Civil Engineering Reference
In-Depth Information
Chapter 6
Corrosion Protection
6.1 INTRODUCTION
Cathodic protection of the offshore structure is the responsibility of the offshore
structure engineer. This chapter presents cathodic protection (CP) design meth-
odology and what information the CP designer needs from the structural engi-
neer as well as the sensitivity of the data provided for the output. Because most
of the structure is affected by the cathodic protection, it is important to know the
appropriate time and the methods available to assist in anode retrofitting of
projects worldwide, especially due to maturation of existing offshore steel
platforms.
Corrosion is a complex concept. Therefore, the aim here is to describe cor-
rosion simply, in a form emphasizing only those aspects that are important in
understanding corrosion of offshore structures.
In general, a key factor in corrosion is the environmental condition sur-
rounding the structure. The definition and characteristics of this variable can
be complex. Practically, it is important to keep in mind that the environment
changes with time and conditions. It is also important to realize that the envi-
ronment that actually affects a metal is the micro-environment at the local sur-
face of the metal. It is indeed the reactivity of the local environment that
determines the real corrosion damage. Thus, an experiment that investigates
only nominal environmental conditions without consideration of local effects,
such as flow, pH cells, deposits, and galvanic effects, is useless for lifetime
prediction.
Corrosion is usually identified by the appearance of rust on the steel surface.
The chemical reactions driving the corrosion process are caused by chloride
attack. When corrosion of steel occurs, electrons in water cause the anodic
reaction.
A steel offshore structure is exposed to saline water during its whole life-
time. The effect of the water on the integrity of the materials is thus important.
Since steels are the metallic materials most commonly exposed to water, aqu-
eous corrosion is discussed here, with a special focus on the reactions of iron
(Fe) with water (H
2
O). Metal ions go into solution at anodic areas in an amount
chemically equivalent to the reaction at cathodic areas, as shown in
Figure 6.1
.
In corrosion of steel, the following reaction usually takes place at anodic areas:
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