Tungsten (W) is a metallic element known for its extreme properties, including the highest melting point of any metal and exceptional hardness. Pure tungsten metal does not rust in the traditional sense that iron does, where a flaky, red-brown oxide forms and continuously degrades the material. This distinction arises from the fundamental differences in how these elements react with oxygen and moisture.
Why Tungsten Does Not Rust
Rusting is the specific electrochemical oxidation of iron, occurring when the metal reacts with both oxygen and water. The resulting iron oxide is porous and flakes off, exposing fresh metal beneath to continue the degradation cycle. Tungsten, however, is a non-reactive element at room temperature and is highly resistant to water and most common acids and bases.
When pure tungsten is exposed to air, it immediately undergoes passivation. This involves the metal reacting with oxygen to form a thin, dense, and non-porous layer of tungsten trioxide (\(WO_3\)) on its surface. This protective oxide layer adheres tightly to the underlying metal, effectively sealing it off from the external environment. The stable \(WO_3\) layer prevents any further reaction, halting the corrosion process.
Conditions That Cause Tungsten Corrosion
While highly resistant under normal conditions, tungsten is not completely immune to chemical attack and can corrode under specific, harsh circumstances. The most common exception is high-temperature oxidation. If the metal is exposed to air at temperatures exceeding 400°C (752°F), the protective oxide layer is compromised, and the tungsten begins to oxidize rapidly.
Tungsten is also susceptible to attack by certain aggressive chemicals, typically at elevated temperatures. It is not resistant to a mixture of hydrofluoric and nitric acids, which can dissolve the passive oxide layer. Furthermore, molten salts and hot, concentrated nitric acid can also cause chemical degradation.
The Case of Tungsten Carbide
Many people encounter tungsten not as the pure element but as tungsten carbide (WC), a compound commonly used in cutting tools and jewelry. Tungsten carbide itself is a ceramic compound that is chemically inert and extremely hard. However, the tungsten carbide grains are typically held together by a metallic binder, usually cobalt or sometimes nickel.
The corrosion resistance of the finished product is determined by the vulnerability of this binder metal, not the carbide itself. When exposed to corrosive agents like strong acids, the binder metal can chemically dissolve in a process known as “binder leaching.” This leaching removes the metallic glue, leaving behind a porous, weakened skeleton of tungsten carbide grains. Although the carbide remains, the structural integrity of the composite material is lost, causing the item to appear degraded.