as ingle metal, because of the specific action of the inhibitor,m ay fail to
control corrosion if dissimilar metals are in contact. The behaviorofazincsteel couple in sodiumb enzoate solution was studied by Brasher and
Mercer.
33
Steel is protected in benzoate solution, but usually it corrodes
when coupled to zinc,a si ng alvanized iron, in thats olution. An
interpretation of the phenomenon was given,
33
emphasizing that rusting
of steel coupledtozinc in benzoate solutioncan be prevented by bubbling air
through the solutionfor one to two days immediately after immersion. After
this time, the system should have become stabilized, and no corrosion of the
steel should take place when the air stream is discontinued. Ve nezela nd
Wranglen
34
studied the steel-copper combination in ahot-water system.The
best inhibitore fficiency was obtained by usingam ixture of benzoate
and nitrite.
It is well known
35
that pitting depends not only on the concentration of
aggressive anions in the solution, but also on the concentration of the nonaggressive anions. For this reason, special attention was paid to the effect of
inhibitive anions in the aggressive solution. The presence of nonaggressive
anions produces differenteffects:
35
(1) shifting the pitting potential to more
positive values; (2) increasing the induction period for pitting;a nd (3)
reducing the number of pits.
In studying the behavior of various anions in pitting, Strehblow
36
found
that pit nucleation on iron electrodes in phthalate buffer containing chloride
ionswas prevented by picrate ions. Strehblow and Titze
37
emphasizedthat
the pitting potential of iron in borate buffer containing chloride ions was
ennobled by the presence of capronate. On the other hand, Ve tter and
Strahblow
38
showed that sulfate ions can reduce the current density at the
pit bottom area, inhibiting the propagation stageo fpitting corrosion.
Dezincification is the most common exampleo fs elective leaching. It is
usually preventedb yu sing less-susceptible alloys.T he phenomenon
can also be minimized by reducing the aggressiveness of the environment
with corrosioni nhibitors. Kravaeva et al.
39
demonstrated that the addition
of surface-active substances such as saponin, dextrin,o rb enzotriazole
can inhibit dezincificationofsingle-phase and diphasic brass in 0.5 NNaCl
and HCls olutions. The highest inhibition efficiency was obtained by
benzotriazole.
3.9S ummary
Corrosioni nhibitorsa re usuallya ble to preventg eneral or uniform
corrosion. However,t hey are very limited in theira bility to prevent
localized corrosion sucha sp itting, crevice corrosion, galvanic corrosion,
dezincification, or stress corrosionc racking.A dditional research work is
beingundertaken in the use of inhibitors to prevent these types of corrosion.
The importance of these studies is realized when it is taken into account that
Corrosion of Linings and Coatings
66
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68
control corrosion if dissimilar metals are in contact. The behaviorofazincsteel couple in sodiumb enzoate solution was studied by Brasher and
Mercer.
33
Steel is protected in benzoate solution, but usually it corrodes
when coupled to zinc,a si ng alvanized iron, in thats olution. An
interpretation of the phenomenon was given,
33
emphasizing that rusting
of steel coupledtozinc in benzoate solutioncan be prevented by bubbling air
through the solutionfor one to two days immediately after immersion. After
this time, the system should have become stabilized, and no corrosion of the
steel should take place when the air stream is discontinued. Ve nezela nd
Wranglen
34
studied the steel-copper combination in ahot-water system.The
best inhibitore fficiency was obtained by usingam ixture of benzoate
and nitrite.
It is well known
35
that pitting depends not only on the concentration of
aggressive anions in the solution, but also on the concentration of the nonaggressive anions. For this reason, special attention was paid to the effect of
inhibitive anions in the aggressive solution. The presence of nonaggressive
anions produces differenteffects:
35
(1) shifting the pitting potential to more
positive values; (2) increasing the induction period for pitting;a nd (3)
reducing the number of pits.
In studying the behavior of various anions in pitting, Strehblow
36
found
that pit nucleation on iron electrodes in phthalate buffer containing chloride
ionswas prevented by picrate ions. Strehblow and Titze
37
emphasizedthat
the pitting potential of iron in borate buffer containing chloride ions was
ennobled by the presence of capronate. On the other hand, Ve tter and
Strahblow
38
showed that sulfate ions can reduce the current density at the
pit bottom area, inhibiting the propagation stageo fpitting corrosion.
Dezincification is the most common exampleo fs elective leaching. It is
usually preventedb yu sing less-susceptible alloys.T he phenomenon
can also be minimized by reducing the aggressiveness of the environment
with corrosioni nhibitors. Kravaeva et al.
39
demonstrated that the addition
of surface-active substances such as saponin, dextrin,o rb enzotriazole
can inhibit dezincificationofsingle-phase and diphasic brass in 0.5 NNaCl
and HCls olutions. The highest inhibition efficiency was obtained by
benzotriazole.
3.9S ummary
Corrosioni nhibitorsa re usuallya ble to preventg eneral or uniform
corrosion. However,t hey are very limited in theira bility to prevent
localized corrosion sucha sp itting, crevice corrosion, galvanic corrosion,
dezincification, or stress corrosionc racking.A dditional research work is
beingundertaken in the use of inhibitors to prevent these types of corrosion.
The importance of these studies is realized when it is taken into account that
Corrosion of Linings and Coatings
66
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68