Wet or Electrochemical corrosion, Mechanism of electrochemical corrosion, Evolution of hydrogen and absorption of oxygen type cathodic reaction, Distinction between dry and wet corrosion.

1. Prepared by:
Prof Sarala Prasanna Pattanaik
Assistant Professor
Department of Chemistry

3. • Wet or Electrochemical Corrosion
• Mechanism of Wet or Electrochemical Corrosion.
• Types of Cathodic reaction:
• Evolution of H2 and absorption of O2 type.
• Distinction between Dry and Wet corrosion.

4. Rusting of Iron in presence of Electrolyte:-
Fe → Fe2+ + 2e– (oxidation)
1/2O2 + H2O + 2e– → 2OH– (reduction)
Overall:- Fe + 1/2O2 + H2O → Fe2+ + 2OH– → Fe(OH)2
Or, 2Fe + O2 + 2H2O → 2Fe2+ + 4OH– → 2Fe(OH)2
In the presence of excess Oxygen:-
4Fe(OH)2 + O2 + 2H2O → 4 Fe(OH)3 or 2Fe2O3. 3H2O
Metals when exposed to the acidic solutions:
4Al + 3H2SO4 → 2Al2(SO4)3 + 3H2
Fe + H2SO4 → FeSO4 + H2
Zn + H2SO4 → ZnSO4 + H2
Zn + 2HCl → ZnCl2 + H2
2Al + 6HCl→ 2AlCl3 + 3H2

5. Wet or Electrochemical Corrosion
 Occurs:-
• When a metal is in contact with moist air or any electrolyte or liquid medium.
• When the parts of a metal become differently aerated or exposed to different concentration of
Oxygen.
• When two different metals are partially immersed in a solution or their respective salt solutions.
• Chemically non uniform surfaces of metals behave like electrochemical cells in the presence of
water containing dissolved O2 & CO2.
• When a metal is stressed at sharp corners, bends, pits, crevices, rivets, joints etc and exposed to a
specific electrolytic medium.
• Corrosion occurs at anodic areas only while cathodic area is said to be protected.
 Mechanism:-
• Involves Oxidation – Reduction or Red - ox reaction taking place separately at anode and cathode
in contact with each other.
• Corrosion occurs through the mechanism of electrochemical reaction.
• It is a fast process and occurs on heterogeneous metallic surface only.
• Electrons move from anode to cathode through the metal or through the corrosive medium.
• Depending on the nature of corroding environment, electrons released at anode are consumed at
the cathodic area by two ways:
 Evolution of H2 type cathodic reaction
 Absorption of O2 type cathodic reaction

6. Evolution of H2 type Cathodic reaction
Mechanism:-
• Occurs when a metal is exposed to or comes in contact with an acidic medium.
• Metallic part at anodic area undergo dissolution with the release of electrons.
• Electrons flow through the metal from anode to the cathode, where H+ ions of
the acidic solution are liberated as H2.
• Such corrosion involves displacement of H+ ions from the acidic solution by the
metal ions due to which pH of the acidic solution increases. Hence all the metals
above the hydrogen in Electrochemical Series have a tendency to get dissolved in
acidic solutions with evolution of hydrogen.
• In such type of Corrosion, anode is usually large metallic area where as cathode
is small metallic area.
Reactions:- Fe(s) + 2HCl → FeCl2 + H2(g)
Reaction at anode:- Fe(s) → Fe+2 + 2e- (Oxidation)
Reaction at cathode:- 2H+ + 2e- → H2(g) (Reduction)
Overall Reaction:- Fe(s) + 2H+ → Fe+2 + H2(g)

7. Mechanism of Corrosion by H2 Evolution

8. Mechanism of Corrosion by H2 Evolution

9. Absorption of O2 type Cathodic reaction
Mechanism:-
• Occurs when a metal is exposed to a basic or neutral solution (such as salt solution) in
the presence of oxygen.
• Due to the presence of oxygen, the surface of the metal is usually coated with a thin
layer of oxide. If the oxide layer develops some cracks, anodic area is created while
rest of the well metal part behaves as cathode.
• Oxidation occurs at the anodic area while reduction occurs at cathode and electrons
flow through the metal from anode to the cathode with the absorption of O2.
• In such type of Corrosion, anode is usually small metallic area where as cathode is
large well metal parts.
Reaction at anode:- Fe(s) → Fe+2 + 2e- (Oxidation)
Reaction at cathode:-1/2O2 + H2O + 2e– → 2OH– (reduction)
Overall Reaction:-Fe + 1/2O2 + H2O → Fe2+ + 2OH– → Fe(OH)2
Or, 2Fe + O2 + 2H2O → 2Fe(OH)2
In the presence of excess Oxygen:- 4Fe(OH)2 + O2 + 2H2O → 4 Fe(OH)3 or 2Fe2O3. 3H2O

10. Mechanisms of Corrosion by O2 Absorbtion

11. Difference:- Dry & Wet Corrosion
DRY or CHEMICAL CORROSION WET or ELECTROCHEMICAL CORROSION
It occurs due to direct chemical attack of atmospheric gases on the
exposed metallic surface.
It occurs when a metal is exposed to a conducting liquid or electrolytic medium
or moisture due to the formation of tiny electrochemical cells.
It occurs in dry condition in the absence of any conducting liquid or
electrolytic medium or moisture.
It occurs in wet condition in the presence of any conducting liquid or
electrolytic medium or moisture.
It is a slow process. It is a fast process.
It takes place at low or high temperature depending on the nature of the
metal.
It takes place under normal condition of temperature.
It is a self controlled process. It is a continuous process.
Corrosion occurs through the mechanism of adsorption. Corrosion occurs through the mechanism of electrochemical reaction.
Corrosion occurs on both homogeneous and heterogeneous metallic
surfaces.
Corrosion occurs on heterogeneous metallic surfaces only.
Corrosion on exposed metallic surface is uniform and occurs on the entire
surface exposed.
Corrosion on exposed metallic surface is non uniform, highly localised and
occurs at anode only.
Corrosion does not involve the formation of separate anodic and cathodic
area.
Corrosion involves the formation of separate anodic and cathodic area in
contact with each other.
Corrosion product accumulates uniformly on the exposed metallic surface.
Corrosion occurs at anode but the corrosion product accumulates near the
cathode or somewhere in between anode and cathode.
Extent of corrosion depends on the affinity between the metal and gases
and also on the nature of the product formed.
Extent of corrosion depends on the potential difference between the anode and
cathode and also on the nature of the corroding medium.
Oxidation corrosion is an example of dry or chemical corrosion.
Galvanic or Bimetallic corrosion, Differential aeration corrosion, Pitting
corrosion, Waterline corrosion, Crevice corrosion, Stress corrosion etc are
examples of wet or electrochemical corrosion.

12. Text books references
1. Jain P C and Jain M: Engineering Chemistry (15th Edition) 2006
Dhanpat Rai Publishing Company, NewDelhi.
2. Dara S.S. & Umare S.S. A Text Book of Engineering Chemistry(12th
Edition ) 2008 S.Chand Publishing Company, New Delhi
3. Chawla Shashi: A text book of Engineering Chemistry (3rd Edition)
2010 Dhanpat Rai Publishing Company, New Delhi.
4. Palanna O G : A text book of Engineering Chemistry(4th Reprint)
2012 McGraw Hill, New Delhi
5. Sharma BK, Industrial Chemistry (16th Edition), 2014, Krishna
Prakashan Media (P) ltd. Meerut.