Gold does not rust in water or air, a fact rooted in the element’s unique chemical stability. Rust is a specific form of corrosion that occurs only with iron and its alloys. Gold is classified as a noble metal, meaning it is highly resistant to chemical attack, including the oxidation process that causes degradation in most other metals.
The Chemical Process of Rust and Corrosion
Corrosion is the gradual breakdown of materials, typically metals, through chemical reactions with their environment. This process is electrochemical, involving the flow of electrons from the metal to an external substance. The metal is oxidized as it loses electrons, forming a new chemical compound on its surface. Rust is the common term for the corrosion of iron (Fe) or iron alloys like steel. The formation of rust, which is hydrated iron(III) oxide, requires the simultaneous presence of both oxygen and water. Water acts as an electrolyte, facilitating the transfer of electrons and accelerating the reaction. Rust is porous and flaky, failing to form a protective layer, which allows corrosion to continue deeper into the metal.
Gold’s Status as a Noble Metal
Gold’s exceptional resistance to oxidation and corrosion stems from its classification as a noble metal. Noble metals, including gold (Au), platinum, and palladium, are chemically inert; they do not readily react with oxygen, moisture, or most acids under normal conditions. Pure gold does not form a stable oxide layer because its atoms have a very low affinity for oxygen. Gold’s stability is explained by its unique atomic structure and high ionization energy. This energy barrier means that gold atoms do not easily give up their electrons to form chemical bonds with other elements, such as oxygen or sulfur. Gold is positioned near the bottom of the electrochemical series, confirming its reluctance to be oxidized.
Why Gold Alloys Tarnish
The common observation of gold jewelry losing its shine, known as tarnishing, occurs because most items are not pure 24-karat gold. Pure gold is too soft for practical use, so it is mixed with base metals like copper, silver, or zinc to create an alloy for increased hardness and durability. These alloying metals are much more chemically reactive than gold itself. It is these base metals within the alloy that react with environmental factors, such as oxygen, moisture, and sulfur compounds. The reaction forms a thin layer of silver sulfide or copper oxide on the surface, which appears as a dull, dark discoloration. Higher-karat gold, such as 18-karat (75% gold), is more resistant to tarnishing than lower-karat gold, like 10-karat, because it contains a smaller proportion of the reactive base metals.