Tungsten (W) is a dense, silver-gray metal known for its exceptional properties, including the highest melting point of any element at 3,410°C. This refractory nature makes it a popular material in industrial applications like heating elements and wear-resistant tools. Tungsten has also gained popularity in jewelry, often as tungsten carbide, due to its hardness and durability. This article explores the chemical reasons behind tungsten’s behavior when exposed to water and whether it is susceptible to tarnishing.
Understanding Surface Reactions
Tarnishing is a form of surface corrosion that results in the loss of a metal’s luster, typically creating a dull, darkened film. This change occurs when the metal undergoes a chemical reaction with airborne substances, such as oxygen or sulfur compounds, or with chemicals dissolved in water. The reaction forms a thin surface layer, such as an oxide or a sulfide, which does not penetrate deeply into the bulk material.
Common metals like silver and copper tarnish readily by reacting with sulfur in the atmosphere to form black surface compounds. Corrosion is a broader term describing the gradual deterioration of a metal due to a reaction with its environment, often leading to structural breakdown.
Tungsten’s Stability Under Normal Conditions
Pure tungsten metal displays chemical inertness, meaning it is largely non-reactive with water, air, or oxygen at ambient temperatures. This stability is a direct consequence of its high melting point and strong atomic bonds, which resist the energy required to initiate a chemical change. Pure tungsten will not tarnish or corrode when exposed to standard water sources, including tap water, swimming pool water, or seawater.
The metal’s stability is enhanced by a process called passivation, which occurs naturally upon exposure to air. Tungsten spontaneously forms a thin layer of tungsten trioxide (\(\text{WO}_3\)) on its surface. This oxide layer acts as a protective shield, effectively blocking water and oxygen molecules from reaching the underlying bulk metal.
This protective \(\text{WO}_3\) film is highly insoluble and chemically stable in neutral and acidic aqueous environments, covering the range of most everyday water exposure. This stability allows a tungsten item, such as a ring, to retain its polished appearance and luster even with continuous daily wear involving contact with moisture. High-quality jewelry crafted from tungsten carbide inherits this same resistance to surface change.
When Tungsten’s Surface Might Change
While pure tungsten is stable, two primary factors can lead to a change in its surface appearance, neither of which is true tarnishing by water. The first involves exposure to chemically extreme conditions that are far outside the scope of daily life. Tungsten can be attacked and dissolved by highly aggressive chemical mixtures, such as a combination of concentrated nitric and hydrofluoric acids, which are used in specialized industrial cleaning.
Tungsten will also undergo significant oxidation, or corrosion, if exposed to very high heat, typically above 600°C. At these “red-hot” temperatures, the protective oxide layer breaks down, and the metal reacts readily with oxygen or even water vapor to form a visible, non-protective oxide layer. This condition is irrelevant for jewelry or standard industrial use, but demonstrates the metal’s theoretical limit.
Binding Agents and Residue
The most common reason a consumer might believe their tungsten item is tarnishing is due to the presence of a binding agent in the alloy. Lower-quality or industrial-grade tungsten carbide often uses cobalt as a binder, a cheaper metal that is susceptible to oxidation and reaction with skin oils and moisture. When the cobalt binder reacts, it can cause a dark discoloration that appears like tarnish, but this is the cobalt reacting, not the tungsten. High-quality, jewelry-grade tungsten carbide avoids this issue by using nickel as a binder, which is chemically inert and does not react with water or skin oils. Additionally, a perceived dullness or surface change is sometimes simply the buildup of residue, such as soap scum, lotion, or dirt, which can be easily cleaned off and is not a permanent chemical alteration of the metal itself.