Tarnishing is a discoloration of the surface, often black in color, and this is due to a corrosion
reaction. General corrosion is not a major concern for precious metals, but low-karat golds and silver do often undergo tarnishing. Other alloys, such as 21K and 22K golds, have been shown to tarnish, but this is less common.
The tarnishing layer is often an oxide or sulfide caused by a chemical reaction between the metal alloy and the environment.
Causes of Tarnishing
Tarnishing requires the presence of sulfur, moisture and oxygen, and often chlorine. Sulfur is normally present in the atmosphere, typically as sulfur dioxide, and particularly in industrially polluted atmospheres.
In consumer environments, sulfur-containing substances include:
Human perspiration – sulfurous compounds and sodium chloride (salt) will be present in human sweat and saliva. These exacerbate any corrosion.
Food – Onions, eggs, fruit juices, pickles, and spices contain a lot of sulfur. The kitchen is not safe for tarnish-prone jewelry.
Perfumes and deodorants, household cleaners, etc.: Many sprays contain sulfurous propellants and active agents.
Jewelry storage – Glues and linings to boxes often contain sulfur compounds which can slowly leach or evaporate. Many papers are bleached in chlorine.
Microporosity in cast jewelry – Surface microporosity may trap pickling solutions, many based on sulfuric acid, during final finishing. During use, they may absorb perspiration, perfumes, etc., leading to localized tarnishing, which can spread.
In general, tarnishing is unsightly but is not a hazard to health, although it can result in black smudging of the skin, which is undesirable but not harmful.
Tarnishing reaction in silver
For both sterling and other fineness silvers, tarnishing is an accepted problem. It is caused primarily by sulfur-based corrosion reactions but can also occur in chloride-rich environments. The presence of moisture is also important.
The corrosion mechanism is complex but is based on the reaction:
$\rm 2Ag + H_2S + \frac{1}{2}O_2 \to Ag_2S + H_2O$
Silver sulfide, which is black, is thermodynamically more stable than silver oxide.
Copper in the silver alloy can also be involved, and we should note that copper oxides are black or red.
The alloy microstructure and composition play a role in such corrosion reactions. Where two phases are present, or there is chemical segregation in the casting, electrolytic cells can be set up on the surface in the presence of a corrosive environment, and corrosion is accelerated.
Tarnish-resistant Silver
There are many new Sterling and other silver alloys on the market that claim improved tarnish resistance. Most are based on modified silver-copper alloys with minor alloying elements added, such as germanium and silicon. These generally act to form transparent oxides on the surface in preference to the black silver-copper sulfide-based tarnish layers. There are also a number of efforts to add gold and platinum-group metals to improve the tarnish resistance and strength.
Ultimately, all silver will tarnish, we can only delay the onset and lower the rate of tarnish, and different alloys will perform better in different environments.
Tarnishing in Low-Karat Gold
For the karat golds, the silver and copper content are involved in the tarnishing reaction since gold is the noblest of all metals and does not corrode. The areas with lower gold content will corrode preferentially in two-phase alloys, so single-phase alloys are preferred.
To prevent tarnishing in low-karat gold alloys, there are some general, broad rules:
Tarnishing occurs more readily as the gold content is reduced.
Tarnishing occurs more readily as the silver/copper ratio increases at the same gold content. In two-phase alloys, the silver-rich phase tarnishes preferentially.
Multi-phase alloys tarnish more readily than single-phase alloys of the same gold content. Special attention should be paid to heat treatments.
Zinc additions generally improve tarnish resistance, often through stabilizing a single-phase structure. However, in the absence of silver, high-zinc low-karat golds are more susceptible to tarnishing.
The susceptibility of alloys to tarnishing will also depend on the severity of the corrosive environment. If severe enough, even high-karat gold can tarnish!
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