2,315
edits
Changes
→13.5.4 Corrosion Testing
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 ]].
==<!--13.1 -->Terms and Definitions==
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:
Availability (for use) defines the probability of a system or switching device to be in a functional stage at a given time
Both, availability and reliability, are guaranteed for a pre-determined time span and/or a specific number of switching operations. This means they warrant the life expectancy of a switching device. At the end of the live span the failure rate exceeds pre-defined limit values.
Electrical life is the number of operations reached under a given electrical load under specified operating conditions.
Since the criteria which determine the electrical life of switching contacts depend depends on the type of switching devices they are used in, they are described in more detail under the testing procedures in information and power engineering.
==<!--13.2-->Testing of Contact Surface Layers==
The following descriptions are mainly applicable to electroplated contact coatings which are of the most economical importance in contact applications. They also apply however in similar form to surface layers which have been created by mechanical cladding or by sputtering.
==<!--13.3 -->Test Procedures for the Communications Technology==
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.
One must differentiate between static tests (for ex. contact resistance) and dynamic ones (for ex. electrical life). In certain electromechanical components and switching devices the contacts can be exposed to both, static and dynamic stresses (for ex. connectors, relays, switches, pushbuttons, circuit breakers). For statically stressed components the life expectancy is usually expressed as a time period, i.e. hours, while for dynamically stressed ones the expected functional life is defined as numbers of operations or switching cycles.
==<!--13.4-->Testing Procedures for Power Engineering==
The testing assessment of contact materials is performed using metallurgical test methods as well as switching tests in model test set-ups and in commercial switching devices. While physical properties, such as melting and boiling point, electrical contacts conductivity etc. are fundamental for power engineering applications serves on the one hand selection of the continuous quality assurance, on base metals and the other one additional components of the new and improvement development efforts for contact materials. To optimize , they cannot provide a clear indication of the contact and switching performance contact materials behavior. Metallurgical evaluations and tests are used primarily for determining material and device designs have to complement each otherworking defects. The success of such optimizing is proven actual contact and switching behavior can however only be determined through switching testsin a model switch or preferably in the final electromechanical device.
*Make capacity
*Temperature rise
The following chapters are limited to metallurgical analysis and the testing of the most important properties of switching devices such as electrical life, temperature rise, and switching capacity.
Main Articel: [[Testing Procedures for Power Engineering| Testing Procedures for Power Engineering]]
==<!--13.5 -->Corrosion Testing==
===<!--13.5.1 -->Definition of “Corrosion”===
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).
===<!--13.5.2 -->Special Types of Corrosion===
*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 />
*Selective corrosion: <br />Preferred corrosion in specific microstructure areas (for example: loss of zinc in brasses with formation of copper enrichments).<br />
*Air access corrosion: <br />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 />
*Tension stress corrosion: <br />Crack formation of stress corrosion sensitive materials which are under mechanical pull stresses while exposed to corrosive media. Especially affected are zinc containing copper alloys (brasses) under the influence of ammonia or nitrates.<br />
*Hydrogen corrosion: <br />Cathodic reduction of H to H<sub>2</sub> (in acidic solutions) under conditions where the potential of the metal is less precious.<br />
*Fretting (frictional) corrosion: <br />Enrichment of oxide particles of non-precious metal (especially tinned) surfaces during relative movements at small ampliyudes amplitudes (< 100 μm). They occur through transfer of oscillation ot of thermal displacement energy because of the difference in thermal expansion of the two contacting metals. This effect can be especially detrimental in connectors with tin plated surfaces, such as for example in automotive applications.<br />
*Fatigue corrosion: <br />Fatigue fracture during repeated mechanical stresses in corrosive environments. This effect is often observed on in brittle electroplated surface coatings that are exposed to repeated cycling alternations between mechanical stresses stress 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 />
===<!--13.5.3 -->Electrochemical Potentials=== Corrosion effects are mainly governed by the electrode potential of the respective metals. The electrochemical potential table gives a measure of the 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">
<caption>'''<!--Table 13.3: -->Electrode Potential of Metals'''</caption>
{| class="twocolortable" style="text-align: left; font-size: 12px"
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
components used in the information processing technology. Causes for the formation of tarnish film on electrical contacts include for example the presence of corrosive gases such as H<sub>2S2</sub>S, SO<sub>2</sub>, NO<sub>x</sub>, O<sub>3</sub>, Cl<sub>2</sub>, and NH<sub>3</sub> (<xr id="tab:Typical Corrosive Gas Concentrations (ppm) Near Industrial Facilities"/> <!--(Tab. 13.4)-->) in industrial environments.
<figtable id="tab:Typical Corrosive Gas Concentrations (ppm) Near Industrial Facilities">
<caption>'''<!--Table 13.4: -->Typical Corrosive Gas Concentrations (ppm) Near Industrial Facilities'''</caption>
{| class="twocolortable" style="text-align: left; font-size: 12px"
|0.2<br />0.2
|-
|Smell threshold<br />MAK-value[[#text-reference|<sup>1)</sup>]]<br />Life threatening
|0.18<br />2.0<br />400
|0.02<br />10.0<br />700
|}
</figtable>
<div id="text-reference">1)MAK-Value: maximum allowable concentration in the workplace</div>Corrosion tests – also called environmental – on electrical contacts in natural environments must be critically evaluated because they are the rather time consuming.
<figtable id="tab:Some Standardized Corrosion Tests for Electrical Contacts">
<caption>'''<!--Table 13.5: -->Some Standardized Corrosion Tests for Electrical Contacts'''</caption>
{| class="twocolortable" style="text-align: left; font-size: 12px"
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.
Battelle (the Battelle Institute) has, for different applications, defined four climate classes which reflect the corrosion behavior of porous electroplated gold surfaces. Such gold layers are often used in connectors for the telecommunications and information technology ''(Table <xr id="tab:Classification of Corrosion Effects According to Battelle"/><!--(Tab. 13.5, )--> and <xr id="fig:Influence of the corrosive gas concentration for four classes"/><!--(Fig. 13.14)''-->).
The dominant corrosion effects for thin gold coatings are pore corrosion and at higher gas concentrations creep corrosion from the base materials onto the coating, starting at the boundary line between non-precious base metal and contact layer.
The measurement of contact resistance allows an indirect classification of corrosion product layers. While the analysis of thicker corrosive product layers in the range of 0.1 – 1 μm can be performed by classic methods such as SEM and X-ray microprobe, thinner layers of 10 – 100 nm require the use of ionoptical analysis equipment.
Methods, Nov. 2000
[[de:Prüfverfahren]]
