Electrochemical Corrosion: Cathodic and Anodic Reactions

Posted on Jan 30, 2026 in Chemistry

Mechanism of Cathodic Reaction

Corrosion of metals in aqueous environment takes place by an electrochemical mechanism consisting of two simultaneous reactions:
anodic reaction and cathodic reaction.
The cathodic reaction occurs at the cathodic areas of the metal surface and involves consumption of electrons released by the anodic reaction.

Meaning of Cathodic Reaction

A cathodic reaction is a reduction reaction in which electrons are gained. It does not cause metal dissolution directly, but it is essential for corrosion to continue, because it balances the anodic reaction.

Types of Cathodic Reactions

Depending on the environment, cathodic reactions occur in two main ways:

1. Hydrogen Evolution Reaction (Acidic Medium)

This reaction occurs when corrosion takes place in an acidic solution.

Mechanism

1.Hydrogen ions (H^+) are present in the solution 2.These ions migrate to the cathodic area 3.Hydrogen ions gain electrons 4.Hydrogen gas is liberated Cathodic reaction

Example

Corrosion of iron in dilute acids.

2. Oxygen Absorption (Oxygen Reduction) Reaction (Neutral or Alkaline Medium)

This reaction occurs in neutral or alkaline solutions, where dissolved oxygen is present.

Mechanism

1.Oxygen dissolves in water 2.Oxygen diffuses to the cathodic region 3.Oxygen gains electrons and reacts with water 4.Hydroxyl ions (OH^-) are formed Cathodic reaction

Example

Atmospheric corrosion of iron in moist air.
Role of Cathodic Reaction in Corrosion •Consumes electrons released by anodic reaction •Maintains electrical neutrality •Controls the rate of corrosion •Without cathodic reaction, corrosion cannot proceed

Mechanism of Anodic Reaction

Corrosion of metals in aqueous environments occurs by an electrochemical process involving two simultaneous reactions: anodic reaction and cathodic reaction. The anodic reaction is the place where actual metal loss takes place.

Meaning of Anodic Reaction

An anodic reaction is an oxidation process in which a metal atom loses electrons and goes into the solution in the form of metal ions.      This reaction is responsible for the dissolution of metal, hence it is the main cause of corrosion.

Anodic Reaction Process (Step-by-Step)

1.Certain areas on the metal surface become anodic regions 2.Metal atoms at these regions lose electrons 3.Metal atoms convert into positively charged ions 4.Electrons released travel through the metal to the cathodic areas

General Anodic Reaction

For a metal M:

M \rightarrow M^{n+} + ne^-

Example:

Anodic Reaction of Iron

When iron corrodes, the anodic reaction is:
Fe \rightarrow Fe^{2+} + 2e^- •Iron atoms dissolve into solution as ferrous ions •Electrons move to cathodic sites

Characteristics of Anodic Reaction •It is an oxidation reaction •Always involves loss of electrons •Occurs at anodic areas of metal surface •Causes actual metal damage •Rate of corrosion depends on anodic activity Factors Affecting Anodic Reaction •Nature of metal •Surface condition •Presence of impurities •Electrolyte concentration •Temperature Importance of Anodic Reaction in Corrosion •Determines corrosion rate •Responsible for metal thinning and failure •Cannot occur alone; must be accompanied by cathodic reaction

Burgers Vector –

How do Dislocation in crystals determined by Burghers Vector ?

The Burgers vector is a vector quantity that represents the magnitude and direction of lattice distortion produced by a dislocation in a crystal. It is denoted by              and is used to characterize and identify dislocations.
In simple words, the Burgers vector tells us how much and in which direction the crystal lattice is distorted due to a dislocation.

Burgers Circuit (How Burgers Vector is Obtained)

To determine the Burgers vector, a Burgers circuit is drawn: 1.Choose a point in a perfect crystal region 2.Move equal steps around a closed loop (right, up, left, down) 3.Repeat the same steps around a region containing a dislocation 4.The circuit fails to close 5.The vector required to close the circuit is the Burgers vector      This closure failure occurs only because of the dislocation.

Types of Dislocations and Burgers Vector :

1. Edge Dislocation •Extra half-plane of atoms is present •Burgers vector is perpendicular to the dislocation line 2. Screw Dislocation •Crystal planes form a helical structure •Burgers vector is parallel to the dislocation line 3. Mixed Dislocation •Combination of edge and screw dislocation •Burgers vector has both parallel and perpendicular components

How Dislocations in Crystals are Determined by Burgers Vector
Dislocations are identified and analysed using the Burgers vector in the following ways:
1.Type of dislocation •If                                  dislocation line → Edge dislocation •If                                  dislocation line → Screw dislocation 2.Magnitude of lattice distortion •Larger Burgers vector → greater lattice strain 3.Slip direction and slip system •Burgers vector gives the direction of slip in a crystal 4.Energy of dislocation •Dislocation energy is proportional to |\vec{b}|^2 5.Mechanical behaviour •Strength, plastic deformation, and work hardening depend on Burgers vector Importance of Burgers Vector •Helps in identifying dislocation type •Determines plastic deformation behaviour •Used in crystal defect analysis

Hardening Methods (Through Bulk Hardening)

GROUP–I : HARDENING METHODS (Through / Bulk Hardening)

👉 In these methods, the entire cross-section or a considerable depth of the material is hardened.

1. Direct Hardening (Quenching)

•Steel is heated above the critical temperature •Then rapidly cooled in water, oil, or brine •Produces high hardness and strength •Core also becomes hard (less toughness)

Applications:

cutting tools, chisels, punches

2. Flame Hardening •Surface heated by gas flame •Immediately followed by quenching •Depth of hardness is more than chemical case hardening •No change in chemical composition Applications:
rails, gears, large machine parts

3. Induction Hardening •Surface heated using high-frequency electric current •Rapid heating and quenching •Accurate and controlled process •Suitable for mass production Applications:
shafts, camshafts, crankshafts

Hardening Methods (Case/Surface Hardening)

CASE HARDENING METHODS (Surface Hardening)

👉 In these methods, only the surface (case) is hardened, while the core remains soft and tough. Mainly used for low-carbon steels.

🔹 A. Chemical Case Hardening Methods
1. Carburizing •Steel heated in carbon-rich medium •Carbon diffuses into surface •Followed by quenching •Produces hard case, tough core Applications:
gears, shafts, cams

2. Cyaniding •Steel heated in molten cyanide bath •Both carbon and nitrogen added •Thin but very hard case •Fast process Applications:
bolts, screws, small parts

3. Carbonitriding •Carbon + nitrogen added from gaseous atmosphere •Lower temperature than carburizing •Less distortion Applications:
automotive parts, fasteners

4. Nitriding •Nitrogen added by heating steel in ammonia gas •No quenching required •Very hard and wear-resistant surface Applications:
crankshafts, dies, valve parts

  B. Thermal / Physical Case Hardening Methods

5. Flame Hardening •Local surface heating by flame •Immediate quenching •Hard surface with tough core

6. Induction Hardening •Surface heated by induced current •Quenching follows •Clean and fast process