Gold is the most non-reactive of all metals and is benign in all natural and industrial environments. Gold never reacts with oxygen (one of the most active elements), which means it will not rust or tarnish. Yet this is precisely what Russian commercial precious metal trading company, International Reserve Payment System, discovered on thousands of (allegedly) 999 gold coins "St George" issued by the Central Russian Bank (reference).
Gold tarnish is usually very thin and shows up as a darkening of reflecting surfaces. Gold has a many unique properties that make it the perfect metal for many industrial uses. Some notable properties of gold are: resistance to corrosion, electrical conductivity, ductility (the extent it can be deformed plastically without fracture), malleability (how well it can be flattened into thin sheets without cracking), infrared reflectivity, and thermal conductivity. For these reasons, gold is used in many electronic products such as computers, high end technical equipment, spacecraft, and satellites (reference).
Gold is among the most electrically conductive of all metals. Since electricity is basically the flow of charged particles in a current, metals that are conductive allow this current to flow unimpeded. Gold is able to convey even a tiny electrical current in temperatures varying from -55° to +200° centigrade. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook.
Gold finishes, if pore-free, protect the substrate metal from corrosion. If the finish is not pore-free, the underlying metal is exposed to the environment and so is more likely to corrode. For example, the mercury gilding process, which is based on applying a mercury-gold amalgam to a metal followed by heating to remove the mercury, produces a porous gold layer. The metal corrosion rate through pores in the gold is often accelerated relative to the rate in the absence of a gold layer. This is a galvanic effect caused by the noble (cathodic) gold layer in contact with an active (anodic) substrate. Two common substrate metals are copper (Cu) and silver (Ag).
Under atmospheric conditions, copper and silver react with reduced sulfur-containing gases, mainly hydrogen sulfide (H2S) and carbonyl sulfide (COS), to form copper sulfide (Cu2S) and silver sulfide (Ag2S), respectively. These sulfides grow through pores and produce dark spots that can spread over the gold surface. Copper sulfide spreads at a slower rate than silver sulfide. Copper is also susceptible to attack by volatile organic acids such as acetic acid. Under burial conditions, gilded objects can be exposed to water containing dissolved oxygen, carbon dioxide, and chlorides. Such conditions result in chlorargyrite (AgCl, also called cerargyrite or horn silver) forming on exposed silver, and cuprite (Cu2O), nantokite (CuCl), and basic copper compounds (e.g., malachite, Cu2(CO3)(OH)2) forming on exposed copper. Nantokite plays a key role in bronze disease (reference).
Possible causes include: (reference)
- Perspiration (everyone's body chemistry is different, hence this is why some are more susceptible than others); for women, the time of the month can influence their body chemistry.
- Perfume, hair or deodorant sprays,
- Tarnishing during storage (storage boxes may contain organic sulfur compounds),
- Leaching of acid/ cleaning solutions from surface microporosity from cast jewelry; this causes corrosion locally (such porosity may even trap perspiration during wear, causing local corrosion)
- Preparation of vegetables such as onions and spices (many foodstuffs contain
sulfur compounds and others are also acidic).
Another possible mechanism may be surface micro-porosity on the surface of investment (lost wax) cast items. This porosity may trap acids and other cleaning solutions, sprays, or perspiration and cause a local corrosion which 'creeps' over the surface of the item. The tarnish films formed are generally harmless although unsightly and may lead to a black smudging of the skin. Such films can be easily polished off by a jeweler to restore the bright gold color.
The wire bonding in some microelectronic applications can promote the formation of gold-aluminum intermetallics. The two most common forms of intermetallics go by the sinister-sounding names of white plague and purple plague. During the high-temperature environment of the wire bonding process, gold components and aluminum components can “fuse” (similar to an alloy) to create intermetallics. White plague (described chemically as Au5Al2) has low electrical conductivity. If enough of it forms, the resulting electrical resistance can cause a total failure of the component.
Purple plague (described chemically as AuAl2) is actually used in jewelry, but it’s problematic when it appears in electronics. As purple plague forms, it reduces in volume. This creates cavities in the metal surrounding the purple plague, which increases electrical resistance and structurally weakens the wire bonding. To avoid introducing intermetallics into circuitry, gold and aluminum components must be bonded together without using heat. Ultrasonic welding (similar, but unrelated to ultrasonic assaying) is a common choice. Without it, circuitry must be designed using only aluminum-to-aluminum or gold-to-gold junctions (reference).