High Temperature Corrosion : High temperature corrosions are named as follows:
- sulfidic corrosion
- sulfidic corrosion without hydrogen present
- sulfudic corrosion with hydrogen present
- naphthenic acids
- fuel ash
- oxidation
1. Sulfidic corrosion:
Corrosion by various sulfur compounds at temperatures between 260 and 540°C is a common
problem in many petroleum-refining processes and in petrochemical processes. Corrosion is in the form of uniform thining, localized attack, or errosion corrosion. Nickel and nickel rich alloys are rapidly attacked by sulfur compounds at elevated temperatures, while chromium containing steels provide excellent corrosion resistance (as does aluminum). The combinations of hydrogen sulfide and hydrogen can be particularly corrosive, and as a rule, austenitic stainless steels are required for effective corrosion control.
2. Sulfidic corrosion without hydrogen present:
This type of corrosion occurs in various components of crude distillation units, catalytic cracking
units, hydrotreating and hydrocracking units upstream of hydrogen injection line.
Preheat-exchanger tubes, furnace tubes, and transfer lines are generally made from carbon steel,
as is corresponding equipment in the vacuum distillation section. The lower shall of distillation
towers, where temperatures are above 230°C is usually lined with stainless steel containing 12% Cr
such as Type 405.
Metal skin temperature, rather than flow stream temperatures, shall be used to predict corrosion rates when significant differences between the two arise. For example metal temperatures of
furnace tubes are typically 85 to 110°C higher than the temperature of the hydrocarbon stream
passing through the tubes. Furnace tubes normally corrode at a higher rate on the hot side (fire
side) than on the cool side (wall side).
3. Sulfidic corrosion with hydrogen present:
The presence of hydrogen in, for example, hydrotreating and hydrocracking operations, increases
the severity of hightemperature sulfidic corrosion. Hydrogen converts organic sulfur compounds in
feed stocks to hydrogen sulfide; corrosion becomes a function of H2S concentration.
Down stream of hydrogen injection line, low-alloy steel piping usually requires aluminizing in order
to minimize sulfidic corrosion. Alternatively Type 321 (S32100) stainless steel can be used. Tubes
in the preheat furnace are aluminized low-alloy steel, aluminized 12% Cr stainless steel.
Reactors are usually made of 2.25 Cr-1 Mo steel, either with a Type 347 (S34700) stainless steel
weld overlay or an internal factory lining. Reactor internals are often Type 321 stainless steel.
4. Naphthenic acids:
These organic acids are present in many crude oils. The general formula may be written as R(CH2)n
COOH, where R is usually cyclopentane ring.. This acid is corrosive only at temperature above 230°C in the range of 1 to 6 neutralization number encountered with crude oil and various side-cuts. At any given temperature, corrosion rate is proportional to neutralization number. Corrosion rate triples with each 55°C increase in temperature. In contrast to high-temperature sulfidic corrosion, no protective scale is formed, and low-alloy and stainless steels containing up to 12% Cr provide no benefits
whatsoever over carbon steel. The presence of naphthenic acids may accelerate high-temperature
sulfidic corrosion that occurs at furnace headers, elbows, and tees of crude distillation units because
of unfavorable flow conditions.
Severe naphthenic acid corrosion (in the form of pitting) has been experienced in the vacuum
towers of crude distillation units in the temperature zone of 290 to 345°C and sometimes as low as
230°C. Attack is often limited to the inside and very top of the outside surfaces of bubble caps.
Alloy 20 (N08020) and titanium Grade 2 (R50400) are also resistant to naphthenic
acid corrosion. In contrast, aluminized carbon steel tray components, such as bubble caps, have
performed poorly.
5. Fuel ash:
Corrosion by fuel ash deposits can be one of the most serious operating problems with boiler and
preheat furnaces. All fuels except natural gas contain certain inorganic contaminants that leave the
furnace with products of combustion.These will deposit on heat-receiving surfaces, such as
superheater tubes, and after melting can cause severe liquid-phase corrosion. Contaminant of this
type include various combinations of vanadium, sulfur, and sodium compounds. Fuel ash corrosion
is most likely to occur when residual fuel oil (Bunker C fuel) is burned.
In particular, vanadium pentoxide vapor (V2O5) reacts with sodium sulfate (Na2SO4) to form sodium
vanadate (Na2O-6 V2O5). The latter compound reacts with steel, forming a molten slag that runs off
and exposes fresh metal to attack.
Corrosion increases sharply with increasing temperature and vanadium content of fuel. If the
vanadium content in the fuel oil exceeds 150 ppm, the maximum tube wall temperature should be
limited to 650°C. Between 20 and 150 ppm V, maximum tube wall temperatures can be between
650 and 845°C depending on sulfur content and the sodium-vanadium ratio of the fuel oil. With 5 to
20 ppm V, the maximum tube wall temperature can exceed 845°C.
In general, most alloys are likely suffer from fuel ash corrosion. However, alloys with high chromium
and nickel contents provide the best resistance to this type of attack. Sodium vanadate corrosion
can be reduced by firing boilers with low excess air (<1%). This minimizes formation of sulfur
trioxide in the firebox and produces high-melting slages containing vanadium tetroxide and trioxide
rather than pentoxide. In the temperature range of 400 to 480°C boiler tubes are corroded by alkali
pyrosulfates such as sodium pyrosulfate and potassium pyrosulfate, when appreciable
concentrations of sulfur trioxide are present.
6. Oxidation:
Carbon steels, low-alloy steels and stainless steels react at elevated temperatures with oxygen in
the surrounding air and become scaled. Nickel alloys can also become oxidized, especially if
spalling of scale occur. The oxydation of copper alloys usually is not a problem, because these are
rarely used where operating temperatures exceed 260°C. Alloying with both chromium and nickel
increases scaling resistance. Stainless steels or nickel alloys except alloy 400 (N04400), are
required to provide satisfactory oxidation resistance at temperatures above 705°C.
Thermal cycling, applied stresses, moisture and sulfur-bearing gases will decrease scaling
resistance.
High temperature oxidation is limited to the outside surfaces of furnace tubes, tube hangers and
other parts that are exposed to combustion gases containing excess air.
At elevated temperatures, steam decomposes at metal surfaces to hydrogen and oxygen and may
cause steam oxidation which is more severe than air oxidation at the same temperature. Fluctuating
steam temperatures tend to increase the rate of oxidation by causing scale to spall and thus expose
fresh metal to further attack.
- sulfidic corrosion
- sulfidic corrosion without hydrogen present
- sulfudic corrosion with hydrogen present
- naphthenic acids
- fuel ash
- oxidation
1. Sulfidic corrosion:
Corrosion by various sulfur compounds at temperatures between 260 and 540°C is a common
problem in many petroleum-refining processes and in petrochemical processes. Corrosion is in the form of uniform thining, localized attack, or errosion corrosion. Nickel and nickel rich alloys are rapidly attacked by sulfur compounds at elevated temperatures, while chromium containing steels provide excellent corrosion resistance (as does aluminum). The combinations of hydrogen sulfide and hydrogen can be particularly corrosive, and as a rule, austenitic stainless steels are required for effective corrosion control.
2. Sulfidic corrosion without hydrogen present:
This type of corrosion occurs in various components of crude distillation units, catalytic cracking
units, hydrotreating and hydrocracking units upstream of hydrogen injection line.
Preheat-exchanger tubes, furnace tubes, and transfer lines are generally made from carbon steel,
as is corresponding equipment in the vacuum distillation section. The lower shall of distillation
towers, where temperatures are above 230°C is usually lined with stainless steel containing 12% Cr
such as Type 405.
Metal skin temperature, rather than flow stream temperatures, shall be used to predict corrosion rates when significant differences between the two arise. For example metal temperatures of
furnace tubes are typically 85 to 110°C higher than the temperature of the hydrocarbon stream
passing through the tubes. Furnace tubes normally corrode at a higher rate on the hot side (fire
side) than on the cool side (wall side).
3. Sulfidic corrosion with hydrogen present:
The presence of hydrogen in, for example, hydrotreating and hydrocracking operations, increases
the severity of hightemperature sulfidic corrosion. Hydrogen converts organic sulfur compounds in
feed stocks to hydrogen sulfide; corrosion becomes a function of H2S concentration.
Down stream of hydrogen injection line, low-alloy steel piping usually requires aluminizing in order
to minimize sulfidic corrosion. Alternatively Type 321 (S32100) stainless steel can be used. Tubes
in the preheat furnace are aluminized low-alloy steel, aluminized 12% Cr stainless steel.
Reactors are usually made of 2.25 Cr-1 Mo steel, either with a Type 347 (S34700) stainless steel
weld overlay or an internal factory lining. Reactor internals are often Type 321 stainless steel.
4. Naphthenic acids:
These organic acids are present in many crude oils. The general formula may be written as R(CH2)n
COOH, where R is usually cyclopentane ring.. This acid is corrosive only at temperature above 230°C in the range of 1 to 6 neutralization number encountered with crude oil and various side-cuts. At any given temperature, corrosion rate is proportional to neutralization number. Corrosion rate triples with each 55°C increase in temperature. In contrast to high-temperature sulfidic corrosion, no protective scale is formed, and low-alloy and stainless steels containing up to 12% Cr provide no benefits
whatsoever over carbon steel. The presence of naphthenic acids may accelerate high-temperature
sulfidic corrosion that occurs at furnace headers, elbows, and tees of crude distillation units because
of unfavorable flow conditions.
Severe naphthenic acid corrosion (in the form of pitting) has been experienced in the vacuum
towers of crude distillation units in the temperature zone of 290 to 345°C and sometimes as low as
230°C. Attack is often limited to the inside and very top of the outside surfaces of bubble caps.
Alloy 20 (N08020) and titanium Grade 2 (R50400) are also resistant to naphthenic
acid corrosion. In contrast, aluminized carbon steel tray components, such as bubble caps, have
performed poorly.
5. Fuel ash:
Corrosion by fuel ash deposits can be one of the most serious operating problems with boiler and
preheat furnaces. All fuels except natural gas contain certain inorganic contaminants that leave the
furnace with products of combustion.These will deposit on heat-receiving surfaces, such as
superheater tubes, and after melting can cause severe liquid-phase corrosion. Contaminant of this
type include various combinations of vanadium, sulfur, and sodium compounds. Fuel ash corrosion
is most likely to occur when residual fuel oil (Bunker C fuel) is burned.
In particular, vanadium pentoxide vapor (V2O5) reacts with sodium sulfate (Na2SO4) to form sodium
vanadate (Na2O-6 V2O5). The latter compound reacts with steel, forming a molten slag that runs off
and exposes fresh metal to attack.
Corrosion increases sharply with increasing temperature and vanadium content of fuel. If the
vanadium content in the fuel oil exceeds 150 ppm, the maximum tube wall temperature should be
limited to 650°C. Between 20 and 150 ppm V, maximum tube wall temperatures can be between
650 and 845°C depending on sulfur content and the sodium-vanadium ratio of the fuel oil. With 5 to
20 ppm V, the maximum tube wall temperature can exceed 845°C.
In general, most alloys are likely suffer from fuel ash corrosion. However, alloys with high chromium
and nickel contents provide the best resistance to this type of attack. Sodium vanadate corrosion
can be reduced by firing boilers with low excess air (<1%). This minimizes formation of sulfur
trioxide in the firebox and produces high-melting slages containing vanadium tetroxide and trioxide
rather than pentoxide. In the temperature range of 400 to 480°C boiler tubes are corroded by alkali
pyrosulfates such as sodium pyrosulfate and potassium pyrosulfate, when appreciable
concentrations of sulfur trioxide are present.
6. Oxidation:
Carbon steels, low-alloy steels and stainless steels react at elevated temperatures with oxygen in
the surrounding air and become scaled. Nickel alloys can also become oxidized, especially if
spalling of scale occur. The oxydation of copper alloys usually is not a problem, because these are
rarely used where operating temperatures exceed 260°C. Alloying with both chromium and nickel
increases scaling resistance. Stainless steels or nickel alloys except alloy 400 (N04400), are
required to provide satisfactory oxidation resistance at temperatures above 705°C.
Thermal cycling, applied stresses, moisture and sulfur-bearing gases will decrease scaling
resistance.
High temperature oxidation is limited to the outside surfaces of furnace tubes, tube hangers and
other parts that are exposed to combustion gases containing excess air.
At elevated temperatures, steam decomposes at metal surfaces to hydrogen and oxygen and may
cause steam oxidation which is more severe than air oxidation at the same temperature. Fluctuating
steam temperatures tend to increase the rate of oxidation by causing scale to spall and thus expose
fresh metal to further attack.
n this article we will focus on the most popular precious metals used for jewelry: gold, platinum and the fast growing in popularity palladium. Pure gold and platinum are soft metals which are extremely dense. pure gold is about two and half times heavier than iron and pure platinum is just under three times.
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