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CORROSION INHIBITOR FUNDAMENTALS

Inhibition - preventive measure against corrosion

Primarily chemical compounds

Inhibitors can be classified on the basis:

	Electrochemical reaction reduction
	Chemical nature of the compound
	Characteristics
	Technology field of application domain, P : Passive domain, I : Immune domain

Different modes of inhibiting electrode reactions (by Lorenz & Mansfield), distinguishing interface and interphase retarding mechanisms:

	By geometric blocking effect of the electrode surface due to adsorption of a stable inhibitor at a relatively high degree of coverage of metal surface.
	By a blocking effect of active surface sites due to adsorption of a stable inhibitor at a relatively low degree of coverage.
	By a reactive coverage of the metal surface. In this case adsorption process is followed by electrochemical or chemical reactions of the inhibitor at the interface.

Inhibition Mechanism :

Process of adsorption of inhibitor on the metal surfaces influenced by

	Nature & surface charge of metal
	Chemical structure of the inhibitor
	Type of aggressive electrolyte

Principle type of interaction between organic inhibitor and metal

	Physical – electrostatic adsorption
	Chemisorption

Adsorption of inhibitor involves water molecules adsorbed at the metal surface:

	Org_(sol) + nH₂o_(ads) = Org_(ads) + nH₂o_(sol)
	Where n is number of water molecules removed from the metal surface for each molecule of inhibitor adsorbed.
	Interaction energy between inhibitor & the metal surface is greater than interaction energy between water & metal

Physical Adsorption

Electrostatic attraction between inhibiting organic ions (dipoles) & electrically charged metal surface

	Surface charge due to electric field at the outer Helmholtz plane of electrical double layer at the metal/solution interface
	Degree of inhibiting increases due to inhibitor solution containing absorbable ions creating oriented dipoles
	Importance of inhibitor's structure influencing the electrical charge of organic ions:
	Hydrocarbon chain length
	Nature & position of substituents in aromatic rings
Electrostatic adsorption process has low activation energy & is independent of temperature

Chemisorption

	Process of charge sharing/transfer from inhibitor to metal surface forming a coordinate type of bond.
	Reaction is slow, needs higher activation energy and depends on temperature
	Bond depends on nature of metal and inhibitor
	Electron transfer occurs with transition metal having vacant low-energy electron orbital
	Similarly if inhibitor has heteroatom, lone-pair electron transfer is favoured
	Most organic inhibitor have at least one functional group as a reaction center
	Strength of the adsorption bond depends on
		Heteroatom electron density
		Functional group polarizability
	The inhibition efficiency of homologous series of organic substances differing only in heteroatom is:
	Thus Chemisorption depends on
		Electronic structure (o electron density, the order of the o bond line, the index of free valence and the charges of atoms in the free and adsorbed state)
		Projected molecular area
		Molecular weight
		Molecular configuration

Interaction between Adsorbed Inhibitor:

	Lateral interactions between inhibitor molecules influences inhibition efficiency
	Attractive interaction means stronger adsorption and higher inhibition efficiency
	Longer hydrocarbon chain gives positive effect due to van der Waals forces
	Corrosion protection increases markedly as the polymer film grows from nearly two dimensional chemiadsorbed layers to films up to several hundred angstrom thick

Organic Inhibitor Acts through:

	Changes in electrical double layer
	Formation of physical barrier
	Reduction of metal reactivity
	Participation of the inhibitor in partial electrochemical reactions (anodic & cathodic)

Main Types:

Metallic

	Cathodic sputtering			Fusion Bonding
	Diffusion Coating			Gas Plating
	Electrophoresis Deposition			Hot Dipping
	Electrolysis Plating			Metal Cladding
	Electroplating			Plasma Spraying
	Explosion Bonding			Vacuum & Vapour Deposition
	Flame Spraying

Polymeric

	Lacquers
	Varnishes
	Paints
	Corrosion between the polymeric coating has the same process of electrochemical

Process of corrosion is :

	Transport through coating
	Development of aqueous phase at interface
	Activation of anodic and cathodic cells
	Deterioration

Polymeric Coating

Types of corrosion beneath the coatings on metal:

Conversion Coating

Cementitious