For many purposes, tinplate has been replaced by galvanised (zinc-coated) vessels, though not for cooking as zinc is toxic, where stainless steelis often used. Zinc protects iron electrolytically, that is, the zinc will oxidise and turn to a white powder to preserve the iron, whereas tin will only protect the iron if the tin-surface remains unbroken, as it electrolytically cannibalises unprotected iron to preserve itself.
There are two processes for the tinning of the black plates: hot-dipping and electroplating.
Hot-dipping
Tinplate made via hot-dipped tin plating is made by cold rolling steel or iron, pickling or remove any scale, annealing to remove any strain hardening, and then coating it with a thin layer of tin. Originally this was done by producing individual or small packs of plates, which became known as the pack mill process. In the late 1920s strip mills began to replace pack mills, because they could produce the raw plates in larger quantities and more economically.
Electroplating
In electroplating, the item to be coated is placed into a container containing a solution of one or more tin salts. The item is connected to anelectrical circuit, forming the cathode (negative) of the circuit while an electrode typically of the same metal to be plated forms the anode(positive). When an electrical current is passed through the circuit, metal ions in the solution are attracted to the item. To produce a smooth, shiny surface, the electroplated sheet is then briefly heated above the melting point of tin. Most of the tin-plated steel made today is then further electroplated with a very thin layer of chromium to prevent dulling of the surface from oxidation of the tin.
