Reliability Research on Tungsten Contacts

1. The dark brown gum on tungsten contacts surface is --- H₂WO₄ (which can be dissolved in water, its aqueous solution is gray, after dewatering, it would turn into brown oxide dura --- WO₃) is a compound which is generated by electrochemical corrosion under high temperatures and high humidity conditions.
2. The root causes of electrochemical corrosion: the potential difference (0.54V) between contact wafer (tungsten) and the base (copper) is larger.
Electrochemical corrosion refers to a corrosion galvanic cell, in which metal and electrolyte act as an electrode composition.
Deoxygenating methods: the common deoxidize methods of vacuum, heating powder, iron scrap, resin and chemical agent methods.      
3. In order to improve the reliability of the contact of tungsten contacts
(1) Minimize the electrode potential difference between the contact wafer and the copper base. Under normal weather conditions, the contact potential difference between the electrodes can not be more than 0.5V; in bad weather, can not be greater than 0.25V.
Electrode potential refers to, in the electrolyte solution (water), there are potential difference generated between the metal (tungsten) surface and the solution, that is, the electrode potential of the metal in the solution.
(2) The combination of W and Ni or tungsten-rhenium 10 - nickel, then weld them into contact, can improve the reliability of tungsten contacts, which can prevent the formation of oxide film on tungsten contacts surface.
4. When the load current is not very large, use other materials, such as silver and copper, to take the place of tungsten as a contact.



 

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