Corrosion Reaction on Single Metal
Corrosion is a natural
process, which converts a refined metal to a more chemically-stable form, such
as its oxide, hydroxide, or sulfide. The mechanisms of corrosion are the same
at a microscopic level and various microstructures, composition, and mechanical
design issues lead to different manifestations of corrosion. For example, a
single piece of metal, Fe, when comes in contact with acid, HCl, small galvanic
cells may be set up on the surface. Each galvanic cell consists of anode and
cathode regions. The interaction taking place in these two regions is as
follows.
Reaction at the anode:
Fe on the iron liberates two electrons to the metal and itself becomes Fe++
ion. Since the Fe++ ion is soluble in water it is released in the
medium. This causes corrosion of the iron surface.
Reaction at the cathode:
The released electron is conducted through the metal piece to the cathode
region. Two electrons are supplied to two protons (H+) to form two
atoms of hydrogen. Hydrogen atoms being unstable, two H atoms combine to form a
stable molecule H2. In the absence of acid, water itself dissociates
to generate an H+ ion.
2H+ + 2e−
→ H2 …(1)
Hydrogen (H2)
forms bubbles on the metal surface. If the rate of hydrogen formation is very
slow then a film of H2 bubbles will be formed that will slow down the cathode
reaction, hence the rate of corrosion will slow down. If the rate of hydrogen
production is very high then hydrogen molecules cannot form the film on the
surface. So the corrosion proceeds rapidly.
Corrosion Reactions between Metals
As is known corrosion of
metals is an electrochemical reaction involving changes in metal as well as the
environment in contact with the metal. If two metals come in contact with a
common aqueous medium then one metal will form an anode and the other will form
the cathode. Now, if both the metals are connected with a wire the reaction
will proceed. Anode metal will be corroded and hydrogen will form at the
cathode.
For example, if zinc and
a copper plate are immersed in an acidic medium then zinc will form an anode
and will be corroded while hydrogen will be formed at the copper plate.
Anode reaction: Zn → Zn++
+ 2e− … (2)
Cathode reaction: 2H+
+ 2e− → H2 …(3)
So anode will be corroded
and hydrogen will be evolved at the cathode.
Corrosion Involving Oxygen
The oxygen dissolved in
the electrolyte can react with accumulated hydrogen to form water. Depletion
(reduction) of the hydrogen layer allows corrosion to proceed.
At cathode: O2
+ 2H2 → 2H2O … (4)
The above reaction takes
place in an acid medium. When the medium is alkaline or neutral oxygen is
absorbed. The presence of moisture also promotes corrosion.
Factors Influencing Corrosion:
(a) The pH of the
solution: Iron dissolves rapidly in acidic pH. Aluminum and
zinc dissolve both in acidic and alkaline pH. Noble metals such as gold and
platinum are not affected by pH.
(b) Oxidizing agents:
Oxidizing agents may accelerate the corrosion of one class of materials whereas
retard another class. For example, O2 reacts with H2 to
form water. When H2 is removed corrosion is accelerated. The
presence of Cu in NaCl solution also follows this mechanism. Oxidizing agents
form a surface oxide (like Aluminium oxide) and make the surface more resistant
to chemical attack.
(c) Velocity:
When the corrosive medium moves at a high velocity along the metallic surface,
the rate of corrosion increases. This is due to the rapid formation and washing
away of corrosion products to expose new surfaces to corrosion reactions. The
corrosion is rapid in the bends in the pipes, propellers, agitators, and pumps.
Due to high velocity, the accumulation of insoluble films on the surface is
prevented.
(d) Surface films:
Thin oxide films are formed on the surface of stainless (rusting). These films
absorb moisture and increase the rate of corrosion. For example, zinc oxide
forms porous films. The fluid medium can enter inside the surface and thus
corrosion continues. Nonporous films of chromium oxide or iron oxide prevent
corrosion. Grease films protect the surface from direct contact with corrosive
substances.
Make sure you also check our other amazing article on : Methods of Prevention of Corrosion?