An alloy is a material formed by mixing two or more elements to enhance specific properties like strength or corrosion resistance. Alloys are not immune to change, and they definitively change color over time. This discoloration is a naturally occurring process resulting from chemical reactions on the alloy’s exposed surface. The alloy’s components and the conditions it encounters determine the rate and type of color alteration observed.
Understanding Oxidation and Tarnishing
The most common process leading to color change in alloys is tarnishing, a mild form of surface corrosion. This phenomenon occurs when the alloy’s surface metals react slowly with gases in the atmosphere, primarily oxygen and sulfur compounds. This chemical interaction forms a very thin surface film, often called a patina, which is usually a metal oxide or metal sulfide.
In sterling silver, the copper component reacts with airborne sulfur to create silver sulfide, a black compound that causes dark tarnish. Copper-based alloys like brass and bronze develop a brownish or greenish-brown patina as the copper slowly oxidizes. While this patina dulls the original metallic luster, it often acts as a protective layer, slowing down further reaction of the underlying metal.
Corrosion Caused by Environmental Exposure
Corrosion differs from simple tarnishing as it involves a more aggressive, wet chemical reaction that can lead to a severe breakdown of the surface. This accelerated discoloration is often triggered by contact with moisture, salts, and acidic substances, such as those found in human sweat. Sweat contains chloride ions and lactic acid, which act as electrolytes, significantly speeding up the chemical dissolution of certain metals in the alloy.
Copper-containing alloys are particularly susceptible to reaction with sweat, leading to the formation of copper salts. These salts are typically blue or green and are responsible for the noticeable greenish-blue residue that can transfer onto the skin or clothing. If an alloy contains nickel, amino acids in sweat can cause the nickel to leach out and react, forming a dark, often black, compound on the surface. Unlike the slow patina of tarnish, this corrosion can cause visible discoloration quickly.
Color Changes from Worn Surface Coatings
Not all color change results from the underlying alloy reacting; sometimes, the surface coating is the culprit. Many affordable or fashion items are made from a base alloy covered with a thin layer of a more desirable metal, a process known as plating. Metals like gold or rhodium are frequently used for plating to give the item a bright finish.
This protective plating layer is subject to mechanical wear from abrasion and friction during everyday use. Activities like washing hands, rubbing against clothing, or contact with chemicals can cause the plating to thin out. Once the layer of gold or rhodium wears away, the underlying base alloy, often a copper or nickel mixture, is abruptly exposed. This physical loss of the surface coating results in a sudden color shift, such as a gold item revealing a silver or copper tone beneath.
Maintaining Alloy Appearance
Protecting an alloy’s appearance requires minimizing its exposure to the reactive elements that cause discoloration. Storing items in a cool, dry place, ideally in an airtight container or an anti-tarnish pouch, reduces contact with airborne sulfur and moisture. Avoid wearing alloy items while exercising, swimming, or using household cleaning products, as these activities introduce corrosive chemicals and high concentrations of salt.
After wearing, gently wiping the alloy with a soft, lint-free cloth removes any residual moisture or sweat before storage. For minor tarnish, a gentle polish with a specialized metal cleaner can restore the luster by removing the thin surface film. If discoloration is due to worn plating, the only effective solution is professional replating, which involves applying a new layer of the original metal coating.