Corrosion Inhibitors
315
near the critical value in the presence of Cl − or SO 4
2− ions. In this regard, nitrites
are less sensitive to Cl – than to SO 4
2− contrary to the situation for chromates.
Zinc molybdate has been suggested as an inhibiting pigment for paints,
being white instead of the characteristic yellow of chromates. It is also said
to be less toxic than chromates.
10.2.2 Organic inhibitors
A variety of organic compounds are used as inhibitors. Common among
them are amines, imines, thiourea, mercaptans, guanidine, and aldehydes.
These compounds are chemisorbed on the metal surface, forming a monolayer that interferes with both the anodic and cathodic processes, although
in many cases the effect is equal. The chemisorption is effected through the
presence of a polar group in the molecular structure by which the molecules
can attach themselves to the metal surface. These include sulfur, nitrogen,
amine, phosphorous, and hydroxyl groups.
Cationic inhibitors (such as amines) or anionic inhibitors (such as sulfonates) will be adsorbed preferentially, depending on whether the metal is
negatively or positively charged with respect to the solution. Amines show
better performance as inhibitors for steel in phosphoric acid in the presence
of iodides. The explanation for this synergism is that the adsorption of iodide
ions shifts the surface charge of steel to more negative values, where the
adsorption of positively charged amines is favored. The fact that a certain
TabLE 10.1
Effect of Chromate Concentration, Chlorides, and Temperature on
Corrosion of Mild Steel
Velocity of Spec: 37 cm/s: 14-day tests
Na 2 Cr 2 O 7 ·2H 2 O (g/L)
0
0.1
0.5
1.0
%NaCl Temp. (°C)
Corrosion Rate (ipy)
0
20
0.021
0.0001
0.0001
0.0000
75
0.036
0.014 a
0.0004
0.0002
95
0.017
0.011 a
0.0004
0.0000
0.002
20
0.026
0.0006
0.0000
0.0000
75
0.067
0.005 a
0.0002
0.0000
95
0.021
0.017 a
0.005 a
0.0003
0.05
20
0.031
0.0012
0.0015
0.0008
75
0.085
0.002
0.003
0.002
95
0.023
0.007 a
0.005 a
0.002
3.5
20
0.024
0.0017
0.0016
0.0015
22.0
20
0.007
0.0009
0.0006
0.0013
a Pitted.
315
near the critical value in the presence of Cl − or SO 4
2− ions. In this regard, nitrites
are less sensitive to Cl – than to SO 4
2− contrary to the situation for chromates.
Zinc molybdate has been suggested as an inhibiting pigment for paints,
being white instead of the characteristic yellow of chromates. It is also said
to be less toxic than chromates.
10.2.2 Organic inhibitors
A variety of organic compounds are used as inhibitors. Common among
them are amines, imines, thiourea, mercaptans, guanidine, and aldehydes.
These compounds are chemisorbed on the metal surface, forming a monolayer that interferes with both the anodic and cathodic processes, although
in many cases the effect is equal. The chemisorption is effected through the
presence of a polar group in the molecular structure by which the molecules
can attach themselves to the metal surface. These include sulfur, nitrogen,
amine, phosphorous, and hydroxyl groups.
Cationic inhibitors (such as amines) or anionic inhibitors (such as sulfonates) will be adsorbed preferentially, depending on whether the metal is
negatively or positively charged with respect to the solution. Amines show
better performance as inhibitors for steel in phosphoric acid in the presence
of iodides. The explanation for this synergism is that the adsorption of iodide
ions shifts the surface charge of steel to more negative values, where the
adsorption of positively charged amines is favored. The fact that a certain
TabLE 10.1
Effect of Chromate Concentration, Chlorides, and Temperature on
Corrosion of Mild Steel
Velocity of Spec: 37 cm/s: 14-day tests
Na 2 Cr 2 O 7 ·2H 2 O (g/L)
0
0.1
0.5
1.0
%NaCl Temp. (°C)
Corrosion Rate (ipy)
0
20
0.021
0.0001
0.0001
0.0000
75
0.036
0.014 a
0.0004
0.0002
95
0.017
0.011 a
0.0004
0.0000
0.002
20
0.026
0.0006
0.0000
0.0000
75
0.067
0.005 a
0.0002
0.0000
95
0.021
0.017 a
0.005 a
0.0003
0.05
20
0.031
0.0012
0.0015
0.0008
75
0.085
0.002
0.003
0.002
95
0.023
0.007 a
0.005 a
0.002
3.5
20
0.024
0.0017
0.0016
0.0015
22.0
20
0.007
0.0009
0.0006
0.0013
a Pitted.