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Steel Designers' Manual - 6th Edition (2003)
Metallic coatings 1037
the steel surface and cannot be removed by conventional dry blast cleaning
methods. Wet abrasive blasting has proved to be particularly useful on offshore
structures and prior to maintenance painting for structures in heavily polluted
environments.
(4) Acid pickling
This process involves immersing the steel in a bath of suitable inhibited acids
which dissolve or remove mill-scale and rust but do not appreciably attack the
exposed steel surface. It can be 100% effective. Acid pickling is normally used
on structural steel intended for hot-dip galvanizing but is now rarely used as
pre-treatment before painting.
35.5 Metallic coatings
There are four commonly used methods of applying metal coating to steel surfaces:
hot-dip galvanizing, thermal (metal) spraying, electroplating and sherardizing. The
latter two processes are not used in structural steelwork but are used for fittings,
fasteners and other small items.
In general the corrosion protection afforded by metallic coatings is largely
dependent upon the choice of coating metal and its thickness and is not greatly influenced
by the method of application.
35.5.1 Hot-dip galvanizing
The most common method of applying a metal coating to structural steel is by
galvanizing.
The galvanizing process involves the following stages:
(1) Any surface oil or grease is removed by suitable degreasing agents.
(2) The steel is cleaned of all rust and scale by acid pickling. This may be preceded
by blast-cleaning to remove scale and roughen the surface but such surfaces are
always subsequently pickled in inhibited hydrochloric acid.
(3) The cleaned steel is then immersed in a fluxing agent to ensure good contact
between the zinc and steel during immersion
(4) The cleaned and fluxed steel is dipped into a bath of molten zinc at a temperature
of about 450°C at which the steel reacts with the molten zinc to form a
series of zinc/iron alloys on its surface.
(5) As the steel workpiece is removed from the bath a layer of relatively pure zinc
is deposited on top of the alloy layers.
As the zinc solidifies it assumes a crystalline metallic lustre, usually referred to as
spangling. The thickness of the galvanized coating is influenced by various factors