NACE Resource Center

Resource Library - (Materials Selection)

Information Module \| Mechanical properties \| Fabricability \| Availability \| Cost \| Weldability \| Corrosion resistance \| Information sources

Silver Corrosion

Silver (Chemical symbol Ag) is a brilliant gray white metal that is quite soft and malleable. It is quite ant resist to corrosion and does not oxidize easily, although it readily forms a surface tarnish of silver sulfide. Of all the metals, it is the best conductor of electricity. Due to these qualities (and its relative scarcity), it is often classified along with gold and platinum as a precious metal. Silver's primary use is in photographic paper and film 28% (commercial photography, medical, dental and industrial X-rays and graphic arts), jewellery and electronics 25% (connectors, contacts and batteries). The main source of silver is in lead ore, although it can also be found associated with copper, zinc and gold and produced as a by-product of base metal mining activities. Mexico, Peru and the USA are major producers of silver. There are many substitutes for silver in a variety of applications. Most importantly is the advance of digital imaging that no longer requires image development using silver related products. The use of stainless steel for ornamental tableware is replacing traditional silver. Aluminum and rhodium can be used instead of silver in mirrors and other reflective applications.

Electrical Contacts

Silver plating is widely used on contacts and other conductive parts in electrical apparatus such as switchgear and motor control centers because of the superior conductivity and longevity. The silver is found on the bus, in the circuit breaker, in protective relays, auxiliary relays, control switches, and test switches. The decomposition of the contact surfaces leads to an increase of the contact resistance and consequently to a rise in temperature, and eventually to the failure.

'Red Plague'

When silver is plated over copper there can be an accelerated corrosion of the copper, through galvanic action, at pinholes or breaks in the silver plating. It is then susceptible to the formation of cuprous oxide when stored or used in a moist or high humidity environment. This corrosion is known as "red plague" and is identifiable by the presence of a brown-red powder deposit on the exposed copper. The environment for this wire must be controlled.

Red plague is created by normally occurring (without the presence of a bias voltage) galvanic corrosion between silver and copper. Red plague may develop when porous silver plating allows moisture and oxygen to invade the silver-copper interface. Project quality personnel must impose proper requirements on procurement of silver-plated wire. Please check the procurement specification to ensure that wire being procured is not silver-plated. If it is silver-plated, then impose the following quality requirements:

1. Each lot of wire shall be traceable to the silver-plating bath;
2. Dry processing of insulation and dry dielectric testing is required;
3. Finished wire shall be shipped and stored with the ends capped to prevent diffusion of air and water vapor into the wire through open ends
4. Water quenching was not used. Only an oil quench, dry processing and sealing are to be used. Such sealing is to include end capping and/or desiccation as soon as possible.