Under what circumstances is galvanic corrosion a concern in electrical work?

Question

I'm curious why contact between dissimilar metals in electrical work is seems to only be a problem in some cases. For example, one must use antioxidant when joining aluminum and copper wiring in house wire and distribution panels. But what about the following cases:

• Copper wire attached to brass and steel terminal screws in receptacles
• Silver-colored (aluminum?) wire in light fixtures joining copper house wire
• Various kinds of screws mounting metal junction boxes to studs

Why are those cases not a concern?

Answer 1

Designers of such devices often select their materials carefully to avoid galvanic corrosion, and prohibit attaching materials that will cause corrosion. You may sometimes see devices marked Cu-only or copper-only: this means that you're only allowed to put a copper wire under that screw, and using something else may cause issues. And when using a material that would usually be susceptible to galvanic corrosion, it's sometimes possible to mitigate that risk by coating it in something else that's not -- nickle and tin coatings are common on electrical equipment (the silver-color wires you see in lighting fixtures is more likely tinned copper than aluminum). Finally, for some applications (i.e. junction boxes), you just accept that some corrosion will happen, and design things to still function even when corroded.

Answer 2

Galvanic corrosion is a problem when the two metals are wet or damp , water being necessary to conduct current causing corrosion. In some conditions of humidity and temperature change ,a small amount of water can condense ,promoting corrosion. Any time aluminum is connected to another metal there can be corrosion. I once had a home with a damp crawl space, all copper wire in galvanized conduit . When I worked in the boxes I wondered why there was white powder zinc oxide . Until one day under the right,or wrong, conditions I opened a box and several drops of water dripped out of the conduit.Aluminum will oxidize by itself in air depending on humidity.