In the photo is a closeup of one of my earbuds that I have used for a few years.

One of the contact surfaces (to the right in picture) is gold-colored and quite shiny. The other one is more brownish in color and not shiny. When the earbuds were new, both contact surfaces were gold-colored and shiny.


What has happened?

About the earbuds:

  1. They are true wireless in-ears, so contact surfaces are exposed to earwax. In the photo they are carefully cleaned using isopropyl alcohol
  2. Charging relies on two (gold-colored) pogo pins in the charging case to make contact with the two contact surfaces on the earbud (previously mentioned earwax is sometimes a problem...)
  3. They are among the cheapest earbuds of similar type/model on the market
  4. I have used them daily for about 2-3 years


  1. What surface finish has likely been used?
  2. Does the difference in surface wear/color/finish after use have to do with positive vs negative terminal?
  3. If the answer to question 2 is yes -- what's the process? What happens exactly?
  4. If the answer to question 2 is no -- then what's the cause?
  5. Could this have been avoided with a different surface finish on contact surface and/or pogo pin?
  • \$\begingroup\$ The image is hard to interpret without some scale reference. What's the diamater of those pads? \$\endgroup\$ Commented Aug 31, 2023 at 18:36
  • \$\begingroup\$ contact surfaces are exposed to earwax If only. The contact surfaces are exposed to earwax, sweat, metabolic waste from bacteria, environmental contaminants - including abrasive dust particles. And finally, they are subject to abrasion each time they come in contact with the pogo pins. The presumed gold coating may be slightly porous - enough to initiate pitting corrosion. It'd be nice to see those under an electron microscope. It's a pretty good question! \$\endgroup\$ Commented Aug 31, 2023 at 18:39
  • \$\begingroup\$ The pads are about 1 mm in diameter and center-to-center distance approximately 7-8 mm \$\endgroup\$
    – Filippa
    Commented Sep 1, 2023 at 8:16
  • \$\begingroup\$ Is pitting corrosion related to galvanic corrosion? And how come on only one of the pads? \$\endgroup\$
    – Filippa
    Commented Sep 1, 2023 at 8:20

1 Answer 1

  1. The surface finish is likely gold, though there are several different kinds of gold plating. The cheapest is "Electroless Nickel Immersion Gold" (ENIG) which gives a thin soft gold coat. The better is "Hard Gold", where the copper is first electroplated with hard nickel, then over-plated with a thick layer of hard gold.

I would guess these earbuds may well just be a thin immersion gold flash. In that case the gold film will likely be porous, exposing based metal in places.

  1. & 3) The discoloration is most likely related to sweat/moisture providing an electrolyte, from the charger contact, through the gold pores, down to the base metal of the ear bud. The charging current may then set up an electic field which will lead to attacking of the base metal. This will be polarity sensitive, with either salts coming up from the bud, or dissolved copper from the charger-side contact being redeposited.

  2. n/a

  3. Yes. Thicker gold plating, with a thick nickel underplate, is more than capable of providing very stable contacts under fairly aggressive condition. However, it is more expensive, and for low margin products a cheaper finish will typically be used.

  • 1
    \$\begingroup\$ Is this an example of galvanic corrosion? And if they would had used a more durable surface finish --> both pogo pin and contact surface on earbud would be of the same material even after use/wear --> no galvanic corrosion? \$\endgroup\$
    – Filippa
    Commented Sep 1, 2023 at 8:09
  • \$\begingroup\$ I suspect it is electrolytic corrosion (driven by an external voltage) as opposed to galvanic (driven by the battery formed by two dissimilar metals and an electrolyte) but very close. The issue is the porous gold meaning the base copper can be attacked. \$\endgroup\$
    – colintd
    Commented Sep 1, 2023 at 8:23
  • \$\begingroup\$ Regarding the polarity. I assume the charging case automatically turns off when the earbuds are fully charged, but the earbuds will remain in the case until I want to use them next. If they used a high-side switch, the brownish pad is the positive terminal, or? And the still gold and shiny pad is GND (pogo pin and pad still at same potential even after charging is switched off)? \$\endgroup\$
    – Filippa
    Commented Sep 1, 2023 at 8:31

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