Electrochemical corrosion mechanism of alkaline absorption in alkalne medium

1. Electrochemical
Corrosion
Mechanism of oxygen absorption in alkaline medium
Presented by : ASWIN

2. Introduction: What is corrosion and types of Corrosion
What is corrosion ?
Corrosion is defined as the “destruction or deterioration of a metal or
Its alloy and consequent loss of metal, caused due to direct chemical
action or electrochemical reactions with its environment”.
Types of corrosion :
depending on medium in which corrosion occurs :
1- Dry Corrosion (explained by chemical theory)
2- Wet corrosion (explained by electrochemical
theory)

3. Rusting of iron
Rusting of Aluminium

4. 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.

5.  Occurs
o When a metal is contact with moist air or any liquid
medium
o When two diff. metals are partially immersed in a soln.
o Chemically non- uniform surfaces of metals behave like
electrochemical cells in the presence of water containing
dissolved O2 &CO2
oAlways 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 .
(i)Evolution of H2
(ii)Absorption of O2

6. Absorption of O2
This type of corrosion takes place in neutral or basic medium in the
presence of oxygen. The oxide of iron covers the surface of the iron.
The small scratch on the surface creates small anodic area and rest
of the surface acts as cathodic area

7. Anode:
M------—> Mn+
+ ne_
Cathode:
O2 (g) + 4e- + 2H2O ------—> 4OH-
Overall reaction:
4 M +2nH2O ------—> 4Mn+ + 4OH-

9. EXAMPLES :
RUSTING OF IRON IN ALKALINE MEDIUM:
This type of corrosion takes place in neutral or basic medium in the presence of oxygen. The oxide of
iron covers the surface of the iron. The small scratch on the surface creates small anodic area and rest
of the surface acts as cathodic area
At anode:
Fe Fe++ + 2e-- (oxidation)
At cathode:
O2+H2O+ 2e 2OH (reduction)
Fe + O2 +H20 Fe+++ 2OH or Fe(OH)2

10. White Rust in Zinc
Reaction similar to rusting occurs when zinc is
exposed to water or moist air followed by
natural drying.
The resulting zinc oxide is the whitish deposit
seen on galvanized pails, rain gutters, and
imperfectly chrome plated bathroom faucets It
also familiarly called 'white rust' a non-
protective and even destructive form of
corrosion that attacks incompletely passivated
galvanized steel material or galvanized
components subjected to marine
atmospheres.
2 Zn + 2 H2O + O2 Zn(OH)2 (ppt)
Zn(OH)2 ZnO + H2O

11. References:
Slideshare.net
Corrosion by Dr swastika das
Theories of corrosion by Sheama Farheen Savanur
https://corrosion-doctors.org/Chemistry-of-Corrosion/corrosion-in-neutral.htm
Abbreviations:
ppt – precipitate