Corrosion Inhibitors
319
It is generally assumed that the first stage in the action mechanism of inhibitors in aggressive acid media is adsorption of the inhibitors onto the metal surface. This adsorption process is influenced by the nature and surface charge 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 inhibitor and a metal surface.
10.3.1 Physical adsorption
Physical adsorption is the result of electrostatic attractive forces between
inhibiting organic ions or dipoles and the electrically charged surface of the
metal. The surface charge of the metal is due to the electric field at the outer
Helmholtz plane of the electrical double layer existing at the metal/solution
interface. The surface charge can be defined by the potential of the metal
(E corr ) vs. its zero charge potential (ZCP) (E q=0 ). 12 When the difference E corr
– E q=0 = ø is negative, carbon adsorption is favored. Adsorption of anions is
favored when ø becomes positive. This behavior is related not only to compounds with formal positive or negative charge, but also to dipoles whose
orientation is determined by the value of the ø potential.
According to Antropov, 16 at equal values of ø for different metals, similar
behavior of a given inhibiting species should be expected in the same environment. This has been verified for adsorption of organic charged species on
mercury and iron electrodes at the same potential for both metals.
In studying the adsorption of ions at the metal/solution interface, it was
first assumed that ions maintained their total charge during the adsorption,
giving rise in this way to a pure electrostatic bond. Lorenz 17–19 suggested that
a partial charge is present in the adsorption of ions; in this case, a certain
amount of covalent bond in the adsorption process must be considered. The
partial charge concept was studied by Vetter and Schulze, 20–22 who defined
as electrosorption valency the coefficient for 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.
Considering the concepts discussed above in relation to corrosion inhibition, when an inhibited solution contains adsorbable anions, such as halide
ions, these adsorb onto the metal surface by creating oriented dipoles and
consequently increase the adsorption of the organic cations on the dipoles.
In these cases, a positive synergistic effect arises; therefore, the degree of
inhibition in the presence of both adsorbable anions and inhibitor cations is
higher than the sum of the individual effects. This could explain the higher
inhibition efficiency of various organic inhibitors in hydrochloric acid solutions compared to sulfuric acid solutions (see Table 10.2).
319
It is generally assumed that the first stage in the action mechanism of inhibitors in aggressive acid media is adsorption of the inhibitors onto the metal surface. This adsorption process is influenced by the nature and surface charge 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 inhibitor and a metal surface.
10.3.1 Physical adsorption
Physical adsorption is the result of electrostatic attractive forces between
inhibiting organic ions or dipoles and the electrically charged surface of the
metal. The surface charge of the metal is due to the electric field at the outer
Helmholtz plane of the electrical double layer existing at the metal/solution
interface. The surface charge can be defined by the potential of the metal
(E corr ) vs. its zero charge potential (ZCP) (E q=0 ). 12 When the difference E corr
– E q=0 = ø is negative, carbon adsorption is favored. Adsorption of anions is
favored when ø becomes positive. This behavior is related not only to compounds with formal positive or negative charge, but also to dipoles whose
orientation is determined by the value of the ø potential.
According to Antropov, 16 at equal values of ø for different metals, similar
behavior of a given inhibiting species should be expected in the same environment. This has been verified for adsorption of organic charged species on
mercury and iron electrodes at the same potential for both metals.
In studying the adsorption of ions at the metal/solution interface, it was
first assumed that ions maintained their total charge during the adsorption,
giving rise in this way to a pure electrostatic bond. Lorenz 17–19 suggested that
a partial charge is present in the adsorption of ions; in this case, a certain
amount of covalent bond in the adsorption process must be considered. The
partial charge concept was studied by Vetter and Schulze, 20–22 who defined
as electrosorption valency the coefficient for 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.
Considering the concepts discussed above in relation to corrosion inhibition, when an inhibited solution contains adsorbable anions, such as halide
ions, these adsorb onto the metal surface by creating oriented dipoles and
consequently increase the adsorption of the organic cations on the dipoles.
In these cases, a positive synergistic effect arises; therefore, the degree of
inhibition in the presence of both adsorbable anions and inhibitor cations is
higher than the sum of the individual effects. This could explain the higher
inhibition efficiency of various organic inhibitors in hydrochloric acid solutions compared to sulfuric acid solutions (see Table 10.2).