A variety of low-voltage electronic devices may fail to operate properly because of exposure to certain corrosive, gaseous environments. Susceptible items include consumer products such as microwave ovens, audio equipment, TV's and personal computers as well as many types of industrial sensors and automatic control devices. Failure generally occurs because of specific application factors. These include the severity of the environment, the presence or absence of atmospheric sealing of components within the device and the metals used at critical internal locations. Frequently complete failure doesn't occur but normal operation is erratic.
Typically these problems occur due to raised electrical resistance at contact junction points within the device that would normally allow essentially unrestricted passage of electrical current. Particular chemical constituents in the air attack common metals used at contact points and form very thin films, i.e., corrosion products, which increase electrical resistance. Either failure of the device or irregular function then results. Low electrical resistance at contact points is essential in many electronic devices because they operate at relatively low voltages. Voltage is the driving force for electric current flow across an electrical contact point just as pressure is the driving force for delivering water through the resistance offered by a given length or diameter of piping.
Corrosion product film thickness as low as several hundred angstrom units can create irregular operation of low-voltage electronic devices in even relatively low humidity gaseous environments. One angstrom equals 3.9 X 10-9 inch (or 1 X 10-7 millimeter). Obviously such thicknesses are not what most persons generally consider as evidence of corrosion. Copper and silver are normally the preferred metals used in electronic contacts. Both of these metals (and some others) are subject to corrosive attack and thus formation of these very thin corrosion product films in atmospheric air containing parts per billion (ppb) levels of reduced sulfur compounds. Copper is also subject to attack when ppb levels of chlorine gas or chlorides, nitrogen oxides or ozone are present in air. High humidity, i.e., above about 50%, in air that also contains one or more of these contaminants greatly accelerates the rate of corrosion. Often in industrial applications synergistic interactions between the several gaseous contaminants cited - with or without high humidity - intensify the attack.
The Instrumentation, Systems and Automation Society (ISA) has developed a widely used engineering standard (ISA-71.04-1985) that provides more details on these effects. It also provides practical guidance on rating the relative corrosivity of gaseous atmospheres in terms of ppb quantities of contaminants present and the thickness of corrosion films formed on copper test coupons exposed to such conditions.
Gerald O. Davis, PE, President and co-owner of DM&ME, has over 40 years experience in Materials Engineering and Business. Mr. Davis is a Forensic Expert in Materials Usage, Corrosion, Metallurgy, Mechanical Failure, & Root-Cause Failure Analysis. His recent background includes work as a corrosion researcher, senior engineer, and program manager for Battelle Memorial Institute, DNV, Inc., Henkels & McCoy, Inc., respectively and, since 2004, as president of DM&ME.
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