The question of whether silver can rust in water stems from a misunderstanding of how metals degrade. Silver is a noble metal, meaning it exhibits high resistance to chemical attack and does not undergo the same corrosive process as iron. The familiar reddish-brown degradation known as rust is chemically specific to iron and its alloys. The discoloration observed on silver is a fundamentally different chemical reaction called tarnishing.
The Chemistry of Rust Versus Tarnish
Rust is the common term for the corrosion of iron (Fe), which involves an electrochemical reaction where iron metal oxidizes in the presence of both oxygen and water. This process results in the formation of hydrated iron(III) oxide, a flaky, reddish-brown substance. Water is necessary for the transfer of electrons, making it a prerequisite for iron to rust.
Silver (Ag) is far less reactive with oxygen and water. It does not possess the iron atoms required to form iron oxide, meaning the term “rust” cannot chemically apply to silver. Silver degradation occurs through a sulfidation process, not an oxidation reaction involving oxygen and water. This process results only in a surface film, unlike iron which can crumble away entirely.
What Causes Silver to Tarnish
The degradation seen on silver is tarnish, a chemical reaction involving airborne sulfur compounds, not oxygen. Silver reacts primarily with hydrogen sulfide gas, which is naturally present in the atmosphere at trace levels. This reaction forms a thin layer of silver sulfide, a dense black or dark brownish-gray compound.
Silver sulfide causes the characteristic darkening on the metal’s surface. Sulfides that accelerate this process come from various common sources. These include air pollution, rubber products, certain paints, wool, and some foods like eggs.
The Role of Water and Environment
While water alone does not directly cause silver to tarnish, it acts as an accelerator and carrier. High humidity or moisture provides a medium that dissolves and carries corrosive sulfur compounds to the silver surface. Water also acts as an electrolyte, which speeds up the chemical reaction between silver and sulfur-containing gases.
Exposure to water containing dissolved chemicals is particularly damaging to silver. Saltwater and chlorinated water, such as in swimming pools, contain ions that significantly accelerate tarnish formation. Furthermore, the copper alloy often mixed with silver in sterling silver is prone to oxidation when exposed to moisture, further contributing to discoloration.
Practical Steps for Silver Preservation
To minimize tarnishing, reduce the silver’s exposure to both sulfur compounds and moisture. Storing silver in a low-humidity environment, such as an airtight container or a jewelry box, slows the reaction rate. Specialized anti-tarnish strips or cloths contain chemicals designed to absorb sulfur gases from the air around the silver.
Avoid letting silver come into direct contact with sulfur-containing items like rubber bands, wool felt, or certain cardboard boxes. If silver does become tarnished, the silver sulfide film can be safely removed without harsh abrasives. Simple home methods, such as using aluminum foil and a baking soda solution, can reverse the sulfidation reaction and restore the surface.