Pure tungsten, a highly dense and refractory metal, does not rust or tarnish under normal ambient conditions. Rust is the oxidation of iron, a process impossible in pure tungsten. Tungsten and its alloys are prized for their exceptional resistance to chemical degradation. Confusion about its durability often stems from the behavior of binding agents used in commercial products.
Inherent Stability of Tungsten Metal
The elemental nature of tungsten (W) provides it with remarkable chemical robustness. Tungsten is classified as a refractory metal because it possesses the highest melting point of any known element, approximately 3422°C. This property is linked to the strong covalent bonds between its atoms, which also contribute to its extremely high density.
At standard room temperature and pressure, pure tungsten is chemically non-reactive and remains inert when exposed to water, oxygen, and most common acids and bases. While tungsten can oxidize, this reaction requires significantly high temperatures, generally beginning around 400°C and becoming substantial above 600°C. Under these extreme conditions, the metal reacts with oxygen to form tungsten trioxide (WO3).
The tungsten trioxide that forms on the metal surface is dense and stable, unlike flaky iron oxide. This oxide layer acts as a passivation barrier, effectively shielding the underlying tungsten metal from further oxidation. This self-limiting protective layer is why pure tungsten maintains its integrity in environments that would rapidly degrade other metals.
Defining Chemical Degradation
The terms “rust” and “tarnish” are technically inaccurate when applied to tungsten because they describe specific chemical reactions the metal does not undergo. Rust is strictly defined as the electrochemical corrosion of iron, resulting in the formation of hydrated iron(III) oxides. Since tungsten contains no iron, the process of rusting is chemically impossible.
Tarnish refers to a thin film of corrosion that forms on the surface of more reactive metals, such as silver or copper, usually involving sulfur compounds. This process typically results in a dull or darkened surface layer. Tungsten’s highly stable surface layer prevents this kind of surface reaction under normal circumstances. The element’s high chemical stability means it does not readily form the reactive surface compounds associated with tarnishing.
Binder Breakdown in Tungsten Alloys
The material commonly sold as “tungsten” for consumer products like jewelry is actually tungsten carbide (WC), a ceramic compound. Since tungsten carbide powder is extremely hard but brittle, a metallic binder must be added to hold the particles together and provide toughness. The choice of this binder metal determines the product’s long-term corrosion resistance and is responsible for most perceived degradation.
The most common binder used in lower-cost tungsten carbide is cobalt. While cobalt provides excellent strength, it is susceptible to chemical attack from moisture, skin oils, cleaning agents, and strong acids. When the cobalt binder reacts with these substances, it can dissolve or leach out from the alloy structure. This leaching leaves microscopic pits and causes discoloration that appears like pseudo-rust or tarnish, weakening the surface.
Higher-quality tungsten carbide alloys substitute nickel for cobalt as the binder metal. Nickel-binder tungsten carbide offers significantly superior corrosion resistance, making it the preferred choice for applications in chemically harsh environments, such as marine or industrial settings. The nickel forms a more stable oxide film than cobalt, which better protects the underlying alloy structure from corrosive agents.
Consumers can prevent the surface degradation of cobalt-bound products by removing them before exposure to aggressive chemicals, such as household bleach or strong hand soaps.