Corrosion is the phenomenon of metals losing their metallic properties as a result of electrochemical reactions with the environment. This is one of the most damaging events over time to metal materials. Due to corrosion, the bridges and buildings we build can collapse and our pipelines can be damaged. When designing any design using metals, corrosion is an important factor to consider; because it directly concerns the building safety and the life of the structure.
It is difficult to observe the effect of corrosion in
short time periods; however, as the time scale increases, it is possible to see
the damage caused by corrosion much more clearly: The most famous example of
this is undoubtedly the Statue of Liberty in New York City, USA. The Statue of
Liberty was originally a brownish color when it was gifted to the United States
by France. However, with the effect of corrosion over time, the metal sculpture
has turned into a green-like color that we know today and makes the sculpture
famous.
Many of the metals we use are found in nature bonded
with sulfur or oxygen; because this form is two of the most stable states of
chemical compounds. To remove these compounds from this form and turn them into
"pure metals" requires energy at very high temperatures. Sulfurous
and oxidized metal compounds are stable and inert; however, metals purified by
giving high energy are much more mobile and they try to return to their former
more stable state by giving out this energy in their structure. This is the
main reason for the corrosion of the metals we use in our structures.
The ease with which metals corrode differs from
element to element; but in general the more active a metal is, the more easily
it corrodes. As long as the metal surface is not passivated, we can determine
the corrosion activity by looking at standard electrode potentials.
All ionic solutions and natural waters can cause
corrosion as electrolytes (free ion containing and conductive media). In
addition, the water vapor in the humid air condenses on the surface of the
metals, creating a favorable environment for corrosion.
Electrochemical
Corrosion
In essence, we can compare corrosion to the galvanic
battery (or galvanic cell), which we have described in detail before. If you
remember, galvanic cells are basically batteries that can generate electrical
energy from chemical energy. Electric current is obtained by the reduction and
oxidation reactions that take place in the galvanic cell.
In the case of electrochemical corrosion, different
parts of a metal act as anode and cathode. In other words, some of the same
material shows the anode feature, while another part shows the cathode feature,
and a redox reaction takes place between these two pieces. As a result of this
event, the structure of the parts of the metal that are oxidized by acting as
an anode or reduced by acting as a cathode is destroyed. For example, oxidation
can be represented by the formula:
X→Xn++ne-
At the cathode, the "half-cell" where
reduction takes place, there are two different possibilities:
- Hydrogen reduction
occurs if the environment is acidic (pH<7):
2H++2e-→H2
- If the environment
is neutral and there is dissolved oxygen in the environment, the oxygen
gains electrons to form hydroxyl:
½O2+H20+2e-→s2OH-
Natural water bodies such as lakes, streams and seas
are generally basic and can cause serious corrosion in the long run when they
come into contact with metal because they have electrolyte properties.
In the corrosion cell shown above, different regions
of the same metal acted as both anode and cathode. While the metal dissolves at
the anode, the hydroxyl ion is released at the cathode. Then, the iron meets
with the released hydroxyl ion to form the more stable iron hydroxide
(Fe(OH)2).
· The reaction
that takes place in the part that acts as the anode:Fe→Fe2++ 2e-
- The reaction that takes place in the part that acts as the cathode:½O2+
H20 + 2e-→2OH-
- Net reaction: Fe + ½
O2 + H2O → Fe(OH)2
In the presence of sufficient oxygen in the environment,
iron(II) hydroxide comes to iron(III) hydroxide.
2Fe(OH)2 + ½O2 + H2O
→ 2Fe(OH)3
Corrosion
Spontaneity
As in every case, we can take help from the laws of
thermodynamics in order to determine in advance whether corrosion will occur on
purpose or not. For this, we need to calculate the change in Gibbs free energy.
We can say that if the change in Gibbs free energy (ΔΔG) is less than zero, the
event may occur voluntarily, and if it is greater than zero, it will not
happen.
Korozyonun gerçekleştiği zemini bir elektrokimyasal
hücre olarak kabul ettiğimiz için kullanacağımız ifade şudur:
ΔG=−nFEpil
Since our cell is not in standard conditions (0oC,
1 atm), we need to calculate the cell potential using the Nernst equation.
E=E0−RTnF∗lnQ
After calculating the cell potential using
this equation, we can predict whether corrosion will occur when we find the
Gibbs free energy change.
Types of
Corrosion
Corrosion is also divided into different types
according to the way it occurs. In this section, we will examine these types
step by step.
Uniform
(Single Type, Regular) Corrosion
Uniform corrosion is a type of corrosion that occurs
at the same rate in all parts of the metal. This type of corrosion rarely
causes major problems, as it is somewhat easier to measure and predict than
others. In most cases, it will only cause problems in terms of appearance.
Pit Corrosion
If cavities occur on the metal surface due to
corrosion, this is called pitting corrosion. This type of corrosion, unlike
uniform corrosion, does not occur at the same rate in all parts of the surface.
The anode and cathode are strictly separated from each other. The anode, that
is, the half-cell where the amplification takes place, is inside the pit, while
the area outside the pit acts as the cathode. In general, when the surface
protection layer is damaged or cracked, some of the metal becomes the cathode. It
is a very dangerous species as it has the potential to completely pierce metal.
Pitting corrosion can be caused by:
- Turbulent fluid flow
- Uneven protective
coating
- Cracks in the
protective coating
- Exposure of the
protective coating to a strong chemical
- Uneven pressure
- Scratches, scrapes
and small chips
Galvanic
Corrosion
Galvanic corrosion is a type of corrosion that occurs
due to the joining of two different metals or taking place in the same
electrolyte. In this cell, the metal with electronegative potential (or the
more electronegative metal if both are electronegative) acts as the anode. As
an example, we can consider the electrode wear phenomenon in galvanic
batteries. . For galvanic corrosion, three conditions must be met
simultaneously:
- Different types of metals: Galvanic corrosion is possible when two
different types of metal are in contact.
- Presence of electrolyte: The contact area must be wetted with an aqueous
liquid to ensure ionic conduction. Otherwise galvanic corrosion will not
be possible.
- Electrical continuity between two metals: : Electrical continuity between metals can be achieved by direct
contact or a connection between two metals such as a bolt.
Crack
Corrosion
Crevice corrosion may occur due to a difference in ion
concentration due to cracks or voids on the metal surface. Usually the starting
fuse is ignited by a difference in the concentration of oxygen. Crevice
corrosion can occur at lower temperatures than pitting corrosion. Proper joint
design is a useful measure in minimizing crevice corrosion.
Intergranular
Corrosion
At the microscopic level, metals and alloys have
small, distinguishable regions called 'grains'. The impure areas between these
particles, heat treatment and welding can cause changes in metal composition, which
can cause intergranular corrosion. An example of this is iron grains in
aluminum.
One of the most common examples of intergranular
corrosion is seen in stainless steels. Stainless steel becomes susceptible to
corrosion at temperatures between 500-800°C.
Erosion
Corrosion
Erosion corrosion occurs as a result of the flow of a
corrosive fluid on the metal surface. Since the flow is continuous, the
products formed are dragged by the flow and the surface of the metal is
constantly exposed to the corrosive fluid. The amount of wear is directly
related to the flow rate.
Stress
Corrosion
Stress corrosion is the result of a combination of
tensile stress and a corrosive environment that usually exists at elevated
temperatures. It is also used for cold forming, welding, machining, grinding,
etc. It can also be caused by the stress applied during the manufacturing
process. As a result, cracks form on the metal surface and accelerate
corrosion.
Measurement
of Corrosion Rate
The measurement of the corrosion rate is important for
preventing corrosion and predicting the damage that may occur. It is defined as
the mass of metal that moves away from the unit surface area per unit time. In
general, grams per square decimeter per year (g/dm2×years) are used
for this. As another unit, the decrease in metal thickness over a certain
period of time (mm/year) can also be used.
How to
Prevent Corrosion?
It is possible to prevent corrosion or reduce the rate
of corrosion with some precautions. For this, we basically need to cut the
interaction of the surface with oxygen and corrosive substances. . Some of
those:
- Painting
- Lubrication
- Plastic coating
Another method is to use another metal to be
sacrificed. In this method, the surface is covered with another more reactive
metal. The more reactive metal oxidizes more easily, so while the base metal is
protected, another metal is sacrificed. In this way, the material and structure
to be protected will not be eroded.
Conclusion
Corrosion is one of the most destructive and important
natural events that threaten building safety. For this reason, the rate of
corrosion of the material used in the construction of any structure, its
activity and how it will be protected against external factors are the basic
questions to be considered.
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