Corrosion Inhibitors
313
of iron in contact with chromate solutions (<0.0001 ipy), it is estimated that
i passive is less than 0.3 µamp/cm 2 . The rate of reduction increases with factors
that increase i passive such as the H + concentration, temperature, and Cl – concentration. It is found in practice that less chromate is consumed as exposure
time increases, which may be caused, in part, by a secondary film of oxides
eventually covering the metal, thereby exposing less surface at which the
passive film requires repair.
For optimum inhibition, the concentration of passivator must exceed a
certain critical value. Below this concentration, passivators behave as active
depolarizers and increase the corrosion rate in localized areas (pits). Lower
concentrations of passivator correspond to more active values of the oxidation-reduction potential and eventually the cathodic polarization curve intersects the anodic curve in the active region instead of in the passive region
(Figure 10.1).
The critical concentration for CrO 4
2− , NO 2
− , MoO 4
2− , or WO 4
2− is about 10 −3
to 10 −4 M. 16 A concentration of 10 −3 M Na 2 CrO 4 is equivalent to 0.016%, or 160
ppm. Chloride ions and elevated temperatures increase i critical as well as i passive ,
which in effect raise the critical passivator concentration to higher values.
Should passivator concentration fall below the critical value in stagnant
areas (e.g., at threads of a pipe or at crevices), the active potential of such
areas in galvanic contact with passive areas elsewhere of noble potential promotes corrosion (pitting) at the active areas (passive-active cells). For this
reason, it is important to maintain the concentration of passivators above
the critical value at all portions of the inhibited system by the use of stirring,
rapid flow rates, and avoidance of crevices or of surface films of grease and
other dirt. Because consumption of passivators increases with an increase in
chloride and sulfate ions, it is also essential to maintain as low a concentration of these ions as possible.
This is the most effective, and consequently, the most widely used type
of inhibitor. Chromatics are the least expensive inhibitors for use in water
systems and are widely used in the recirculating-cooling systems of internal
combustion engines, rectifiers, and cooling towers. Sodium chromate in concentrations of 0.04 to 0.1% is used for this purpose. At higher temperatures
or in freshwater that has chloride concentrations above 10 ppm, higher concentrations are required. If necessary, sodium hydroxide is added to adjust
the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a
concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that a periodic colormetric analysis be conducted to prevent this from
happening.
Recent environmental regulations have been imposed on the use of chromates. They are toxic, and in prolonged contact with the skin can cause a
rash. It is usually required that the Cr 6+ ion be converted to Cr 3+ before discharge. The Cr 3+ ion is insoluble and can be removed as a sludge, whereas the
Cr 6+ ion is water soluble and toxic. Even so, the Cr 3+ sludge is classified as a
hazardous waste and must be constantly monitored. Because of the chromate
313
of iron in contact with chromate solutions (<0.0001 ipy), it is estimated that
i passive is less than 0.3 µamp/cm 2 . The rate of reduction increases with factors
that increase i passive such as the H + concentration, temperature, and Cl – concentration. It is found in practice that less chromate is consumed as exposure
time increases, which may be caused, in part, by a secondary film of oxides
eventually covering the metal, thereby exposing less surface at which the
passive film requires repair.
For optimum inhibition, the concentration of passivator must exceed a
certain critical value. Below this concentration, passivators behave as active
depolarizers and increase the corrosion rate in localized areas (pits). Lower
concentrations of passivator correspond to more active values of the oxidation-reduction potential and eventually the cathodic polarization curve intersects the anodic curve in the active region instead of in the passive region
(Figure 10.1).
The critical concentration for CrO 4
2− , NO 2
− , MoO 4
2− , or WO 4
2− is about 10 −3
to 10 −4 M. 16 A concentration of 10 −3 M Na 2 CrO 4 is equivalent to 0.016%, or 160
ppm. Chloride ions and elevated temperatures increase i critical as well as i passive ,
which in effect raise the critical passivator concentration to higher values.
Should passivator concentration fall below the critical value in stagnant
areas (e.g., at threads of a pipe or at crevices), the active potential of such
areas in galvanic contact with passive areas elsewhere of noble potential promotes corrosion (pitting) at the active areas (passive-active cells). For this
reason, it is important to maintain the concentration of passivators above
the critical value at all portions of the inhibited system by the use of stirring,
rapid flow rates, and avoidance of crevices or of surface films of grease and
other dirt. Because consumption of passivators increases with an increase in
chloride and sulfate ions, it is also essential to maintain as low a concentration of these ions as possible.
This is the most effective, and consequently, the most widely used type
of inhibitor. Chromatics are the least expensive inhibitors for use in water
systems and are widely used in the recirculating-cooling systems of internal
combustion engines, rectifiers, and cooling towers. Sodium chromate in concentrations of 0.04 to 0.1% is used for this purpose. At higher temperatures
or in freshwater that has chloride concentrations above 10 ppm, higher concentrations are required. If necessary, sodium hydroxide is added to adjust
the pH to a range of 7.5 to 9.5. If the concentration of chromate falls below a
concentration of 0.016%, corrosion will be accelerated. Therefore, it is essential that a periodic colormetric analysis be conducted to prevent this from
happening.
Recent environmental regulations have been imposed on the use of chromates. They are toxic, and in prolonged contact with the skin can cause a
rash. It is usually required that the Cr 6+ ion be converted to Cr 3+ before discharge. The Cr 3+ ion is insoluble and can be removed as a sludge, whereas the
Cr 6+ ion is water soluble and toxic. Even so, the Cr 3+ sludge is classified as a
hazardous waste and must be constantly monitored. Because of the chromate