In article . com
peter wrote:
What kind of plating would that be? I thought all electrical kettle's
heating elements are simply metallic (stainless steel?)

Probably something like nickel plated copper. Steel would be crap at
conducting the heat into the water.

Does it mean my kettle is no good any more to produce any "healthy"
drinking water? Can those black stuff be removed with vinegar, acid,
etc.?

It's the acid that caused the copper to start corroding in the first
place ...