1.  Overall electrochemical reaction : at least one oxidation and one reduction
reaction
 All electrons generated through oxidation must be consumed by reduction.
Oxidation reaction
for metal M
Oxidation reaction
1

2. Reduction reaction
• Reduction of hydrogen ions in an acid solution.
• Reduction reaction in an acid solution containing dissolved oxygen
• Reduction reaction in a neutral or basic solution containing dissolved
oxygen
• Reduction of a multivalent metal ion to a lower valence state
• Reduction of a metal ion to its electrically neutral atom
2

3. 3
COMPARISON OF CHEMICAL & ELECTROCHEMICAL CORROSION
S.No Chemical Corrosion Electrochemical Corrosion
1 Occurs in dry conditions Wet conditions
2 Takes place by direct chemical
attack of atmos. gases like O2,
CO2, SO2 etc.
Electrochemical attack of corrosive
environment-setting up of cells
3 Takes place on homogeneous
& heterogeneous metal
surfaces
Takes place on heterogeneous metal
surfaces only
4 Corrosion product is formed at
the place of corrosion
Corrosion product is formed near the
cathode
5 Corrosion is Uniform Non-uniform, when anodic area is small
corrosion takes place more frequently
6 Slow Process Fast Process
7 Involves chemical reaction Involves electrochemical reaction

4. 4
Electrochemical Corrosion
1. Galvanic
2. Pitting
3. Concentration Cell or differential aeration
4. Water line
5. Crevice corrosion
6. Stress corrosion
7. Intragranular

5. 5
GALVANIC CORROSION
(Bimetallic Corrosion)
 Takes place when different metals like Zn & Cu are in contact and jointly
exposed to corrosive environment
 The metal having lower reduction potential (acts as anode) in electrochemical
series will undergo oxidation and get corroded
e.g.. Zn and Cu in contact with each other
At Anode: Zn → Zn2⁺ + 2e⁻ oxidation (loss of electrons)
At Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ reduction (oxygen absorption)
2H+ +2e- →H2 (hydrogen evolution)
 The difference in reduction potential of a metals form the basis of galvanic corrosion
 Corrosion occurs on anodic part while the cathodic part is protected.

6. 6
Electrochemical series compares electrode
potentials of metals and non-metals with
hydrogen electrode under standard
conditions.
A metal having low std red. potential will act
as anode and undergo corrosion when
coupled to another metal having higher std
reduction potential.
e.g.. Zn (-0.76 V) corrodes faster than Cu
(+0.34 V), Fe (-0.44V), Ag (+0.80V)
Low potential
Other Examples of Galvanic corrosion-
(i) Tin coating on copper vessel, (ii) Steel pipes connected to copper plumbing.
Platinum
Gold
Graphite
Titanium
Silver
316 Stainless Steel (passive)
Nickel (passive)
Copper
Nickel (active)
Tin
Lead
316 Stainless Steel (active)
Iron/Steel
Aluminum Alloys
Cadmium
Zinc
Magnesium
moreanodic
(active)
morecathodic
(inert)
High potential

7. DIFFERENTIAL AERATION CORROSION
Occurs when one part of the metal gets different concentration of
oxygen in comparison to another part.
 Anode: poorly oxygenated part
 Cathode: highly oxygenated part
 Conducting medium acts as electrolyte.
 Flow of current takes place causing corrosion at anode.
7

8. 8
 Zn rod is partially dipped in dilute NaCl.
 Part of metal below the solution which has poor concentration of O₂ acts as anode.
 Part of metal above solution having good concentration of O₂ acts as cathode.
 Difference in potential thus created causes the flow of e⁻s.
Anode: Zn → Zn²⁺ + 2e⁻ (oxidation)
Cathode: ½ O₂ +H₂O + 2e⁻ → 2OH⁻ (reduction)
Net reaction: Zn²⁺ + 2OH⁻ → Zn(OH)₂

9. PITTING CORROSION
9

10. WATER LINE CORROSION
10
Water storage metallic tanks along
the level in tank
In Marine Ships

11. 11
CREVICE CORROSION
Takes place within crevices (narrow openings) or other shielded areas on metal
surfaces such as bolts, nuts, washers and rivets.

12. 12
STRESS CORROSION
 The metallic substances are subjected to uneven stresses during manufacturing
operations such as bending, welding, pressing, hammering.
 Parts have not been cooled properly (non-uniform cooling) during fabrication causes-
stress
 The areas which are more stressed become anode and corroded faster
 It occurs due to combined effect of tensile stress & corroding environment
e.g.. Brass undergoes stress corrosion in presence of ammonia

13. 13
GRANULAR CORROSION
 Metals and alloys have granular structure
 Each grain has clearly define boundary that
chemically differ from metal within the grain
centre