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Under Coating Corrosion of Silver Plated Copper in a Circuit Breaker

ENVIRONMENT:

Fossil Fuel Power Plant

EQUIPMENT:

Contact Finger - Vacuum Circuit Breaker
MATERIAL: Selectively Silver Plated Copper
OPERATING TEMPERATURE: 75 to 90C

FAILURE:

Under Coating Corrosion
 

The circuit breakers were installed in a vented air-conditioned trailer. Normally, these contacts cycled once daily, on in the morning and off at night. When energized, the operating temperature inside the box was 75C to 90C. When cycled off, the box was cooled to ambient temperature overnight. The contacts carried either 4160 or 13800 V AC.

The contacts were silver plated only along the edges. This left large amounts of exposed copper. A visual examination revealed two basic areas of interest (Figure 1). Area 1 is a silvery metallic color (typical of silver plate). Area 2 is green in color (typical of corroding copper).

Electron Beam X-Ray Analysis was performed to identify the elemental components of these area. Area 1 was mostly silver and Area 2 was predominantly copper and potassium with silver present also.

X-Ray diffraction was used to identify the compounds in the green deposits (Area 2). The major component was copper carbonate hydroxide (typical of corroding copper) and the minor component was potassium copper cyanide. The presence of cyanide is not a natural occurrence and could only be the result of contamination, mostly likely as a result of the plating process. Analysis of a new contact finger confirmed that the cyanide was present before installation and that corrosion similar to that observed on the used fingers had already begun.

A cross-section of a used contact finger revealed that corrosion is occurring under the silver late causing it to spall off (Figure 2). The cyanide had established a galvanic cell between the silver plate and the copper substrate preferentially attacking the copper substrate.

 
Figure 1. Surface of exposed contact finger; Area 1 is predominately silver while Area 2 is predominately copper. (4X Original Magnification) Figure 2. Cross-section of corroding copper in Area 2 (500x Original Magnification with Dichromate Etch)

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