Zinc plating, or electro-galvanising, deposits a thin layer of zinc on a steel part by electrolysis: immersed in a bath containing zinc salts, the part acts as the cathode and gradually becomes coated with metal. The usual thickness runs from a few micrometres to around twenty micrometres, with great consistency. The deposit is then passivated: this conversion treatment, now based on trivalent chromium, delays white corrosion of the zinc and gives the finish (clear, bluish, iridescent yellow or black). Hexavalent chromium baths, restricted by regulation, have largely been abandoned. This treatment has nothing in common with the passivation of stainless steel, which shares the same name.
Zinc plating is the usual solution for fasteners, washers, small mechanical parts and light sheet metal, because dimensions barely change. The protection remains moderate: suited to indoor use or mildly aggressive atmospheres, it is judged by salt spray tests (NF EN ISO 9227), expressed in hours before white rust and then red rust appear. Zinc deposits fall notably under standard NF EN ISO 2081, fasteners under ISO 4042. For a harsher environment, choose a zinc-nickel alloy, or hot-dip galvanising for large structures, which can then be painted by powder coating.
One precaution concerns high-strength steels: electrolysis releases hydrogen that can embrittle them, hence a baking step after plating. Flag this in the request for high-strength-class screws and for parts hardened by quench hardening.
In a quote request
Specify the material and its hardness, the type of passivation and the colour, the zinc thickness targeted, the areas to mask (threads, bearing surfaces) and the salt spray resistance required. State whether dimensions apply before or after plating and whether a thickness check is expected. The surface treatment guide compares zinc plating with the other options.