Tungsten (element W) is one of the most robust materials known, defined by its high density, tensile strength, and the highest melting point of any pure metal at 3422 °C (6192 °F). These properties lead to its use in demanding applications like light bulb filaments, X-ray targets, and high-temperature furnace components. However, even this refractory metal is susceptible to degradation under specific, often extreme, chemical, thermal, and mechanical conditions. Understanding the environments that overcome its stability pinpoints the precise mechanisms that cause tungsten to fail.
Chemical Attack and Corrosion
Tungsten resists most common acids and bases at room temperature, surviving exposure to sulfuric, hydrochloric, and nitric acid mixtures. Chemical destruction requires highly energetic environments or specialized reagents that overcome the metal’s inherent passivity. This aggressive attack often involves strong oxidizing agents or highly concentrated alkaline solutions at elevated temperatures.
The most effective chemical destroyer is typically a mixture of nitric acid and hydrofluoric acid, which rapidly dissolves the metal. Molten salts, such as hot mixtures of potassium nitrate, nitrite, or chlorate, also aggressively attack tungsten. These salts act as powerful oxidizing fluxes, breaking down the metal structure.
A common mode of degradation is high-temperature oxidation in the presence of air or oxygen. Tungsten resists air exposure up to about 400 °C, but above this temperature, it reacts to form tungsten trioxide (\(\text{WO}_3\)). As temperature increases, the trioxide layer becomes non-protective and begins to sublime (or vaporize), becoming significant above \(1300\text{ }^\circ\text{C}\). This vaporization removes the metal layer by layer, causing rapid mass loss and structural failure.
Strong alkaline solutions also pose a threat, particularly as the temperature rises. At a pH greater than 6, tungsten’s corrosion resistance decreases because the metal or its oxide layer dissolves to form soluble tungstate anions. Furthermore, the halogen fluorine reacts with tungsten even at room temperature to form volatile tungsten hexafluoride. Other halogens require only slightly elevated temperatures (around \(250\text{ }^\circ\text{C}\)) to initiate a destructive reaction.
Thermal Destruction and Sublimation
The thermal stability of tungsten is defined by its melting point of \(3422\text{ }^\circ\text{C}\). Achieving this temperature requires immense energy input, typically only possible in highly specialized environments like electron beam furnaces or plasma torches. In these conditions, the metal lattice absorbs enough energy to transition into a liquid state.
Destruction often occurs not through melting, but through sublimation, where a solid turns directly into a gas. Although tungsten has the lowest vapor pressure of all metals, at extremely high temperatures in a vacuum or inert atmosphere, atoms gain enough thermal energy to escape the solid structure. This solid-state vaporization begins to become measurable around \(2600\text{ }^\circ\text{C}\) in a vacuum, causing the material to erode.
The rate of sublimation is directly proportional to temperature, leading to significant material loss in high-vacuum applications, such as incandescent light bulb filaments. This material loss directly degrades the component’s structure, even if the bulk material never reaches its melting temperature. This mechanism of destruction is purely thermal, relying on the energy of the environment.
Mechanical Fracture and Brittleness
While tungsten is strong and hard, it is not ductile, meaning it cannot be easily deformed without breaking. This lack of ductility makes the metal inherently brittle, particularly at lower temperatures, which is the primary mode of mechanical destruction. The transition from ductile behavior to brittle fracture occurs at a high ductile-to-brittle transition temperature (DBTT), often exceeding \(300\text{ }^\circ\text{C}\).
The brittle nature means that tungsten fails catastrophically by cracking and shattering instead of bending under stress. Failure is often initiated by pre-existing microstructural defects, such as impurities, which act as stress concentrators. When a high-impact force or sudden load is applied, the energy cannot be dissipated through plastic deformation, leading to rapid crack propagation and cleavage fracture.
Another significant mechanical failure mechanism is thermal shock, involving rapid and severe changes in temperature. Because tungsten has a relatively low coefficient of thermal expansion, sudden cooling or heating creates extreme internal stresses between the surface and the core. These differential stresses exceed the material’s limited ability to deform, causing surface cracks to initiate and quickly propagate, resulting in complete structural failure.