"crystal growth on electrical switch components"

Question:

"Issue: Micro switch failed, symptom was a short circuit (false alarm). This is the third switch to fail in 14 years. Previous failures were not investigated. Application: Micro switch is part of a level switch used to detect flooding in an underground vault. Low voltage DC alarm circuit. Environment: Damp concrete vault. Troubleshooting revealed less than 1K Ohm resistance across open contacts of the switch. Inspection of the failed switch resulted in the following observations: 1. External tin plated solder lugs have a white crystal growth near brass rivets. The crystal has small lumps and some whiskers. Also several small blue-green spots on growth. No growth observed near soldered connection. Soldered connection has flux residue. 2. Internal brass components have same type white crystal growth with whiskers 1/4" in length. The whiskers appear to be the cause of the short circuit. Near the center of one brass rivet there is pitting with crystal growth in the center. Around the edges of the pitting the metal appears to be copper colored. Crystal growth appears on various spots on the brass components and there is one spot with a crystal on the bakalite case that appears to have no contact with metal. The spring mechanism is the common switch contact and is a copper colored alloy that is riveted to a brass contact. 3. The unused switch contact has no internal or external corrosion. Questions: 1. Does brass dezincification have crystal growth with whiskers? I have heard of tin whiskers. 2. What promotes this growth? 3. Is this galvanic?"

Answer:

In answer to your questions: 1. I have not ever observed white whisker growth associated with dezincification of brass. Furthermore, the environment, as you described it, is not particularly associated with dezincification. 2. Can't answer your growth question. Have you considerd analyzing the material for chemical composition? I would suggest energy dispersive spectrometry (EDS) as a start. 3. Galvanic corrosion requires the electrical coupling of two different metals or alloys of different potentials. It also requires an aqueous corrosive environment. From your description of the circuit, I cannot determine if the brass is in electrical contact with the tin-coated solder or another metal.

 

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