314
Fundamentals of Corrosion
ions cost of conversion, the constant monitoring required, and the disposal
of the hazardous waste, the economics of the use of these inhibitors are not
as attractive as they formerly were.
Because most antifreeze solutions contain methanol or ethylene glycol,
chromates cannot be used for this application because they have a tendency to
react with organic compounds. In these applications, borax (Na 2 B 4 O 7 10H 2 O)
that has added sulfonated oils to produce an oily coating and mercaptobenzothiazole are used. The latter material is a specific inhibitor for copper.
Nitrates are also used in antifreeze-type cooling water systems because
they have little tendency to react with alcohols or ethylene glycol. Because
they are gradually decomposed by bacteria, they are not recommended for
use in cooling tower waters. Another application for nitrites is as a corrosion
inhibitor for the internal surfaces of pipelines used to transport petroleum
products or gasoline. Such inhibition is accomplished by continuously injecting 5 to 30% sodium nitrite solution into the line.
At lower temperatures, such as in underground storage tanks, gasoline
can be corrosive to steel as dissolved water is released. This water, in contact
with large quantities of oxygen dissolved in the gasoline, corrodes the steel
and forms large quantities of rust. The sodium nitrite enters the water phase
and effectively inhibits corrosion.
Nitrites are also used to inhibit corrosion by cutting-oil-water emulsions
used in the machining of metals.
Passivating inhibitors can usually cause pitting and accelerate corrosion
when concentrations fall below minimum limits. For this reason, it is essential that constant monitoring of the inhibitor concentration take place.
Chromates are applied mostly as inhibitors for recirculating the cooling
water of internal combustion engines, rectifiers, and cooling towers. The
concentration of Na 2 CrO 4 used for this purpose is about 0.04 to 0.1%, the
higher concentrations being employed at higher temperatures or in freshwaters of chloride concentrations above 10 ppm. The pH is adjusted, if necessary, to 7.5 to 9.5 by the addition of NaOH. Periodic colormetric analysis is
required to ensure that the concentration remains above the critical 10 −3 M
or 0.016% Na 2 CrO 4 . Sometimes, combinations of chromates and polyphosphates or other inhibitors permit the concentration of chromates to fall below
the critical level. This results in some sacrifice of inhibiting efficiency but
with adequate protection against pitting for the treatment of very large volumes of water employing cooling towers.
Corrosion rates of mild steel as a function of chromate and chloride concentration at various temperatures are shown in Table 10.1 Such data in the
region of the critical chromate concentration are not readily reproduced
because of erratic pitting behavior.
Nitrites are inhibitors only above about pH 6.0. In more acidic environments,
they tend to decompose, forming volatile nitric oxide and nitrogen peroxide. In
common with other passivators, they tend to induce pitting at concentrations
Fundamentals of Corrosion
ions cost of conversion, the constant monitoring required, and the disposal
of the hazardous waste, the economics of the use of these inhibitors are not
as attractive as they formerly were.
Because most antifreeze solutions contain methanol or ethylene glycol,
chromates cannot be used for this application because they have a tendency to
react with organic compounds. In these applications, borax (Na 2 B 4 O 7 10H 2 O)
that has added sulfonated oils to produce an oily coating and mercaptobenzothiazole are used. The latter material is a specific inhibitor for copper.
Nitrates are also used in antifreeze-type cooling water systems because
they have little tendency to react with alcohols or ethylene glycol. Because
they are gradually decomposed by bacteria, they are not recommended for
use in cooling tower waters. Another application for nitrites is as a corrosion
inhibitor for the internal surfaces of pipelines used to transport petroleum
products or gasoline. Such inhibition is accomplished by continuously injecting 5 to 30% sodium nitrite solution into the line.
At lower temperatures, such as in underground storage tanks, gasoline
can be corrosive to steel as dissolved water is released. This water, in contact
with large quantities of oxygen dissolved in the gasoline, corrodes the steel
and forms large quantities of rust. The sodium nitrite enters the water phase
and effectively inhibits corrosion.
Nitrites are also used to inhibit corrosion by cutting-oil-water emulsions
used in the machining of metals.
Passivating inhibitors can usually cause pitting and accelerate corrosion
when concentrations fall below minimum limits. For this reason, it is essential that constant monitoring of the inhibitor concentration take place.
Chromates are applied mostly as inhibitors for recirculating the cooling
water of internal combustion engines, rectifiers, and cooling towers. The
concentration of Na 2 CrO 4 used for this purpose is about 0.04 to 0.1%, the
higher concentrations being employed at higher temperatures or in freshwaters of chloride concentrations above 10 ppm. The pH is adjusted, if necessary, to 7.5 to 9.5 by the addition of NaOH. Periodic colormetric analysis is
required to ensure that the concentration remains above the critical 10 −3 M
or 0.016% Na 2 CrO 4 . Sometimes, combinations of chromates and polyphosphates or other inhibitors permit the concentration of chromates to fall below
the critical level. This results in some sacrifice of inhibiting efficiency but
with adequate protection against pitting for the treatment of very large volumes of water employing cooling towers.
Corrosion rates of mild steel as a function of chromate and chloride concentration at various temperatures are shown in Table 10.1 Such data in the
region of the critical chromate concentration are not readily reproduced
because of erratic pitting behavior.
Nitrites are inhibitors only above about pH 6.0. In more acidic environments,
they tend to decompose, forming volatile nitric oxide and nitrogen peroxide. In
common with other passivators, they tend to induce pitting at concentrations