Nickel is a silvery-white transition metal, often valued for its strength, ductility, and ability to resist corrosion. The simple answer to whether nickel reacts with water is that under normal, everyday conditions, it does not undergo any significant chemical change. This stability is why the metal is so widely used in numerous industrial and consumer applications. The reason for this inertness lies not in nickel’s lack of reactivity, but in a thin, naturally occurring defensive shield that forms on its surface.
Nickel’s Stability in Liquid Water
The stability of nickel in neutral liquid water is due to a process called passivation. When nickel is exposed to air or water that contains dissolved oxygen, it instantly forms a microscopically thin layer of nickel oxide (\(\text{NiO}\)) on its surface. This layer is non-porous, extremely hard, and tightly adheres to the underlying metallic nickel.
This protective film acts as an impervious barrier, separating the bulk nickel metal from the surrounding water molecules and oxygen. The nickel oxide layer is chemically inert and prevents the water from reaching the reactive metal underneath, effectively halting any further oxidation or corrosion. Because the layer is tenacious and self-healing, it maintains the metal’s integrity in typical room-temperature environments, even when the water is slightly acidic or basic.
This mechanism differs from the flaking, bulky rust that forms on iron, which allows continuous exposure and degradation of the underlying metal. Nickel is insoluble in both cold and hot water under standard pressures. This resistance is maintained in most neutral or basic aqueous environments, preventing the metal from dissolving or releasing nickel ions into the solution.
Extreme Conditions Required for Nickel Reactivity
While nickel is highly resistant to water under normal conditions, its stability can be overcome in extreme environments involving high heat or aggressive chemicals. One such condition is exposure to superheated steam, which can force a reaction at elevated temperatures, typically above 400°C. In this high-energy environment, the water molecules possess enough thermal energy to react with the nickel, breaking down the passive layer and forming new compounds.
The reaction with steam produces nickel oxide (\(\text{NiO}\)) and releases hydrogen gas (\(\text{H}_2\)), demonstrating that a reaction is possible when the energy barrier is lowered by heat. This is a consideration in applications like steam power plants, where nickel alloys are specifically designed to manage these high-temperature, high-pressure water environments.
The protective oxide layer can also be compromised by highly acidic environments, which are chemically aggressive rather than thermally aggressive. Dilute strong acids, such as sulfuric or hydrochloric acid, can slowly dissolve the nickel metal. This process releases nickel ions (\(\text{Ni}^{2+}\)) into the solution while generating hydrogen gas.
Strong oxidizing acids, such as concentrated nitric acid, can initially reinforce the passive layer, but under certain conditions, they will dissolve the metal. This dissolution occurs because the acid can strip away the protective oxide and then readily react with the exposed nickel.
Why Nickel is Corrosion Resistant in Everyday Use
Nickel’s ability to resist corrosion makes it a widely adopted material. The metal’s durability and chemical inertness are leveraged in the manufacturing of various alloys, which are typically more resistant than pure nickel. For instance, nickel is a component in stainless steel, where it enhances resistance to rusting and pitting, especially in environments exposed to water or moisture.
The metal is commonly used in coinage, such as cupro-nickel alloys, which are selected for their longevity and resistance to the wear and tear of being handled and exposed to environmental moisture. In plumbing and hardware, nickel plating is often applied to brass or zinc fixtures to provide a bright, durable, and corrosion-free surface. This plating ensures that fittings maintain their appearance and structural integrity despite constant contact with water.
Furthermore, the corrosion resistance of nickel alloys is utilized in demanding industrial settings where contact with aggressive fluids is common. Components in the marine industry, like seawater handling equipment, and in chemical processing plants rely on nickel-based alloys to prevent degradation and ensure long-term operational reliability.