Most corrosion problems are not caused by hydrocarbons but by various inorganic compounds such
as water, hydrogen sulfide (H2S), hydrochloric acid (HCl), hydrofluoric acid , sulfuric acid (H2SO4 )
and caustic. There are two main sources of these compounds:
- feed-stock contaminants and
- process chemicals.
1. Low temperature corrosion by feed-stock contaminants
The cause of the refinery corrosion is the presence of contaminants in crude oil as it is produced.
Corrosive hydrogen chloride evolves in crude preheat furnaces from relatively harmless magnesium
and calcium chloride entrained in crude oil. In petrochemical plants, certain corrosives may have
been introduced from upstream refinery and other process operations, other corrosives can form
from corrosion products after exposure to air during shut-down: polythionic acids fall into this
category. Corrosive contaminants are as follows:
- air
- water
- hydrogen sulfide
- hydrogen chloride
- nitrogen compounds
- sour water
- polythionic acids
Air:
Air may enter to many potential inlets equipments of during prolong shutdown of plant. In general, the air contamination of hydrocarbon streams has been more detrimental with regard to fouling than corrosion. However, air contaminant has been sited as a cause of accelerated corrosion in vacuum towers and vacuum transfer lines, also accelerate overhead corrosion of crude distillation towers.
Water:
Water content of crude oils and water originated with stripping steam for fractionation towers
hydrolyses certain inorganic chlorides to hydrogen chloride, and is responsible for various forms of
corrosion in fractionation tower overhead systems. It is a good practice to keep equipment dry in
order to minimize corrosion.
Combination of moisture and air enters into storage tanks during normal breathing as a result of
pumping and changes in temperature. Corrosion of tank bottoms occurs mostly with crude oil tanks
which is caused by water and salt entrained in the crude oil. A layer of water usually settles out and
can become highly corrosive.
Hydrogen sulfide is the main constituent of refinery sour waters and can cause severe corrosion
problems in overhead of certain fractionation towers, in hydrocracker and hydrotreater effluent
streams in vapor recovery of Fluid Catalytic Cracking (F.C.C) Units, in sour water stripping units and
in sulfur recovery units. Carbon steel has fairly good resistance to aqueous sulfide corrosion
because a protective film of FeS is formed to avoid hydrogen stress cracking (sulfide cracking),
hard welds (above 200 HB) must be avoided, if necessary, through suitable postweld heat
treatment. Excessive localized corrosion in vessels can be resolved by selective lining with alloy
400 , but this alloy can be less resistant than carbon steel to aqueous sulfide corrosion at
temperatures above 150°C. If significant amounts of chlorides are not present, lining vessels with
Type 304 (S30400) stainless steel can be considered.
In refineries, corrosion by hydrogen chloride is primarily a problem in crude distillation units, and to
lesser degree in reforming and hydrotreating units. In petrochemical plants, HCl contamination can
be present in certain feed stocks
Ammonia is also produced in ammonia plants to become a raw material for the manufacture of urea
and other nitrogenbase fertilizers. Ammonia in synthesis gas at temperatures between 450 and
500°C causes nitriding of steel components. When synthesis gas is compressed to up to 34.5 MPa
(5000 psig) prior to conversion, corrosive ammonium carbonate is formed, requiring various
stainless steels for critical components. Condensed ammonia is also corrosive and can cause SCC
of stressed carbon steel and low-alloy steel components.
Polythionic acids:
Combustion of H2S in refinery flares can produce polythionic acids of type H2SxOy (including
sulfurous acid) and cause severe intergranular corrosion of flare tips made of stainless steels and
high-nickel alloys. Corrosion can be minimized by using nickel alloys such as alloy 825 or
alloy 625
2. Low temperature corrosion by process chemicals:
Severe corrosion problems can be caused by process chemicals, such as various alkylation
catalysts, certain alkylation by-products, organic acid solvents used in certain petrochemical
process, hydrogen chloride stripped off reformer catalyst, and caustic and other neutralizers that
ironically, are added to control acid corrosion. Another group of process chemicals
that are corrosive, or become corrosive, is solvents used in treating and gas-scrubbing operations.
These chemicals are as follows:
- acetic acid
- aluminum chloride
- organic chloride
- hydrogen fluoride
- sulfuric acid
- caustic
- amine
- phenol
Acetic acid :
Corrosion by acetic acid can be a problem in petrochemical process units for the manufacture of
certain organic intermediates such as terephthalic acid. Various types of austenitic stainless steels
are used, as well as alloy C-4 , alloy C-276 and titanium, to control corrosion by acetic acid in the presence of small amount of hydrogen bromide or hydrogen chloride.Small amount of water in acetic acid can have a significant influence on corrosion. Type 304 (S30400) stainless steel has sufficient resistance to the lower concentration of acetic acid up to the boiling point. Higher concentration can be handled by type 304 stainless steel if the temperature is below 90°C.Corrosion by acetic acid increases with temperature. Bromide and chloride contamination causes pitting and SCC, while addition of oxidizing agents, including air, can reduce corrosion rates by several orders of magnitude.
Aluminum chloride :
Certain refining and petrochemical processes such as butane, isomerization, ethylbenzene
production and polybutene production, use aluminum chloride as a catalyst. Aluminum chloride is
not corrosive if it is kept absolutely dry otherwise it hydrolyzes to hydrochloric acid. During shutdown
equipment shall be opened for the shortest possible time. Upon closing, the system shall be
dried with hot air followed by inert gas blanketing. Equipment that is exposed to hydrchloric acid
may require extensive lining with nickel alloys, such as alloy 400 (N04400), B-2 (N10665), C-4
(N06455), or C-276 (N10276).
Hydrogen fluoride:
Some alkylation processes use concentrated HF instead of H2SO4 as the catalyst. In general, HF is
less corrosive than HCl because it passivates most metals by the formation of protective fluoride
films. If these films are destroyed by diluted acid, severe corrosion occurs. Therefore as long as
feed stocks are dry, carbon steel-with various corrosion allowances-can be used for vessels, piping,
and valve bodies of hydrofluoric acid alkylation units. All carbon steel welds that contact HF, shall be PWHT (postweld heat treated).
Fractionation towers shall have Type 410 (S41000) stainless steel tray valves and bolting for
desiobutanizer tower tray valve and bolting, alloy 400 (N04400) is recommended. Corrosion
problems in HF alkylation units occur after shutdown because pockets of water have been left in the
equipment. It is very important that equipment be thoroughly dried by draining all low spots and by
circulating hydrocarbon before the introduction of HF catalyst at start-up.
Sulfuric acid :
Certain alkylation units use essentially concentrated sulfuric acid as the catalyst; some of this acid
is entrained in reactor effluent and must be removed by neutralization with caustic and scrubbing
with water. Acid removal may not be complete, however, and trace of acid-at various concentrations
(in terms of water)-remain in the stream.
Dilute sulfuric acid can be highly corrosive to carbon steel, which is the principal material of
construction for sulfuric acid alkylation units. Because the boiling point of sulfuric acid depending on
concentration ranges from 165 to 315°C, depending on concentration entrained acid usually ends
up in the bottom of the first fractionation tower and reboiler following the reactor; this is where the
entrained acid becomes concentrated.
Acid concentration above 85% by weight are not corrosive to carbon steel if temperatures are below
40°C. Cold-worked metal (usually bends) shall be stress relieved. Under ideal operating conditions,
few, if any, corrosion and fouling problems occur.
Carbon steel depends on a film of iron sulfate for corrosion resistance, and if its film is destroyed by
high velocities and flow turbulence, corrosion can be quite severe.
Phenol
Phenol (carbolic acid) is used in refineries to convert heavy, waxy distillates obtained by crude oil
distillation into lubricating oils. As a rule, all components in the treating and raffinate recovery
sections, except tubes in water-cooled heat exchangers, are made from carbon steel. If water is not
present, few significant corrosion problems can be expected to occur in these sections. In the exact
recovery section severe corrosion can occur, especially where high flow turbulence is encountered.
As a result, certain components require selective alloying with Type 316 (S31600) stainless steel.
Typically stainless steel liners are required for the top of the dryer tower, the entire phenol flash
tower, and various condenser shells and separator drums that handle phenolic water.
Tubes and headers in the extract furnace shall also be made of Type 316 (S31600) stainless steel
with U-bends sleeved with alloy C-4 (N06455) on the outlet side to minimize velocity accelerated
corrosion.
as water, hydrogen sulfide (H2S), hydrochloric acid (HCl), hydrofluoric acid , sulfuric acid (H2SO4 )
and caustic. There are two main sources of these compounds:
- feed-stock contaminants and
- process chemicals.
1. Low temperature corrosion by feed-stock contaminants
The cause of the refinery corrosion is the presence of contaminants in crude oil as it is produced.
Corrosive hydrogen chloride evolves in crude preheat furnaces from relatively harmless magnesium
and calcium chloride entrained in crude oil. In petrochemical plants, certain corrosives may have
been introduced from upstream refinery and other process operations, other corrosives can form
from corrosion products after exposure to air during shut-down: polythionic acids fall into this
category. Corrosive contaminants are as follows:
- air
- water
- hydrogen sulfide
- hydrogen chloride
- nitrogen compounds
- sour water
- polythionic acids
Air:
Air may enter to many potential inlets equipments of during prolong shutdown of plant. In general, the air contamination of hydrocarbon streams has been more detrimental with regard to fouling than corrosion. However, air contaminant has been sited as a cause of accelerated corrosion in vacuum towers and vacuum transfer lines, also accelerate overhead corrosion of crude distillation towers.
Water:
Water content of crude oils and water originated with stripping steam for fractionation towers
hydrolyses certain inorganic chlorides to hydrogen chloride, and is responsible for various forms of
corrosion in fractionation tower overhead systems. It is a good practice to keep equipment dry in
order to minimize corrosion.
Combination of moisture and air enters into storage tanks during normal breathing as a result of
pumping and changes in temperature. Corrosion of tank bottoms occurs mostly with crude oil tanks
which is caused by water and salt entrained in the crude oil. A layer of water usually settles out and
can become highly corrosive.
Hydrogen sulfide is the main constituent of refinery sour waters and can cause severe corrosion
problems in overhead of certain fractionation towers, in hydrocracker and hydrotreater effluent
streams in vapor recovery of Fluid Catalytic Cracking (F.C.C) Units, in sour water stripping units and
in sulfur recovery units. Carbon steel has fairly good resistance to aqueous sulfide corrosion
because a protective film of FeS is formed to avoid hydrogen stress cracking (sulfide cracking),
hard welds (above 200 HB) must be avoided, if necessary, through suitable postweld heat
treatment. Excessive localized corrosion in vessels can be resolved by selective lining with alloy
400 , but this alloy can be less resistant than carbon steel to aqueous sulfide corrosion at
temperatures above 150°C. If significant amounts of chlorides are not present, lining vessels with
Type 304 (S30400) stainless steel can be considered.
In refineries, corrosion by hydrogen chloride is primarily a problem in crude distillation units, and to
lesser degree in reforming and hydrotreating units. In petrochemical plants, HCl contamination can
be present in certain feed stocks
Ammonia is also produced in ammonia plants to become a raw material for the manufacture of urea
and other nitrogenbase fertilizers. Ammonia in synthesis gas at temperatures between 450 and
500°C causes nitriding of steel components. When synthesis gas is compressed to up to 34.5 MPa
(5000 psig) prior to conversion, corrosive ammonium carbonate is formed, requiring various
stainless steels for critical components. Condensed ammonia is also corrosive and can cause SCC
of stressed carbon steel and low-alloy steel components.
Polythionic acids:
Combustion of H2S in refinery flares can produce polythionic acids of type H2SxOy (including
sulfurous acid) and cause severe intergranular corrosion of flare tips made of stainless steels and
high-nickel alloys. Corrosion can be minimized by using nickel alloys such as alloy 825 or
alloy 625
2. Low temperature corrosion by process chemicals:
Severe corrosion problems can be caused by process chemicals, such as various alkylation
catalysts, certain alkylation by-products, organic acid solvents used in certain petrochemical
process, hydrogen chloride stripped off reformer catalyst, and caustic and other neutralizers that
ironically, are added to control acid corrosion. Another group of process chemicals
that are corrosive, or become corrosive, is solvents used in treating and gas-scrubbing operations.
These chemicals are as follows:
- acetic acid
- aluminum chloride
- organic chloride
- hydrogen fluoride
- sulfuric acid
- caustic
- amine
- phenol
Acetic acid :
Corrosion by acetic acid can be a problem in petrochemical process units for the manufacture of
certain organic intermediates such as terephthalic acid. Various types of austenitic stainless steels
are used, as well as alloy C-4 , alloy C-276 and titanium, to control corrosion by acetic acid in the presence of small amount of hydrogen bromide or hydrogen chloride.Small amount of water in acetic acid can have a significant influence on corrosion. Type 304 (S30400) stainless steel has sufficient resistance to the lower concentration of acetic acid up to the boiling point. Higher concentration can be handled by type 304 stainless steel if the temperature is below 90°C.Corrosion by acetic acid increases with temperature. Bromide and chloride contamination causes pitting and SCC, while addition of oxidizing agents, including air, can reduce corrosion rates by several orders of magnitude.
Aluminum chloride :
Certain refining and petrochemical processes such as butane, isomerization, ethylbenzene
production and polybutene production, use aluminum chloride as a catalyst. Aluminum chloride is
not corrosive if it is kept absolutely dry otherwise it hydrolyzes to hydrochloric acid. During shutdown
equipment shall be opened for the shortest possible time. Upon closing, the system shall be
dried with hot air followed by inert gas blanketing. Equipment that is exposed to hydrchloric acid
may require extensive lining with nickel alloys, such as alloy 400 (N04400), B-2 (N10665), C-4
(N06455), or C-276 (N10276).
Hydrogen fluoride:
Some alkylation processes use concentrated HF instead of H2SO4 as the catalyst. In general, HF is
less corrosive than HCl because it passivates most metals by the formation of protective fluoride
films. If these films are destroyed by diluted acid, severe corrosion occurs. Therefore as long as
feed stocks are dry, carbon steel-with various corrosion allowances-can be used for vessels, piping,
and valve bodies of hydrofluoric acid alkylation units. All carbon steel welds that contact HF, shall be PWHT (postweld heat treated).
Fractionation towers shall have Type 410 (S41000) stainless steel tray valves and bolting for
desiobutanizer tower tray valve and bolting, alloy 400 (N04400) is recommended. Corrosion
problems in HF alkylation units occur after shutdown because pockets of water have been left in the
equipment. It is very important that equipment be thoroughly dried by draining all low spots and by
circulating hydrocarbon before the introduction of HF catalyst at start-up.
Sulfuric acid :
Certain alkylation units use essentially concentrated sulfuric acid as the catalyst; some of this acid
is entrained in reactor effluent and must be removed by neutralization with caustic and scrubbing
with water. Acid removal may not be complete, however, and trace of acid-at various concentrations
(in terms of water)-remain in the stream.
Dilute sulfuric acid can be highly corrosive to carbon steel, which is the principal material of
construction for sulfuric acid alkylation units. Because the boiling point of sulfuric acid depending on
concentration ranges from 165 to 315°C, depending on concentration entrained acid usually ends
up in the bottom of the first fractionation tower and reboiler following the reactor; this is where the
entrained acid becomes concentrated.
Acid concentration above 85% by weight are not corrosive to carbon steel if temperatures are below
40°C. Cold-worked metal (usually bends) shall be stress relieved. Under ideal operating conditions,
few, if any, corrosion and fouling problems occur.
Carbon steel depends on a film of iron sulfate for corrosion resistance, and if its film is destroyed by
high velocities and flow turbulence, corrosion can be quite severe.
Phenol
Phenol (carbolic acid) is used in refineries to convert heavy, waxy distillates obtained by crude oil
distillation into lubricating oils. As a rule, all components in the treating and raffinate recovery
sections, except tubes in water-cooled heat exchangers, are made from carbon steel. If water is not
present, few significant corrosion problems can be expected to occur in these sections. In the exact
recovery section severe corrosion can occur, especially where high flow turbulence is encountered.
As a result, certain components require selective alloying with Type 316 (S31600) stainless steel.
Typically stainless steel liners are required for the top of the dryer tower, the entire phenol flash
tower, and various condenser shells and separator drums that handle phenolic water.
Tubes and headers in the extract furnace shall also be made of Type 316 (S31600) stainless steel
with U-bends sleeved with alloy C-4 (N06455) on the outlet side to minimize velocity accelerated
corrosion.
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