Basic Environmental Testing Procedures Part 2: Tests Guidance to Test Kd: Hydrogen Sulphide Test for Contacts and Connections
|Publication Date:||1 January 1982|
|ICS Code (Environmental testing):||19.040|
Object and scope of the test
Types of contacts und connections
As this test is specifically intended for certain types of contacts and connections (other than those of the welded or soldered type), a short description of these types of contacts and connections is considered to be useful.
Contacts and connections may be divided into two types and could be described as permanent or temporary. In both cases, metal surfaces are held together by an external force.
In the case of permanent connections, the force is very great and will usually cause permanent deformation of the metals and it is possible that a form of local welding takes place. Such connections are not intended to be made and broken during their lifetime. Examples of permanent connections are crimp and wrap joints.
With temporary connections, the force holding the metals in contact is by comparison light and they are of course designed to be made and broken possibly very many times during their lifetime. Examples of temporary connections are : connectors, switches and relays. In temporary connections the areas of metal which make contact with each other are in some cases referred to as contacts.
The contacts or contact areas in temporary connections will be made of various metals according to duty and application. Most metals-with the exception of precious metals-suffer from atmospheric corrosion. When contact materials corrode, contact resistance increases. The extensive use of precious metal contacts would be costly, so it is common in many applications to use precious metal alloys or coatings of precious metal or alloys over base metals for contact materials.
In the case of permanent joints, it is not normal to use precious metals and some general corrosion of external surfaces by hydrogen sulphide must be expected. But, in a properly. designed and made crimp or wire wrap joint, corrosion does not occur between the contact surfaces due to the cold weld and high pressure. However, in joints that are poorly made or weakened as a result of thermal cycling, corrosive gas will penetrate into those contact areas with a resultant increase in contact resistance.
Object of the test
The test has been devised to assess the consequence of tarnishing of silver and some of its alloys. It has been largely validated by laboratory and field tests on silver, though limited tests have also been carried out on components with contacts of some silver alloys.
When the same tarnish test is appropriate to test specimens containing different contact materials, then the test conditions may give rise to different accelerations (see Clause 5), and considerable experience and experimentation may be required to assess relative results. Contact alloys of silver and palladium represent a case where the test is appropriate but where such care is required.
When the test is used for contacts and connections involving silver alloys and silver in conjunction with other materials the test is expected to give more realistic results for those involving silver together with a more precious metal than it will for silver alloys (or structures) involving significant quantities of base metals. The following cases are given by way of example :
- Gold contacts are largely unaffected by the test.
- Contacts involving gold layers over silver or gold contacts in the close proximity of silver will be affected owing to the phenomena of creeping silver sulphide. Both these effects are true reflections of field behaviour.
- Copper and alloys containing a high percentage of copper (such as phosphor bronze or brass) are heavily tarnished in the test atmosphere owing to the formation of copper sulphides. However, this type of tarnishing rarely predominates in practice, where oxide formation inhibits sulphide growth. Hence the test is inappropriate in these cases, if the tarnishing behaviour is required to resemble that occurring naturally.