CORROSION AND ITS
PREVENTION
(2.3,2.4)
24/PCHA/111
FATHIMA NOURIN M S
,What is Corrosion?
Corrosion is the deterioration or destruction of metals and alloys in the
presence of an environment by chemical or electrochemical reaction. In simple
terms, corrosion processes involve reaction of metals with environment species.
For example : Rusting of Iron, Tarnishing of silver.
❖ FACTORS INFLUENCING CORROSION
Corrosion depends on two most important factors they are:
1. Nature of Metal
2. Nature of Environment.
1.NATURE OF METAL :
Position of metal in Galvanic Series:
• The metal which is placed at higher position in the series are more
reactive and undergoes corrosion.
• The metal in higher position in the galvanic series will act as the ‘anode’
and the metal in lower position in galvanic series will act as the ‘cathode’ .
• The rate and severity of corrosion, depend upon the difference in their
position.
• Greater is the difference, the faster is the corrosion of anodic metal.
Eg. When iron pipes are connected to copper pipes, iron corrodes quickly.
• Metals that are close to each other in series will have fewer tendency to
corrode. Eg. Minimal corrosion in copper pipes and brass fittings.
• Noble metals are least susceptible to corrosive attack.
Eg. Gold,Platinum.
Relative Area of Anode and Cathode:
• When 2 dissimilar metals are in contact, the corrosion of the anodic part
is directly proportional to the ratio of areas of cathodic part and anodic
part.
• Rate of Corrosion α Area of Cathode
Area of Anode
, • Smaller the area of anode, faster is the rate of corrosion.
• This is because the current density at a smaller anodic area is much
greater and the demand for electrons can be met by smaller anodic area
only by undergoing corrosion more rapidly.
eg. Steel screw (small anode) in copper plate( large cathode).
• Larger the anodic area, slower the rate of corrosion.
Purity of Metal :
• Pure metal resists corrosion.
• Presence of impurities, generally leads to the formation of tiny
electrochemical cells and therefore corrosion occurs. (Increasing
corrosion in the anodic part.)
• More the impurity, greater is the rate of corrosion.
Physical State of Metal:
• Rate of corrosion is influenced by physical state of metal i.e., grain
size, stress, orientation of crytals etc.
• Smaller the grain size of metal, greater will be its corrosion.
Eg. Steel gets corroded more easily than cast irons because grains in
steels are of smaller size.
• Area under stress even in pure metal, tend to be anodic and corrosion
takes place at these stressed areas.
• Stressed part is anodic and unstressed part is cathodic.
Nature of Oxide film:
• Every metal is surrounded by its corresponding oxide layers.
• Specific Volume Ratio = Volume of oxide layer
Volume of metal
• Greater the SVR ,lesser is the corrosion rate.
• Lesser the SVR, Greater is the corrosion rate.
• Corrosion takes place in porous film (non-protective).
• No corrosion takes place in non-porous film/dense adherent film
(protective).
• SVR of Chromium is 2 and SVR of Nickel is 1.6.
Oxidation corrosion of Cr is lesser compared to Ni.
PREVENTION
(2.3,2.4)
24/PCHA/111
FATHIMA NOURIN M S
,What is Corrosion?
Corrosion is the deterioration or destruction of metals and alloys in the
presence of an environment by chemical or electrochemical reaction. In simple
terms, corrosion processes involve reaction of metals with environment species.
For example : Rusting of Iron, Tarnishing of silver.
❖ FACTORS INFLUENCING CORROSION
Corrosion depends on two most important factors they are:
1. Nature of Metal
2. Nature of Environment.
1.NATURE OF METAL :
Position of metal in Galvanic Series:
• The metal which is placed at higher position in the series are more
reactive and undergoes corrosion.
• The metal in higher position in the galvanic series will act as the ‘anode’
and the metal in lower position in galvanic series will act as the ‘cathode’ .
• The rate and severity of corrosion, depend upon the difference in their
position.
• Greater is the difference, the faster is the corrosion of anodic metal.
Eg. When iron pipes are connected to copper pipes, iron corrodes quickly.
• Metals that are close to each other in series will have fewer tendency to
corrode. Eg. Minimal corrosion in copper pipes and brass fittings.
• Noble metals are least susceptible to corrosive attack.
Eg. Gold,Platinum.
Relative Area of Anode and Cathode:
• When 2 dissimilar metals are in contact, the corrosion of the anodic part
is directly proportional to the ratio of areas of cathodic part and anodic
part.
• Rate of Corrosion α Area of Cathode
Area of Anode
, • Smaller the area of anode, faster is the rate of corrosion.
• This is because the current density at a smaller anodic area is much
greater and the demand for electrons can be met by smaller anodic area
only by undergoing corrosion more rapidly.
eg. Steel screw (small anode) in copper plate( large cathode).
• Larger the anodic area, slower the rate of corrosion.
Purity of Metal :
• Pure metal resists corrosion.
• Presence of impurities, generally leads to the formation of tiny
electrochemical cells and therefore corrosion occurs. (Increasing
corrosion in the anodic part.)
• More the impurity, greater is the rate of corrosion.
Physical State of Metal:
• Rate of corrosion is influenced by physical state of metal i.e., grain
size, stress, orientation of crytals etc.
• Smaller the grain size of metal, greater will be its corrosion.
Eg. Steel gets corroded more easily than cast irons because grains in
steels are of smaller size.
• Area under stress even in pure metal, tend to be anodic and corrosion
takes place at these stressed areas.
• Stressed part is anodic and unstressed part is cathodic.
Nature of Oxide film:
• Every metal is surrounded by its corresponding oxide layers.
• Specific Volume Ratio = Volume of oxide layer
Volume of metal
• Greater the SVR ,lesser is the corrosion rate.
• Lesser the SVR, Greater is the corrosion rate.
• Corrosion takes place in porous film (non-protective).
• No corrosion takes place in non-porous film/dense adherent film
(protective).
• SVR of Chromium is 2 and SVR of Nickel is 1.6.
Oxidation corrosion of Cr is lesser compared to Ni.
