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Steel Designers' Manual - 6th Edition (2003)
Chapter 35
Corrosion and corrosion
prevention
by ROGER HUDSON and KEN JOHNSON
35.1 The corrosion process
35.1.1 Introduction
The specification of cost-effective protective treatments for structural steelwork
should not present a major problem for most common applications if the factors
that affect durability are appreciated. Primarily it is important to recognize and
define the corrosivity of the environment to which the structure is to be exposed to
enable the specification of an appropriate protective system. Many structures are in
relatively low risk category environments and therefore require minimal treatment.
Conversely, a steel structure exposed to an aggressive environment needs to be protected
with a durable system that may require maintenance for extended life.
The optimum protection treatment combines good surface preparation with
suitable coating materials for a required durability at a minimum cost. Modern practices
applied according to the relevant industry standards provide an opportunity
to achieve the desired protection requirements for specific structures.
Much guidance for the protection of steel structures has, over the years, been
sought from BS 5493: 1977 Code of Practice for Protective Coating of Iron and Steel
Structures Against Corrosion. This document has now been superseded by a series
of new ISO standards. One of the most important is ISO 12944 Paints and Varnishes
– Corrosion Protection of Steel Structures by Protective Paint Systems.This standard,
which is published in eight parts, should be referred to when drafting protection
specifications for steel structures. Part 5 of the series Protective paint systems contains
a range of paint coatings and systems for different environmental categories
that are defined in Part 2 Classification of environments. However, specifiers concerned
with UK projects should be aware that not all of the paints listed are ‘compliant’
with current national environmental legislation, and further advice should
be sought from the paint manufacturer.
35.1.2 General corrosion
Most corrosion of steel can be considered as an electrochemical process which
occurs in stages. Initial attack occurs at anodic areas on the surface, where ferrous
ions go into solution. Electrons are released from the anode and move through the
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