Corrosion handbook

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Sherardizing / thermal diffusion
Process description
Sherardizing is a method of zinc coating utilizing a thermal diffusion
process. The steel parts are placed in a drum containing Zn
powder and then heated to temperatures above 320 °C. The Zinc
is not liquid, and the coating forms by thermal diffusion of the Zn
powder into the steel parts.
Coating thickness The achievable coating thickness ranges up to 45 µm.
Other features
These coatings consist mainly of a ZnFe alloy which offers very
good protection against corrosion that can be compared with hot-
dip galvanizing at the same thickness. Even on complex threaded
parts, this process produces tough and uniform coatings.
Multilayer coatings / duplex-coated carbon steel
When the corrosion protection provided by the metallic coating is not sufficient,
the parts can be further protected by additional coatings, mainly organic paint
with or without metallic flakes.
An example of this is the multilayer coating on fasteners consisting of an
electroplated Zn alloy coating with an additional organic top coat (see Fig. 32).
3.2 Corrosion behavior of stainless steel
Steel alloyed with at least 10 % chromium is called stainless steel. The addition
of chromium causes the formation of a stable, very thin (few nanometers) oxide
layer (passivation layer) on the surface. Stainless steel therefore does not readily
corrode or stain when in contact with water like carbon steel does.
However, under some circumstances, the passivation layer can break down
causing local attack such as pitting corrosion (see 1.2.2). Pitting corrosion, as the
predominant form of corrosion of stainless steel, does not allow lifetime prediction
as is possible with zinc coatings. In general, for a given application, a grade of
stainless steel which is stable and does not show any corrosion in the given
environment has to be selected.
The resistance of stainless steels against pitting corrosion can be roughly
estimated by the PREN (pitting resistance equivalent number). The PREN is
based on the chemical composition of steel, taking into account the amount of
chromium, molybdenum and nitrogen. In literature, various equations for this
calculation are given. The most common equations are:
PREN = %Cr + 3.3 x %Mo
(for stainless steels Mo < 3%)
PREN = %Cr + 3.3 x %Mo + 30 x %N
(for stainless steels Mo ≥ 3%)
Top coat for chemical resistance
Zn-alloy electroplating for cathodic protection
Hardened carbon steel
Fig. 32: Illustration of multilayer coating
on fasteners and connectors.