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→Corrosion Testing
*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 provides gives a measure for 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.
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>2</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>
<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.
During different times of the year , temperature and relative humidity changes as well as changes in the concentration of corrosive gases can have significant influences on the formation of corrosion products.
Therefore , research and quality assurance efforts have centered for many years on developing test methods for electrical contacts which can predict in an accelerated time frame the corrosion behavior of electrical contacts in different corrosive atmospheres with reasonable accuracy.
Single components corrosive test atmospheres and testing with two gas exposures following each other , provide only limited validity. Flowing gas testatmospheres 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 Facilities"/><!--(Tab. 13.4)-->).
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 (<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)-->).
</figtable>
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.
<figure id="fig:Influence of the corrosive gas concentration for four classes">
[[File:Influence of the corrosive gas concentration for four classes.jpg|right|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>
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.
