Electrode-Solution Interface
When a metal electrode is immersed in an electrolyte solution, an
interfacial region is formed between the solid metal and the liquid
electrolyte. This region is known as the electrode–solution interface. It
plays a fundamental role in electrochemistry because all
electrochemical reactions occur at this boundary.
At the interface, metal atoms may dissolve into solution as ions or ions
from solution may deposit onto the metal surface. This leads to charge
separation and the formation of an electrical double layer. The electrical
double layer consists of the Inner Helmholtz Plane (IHP), Outer
Helmholtz Plane (OHP), and Diffuse Layer. The potential decreases
gradually from the metal surface into the bulk solution.
Corrosion is a heterogenous surface-based phenomenon occurring at
this interface. The nature of the interaction at this boundary indicates
the rate and mechanism of degradation.
Types of interfaces
1. Metal-Electrolyte interface
This is the most common interface, where a metal or alloy
is immersed in a conducting solution. Eg: water, acids, salt
solutions. This interface is the site of electrochemical
reactions-anodic dissolution(corrosion) and cathodic
reduction.
2. Metal-Gas interface (Dry corrosion)
It occurs in the absence of moisture, where metals react
directly with gases like O2, SO2, H2S etc. at higher
temperature to form oxide films.
3. Metal-Soil interface
This interface involves underground pipes or structures,
where moisture, pH and oxygen content in the soil
determine the corrosion rate.
4. Metal-surface coating interface
The stability of this interface determines how well the
coating protects the metal from the environment.
, The interface itself is rarely static; it can be due to environmental or
electrochemical factors.
i. Electrochemical Double Layer
At a metal-electrolyte interface, an electric double layer
forms where charges are separated(ions in solution vs
charges on the metal)
ii. Passive Layer
A protective, non-porous layer (eg. Cr2O3 on stainless steel)
formed at the surface that slows down further corrosion.
iii. Active-Passive interface
Localized areas on a metal surface where part of the metal
is active (corroding) while adjacent parts are passive
(protected) often causing severe localized pitting.
iv. Interface-Interphase inhibitor films
In interface inhibition, the adsorption of inhibitors forming
2D layers, subdivided into geometric blocking, deactivating or
reactive 2D coverage. In interphase inhibition, the 3D layers
formed by inhibitors that cover the entire surface such as polymer
coatings or thick corrosion products.
Factors affecting the interface
a. Surface heterogeneity
Grain boundaries are more acidic (reactive) than grain
interiors leading to intergranular corrosion.
b. Fluid dynamics
High velocity fluids can remove protective films at the
interface causing erosion-corrosion.
c. Surface geometry
Cervices (eg: under gaskets) create unique, stagnant,
oxygen-depleted microenvironment
Types of polarization
▪ Activation polarization
Activation polarization is caused by a slow electrode reaction or
another way the electrical reaction for which an activation energy in the
form of potential is required for the reaction to proceed. When current
When a metal electrode is immersed in an electrolyte solution, an
interfacial region is formed between the solid metal and the liquid
electrolyte. This region is known as the electrode–solution interface. It
plays a fundamental role in electrochemistry because all
electrochemical reactions occur at this boundary.
At the interface, metal atoms may dissolve into solution as ions or ions
from solution may deposit onto the metal surface. This leads to charge
separation and the formation of an electrical double layer. The electrical
double layer consists of the Inner Helmholtz Plane (IHP), Outer
Helmholtz Plane (OHP), and Diffuse Layer. The potential decreases
gradually from the metal surface into the bulk solution.
Corrosion is a heterogenous surface-based phenomenon occurring at
this interface. The nature of the interaction at this boundary indicates
the rate and mechanism of degradation.
Types of interfaces
1. Metal-Electrolyte interface
This is the most common interface, where a metal or alloy
is immersed in a conducting solution. Eg: water, acids, salt
solutions. This interface is the site of electrochemical
reactions-anodic dissolution(corrosion) and cathodic
reduction.
2. Metal-Gas interface (Dry corrosion)
It occurs in the absence of moisture, where metals react
directly with gases like O2, SO2, H2S etc. at higher
temperature to form oxide films.
3. Metal-Soil interface
This interface involves underground pipes or structures,
where moisture, pH and oxygen content in the soil
determine the corrosion rate.
4. Metal-surface coating interface
The stability of this interface determines how well the
coating protects the metal from the environment.
, The interface itself is rarely static; it can be due to environmental or
electrochemical factors.
i. Electrochemical Double Layer
At a metal-electrolyte interface, an electric double layer
forms where charges are separated(ions in solution vs
charges on the metal)
ii. Passive Layer
A protective, non-porous layer (eg. Cr2O3 on stainless steel)
formed at the surface that slows down further corrosion.
iii. Active-Passive interface
Localized areas on a metal surface where part of the metal
is active (corroding) while adjacent parts are passive
(protected) often causing severe localized pitting.
iv. Interface-Interphase inhibitor films
In interface inhibition, the adsorption of inhibitors forming
2D layers, subdivided into geometric blocking, deactivating or
reactive 2D coverage. In interphase inhibition, the 3D layers
formed by inhibitors that cover the entire surface such as polymer
coatings or thick corrosion products.
Factors affecting the interface
a. Surface heterogeneity
Grain boundaries are more acidic (reactive) than grain
interiors leading to intergranular corrosion.
b. Fluid dynamics
High velocity fluids can remove protective films at the
interface causing erosion-corrosion.
c. Surface geometry
Cervices (eg: under gaskets) create unique, stagnant,
oxygen-depleted microenvironment
Types of polarization
▪ Activation polarization
Activation polarization is caused by a slow electrode reaction or
another way the electrical reaction for which an activation energy in the
form of potential is required for the reaction to proceed. When current
