Changes

The procedures and standards for testing electrical contacts described below are mostly concentrated on contact applications in electromechanical devices. Since the range of applications for electrical contacts is very broad, a complete description of all relevant test procedures would extend the scope of this chapter of the Data Book. Therefore we limited the content here to contact coatings and switching contacts for information and power engineering. Because of the ongoing miniaturization of electromechanical devices the testing for effects of corrosive influences by the environment play an important role. Special testing procedures, such as these for brazed, soldered, and welded contact joints are covered already in chapter 3 [[Manufacturing Technologies for Contact Parts|Manufacturing Technologies for Contact Parts ]]

Every technical device has to fulfill a series of requirements. Some of those which are important for agreement between contact manufacturer and user are part of DIN 40042 standard and described here in a summarized version:

For applications at low switching loads contact layers with thicknesses in the range of just a few micrometers are widely used. For testing such thin layers the actual coating properties must be distinguished from the functional properties. Coating properties include, besides others, porosity, hardness, and ductility.

Testing of the contact behavior in the communications technology is usually performed on the actual devices such as for example in relays. Experience has shown that the interaction between all design and functional parameters such as contact forces, relative movement, and electrical loads, are determining the failure mode. Therefore only statistical performance tests on a larger number of switching devices lead to meaningful results.

The testing of electrical contacts for power engineering applications serves on the one hand the continuous quality assurance, on the other one the new and improvement development efforts for contact materials. To optimize the contact and switching performance contact materials and device designs have to complement each other. The success of such optimizing is proven through switching tests.

The definition of corrosion can be found in DIN 500900 Part 1 as follows: Reaction of a metallic material with its environment, which produces a

During corrosive influences metal is dissolved. This metal loss can be uniformly spread out over a certain area or be limited to locally smaller spots. This process usually proceeds with constant speed up to the total material loss, or after certain reaction times a natural corrosion limiting surface layer can be formed (i.e. on aluminum).

*Contact corrosion: <br />Corrosion of a metal object after coming into physical contact with another metallic body. This can occur also on metallic impurities in alloys, on chemically and physically heterogeneous surfaces and on heterogeneous solutions on homogeneous surfaces, as well as through contacting a metal object by non-metallic materials through formation of corrosion compounds.<br />

*Fatigue corrosion: <br />Fatigue fracture during repeated mechanical stresses in corrosive environments. This effect is often observed on brittle electroplated surface coatings that are exposed to repeated cycling between mechanical stresses and corrosive chemicals. Air access corrosion: Through differences in the amount of exposure to air or oxygen surface areas of a metal are becoming cathodes at the more exposed spots and therefore corrode less than those protected (for example: gap corrosion in screw or press connections).<br />

Corrosion effects are mainly governed by the electrode potential of the respective metals. The electrochemical potential table provides a measure for corrosion resistance. Non-precious (corrosion prone) metals are characterized by a negative, precious (corrosion resistant) metals by a positive normal potential against hydrogen.

<figtable id="tab:Electrode Potential of Metals">

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Corrosion products on the surface of electrical contacts can reduce the reliability of contact making significantly by, for example, higher contact

resistance, which will negatively affect the transmission of current and data signals. This can cause major problems in electromechanical contact

<figtable id="tab:Typical Corrosive Gas Concentrations (ppm) Near Industrial Facilities">

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Single components corrosive test atmospheres and testing with two gas exposures following each other provide only limited validity. Flowing gas test

<figtable id="tab:Some Standardized Corrosion Tests for Electrical Contacts">

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The differences in the corrosive gas concentrations and the test durations are dependent on the end application of the contact components and the

assessment of the exposure parameters.

<figtable id="tab:Classification of Corrosion Effects According to Battelle">

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