Generally,itisassumed that the firststage in the action mechanism of the
inhibitors in aggressive acidm edia is adsorption of the inhibitors onto
the metal surface. The process of adsorption of inhibitors is influenced by
the nature and surfacecharge of the metal, by the chemical structure of the
inhibitor,and by the type of aggressive electrolyte. Physical (or electrostatic)
adsorption and chemisorption are the principal types of interaction between
an organic inhibitorand am etal surface.
3.4.1P hysical Adsorption
Physical adsorption is the result of electrostatic attractive forces between
inhibiting organic ionso rd ipoles and the electrically charged surface of
the metal.T he surfacec harge of the metal is due to the electric field at the
outer Helmholtz planeo ft he electrical doublel ayer existing at the metal/
solution interface. The surface charge can be defined by the potentialo f
the metal ( E corr )v s. its zero-charge potential (ZCP) ( E q Z 0 ).
12
When the
difference E corr K E q Z 0 Z f is negative, cation adsorption is favored.
Adsorption of anions is favored when f becomes positive. This behavior
is relatedn ot only to compounds with formal positive or negative charge,
but also to dipoles whose orientation is determined by the value of the
f potential.
AccordingtoAntropov,
12
at equal values of f for different metals, similar
behavior of ag iven inhibiting speciess hould be expected in the same
environment. Thish as been verified for adsorption of organic charged
species on mercurya nd iron electrodes at thes amep otential for
bothm etals.
In studyingt he adsorption of ions at the metal/solution interface, it was
first assumed that ions maintained their total charge during the adsorption,
givingr ise in this way to ap ure electrostatic bond. Lorenz
13–15
suggested
that ap artial charge is present in the adsorption of ions; in this case,
ac ertain amount of covalent bondi nt he adsorption process must be
considered.T he partialc hargec oncept wass tudied by Vettera nd
Schulze,
16–19
whod efined as electrosorption valency the coefficientf or
the potential dependence and charge flow of electrosorption processes.
The term electrosorption valency was chosen because of its analogy with the
electrode reaction valency that enters into Faraday’s law as well as the
Nernst equation.
Consideringt he concepts discusseda bove in relation to corrosion
inhibition, when an inhibited solutionc ontainsa dsorbable anions, such as
halide ions, these adsorb on the metal surfaceb yc reating oriented dipoles
and, consequently,i ncrease the adsorption of the organic cations on the
dipoles. In these cases, ap ositive synergistic effect arises; therefore, the
degree of inhibition in the presence of both adsorbable anions andinhibitor
cations is highert han the sum of the individual effects. This could explain
the higher inhibition efficiency of variousorganic inhibitors in hydrochloric
acid solutions compared to sulfuric acid solutions (Table 3.1).
20
Corrosion of Linings and Coatings
60
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68
inhibitors in aggressive acidm edia is adsorption of the inhibitors onto
the metal surface. The process of adsorption of inhibitors is influenced by
the nature and surfacecharge of the metal, by the chemical structure of the
inhibitor,and by the type of aggressive electrolyte. Physical (or electrostatic)
adsorption and chemisorption are the principal types of interaction between
an organic inhibitorand am etal surface.
3.4.1P hysical Adsorption
Physical adsorption is the result of electrostatic attractive forces between
inhibiting organic ionso rd ipoles and the electrically charged surface of
the metal.T he surfacec harge of the metal is due to the electric field at the
outer Helmholtz planeo ft he electrical doublel ayer existing at the metal/
solution interface. The surface charge can be defined by the potentialo f
the metal ( E corr )v s. its zero-charge potential (ZCP) ( E q Z 0 ).
12
When the
difference E corr K E q Z 0 Z f is negative, cation adsorption is favored.
Adsorption of anions is favored when f becomes positive. This behavior
is relatedn ot only to compounds with formal positive or negative charge,
but also to dipoles whose orientation is determined by the value of the
f potential.
AccordingtoAntropov,
12
at equal values of f for different metals, similar
behavior of ag iven inhibiting speciess hould be expected in the same
environment. Thish as been verified for adsorption of organic charged
species on mercurya nd iron electrodes at thes amep otential for
bothm etals.
In studyingt he adsorption of ions at the metal/solution interface, it was
first assumed that ions maintained their total charge during the adsorption,
givingr ise in this way to ap ure electrostatic bond. Lorenz
13–15
suggested
that ap artial charge is present in the adsorption of ions; in this case,
ac ertain amount of covalent bondi nt he adsorption process must be
considered.T he partialc hargec oncept wass tudied by Vettera nd
Schulze,
16–19
whod efined as electrosorption valency the coefficientf or
the potential dependence and charge flow of electrosorption processes.
The term electrosorption valency was chosen because of its analogy with the
electrode reaction valency that enters into Faraday’s law as well as the
Nernst equation.
Consideringt he concepts discusseda bove in relation to corrosion
inhibition, when an inhibited solutionc ontainsa dsorbable anions, such as
halide ions, these adsorb on the metal surfaceb yc reating oriented dipoles
and, consequently,i ncrease the adsorption of the organic cations on the
dipoles. In these cases, ap ositive synergistic effect arises; therefore, the
degree of inhibition in the presence of both adsorbable anions andinhibitor
cations is highert han the sum of the individual effects. This could explain
the higher inhibition efficiency of variousorganic inhibitors in hydrochloric
acid solutions compared to sulfuric acid solutions (Table 3.1).
20
Corrosion of Linings and Coatings
60
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68