in concentrations of 0.04–0.1%,i su sedf or this purpose. At higher
temperatures or in freshwater that has chloride concentrations above
10 ppm,higher concentrationsare required. If necessary,sodium hydroxide
is added to adjust the pH to ar ange of 7.5–9.5. If the concentration of
chromatef alls belowaconcentrationo f0 .016%, corrosionw illb e
accelerated. Therefore, it is essential that periodic colorimetric analysis be
conducted to prevent this from occurring.
Recente nvironmental regulations have been imposed on the use of
chromates. Theyare toxic and, on prolonged contact with the skin,can cause
ar ash.I ti su sually required that the Cr
6 C ion be converted to Cr
3 C before
discharge. The Cr
3 C
ion is insoluble and can be removed as as ludge,
whereas, the Cr
6 C
ion is water-soluble and toxic. Even so, the Cr
3 C
sludgeis
classified as ahazardous waste and must be constantly monitored. Because
of the chromate ions’ cost of conversion, the constantm onitoringr equired,
and the disposal of the hazardous wastes, the economics of the use of these
inhibitors are not as attractive as they formerlyw ere.
Because most antifreeze solutions contain methanol or ethylene glycol,the
chromates cannotb eu sed in this applicationb ecause the chromates have
at endencyt or eactw ith organic compounds. In these applications, borax
(Na 2 B 4 O 7 $ 10H 2 O) that has added sulfonated oils to produce an oilycoating,
and mercaptobenzothiazole are used. The latter material is as pecific
inhibitor for the corrosionofc opper.
Nitrites are also used in antifreeze-type cooling water systemsb ecause
they have little tendency to react with alcoholso re thylene glycol. Because
they are gradually decomposed by bacteria, they are not recommended for
use in cooling tower waters.Another application for nitrites is as acorrosion
inhibitor of the internal surfaces of pipelines used to transport petroleum
products or gasoline. Such inhibition is accomplished by continuously
injecting a5–30% sodium nitrite solutioninto the line.
At lower temperatures, such as in underground storage tanks, gasoline
can be corrosive to steel as dissolvedwater is released. Thiswater,incontact
with the large quantitieso fo xygend issolved in the gasoline, corrodes the
steel and forms large quantitiesofrust. The sodium nitrite enters the water
phasea nd effectively inhibits corrosion.
The nitrites are also used to inhibit corrosionb yc utting oil-water
emulsions used in the machining of metals.
Passivation inhibitors can actually cause pitting and accelerate corrosion
when concentrations fall below minimum limits. For this reason, it is essential
that constantmonitoring of the inhibitor concentration be performed.
3.3.2O rganic Inhibitors
These materials buildu paprotective film of adsorbed molecules on
the metal surfacet hat provide ab arriert ot he dissolution of the metal in
the electrolyte. Since the metal surfacec overed is proportional to the
inhibitor concentrates, the concentration of the inhibitor in the medium is
Corrosion of Linings and Coatings
58
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68
temperatures or in freshwater that has chloride concentrations above
10 ppm,higher concentrationsare required. If necessary,sodium hydroxide
is added to adjust the pH to ar ange of 7.5–9.5. If the concentration of
chromatef alls belowaconcentrationo f0 .016%, corrosionw illb e
accelerated. Therefore, it is essential that periodic colorimetric analysis be
conducted to prevent this from occurring.
Recente nvironmental regulations have been imposed on the use of
chromates. Theyare toxic and, on prolonged contact with the skin,can cause
ar ash.I ti su sually required that the Cr
6 C ion be converted to Cr
3 C before
discharge. The Cr
3 C
ion is insoluble and can be removed as as ludge,
whereas, the Cr
6 C
ion is water-soluble and toxic. Even so, the Cr
3 C
sludgeis
classified as ahazardous waste and must be constantly monitored. Because
of the chromate ions’ cost of conversion, the constantm onitoringr equired,
and the disposal of the hazardous wastes, the economics of the use of these
inhibitors are not as attractive as they formerlyw ere.
Because most antifreeze solutions contain methanol or ethylene glycol,the
chromates cannotb eu sed in this applicationb ecause the chromates have
at endencyt or eactw ith organic compounds. In these applications, borax
(Na 2 B 4 O 7 $ 10H 2 O) that has added sulfonated oils to produce an oilycoating,
and mercaptobenzothiazole are used. The latter material is as pecific
inhibitor for the corrosionofc opper.
Nitrites are also used in antifreeze-type cooling water systemsb ecause
they have little tendency to react with alcoholso re thylene glycol. Because
they are gradually decomposed by bacteria, they are not recommended for
use in cooling tower waters.Another application for nitrites is as acorrosion
inhibitor of the internal surfaces of pipelines used to transport petroleum
products or gasoline. Such inhibition is accomplished by continuously
injecting a5–30% sodium nitrite solutioninto the line.
At lower temperatures, such as in underground storage tanks, gasoline
can be corrosive to steel as dissolvedwater is released. Thiswater,incontact
with the large quantitieso fo xygend issolved in the gasoline, corrodes the
steel and forms large quantitiesofrust. The sodium nitrite enters the water
phasea nd effectively inhibits corrosion.
The nitrites are also used to inhibit corrosionb yc utting oil-water
emulsions used in the machining of metals.
Passivation inhibitors can actually cause pitting and accelerate corrosion
when concentrations fall below minimum limits. For this reason, it is essential
that constantmonitoring of the inhibitor concentration be performed.
3.3.2O rganic Inhibitors
These materials buildu paprotective film of adsorbed molecules on
the metal surfacet hat provide ab arriert ot he dissolution of the metal in
the electrolyte. Since the metal surfacec overed is proportional to the
inhibitor concentrates, the concentration of the inhibitor in the medium is
Corrosion of Linings and Coatings
58
CAT8247—CHAPTER 3—6/10/2006—19:23—KADAMBADI—XMLMODEL B–p p. 55–68