What chemical reaction happens on worn out metal handles? I randomly came across a picture of this. It's the comparison between right handed and left handed people. As more people use the door handle, the metal becomes more faded and turns gold color. What is this gold color underneath and why does friction between your fingers and the door handle cause this reaction?
It looks like a brass ( a class of alloys, containing mostly copper and zinc, being golden-like ), which surface was covered by protective plating , possibly by chrome or nickel(*). It was probably worn out mechanically and/or by corrosion due acidic sweat.
(*)Nickel plating could be worn out easier than hard and resistant chrome.