Platinum Alloys
Platinum is the metal that does not react: it melts at 1,768 °C, does not oxidize at any temperature in air, dissolves only in aqua regia and a few hot mixed acids, and is inert in the body. It is dense (21.45 g/cm³), soft when pure (about 40 HV, 125–165 MPa tensile annealed) and very ductile. Rhodium, iridium, ruthenium and cobalt are alloyed in at 5–30 % to add hardness and creep strength: Pt90Rh10 is the glass-industry and thermocouple alloy, Pt90Ir10 the hard electrode and standards alloy, and Pt95Ru5 and Pt95Co5 the 950 jewelry alloys.
Popular platinum grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| Pt950 | 130 | 310 | 20.1 | Poor | Excellent | Platinum jewelry; 95 % hallmark standard |
| Pt99.95 | 45 | 140 | 21.45 | Poor | Excellent | Crucibles, electrodes, thermocouple negative leg |
| Pt95Ru5 | 175 | 380 | 20.67 | Poor | Very good | Machining and die-struck jewelry alloy |
| Pt95Co5 | 250 | 480 | 20 | Fair | Excellent | Casting alloy with the best mould fill |
| Pt90Ir10 | 210 | 380 | 21.54 | Poor | Very good | Pacemaker electrodes, marker bands, standards |
| Pt90Rh10 | 170 | 320 | 19.97 | Poor | Very good | Glass bushings, crucibles, Type S thermocouple |
| Pt87Rh13 | 190 | 380 | 19.6 | Fair | Very good | Type R thermocouple positive leg |
Choose platinum for
Laboratory crucibles, dishes and electrodes; glass-fibre bushings, stirrers and melting hardware; Type R, S and B thermocouples; catalyst gauze and electrochemical anodes; pacemaker, neurostimulator and catheter electrodes and radiopaque marker bands; and fine jewelry that must stay white and never wear through to another metal.
Look elsewhere when
The temperature stays below about 1,000 °C in air, where a nickel or cobalt alloy such as L605 survives at a fraction of the cost; the part is an anode or electrode that can be platinum-plated titanium instead of solid metal; the medium is a hot acid rather than a hot gas, where Pure Ta or Zr 702 resist as well; the jewelry buyer wants a lighter, cheaper white metal (Pd950); or the thermocouple runs above 1,700 °C, where a W-26Re couple takes over in vacuum.
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Platinum by application
- Laboratory and glass hardware
- Pt99.95 crucibles and dishes for analysis and for melting optical glass; Pt90Rh10 for bushings, stirrers and crucibles that carry load hot; Pt95Rh5 gauze as the catalyst in ammonia oxidation.
- Thermocouples
- Pt99.95 with Pt90Rh10 for Type S (to 1,600 °C), with Pt87Rh13 for Type R; Pt70Rh30/Pt94Rh6 for Type B to 1,700 °C with the best drift resistance above 1,200 °C.
- Medical electrodes and markers
- Pt90Ir10 and 90Pt-10Ir for pacing and neurostimulation electrodes, guidewire tips and marker bands — radiopaque, inert, hard enough to machine and crimp; 85Pt-15Ir where more stiffness is needed.
- Jewelry
- Pt950 in the Pt95Ru5 version for rings and settings that are machined or hand-made, Pt95Co5 for cast pieces, PtRu10 and Pt950/Pt900/Pt850 for the lower-fineness hallmarks used in some markets.
Working with platinum
- Machining
- Soft, tough and gummy; the ruthenium and iridium alloys machine best. Sharp polished carbide, high speed, light cuts and a light oil, with total chip recovery — platinum swarf is refined, never discarded. Parts are machined slightly oversize and hand-finished.
- Casting and forming
- Pt95Co5 is the casting alloy: induction-melted at about 1,750 °C in a zirconia crucible and centrifugally or vacuum-cast, since ordinary quartz investment and crucibles contaminate it. Pt99.95, Pt90Rh10 and the 950 alloys deep-draw, spin and roll like a stiff copper; anneal at 900–1,000 °C.
- Joining
- Platinum welds cleanly by TIG, laser and resistance welding without flux or filler, and the joint is as inert as the metal — the reason platinum jewelry is welded rather than soldered. Never heat platinum on steel, in contact with base metals or in a reducing flame: iron, lead, phosphorus, silicon and carbon embrittle it.
- Finishing
- Polishes to a bright white mirror that needs no plating and does not tarnish; a satin or brushed finish holds up well because platinum wears by displacement, not by loss of metal. Hallmarking and assay on request; every job is weighed in and out.
Other precious metal series
Platinum Alloys — frequently asked
What is platinum?
Platinum is the metal that does not react: it melts at 1,768 °C, does not oxidize at any temperature in air, dissolves only in aqua regia and a few hot mixed acids, and is inert in the body. It is dense (21.45 g/cm³), soft when pure (about 40 HV, 125–165 MPa tensile annealed) and very ductile. Rhodium, iridium, ruthenium and cobalt are alloyed in at 5–30 % to add hardness and creep strength: Pt90Rh10 is the glass-industry and thermocouple alloy, Pt90Ir10 the hard electrode and standards alloy, and Pt95Ru5 and Pt95Co5 the 950 jewelry alloys.
Which grade of platinum should I use?
Pt99.95 for laboratory crucibles, dishes, electrodes and the negative leg of Type R and S thermocouples — the pure metal, chosen for purity rather than strength. Pt90Rh10 when the same crucible or bushing must carry load at 1,400–1,600 °C: rhodium raises creep strength several-fold, and Pt90Rh10 is also the positive leg of Type S; Pt87Rh13 is the Type R leg and Pt70Rh30/Pt94Rh6 the Type B pair. Pt90Ir10 for pacemaker electrodes, marker bands, spark-plug tips and reference standards — about three times the hardness of pure platinum with the same inertness; 85Pt-15Ir where it must be harder still. Pt950 is the jewelry standard: Pt95Ru5 for machining, die-striking and hand-fabrication, Pt95Co5 for casting because it fills a mould best, Pt95W5 for springs and hard settings. Pt95Rh5 gauze is the ammonia-oxidation catalyst in nitric-acid plants.
Which grade of platinum is the strongest?
B541 has the highest yield strength in the series at 850 MPa; Pt70Rh30/Pt94Rh6 follows at 250 MPa. Values are for each grade's default condition.
Which grade of platinum is easiest to machine?
B541 (machinability fair, 42/100), then B522 Class II (40/100).
Which Pt950 alloy — ruthenium, cobalt or iridium?
Pt95Ru5 for anything machined, die-struck or hand-fabricated: it is the hardest of the three (about 130 HV), takes a crisp finish and is the European and US default for rings. Pt95Co5 for casting — the cobalt gives the best fluidity and mould fill, at the price of slight magnetism and a darker oxide during soldering. Pt95Ir5 is softer and is used where the piece will be formed by hand; 90Pt-10Ir is a 900 alloy that is harder still and once was the American jewelry standard.
Why platinum rather than nickel alloy for a hot part?
Because above about 1,100 °C in air nothing metallic except a platinum-group alloy stays clean and ductile. Nickel and cobalt alloys scale, creep and lose section; platinum and Pt90Rh10 sit in molten glass at 1,400–1,600 °C for months. Below 1,000 °C, or in a vacuum, the case for platinum disappears and L605, Inconel 601 or Pure Mo is the answer. Rhodium is added specifically for creep: Pt90Rh10 holds several times the load of pure platinum at 1,500 °C.