A metal alloy is a uniform mixture of two or more elements, with at least one being a metal. Alloying enhances properties like strength, durability, and corrosion resistance that pure metals often lack. The answer to whether metal alloys tarnish is generally yes, but the extent depends entirely on the specific elements combined. Tarnish is a discoloration resulting from a chemical reaction between the metal surface and airborne compounds like oxygen or sulfur. This reaction forms a thin chemical film on the outermost layer, dulling the material’s natural luster.
Tarnish Versus Corrosion
Tarnish is a surface discoloration resulting from a thin film, often a metal oxide or sulfide, forming on the exterior. This chemical reaction is typically self-limiting, meaning the thin layer seals and protects the underlying bulk metal from further reaction. Tarnish is a superficial issue affecting appearance, not structural integrity. Corrosion, in contrast, involves a deeper chemical process leading to the deterioration and breakdown of the metal’s bulk material. A common example is rust on iron, which continues to flake away and expose new metal to the environment.
The Role of Non-Noble Metals in Alloys
An alloy’s tendency to tarnish relates directly to the chemical reactivity of its components. Noble metals, such as pure gold, platinum, and rhodium, are highly resistant to chemical change and do not tarnish under normal conditions. However, they are often mixed with more reactive “non-noble” metals like copper, zinc, and silver to add strength, change color, or reduce cost. When the alloy is exposed to the environment, these non-noble components act as the primary reactive sites. For instance, copper readily reacts with sulfur compounds in the air, forming copper sulfides or oxides that appear as the discolored tarnish layer while the bulk of the alloy remains intact.
Tarnishing Profiles of Common Alloys
Sterling Silver
Sterling silver is a well-known tarnishing alloy, typically composed of 92.5% silver and 7.5% copper. The copper component is the main culprit in the tarnishing process. The alloy reacts readily with hydrogen sulfide and other sulfur compounds in the air, forming a dark layer of silver sulfide on the surface. This sulfidation results in the characteristic black or dark brown discoloration seen on jewelry and silverware.
Brass and Bronze
Brass, an alloy of copper and zinc, and bronze, an alloy of copper and tin, both tarnish due to their high copper content. Copper reacts with oxygen and moisture to create copper oxides and carbonates. This reaction often produces a noticeable green or brownish-green film, commonly referred to as a patina or verdigris. The rate of tarnish can be accelerated by high humidity or exposure to pollutants.
Stainless Steel
Stainless steel alloys, which include iron, chromium, and nickel, are highly resistant to tarnish. This resistance comes from the chromium content, which immediately reacts with oxygen to form an invisible, thin layer of chromium oxide. This stable, self-repairing oxide layer is called a passive film. The passive film effectively shields the underlying metal from further chemical reaction, ensuring stainless steel exhibits minimal tarnish and is largely protected from deeper corrosion.
Gold Alloys
Pure 24-karat gold does not tarnish, but gold alloys (such as 10-karat or 14-karat) do because they contain less gold and more reactive filler metals. The extent of tarnish depends on the karat percentage, with lower karat pieces having a higher risk. These alloys frequently contain copper and silver, which react with sulfur and oxygen to cause discoloration. The tarnish is a result of the silver or copper components reacting, appearing as dark spots or a general dullness on the surface.
Slowing the Tarnish Process
Preventing tarnish involves creating barriers between the reactive metals in the alloy and the environmental compounds that cause the discoloration. One effective strategy is controlling the storage environment. Storing items in airtight containers, such as sealed plastic bags or specialty anti-tarnish pouches, significantly limits exposure to airborne sulfur and moisture. Placing a desiccant packet or anti-tarnish strip inside the container can further absorb any residual moisture or sulfur-containing gases.
Minimizing Chemical Contact
It is important to minimize contact with common household chemicals and body secretions. Sweat, perfumes, lotions, and cleaning products containing chlorine or ammonia accelerate the chemical reactions that cause tarnish. Wiping down metal items with a soft, dry cloth immediately after wearing them removes oils and moisture before a reaction can begin. For highly susceptible items, applying a thin, protective coating, such as a microcrystalline wax or a clear lacquer, creates a physical shield against the atmosphere.