Corrosion, Its causes, affecting factors and
its Prevention
Definition of Corrosion
The slow, continuous
and spontaneous destruction of metals at their exposed surface due to their
interaction with atmospheric oxygen and water involving the conversion of
uncombined metal into its oxide or hydrated oxide or carbonates by the harmful
and undesirable combined action of atmosphere or any chemical agent is called
corrosion. It is essentially an Electro-Chemical
Oxidation-Reduction Reaction.
Metal + Air + Water → Metal Oxide
Metals are eaten away
or corroded by the process of corrosion on their exposed surface followed by
the corrosion of the inner surface and then continues up to certain depth.
Metals to be effected
Corrosion affects
almost all metals except noble metals like Ag, Au and Pt. corrosion is said to have been taken
place when
1. Iron gets coated with a brittle reddish
brown flaky rusty layer.
2. Copper develops green coloured deposits.
3.
Zinc
is coated with whitish deposits.
4. Aluminium surface becomes dull and loses its
shine.
5. Silver gets tarnished i.e. it loses its shine.
Rate of Corrosion
Different metals
corrode at different rates on exposure to the atmosphere or water or any
reactive agent. e.g.
(1). Alkali metals like Na and K readily tarnish
in air.
(2). Alkaline earth metals like Ca corrodes
quickly.
(3). Iron gets rusted slowly.
(4). Other less reactive metals like Sn, Pb and Cu
corrode very slowly.
Adverse effects of Corrosion
Corrosion causes
enormous damage to the buildings, bridges, ships and many other things made up
of metals especially iron. e.g.
1. It reduces the mechanical strength of metals
making them weak and brittle.
2.
It
decreases conductivity of metals for heat and electricity.
3. It reduces resistance of metals for acids and
alkalis.
Cause of Corrosion
The corrosion of
metals is due to combined effect of atmospheric oxygen and water which is
generally accelerated by the presence of CO2, acids and gaseous
pollutants like SO2 in the air.
Rusting of iron
Rusting is the
process of corrosion of iron which is an electrochemical redox reaction during
which iron is covered or coated by combining with atmospheric moist oxygen in
presence of water developing a porous, thin, flaky and easily detachable
Reddish Brown Deposits called Rust, which consists of hydrated ferric oxide Fe2O3.nH2O.
4Fe
|
+
|
3O2 + 2nH2O
|
→
|
2Fe2O3.nH2O
|
Conditions favouring Rusting
Following
conditions favour the rusting of iron
1.
|
Presence
of moisture
|
|
2.
|
Presence
of oxygen or air.
|
|
3.
|
Presence of
weakly acidic atmosphere.
|
|
4.
|
Presence
of electrolyte (like NaCl, H2SO4)
|
|
7.
|
Presence
of dissolved salts in water. (e.g. iron corrodes faster in sea water)
|
|
5.
|
Presence
of impurity in iron.
|
|
6.
|
Presence
of reactive gases like CO2, SO2, NO2 etc.
|
|
8.
|
Presence
of uneven surface of iron.
|
|
9.
|
Presence of rust
itself (auto catalysis).
|
|
10.
|
High temperature
|
|
Prevention from Corrosion
The protection of
iron from rusting can be brought about by:
1.
|
By electroplating
|
e.g.;
|
Nickel plating, chromium plating etc.
|
2.
|
By protective metallic coating;
|
e.g.;
|
galvanizing (coating with zinc), tin plating
|
3.
|
By alloying with other metals;
|
e.g.;
|
stainless steel (Fe, Cr and Ni).
|
4.
|
Non-metallic
material coating;
|
e.g.;
|
Paint,
oils, grease, varnish, plastic emulsion.
|
5.
|
By other
coating; phosphate coating,
|
e.g.;
|
concrete coating, Fe3O4
coating.
|
6.
|
By using corrosion inhibitors;
|
e.g.;
|
K2CrO4.
|
Tin Plating
Definition
The process of
depositing or coating thin layer of metallic tin on base metals (e.g. Fe, Cu)
to protect them from Corrosion is called Tin Plating.
Methods of Tinning the Metals
The process of
tinning may be carried out by 2 methods:
1. Mechanical Tin Plating.
2. Electrolytic Tin Plating.
1. Mechanical Tin Plating
(a) Tinning of Sheets
(i) The iron sheet
which is to be tinned is cleaned thoroughly by dipping in dilute H2SO4
to remove oxide layer.
(ii) The sheets, after washing and drying, then dip
in the bath containing Molten Tin (on the surface of which floats a flux, ZnCl2).
(iii) After the process of completion of tinning, the
tinned sheets of iron are then passed through Hot Roller to give uniform
coating by removing superfluous tin.
(b) Tinning of
Utensils
(i) The utensils are cleaned by ammonium
chloride.
(ii) After cleaning, they are heated
strongly.
(iii) A small quantity of tin is rubbed on the
utensils. It produces a thin layer of
tin all over the utensils.
2. Electrolytic Tin Plating
(i) The
process is carried out in an electrolytic cell.
(ii) Cathode is made up of the metallic object
which is to be coated.
(iii) Anode is made up of the tin plate.
(iv) Both electrodes are immersed in electrolytic
mixture which consists of salt of tin such as stannous chloride (SnCl2)
and hydrochloric acid (HCl).
(v) On passing electric current, oxidation takes
place at anode where tin atoms change to stannous ions by the loss of two
electrons.
Sn
|
–
|
2e–
|
→
|
Sn2+
|
(Stannous ion)
|
––––
|
Oxidation
|
(vi) These Sn2+ ions migrate towards
cathode which is to be coated, where Sn2+ ions are reduced by
the gain of two electrons to form tin metal which is deposited over
metallic object.
Sn2+
|
+
|
2e–
|
→
|
Sn
|
––––
|
Reduction
|
Stainless Steel
Definition
Stainless steel is a term applied to the alloy of iron with chromium and nickel containing very
low percentage of carbon which shows resistance to corrosion.
Composition
Stainless
steel contains 0.15–2.0% of carbon.
In addition, some metals chromium or nickel are added to give the steel of
desired quality and property.
Properties
1. It is hard, heat resistant and has high tensile
strength and it is practically non-oxidisable.
2. It gives resistance to corrosion and usually
considered as rust proof.
Types of Steel
There are three types of stainless steel according to
%ages of different constituents.
1. Mild Steel (Low
carbon steels) (13% Cr,
5% Ni and 0.1–0.4% carbon)
2. Medium Steel
(Medium Carbon Steels) (17% Cr, 6% Ni
and 0.2–0.6% carbon)
3. Super or Hard
Steels (High Carbon Steels) (18% Cr, 8% Ni
and 0.18% carbon)
1. Mild Steel (Low carbon steels)
It
contains 13% Cr, 5% Ni and 0.1–0.4%
carbon. It is soft, malleable and ductile. It is used in making boiler plates,
rods and nut bolts.
2. Medium Steel (Medium Carbon
Steels)
It contains 17% Cr, 6% Ni and 0.2–0.6% carbon. It
is used in preparing Axle, Beams.
3. Super or Hard Steels (High Carbon
Steels)
It
contains 18% Cr, 8% Ni and 0.18%
Carbon. It is used in making surgical equipments, blades and automobile parts.
Silvering of Mirrors
Definition of Silvering of Mirrors
The process of
deposition of thin and uniform layer of silver on a clean glass surface by the
reduction of silver ions by an organic compound forming light reflecting
surface (mirror) is called Silvering of Mirrors. This process was first
discovered by Sliebig in 1938.
Principle of Process
The
process is based on the reduction of Ammonical Silver Nitrate (Tollen’s
Reagent, a silver complex, which acts as oxidizing agent) by organic compounds
containing aldehydic group (–CHO) i.e. glucose, formaldehyde, tartarates (acts
as reducing agent) to metallic silver. Aldehyde gets oxidized to carboxylic
acid. Silver formed is deposited in the form of mirror on the walls of glass.
Solutions required
1. Ammonical
Silver Nitrate Solution (Tollen’s Reagent)
It is prepared by
adding NH4OH solution to AgNO3 solution, till the brown
precipitate of Ag2O first formed is re-dissolved.
2. Solution
of Reducing Agent
Glucose or
Formaldehyde (HCHO) solutions are reducing agent. Rochelle salt (sodium
potassium tartarate) can also be used as Reducing Agent.
Method of Silvering the Glass
The process of
silvering of mirror is summarized as:
1. The glass plate to be silvered is cleansed with
alcoholic KOH, then washed with
distilled water and dried.
2. The clean glass plate to be silvered is brought
in contact with ammonical silver nitrate solution, over which an equal amount
of organic reducing agent such as formaldehyde is poured.
3. The reduction of silver ions immediately begins
and metallic silver produced is deposited on the glass as a thin film.
4. The plate is washed with water, dried and
polished.
5. The back of
glass is coated with a mixture of red lead (Pb3O4) and
turpentine (a varnish); to get a looking glass.
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