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10
Corrosion Inhibitors
Corrosion of metallic surfaces can be controlled or reduced by the addition of chemical compounds to the corrodent. This form of corrosion control is called inhibition and the compounds added are known as corrosion
inhibitors. These inhibitors will reduce the rate of either anodic oxidation,
cathodic reduction, or both. The inhibitors themselves form a protective film
on the surface of the metal. It has been postulated that the inhibitors are
adsorbed onto the metal surface either by physical (electrostatic) adsorption
or chemisorption.
Physical adsorption is the result of electrostatic forces between the organic
ions and the electrically charged metal surface. Chemisorption is the transfer, or sharing, of the inhibitor molecule’s charge to the metal surface, forming a coordinate-type bond. The adsorbed inhibitor reduces the corrosion
rate of the metal surface either by retarding the anodic dissolution reaction
of the metal, by the cathodic evolution of hydrogen, or both.
Inhibitors can be classified in many different ways according to:
1. Their chemical nature (organic or inorganic substances)
2. Their characteristics (oxidizing or nonoxidizing compounds)
3. Their technical field of application (pickling, acid cleaning, descaling, cooling water systems, etc.)
The most common and widely known use of inhibitors is their application
in automobile cooling systems and boiler feed waters.
By considering the electrochemical nature of corrosion processes, constituted by at least two electrochemical partial reactions, inhibition may be
defined on an electrochemical basis. Inhibitors will reduce the rates of either
or both of these partial reactions (anodic oxidation and/or cathodic reduction). As a consequence, there can be anodic, cathodic, and mixed inhibitors.
Inhibitors can be used in electrolytes at different pH values, from acid to
near-neutral or alkaline solutions. Because of the very different situations
created by changing various factors such as medium and inhibitor in the
system, metal/aggressive medium/inhibitor, various inhibition mechanisms
must be considered. 1–6
An accurate analysis of the different modes of inhibiting electrode reactions, including corrosion, was carried out by Fisher. 7 He distinguished
among various mechanisms of action such as:
10
Corrosion Inhibitors
Corrosion of metallic surfaces can be controlled or reduced by the addition of chemical compounds to the corrodent. This form of corrosion control is called inhibition and the compounds added are known as corrosion
inhibitors. These inhibitors will reduce the rate of either anodic oxidation,
cathodic reduction, or both. The inhibitors themselves form a protective film
on the surface of the metal. It has been postulated that the inhibitors are
adsorbed onto the metal surface either by physical (electrostatic) adsorption
or chemisorption.
Physical adsorption is the result of electrostatic forces between the organic
ions and the electrically charged metal surface. Chemisorption is the transfer, or sharing, of the inhibitor molecule’s charge to the metal surface, forming a coordinate-type bond. The adsorbed inhibitor reduces the corrosion
rate of the metal surface either by retarding the anodic dissolution reaction
of the metal, by the cathodic evolution of hydrogen, or both.
Inhibitors can be classified in many different ways according to:
1. Their chemical nature (organic or inorganic substances)
2. Their characteristics (oxidizing or nonoxidizing compounds)
3. Their technical field of application (pickling, acid cleaning, descaling, cooling water systems, etc.)
The most common and widely known use of inhibitors is their application
in automobile cooling systems and boiler feed waters.
By considering the electrochemical nature of corrosion processes, constituted by at least two electrochemical partial reactions, inhibition may be
defined on an electrochemical basis. Inhibitors will reduce the rates of either
or both of these partial reactions (anodic oxidation and/or cathodic reduction). As a consequence, there can be anodic, cathodic, and mixed inhibitors.
Inhibitors can be used in electrolytes at different pH values, from acid to
near-neutral or alkaline solutions. Because of the very different situations
created by changing various factors such as medium and inhibitor in the
system, metal/aggressive medium/inhibitor, various inhibition mechanisms
must be considered. 1–6
An accurate analysis of the different modes of inhibiting electrode reactions, including corrosion, was carried out by Fisher. 7 He distinguished
among various mechanisms of action such as: