360
Fundamentals of Corrosion
in alloy selection in the naphtha and gas oil desulfurization unit and the
hydrocracking unit of a refinery.
At high temperatures, high-pressure hydrogen diffuses into metals and
reacts with the carbon steel to form methane. The reaction causes fissuring
and blistering, as well as decarburization of the steel, with a consequent loss
in ductility. Then catastrophic failure can occur without warning and with
no visible deterioration of the metal.
Hydrogen is also produced by aqueous acid corrosion at low temperatures.
In this case, hydrogen atoms, or ions, form at the metal surface, rapidly diffuse
inward, and then form hydrogen molecules at defects or dislocations within
the metal. Because these molecules cannot diffuse out of the metal at low temperatures, pressures of several pounds (psi) can build up and cause cracks,
blisters, and serious ruptures. Such damage is particularly severe in the fractionation section of fluid catalytic cracking units or vapor recovery units where
there are cyanides and hydrogen sulfide, both of which accelerate the diffusion of hydrogen into steel. Welds, which are often significantly stronger than
the base metal, are especially susceptible to cracking from this hydrogen.
The strong acids and bases required in many refining processes can be
particularly corrosive. These include the sulfuric and hydrochloric acids
used as alkylation catalysts and the aluminum chloride plus hydrochloric
acid used as both an alkylation and isomerization catalyst. Caustic soda is
used to remove entrained acids from products, and the several bases such
as monoethanolamine and potassium carbonate used to remove hydrogen
sulfide and carbon dioxide from fuel gases. Although the bases alone are not
corrosive, the dissolution and release of acid gases can cause severe corrosion, hydrogen blistering, and stress corrosion cracking. The amount of acid
gases dissolving in the water that condenses on the storage tank roofs or separates to the bottom of the tanks usually determines the life of such vessels
and the amount of maintenance required (ASTM 1973 Symposia STP-558).
Naphthenic acids are initially present in most crudes at very low concentrations but can lead to corrosion in stainless steel sections of the vacuum distillation tower where type 410SS is used. The identification of naphthenic acid
corrosion may be confused with high-temperature sulfide corrosion because
of the similarity of the sharp-edge pits and grooves. In the fractionation process, sufficiently high local concentrations of the naphthenic acids can occur
at temperatures between 450ºF and 650ºF to corrode stainless steels that normally resist sulfide corrosion. The metal naphthanates are soluble in oil, so
the scale on the corroded metal is iron sulfide, which further complicates
diagnosis. What constitutes a dangerous concentration of naphthenic acids
is not clear but authorities suggest that corrosion will occur if the neutralization number is above 0.5 mg KOH/g of crude.
Perhaps the most pervasive sources of corrosion are the large amounts of
water and steam required in refineries. Cooling water, both once-through
and circulating from cooling towers (generally several times the volume of
the crude being processed), is responsible for both corrosion and scaling of
Fundamentals of Corrosion
in alloy selection in the naphtha and gas oil desulfurization unit and the
hydrocracking unit of a refinery.
At high temperatures, high-pressure hydrogen diffuses into metals and
reacts with the carbon steel to form methane. The reaction causes fissuring
and blistering, as well as decarburization of the steel, with a consequent loss
in ductility. Then catastrophic failure can occur without warning and with
no visible deterioration of the metal.
Hydrogen is also produced by aqueous acid corrosion at low temperatures.
In this case, hydrogen atoms, or ions, form at the metal surface, rapidly diffuse
inward, and then form hydrogen molecules at defects or dislocations within
the metal. Because these molecules cannot diffuse out of the metal at low temperatures, pressures of several pounds (psi) can build up and cause cracks,
blisters, and serious ruptures. Such damage is particularly severe in the fractionation section of fluid catalytic cracking units or vapor recovery units where
there are cyanides and hydrogen sulfide, both of which accelerate the diffusion of hydrogen into steel. Welds, which are often significantly stronger than
the base metal, are especially susceptible to cracking from this hydrogen.
The strong acids and bases required in many refining processes can be
particularly corrosive. These include the sulfuric and hydrochloric acids
used as alkylation catalysts and the aluminum chloride plus hydrochloric
acid used as both an alkylation and isomerization catalyst. Caustic soda is
used to remove entrained acids from products, and the several bases such
as monoethanolamine and potassium carbonate used to remove hydrogen
sulfide and carbon dioxide from fuel gases. Although the bases alone are not
corrosive, the dissolution and release of acid gases can cause severe corrosion, hydrogen blistering, and stress corrosion cracking. The amount of acid
gases dissolving in the water that condenses on the storage tank roofs or separates to the bottom of the tanks usually determines the life of such vessels
and the amount of maintenance required (ASTM 1973 Symposia STP-558).
Naphthenic acids are initially present in most crudes at very low concentrations but can lead to corrosion in stainless steel sections of the vacuum distillation tower where type 410SS is used. The identification of naphthenic acid
corrosion may be confused with high-temperature sulfide corrosion because
of the similarity of the sharp-edge pits and grooves. In the fractionation process, sufficiently high local concentrations of the naphthenic acids can occur
at temperatures between 450ºF and 650ºF to corrode stainless steels that normally resist sulfide corrosion. The metal naphthanates are soluble in oil, so
the scale on the corroded metal is iron sulfide, which further complicates
diagnosis. What constitutes a dangerous concentration of naphthenic acids
is not clear but authorities suggest that corrosion will occur if the neutralization number is above 0.5 mg KOH/g of crude.
Perhaps the most pervasive sources of corrosion are the large amounts of
water and steam required in refineries. Cooling water, both once-through
and circulating from cooling towers (generally several times the volume of
the crude being processed), is responsible for both corrosion and scaling of