. Above roughly 260 °C tantalum oxidises rapidly in air or any oxidising atmosphere and the oxide is not protective; over the same range it also absorbs oxygen, nitrogen and hydrogen directly from the atmosphere, all of which embrittle it. Tantalum's 2996 °C melting point is therefore only usable under vacuum or inert gas, where it performs superbly to well over 2000 °C. For oxidising service above 260 °C the part must be coated, and coating life then governs the design.
There is no ASTM fastener specification for tantalum. ASTM F468 and F467, which govern nonferrous bolts, screws, studs and nuts, carry copper, nickel, aluminium and titanium alloys — tantalum is not among them. Tantalum fasteners are manufactured from material certified to the mill product standards (B365 rod and wire, B708 plate and sheet, B521 tube, B364 ingot) with thread conformance to the applicable ASME or ISO standard and acceptance criteria agreed on the order. Anyone quoting a tantalum bolt “to F468” is quoting something that does not exist.
Two things govern tantalum fabrication. The first is that it is exceptionally ductile with a low work-hardening rate — annealed unalloyed tantalum takes 50% elongation and 90% cold reduction without intermediate annealing — so Exotic Alloy forms rather than cuts wherever the geometry allows, and rolls threads rather than cutting them. The second is that tantalum getters oxygen, nitrogen and hydrogen avidly above about 300 °C, so every thermal operation runs under vacuum or high-purity inert gas. That is a requirement, not a preference: a weld or anneal made with inadequate shielding is embrittled whatever it looks like, and weld colour is the first-pass check — bright silver is acceptable, any straw, blue or grey tint is not. Machining is the hard part: tantalum smears, galls and tears rather than shearing, so sharp high-rake tooling, rigid setups and heavy chlorinated or sulphurised lubricant are all necessary, and swarf is collected and returned because the metal is far too valuable to scrap.
Mechanical values are ASTM B708 specified minimums for annealed sheet and strip under 1.52 mm (R05252: under 3.18 mm), not typical figures.
| Grade | Nominal Addition | Production Route | O max | UTS min | YS min | Elong. min | Chosen For |
|---|---|---|---|---|---|---|---|
| R05200 | None — unalloyed | EB or vacuum-arc melted | 0.015% | 207 MPa | 138 MPa | 20% | Benchmark acid resistance |
| R05240 | 35.0–42.0% Nb | EB or vacuum-arc melted | 0.020% | 241 MPa | 138 MPa | 25% | Lower density and cost |
| R05252 | 2.0–3.5% W | EB or vacuum-arc melted | 0.015% | 276 MPa | 207 MPa | 20% | Strength, corrosion resistance intact |
| R05255 | 9.0–11.0% W | EB or vacuum-arc melted | 0.015% | 482 MPa | 414 MPa | 15% | Structural and high-temperature strength |
| R05400 | None — unalloyed | Powder-metallurgy consolidation | 0.030% | 207 MPa | 138 MPa | 20% | Availability and cost, unalloyed |
Density runs 16.6 g/cm³ for the unalloyed grades and 16.9 g/cm³ for Ta-10W — roughly twice that of steel; R05240 is the exception at approximately 12.1 g/cm³, about 27% lower, which is a large part of why that grade exists. None of these alloys is hardenable by heat treatment: they are single-phase body-centred-cubic solid solutions, so strength comes from the tungsten addition and from cold work only. Note that R05240 is not corrosion-equivalent to unalloyed tantalum — niobium's oxide film is less robust and the alloy is measurably weaker in hot concentrated sulfuric and hydrochloric acid, so it must be qualified against the actual process stream rather than substituted on an assumption of equivalence. ASTM specifies no physical properties for R05240 or R05252; where density or melting point is a design input, request the certified mill data for the heat supplied.
We are committed to delivering zero-defect tantalum fasteners. All products undergo rigorous testing including PMI (Positive Material Identification), interstitial analysis for carbon, oxygen, nitrogen and hydrogen against the controlling ASTM limits, hydrogen-content verification, dimensional inspection and material traceability with EN 10204 3.1 certification. Material to ASTM F560 surgical implant limits is available on request.
Exotic Alloy exports tantalum fasteners and machined components to chemical processing, pharmaceutical, acid regeneration, vacuum furnace, aerospace and medical device clients worldwide. Our dedicated logistics team ensures secure export packaging and timely deliveries by sea and air freight with full customs documentation support.