TIN (Titanium Nitride) The first coating to be used successfully to machine steel in industry and still the most recognized, distinguished by it's attractive bright gold colour,
The "gunmetal" is actually TiAIN which actually is Titanium Nitride and Aluminum, I realize it sounds stupid, but by adding Aluminum to certain metel compositions, it actually makes them harder and more durable as well as disapating heat quicker, The Gold with Cobalt is probably also a Positive Vapor Deposition and not actually an electroplating.
I imagine that most any of the "platings" that are TIN or TiAIN coatings are vapor depositions, and not electroplated like the 24kt Gold , Rhodium or Platinum
Rhodium and Paladium are two of the hardest of a class of metals known as Noble Metals same class as gold silver platinum, and rhodium.
Electro plating or batch plating usually only deposits a coating of less than .0000002 I think I got the 0s right, its about200/millionths of an inch in thickness for average good quality gold plating, flash platings are a lot thinner.[8][

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I'm going to make some sawdust now that I've confused old Hosspen[

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Ken