Corrosion
Steel
has a great many practical applications due to its strength and the ease with
which it can be worked. However, a disadvantage of this metal is that it is
subject to corrosion, i.e. rust formation. Steel consists largely of iron and
small quantities of carbon, silicon, manganese, aluminium, nickel, chromium,
copper, molybdenum and vanadium. The exact composition depends upon the
specific target application. The resistance to corrosion also depends largely
on the specific composition of the steel.
The
production of steel components is based on rolling out large blocks of steel at
high temperatures. During the cooling process, the metal surface oxidizes
leading to the formation of oxide scale or mill scale. Steel plates with mill
scale are first immersed in a pickling bath to remove the mill scale, after
which the plate is rolled out at room temperature to the desired thickness.
After the resulting sheets have gone through a heat treatment in an oxygen-free
environment, they are covered in a film of oil.
As
mentioned above, steel reacts rather quickly with oxygen at high temperatures.
At temperatures under 400°C, the reaction between steel and oxygen proceeds
very slowly. The fact that steel corrodes relatively quickly at room
temperature is based on chemical reactions involving water.
Corrosion is therefore an undesirable surface-based electrochemical or
chemical process that damages metal, and it can be combated in many different
ways. One of the most widely used methods is the application of suitable
coating systems, generally in the form of paints. A suitable coating system is
capable of blocking moisture and oxygen from contact with the metal surface,
thereby preventing corrosion. In addition, a suitable coating system can
greatly influence the factors which reduce the speed at which corrosion takes
place. As a result, a suitable coating system can significantly increase the
value of the metal concerned by extending its usable lifespan and significantly
improving its appearance. In this regard, it should also be noted that the form
and design of metal objects also impact the ability of a coating system to
prevent corrosion.
The
following characteristics have a negative impact in terms of corrosion:
the presence of sharp edges and corners
places where water and dirt can accumulate
discontinuous welding seams, allowing moist air to penetrate beneath the
coating
cracks and other difficult to reach places
4.1 Protecting metal
If a metal object is exposed outdoors to the climate, it will rust. In
order to present rust formation, the surface of the object is treated in such a
fashion as to provide it with protection. The level of protection should be in
line with the expected or required life span of the object. An additional and
significant benefit is that this type of protection generally also improves the
appearance of the metal object. As a result, this type of surface treatment
makes the metal more suitable for use and gives added value to the product.
In order to make a choice from the many similar types of surface
treatments available, we need to look at the intended use of the object. It is
this use which largely determines what conditions and factors the object
concerned will be exposed to. The most important factors upon which such a
choice can be based involve:
desired lifespan
influence of climate
influence of mechanical factors
4.2 Desired lifespan
The surface treatment chosen
should provide a level of protection in line with the lifespan of the object.
Objects with a very long life span need to be reconditioned several times. This
is, for example, the case with steel constructions, ships, bridges, and
building façades. For such types of capital goods, one needs to choose a
protective system that will provide protection for as long as possible.
4.3 Influence of climate
The climate greatly impacts
the lifespan of a coating system.
When used outdoors, a coating system will be exposed to the influence of
ultraviolet light, which is very destructive for a great many binding agents.
However, this effect can largely be circumvented via the use of plate- and
needle-shaped pigments such as aluminium and micaceous iron oxide.
4.4 Moisture
Continual fluctuations in humidity exert a great deal of pressure on a
coating system. These types of conditions occur, for example, when it rains and
in day-night cycles during which temperature fluctuations continually lead to
the formation of condensation.
4.5 Corrosive influences
Contaminants from the atmosphere can also exert a great deal of pressure on a coating system. Such contaminants are common in industrial and maritime climates. At present, the situation is such that the climate in all Western countries can be considered as being contaminated.
