Corrosion handbook

4 06/21
1 BASICS OF CORROSION
1.1 What is corrosion?
Corrosion is the physicochemical interaction between a metal and its environment,
which results in changes in the metal’s properties and may lead to significant
functional impairment of the metal, the environment, or the technical system of
which they form a part (see ISO 8044:2010).
We only talk about corrosion when there is a change in the metal’s or system’s
properties that may lead to an undesirable outcome. This can range from simple
cosmetic damage to complete failure of technical systems potentially causing
great economic damage and even present a hazard to people.
The commonly used metals in construction and engineering like steel, aluminum
and zinc have a tendency to return to its original state as found in nature. For this
reason, corrosion can be regarded as metal-winning in reverse. Fig. 1 shows how
this looks like when iron is extracted from iron ore.
metal winning
requires energy
Fe
2O3 2 Fe + ³⁄₂ O2
unrefined, natural state
(stable)
corrosion reaction
refined, metallic state
(unstable)
Fig 1: Chemical reactions of iron during corrosion and the metal-winning process.
Types of corrosion reactions
By far the most common type of the corrosion reaction is electrochemical
in nature. Such reactions imply a transfer of electrons from the metal via an
environment that can conduct ions, such as a water film on its surface.
In rarer cases corrosion reactions can also be purely chemical, as the reaction of
a metal surface with hot gases or liquid salts.
The overall electrochemical corrosion reaction can be separated into two partial
reactions:
• Metal dissolution, also known as oxidation or anodic reaction
Fe → Fe
2+
+ 2 e-
• Reduction or cathodic reaction
In neutral environment and with a sufficient supply of oxygen the cathodic
reaction is as follows:
O
2 + 2 H2O + 4 e
-
→ 4 OH
-
This is the most important cathodic reaction we must consider when corroding
our parts in common environmental conditions (also called atmospheric
corrosion).
In acidic environment and with the lack of oxygen following reaction may occur:
2H
+
+ 2e
-
→ H2
Even if this reaction, as a partial reaction of corrosion, occurs much less
frequently in atmospheric environments compared to the oxygen reduction, it
must nevertheless be taken into account, since the formation of hydrogen here
can lead to embrittlement of the metals concerned (see chapter on forms of
corrosion).