"Gold" plated connectors....When and why?

[Pjotr]

Contact resistance, especially at very low voltages.

I allways wondered why industrial audio connectors are silver plated.

Industrial audio connectors aren't silver plated. It became a stupid fashion for headphone sockets on consumer equipment, but it corrodes too much for any serious audio use. Cheap industrial connectors are gold plated, and the good ones for small signal things like mics use expensive exotic alloys.

The original Cannon X, XL and later XLR connectors were silver plated, sure! Just for their low contact resistance and low thermal voltage. The scraping action is more than adequate to "clean" the contacts when mating. Many connectors are very thin gold plated (< 1um) just for preservation to avoid oxidation laying on the shelf. When used several times the gold is scraped off anyway then.

[Tac Eht Xilef]

You cannot plate anything over everything. That's why ENIG on the PCBs consists of gold over nickel. It won't hold on bare copper. The same with connectors, there is nickel layer underneath the gold.

AIUI, gold plates onto copper quite well - the problem is it alloys relatively quickly due to migration of the copper and (above a couple of %, if memory serves) the alloy becomes a worse conductor than pure gold (and, above ~20% Cu, worse than nickel). Hence the intermediate plating of nickel, which acts as a barrier to electromigration( & also adds a degree of wear resistance).