Corrosion Inhibitors
325
It is well known that pitting depends not only on the concentration of the
aggressive anions in the solution, but also on the concentration of the nonaggressive anions. For this reason, special attention must be paid to the effect
of inhibitor anions in the aggressive solution. The presence of nonaggressive
anions produces different effects:
1. Shifting the pitting potential to more positive values
2. Increasing the induction period for pitting
3. Reducing the number of pits
It has been found that pit nucleation on iron electrodes in phthalate buffer
containing chloride ions was prevented by pictate ions. The pitting potential
of iron in borate buffer containing chloride ions is enhanced by the presence
of capronate. On the other hand, sulfate ions can reduce the current density
at the pit bottom area, inhibiting the propagation stage of pitting corrosion.
Dezincification is the most common example of selective leaching. It is usually prevented by using less-susceptible alloys. The phenomenon can also be
minimized by reducing the aggressive of the environment with corrosion
inhibitors. The addition of surface-active substances such as saponin, dextrin, or benzotriazole can inhibit dezincification of single-phase and dibasic
brass in 0.5 N NaCl and HCl solutions. The highest inhibition efficiency was
obtained by benzotriazole.
Mild steels are susceptible to SCC in nitrate, caustic, and carbonate solutions. It has been demonstrated that the presence of inorganic ions (e.g.,
nitrate or chromate) or organic chemicals (e.g., acetate, benzoate) can inhibit
the localized attack.
Research on SCC inhibition with austenitic stainless steels in magnesium chloride solutions has been performed by Lee and Uhlig 27 and
Pinard 28 . In the presence of small concentrations of piperidine, n-decylamine, and other nitrogen-containing substances, it was possible to noticeably
lengthen the failure time of AISI 304 wires under constant load in boiling
chloride solutions.
The influence of some organic substances on the stress corrosion cracking of austenitic stainless steels in sulfuric acid solutions containing chlorides or in dilute hydrochloric acid solutions at room temperature has also
been studied. Very promising results have been obtained with benzonitrile,
2-mercaptobenzimidazole, 2-mercaptobenzothiozole, and thiourea deratives,
depending on the type of aggressive medium. The overall results support
the idea that SCC of austenitic stainless steels in acid chloride media occurs
on the alloys in the active state and is related to the presence of an adsorbed
layer of chloride ions. The stress brings “bare” metal areas, which are more
active than the surrounding areas, into contact with the solution. A very
localized attack can thus occur with the consequent formation of a crack.
In this way, only organic substances able to rapidly adsorb onto the surface
325
It is well known that pitting depends not only on the concentration of the
aggressive anions in the solution, but also on the concentration of the nonaggressive anions. For this reason, special attention must be paid to the effect
of inhibitor anions in the aggressive solution. The presence of nonaggressive
anions produces different effects:
1. Shifting the pitting potential to more positive values
2. Increasing the induction period for pitting
3. Reducing the number of pits
It has been found that pit nucleation on iron electrodes in phthalate buffer
containing chloride ions was prevented by pictate ions. The pitting potential
of iron in borate buffer containing chloride ions is enhanced by the presence
of capronate. On the other hand, sulfate ions can reduce the current density
at the pit bottom area, inhibiting the propagation stage of pitting corrosion.
Dezincification is the most common example of selective leaching. It is usually prevented by using less-susceptible alloys. The phenomenon can also be
minimized by reducing the aggressive of the environment with corrosion
inhibitors. The addition of surface-active substances such as saponin, dextrin, or benzotriazole can inhibit dezincification of single-phase and dibasic
brass in 0.5 N NaCl and HCl solutions. The highest inhibition efficiency was
obtained by benzotriazole.
Mild steels are susceptible to SCC in nitrate, caustic, and carbonate solutions. It has been demonstrated that the presence of inorganic ions (e.g.,
nitrate or chromate) or organic chemicals (e.g., acetate, benzoate) can inhibit
the localized attack.
Research on SCC inhibition with austenitic stainless steels in magnesium chloride solutions has been performed by Lee and Uhlig 27 and
Pinard 28 . In the presence of small concentrations of piperidine, n-decylamine, and other nitrogen-containing substances, it was possible to noticeably
lengthen the failure time of AISI 304 wires under constant load in boiling
chloride solutions.
The influence of some organic substances on the stress corrosion cracking of austenitic stainless steels in sulfuric acid solutions containing chlorides or in dilute hydrochloric acid solutions at room temperature has also
been studied. Very promising results have been obtained with benzonitrile,
2-mercaptobenzimidazole, 2-mercaptobenzothiozole, and thiourea deratives,
depending on the type of aggressive medium. The overall results support
the idea that SCC of austenitic stainless steels in acid chloride media occurs
on the alloys in the active state and is related to the presence of an adsorbed
layer of chloride ions. The stress brings “bare” metal areas, which are more
active than the surrounding areas, into contact with the solution. A very
localized attack can thus occur with the consequent formation of a crack.
In this way, only organic substances able to rapidly adsorb onto the surface