Corrosion Science Chemistry Research Article | Page 7

The Effects and Economic Impact of Corrosion
and platinum, to the low corrosion resistance of active metals, for exam-
ple, sodium and magnesium. Furthermore, the corrosion resistance of a
metal strongly depends on the environment to which it is exposed, that
is, the chemical composition, temperature, velocity, and so forth.
The general relation between the rate of corrosion, the corrosivity of
the environment, and the corrosion resistance of a material is:
corrosivity of environment
= rate of corrosive attack
corrosion resistance of metal
For a given corrosion resistance of the material, as the corrosivity of
the environment increases, the rate of corrosion increases. For a given
corrosivity of the environment, as the corrosion resistance of the mate-
rial increases, the rate of corrosion decreases. Often an acceptable rate of
corrosion is fixed and the challenge is to match the corrosion resis- tance
of the material and the corrosivity of the environment to be at or below
the specified corrosion rate. Often there are several competing materials
that can meet the corrosion requirements, and the material se- lection
process becomes one of determining which of the candidate ma- terials
provides the most economical solution for the particular service.
Consideration of corrosion resistance is often as important in the se-
lection process as the mechanical properties of the alloy. A common so-
lution to a corrosion problem is to substitute and alloy with greater cor-
rosion resistance for the alloy that has corroded.
Coatings
Coatings for corrosion protection can be divided into two broad
groups—metallic and nonmetallic (organic and inorganic). With either
type of coating the intent is the same, that is, to isolate the underlying
metal from the corrosive media.
Metallic Coatings. The concept of applying a more noble metal coat-
ing on an active metal takes advantage of the greater corrosive resis-
tance of the noble metal. An example of this application is tin-plated
steel. Alternatively, a more active metal can be applied, and in this case
the coating corrodes preferentially, or sacrificially, to the substrate. An
example of this system is galvanized steel, where the sacrificial zinc
coating corrodes preferentially and protects the steel.
Organic Coatings. The primary function of organic coatings in corro-
sion protection is to isolate the metal from the corrosive environment. In
addition to forming a barrier layer to stifle corrosion, the organic coating
can contain corrosion inhibitors. Many organic coating formu- lations
exist, as do a variety of application processes to choose from for a given
product or service condition.
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