PtIR

PtIr, short for platinum–iridium alloy, is a binary alloy of platinum and iridium. The most widely used formulation is platinum–iridium with about 90 percent platinum and 10 percent iridium (Pt-10Ir). Other compositions have been used depending on the desired balance of hardness, oxidation resistance, and ductility.

PtIr combines the advantageous properties of both elements. It is very dense and has a high melting point, with improved hardness and creep resistance attributed to the iridium content. The alloy preserves platinum’s excellent corrosion resistance and chemical inertness, including resistance to many acids, making it suitable for high-purity and high-temperature applications. Iridium also helps reduce deformation under load, which is important for long-term stability.

Historically significant, PtIr played a central role in metrology as the material of the International Prototype Kilogram (IPK), cast in Pt-90Pt-10Ir. The IPK served as the standard mass from the late 19th century until the redefinition of the kilogram in 2019, which established the unit in terms of the Planck constant. Despite this redefinition, PtIr weights remain valued for calibration and reference standards due to their dimensional stability and low creep.

In addition to legacy metrology uses, PtIr is used in specialized laboratory weights, certain high-temperature crucibles, and components requiring durable, corrosion-resistant, high-temperature service. The alloy is produced by careful melting and alloying to achieve a uniform distribution of iridium within the platinum matrix.