CAUSE AND EFFECT
Cause and effect is a
relationship between events or things, where one is the result of the other or
others. This is a combination of action and reaction.
CORROSION
A.
The Meaning of Corrosion
In general, corrosion is the destruction of metal objects
due to environmental influences. The corrosion process can be explained
electrochemically, for example in the process of ironing which forms iron oxide
(F2O3.nH2O). Electrochemically, the process of
ironing is an iron oxidation event of iron by oxygen from the air.
Corrosion in iron due to contact with water. In
the iron there is anode and there is a cathode.
Anode :
F(s) → Fe2+ (aq) + 2e- Eo = +0,44 V
Codes : O2 (g) + 2H2O (l) + 4e-
→ 4OH- (aq) Eo = +0.4 V
Cell
reaction : 2Fe(s) + O2(g)
+ 2H2O(l) → 2Fe2 + (aq) + 4OH-
(aq) Eo = +0.84 V
The Fe2 + ion then undergoes further oxidation by
reaction:
Fe2+(aq)
+ O2(g) + (4 + 2n)H2O(l) → 2Fe2O3.nH2O
+ 8H+ (aq)
Based on the potential value of the
reaction, iron is an easily corrosive metal. Other metals with electrode
potential values greater than 0.4 V will be difficult to corrosive, since
they will produce Eoreact <0 (negative) when in contact with oxygen in the
air. The metals of silver, platinum, and gold have electrode potentials greater
than 0.4 V so it is difficult to experience corrosion.
B. Causes
of corrosion
1. Water and air humidity
Judging from
the reaction that occurs in the corrosion process, water is one important
factor for the ongoing corrosion. Humid air that contains moisture will
accelerate the process of corrosion.
2. Electrolyte
Electrolyte
(acid or salt) is a good medium for the transfer of charge. This results
in the electrons being more easily attached to the oxygen in the air. Rain
water contains acid, while salt water contains salt. Therefore rainwater and
seawater are the main causes of corrosion.
3. Uneven metal
surfaces
Uneven
metal surfaces facilitate the occurrence of poles of charge, which will
eventually act as anode and cathode. Smooth and clean metal surfaces will cause
difficult corrosion, because the poles that will act as anode and cathode are difficult
to form.
4. The formation of
electrochemical cell
If
two different metals are potentially tangent to the aqueous or moist
environment, electrochemical cells may form directly. The lower potential metal
will immediately release the electrons when in contact with a higher potential
metal and will be oxidized by oxygen (O2) from the air. This results
in faster corrosion occurring in potentially low metals, while potentially high
metals are even more durable. For example, a rivet made of copper to connect
iron would cause the iron around the rivet to rust more quickly.
C. Corrosion
Prevention
Corrosion events are difficult to prevent, but can be inhibited in the
following ways:
1. Controlling the atmosphere to keep
it from moisture and plenty of oxygen, for example by making the air
environment free of oxygen by passing CO2 gas.
2. Prevent metals intersect with oxygen in
the air as well as water. Prevention is done in the following way.
a. Paint it
The paint layer prevents direct iron contact with oxygen and water. Only
if the paint is scratched or peeling, then corrosion begins to occur and can
spread under the paint that is still intact. Examples that use this technique
are on boats, bridges and cars.
b. Gives oil, oil or fat
The oil and grease layers prevent direct
contact of iron with oxygen and water and should be applied periodically. Examples
that use this technique are on the moving parts of the machine, like a car
engine.
c. Gives a plastic coating
The plastic layer prevents direct iron
contact with oxygen and water. Only if the plastic is peeling off,
corrosion begins to occur. Examples that use this technique are on kitchen
items, such as dryer racks.
d. Galvanizing
Galvanization is coating metal with zinc
(for example, zinc roof). Zinc (Zn) coating can prevent direct metal
contact with oxygen and water. In addition, the oxidized Zn, resulting in Zn
(OH)2 can react further with CO2 in the air to form a
very strong Zn(OH)2.xZnCO3 oxide layer. When Zn coating
is scratched, Zn can still protect iron because Zn (Eo = -0.76 V) is more
easily oxidized than Fe (Eo = -0.44 V). Examples that use this technique are on
iron supports for building construction and bridges.
e. Electroplating
Electroplating is metal coating with other
metals using electrolysis method. For example, coating with nickel metal
(veernikel), chromium (eg water tap), tin (eg food cans), and lead (eg drinking
water pipes).
f. Chrom plating / Cr
The Cr layer prevents direct metal contact with oxygen and water. In
addition, Cr is oxidized to form a very strong layer of Cr2O3
oxide that can protect the Fedi metal beneath it. When scratched, the Cr layers
can still protect the iron because Cr (Eo = -0.74V) is more easily oxidized
than Fe (Eo = -0.44 V).
g. Tin plating / Sn
The Sn layer can prevent metal direct
contact with oxygen and water. However, Sn (Eo = -0.14 V) is less
reactive than Fe (Eo = -0.44 V). Thus, when the Sn layer is scratched, the iron
underneath starts corrosion.
h. Sherardizing
Sherardizing is reacting metal with
phosphoric acid so that the metal surface is covered with phosphate (Fe3(PO4)2).
For example, car body.
3. Cathodic
Protection
Cathodic protection is made by connecting a metal to be protected by
another metal having a very low electrode potential (usually Mg). When
oxidation occurs, the protected metal will immediately attract electrons from
the protective metal so that oxidation will take place on the protective metal.
Because the protective metal is oxidized, it can eventually be exhausted and
should always be replaced with new ones periodically.
What should we do to decrease a corrosion?
BalasHapusHai Indri, Thanks questions. Corrosion events are difficult to prevent, but can be inhibited in the following ways:
Hapus1. Controlling the atmosphere to keep it from moisture and plenty of oxygen, for example by making the air environment free of oxygen by passing CO2 gas.
2. Prevent metals intersect with oxygen in the air as well as water.Prevention is done in the following way Paint it,Gives oil,Gives a plastic coating,Galvanizing,Electroplating,Chrom plating / Cr,Tin plating / Sn, and Sherardizing
3.Cathodic protection is made by connecting a metal to be protected by another metal having a very low electrode potential (usually Mg)
If we stir a drink using a rusty spoon, but we do not know if the spoon has rusted, and then we drink. What is the effect of our body?
BalasHapusHi Wahyu, thanks for question. If we drink water stirred with a spoon rusted or contaminated by rust it will arise a bad effect for the body because if consumed enough it will be toxic. Because the mercury contains iron metal that can harm health if consumed
HapusElectrolytes (acid or salt) are good medium for charge transfer, explain and give electrolyte samples used as charge transfers
BalasHapusElectrolytes (acid or salt) are good medium for charge transfer because the electrolyte solution easily releases the positive and negative charges. Examples of electrons in electrolyte solutions are more easily attached to oxygen in the air and can corrode.
Hapuswhy is aluminium metal including reactive metal resistant to airbone corosion ?
BalasHapusHi Elta, thank questions. Aluminum is known as a corrosion resistant material. Actually, rusted aluminum quickly forms aluminum oxide (Al2O3). This is caused by the passivation phenomenon, ie the process of forming a layer of aluminum oxide (Al2O3) on the surface of aluminum metal as soon as the metal is exposed by free air. This layer of aluminum oxide prevents further oxidation. However, passivation can occur more slowly if combined with more cathodic metals, as it can prevent aluminum oxidation.
HapusHow can an iron surface form anode and cathode parts?
BalasHapus