3
Corrosion Inhibitors
3.1I ntroduction
Corrosion of metallic surfaces can be reduced or controlled 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 willr educe the rate of eithera nodic oxidation or cathodic
reduction, or both. The inhibitors themselves form ap rotective film on the
surfaceofthe metal. It has been postulated that the inhibitors areadsorbed
into the metals urface either by physical (electrostatic) adsorption
or chemosorption.
Physical adsorption is the result of electrostatic attractivef orces between
the organic ions and the electrically charged metal surface. Chemosorption
is the transfer,o rs haring of the inhibitor molecule’s charge to the metal
surface, forming acoordinate-type bond. The adsorbed inhibitorreduces the
corrosion rate of the metal surfaceeitherbyretardingthe anodic dissolution
reaction of the metal by the cathodic evolution of hydrogen, or both.
Inhibitors can be used at pH values of acid from near neutral to alkaline.
They can be classified in manyd ifferent ways accordingt o
1. Their chemical nature (organic or inorganic substances)
2. Their characteristics (oxidizing or nonoxidizing compounds)
3. Theirt echnical field of application( pickling,d escaling,a cid
cleaning, cooling water systems, and the like)
The mostcommon and widely known use of inhibitors is their application
in automobile cooling systems and boilerfeedwaters.
By consideringt he electrochemicaln atureo fc orrosion processes,
constituted by at least two electrochemical partial reactions, inhibition
may be defined on an electrochemical basis. Inhibitors willreduce the rates
CAT8247—CHAPTER 3—6/10/2006—19:22—KADAMBADI—XMLMODEL B–p p. 55–68
55
Corrosion Inhibitors
3.1I ntroduction
Corrosion of metallic surfaces can be reduced or controlled 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 willr educe the rate of eithera nodic oxidation or cathodic
reduction, or both. The inhibitors themselves form ap rotective film on the
surfaceofthe metal. It has been postulated that the inhibitors areadsorbed
into the metals urface either by physical (electrostatic) adsorption
or chemosorption.
Physical adsorption is the result of electrostatic attractivef orces between
the organic ions and the electrically charged metal surface. Chemosorption
is the transfer,o rs haring of the inhibitor molecule’s charge to the metal
surface, forming acoordinate-type bond. The adsorbed inhibitorreduces the
corrosion rate of the metal surfaceeitherbyretardingthe anodic dissolution
reaction of the metal by the cathodic evolution of hydrogen, or both.
Inhibitors can be used at pH values of acid from near neutral to alkaline.
They can be classified in manyd ifferent ways accordingt o
1. Their chemical nature (organic or inorganic substances)
2. Their characteristics (oxidizing or nonoxidizing compounds)
3. Theirt echnical field of application( pickling,d escaling,a cid
cleaning, cooling water systems, and the like)
The mostcommon and widely known use of inhibitors is their application
in automobile cooling systems and boilerfeedwaters.
By consideringt he electrochemicaln atureo fc orrosion processes,
constituted by at least two electrochemical partial reactions, inhibition
may be defined on an electrochemical basis. Inhibitors willreduce the rates
CAT8247—CHAPTER 3—6/10/2006—19:22—KADAMBADI—XMLMODEL B–p p. 55–68
55