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 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==
Single components corrosive test atmospheres and testing with two gas exposures following each other, provide only limited validity. Flowing gas test
atmospheres with four components have proven to be the most likely ones to realistically simulate long term natural corrosive gas exposure (<xr id="tab:Typical Corrosive Gas Concentrations (ppm) Near Industrial FacilitiesSome Standardized Corrosion Tests for Electrical Contacts"/><!--(Tab. 13.45)-->).
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 (<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)-->).
<figure id="fig:Influence of the corrosive gas concentration for four classes">
[[File:Influence of the corrosive gas concentration for four classes.jpg|left|thumb|Figure 1: Influence of the corrosive gas concentration for four classes (I–IV) on the contact resistance of a porous gold layer as a function of the exposure time (Battelle)]]
</figure>
<br style="clear:both;"/>
<figtable id="tab:Classification of Corrosion Effects According to Battelle">
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.
