Download - Corrosion Mechanisms - Material Selection and Corrosion Control (Technip Italy)

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


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


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

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

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


Table from ASM Vol 13 Corrosion


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

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

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


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.


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


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


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


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


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


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 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


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 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


Download - Corrosion Mechanisms - Material Selection and Corrosion Control (Technip Italy)

Top Related