Transition Metal
Iridium
The rarest, hardest metal in the entire engine — used in a spot no bigger than a grain of sand on every spark plug.
- Atomic Number
- 77
- Atomic Mass
- 192.22 u
- Melting Point
- 2,446°C
- Density
- 22.56 g/cm³
Overview
Iridium is one of the rarest elements in Earth’s crust, and among the densest and most corrosion-resistant metals known. In an engine, it appears in the smallest possible quantity for the biggest possible reason to justify the cost: a tiny welded tip, often well under a millimeter across, on the center electrode of a spark plug.
Atomic Structure & Properties
Iridium’s electron configuration is [Xe] 4f¹⁴ 5d⁷ 6s², placing it deep in the third row of transition metals alongside platinum and osmium. Its extremely tight, tightly-bonded lattice is what gives iridium its combination of hardness, an exceptionally high melting point, and near-total resistance to chemical attack — it’s one of the only metals that essentially doesn’t corrode under normal conditions.
Face-Centered Cubic (FCC) Lattice
Iridium forms a face-centered cubic structure, the same family as aluminum and copper, but packed far more tightly and bonded far more strongly — a direct result of its position deep in the transition metal series. This is part of why iridium is both extremely hard and extremely dense for its atomic size.
Iridium’s resistance to oxidation and chemical attack even at extreme temperature is almost unmatched among metals. A spark plug tip experiences repeated electrical arcing, momentary combustion temperatures well over 1,000°C, and constant exposure to combustion byproducts — conditions that would erode most metals within a fraction of the mileage an iridium tip survives.
Why Engines Use Iridium
A spark plug’s electrode needs to survive thousands of tiny electrical discharges per minute, at extreme temperature, without eroding away — erosion widens the spark gap over time, which weakens and eventually kills the spark. Iridium’s hardness and heat tolerance let manufacturers shape the electrode into a much finer point than older nickel or platinum designs, which produces a more precise, more reliable spark and lets the plug go 120,000 miles between replacements instead of 30,000.
Where You’ll Find It
On the Toyota A25A-FKS 2.5L, iridium appears in the following parts:
- Spark Plugs Iridium-Tipped Center Electrode
As more engines are added to the site, every part using iridium will link back here.
Common Questions
Why use such a rare, expensive metal for something as small as a spark plug tip?
Because the amount actually used is tiny, often well under a milligram per plug, so the material cost stays reasonable even though iridium itself is expensive per ounce. The payoff is a spark plug that lasts roughly 3-4 times longer than a standard nickel plug, which more than offsets the higher unit price over the plug’s service life.
Is iridium rarer than gold?
Yes, significantly. Iridium is one of the rarest elements in Earth’s crust, rarer than gold or platinum, largely because most of it sank into the planet’s core during Earth’s early formation rather than staying in the accessible crust.
Why doesn’t iridium corrode like other metals?
Iridium’s electron structure creates unusually strong, stable metallic bonding, which makes it extremely resistant to chemical reactions with oxygen and acids that would corrode most other metals. It’s considered one of the most corrosion-resistant metals known, even at high temperatures.
See where Iridium sits on the Periodic Table
View all 118 elements and explore the ones used across every engine on this site.