STEEL INDUSTRY

GUIDANCE NOTES
The information given in this Steel Industry Guidance Note is for general information
only and the reader should always seek specific advice on any particular issue.
D158 SIGNS 16/12/05 2:03 pm Page 1
SN14 04/2007
The prevention of corrosion
on structural steelwork
The cost effective corrosion protection of structural steelwork should present little difculty for
common applications and environments if the factors that affect durability are recognised at the
outset. This note aims to give speciers an insight into the factors involved.
In dry heated interiors no special precautions are necessary. Where precautions are required
modern durable protective coatings are available which, when used appropriately, provide
extended maintenance intervals and improved performance.
The corrosion process
Most corrosion of steel can be considered as an
electrochemical process that occurs in a series of
consecutive stages. The details of this process can be
summarised by the following equation:-
4Fe + 3O
2
+ 2H
2
O = 2Fe
2
O
3
.H
2
O
(Iron/Steel) + (Oxygen) + (Water) = Rust
From this it can be seen that for iron and steel to corrode it
is necessary to have the simultaneous presence of water
and oxygen. In the absence of either, corrosion does not
occur.
What affects the rate of corrosion?
The principle factors that determine the rate of corrosion
of steel in air are the time of wetness and the presence of
atmospheric pollution typically present as suplhates and
chlorides.
Time of wetness
This is the proportion of total time during which the
surface is wet, due to rainfall, condensation, etc.
Sulphates
These originate from sulphur dioxide gas that is
produced during the combustion of fossil fuels,
Chlorides
These are mainly present in marine environments. The
highest concentrations of chlorides are to be found in
coastal regions and there is a rapid reduction when
moving inland.
Both sulphates and chlorides increase corrosion rates.
They react with the surface of the steel to produce
soluble salts of iron that can concentrate in pits and are
themselves corrosive.
Because of variations in atmospheric environments,
corrosion rate data cannot be generalised, however,
environments and corresponding corrosion rates are
broadly classied in BS EN ISO 12944 Part 2 and ISO
9223
The effect of design on corrosion prevention
In external or wet environments, design can have an
important bearing on the corrosion of steel structures. In
dry heated interiors no special precautions are necessary.
The prevention of corrosion should therefore be taken into
account during the design stage of a project. The main
points to be considered are:
To avoid the entrapment of moisture and dirt
The key here is to avoid the creation of cavities and
crevices; so welded joints are preferable to bolted
joints. Lap joints should be avoided or sealed where
possible. Additionally drainage holes to prevent
standing water may have to be incorporated.
Coating application
The design should ensure that the selected protective
coatings can be applied efciently. Typically this
might involve ensuring adequate access for painting
or adding drain/vent holes to sealed components,
which will be subject to hot dip galvanizing.
The application of protective coatings
Surface Preparation
The surface preparation of steel is concerned with the
removal of mill-scale, rust and other contaminants to
provide a satisfactory substrate for coating and is generally
considered to be a two stage process
Further sources of Information
D158 SIGNS 16/12/05 2:03 pm Page 2
Key Points
The rst stage of any surface preparation is to remove
residues of grease, oil or marking inks. The second stage
is to remove any mill scale and rust and is generally done
by either hand and power tool cleaning or abrasive blast
cleaning.
Painting
Painting is the principle method of protecting structural
steelwork from corrosion.
Paints are made by mixing, pigments (the coloured part),
binders (the lm forming component) and the solvent
(which dissolves the binder).
Paints are usually applied one coat on top of another and
each coat has a specic function or purpose.
The primer is applied directly onto the cleaned steel
surface. Its purpose is to wet the surface and to provide
good adhesion for subsequently applied coats. In the
case of primers for steel surfaces, these are also usually
required to provide corrosion inhibition.
The intermediate coats (or undercoats) are applied to
build the total lm thickness of the system. Generally,
the thicker the coating the longer the life and this may
involve the application of several coats.
The nishing coats provide the rst line of defence against
the environment and also determine the nal appearance
in terms of gloss, colour, etc.
Hot dip galvanizing.
The most common method of applying a metal coating to
structural steel is by hot-dip galvanizing.
Following surface preparation as described earlier the
galvanizing process involves the following stages:
i. The cleaned steel is immersed in a uxing agent to
ensure good contact between the steel and zinc
during the galvanizing process.
ii. The cleaned and uxed steel is dipped into a bath of
molten zinc at a temperature of about 450C. At this
temperature, the steel reacts with the molten zinc to
form a series of zinc/iron alloys integral with the steel
surface.
iii. 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 solidies it usually assumes a crystalline metallic
lustre, often referred to as spangling. The thickness of
the galvanized coating is inuenced by various factors
including the size and thickness of the workpiece, the
steel surface chemistry and the surface preparation of the
steel.
Weathering Steels
Weathering steels are high strength, low alloy weldable
structural steels that possess good weather resistance
in many atmospheric conditions without the need for
protective coatings. They contain up to 2.5% of alloying
elements, e.g. chromium, copper, nickel and phosphorous.
On exposure to air, under suitable conditions, they form an
adherent protective rust patina. This acts as a protective
layer that, with time, causes the corrosion rate to reduce
until it reaches a low terminal level, usually between 2-5
years.
1. In dry heated interiors no special precautions are
necessary.
2. The corrosion of steel can be considered as an
electrochemical process
3. For steel to corrode it is necessary to have the
simultaneous presence of water and oxygen.
4. The principle factors that determine the rate of
corrosion of steel in air are the time of wetness and
the presence of atmospheric pollution.
5. The prevention of corrosion should therefore be taken
into account during the design stage of a project.
6. Painting is the principle method of protecting structural
steelwork from corrosion.
7. Hot dip galvanizing is the most common method of
applying a metal coating to structural steel
8. Weathering steels are high strength, low alloy weldable
structural steels that possess good weather
1. The prevention of corrosion on structural
steelwork, Corus. Available from
www.corusconstruction.com.
2. BS EN ISO 12944 Paints and varnishes. Corrosion
protection of steel structures by protective paint
systems, BSI.
3. BS 7773 Code of Practice for Cleaning and
Preparation of Metal Surfaces.
For information on the specication of
galvanizing contact the Galvanizing Association
- www.galvanizing.org.uk