2,808
edits
Changes
→13.5.4 Corrosion Testing
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>2S</sub>, SO<sub>2</sub>, NO<sub>x</sub>, O<sub>3</sub>, Cl<sub>2</sub>, and NH<sub>3</sub> ''(Table 13.4)'' in industrial environments.
Table 13.4: Typical Corrosive Gas Concentrations (ppm) Near Industrial Facilities
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 test
atmospheres with four components have proven to be the most likely ones to realistically simulate long term natural corrosive gas exposure ''(Table 13.4)''.
Table 13.5: Some Standardized Corrosion Tests for Electrical Contacts
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 13.5, Fig. 13.14)''.
Table 13.5: 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.
Fig. 13.14:
Influence of the corrosive gas
concentration for four classes ( – )
on the contact resistance of a porous
gold layer as a function of the exposure
time (Battelle)
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.
