Definition

During hot-dip zinc-coating a blank or finished work piece from iron or steel is dipped or pulled through molten zinc at roughly 450 °C / 840 °F. By also dipping inner parts, welding seams and other inaccessible places can be reached. At these temperatures, a resistant alloy layer of iron and zinc forms on the contact surface and above it, is a very well adhering layer of pure zinc. The latter is relatively thick (between 50 and 150 µm) as opposed to when electrochemically galvanized.

In its fresh state, the zinc layer has a bright and shiny appearance. Then it looks metallic-crystalline for awhile. Over time it becomes dark grey due to corrosion of the zinc which, when exposed to atmospheric oxygen, yields a relatively weatherproof protective layer of zinc oxide and zinc carbonate. Furthermore, zinc is more of a base metal than iron (see electromotive series) and therefore serves as a sacrificial anode, which protects the iron underneath from corrosion until it is itself fully corroded. Because of these two material properties, such a think zinc layer can provide an economical rust prevention for decades without maintenance.

This is true for even the strongest corrosion exposure which in Germany are difficult to find (e.g. C4 according to the corrosion categories in DIN EN ISO 14713). More likely the average is C2 or C3.

The length of protection of a hot-dip zinc-coating can be improved by an additional coat of paint („duplex system“).

Specifications