This Conceptual Power Point project consists of a grieve description about the science of Corrosion, perfect for covering whole topic for short period of time. The project consists of basic knowledge regarding corrosion, its type of causes that is dry and wet corrosion. It also summarizes about different types of differential corrosion and its remedies. Its allover a good conceptual ppt for students of chemistry in their prelims. It's good for the ones achieving for engineering degrees.

1. CORROSION CHEMISTRY:
CAUSES AND REMEDY
Represented By :
Arijit Dhali

2. INTRODUCTION TO CORROSION
Corrosion can be defined as the process by which the metals have a tendency to stay in
the combined state such as oxides, carbonates, sulphides, chlorides etc.
Metallic Ore (Stable State) Pure Metal (Unstable State)
Extraction of
Metal
Corrosion Reaction
(with environmental elements
)
Change in properties of metal :
• Weakening of materials due to loss of cross sectional area.
• Decaying of surface of the materials.
• Loss of properties like malleability and ductility.
Common examples of corrosion :
• Rusting of iron : Iron is exposed to atmospheric conditions to form reddish brown
scale of Fe3O4.
• Tarnishing of silver : Metallic silver is exposed to atmospheric conditions to form
black coating of Ag2O.

3. THEORIES OF CORROSION
(Dry) Chemical Corrosion
It involves direct chemical attack of atmospheric
gases like CO2, O2 , H2S, SO2 , halogen, moisture
and inorganic acid vapours on metal .
Example, tarnishing of silver ware in H2S laden air.
There are 3 types of (dry) chemical corrosion :
1. Oxidation Corrosion : Takes place by direct
action of oxygen at high & low temperature in
absence of moisture.
2. Corrosion by Other Gases : The occurrence of
corrosion due to oxidation of other gases like
CO2, SO2, Cl2, H2S, F2 on metal.
3. Liquid – Metal Corrosion : The corrosion
occurs due to action of flowing liquid at high
temperature on solid metals or alloys.
(Wet) Electrochemical Corrosion
It occurs due to setting up of a large number of tiny
galvanic cells in metals in presence of an impurity
as well as in presence of moisture. Generally
impurity (more active metal) act as anode and
original metal act as cathode so anode is the area
where corrosion occurs. Example, rusting of iron in
moist atmosphere.
There are 2 types of (wet) electrochemical corrosion
reaction :
1. Anodic Reaction.
2. Cathodic Reaction.
• Evolution of Hydrogen
• Absorption of Oxygen

4. (DRY) CHEMICAL CORROSION
 Occurrence :
 Due to direct chemical reaction of atmospheric gases.
 Due to molten metal in contact with metal surface.
 Types :
 Oxidation corrosion
 Corrosion by other gases
 Liquid-Metal Corrosion
 Pilling – Bed Worth Rule :
A protective and Non-Porous metal oxide layer has
volume equal to or greater than the volume of metal
from which it is formed. A Non-Protective and Porous
metal oxide layer has volume lesser than the volume of
metal from which it is formed.
Specific Volume Ratio : Volume of oxide formed
Volume of metal
 Corrosion by Other Gases :
• Ag + Cl AgCl (Non –Porous Layer)
• Fe + H2S FeS ( Porous Layer) + H2
• Sn + 2Cl2 SnCl4 (Volatile Layer)
 Liquid-Metal Corrosion :
 Occurs due to the action of flowing liquid metal at high
temperature on solid metals or alloys.
 Observed in nuclear reaction where Na metal used as a
coolant leads to corrosion of Cd.
(Dry) Chemical Corrosion Visual Representation
 Oxidation corrosion :
 Occurrence :
• Due to direct chemical reaction of atmosphere O2 with
metal surface forming metal oxide.
• Absence of moisture.
• Increases with increase in temperature.
 Mechanism :
• On exposure to atmosphere , metal gets oxidized to
form metal ions
M(S) M+n + ne-
• Electrons lost by metal are taken up by Oxygen to form
oxide ions
½O2 + 2e- O2-
M + n ½O2 2M+n + nO2-
M2On
(Metal

5. (WET) ELECTROCHEMICAL CORROSION
 Occurrence :
 When a metal is contact with moist air or any liquid
medium.
 When two different metals are partially immersed in a
solution.
 Chemically non- uniform surfaces of metals behave like
electrochemical cells in the presence of water containing
dissolved O2 & CO2.
 Always occurs at anodic areas.
 Mechanism :
 Involves oxidation- reduction process
 Depending on the nature of corroding environment,
electrons released at anode are consumed at the
cathodic area by two ways :
 Evolution of H2.
 Absorption of O2.
 Absorption of O2 :
 Anode :
M(S) M+n + ne- (Oxidation)
 Cathode :
O2 + 4e- + 2H2O 4OH- (Reduction)
 Overall Reaction :
4M(S) + 2nH2O 4M+n + 4OH-
 Evolution of H2 :
 Anode :
M(S) M +n + ne (Oxidation)
 Cathode :
2H+ + 2e- H2(g) (Reduction)
 Overall Reaction :
2M(S) + 2nH+ 2M+n + nH2 (g)

6. TYPES OF CORROSION
Galvanic Corrosion
When two different metals are present in contact with each
other in conducting medium.
Concentration Cell Corrosion
Occurs due to different in potential aerated areas. Part of
metal exposed to air is more oxygenated part & acts as
CATHODE. Part of metal immersed in electrolyte is poorly
oxygenated & acts as ANODE.
Stress Corrosion
It is the combined effect of mechanical stress and corrosive
environment of material.
Pitting Corrosion
It is formed as a result of pit and cavities, localized attack
and formed by cracking protective coating.

7. FACTORS EFFECTING CORROSION
Nature of metal
• Purity of metal.
• Physical state of metal.
• Position of metal in Galvanic series.
• Nature of Oxide film.
• Nature of products of corrosion.
• Relative areas of Cathode & Anode.
Nature of Corroding Environment
• Temperature and Humidity.
• Effects of pH.
• Presence of impurities in atmosphere.
• Nature of electrolyte.
• Solubility of products of corrosion.

8. CORROSION CONTROL
The common methods for controlling corrosion are :
 Protection by proper design and fabrication procedure.
 Proper material selection.
 Change in environmental condition.
 Modifying the properties of metal.
 Application of inhibitors.
 Use of protective coatings.
 Cathodic Protection or Electrochemical Protection :
 Galvanic Protection : In galvanic protection or sacrificial anodic protection, the metallic structure to be
protected is connected by a wire to a more anodic metal.
 Impressed Current Cathodic Protection : In this method , an impressed current is applied in the opposite
direction to nullify the corrosion current, and converting the corroding metal from anode to cathode.
 Passivation : It is the phenomenon in which a metal exhibits a much higher corrosion resistance than
expected from its position in the electrochemical series.

9. Reference :
• Chemistry I By : Gourkrishna Dasmohapatro
• Link : https://www.learnpick.in/prime/documents/ppts/details/4116/corrosion