Rhodium, a rare and silvery-white metal, does not rust. It is classified as a Platinum Group Metal (PGM) and a noble metal, placing it among elements that resist chemical degradation. Its primary application involves electroplating other metals, such as white gold and sterling silver, to enhance their durability and provide a brilliant, tarnish-resistant finish. This high stability makes it valued in applications ranging from automotive catalytic converters to high-end electronics.
Understanding Rust Versus Oxidation
The question of whether rhodium rusts requires a clear distinction between “rust” and “oxidation.” Rust is a specific form of corrosion that exclusively affects iron and its alloys, such as steel. It results from iron reacting with oxygen and water to form hydrated iron(III) oxide, the flaky, reddish-brown substance.
Oxidation is a broader chemical reaction where a substance loses electrons, often involving oxygen. While all rusting is a form of oxidation, not all oxidation is rust. Rhodium is highly resistant to this process under normal atmospheric conditions, meaning it does not readily tarnish.
Why Rhodium Resists Corrosion
Rhodium’s resistance to corrosion is rooted in its identity as a noble metal and a member of the Platinum Group Metals. These classifications indicate low chemical reactivity. Its stable electronic configuration and strong metallic bonds make it chemically inert under standard environmental exposure. This inertness means rhodium does not readily react with common corrosive agents like oxygen and moisture at room temperature.
This stability allows rhodium to maintain its reflective surface without forming an oxide layer. When used as plating, it protects the underlying metal, significantly extending the lifespan of the coated item, which is beneficial in high-demand environments like industrial equipment.
What Can Affect Rhodium
While rhodium is extremely stable, it can be forced to react under specific, extreme conditions. The most common way to make rhodium oxidize is by exposing it to extremely high temperatures. The metal remains unaffected by air and water up to approximately 600°C (1,112°F), but above this point, it can begin to form rhodium(III) oxide.
This high-temperature oxidation is typically only relevant in specialized industrial or laboratory settings. Rhodium is also exceptionally resistant to most common acids, including nitric acid and aqua regia, especially below 100°C. However, the metal will react with certain powerful chemical agents, such as molten potassium bisulfate or concentrated sulfuric acid.