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Summary:
Dealing with Corrosion With
trilingual summary
Corrosion -- the electrochemical erosion of metals -- damages
electrical apparatus in several ways. Fastener corrosion interferes
with disassembly when that becomes necessary for equipment
servicing. Damage to shaft and bearing surfaces can cause wear and
vibration. Spreading from corrosion sites, fatigue cracks can lead
to premature failure in highly-stressed machine parts. Corroded
enclosures foster leakage of dielectric fluids or contamination of
electrical devices. Switching devices may be overheated or disabled
by contact corrosion, which may also lead to excessive voltage drop,
phase unbalance, or destructive arcing.
The process itself is similar to that in a battery. Whenever two
electrically dissimilar conductive materials are in contact with a
liquid electrolyte, electrons and ions migrate between the
conductors, eroding one or both of them and yielding gases or
non-metallic byproducts.
Metals can be protected against corrosion by sealing (applying
protective coatings that prevent contact with liquids); by
substitution (using a different material); or by sacrifice (linking
the protected material to, or coating it with, another metal that
corrodes more readily.
Coatings include primers and paints, of both organic and inorganic
materials, such as phenolics, epoxies, and chromates. Plating is
also used. Careful preparation of the protected surface is required
to ensure that moisture (always a potential electrolyte) is not
trapped beneath the coating, and that the coating will adhere
tightly).
Another kind of coating can be supplied by vapor-action inhibitors.
Usually supplied in small capsules that can be mounted inside
equipment enclosures, these release a vapor that forms a molecular
film on metal surfaces to ward off moisture.
Substitution often means using stainless alloys rather than carbon
steel (particularly for motors in food processing industries).
Bronze may be substituted for aluminum (or plastic for metal)
cooling fans in totally-enclosed motors. However, what is effective
in one environment may be a poor choice in another.
The most common examples of sacrifice are zinc galvanizing for steel
sheets and wire, and the attachment of sacrificial zinc anodes to
steel piping or structural members buried underground. Corrosion of
the iron involved commences only after the zinc has been consumed.
Water has been called the universal solvent. It might also be
considered a universal electrolyte. Keeping electrical apparatus
dry, though often difficult, therefore remains the most effective
way to prevent corrosion.
From
"Dealing with Corrosion" ...by
Richard L. Nailen, EA Engineering Editor -
published in Electrical Apparatus December 2003.
© 2003
Barks Publications, Inc. All Rights Reserved.
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