corrosion in the refining industry

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

Low Temperature Corrosion

Also known as:

• Aqueous corrosion

• Wet corrosion

• Electrochemical 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


Iron + Water

Anode: Fe Fe2+ + 2e-

Cathode:O2 + 2H2O + 4e- 4OH-


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-


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


The determination of the anode/cathode depend on reactivity of

species in contact.


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.


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


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.


In oil refining processes, the corrosion is not caused by the actual Hydrocarbons but by the inorganic impurities contained within the raw

crude oil!


Some corrosives:

• Sulfur• Napthenic acids• Polythionic acids• Chlorides• CO2

• NH3

• Cyanides• HCl• Phenols


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)


The high temperature corrosion reactions are as follows:

Metal gets oxidized

M Mn+ + ne-

Oxygen gets reduced

1/2O2 + 2e- 2O-


Therefore, the overall reaction may be written as:

M + 1/2O2 MO



M + 1/2O2 MO

Metal Oxide



M + 1/2O2 MO

Metal Oxide

Most metals will react with Oxygen at high temperatures to form an oxide layer.


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.


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


Oxide scales can be:

• Protective

• Non-protective



• Protective prevents from further oxidation as the layer thickens.

• Non-protective has no effect as layer thickens


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.



• Protective prevents from further oxidation as the layer thickens.

• Non-protective has no effect as layer thickens


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

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.

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.

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.

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

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

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

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

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

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.

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

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

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

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

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

corrosion in the refining industry

Documents

fundamentals of corrosion

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