134649306-material-selection-corrosion-control-pdf.pdf
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corrsionTRANSCRIPT
CORROSION MECHANISMSCORROSION MECHANISMSMATERIAL SELECTION ANDMATERIAL SELECTION AND
CORROSION CONTROLCORROSION CONTROLIN REFINERYIN REFINERY
Flavio CifàMichele Scotto di Carlo
2
““Corrosion is defined as the destruction orCorrosion is defined as the destruction or
deterioration of a material because ofdeterioration of a material because of
reaction with its environment”reaction with its environment”
Mars G. Fontana
3
n CORROSION AND DEGRADATION MECHANISMSèCORROSION PROCESSES KINETICèLOW TEMPERATURE DEGRADATION MECHANISMS
l GENERAL CORROSION– CO2 corrosion– Wet hydrogen sulfide corrosion
l GALVANIC CORROSIONl PITTING CORROSIONl CREVICE CORROSIONl UNDER DEPOSIT CORROSIONl STRESS CORROSION CRACKING
– Chloride stress corrosion cracking (CSCC)– Sulfide stress cracking (SSC)– Alkaline stress corrosion cracking (ASCC)– Caustic cracking– Amine cracking– Cracking in H2O-CO-CO2 systems
CONTENTS
4
èLOW TEMPERATURE CORROSION MECHANISMS (CONTINUE)l SENSITIZATION AND WELD DECAY CORROSION (INTEGRANULAR)
– Sensitization– Weld Decay– knife line attack– Polythionic Acid Stress corrosion Cracking (PASSC)
l EROSION CORROSIONl MICROBIOLOGICALLY INDUCED CORROSIONl CORROSION UNDER INSULATIONl HYDROGEN DAMAGE
èHIGH TEMPERATURE CORROSION MECHANISMSl NAPHTENIC ACID CORROSION
l HIGH TEMPERATURE OXIDATION
l SULFIDATION
l HIGH TEMPERATURE HYDROGEN DAMAGE
CONTENTS
5CONTENTS
nMATERIALS AND CORROSION PROTECTIONè MATERIAL SELECTION GUIDELINEè CARBON STEELè LOW ALLOYED STEELSè STAINLESS STEELSè COPPER ALLOYSè NICKEL ALLOYSè TITANIUM ALLOYSè POLIMERIC MATERIALSè CATHODIC PROTECTION
nMATERIAL SELECTION AND CORROSION CONTROLIN REFINERY UNITSè DESALTERè ATMOSPHERIC DISTILLATION UNITè VACUUM DISTILLATION UNITè AMINE UNITè HYDRODESULPHURIZATION UNITè SOUR WATER STRIPPER UNIT
CORROSION AND DEGRADATIONCORROSION AND DEGRADATIONCORROSION AND DEGRADATIONCORROSION AND DEGRADATION MECHANISMS MECHANISMS MECHANISMS MECHANISMS
General criteria
7
n STATIONARY KINETICSSteady corrosion rate which often allows:
è corrosion rate prediction trough laboratory tests, bibliographic data
and estimation models.è Monitoring on stream and off stream
è Upset conditions are not decisive on corrosion process
n INCUBATION PERIOD KINETICSIt presents an incubation period which closeswith high corrosion rate (cracking).
è “upset conditions” are decisive.è The incubation period may be very short (h!!!)è The corrosion process once started (t > ti) continues up to the
rupture independently from the incubation conditions persistence.
• Whenever “upset conditions” are decisive for the describedcorrosion mechanism they will be clearly highlighted with
CORROSION KINETICS
R corr
Timeti
Stationary
Incubation period
UPSET
8LOW/HIGH TEMPERATURE CORROSION
LOW TEMPERATURE CORROSION
n Temperature < 260°C
n Aqueous phase and presence of ionic species
HIGH TEMPERATURE CORROSION
n Temperature > 260°C
n Aqueous phase not necessary
CORROSION AND DEGRADATIONCORROSION AND DEGRADATIONCORROSION AND DEGRADATIONCORROSION AND DEGRADATION MECHANISMS MECHANISMS MECHANISMS MECHANISMS
Low temperature corrosion mechanisms
10 LOW TEMPERATURE DEGRADATION MECHANISMS
nn GENERAL CORROSIONGENERAL CORROSIONnn GALVANIC CORROSIONGALVANIC CORROSIONnn PITTING CORROSIONPITTING CORROSIONnn CREVICE CORROSIONCREVICE CORROSIONnn UNDER DEPOSIT CORROSIONUNDER DEPOSIT CORROSIONnn STRESS CORROSION CRACKINGSTRESS CORROSION CRACKINGnn SENSITIZATION AND WELD DECAY CORROSIONSENSITIZATION AND WELD DECAY CORROSION
(INTEGRANULAR CORROSION)(INTEGRANULAR CORROSION)nn EROSION CORROSIONEROSION CORROSIONnn MICROBIOLOGICALLY INDUCED CORROSIONMICROBIOLOGICALLY INDUCED CORROSIONnn CORROSION UNDER INSULATIONCORROSION UNDER INSULATIONnn HYDROGEN DAMAGEHYDROGEN DAMAGE
11GENERALIZED CORROSION AT LOW TEMPERATURE
n ANODE “location where metaldissolution takes place (i.e.Fe→→Fe2+)”
n CATHODE: “location where O2, H+
or metal reduction takes place(i.e. Fe3+→→ Fe2+)”
n No specific location for anodeand cathode
n Anode and cathode move withtime
n Can be monitored, measured andpredicted
12GENERALIZED CORROSION AT LOW TEMPERATURE
Can be uniform or not
CONTROL:n Select proper metallurgyn Corrosion allowance (function ofcorrosion rate and required lifetime)n Inhibitorn Cathodic Protectionn Monitoring
Some metal-environment combinationsknown to results in general corrosion:CS - dilute mineral acidCS - CO2 and/or H2S in aqueous phaseCS - seawaterSS - organic acid at high T (i.e. 100- 200 °C)Ti - concentrated sulfuric acid
Corrosion rate of various alloys inboiling mixtures of 50% acetic acidand varying proportions of formic acid.Test time 1+3+3 days. (by SANDVIK)
13
An example of generalized corrosion at low temperature is CO2
corrosion on carbon steel.Requires a presence of aqueous phase and it’s due to the low pH.
It can be tentatively predicted using a softwareIt’s a function of:n PCO2
n Temperaturen System Fluid dynamics(influences scale stability )n Presence of H2S and/or organic acidn O2 content
CONTROL: it can be controlled with CS + CA up to corrosion rate (CR)0.6mm/y. For higher CR upgrade metallurgy to 304 (316 not necessary)
GENERALIZED CORROSION AT LOW TEMPERATURE- CO2
14GENERALIZED CORROSION AT LOW TEMPERATURE - H2S
Another example of generalizedcorrosion at low temperature oncarbon steel is Wet HydrogenSulphide corrosion. (Note:includes also risk of SSC andhydrogen damage).
It requires a presence of aqueousphase and it’s due to the low pHand to the reaction between S andFe (formation of FeS scale)
The stability of FeS scale isinfluenced by pH and presence ofcontaminants (i.e. CN-)
The temperature rise increase CR
CR is hardly predictable NOTE: CR is influenced also by pH, finemetal composition, presence ofcontaminants (i.e. CN), etc... (by NACE)
15
H (atomic) can diffuse into themetal causing:n crackingn blisteringn embrittlement(see also SSC andHydrogen damage)
CONTROL: Wet H2S general corrosion can be controlled with CS + CAup to corrosion rate (CR) 0.6mm/y. For higher CR upgrade metallurgyto SSThe phenomenology related to hydrogen attack are taken into accountrequiring HIC resistant specs (composition + test NACE TM 0284).Note: consider as valid alternative SS cladding instead of CS HICresistant
GENERALIZED CORROSION AT LOW TEMPERATURE - H2S
Graphby
UOP
16GENERALIZED CORROSION AT LOW TEMPERATURE
17GALVANIC CORROSION
n Preferential corrosion of onemetal of two or moreelectrically connecteddissimilar
n It requires an aqueousenvironment which iscorrosive to at least one metaland with a non negligibleconductivity
n It’s related to the ∆∆V betweenthe metals in the consideredenvironment (i.e. see galvanicseries in seawater).
18
ALL the following parameter have to be verified to evaluate risk ofgalvanic corrosion
n Verify the allowable ∆∆V:è if it is not significant (i.e. the coupled metals are close in the
galvanic series measured in the considered environment) don’tworry about CG
n Verify the medium corrosivity:è if the fluid is not aggressive towards at least one of two coupled
metals (i.e CS - SS in neutral deoxygenated water) CG is not aproblem
n Verify the fluid conductivity:è if it is very low (i.e. demi water of hydrocarbons) CG are not an
issue
n Verify cathodic/anodic areas:è if the cathodic area is << of anodic area (don’t forget to consider
lining!!) galvanic corrosion can be tolerated (i.e. SS bolting onCS flange)
GALVANIC CORROSION
19
CONTROL:n Ratio cathodic/anodic areas (if the ratio increase the CR↑↑).n Control environment (i.e. pH↑↑, remove O2... )n Use of coating (either on both surfaces or on cathodic surface,
NEVER only on anodic surface)n Use insulation kit to break electrical continuityn Cathodic Protection
Metal coupling that can generate GC(the first is attached):CS-SS CS-Copper alloy CS-TiCS-Hastelloy SS-Ti SS - Hastelloy
GALVANIC CORROSION
Insulation kit
20GALVANIC CORROSION
21GALVANIC CORROSION
22GALVANIC CORROSION
23PITTING CORROSION
n PITTING: form of extremelylocalized attack that results in holein the metal. One of the mostdangerous and insidious form ofcorrosion.
n It causes equipment to fail becauseof perforation with only a smallweight loss
n Normally occurs in active/passivemetals (i.e. SS series 300) inpassive state
n Requires depassivating species(i.e. chloride or other halides )
n Worse problem at low velocity andhigh T
n Hard to detect and/or predict
UPSET
24
CONTROL:
n avoid metal/environmentcombination susceptible topitting
n check environmentalconditions especiallyè [Cl-] o [X-]è Temperatureè O2
è Minimum fluid velocity
A parameter to evaluate pittingresistance of SS is PREN (pittingresistance equivalents number):
PREN = Cr + 3.3 Mo + 16N
Critical pitting temperatures (CPT) for SAF 2205,AISI 304 and AISI 316 at varying concentrations ofsodium chloride (potentiostatic determination at+300 mV SCE), pH»6.0 (by SANDVIK)
PITTING CORROSION
UPSET
25PITTING CORROSION
Examples of metals susceptibleto pitting in chloridesenvironment:
n SS (Ferritic, Austenitic,Duplex)
n Fe-Ni-Cr Alloy (Incoloy)
n Aluminum Alloy
n Copper Alloy
Immune Very resistant Resistant Acceptable Not AcceptableTi 90/10 Cu/Ni 70/30 Cu/Ni Monel SS series 400
Alloy C Admiralty brass Tin 316 (+ CP) 304Alloy 625 Al bronze Alloy 825 Nickel
Alloy 20
Pitting resistance in seawater
UPSET
26PITTING CORROSION
27PITTING CORROSION
28
Selective corrosion in crevice
n CC requires a stagnant zonewhere it’s possible to developdifferent conditions from bulk(inhibitor, oxygen, pH, Cl-)
n CC requires an aggressiveenvironment (i.e. presence ofchloride)
n If temperature ↑↑ crevicelikelihood ↑↑
CREVICE CORROSION
UPSET
29CREVICE CORROSION
CONTROL:n Use materials less sensitive to pitting (the corrosion mechanisms
are similar therefore a material resistant to pitting corrosion is alsoresistant to crevice corrosion. See slide 99)
n avoid stagnant zonen don’t use threaded connectionsn control O2 contentSome materials susceptible to CC: èSSèNi alloyèTi
Preferentially locations for CC:n Flanged connectionn Tube/Tubesheet connectionn Threaded connectionsn Plate Heat Exchangers
UPSET
30CREVICE CORROSION
31CREVICE CORROSION
32UNDER DEPOSIT CORROSION
Corrosion enhanced by thepresence of scales (can beaggressive i.e NH4Cl or not)
Under deposit corrosion resultsfrom difference between local andbulk environment (i.e oxygen, pH,presence of aggressive ions Cl. Seealso crevice and pitting corrosion)
If chloride are present H+ “drawn”under deposits (pH drops below 4increasing corrosion rates)
Most common materials aresubjected to UDC including CS,austenitic SS, nickel alloy (Inconel625, Hastelloy and Ti are veryresistant)
33UNDER DEPOSIT CORROSION
Refinery examples:any location in which scaling and/orfouling occur especially if chloride oroxygen are present
CONTROL TECHNIQUESn treat the source of the problem (i.e.corrosion or fouling)n design equipment to minimizedeposition. Metallurgy may solvecorrosion problem but not performancelossn antifoulant may be helpful
34
AMMONIUM BISULFIDE
n frost from gas to solid at a temperature depending on NH3 H2Sconcentration
n Is corrosive vs CS but not vs SS or higher alloyn Causes very rapid fouling
Refinery examplesn REACs (hydrotreaters/hydrocrackers)n Crude unit overheadn FCC (overhead in separator section)
CONTROLn wash waterèuse continuous washing (20%min water not vaporized)è inject upstream of ammonium bisulfide dew point
n Use balanced piping for REACsn Upgrade metallurgy
UNDER DEPOSIT CORROSION
35
AMMONIUM CHLORIDEn frost from gas to solid at a temperature depending on NH3 HCl
concentrationn Is corrosive vs CS and SS. Ti and Inconel 625 may offer sufficient
protectionn Causes very rapid foulingREFINERY EXAMPLESn Crude unit overheadn hydrotreaters (REACs, overhead in separator section)n Catalytic reformer (REAC, separator, stabilizer, recycle gas
compressor)n FCC (overhead in separator section)
CONTROLn wash waterèuse continuous washing (20%min water not vaporized)è inject upstream of ammonium chloride dew point
n Use balanced piping for REACsn Upgrade metallurgy (expensive solution)
UNDER DEPOSIT CORROSION
36STRESS CORROSION CRACKING
Cracking corrosive process that requires the simultaneous
presence of:
n Material in passive state susceptible to attack
nAggressive environment
nstress state
èresidual (i.e. welds)
èapplied (i.e. bends)
UPSET
37STRESS CORROSION CRACKING
Table from ASM Vol 13 Corrosion
38STRESS CORROSION CRACKING
Main type of SCC
n Chloride stress corrosion cracking (CSCC)
n Sulphide stress cracking (SSC)
n Alkaline stress corrosion cracking (ASCC)
n Polythionic Acid Stress Corrosion Cracking (PASSC)
n Cracking in H2O-CO-CO2 system
UPSET
39CHLORIDE STRESS CORROSION CRACKING
Material susceptible to CSCC
n austenitic SS, duplex , ferritic
(sensibilized)
n Fe-Cr-Ni alloy (Incoloy)
n Copper alloy
n Bronze/Brasses
n Aluminum
n Cobalt alloy (i.e. Stellite)
View of chloride stress corrosion cracking in a 316stainless steel chemical processing piping system.Chloride stress corrosion cracking in austeniticstainless steel is characterized by the multi-branched "lightning bolt" transgranular crackpattern. (Mag: 300X)
UPSET
40CHLORIDE STRESS CORROSION CRACKING
SS serie 300
CONTROL:n limit O2 contentn limit stress (∃∃ threshold value)n Control temperaturen Control pHN.B. H2S lowers CSCC limits
SCC resistance in oxygen-bearing (abt. 8 ppm)neutral chloride solutions. Testing time 1000hours. Applied stress equal to proof strength attesting temperature. (by SANDVIK)
For SCC Ni content isfundamental
UPSET
41SULPHIDE STRESS CRACKING
H2S SSC Cracks in a 17-4 pHstainless steel
SSC is defined as cracking of a metal underthe combined action of tensile stress andcorrosion in the presence of water and H2SSSC is a form of hydrogen stress crackingresulting from absorption of atomic hydrogenthat is produced by the sulfide corrosionreaction on the metal surfaceSSC is influenced by:n Chemical composition (P,S,Mn), hardness,metal thermal treatmentnTotal tensile stress (applied plus residual)
n Hydrogen flux (function of [H2S], pH, CN-,etc..)
n Time (Note: short term conditions i.e.shutdowns can be sufficient)
n Temperature (increase H2S dissociationand H diffusion)
UPSET
42
Some environmental conditions known to cause SSC are thosecontaining free water (in liquid phase) and:n >50 ppmw dissolved H2S in the free water orn free water pH<4 and some dissolved H2S present orn free water pH>7,6 and 20ppmw dissolved HCN in the water andsome dissolved H2S presentn >0.0003 MPa absolute partial pressure H2S in the gas in processeswith a gas phase
CONTROL:For Refinery apply NACE MR0103For upstream (oil and gas production) apply NACE MR0175
Note: Pay attention to thermodynamic model used in the simulatorsand to hypothesis to calculate % H2S in free water
SULPHIDE STRESS CRACKING
UPSET
43ALKALINE CRACKING
cracking in caustic environment
carbonate cracking
cracking in amine environment
Main materials involved:n Carbon steeln Low alloy steeln Stainless steeln Copper alloy
UPSET
44
Cracking due toexposition of CS to hotcaustic solution (i.e.NaOH, KOH)
CONTROL: use thematerials indicated onCaustic Soda ServiceGraph (see also SR) byNACE
Note: If for the serviceaustenitic SS has beenspecified, checkchloride concentrationand T max.
CAUSTIC CRACKING
UPSET
Caustic Soda Service Graph by NACE
45STRESS CORROSION CRACKING
46STRESS CORROSION CRACKING
47STRESS CORROSION CRACKING
48STRESS CORROSION CRACKING
49STRESS CORROSION CRACKING
50
Cracking caused by amine (mainly due to dissolved CO2 e H2S).Amine cracking happens preferentially in the heat affected zone (HAZ).Lean amine is not corrosive vs CS and it shows less probability tocause cracking.MEA is more aggressive than DEA o MDEAIf temperature ↑↑ cracking likelihood ↑↑ (consider also short termcondition, i.e. Steam out)
CONTROL:èSR (included PWHT) in accordance with API 945 (595 °C < T <
649°C, min holding time 1h)èhardness < 200HRB
SR is suggested, function of used amine, at the following operating T:nMEA : all operating TnDEA: T > 60°CnMDEA : T> 82°C
AMINE CRACKING
UPSET
51CRACKING IN CO-CO2-H2O SYSTEMS
It can happen in pressure system with the simultaneous presence ofCO-CO2-H2On low T (maximum risk in the range 20-60°C)n minimum CO and CO2 pressure required
CONTROL: Check environmental conditions (T, water, PCO & PCO2)
Use SS (12 Cr o 304; 316 not necessary)Range of SCC susceptibi l i ty
0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
1 2 0 0
1 4 0 0
0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0
CO2 part ia l pressure (kPa)
CO
par
tial p
ress
ure
(kPa
)
Published data SASOL Mossgas
52SENSITIZATION ISSUES (INTERGRANULAR CORROSION)
Main degradation forms related:n Sensitizationn Weld decayn Knife line attackn Polythionic acid stress corrosion
cracking
MECHANISM:
1) A high temperature exposureallows the reaction betweenCr and C.
2) Cr carbides precipitates at grainboundaries.
3) Cr depletion in areas surroundingto grain boundaries. (when Crbelow 12% the steel is no more SSand corrode like CS).
53SENSITIZATION AND WELD DECAY
Sensitizationn is not a corrosion mechanism but the Cr depletion may generate
intergranular attack.n May occur rapidly due to: weld, heat treatment and operating
temperature.n The sensitization range (temperature and time) is related to the
material.
Weld decayn The Cr depletion is related to the heating in areas surrounding the weld.n Varies with welding conditionsn varies with distance form the weld
Knife line attackn Same mechanism of weld decayn on chemically stabilized material
Heat affected zone (HAZ)
54SENSITIZATION CONTROL TECHNIQUES
Sensitization control
n Materials selection:ènormal and high carbon grades: Carbon content 0,03 % - 0,10l Ferrous (i.e. 304/316) and Ni-Cr alloys
Subjected to sensitization.è low carbon grades: below 0,03 %l i.e. 304L, 316 L, Hastelloy C-276
Do not sensitize under welding conditions but are subjected to sensitization under operating conditions
èChemically stabilized material (Nb or Ti)l I.e. 321, 347, Incoloy 801, 825, alloy G, Inconel 625
Ni and Ti form carbides avoiding Cr depletion.Thermal treatment (stabilization) avoidssensitization over long term exposure.
l Stabilization heat treatment should be recommended
n Procedure mistakesèCleaning with oily rag before welding introduces CèPWHT in the sensitizing time-temperature range
55INTERGRANULAR CORROSION
56
Intergranular corrosion and crackingcaused by the simultaneous presenceof:n Sulfide scalen Sensitized materialn Oxygenn Stress (residual or applied)n Water
n Polythionic Acids (H2SxOy) form(usually during shut down) for reactionof sulfide scale with H2O e O2
Main material subjected to sensitization:n Austenitic or Ni alloy (also low carbonor stabilized) operating at high T (i.e. 370°C < T < 815°C for 304/316)n Austenitic or Ni alloy (not stabilized)welded
Polythionic acid stress corrosion cracking oftype 310 stainless steel. The item wasexposed to sulfur containing natural gas in acontinuous flare
POLYTHIONIC ACID STRESS CORROSION CRACKING (PASCC)
UPSET
57POLYTHIONIC ACID STRESS CORROSION CRACKING (PASCC)
Refinery examples:n hydrodesulfurizersn hydrocrackersn hydrogen reformersn FCCn Fired heaters (both external and internal)
CONTROL: follow guideline NACE RP0170
n Exclusion of oxygen (air) and water by using a dry nitrogen purgen Alkaline washing with soda ash. Avoid washing of zone that can’t
be drainedn Exclusion of water by using a dry purge with a dew point lower
than -15°C
UPSET
58CORROSION UNDER INSULATION
For further information on CUI see NACE RP 0198
59
Atomic hydrogen even produced by low temperature corrosionphenomena may diffuse through metal surface causing hydrogendamage.
Hydrogen damage is recognized under various forms:
n Blisteringn Hydrogen Induced Cracking (HIC)n Stress Oriented Hydrogen Induced Cracking (SOHIC)n Hydrogen Embrittlementn High Temperature Hydrogen Attack
HYDROGEN DAMAGE
UPSET
60BLISTERING-HIC-SOHIC
UPSET
Main steps of blistering and HICn Atomic hydrogen diffuses inside the
metal bulkn Inside the metal atomic hydrogen
meets the voids (rolling defects) andinclusions (MnS) and re-combines inmolecular hydrogen (H2)
n Gradually, molecular hydrogencollected in voids and inclusionsincreases the pressure reaching upto 10 GPa.
n The elevated pressure evidenced bysurface blistering may lead to local(stepwise) and complete rupture ofthe plate.
n SOHIC is related to residual stressespresents in the metal.
61
Influencing and control parameters:
n Chemical composition of the process fluids (presence H2O,pH, H2S, CN, As, Sb)
n Voids and inclusions presence
n Metal chemical composition and thermal treatments.
n Residual stresses (only for SOHIC)
n Construction and welding and test procedure accordingstandards. (NACE MR 0175, NACE 0103, NACE TM0284, API945)
BLISTERING-HIC-SOHIC
UPSET
62BLISTERING-HIC-SOHIC
63
Embrittlement caused by thehydrogen diffusion through themetals.Possible Hydrogen sources:n General corrosionn Galvanic corrosionn Overprotection of cathodicprotection.
Influencing factors:n Enhanced by CN, As, Sbpresence.n May occur on CS, alloyed steels,nickel alloys, Titanium (T > 71° C)Copper alloys are consideredimmune
HYDROGEN EMBRITTLEMENT
UPSET
64HYDROGEN DAMAGE
Critical areas:è“Rich” section of amine units.èSour water stripper.èHydrodesolfurization units.èFCC units.
Hydrogen damage control:n Appropriate material selectionèReduction the allowable metal inclusions (S, Mn and P content).èCa and rare earth addition (shape control of residual inclusions).èSteel HIC resistance according NACE TM0284.
n Optimization of process conditions (i.e. H2O, pH)n Construction and welding according standard (i.e. NACE MR0175
NACE 0103).n Correct cathodic protection design and operation.n Use of insulation kit for different metals in electrical contact.
UPSET
65EROSION-CORROSION
♦ Degradation mechanismaccelerated by flow conditions of acorrosive fluid in contact with metalsurface
♦ Mechanism:
Corrosive fluid reacting with metalcreates a film scale
Fluid removes mechanically thescale exposing uncorroded metal
Material 1ft/sec 4 ft/sec 27 ft/secCS 34 72 254Ad. Brass 2 20 17070-30 Cu Ni (0.05% Fe) 2 - 19970-30 Cu Ni (0.5% Fe) < 1 <1 39
Typical corrosion rates in seawater mdd
66EROSION-CORROSION
Factors influencing erosion-corrosion:n Velocity and fluid turbulencen Temperaturen Multiphase flown Suspended solidn Galvanic effect (i.e.: CS-SS andCS-CuNi in seawater)
CONTROL:
n Material Selection or lining (i.e. Cu-Ni 66-30-2-2 instead of Cu-Ni 70/30).
n Check allowable velocity
n Localized preventive measures (i.e. ferrule on tubes inlet).
n Change environment (i.e. inhibitor, filtering, temperature).
67EROSION-CORROSION
68
69MICROBIOLOGICAL INDUCED CORROSION (MIC)
MIC refers to corrosioninfluenced by the presenceand activities ofmicroorganisms and/or theirmetabolitesMicroorganism (i.e. fungi,bacteria or algae) can beaerobic or anaerobic
Generally MIC shows jeopardized attack on CS, localized on SS (i.e.pitting)Microorganism’s growth is influenced by pH, temperature and “food”availability (peak between 30 e 40 °C)Stagnant zone increase attack severity
70MICROBIOLOGICAL INDUCED CORROSION (MIC)
Refinery examples:
n Cooling water systems
n Water layer in tanks
n Following hydrotesting
CONTROL:
n Use Biocide addition
n High thick Coating (i.e. coal tar)
n Cathodic Protection (+950 mV)
n High quality hydrotest water
n Avoid wet dead legs
71
CORROSION AND DEGRADATIONCORROSION AND DEGRADATIONCORROSION AND DEGRADATIONCORROSION AND DEGRADATION MECHANISMS MECHANISMS MECHANISMS MECHANISMS
High temperature corrosion mechanisms
73HIGH TEMPERATURE CORROSION MECHANISMS
nn NAPHTENIC ACID CORROSIONNAPHTENIC ACID CORROSION
nn HIGH TEMPERATURE OXIDATIONHIGH TEMPERATURE OXIDATION
nn SULFIDATIONSULFIDATION
nn HIGH TEMPERATURE HYDROGEN DAMAGEHIGH TEMPERATURE HYDROGEN DAMAGE
74NAPHTENIC ACID CORROSION
Generalized corrosion at high T (230-400 °C) caused by naphtenic acids forcrude with TAN > 0.5 (ASTM D 974T.A.N. as mg KOH/g) or TAN > 0.35 forsome licensor.
∃∃ several type of naphtenic acids
Naphtenic acids are very aggressive especially close to their boilingpoints (thus can attack selectively some locations of the unit )
n Metallurgy: CS and Cr alloy (i.e. 5 - 9 - 12Cr o 304/316 std) arereadily attackedn Sulfur content: especially at low fluid velocity, sulfur can mitigatecorrosive attackn Velocity: high velocity (>2.7 m/s) increases corrosion rate
75
Refinery examples:n Heaters and Transfer Line in CDUn Diesel section of CDU column (pump-around)n Atmospheric column residuen Vacuum column residuen Gas oil section of VDU
CONTROL:n N.B. Check TAN for each cut with operating temperature in the
range 260 - 400°Cn Stainless steel 317 o 316 with Mo 2.5%minn Monitoring + inhibitor (only for short run)n Use blending to reduce TANn Neutralization with NaOH (pay attention on caustic embrittlement)
NAPHTENIC ACID CORROSION
76
Microstructure of iron oxides formed on iron byhigh-temperature oxidation in air
Generalized corrosion caused bydirect oxidation of base material(liquid water not required)
Oxidation issuesn O2 Concentrationn Alloy compositionn Metal temperature
The source of O2 can be alsosteam or CO2
The scale compositioninfluences CR
HIGH TEMPERATURE OXIDATION
77
Refinery examples:n Heatersn Boilers
CONTROL:n Improve metallurgy(with alloy containingCr, Ni, Si, Al)
Control environmental conditions, especially:
n Sulfur (Increase corrosion rate)
n metals (i.e. V which causes V2O5 formation) in fuel
n Temperature (if scale ↑↑ thermal exchange↓↓ and lifetime↓↓)
HIGH TEMPERATURE OXIDATION
78SULPHIDATION
Reaction between Sulfur and metal or alloy at high temperature.
Can cause generalized corrosion @ T>260 °C
CR is influenced mainly by T and %S (or H2S)
Refinery examples:
n Topping and Vacuum (@ T >260°C)
n HDS (hot heat exchangers, heaters and reactor)
n Sulphur Recovery Unit
%Cr is fundamental to resist to sulfidation attack. Generally lowchrome alloy are used with %Cr higher and higher (1.25-2.25-5-7-9Cr) up to stainless steel (as 12Cr like 405 and 410) or austenitic(304 or 316)
79
CONTROL:Use appropriate metallurgy considering CR calculated by availablecurves (function of metal T, alloy composition and %S for Mc Conomyor H2S for Couper Gorman)n Mc Conomy (API) based on total sulfur content:
SULPHIDATION
80SULPHIDATION
CONTROL:Use Couper Gorman for fluid containing high H2 and H2S concentration(see also Nelson curves on API 941 for HTHA)Available for several material (i.e. CS, low Cr alloy and SS)
Couper Gorman Curves for carbon steel and 18-8 stainless steel
81HIGH TEMPERATURE HYDROGEN ATTACK (HTHA)
High temperature hydrogen can attacksteels in two ways :
n Surface decarburization (slight,localized reduction in strength andhardness and an increase in ductility)
n Internal decarburization and fissuring(CH4 formation and high localizedstresses which lead to the formation offissures, cracks or blister in the steel)
Factors influencing HTHA:n Temperaturen H2 pressuren Stress (i.e. welds)n Time (∃∃ incubation period)
Hydrogen attack corrosion and crackingon the ID of an 1800 psig carbon steelboiler tube.
82
CONTROL:
Use Cr-Mo alloy instead of CS (reducesthe amount of available carbide)
SS are practically immune from HTHA
For CS and Cr-Mo alloy refer to API 941
Note:
èC-0.5Mo, usually, is not allowed inH2 service
èCladding should not be consideredas material resistant to HTHA(therefore also base material haveto be resistant)
Solids deposition and hydrogen attackcorrosion at the ID weld in an 1800 psigcarbon steel boiler tube. The arrowmarks the direction of flow. (~1X)
HIGH TEMPERATURE HYDROGEN ATTACK (HTHA)
83HIGH TEMPERATURE HYDROGEN ATTACK (HTHA)
N.B. Add safety margin, below the relevant curve,when selecting steels (11 °C min)
Nelson curves (API 941)
84STATISTIC RELEVANCE OF CORROSION FAILURES
Types of corrosion failures (duPont)
General27%
Erosion-corrosion7%
Corrosion fatigue3%
Intergranular10%
Pitting14%
Weld corrosion5%
Stress Corr. Cracking
24%
others corrosion 6%
High temperature corrosion
2%
Crevice2%
nCorrosion causes the 55%of the failures in chemicalplants (the remaining 45% ofthe failures are related tomechanical reasons).
n General corrosion and SCCshow the higher occurrencein corrosion failures (in sumthey account 51%).
n Crevice corrosion causesonly 2% of the failures whilepitting the 14%.
nThe sum of Intergranularand weld corrosion isrelevant (15%).
“Il campo della corrosione è con molta aderenza
paragonabile a quello della medicina. Per I materiali, la
corrosione è indubbiamente la più insidiosa delle cause di
decadimento e di morte e al corrosionista si presenta il
compito in genere assai arduo, di diagnosticare il male, di
stabilirne le cause, di prevenirlo ove possibile altrimenti di
reprimerlo o contenerlo in limiti accettabili… [A questo
scopo il corrosionista deve]… pazientemente costruirsi il
suo atlante di anatomia patologica dei materiali esposti ai
più svariati ambianti aggressivi, edificare il corpus della sua
diagnostica, sviluppare una sempre più efficace
farmacologia anticorrosionistica.”
Roberto Piontelli, 1961
MATERIAL SELECTION ANDMATERIAL SELECTION ANDMATERIAL SELECTION ANDMATERIAL SELECTION AND CORROSION CONTROL CORROSION CONTROL CORROSION CONTROL CORROSION CONTROL
Selection criteria, material propertiesand cathodic protection
87MATERIALS AND CORROSION PROTECTION
nn CONDITION ASSESMENT AND MATERIAL SELECTIONCONDITION ASSESMENT AND MATERIAL SELECTION
nn CARBON STEELCARBON STEEL
nn LOW ALLOYED STEELSLOW ALLOYED STEELS
nn STAINLESS STEELSSTAINLESS STEELS
nn COPPER ALLOYSCOPPER ALLOYS
nn NICKEL ALLOYSNICKEL ALLOYS
nn TITANIUM ALLOYSTITANIUM ALLOYS
nn POLIMERIC MAERIALSPOLIMERIC MAERIALS
nn CATHODIC PROTECTIONCATHODIC PROTECTION
88MATERIAL SELECTION
Ver. corrosion protection measures i.e. CPControl galvanic corrosionControl erosion corrosionOn Stream Inspection
4. Engineering, Procurement, Construction
Costs decreaseImprove the reliability of the unit
3. Process andmaterial optimization
Ensure the required service life time2. Material selection
Conditions assessment1. Processdevelopment
Scope of corrosion activitiesPhase sequence
891. CONDITIONS ASSESSMENT
Conditions
Chemical composition
ThermodynamicPhysical
Time extension Probability
TDS &TSS
Environment type(water/oil content)
Contaminants and corrodents
Cl-, H2S, CN-, NH3 …
Chemical composition
ThermodynamicUpset conditions
Physical
Temperature(local)
Fluid dynamic
Pressure
Condensation and dew point
(local)
SolidPrecipitation
Phase settling
OxidizersO2, Cl2, Fe3+, Cu2+…
External conditions
Fire hazard Marine environment
Underground
Atmospheric env.Thermal insulation
902. MATERIAL SELECTION
Material
Joining techniques
Pre-fabrication
dimensions
Galvanic couplings
Material sel. in similarservice within the prj
Heat treating
Procurement time
Fittings
experience in similar units
Density
Corrosion allowance
Strength
Corrosion protection
Experience and literature
Metallurgy
AvailabilityFabricability
Costs
Spare partsConstruction
Conditions Life Time
913. PROCESS AND MATERIAL OPTIMIZATION
ØExam of the whole unit
ØScopeDecrease the project costsAvoid over and under specificationImprove the reliability of the unit
Process development
Conditions assessment
Material selection
Process Engineer
Corrosion Engineer
92CARBON STEEL
n The materialChemical composition based on Fe and C, can be adjusted toimprove the resistance to specific degradation mechanism (i.e. HIC)
n Typical conditions:By far the most common material used up to 400°Cin refineries due primarily to a combination ofstrength, availability, low cost, and resistance to fire.
n Main contaminants and corrodents:Halides (chlorides), sulfides, ammines, dry ammonia, carbonates,CO2+H2O+CO, cyanides, Hydroxides, nitrates, CO2+H2O, acids,oxygenated demi water.
n The degradation mechanisms to be verified:General corrosion, stress corrosion cracking, crevice, underdeposit, under insulation, galvanic attack, hydrogen damage,erosion corrosion, high temperature damage (almost all).
93CARBON STEEL
Specific corrosion protection measures.
n Design according to soda chart, Mc Conomy, Couper Gorman,Nelson where applicable.
n Selection of inhibitors (i.e. acidic water, cooling water, boilingwater).
n Cathodic protection to control general, galvanic, MIC and crevicecorrosion.
n Anodic protection to control general corrosion.
n Polymeric lining (epoxy, PTFE, GRP, rubber) to control corrosion at low temperature.
n PWHT to control SCC.
n Electrical insulation from others metals to controlgalvanic corrosion.
n Water injection to control under deposit corrosion.
94LOW ALLOYED STEEL
The materials:
n Typical conditions:
For high temperatureservice, or hydrogen and
sulfidant atmosphere.
n Present the same contaminants and corrodents of Carbon steel.
n The degradation mechanisms to be verified:
As per CS. Specifically Hydrogen high temperature damage andhigh temperature sulfidation.
n Corrosion prevention measures:
Design according Nelson diagram, Couper Gorman and McConomy to realize correct selection and evaluation of corrosionallowance.
6505% Cr 0,5% Mo
6509% Cr 1% Mo
6252,25% Cr 1% Mo
6001,25% Cr 0,5% Mo
6001% Cr 0,5% Mo
5000,5% Mo
Max Temperature °CChemical Composition
95
n The materials:
n Typical conditions:Acidic and saline water, high temperature and low temperature, waste water,demi water, organic acids.
n Main contaminants and corrodents:
Halides (chlorides), hydroxides (wet and dry), sulfurous acid (onaustenitic), organic acids, Hydrogen sulfide and (by externalside) Vanadium, molten zinc and molten aluminum.
STAINLESS STEEL
Super AusteniticS31254 (254 SMo)20 Cr 18 Ni 6 Mo Cu N
Nickel alloy(Al-6X)20 Cr 24 Ni 6,5 Mo
Super DuplexS32750 (2507)25 Cr 7 Ni Mo N
DuplexS31803 (2205)22 Cr 5 Ni Mo N
316, 316 Ti
304, 304L, 321, 347
405, 410, 410 S
Type
18 Cr 10 Ni Mo
18 Cr 8 Ni
12-13 Cr
Designation
Austenitic
Austenitic
Martensitic, Ferritic
Metallurgy
96STAINLESS STEEL
n The degradation mechanisms to be verified:General corrosion, Pitting, SCC, crevice, galvanic, MIC, erosion
corrosion, weld decay, liquid metal embrittlement.
n Corrosion protection measures:
èDesign taking into account the resistance of the different alloysin considered environment.
èSelection of inhibitors.èThermal treatments to control SCC and intergranular corrosion
cracking.èChemical cleaning (against PASCC) and passivation.èElectrical insulation from others metals to control galvanic
corrosion.
97FERRITIC AND MARTENSITIC STAINLESS STEELS
n 11-13% Chrome (type 405 and 410 S)Primarily used for clad lining
n 11-13% Chrome (type 410)ferritic or martensitic stainless steel extensively applied forstandard trim on process valves, pump impellers, vessel trays, traycomponents and exchanger tubes.
Corrosion resistance
èexcellent resistance to sulfur at high temperature.ègood resistance to hydrogen sulfide at low concentrations and
intermediate temperatures.
98AUSTENITIC STAINLESS STEEL
Variables influencing the behavior of austenitic stainless steelsin salted water:
n Temperature:50° C is accepted as the minimum temperature for theoccurrence of stress corrosion cracking and pitting in slightlysalted water (100-200 ppm).
n Chloride content:In stress relieved structures, the maximum allowed chloridecontent to avoid pitting and crevice (below 50°C) is related tothe alloy
Type Cl-304 100 ppm316 300 ppm
(these limits can be lower for some Licensor i.e. 50 ppm for UOP)
99AUSTENITIC STAINLESS STEEL
n Metallurgyselection is realized considering critical temperature which isthe minimum temperature at which pitting or crevice mayoccur in ferric chloride solution.CSCC occurrence have to be considered separately.
100
n The materials
n The typical applications
Seawater exchangers, water pipes, brackish water equipment.
COPPER ALLOYS
Alloy type Main composition
Aluminium bronze 92% Cu, 8% Al
Aluminium brass 77% Cu, 21% Zn, 2% Al, 0.04% As
Admiralty 71% Cu, 28% Zn, 1% Sn, 0.04% As
90-10 Cu-Ni 10% Ni, 1% Fe, Cu rem.
70-30 Cu-Ni 30% Ni, 1% Fe, Cu rem.
66-30-2-2 Cu-Ni 30% Ni, 2% Fe, 2 %Mn, Cu rem.
101COPPER ALLOYS
nMain degradationmechanisms
Erosion corrosion andimpingement attack, stresscorrosion cracking (inpresence of 1 ppm ofammonia), selective leaching(Immune to hydrogendamage, and preventbiofouling)
nCorrosion protectionmeasurescorrect design accordingstandards (BS MA18 in thegraph).Check ammonia presence(UPSET conditions)Erosion ferrules (in Teflon orspecial Cu Ni alloys Crmodified)
Maximum seawater velocities for continuos flow conditionsm/sec (ref.:BS MA 18)
102TITANIUM ALLOYS
The materials
n Titanium is a reactive metal and as the other materials of thegroup forms spontaneously a superficial oxide film whichensure protection from the environment.
n The corrosion resistance is related to the stability and thecontinuity of the oxide layer (on-off corrosion behavior).
n The reactive metal group is formed by (increasing bycorrosion resistance): Titanium, Zirconium, Niobium andTantalum. The corrosion behavior of these materials shows alarge amount of similarities.
ASTM grade CompositionGr 1,2,3,4 unalloyed (O and N content)Gr 7, 11 0.2 Pd
Gr 12 0.8 Ni 0.3 MoGr 16, 17 0.04 Pd
103TITANIUM ALLOYS
n In which conditions:Seawater and desalinization plant, organic acid, in oxidizing andmildly reducing wet environments.
104TITANIUM ALLOYS
n Main contaminants and corrodents:Wet Fluorides (and halides in high concentration), methanol plus halides,nitric acid fuming, nitrogen tetroxide, gaseous water free halides,chlorinated solvents, concentrated reducing acids.
n Degradation mechanism to be verifiedGeneral corrosion, pitting, crevice, SCC, catastrophic oxidation, galvanic*,hydrogen embrittlement.
105TITANIUM ALLOYS
n Welding of titanium
1) The weld of Chemically Pure and Pd alloys (ASTM gr. 1, 2, 3,4, 7, 11, 16, and 17) shows the same corrosion resistanceas the bulk material.
2) Like all reactive metals at high temperature reacts stronglywith atmospheric oxygen.
3) Can be welded with GTAW or GMAW (same equipment usedfor SS 316 or nickel alloys).
4) Argon or helium have to be be used to protect the weldin welding chamber (shop) or welding shoes (constructionsite).
5) The weld quality verified easily for acceptance• Visual examination of “as weld” surface• hardness measurement is highly sensitive to oxygen pickup
106NICKEL ALLOYS
n Materials
n Advantages:èVery resistant (as a function of specified type) to many
environmentsè In aggressive reducing environments are mandatory selection
n Disadvantages:èHigh cost (GdP will be not so happy!!! )èPossible availability problems for some alloy
Alloy type Main composition
Incoloy 800 33% Ni, 21% Cr, 40%Fe, 0.1% C, 1% Al+Ti
Incoloy 825 43% Ni, 22% Cr, 3% Mo, 2% Cu, 0.04% C, Fe Bal
Inconel 625 43% Ni, 22% Cr, 9% Mo, 3.5% Nb, 0.04% C
Inconel 600 76% Ni, 16% Cr, 8% Fe, 0,2 Cu, 0.08 C
Inconel 601 60% Ni, 23% Cr, 16% Fe, 1% Al Cu, 0.1 C
Hastelloy C-276 57% Ni, 15% Cr, 16% Mo, 1% Fe, 0.02% C
Monel 400 66% Ni, 31% Cu, 1.4% Fe, 0.15% C
107NICKEL ALLOYS
TYPICAL ENVIRONMENTSn Hastelloy C/C276, Inconel 625èHigh resistance to acid (both oxidizing and reducing)èexcellent resistance in chloride and/or H2S environmentèHigh resistance vs underdeposit corrosion
n Inconel 601, Incoloy 800èHigh temperature resistance
n Incoloy 825èHigh resistance in chloride and/or H2S environment (lower than
Hastelloy C-Inconel 625)èHigh resistance vs underdeposit corrosion (but can fail with NH4Cl)
n MonelèHigh resistance to hot alkalisèHigh resistance to acid (especially HF)
108POLYMERIC MATERIALS
n High molecular weight organic materials that can be formed intouseful shapes.
n Can be used for piping and equipment (thermosetters andthermoplastics) or for gaskets (elastomers)
ThermoplasticsPE
PTFEPVC
ThermosettersGlass fiber epoxy resin
Glass fiber vynil ester ep. resinGlass fiber Poly ester ep. resin
ElastomersViton (Flueelastomers)
Kalrez (perfluoelestomers)NBR
Polymeric materialsIn refinery
109THERMOPLASTICS
n Are characterized by the softening withthe increase of temperature and returnto their original hardness when cooled(most are weldable).
n Degradation mechanisms are differentfrom metals:Swelling, softening, loss of mechanicalproperties, hardening and discoloration(no electrochemical mechanismsinvolved). Degradation may be causedby heat, solar exposure and UV.
n For correct material selection anddesign are necessary: life time,temperature (!), environment andpressure.
n Main couple material-environment are:PE(or PP)-water, PVC-mineral acids,PVDF-acids (at higher pressure andtemperature).
Main Materials:Polyethylene (PE)Polypropylene (PP)Polyvinyl chloride (PVC-CPVC)Polyvinylidene Fluoride (PVDF)Teflon (PTFE)
110THERMOPLASTICS
n AdvantagesèExcellent chemical resistance to
water environment,l PTFE can withstand practically all
refinery environments below 200°CèEasy welding and installation (not
for all)èNo protection required in
underground service
n DisadvantagesèRapid decrease of properties with
the temperature increase.èChemical resistance to
hydrocarbonsèNot suitable in fire hazard area
111THERMOSETTERS
n Are characterized by the thermaldegradation when exposed toheating.
n Thermosetters are generally usedas matrix for composite material.Glass is generally used as fiber.
n Same degradation mechanism ofthermoplastic:Swelling, softening, loss ofmechanical properties, hardeningand discoloration. Higherresistance than thermoplastics.
Main matrix Materials:Epoxy resinVinyl ester epoxy resinPhenolic resin
112THERMOSETTERS
Main applications are:Firewater, cooling water, highpressure water lines (special types upto 280 Bar), sewer.
AdvantagesèExcellent chemical resistance to
aqueous environmentèNo protection required in
underground service
Disadvantagesè Installation difficultiesèDesign and installation know howèNot suitable in fire hazard areaèSensitivity to vibrations and
mechanical stresses
113CATHODIC PROTECTION
n HistoryIn 1824 Sir Humphrey Davy discovered that is possible to protect thecopper of royal ships from marine corrosion by electrically coupling itwith iron.
n Basic PrincipleThe metal dissolution is reduced trough the application of cathodiccurrent that may originates from:è the corrosion of a less noble metal (sacrificial cathodic protection)è the conductive anode and ∆∆V (current impressed cathodic
protection)
n Scope of CP applications:è Protect from wet and soil
corrosion coated steel.è Allow the use of carbon
steel avoiding the materialupgrade.
è Minimize the cost of CS coating maintenance.
114CATHODIC PROTECTION
Cathodic protection techniques
n Sacrificial cathodic protectionèUse of anodic metall Magnesium (t< 40°C)l Zinc (t < 40°C)l Aluminum
(Cl- > 1000 ppm or t > 40°C)
èAnode connection with cathodel direct (economical)l trough a electrical resistance
(improve the control and avoidunder and over protection)
èReference electrodesl Allows monitoring and verificationof corrosion for cathodically protected surfaces
115CATHODIC PROTECTION
Cathodic protection techniques
n Impressed current cathodic protectionèanode materiall Ti Mixed metal oxide coatedl High silicon ironl Ceramic electrodes
ècurrent generationl an external DC current
source is necessary
è reference electrodesl the use is mandatory in
conjunction with currentcontrol system
116CATHODIC PROTECTION
n Design parameters
èTemperature (important for anode selection)èpHèChemical composition (Cl- and ions content)èConductivity (high conductivity = aggressive condition)èRedox potential (i.e. oxygen content or other oxidizer presence)èDimensions of the metal surface in contact with conductive
electrolyte.(important! Water level on separators and oil tank internals)
n With the parameters is possible to design the system:
èwhich technique (sacrificial or impressed)èanode selection (Al, Mg, Zn, Ti or Fe-Si-Cr)èanode quantity (related to the required current)èanode distribution (related to the disposition of the surface to
protect)ècurrent system design (only for impressed current)è Insulation kits and resistance bonds disposition
117CATHODIC PROTECTION
n Typical applications
èUnderground and submerged steel surfaces (may berequired by law).
l Bottom tanksl Underground and submerged Pipelinesl Jacket on offshore structuresl underground and submerged steel reinforced concrete
structures
èLow temperature corrosion on the process side (costevaluation).
l Water tanksis preferable to cathodically protect internally lined surfaces
l Water boxes (channels) of Thermal exchangersCP avoids cladding in Cu-Ni alloys in seawater exchangers
l Water-oil separatorsCP avoids the use of stainless steel or ensure lowermaintenance of internal lining
MATERIAL SELECTION ANDMATERIAL SELECTION ANDMATERIAL SELECTION ANDMATERIAL SELECTION ANDCORROSION CONTROL INCORROSION CONTROL INCORROSION CONTROL INCORROSION CONTROL IN
REFINERY UNITSREFINERY UNITSREFINERY UNITSREFINERY UNITS
119MATERIAL SELECTION AND CORROSION CONTROL IN REFINERY UNITS
nn DESALTERDESALTER
nn ATMOSPHERIC DISTILLATION UNITATMOSPHERIC DISTILLATION UNIT
nn VACUUM DISTILLATION UNITVACUUM DISTILLATION UNIT
nn AMINE UNITAMINE UNIT
nn HYDRODESULPHURIZATION UNITHYDRODESULPHURIZATION UNIT
nn SOUR WATER STRIPPER UNITSOUR WATER STRIPPER UNIT
120DESALTER
THE DESALTER CAN BE THE SOURCE OR THE SOLUTION OFREFINERY’S PROBLEMS
121
TIPICAL CORROSION AND FOULING PROBLEMS
n Corrosion of water outlet lines (brine)
n Fouling of inlet heat exchangers (generally due to oxygen
and excessive temperature)
n Remaining problems with desalter aren’t problems in the
desalter itself (affect efficiency and downstream corrosion)
DESALTER
122
n Principal variables (by UOP)
èwash water (4-10%)
èSettling time (30-45min)
èTemperature (90-150°C, high enough to dissolve sediments
and salts)
èDesalting chemicals (0.25 - 1 pint for 1000 barrels)
èAlternating electric field
èValve
è∆∆P (7-15 psig)
èLeveln TARGET: DESALT TO LESS THAN 2 LBS/THOUSAND BARREL (PTB)
n Stripped Water should be used as wash water
DESALTER - OPERATING GUIDELINES
123
TEMPERATURE
n Increasing temperature reduces viscosity and reduces settling time
n Increasing temperature increases water solubility and water
(including dissolved salt) carry over
n Keep inlet heat exchangers below 150 °C
èReduce corrosion rates in exchangers
èReduce fouling in exchangers (minimizing salt precipitation)
DESALTER - OPERATING GUIDELINES
124ATMOSPHERIC DISTILLATION UNIT
125ATMOSPHERIC DISTILLATION UNIT
TYPICAL CORROSION ANDFOULING PROBLEMS
n HCl corrosion in the OVHD
system
èAmmonium Chloride
èAmmonium Bisulfide
n High temperature sulfur
corrosion
n Naphtenic acid corrosion
n Asphaltine/wax/polymer
fouling
n PASCC (300 series SS)
n Wet hydrogen sulphide
126
METALLURGYUse Chrome alloy (solid or lining for high Cr %) for sulfur
resistance (according to McConomy curves) es. 1.25 Cr, 2.25Cr, 5Cr, 9Cr, 12Cr in the bottom section of CDU tower and in the hotside of the heating train
n Use Monel for HCl resistance in the top section of tower (forcladding and trays) and in the OVHD accumulator ifcondensation is expected
n Use 90-10 Cu-Ni for Chloride resistance in the desalter brinen If Naphtenic acid are an issue (Note: check TAN number in cuts)èALL 5 - 9 - 12Cr change to 317 or 316 with 2.5% min Mo (see
also T and TAN)èCarbon steel in gas oil cut may also change to 317 or 316 with
2.5% min MoèMust guard against PASCC of austenitic SS
ATMOSPHERIC DISTILLATION UNIT
127ATMOSPHERIC DISTILLATION UNIT - MSD
NOTE: The indicated selection is not a guideline; it indicates only apossible choice among several solutions as a function of processconditions, corrosion mechanisms involved, lifetime and Prj requirements
128ATMOSPHERIC DISTILLATION UNIT - OPERATING GUIDELINES
CAUSTIC INJECTIONn Inject caustic if necessary to reduce chlorides in OVHD or to
reduce TANèUse fresh 2-3% causticè Inject no more than 4 PTBè Inject to crude no hotter than 150 °Cè Inject at least 5 feet upstream of equipmentèand as close to desalter downstreamas possibleè Inject using a quill
TAIL WATER pHn Operate between pH 5.5- 6.5 in tail watern Use a online pH metern Automate control of corrosion inhibitor injectionn Keep pH meter clean (filming amine, used as inhibitor, can dirty the
instrument)
Injection Quills
129
WASH WATER IN OVERHEAD SYSTEM
n 20% of injected water not vaporized
n water quality not critical, can recirculate
DEW POINT
n Run top of tower above dew point
èWatch for “shock condensation” at point of recycle water inlet
CORROSION INHIBITOR
n Use corrosion inhibitor in the overhead line
n May need to inject neutralizer and film former separately
ATMOSPHERIC DISTILLATION UNIT - OPERATING GUIDELINES
130VACUUM DISTILLATION UNIT
131VACUUM DISTILLATION UNIT
TYPICAL CORROSION ANDFOULING PROBLEMS
n H2S, CO2 corrosion in theOVHD system
n High temperature sulfurcorrosion wherevertemperature exceeds 260°C
n Naphtenic acid corrosionespecially in heater outletand transfer piping
n Asphaltine/wax foulingn Polythionic acid SCC (300
series SS)
132
METALLURGY
n Problem: traces of H2S, CO2, HCl in OVHD systemèSOLUTION: use MONEL mesh for demister
n Problem: traces of corrodents in Vacuum ejectorèSOLUTION: use 316 internals
n Problem: high temperature sulphur corrosion in bottom section oftower and in the heating trainèSOLUTION: use chrome alloy (solid or lining for high Cr %)
according to McConomy curves
n Problem: Naphtenic acid corrosion (for cuts with TAN>0.5)èALL 5 - 9 - 12Cr change to 317 or 316 with 2.5% min Mo
n Problem: Polythionic acid SCC for sensitized materialè follow the recommendation listed in NACE RP 0170
VACUUM DISTILLATION UNIT
133VACUUM DISTILLATION UNIT - MSD
NOTE: The indicated selection is not a guideline; it indicates only apossible choice among several solutions as a function of processconditions, corrosion mechanisms involved, lifetime and Prj requirements
134AMINE UNIT
135
TYPICAL CORROSION AND FOULING PROBLEMSn Tendency for corrosion varies with amine used, concentration and
loadingn Acid gas corrosionèH2S, CO2
èLetdown valve into stripperèOverhead of stripper
n Heat stable amine salts (stronger than H2S)èNot stripped by heat in stripperè Inorganics (Cl-,SO4=, CN-, SO2)l Contaminants in feed
èOrganics (formic, acetic, oxalic)l Feed+oxygenl Pump seals, make up water
èAreas: stripper bottom, reboiler, hot lean amine pipen Thermal degradation of aminesè forms corrosive, acid species (especially in presence of oxygen)
AMINE UNIT
136AMINE UNIT
METALLURGY
n Problem: Amine stress corrosion cracking and hydrogen damageèSOLUTION: PWHT (see also amine SCC) and use killed carbon
steel
n Problem: Acid gas corrosion (Letdown valve/piping into stripperand Overhead of stripper)èSOLUTION: use SS (304 or 316)
n Problem: H2S, CO2in OVHD systemèSOLUTION: use SS for tube condenser and OVHD accumulator
(or CS HIC resistant)
n Problem: sour water in the reflux pumpèSOLUTION: use SS or duplex (as suggested by API 610)
137AMINE UNIT - MSD
NOTE: The indicated selection is not a guideline; it indicates only apossible choice among several solutions as a function of processconditions, corrosion mechanisms involved, lifetime and Prj requirements
138AMINE UNIT - OPERATING GUIDELINE
n Limit amine temperature to 130 °F
èReboiler steam less than 4.5 bar
n Avoid acid gas flashing
èUpgrade metallurgy ifunavoidable
n Keep out oxygen from the system
n Control fluids velocityn FilterèTypically 10-20 µµm (smaller may help, i.e. 2 in series 15µµm-5µµm)èPartially filtration (10-15%) may be sufficientèCarbon filter to remove hydrocarbon and reduce fouling
n Problems come from operating at maximumèAmine concentrationèCirculation rateèRich amine loading
139HYDROTREATER
140
TYPICAL CORROSION AND FOULING PROBLEMS
n Rust from tankage
èoxygen in tank/transport
èPlugs reactor bed
n High temperature sulphur attack (sulphidation)
n High temperature hydrogen attack (HTHA)
n ammonium chloride in hydrogen recycle gas
n ammonium bisulphide
èReactor Effluent Air Cooler (REAC)
n Wet hydrogen sulphide
n PASCC
HYDROTREATER
141
METALLURGY
n Problem: Sulphidation and HTHA on reactor feed (generally fromheater), reactor and effluent piping and exchangersèSOLUTION: use Austenitic SS (321 or 347). Use Chrome alloy for
base material in case of cladded solution
n Problem: Ammonium chloride, Ammonium bisulfide, wet H2S onREAC, piping and accumulatorèSOLUTION: use wash water and/or upgrade material to Incoloy
825, Inconel 625 or Ti. Austenitic 316 may be good to clad waterphase on accumulator. CS is also possible with stringentvelocity limits and monitoring
n Problem: Polythionic acid SCC for sensitized material
è follow the recommendation listed in NACE RP 0170
HYDROTREATER
142HYDROTREATER - MSD
NOTE: The indicated selection is not a guideline; it indicates only apossible choice among several solutions as a function of processconditions, corrosion mechanisms involved, lifetime and Prj requirements
143
n Oxygen in feed (rust in tanks and polymerization fouling)èGas blanket tankagel Nitrogen bestl Natural gas may have air in itl Fuel gas good
èBetter bypass tankage section
n Wash waterècan be continuous (better)or discontinuousèUse balanced exchangerè20% of injected water not vaporizedèVelocity between 2.5 and 6m/s(9 for alloy)èFoul water < 8% NH4HS
HYDROTREATER - OPERATING GUIDELINE
144SOUR WATER STRIPPER
145
TYPICAL CORROSION AND FOULING PROBLEMS
n ammonium chloride
n ammonium bisulphide
èReactor Effluent Air Cooler (REAC)
n Wet hydrogen sulphide
n Hydrogen damage
SOUR WATER STRIPPER
146
METALLURGYn Problem: Sulfide Stress CrackingèSOLUTION: Apply requirements of NACE MR0103 where
necessary
n Problem: Ammonium chloride, Ammonium bisulfide, wet H2S onREAC, piping and accumulator and reflux pump. Erosion corrosionon pumpèSOLUTION:l Use intermittent wash water on REAC and upgrade material to Ti.l Use SS pipe (304 or 316 if chloride are expected) Maintain stream
velocity below 15 m/s on piping.l Austenitic 316 may be good to clad accumulatorl Use Hastelloy C (or alloy 20) on reflux pump to withstand corrosion
and erosion-corrosion
n Problem: Hydrogen damage on feed surge drumèSOLUTION: Use CS HIC resistant or CS SS cladded or lined CS
(+CP)
SOUR WATER STRIPPER
147SOUR WATER STRIPPER
METALLURGYn Problem: Wet H2S Corrosion, Acid gas on tube bottom feed
exchanger, stripping column upper portion and inlet (after valve)èSOLUTION: Use solid SS (304 or 316 if chloride are expected) or
cladding solution.
n Problem: Galvanic corrosion exchanger, stripping column upperportion and inlet (after valve)èSOLUTION: Use solid SS (304 or 316 if chloride are expected) or
cladding solution.
n Problem: Ammonium chloride, Ammonium bisulfide, wet H2SErosion corrosion on charge sour water pumpèSOLUTION:l Use Duplex or Superduplex SS to withstand corrosion and erosion-
corrosion
148SOUR WATER STRIPPER - MSD
NOTE: The indicated selection is not a guideline; it indicates only apossible choice among several solutions as a function of processconditions, corrosion mechanisms involved, lifetime and Prj requirements