Material Selection & Corrosion Control.pdf

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CORROSION MECHANISMS

MATERIAL SELECTION AND

CORROSION CONTROL

IN REFINERY

Flavio Cifà

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“

Corrosion is defined as the destruction or

deterioration of a material because of

reaction with its environment”

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n

CORROSION AND DEGRADATION MECHANISMS

èCORROSION PROCESSES KINETIC

èLOW TEMPERATURE DEGRADATION MECHANISMS

l GENERAL CORROSION

– CO₂corrosion

– Wet hydrogen sulfide corrosion

l GALVANIC CORROSION

l PITTING CORROSION

l CREVICE CORROSION

l UNDER DEPOSIT CORROSION

l 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-CO₂systems

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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 CORROSION

l MICROBIOLOGICALLY INDUCED CORROSION

l CORROSION UNDER INSULATION

l HYDROGEN DAMAGE

èHIGH TEMPERATURE CORROSION MECHANISMS

l NAPHTENIC ACID CORROSION

l HIGH TEMPERATURE OXIDATION

l SULFIDATION

l HIGH TEMPERATURE HYDROGEN DAMAGE

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5 CONTENTS

n

MATERIALS AND CORROSION PROTECTION

è MATERIAL SELECTION GUIDELINE

è CARBON STEEL

è LOW ALLOYED STEELS è STAINLESS STEELS è COPPER ALLOYS è NICKEL ALLOYS è TITANIUM ALLOYS è POLIMERIC MATERIALS è CATHODIC PROTECTION

n

MATERIAL SELECTION AND CORROSION CONTROL

IN REFINERY UNITS

è DESALTER

è ATMOSPHERIC DISTILLATION UNIT è VACUUM DISTILLATION UNIT

è AMINE UNIT

è HYDRODESULPHURIZATION UNIT è SOUR WATER STRIPPER UNIT

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CORROSION AND DEGRADATION

MECHANISMS

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n STATIONARY KINETICS

Steady 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 KINETICS

It presents an incubation period which closes with 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 > t_i) continues up to the rupture independently from the incubation conditions persistence.

• Whenever “upset conditions” are decisive for the described corrosion mechanism they will be clearly highlighted with

CORROSION KINETICS R _corr Time t_i Stationary Incubation period UPSET

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8 LOW/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

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CORROSION AND DEGRADATION

MECHANISMS

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LOW TEMPERATURE DEGRADATION MECHANISMS

n

n GENERAL CORROSIONGENERAL CORROSION

n

n GALVANIC CORROSIONGALVANIC CORROSION

n

n PITTING CORROSIONPITTING CORROSION

n

n CREVICE CORROSIONCREVICE CORROSION

n

n UNDER DEPOSIT CORROSIONUNDER DEPOSIT CORROSION

n

n STRESS CORROSION CRACKINGSTRESS CORROSION CRACKING

n

n SENSITIZATION AND WELD DECAY CORROSIONSENSITIZATION AND WELD DECAY CORROSION (INTEGRANULAR CORROSION)

(INTEGRANULAR CORROSION)

n

n EROSION CORROSIONEROSION CORROSION

n

n MICROBIOLOGICALLY INDUCED CORROSIONMICROBIOLOGICALLY INDUCED CORROSION

n

n CORROSION UNDER INSULATIONCORROSION UNDER INSULATION

n

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11 GENERALIZED CORROSION AT LOW TEMPERATURE

n ANODE “location where metal dissolution takes place (i.e. Fe→Fe→ 2+_)”

n CATHODE: “location where O₂, H+

or metal reduction takes place (i.e. Fe3+→ Fe→ 2+_)”

n No specific location for anode and cathode

n Anode and cathode move with time

n Can be monitored, measured and predicted

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Can be uniform or not CONTROL:

n Select proper metallurgy

n Corrosion allowance (function of corrosion rate and required lifetime)

n Inhibitor

n Cathodic Protection

n Monitoring

Some metal-environment combinations known to results in general corrosion: CS - dilute mineral acid

CS - CO₂ and/or H₂S in aqueous phase CS - seawater

SS - organic acid at high T (i.e. 100- 200 °C) Ti - concentrated sulfuric acid

Corrosion rate of various alloys in boiling mixtures of 50% acetic acid and varying proportions of formic acid. Test time 1+3+3 days. (by SANDVIK)

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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 software

It’s a function of:

n P_CO2

n Temperature

n System Fluid dynamics (influences scale stability )

n Presence of H₂S and/or organic acid

n O₂content

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)

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14 GENERALIZED CORROSION AT LOW TEMPERATURE - H₂S

Another example of generalized corrosion at low temperature on carbon steel is Wet Hydrogen Sulphide corrosion. (Note: includes also risk of SSC and hydrogen damage).

It requires a presence of aqueous phase and it’s due to the low pH and to the reaction between S and Fe (formation of FeS scale)

The stability of FeS scale is influenced by pH and presence of contaminants (i.e. CN-₎

The temperature rise increase CR

CR is hardly predictable _NOTE_{: CR is influenced also by pH, fine}

metal composition, presence of contaminants (i.e. CN), etc... (by NACE)

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H (atomic) can diffuse into the metal causing:

n cracking

n blistering

n embrittlement (see also SSC and Hydrogen damage)

CONTROL: Wet H₂S general corrosion can be controlled with CS + CA up to corrosion rate (CR) 0.6mm/y. For higher CR upgrade metallurgy to SS

The phenomenology related to hydrogen attack are taken into account requiring HIC resistant specs (composition + test NACE TM 0284).

Note: consider as valid alternative SS cladding instead of CS HIC resistant

GENERALIZED CORROSION AT LOW TEMPERATURE - H₂S

Graph by UOP

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17 GALVANIC CORROSION

n Preferential corrosion of one metal of two or more

electrically connected dissimilar

n It requires an aqueous environment which is

corrosive to at least one metal and with a non negligible

conductivity

n It’s related to the ∆∆V between the metals in the considered environment (i.e. see galvanic series in seawater).

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ALL the following parameter have to be verified to evaluate risk of galvanic 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’t worry 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 a problem

n Verify the fluid conductivity:

èif 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 on CS flange)

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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 continuity

n Cathodic Protection

Metal coupling that can generate GC (the first is attached):

CS-SS CS-Copper alloy CS-Ti

CS-Hastelloy SS-Ti SS - Hastelloy

GALVANIC CORROSION

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23 PITTING CORROSION

n PITTING: form of extremely

localized attack that results in hole in the metal. One of the most

dangerous and insidious form of corrosion.

n It causes equipment to fail because of perforation with only a small

weight loss

n Normally occurs in active/passive metals (i.e. SS series 300) in

passive state

n Requires depassivating species (i.e. chloride or other halides )

n Worse problem at low velocity and high T

n Hard to detect and/or predict

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24 CONTROL: n avoid metal/environment combination susceptible to pitting n check environmental conditions especially è [Cl-] o [X-] è Temperature è O₂

è Minimum fluid velocity A parameter to evaluate pitting resistance of SS is PREN (pitting resistance equivalents number):

PREN = Cr + 3.3 Mo + 16N

Critical pitting temperatures (CPT) for SAF 2205, AISI 304 and AISI 316 at varying concentrations of sodium chloride (potentiostatic determination at +300 mV SCE), pH»6.0 (by SANDVIK)

PITTING CORROSION

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Examples of metals susceptible to pitting in chlorides

environment:

n SS (Ferritic, Austenitic, Duplex)

n Fe-Ni-Cr Alloy (Incoloy)

n Aluminum Alloy

n Copper Alloy

Immune Very resistant Resistant Acceptable Not Acceptable

Ti 90/10 Cu/Ni 70/30 Cu/Ni Monel SS series 400

Alloy C Admiralty brass Tin 316 (+ CP) 304

Alloy 625 Al bronze Alloy 825 Nickel

Alloy 20

Pitting resistance in seawater

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Selective corrosion in crevice

n CC requires a stagnant zone where it’s possible to develop different conditions from bulk (inhibitor, oxygen, pH, Cl-₎

n CC requires an aggressive environment (i.e. presence of chloride)

n If temperature ↑↑ crevice likelihood ↑↑

CREVICE CORROSION

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29 CREVICE CORROSION

CONTROL:

n Use materials less sensitive to pitting (the corrosion mechanisms are similar therefore a material resistant to pitting corrosion is also resistant to crevice corrosion. See slide 99)

n avoid stagnant zone

n don’t use threaded connections

n control O₂content

Some materials susceptible to CC:

èSS

èNi alloy

èTi

Preferentially locations for CC:

n Flanged connection

n Tube/Tubesheet connection

n Threaded connections

n Plate Heat Exchangers

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32 UNDER DEPOSIT CORROSION

Corrosion enhanced by the presence of scales (can be aggressive i.e NH₄Cl or not)

Under deposit corrosion results from difference between local and bulk environment (i.e oxygen, pH, presence of aggressive ions Cl. See also crevice and pitting corrosion) If chloride are present H+ “drawn” under deposits (pH drops below 4 increasing corrosion rates)

Most common materials are

subjected to UDC including CS, austenitic SS, nickel alloy (Inconel 625, Hastelloy and Ti are very

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33 UNDER DEPOSIT CORROSION

Refinery examples:

any location in which scaling and/or fouling occur especially if chloride or oxygen are present

CONTROL TECHNIQUES

n treat the source of the problem (i.e. corrosion or fouling)

n design equipment to minimize deposition. Metallurgy may solve

corrosion problem but not performance loss

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AMMONIUM BISULFIDE

n frost from gas to solid at a temperature depending on NH₃ H₂S concentration

n Is corrosive vs CS but not vs SS or higher alloy

n Causes very rapid fouling Refinery examples

n REACs (hydrotreaters/hydrocrackers)

n Crude unit overhead

n FCC (overhead in separator section) CONTROL

n wash water

èuse continuous washing (20%min water not vaporized)

èinject upstream of ammonium bisulfide dew point

n Use balanced piping for REACs

n Upgrade metallurgy

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AMMONIUM CHLORIDE

n frost from gas to solid at a temperature depending on NH₃ HCl concentration

n Is corrosive vs CS and SS. Ti and Inconel 625 may offer sufficient protection

n Causes very rapid fouling REFINERY EXAMPLES

n Crude unit overhead

n hydrotreaters (REACs, overhead in separator section)

n Catalytic reformer (REAC, separator, stabilizer, recycle gas compressor)

n FCC (overhead in separator section) CONTROL

n wash water

èuse continuous washing (20%min water not vaporized)

èinject upstream of ammonium chloride dew point

n Use balanced piping for REACs

n Upgrade metallurgy (expensive solution)

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36 STRESS 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)

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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

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39 CHLORIDE 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 316 stainless steel chemical processing piping system. Chloride stress corrosion cracking in austenitic stainless steel is characterized by the multi-branched "lightning bolt" transgranular crack pattern. (Mag: 300X)

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40 CHLORIDE STRESS CORROSION CRACKING

SS serie 300

CONTROL:

n limit O₂content

n limit stress (∃∃ threshold value)

n Control temperature

n Control pH

N.B. H₂S lowers CSCC limits

SCC resistance in oxygen-bearing (abt. 8 ppm) neutral chloride solutions. Testing time 1000 hours. Applied stress equal to proof strength at testing temperature. (by SANDVIK)

For SCC Ni content is fundamental

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41 SULPHIDE STRESS CRACKING

H₂S SSC Cracks in a 17-4 pH stainless steel

SSC is defined as cracking of a metal under the combined action of tensile stress and corrosion in the presence of water and H₂S SSC is a form of hydrogen stress cracking resulting from absorption of atomic hydrogen that is produced by the sulfide corrosion

reaction on the metal surface SSC is influenced by:

n Chemical composition (P,S,Mn), hardness, metal thermal treatment

nTotal 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 dissociation and H diffusion)

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Some environmental conditions known to cause SSC are those containing free water (in liquid phase) and:

n >50 ppmw dissolved H₂S in the free water or

n free water pH<4 and some dissolved H₂S present or

n free water pH>7,6 and 20ppmw dissolved HCN in the water and some dissolved H₂S present

n >0.0003 MPa absolute partial pressure H₂S in the gas in processes with a gas phase

CONTROL:

For Refinery apply NACE MR0103

For upstream (oil and gas production) apply NACE MR0175

Note: Pay attention to thermodynamic model used in the simulators and to hypothesis to calculate % H₂S in free water

SULPHIDE STRESS CRACKING

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43 ALKALINE CRACKING

cracking in caustic environment carbonate cracking

cracking in amine environment Main materials involved:

n Carbon steel

n Low alloy steel

n Stainless steel

n Copper alloy

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Cracking due to

exposition of CS to hot caustic solution (i.e. NaOH, KOH)

CONTROL: use the materials indicated on Caustic Soda Service Graph (see also SR) by NACE

Note: If for the service austenitic SS has been specified, check

chloride concentration and T max.

CAUSTIC CRACKING

UPSET

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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 to

cause cracking.

MEA is more aggressive than DEA o MDEA

If temperature ↑↑ cracking likelihood ↑↑ (consider also short term condition, 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 T

nDEA: T > 60°C

nMDEA : T> 82°C

AMINE CRACKING

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51 CRACKING IN CO-CO₂-H₂O SYSTEMS

It can happen in pressure system with the simultaneous presence of CO-CO₂-H₂O

n low T (maximum risk in the range 20-60°C)

n minimum CO and CO₂ pressure required

CONTROL: Check environmental conditions (T, water, P_CO & P_CO2) Use SS (12 Cr o 304; 316 not necessary)

R a n g e o f S C C s u s c e p t i b i l i t y 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 C O 2 p a r t i a l p r e s s u r e ( k P a )

CO partial pressure (kPa)

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52 SENSITIZATION ISSUES (INTERGRANULAR CORROSION)

Main degradation forms related:

n Sensitization

n Weld decay

n Knife line attack

n Polythionic acid stress corrosion cracking

MECHANISM:

1) A high temperature exposure allows the reaction between Cr and C.

2) Cr carbides precipitates at grain boundaries.

3) Cr depletion in areas surrounding to grain boundaries. (when Cr

below 12% the steel is no more SS and corrode like CS).

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53 SENSITIZATION AND WELD DECAY

Sensitization

n 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 decay

n The Cr depletion is related to the

heating in areas surrounding the weld.

n Varies with welding conditions

n varies with distance form the weld Knife line attack

n Same mechanism of weld decay

n on chemically stabilized material

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54 SENSITIZATION CONTROL TECHNIQUES

Sensitization control

n Materials selection:

ènormal and high carbon grades: Carbon content 0,03 % - 0,10

l Ferrous (i.e. 304/316) and Ni-Cr alloys

Subjected to sensitization.

èlow 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) avoids

sensitization over long term exposure.

l Stabilization heat treatment should be recommended

n Procedure mistakes

èCleaning with oily rag before welding introduces C

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55 INTERGRANULAR CORROSION

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Intergranular corrosion and cracking caused by the simultaneous presence of:

n Sulfide scale

n Sensitized material

n Oxygen

n Stress (residual or applied)

n Water

n Polythionic Acids (H₂S_xO_y) form

(usually during shut down) for reaction of sulfide scale with H₂O e O₂

Main material subjected to sensitization:

n Austenitic or Ni alloy (also low carbon or 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 of type 310 stainless steel. The item was exposed to sulfur containing natural gas in a continuous flare

POLYTHIONIC ACID STRESS CORROSION CRACKING (PASCC)

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57 POLYTHIONIC ACID STRESS CORROSION CRACKING (PASCC)

n hydrodesulfurizers

n hydrocrackers

n hydrogen reformers

n FCC

n Fired heaters (both external and internal) CONTROL: follow guideline NACE RP0170

n Exclusion of oxygen (air) and water by using a dry nitrogen purge

n Alkaline washing with soda ash. Avoid washing of zone that can’t be drained

n Exclusion of water by using a dry purge with a dew point lower than -15°C

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58 CORROSION UNDER INSULATION

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Atomic hydrogen even produced by low temperature corrosion phenomena may diffuse through metal surface causing hydrogen damage.

Hydrogen damage is recognized under various forms:

n Blistering

n Hydrogen Induced Cracking (HIC)

n Stress Oriented Hydrogen Induced Cracking (SOHIC)

n Hydrogen Embrittlement

n High Temperature Hydrogen Attack

HYDROGEN DAMAGE

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60 BLISTERING-HIC-SOHIC

UPSET

Main steps of blistering and HIC

n Atomic hydrogen diffuses inside the metal bulk

n Inside the metal atomic hydrogen meets the voids (rolling defects) and inclusions (MnS) and re-combines in molecular hydrogen (H₂)

n Gradually, molecular hydrogen collected in voids and inclusions increases the pressure reaching up to 10 GPa.

n The elevated pressure evidenced by surface blistering may lead to local (stepwise) and complete rupture of the plate.

n SOHIC is related to residual stresses presents in the metal.

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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 according standards. (NACE MR 0175, NACE 0103, NACE TM0284, API 945)

BLISTERING-HIC-SOHIC

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62 BLISTERING-HIC-SOHIC

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Embrittlement caused by the hydrogen diffusion through the metals.

Possible Hydrogen sources:

n General corrosion n Galvanic corrosion n Overprotection of cathodic protection. Influencing factors: n Enhanced by CN, As, Sb presence.

n May occur on CS, alloyed steels, nickel alloys, Titanium (T > 71° C) Copper alloys are considered

immune

HYDROGEN EMBRITTLEMENT

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64 HYDROGEN DAMAGE

Critical areas:

è“Rich” section of amine units.

èSour water stripper.

èHydrodesolfurization units.

èFCC units.

Hydrogen damage control:

n Appropriate material selection

èReduction 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.

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65 EROSION-CORROSION

♦ Degradation mechanism

accelerated by flow conditions of a corrosive fluid in contact with metal surface

♦ Mechanism:

Corrosive fluid reacting with metal creates a film scale

Fluid removes mechanically the scale exposing uncorroded metal

Material 1ft/sec 4 ft/sec 27 ft/sec

CS 34 72 254

Ad. Brass 2 20 170

70-30 Cu Ni (0.05% Fe) 2 - 199 70-30 Cu Ni (0.5% Fe) < 1 <1 39

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Factors influencing erosion-corrosion:

n Velocity and fluid turbulence

n Temperature

n Multiphase flow

n Suspended solid

n Galvanic effect (i.e.: CS-SS and CS-CuNi in seawater)

CONTROL:

n Material Selection or lining (i.e. 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).

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69 MICROBIOLOGICAL INDUCED CORROSION (MIC)

MIC refers to corrosion

influenced by the presence and activities of

microorganisms and/or their metabolites

Microorganism (i.e. fungi, bacteria or algae) can be aerobic 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)

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70 MICROBIOLOGICAL INDUCED CORROSION (MIC)

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

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CORROSION AND DEGRADATION

MECHANISMS

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HIGH TEMPERATURE CORROSION MECHANISMS

n

n NAPHTENIC ACID CORROSIONNAPHTENIC ACID CORROSION

n

n HIGH TEMPERATURE OXIDATIONHIGH TEMPERATURE OXIDATION

n

n SULFIDATIONSULFIDATION

n

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74 NAPHTENIC ACID CORROSION

Generalized corrosion at high T (230-400 °C) caused by naphtenic acids for crude with TAN > 0.5 (ASTM D 974 T.A.N. as mg KOH/g) or TAN > 0.35 for some licensor.

∃∃ several type of naphtenic acids

Naphtenic acids are very aggressive especially close to their boiling points (thus can attack selectively some locations of the unit )

n Metallurgy: CS and Cr alloy (i.e. 5 - 9 - 12Cr o 304/316 std) are readily attacked

n Sulfur content: especially at low fluid velocity, sulfur can mitigate corrosive attack

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n Heaters and Transfer Line in CDU

n Diesel section of CDU column (pump-around)

n Atmospheric column residue

n Vacuum column residue

n Gas oil section of VDU CONTROL:

n N.B. Check TAN for each cut with operating temperature in the range 260 - 400°C

n Stainless steel 317 o 316 with Mo 2.5%min

n Monitoring + inhibitor (only for short run)

n Use blending to reduce TAN

n Neutralization with NaOH (pay attention on caustic embrittlement)

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Microstructure of iron oxides formed on iron by high-temperature oxidation in air

Generalized corrosion caused by direct oxidation of base material (liquid water not required)

Oxidation issues

n O₂Concentration

n Alloy composition

n Metal temperature

The source of O₂ can be also steam or CO₂

The scale composition influences CR

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77 Refinery examples: n Heaters n Boilers CONTROL: n Improve metallurgy (with alloy containing Cr, Ni, Si, Al)

Control environmental conditions, especially:

n Sulfur (Increase corrosion rate)

n metals (i.e. V which causes V₂O₅formation) in fuel

n Temperature (if scale ↑↑ thermal exchange↓↓ and lifetime↓↓)

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78 SULPHIDATION

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)

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 low chrome alloy are used with %Cr higher and higher (1.25-2.25-5-7-9 Cr) up to stainless steel (as 12Cr like 405 and 410) or austenitic (304 or 316)

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CONTROL:

Use appropriate metallurgy considering CR calculated by available curves (function of metal T, alloy composition and %S for Mc Conomy or H₂S for Couper Gorman)

n Mc Conomy (API) based on total sulfur content:

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80 SULPHIDATION

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)

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81 HIGH TEMPERATURE HYDROGEN ATTACK (HTHA)

High temperature hydrogen can attack steels in two ways :

n Surface decarburization (slight, localized reduction in strength and hardness and an increase in ductility)

n Internal decarburization and fissuring (CH₄ formation and high localized

stresses which lead to the formation of fissures, cracks or blister in the steel) Factors influencing HTHA:

n Temperature

n H₂pressure

n Stress (i.e. welds)

n Time (∃∃ incubation period)

Hydrogen attack corrosion and cracking on the ID of an 1800 psig carbon steel boiler tube.

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82

CONTROL:

Use Cr-Mo alloy instead of CS (reduces the 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 in H₂ service

èCladding should not be considered as material resistant to HTHA (therefore also base material have to be resistant)

Solids deposition and hydrogen attack corrosion at the ID weld in an 1800 psig carbon steel boiler tube. The arrow marks the direction of flow. (~1X)

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83 HIGH TEMPERATURE HYDROGEN ATTACK (HTHA)

N.B. Add safety margin, below the relevant curve,when selecting steels (11 °C min) Nelson curves (API 941)

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84 STATISTIC RELEVANCE OF CORROSION FAILURES

Types of corrosion failures (duPont)

General 27% Erosion-corrosion 7% Corrosion fatigue 3% Intergranular 10% Pitting 14% Weld corrosion 5% Stress Corr. Cracking 24% others corrosion 6% High temperature corrosion 2% Crevice 2% nCorrosion causes the 55%

of the failures in chemical plants (the remaining 45% of the failures are related to mechanical reasons).

n General corrosion and SCC show the higher occurrence in corrosion failures (in sum they account 51%).

n Crevice corrosion causes only 2% of the failures while pitting the 14%.

nThe sum of Intergranular and weld corrosion is relevant (15%).

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“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.”

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MATERIAL SELECTION AND

CORROSION CONTROL

Selection criteria, material properties

and cathodic protection

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87 MATERIALS AND CORROSION PROTECTION

n

n CONDITION ASSESMENT AND MATERIAL SELECTIONCONDITION ASSESMENT AND MATERIAL SELECTION

n

n CARBON STEELCARBON STEEL

n

n LOW ALLOYED STEELSLOW ALLOYED STEELS

n

n STAINLESS STEELSSTAINLESS STEELS

n

n COPPER ALLOYSCOPPER ALLOYS

n

n NICKEL ALLOYSNICKEL ALLOYS

n

n TITANIUM ALLOYSTITANIUM ALLOYS

n

n POLIMERIC MAERIALSPOLIMERIC MAERIALS

n

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88 MATERIAL SELECTION

Ver. corrosion protection measures i.e. CP Control galvanic corrosion

Control erosion corrosion On Stream Inspection

4. Engineering, Procurement, Construction

Costs decrease

Improve the reliability of the unit 3. Process and

material optimization

Ensure the required service life time 2. Material selection

Conditions assessment 1. Process

development

Scope of corrosion activities Phase sequence

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89 1. CONDITIONS ASSESSMENT Conditions Chemical composition Thermodynamic Physical

Time extension Probability TDS & TSS Environment type (water/oil content) Contaminants and corrodents Cl-, H₂S, CN-_{, NH} 3 … Chemical composition Thermodynamic Upset conditions Physical Temperature (local) Fluid dynamic Pressure Condensation and dew point

(local) Solid Precipitation Phase settling Oxidizers O₂, Cl₂, Fe3+_{, Cu}2+_… External conditions

Fire hazard Marine environment

Underground

Atmospheric env. Thermal insulation

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90 2. MATERIAL SELECTION Material Joining techniques Pre-fabrication dimensions Galvanic couplings

Material sel. in similar service within the prj

Heat treating

Procurement time

Fittings experience in similar units

Density Corrosion allowance Strength Corrosion protection Experience and literature Metallurgy Availability Fabricability Costs Spare parts Construction

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91 3. PROCESS AND MATERIAL OPTIMIZATION

ØExam of the whole unit

ØScope

Decrease the project costs

Avoid over and under specification Improve the reliability of the unit

Process development Conditions assessment Material selection Process Engineer Corrosion Engineer

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92 CARBON STEEL

n The material

Chemical composition based on Fe and C, can be adjusted to

improve the resistance to specific degradation mechanism (i.e. HIC)

n Typical conditions:

By far the most common material used up to 400°C in refineries due primarily to a combination of

strength, 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, under deposit, under insulation, galvanic attack, hydrogen damage, erosion corrosion, high temperature damage (almost all).

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93 CARBON 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, boiling water).

n Cathodic protection to control general, galvanic, MIC and crevice corrosion.

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 control galvanic corrosion.

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94 LOW ALLOYED STEEL

The materials:

n Typical conditions: For high temperature

service, or hydrogen and sulfidant atmosphere.

n Present the same contaminants and corrodents of Carbon steel.

As per CS. Specifically Hydrogen high temperature damage and high temperature sulfidation.

n Corrosion prevention measures:

Design according Nelson diagram, Couper Gorman and Mc

Conomy to realize correct selection and evaluation of corrosion allowance. 650 5% Cr 0,5% Mo 650 9% Cr 1% Mo 625 2,25% Cr 1% Mo 600 1,25% Cr 0,5% Mo 600 1% Cr 0,5% Mo 500 0,5% Mo Max Temperature °C Chemical Composition

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95

n The materials:

n Typical conditions:

Acidic and saline water, high temperature and low temperature, waste water,

demi water, organic acids.

Halides (chlorides), hydroxides (wet and dry), sulfurous acid (on austenitic), organic acids, Hydrogen sulfide and (by external

side) Vanadium, molten zinc and molten aluminum.

STAINLESS STEEL Super Austenitic S31254 (254 SMo) 20 Cr 18 Ni 6 Mo Cu N Nickel alloy (Al-6X) 20 Cr 24 Ni 6,5 Mo Super Duplex S32750 (2507) 25 Cr 7 Ni Mo N Duplex S31803 (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

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96 STAINLESS STEEL

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 alloys in 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.

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97 FERRITIC 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 for

standard trim on process valves, pump impellers, vessel trays, tray components and exchanger tubes.

Corrosion resistance

èexcellent resistance to sulfur at high temperature.

ègood resistance to hydrogen sulfide at low concentrations and intermediate temperatures.

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98 AUSTENITIC STAINLESS STEEL

Variables influencing the behavior of austenitic stainless steels in salted water:

n Temperature:

50° C is accepted as the minimum temperature for the

occurrence of stress corrosion cracking and pitting in slightly salted water (100-200 ppm).

n Chloride content:

In stress relieved structures, the maximum allowed chloride content to avoid pitting and crevice (below 50°C) is related to the alloy

Type

Cl-304 100 ppm 316 300 ppm

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99 AUSTENITIC STAINLESS STEEL

n Metallurgy

selection is realized considering critical temperature which is the minimum temperature at which pitting or crevice may

occur in ferric chloride solution.

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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.

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101 COPPER ALLOYS

nMain degradation mechanisms

Erosion corrosion and

impingement attack, stress corrosion cracking (in

presence of 1 ppm of

ammonia), selective leaching (Immune to hydrogen

damage, and prevent biofouling)

nCorrosion protection measures

correct design according standards (BS MA18 in the graph).

Check ammonia presence (UPSET conditions)

Erosion ferrules (in Teflon or special Cu Ni alloys Cr

modified)

Maximum seawater velocities for continuos flow conditions m/sec (ref.:BS MA 18)

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102 TITANIUM ALLOYS

The materials

n Titanium is a reactive metal and as the other materials of the group forms spontaneously a superficial oxide film which ensure protection from the environment.

n The corrosion resistance is related to the stability and the continuity of the oxide layer (on-off corrosion behavior).

n The reactive metal group is formed by (increasing by

corrosion resistance): Titanium, Zirconium, Niobium and

Tantalum. The corrosion behavior of these materials shows a large amount of similarities.

ASTM grade Composition

Gr 1,2,3,4 unalloyed (O and N content) Gr 7, 11 0.2 Pd

Gr 12 0.8 Ni 0.3 Mo Gr 16, 17 0.04 Pd

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n In which conditions:

Seawater and desalinization plant, organic acid, in oxidizing and mildly reducing wet environments.

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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 verified

General corrosion, pitting, crevice, SCC, catastrophic oxidation, galvanic*, hydrogen embrittlement.

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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 resistance as the bulk material.

2) Like all reactive metals at high temperature reacts strongly with atmospheric oxygen.

3) Can be welded with GTAW or GMAW (same equipment used for SS 316 or nickel alloys).

4) Argon or helium have to be be used to protect the weld in welding chamber (shop) or welding shoes (construction site).

5) The weld quality verified easily for acceptance

• Visual examination of “as weld” surface

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106 NICKEL ALLOYS

n Materials

n Advantages:

èVery resistant (as a function of specified type) to many environments

èIn 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

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107 NICKEL ALLOYS

TYPICAL ENVIRONMENTS

n Hastelloy C/C276, Inconel 625

èHigh resistance to acid (both oxidizing and reducing)

èexcellent resistance in chloride and/or H₂S environment

èHigh resistance vs underdeposit corrosion

n Inconel 601, Incoloy 800

èHigh temperature resistance

n Incoloy 825

èHigh resistance in chloride and/or H₂S environment (lower than Hastelloy C-Inconel 625)

èHigh resistance vs underdeposit corrosion (but can fail with NH₄Cl)

n Monel

èHigh resistance to hot alkalis

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108 POLYMERIC MATERIALS

n High molecular weight organic materials that can be formed into useful shapes.

n Can be used for piping and equipment (thermosetters and thermoplastics) or for gaskets (elastomers)

Thermoplastics

PE

PTFE

PVC

Thermosetters

Glass fiber epoxy resin

Glass fiber vynil ester ep. resin

Glass fiber Poly ester ep. resin

Elastomers

Viton (Flueelastomers)

Kalrez (perfluoelestomers)

NBR

Polymeric materials

In refinery

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109 THERMOPLASTICS

n Are characterized by the softening with the increase of temperature and return to their original hardness when cooled (most are weldable).

n Degradation mechanisms are different from metals:

Swelling, softening, loss of mechanical properties, hardening and discoloration (no electrochemical mechanisms

involved). Degradation may be caused by heat, solar exposure and UV.

n For correct material selection and design are necessary: life time, temperature (!), environment and pressure.

n Main couple material-environment are: PE(or PP)-water, PVC-mineral acids, PVDF-acids (at higher pressure and temperature). Main Materials: Polyethylene (PE) Polypropylene (PP) Polyvinyl chloride (PVC-CPVC) Polyvinylidene Fluoride (PVDF) Teflon (PTFE)

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110 THERMOPLASTICS

n Advantages

èExcellent chemical resistance to water environment,

l PTFE can withstand practically all

refinery environments below 200°C

èEasy welding and installation (not for all)

èNo protection required in underground service

n Disadvantages

èRapid decrease of properties with the temperature increase.

èChemical resistance to hydrocarbons

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111 THERMOSETTERS

n Are characterized by the thermal degradation when exposed to heating.

n Thermosetters are generally used as matrix for composite material. Glass is generally used as fiber.

n Same degradation mechanism of thermoplastic:

Swelling, softening, loss of

mechanical properties, hardening and discoloration. Higher

resistance than thermoplastics.

Main matrix Materials: Epoxy resin

Vinyl ester epoxy resin Phenolic resin

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112 THERMOSETTERS

Main applications are:

Firewater, cooling water, high

pressure water lines (special types up to 280 Bar), sewer.

Advantages

èExcellent chemical resistance to aqueous environment

èNo protection required in underground service

Disadvantages

èInstallation difficulties

èDesign and installation know how

èNot suitable in fire hazard area

èSensitivity to vibrations and mechanical stresses

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113 CATHODIC PROTECTION

n History

In 1824 Sir Humphrey Davy discovered that is possible to protect the copper of royal ships from marine corrosion by electrically coupling it with iron.

n Basic Principle

The metal dissolution is reduced trough the application of cathodic current 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 material upgrade.

è Minimize the cost of CS coating maintenance.

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Cathodic protection techniques

n Sacrificial cathodic protection

èUse of anodic metal

l Magnesium (t< 40°C)

l Zinc (t < 40°C)

l Aluminum

(Cl-_{> 1000 ppm or t > 40°C)}

èAnode connection with cathode

l direct (economical)

l trough a electrical resistance

(improve the control and avoid under and over protection)

èReference electrodes

l Allows monitoring and verification

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Cathodic protection techniques

n Impressed current cathodic protection

èanode material

l Ti Mixed metal oxide coated

l High silicon iron

l Ceramic electrodes

ècurrent generation

l an external DC current

source is necessary

èreference electrodes

l the use is mandatory in

conjunction with current control system

(116)

n Design parameters

èTemperature (important for anode selection)

èpH

èChemical 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)

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n Typical applications

èUnderground and submerged steel surfaces (may be required by law).

l Bottom tanks

l Underground and submerged Pipelines

l Jacket on offshore structures

l underground and submerged steel reinforced concrete

structures

èLow temperature corrosion on the process side (cost evaluation).

l Water tanks

is preferable to cathodically protect internally lined surfaces

l Water boxes (channels) of Thermal exchangers

CP avoids cladding in Cu-Ni alloys in seawater exchangers

l Water-oil separators

CP avoids the use of stainless steel or ensure lower maintenance of internal lining

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MATERIAL SELECTION AND

CORROSION CONTROL IN

REFINERY UNITS

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119

MATERIAL SELECTION AND CORROSION CONTROL IN REFINERY UNITS

n

n DESALTERDESALTER

n

n ATMOSPHERIC DISTILLATION UNITATMOSPHERIC DISTILLATION UNIT

n

n VACUUM DISTILLATION UNITVACUUM DISTILLATION UNIT

n

n AMINE UNITAMINE UNIT

n

n HYDRODESULPHURIZATION UNITHYDRODESULPHURIZATION UNIT

n

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120 DESALTER

THE DESALTER CAN BE THE SOURCE OR THE SOLUTION OF REFINERY’S PROBLEMS

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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)

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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) èLevel

n TARGET: DESALT TO LESS THAN 2 LBS/THOUSAND BARREL (PTB) n Stripped Water should be used as wash water

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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)

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124 ATMOSPHERIC DISTILLATION UNIT

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125 ATMOSPHERIC DISTILLATION UNIT

TYPICAL CORROSION AND FOULING 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)

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126

METALLURGY

Use Chrome alloy (solid or lining for high Cr %) for sulfur

resistance (according to McConomy curves) es. 1.25 Cr, 2.25Cr, 5 Cr, 9Cr, 12Cr in the bottom section of CDU tower and in the hot side of the heating train

n Use Monel for HCl resistance in the top section of tower (for cladding and trays) and in the OVHD accumulator if

condensation is expected

n Use 90-10 Cu-Ni for Chloride resistance in the desalter brine

n 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 Mo

èMust guard against PASCC of austenitic SS

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127 ATMOSPHERIC DISTILLATION UNIT - MSD

NOTE: The indicated selection is not a guideline; it indicates only a possible choice among several solutions as a function of process conditions, corrosion mechanisms involved, lifetime and Prj requirements

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128 ATMOSPHERIC DISTILLATION UNIT - OPERATING GUIDELINES

CAUSTIC INJECTION

n Inject caustic if necessary to reduce chlorides in OVHD or to reduce TAN

èUse fresh 2-3% caustic

èInject no more than 4 PTB

èInject to crude no hotter than 150 °C

èInject at least 5 feet upstream of equipment

èand as close to desalter downstream as possible

èInject using a quill TAIL WATER pH

n Operate between pH 5.5- 6.5 in tail water

n Use a online pH meter

n Automate control of corrosion inhibitor injection

n Keep pH meter clean (filming amine, used as inhibitor, can dirty the instrument)