3.4.2 Chemisorption
Another typeo fm etal/inhibitori nteraction is chemisorption. This process
involves charge sharing or charge transfer from the inhibitor molecules to
the metal surfacei no rder to form ac oordinate type of bond.
The chemisorption process takes place more slowly than electrostatic
adsorption andw ith ah ighera ctivation energy.I td epends on the
temperature, and higher degrees of inhibition should be expected at higher
temperatures. Chemisorption is specific for certain metals and is not
completely reversible.
22
The bonding occuring with electron transfer clearly
depends on the nature of the metal and the nature of the organic inhibitor.
In fact, electron transfer is typical for metals havingv acant, low energy
electron orbitals.C oncerning inhibitors, electron transfer can be expected
with compoundsh avingr elatively loosely bounde lectrons. This situation
may arise because of the presence in the adsorbedi nhibitor of multiple
bonds or aromatic rings, whose electrons have a p character.C learly,e ven
the presence of heteroatoms with lone-pair electrons in the adsorbed
molecule will favor electron transfer.Most organic inhibitors are substances
with at least one functional group regarded as the reaction center for the
chemisorption process. In this case, the strength of the adsorption bond
is relatedt ot he heteroatom electron density and to the functional group
polarizability.F or example, the inhibition efficiency of homologous series
of organic substances differing only in the heteroatom is usually in the
following sequence:
P O SeO S O N O O :
An interpretation may be found in the easier polaribility and lower
electronegativity of the elements on the left of the aboves equence. On this
basis, as urfaceb ond of al ewis acid-base type, normally with the inhibitor
TABLE 3.1
Inhibition Efficiency of Some Pyridinium Derivatives at the Same Molar
Concentration (1! 10
K 4 M) on Armco Iron in Hydrochloric and Sulfuric
Acid Solutions at 258 C
Inhibition Efficiency (%)
Additive
1NHCl
1NH 2 SO 4
n -Decylpyridinium bromide8 7.6
20.0
n -Decyl-3-hydroxypyridiniumb romide
94.8
57.5
n -Decyl-3-carboxypyridinium bromide
92.7
76.5
n -Decyl-3,5-dimethylpyridinium bromide
92.5
30.2
Source:F rom A. Frignani, G. Trabanelli,F .Z ucchi, and M. Zucchini. 1980. Proceedings of the 5th
European Symposium on Corrosion Inhibitors,A nn. Univ.F errara, N.S., Sez. V. Suppl. 7, 1185.
Corrosion Inhibitors
61
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68
Another typeo fm etal/inhibitori nteraction is chemisorption. This process
involves charge sharing or charge transfer from the inhibitor molecules to
the metal surfacei no rder to form ac oordinate type of bond.
The chemisorption process takes place more slowly than electrostatic
adsorption andw ith ah ighera ctivation energy.I td epends on the
temperature, and higher degrees of inhibition should be expected at higher
temperatures. Chemisorption is specific for certain metals and is not
completely reversible.
22
The bonding occuring with electron transfer clearly
depends on the nature of the metal and the nature of the organic inhibitor.
In fact, electron transfer is typical for metals havingv acant, low energy
electron orbitals.C oncerning inhibitors, electron transfer can be expected
with compoundsh avingr elatively loosely bounde lectrons. This situation
may arise because of the presence in the adsorbedi nhibitor of multiple
bonds or aromatic rings, whose electrons have a p character.C learly,e ven
the presence of heteroatoms with lone-pair electrons in the adsorbed
molecule will favor electron transfer.Most organic inhibitors are substances
with at least one functional group regarded as the reaction center for the
chemisorption process. In this case, the strength of the adsorption bond
is relatedt ot he heteroatom electron density and to the functional group
polarizability.F or example, the inhibition efficiency of homologous series
of organic substances differing only in the heteroatom is usually in the
following sequence:
P O SeO S O N O O :
An interpretation may be found in the easier polaribility and lower
electronegativity of the elements on the left of the aboves equence. On this
basis, as urfaceb ond of al ewis acid-base type, normally with the inhibitor
TABLE 3.1
Inhibition Efficiency of Some Pyridinium Derivatives at the Same Molar
Concentration (1! 10
K 4 M) on Armco Iron in Hydrochloric and Sulfuric
Acid Solutions at 258 C
Inhibition Efficiency (%)
Additive
1NHCl
1NH 2 SO 4
n -Decylpyridinium bromide8 7.6
20.0
n -Decyl-3-hydroxypyridiniumb romide
94.8
57.5
n -Decyl-3-carboxypyridinium bromide
92.7
76.5
n -Decyl-3,5-dimethylpyridinium bromide
92.5
30.2
Source:F rom A. Frignani, G. Trabanelli,F .Z ucchi, and M. Zucchini. 1980. Proceedings of the 5th
European Symposium on Corrosion Inhibitors,A nn. Univ.F errara, N.S., Sez. V. Suppl. 7, 1185.
Corrosion Inhibitors
61
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68