Understanding Rust
Rust is a specific form of corrosion affecting iron and its alloys, such as steel. This chemical process, oxidation, occurs when iron reacts with oxygen and moisture. The reaction produces hydrated iron(III) oxides, recognized as rust, which has a reddish-brown, flaky appearance.
The chemical equation for rust formation is Fe + O₂ + H₂O → Fe₂O₃·nH₂O. This reaction is an electrochemical process where iron loses electrons to oxygen, facilitated by the presence of water acting as an electrolyte. Unlike rust, corrosion is a broader term encompassing the degradation of materials due to chemical or electrochemical reactions with their surroundings.
Gold’s Unique Properties
Gold stands apart from most other metals due to its chemical inertness. It is classified as a “noble metal,” a group characterized by their resistance to corrosion and oxidation in moist air. This inherent stability stems from gold’s electronic configuration and its position within the periodic table’s reactivity series.
Gold has a stable electron configuration, its outermost shells nearly full. This characteristic makes gold reluctant to lose or gain electrons, which are necessary for chemical reactions like oxidation. Consequently, gold exhibits very low reactivity, positioned near the bottom of the electrochemical series. It does not readily react with oxygen, sulfur, or most acids under normal atmospheric conditions.
This resistance to chemical degradation means pure gold does not form oxides or other compounds when exposed to air and water. The rarity of its chemical interactions contributes to its enduring value and historical use in coinage and jewelry.
What Appears as “Rust” on Gold
When a “gold” item exhibits discoloration resembling rust, it is not pure gold undergoing oxidation. Such phenomena are attributed to the presence of other metals alloyed with gold, or to gold plating. Gold is combined with base metals to enhance its hardness, durability, or alter its color.
These alloys, such as copper, silver, or nickel, can react with elements in the environment, leading to tarnish or corrosion. For instance, copper within a gold alloy can oxidize and darken, while silver can tarnish due to exposure to sulfur compounds. The karat rating of gold indicates its purity; 24-karat gold is pure, while 14-karat gold contains 14 parts gold and 10 parts other metals, making lower karat items more susceptible to discoloration.
Gold-plated items also show signs of corrosion, but this occurs on the underlying base metal rather than the gold itself. A thin layer of gold covers a less noble metal, like brass or copper, which can corrode if the gold layer wears away, chips, or is scratched. Surface contaminants, including dirt, oils, cosmetics, or chemical residues, can also accumulate on any gold item, creating a darkened appearance that might be mistaken for rust or tarnish.
Maintaining Gold Purity and Appearance
Proper care helps maintain the aesthetic appeal of gold and gold alloy items. Regular cleaning can remove accumulated dirt, oils, and residues that might dull the surface or react with alloyed metals. A soft cloth and mild, non-abrasive soap with warm water are effective for gentle cleaning.
Avoiding exposure to harsh chemicals, such as chlorine bleach, household cleaners, and certain cosmetics, is important as these substances can damage or discolor gold alloys. Chemicals can react with the non-gold components, leading to an undesirable appearance. Storing gold jewelry separately in soft pouches or lined jewelry boxes prevents scratches and minimizes exposure to air and humidity, which can accelerate tarnishing of alloys. Understanding the karat level of a gold item can inform care routines, as lower karat gold requires more diligent maintenance due to its higher proportion of reactive metals.