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Fundamentals of Corrosion
Use of corrosion inhibitors. Caustic added to crude oils before and after
desalting helps suppress chloride hydrolysis and reduces corrosivity by
making the hot, salt-laden desalter water alkaline. Caustic may also remove
some of the naphthenic acids.
Ammonium hydroxide, ammonia, monoethanolamine, diethanolamine,
and morphaline are added to the overhead vapors from distillation towers
to ensure that the condensed water has a pH of 5 or greater. This inhibitor
can completely stop corrosion and provide conditions under which corrosion inhibitors can stop corrosion, providing the salts of the neutralization
reaction do not deposit in the overhead condensers. The amount of neutralizer must be carefully controlled to avoid caustic stress cracking of steel and
ammonia or amine stress corrosion cracking of copper alloys.
Refinery corrosion inhibitors are mostly solid, high molecular weight nitrogen compounds (amides, amines, imidazolines, and their salts with fatty
acids) dissolved in hydrocarbon solvents. They are also generally surfactants
and function by forming protective films on metal surfaces.
Adjustments in process conditions. Corrosion control often can be attained by
simple changes in operating conditions with little or no economic penalty.
For example, the acid gases (HCl, H 2 S, CO 2 , and SO 2 ) are not corrosive at
moderate temperatures, provided there is no aqueous condensation; consequently, raising the gas temperature, lowering the pressure, and installing
insulation or steam-tracing equipment often can eliminate the danger that
the gases will cause severe corrosion or stress corrosion cracking. In areas of
equipment with little or no process flow such as relief valves, bypass lines,
and instrument lines, the accumulation of corrosive gases can be prevented
by a small flow (bleed) of hot process gas or a dry purge gas.
Process corrosion can be minimized if it is possible to reduce the process temperature. This approach, however, is not usually practical. Hightemperature sulfidic corrosion in hot process streams can be monitored with
corrosion probes. Even if there is no practical remedy, at least the probes
measure corrosion without having to shut down the unit. The equipment
life can be estimated and a resistant alloy selected and fabricated to replace
the corroded equipment quickly and economically at a shutdown scheduled
before the equipment becomes defective.
Removal of sulfur and other impurities. This is a specialized kind of hydrogenation in which the quality of the liquid hydrocarbon stream is improved by
subjecting it to mild or severe conditions of hydrogen pressure in the presence of a catalyst. The objective is to convert undesirable or corrosive material
in the feedstock to either desired material or easily disposed of by-products
on a selective basis. As of the early 1980s, about 45% of the crude oil refined
in the United States was hydrotreated. Some applications of hydrotreating
accomplish the following:
Fundamentals of Corrosion
Use of corrosion inhibitors. Caustic added to crude oils before and after
desalting helps suppress chloride hydrolysis and reduces corrosivity by
making the hot, salt-laden desalter water alkaline. Caustic may also remove
some of the naphthenic acids.
Ammonium hydroxide, ammonia, monoethanolamine, diethanolamine,
and morphaline are added to the overhead vapors from distillation towers
to ensure that the condensed water has a pH of 5 or greater. This inhibitor
can completely stop corrosion and provide conditions under which corrosion inhibitors can stop corrosion, providing the salts of the neutralization
reaction do not deposit in the overhead condensers. The amount of neutralizer must be carefully controlled to avoid caustic stress cracking of steel and
ammonia or amine stress corrosion cracking of copper alloys.
Refinery corrosion inhibitors are mostly solid, high molecular weight nitrogen compounds (amides, amines, imidazolines, and their salts with fatty
acids) dissolved in hydrocarbon solvents. They are also generally surfactants
and function by forming protective films on metal surfaces.
Adjustments in process conditions. Corrosion control often can be attained by
simple changes in operating conditions with little or no economic penalty.
For example, the acid gases (HCl, H 2 S, CO 2 , and SO 2 ) are not corrosive at
moderate temperatures, provided there is no aqueous condensation; consequently, raising the gas temperature, lowering the pressure, and installing
insulation or steam-tracing equipment often can eliminate the danger that
the gases will cause severe corrosion or stress corrosion cracking. In areas of
equipment with little or no process flow such as relief valves, bypass lines,
and instrument lines, the accumulation of corrosive gases can be prevented
by a small flow (bleed) of hot process gas or a dry purge gas.
Process corrosion can be minimized if it is possible to reduce the process temperature. This approach, however, is not usually practical. Hightemperature sulfidic corrosion in hot process streams can be monitored with
corrosion probes. Even if there is no practical remedy, at least the probes
measure corrosion without having to shut down the unit. The equipment
life can be estimated and a resistant alloy selected and fabricated to replace
the corroded equipment quickly and economically at a shutdown scheduled
before the equipment becomes defective.
Removal of sulfur and other impurities. This is a specialized kind of hydrogenation in which the quality of the liquid hydrocarbon stream is improved by
subjecting it to mild or severe conditions of hydrogen pressure in the presence of a catalyst. The objective is to convert undesirable or corrosive material
in the feedstock to either desired material or easily disposed of by-products
on a selective basis. As of the early 1980s, about 45% of the crude oil refined
in the United States was hydrotreated. Some applications of hydrotreating
accomplish the following: