GROUP NO 6
Group Members Name
Muhammad Mudeser khalid
Definition of Corrosion
Corrosion is the slow process of decay of the
metal, due to the attack of the atmosphere gases
on the surface of the metal, resulting in the
formation of metallic compounds such oxides,
hydroxides, carbonates, sulphides etc.
FORMS OF CORROSION
1. Dry or Chemical Corrosion:
It is due to the direct chemical action of
environmental atmosphere gases such as O2.
H2S, SO2, N2, halogens or anhydrous
inorganic liquids with metal surfaces.
2. Wet or Electrochemical Corrosion:
It is due to the existence of separate
“anodic” and “cathodic” areas in the system
between which current flows through the
conducting liquid and the anode gets oxidized
WET / AQUEOUS CORROSION
Based on the appearance of the corroded metal,
wet corrosion may be classified as
Uniform or General
Galvanic or Two-metal
Corrosion over the
entire exposed surface
at a uniform rate. e.g..
Maximum metal loss by
Not dangerous. Rate
can be measured in the
When two dissimilar
metals are joined together
and exposed, the more
active of the two metals
corrode faster and the
nobler metal is protected.
This excess corrosion is
due to the galvanic current
generated at the junction
a) Steel plates with copper
rivets in seawater 15 months.
b) ) Copper plates with steel
rivets, same environmental
crevices & shielded areas
on metal surfaces
Small volumes of
caused by holes, gaskets,
surface deposits, lap
A form of extremely
causing holes in the
Most destructive form
Difficult to detect and
Electrochemical Theory of Corrosion
(Mechanism Of Corrosion)
Corrosion is an electrochemical process.
Galvanic cells are setup between dissimilar metals in contact with each
other or between dissimilar parts of he same metal when surrounded by
moist air or liquid.
Anodic area oxidation takes place
Cathodic area reduction takes place.
Metallic ions formed at the anodic part and ions formed at the
cathodic part diffuses towards each other through conducting
medium and the corrosion product is formed somewhere between
the anodic and cathodic areas.
Anodic iron gets oxidised to Fe2+
Mn+ + ne-
The oxygen at the cathode changes to OH- by reduction.
½ O2 + H2O +2e2 OH-
Rate & Extent of corrosion
1. The position of metals in the electrochemical Series
Gold---------------------- +1.15 V
order of std
2. Purity of the metal
A pure metal is more corrosion resistant than an
impure metal. The rate and extent of corrosion
increases with the amount of impurities present.
3.Physical state of the metal
Smaller the size of the metal, more the area under
stress and greater is the corrosion.
4. Solubility and volatility of corrosion product
If the product of corrosion is soluble in the
corroding medium and also is volatile, corrosion
6. Nature of the corroding environment
A.Temperature: Corrosion generally increases
with rise in temperature of environment.
B.Humidity of the air: Rate of corrosion
increases with presence of moisture in the
C.Impurities: Presence of impurities like CO2,
H2S, SO2, acid fumes etc increases corrosion
D.Influence of pH: In acid medium corrosion is
more and in alkaline medium it is less.