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CorrosionTRANSCRIPT
Corrosion In the Refining Industry
By: Wissam El KhatibJune/July 2015
What is Corrosion?
“the deterioration of a material, usually a metal, because of a reaction with its environment.”
-NACE International
What is Corrosion?
Corrosion is a major problem in many industries, constantly trying to be overcome by
engineers.
The Oil Industry
Corrosion is specifically a problem in this industry as refineries contain many different
streams having different operating conditions (temperatures and pressures). Consequently, different degrees and types of corrosion can
occur that must be treated individually.
The Oil Industry
Corrosion is specifically a major problem in the oil refining industry.
• Leads to the deterioration of refinery equipment • Creates safety hazards• Billions of dollars are spent every year on corrosion-related
problems which could be avoided using the fundamentals of corrosion.
Fundamentals and Principles
Types of Corrosion
Refinery corrosion can be classified into:
• Low temperature corrosion
• High temperature corrosion
Types of Corrosion
Refinery corrosion can be classified into:
• Low temperature corrosion
• High temperature corrosion
Low Temperature Corrosion
Also known as:
• Aqueous corrosion
• Wet corrosion
• Electrochemical corrosion
Low Temperature Corrosion
• Occurs in the presence of moisture (usually found where water condenses)
• Occurs at low temperatures (under 260°C)
• Obeys electrochemical laws occurs through simultaneous oxidation and reduction reactions
Low Temperature Corrosion
Iron + Water
Anode: Fe Fe2+ + 2e-
Cathode:O2 + 2H2O + 4e- 4OH-
Low Temperature Corrosion
Consists of two poles:
Anode (-)
The anode is the pole that undergoes corrosion through oxidation (the loss of electrons) and formation of ions. The general chemical equation can be written as follows
M Mn+ + ne-
Low Temperature Corrosion
Consists of two poles:
Cathode (+)
The cathode is the positive pole of the corrosion cell and will get reduced (accepts electrons). Corrosion does not occur at the cathode.
eg:- Hydrogen Evolution
2H+ + 2e- H2
Low Temperature Corrosion
The determination of the anode/cathode depend on reactivity of
species in contact.
Low Temperature Corrosion
Corrosion Rate A measure of the rate with which corrosion occurs. determines whether a material is useable in a certain environment. Measured units of mpy (mills lost per year). Below 5mpy are acceptable generally.
Low Temperature Corrosion
Corrosion Inhibitors designed to decrease the rate of the anodic and/or cathodic reactions, decreasing the overall corrosion rate.
eg:-• Deaerated water will not corrode iron as there is no Oxygen to
be reduced• Adding non-conducting film to metal surfaces
Low Temperature Corrosion
How do Temperature and Concentration affect the corrosion rate?
Temperature: Increases in temperature generally lead to increases in the corrosion rate. Water condensation in Hydrocarbon streams increase at higher temperatures.
Concentration: Increases in concentration in a corrosive environment lead to a higher corrosion rate.
*exception: highly concentrated acids do contain no water and therefore lead to minimal corrosion rates.
Low Temperature Corrosion
In oil refining processes, the corrosion is not caused by the actual Hydrocarbons but by the inorganic impurities contained within the raw
crude oil!
Low Temperature Corrosion
Some corrosives:
• Sulfur• Napthenic acids• Polythionic acids• Chlorides• CO2
• NH3
• Cyanides• HCl• Phenols
Low Temperature Corrosion
Most low temperature corrosives are crude oil contaminants already present and not
removed after primary treatment. Others are picked up in pipelines or during refining
processes.
*N.B. water is found in all crude oil and is nearly impossible to completely separate.
Types of Corrosion
Refinery corrosion can be classified into:
• Low temperature corrosion
• High temperature corrosion
High Temperature Corrosion
• Occurs at temperatures above the environmental dew point
• Occurs in dry conditions (absence of water)
• Gases are typically the corrosive agents
• Also an electrochemical process (oxidation/reduction)
High Temperature Corrosion
The high temperature corrosion reactions are as follows:
Metal gets oxidized
M Mn+ + ne-
Oxygen gets reduced
1/2O2 + 2e- 2O-
High Temperature Corrosion
Therefore, the overall reaction may be written as:
M + 1/2O2 MO
High Temperature Corrosion
Therefore, the overall reaction may be written as:
M + 1/2O2 MO
Metal Oxide
High Temperature Corrosion
Therefore, the overall reaction may be written as:
M + 1/2O2 MO
Metal Oxide
Most metals will react with Oxygen at high temperatures to form an oxide layer.
High Temperature Corrosion
In metal oxides, the rate of ion transfer is slower than the rate of electron transfer and so the rate of corrosion depends on the rate of diffusion of metal or oxygen ions. Consequently,
the rate of high temperature corrosion may be diminished by decreasing the rate of diffusion.
*N.B. In contrast to low temperature corrosion, high temperature corrosion is measured in units of weight gain per year since the scale adheres to the metal surface.
High Temperature Corrosion
Oxide scale formation is influenced by:
• Dissolution of Oxygen atoms in some metals
• Low melting point and high volatilities of oxides
• Existence of grain boundaries in metal and scale
High Temperature Corrosion
Oxide scales can be:
• Protective
• Non-protective
High Temperature Corrosion
Oxide scales can be:
• Protective prevents from further oxidation as the layer thickens.
• Non-protective has no effect as layer thickens
High Temperature Corrosion
Oxide scales can be:
• Protective prevents from further oxidation as the layer thickens.
• Non-protective has no effect as layer thickens
High Temperature Corrosion
If the scale is non-protective, the material will corrode according to a linear rate law as the weight gain is equal for every given amount of time.
High Temperature Corrosion
Oxide scales can be:
• Protective prevents from further oxidation as the layer thickens.
• Non-protective has no effect as layer thickens
High Temperature Corrosion
If the scale is protective, the material will corrode according to a parabolic rate law as corrosion increases at a decreasing rate with time.
Corrosion Mechanisms
There are six classifications of damage or damage mechanisms common to refineries:
1. Metal loss due to general corrosion2. Stress corrosion cracking 3. High-temperature hydrogen attack4. Metallurgical failures5. Mechanical failures6. Other forms of corrosion
*Will be explained in more detail later
Corrosion in Fluid Catalytic Cracking Units
Fluid Catalytic Cracking
The process of cracking heavy oils by using elevated
temperatures, low pressures, and a catalyst.
The process contains many different streams operating at different conditions and so, like most other processes, is subject to different
types of corrosion.
Fluid Catalytic Cracking Components
The components of a Fluid Catalytic Cracking Unit include:
• Riser/Reactor• Regenerator• Flue gas system• Main fractionator
Fluid Catalytic Cracking Components
The components of a Fluid Catalytic Cracking Unit include:
• Riser/Reactor• Regenerator• Flue gas system• Main fractionator
Riser/Reactor
• Preheated gas oil (260-425°C) along with hot regenerated catalyst (675-730°C) are fed.
• High temperatures vaporize gas oil and cracking reaction takes place in riser (2-5 seconds).
• Carbon deposited on catalyst as coke to deactivate.• In the reactor vessel, cyclones separate HC vapors from spent
catalyst.• Vapors sent to main fractionator, while catalyst is further
stripped of HC vapors using steam (in the stripper) and then sent to the regenerator through the spent catalyst standpipe.
Fluid Catalytic Cracking Components
The components of a Fluid Catalytic Cracking Unit include:
• Riser/Reactor• Regenerator• Flue gas system• Main fractionator
Regenerator
• Regenerates catalyst by burning off coke.• Spent catalyst from standpipe is contacted with oxygen, and
combustion begins (650-760°C).• The exothermic combustion is coupled to produce energy for
the endothermic cracking reaction.• Regenerated catalyst and flue gas (CO,CO2,NOx,SOx) are
produced.• Cyclone separates flue gas (directed towards dilute phase)
from the regenerated catalyst (directed towards dense phase).
• Lift gas fluidizes catalyst and sends it to the riser.
Fluid Catalytic Cracking Components
The components of a Fluid Catalytic Cracking Unit include:
• Riser/Reactor• Regenerator• Flue gas system• Main fractionator
Flue Gas System
• Leaves the regenerator at (675-760°C).• These gases can be used for heat recovery since they are high
in energy.• Can pass through heat exchangers to produce additional
steam.• Scrubbers are used to remove pollutants and catalyst particles
which were too small to be removed by the cyclone.
Fluid Catalytic Cracking Components
The components of a Fluid Catalytic Cracking Unit include:
• Riser/Reactor• Regenerator• Flue gas system• Main fractionator
Main Fractionator
• Cools effluent gas and separates the LCO’s and HCO’s from the lighter fractions.
• Lighter streams contain not only methane, ethane, propane, and butane, but also hydrogen, propylene, and butylene.
• Heat is supplied from effluent gas (no reboiler required).• Bottoms temperature of approximately 340-400°C and
overhead temperature of approximately 95-120°C.• Distillate products sent to further fractionation
Materials in FCC
Common materials used in FCC Units:
• Carbon Steel• 1-1/4 Cr low-alloy steel• 5 Cr low-alloy steel• 9 Cr low-alloy steel• 12 Cr stainless steel• 300 series stainless steel• 400 series stainless steel• Alloy 625 nickel-based alloy• Refractory Linings
Materials in FCC
These are steel alloys consisting of Iron and carbon alloyed with different amounts of other metals such as chromium and
molybdenum. This is done to improve the properties such as the steel’s strength, hardness, toughness, wear resistance,
and hardenability. Various types of steel alloys are used in the different components of the FCC in attempt to avoid
corrosion.
Detection Techniques
Corrosion may not always be seen visually, therefore various techniques of inspection are used including:
• Ultrasonic testing (UT) Propagation of ultrasonic waves on metal surface to measure thickness
• Radiographic testing (RT) short wavelength radiation to penetrate materials and measure thickness
• Dye penetrant testing (PT) Uses a penetrant on non-porous materials to detect any damage such as hairline cracks
Corrosion Mechanisms in FCC
Different units of the FCC are subject to different mechanisms of corroding. These mechanisms are stated and will be explained in detail below along
with information on where they are most susceptible to occur and ways in which they can be inspected
and mitigated
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidaation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corrosion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
High-temperature Oxidation
Location• Occurs in the regenerator internals and the flue gas system
(where temperatures exceed 540°C)Inspection• Visual inspection can be used to reveal damage • Ultrasonic testing can be used to determine remaining wall
thicknessControl• A resistant alloy containing sufficient chromium (9 Cr or stainless
steel) is used to control• Internal insulation with refractory is used to keep the metal
surfaces cool
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
High-temperature Sulfidation (H2S attack)
Hydrogen sulfide is produced in the FCC preheater and reactor by thermal decomposition of organic sulfur compounds. It is corrosive to iron and steel above 285°C and 1ppm.
Location• Preheater• Feed Piping to riser after preheater• Reactor• Reaction mix line• Sections of main fractionator above 285°C• Fractionator bottoms, piping, and pumps• Exchangers (>285°C)
High-temperature Sulfidation (H2S attack)
Inspection• Easily detectable by ultrasonic testing (UT) or radiographic testing
(RT) since attack is predictable and uniform.• Mostly occurs in areas where cladding is beyond 12 CrControl• A base metal or cladding overlay with sufficient chromium should
be used rather than carbon steel as it offers higher resistance (5Cr-1/2Mo, 1-1/4 Cr-1/2 Mo, 12 Cr stainless)
• Internal Insulation of metal surfaces with refractory to keep them cool
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
High-temperature Carburization
At temperatures above 540°C, metals can absorb carbon from the atmosphere to form metal carbides. These metal carbides form a layer on the metal which can later bulge away or flake off, reducing metal thickness. This begins with the deposition of coke on the metal surface. Incomplete combustion of coke can also lead to carbon monoxide forming in the flue gas. At high enough temperatures this can also cause carburization.
Location• Reactor internals• Regenerator internals
High-temperature Carburization
Inspection• UT can be used to identify wall thinning Control• The presence of chromium retards carburization in
oxidizing and sulfidizing environments (not in reducing ones).
• Use 1-1/4 Cr-1/2 Mo reactor shells
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
Polythionic Acid Stress Corossion Cracking
Polythionic acids are partially oxidized sulfur acids and are the most common fluids that cause interangular attack. Due to the presence of hydrogen sulfide in the process stream, a thin layer of Iron Sulfide is formed. This oxidizes during shutdowns from the oxidation of Iron Sulfide in the presence of moisture and oxygen to form corrosive polythionic acids.
Location• Regenerator internals• Slide valves• Catalyst withdrawal nozzles• Flue gas lines
Polythionic Acid Stress Corossion Cracking
Inspection• Cracking does not occur very frequently so inspection is not routine • Can be detected visually or using dye penetrant testing (PT)Control• Use alloys that are not prone to sensitization (low carbon varieties
of 300 series SS)• Isolating sensitized stainless steels from Sulfur-derived acids• Preventing polythionic acid formation by:
– Preveting water condensation on 300 series SS above 370°C– Avoiding water washing for dust removal– Using insulated expansion joints– Use internally insulated slide valves or purge with Nitrogen and not steam
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
Catalyst Erosion
Erosion due to the loss/damage of material due to the cutting action of high-velocity solid particles.
Location• Reactor and regenerator shell and internals (specifically in
cyclones)• Catalyst transfer lines• Slide valves• Thermowells• Flue gas lines and coolers• Fractionator bottoms pumps, heat exchangers, valves, piping.
Catalyst Erosion
Inspection• Visual inspection can be used to detect damage • Focus should be put on areas of high-velocity streams
containing catalyst• UT and RT thickness measurementsControl• Reducing turbulence of catalyst and catalyst carryover• Using erosion-resistant refractory linings and hardfacing• Using stainless steel ferrules in flue gas coolers and
fractionator bottoms pumps/exchangers
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
Feed Nozzle Erosion
Location• Occurs in the riser pipe upstream of the regenerate catalyst
entry point and feed spray nozzlesInspection• Visual or RT inspectionControl• Designing to reduce turbulence on riser wall• Using erosion-resistant materials to elongate the life of feed
spray nozzles
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
Refractory Damage
Location• Reactor and Regenerator system and internals and includes:
– Thermal cycling cracks– Loss of anchors– Spalling from poor installation– Insufficient dry-out– Coking
Inspection• Visual inspection during shutdowns
Refractory Damage
Control• Surveying cold-wall equipment onstream using thermography
(pyrometers and infrared analyzers) to identify insulating refractory failure
• Proper refractory selection and application, dry out and anchoring.
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
High-temperature Graphitization
Carbon and carbon-molybdenum steel consists largely of Iron Carbide, which when exposed o very high temperatures (425°C for Carbon and 455°C for carbon-molybdenum) decomposes to ferrite (Fe) and graphite (C). The existence of graphite in the metal decreases properties such as strength and ductility.
Location• Carbon steel reactor cyclone• Fractionator inlet nozzle and shell• Any location where thermal insulation is damaged
(reactor/regenerator internals, catalyst tranfer lines etc.)
High-temperature Graphitization
Inspection• RT• Shear wave UT• Field metallograpy of weldmentsControl• Use of chrome-molybdenum steel (1-1/4 Cr-1/2 Mo) rather
than carbon steel• Use of carbon-molybdenum steel only up to a temperature of
455°C• Use of carbon steel only up to a temperature of 425°C• Insulation with refractory to lower temperature
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
Sigma Phase Embrittlement
Sigma phase is a hard, brittle, non-magnetic phase, containing approximately 50% chromium. Embrittlement leads to an increase in alloys room-temperature tensile strength and hardness along with a decrease in ductility to the point of brittleness. Consequently, cracking is likely to occur during maintenance or cooling from operating temperatures (dissapears over 250°C and reappears below).
Location• Ferrite phase of 300 series SS regenerator internals or flue gas
system components• Cast 300 series SS slide valves exposed to temperature range
of (590-925°C).
Sigma Phase Embrittlement
Inspection• PT inspection for cracks• Field metallography to identify presence/distribution of sigma
phase (difficult).Control • Limiting ferrite content of weld metal to 3-10%• Avoiding shock loads when metal is cold• Using type 304 SS rather than austenitic SS (321,347)• Using internally insulated carbon or low-alloy steel for slide
valves
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
475°C Embrittlement
Location• Occurs in 400 series SS in the temperature range 370-540°C
and 300 series SS welds and cast componentsInspection• PT inspection for cracksControl• Avoiding 300 series SS in high pressure/temperature
environments
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
High-temperature Creep
Metals experiencing stress under the yield point will elastically spring back to its original size after the load is removed, however if stressed beyond the yield point, permanent deformation of the metal will occur. If the stress remains constant, no further deformation occurs. However, at high temperatures, applying a stress the yield point will cause permanent stretching. This phenomenon is known as creep and causes the metal to fail.
Location• Hot wall reactor vessels or cold wall reactor if the refractory fails• Carbon steel reactor cyclones• Regenerators and piping
High-temperature Creep
Inspection• Visual inspection and PT to look for cracking and distortionControl• Using alloy upgrades• Ensuring operating temperatures do not exceed design metal
temperatures• Using stress-analysis techniques to ensure thermal expansion
stresses are accounted for in design
Corrosion Mechanisms in FCC High-temperature Oxidation High-temperature Sulfidation High-temperature Carburization Polythionic Acid Stress Corossion Cracking Catalyst Erosion Feed Nozzle Erosion Refractory Damage High-temperature Graphitization Sigma Phase Embrittlement 475 C Embrittlement High-Temperature Creep Thermal fatigue
Thermal Fatigue
The differential growth between the reactor overhead and the fractionator inlet nozzle is the source of fatigue stress. High stress is placed on the mix line each time the reactor temperature is cycled.
Location• Reaction mix line (specifically at the miters)Inspection• Visual inspection or PT to look for cracksControl• Proper design to avoid cracking• Eliminating mitered joints where stresses concentrate
Citations• 'Corrosion Control In The Refining Industry'. 1 (2006): n. pag. Print. NACE
International.
• "Polythionic Acids." - Corrosion Engineering. N.p., n.d. Web. 05 July 2015.
• "Field Metallography." Field Metallography. N.p., n.d. Web. 05 July 2015.
• "Petroleum Refining Corrosion." The Hendrix Group Resources Special Corrosion
Topics Refining. N.p., n.d. Web. 05 July 2015.
• Wilson, Joseph W. Fluid Catalytic Cracking Technology and Operations. Tulsa, OK: PennWell, 1997. Print.
• "Corrosion Control." Corrosion Control. N.p., n.d. Web. 05 July 2015.
Thank You!
Refining and petrochemical complex in the southern Chinese province of Guangdong