The story
Probably from terne (“colourless, drab, dull”) (as contrasted with the shiny appearance of tinplate) + plate (“layer of a material on the surface of something, plating”).
Etymology from Wiktionary, CC BY-SA 4.0.
terneplate
Today it means: Thin iron or steel sheeting coated with an alloy of lead and tin (or, more recently, zinc and tin), often with some antimony. Full definition →