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Boiler Corrosion IssuesBoiler Corrosion Issues

Harmen BouwmanHarmen Bouwman

Steam system a thin layer of magnetite Steam system … a thin layer of magnetite …

HP t

Attemporator Condensate

HP steam

Steam drumSuperheater

TurbineSuperheater

SteamDeaeratorEvaporatorEconomiser

Condensate StackPolishing

Mud tap

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Mud tap

2

Magnetite formationMagnetite formation

Steel is o idi ed b (p re) waterSteel is oxidized by (pure) water

At high temperature

⇒ layer of magnetite = Fe3O4

Protective barrier against further oxidationg

Black

Thin with little reduction of thermal conductivityThin with little reduction of thermal conductivity

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Magnetite formation – reactionsMagnetite formation – reactionsat elevated temperature - passivation:p p

3 Fe + 4 H2O ⇒ Fe3O4 + 4 H2, protective !l / dat low/moderate temperature:

2 Fe + O2 + 2 H2O ⇒ 2 Fe(OH)2 ⇔ FeO

2 Fe(OH)2 + ½ O2 + H2O ⇒ 2 Fe(OH)3 ⇔ Fe2O3

⇒ ensure proper formation & prevent damage !

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Magnetite – DamageMagnetite – Damage

Damage to protective film byDamage to protective film by

oxygenyg

high alkalinity

idiacidity

corrosion fatigue/mechanical stress

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Improper magnetiteImproper magnetite

Visible after1 dayambient exposureambient exposureas brown stains

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A boiler should A boiler should …

generate steam in a clean circuitcircuit

controlled nucleate boiling

efficient heat transfer

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A boiler might A boiler might …

generate steam in a circuit with heavy deposits (waterwith heavy deposits (water side)

l b bbllarge steam bubbles

less efficient heat transferless efficient heat transfer

higher tube temperatures

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Scales and depositsScales and deposits

scale:l ti l l tirelatively selective

precipitation, hard glassy t i lmaterial

deposit:deposit:precipitated elsewhere, transported and depositedtransported and deposited

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Impact of scales and deposits 2Impact of scales and deposits 2

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Failure causesFailure causes

mechanical nature 81% f ilmechanical nature 81%

• long term overheating 15 %

failure causes

long term overheatingcorrosionoverheating 15 %

• short termh ti 66 %

g15%

corrosion19%

overheating 66 %

corrosion 19%f h h d h t tof which due to

feed water 37 %

short term overheating

66%

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Three major problems in BoilersThree major problems in BoilersDepositsC iCorrosionCarry over

In order to avoid these problems BFW and BW must comply to certain specifications, depending on the pressure and the design of the boilerthe design of the boiler.

BFW/BW impurities are usually controlled by a combination ofExternal treatment, i.e. purificationInternal treatment, i.e. counter measures to reduce or eliminate bj i bl ff f iobjectionable effects of contaminants

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l l dTypical Boiler Feed Water IOW’s

Oxygen < 20 ppbConductivity < 0.2 μS/cm

C diti d ith l til lk li h h tConditioned with volatile alkalies or phosphates pH between 9.5 and 11

Non-volatile : Sodium-phosphate & Causticp pVolatile : Ammonia, Morpholine & Hydrazine

Iron < 50 ppbSilica < 20 ppbSilica < 20 ppb

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Boiler Feed Water

Quality requirements for BFW, BW and steam are specified in different countries by internationally known standardization committees and/or organizations e.g.:

• British Standards Institute (BSI)• British Standards Institute (BSI),• Vereinigung der Technische Überwachungs Vereine (VdTUV ) < 68 bar• Vereinigung Grosskraftswerk Betreiber (VGB) > 64 bar Vereinigung Grosskraftswerk Betreiber (VGB) 64 bar

Shell Global Solutions uses the latest recommendations published by EN 12952.

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BFW spec: EN 12952BFW spec: EN 12952 1212BFW spec: EN 12952BFW spec: EN 12952--1212

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Main Boiler Corrosion IssuespHOxygen CorrosionOxygen CorrosionCaustic corrosionCaustic CrackingFlow Assisted CorrosionErosion/Corrosion-ErosionChelant CorrosionChelant CorrosionScalingCorrosion fatiguegOverheating & Stress RuptureCreepFire-Side CorrosionFlue Gas Corrosion (cold-end corrosion)Condensate Corrosion

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Condensate Corrosion

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Causes of corrosionCauses of corrosionpoor pH control oxygen pitting

poor chemicalfeed control

downtime

inadequateblowdowncontrol

poor boileroxygenin-leakage

mechanicalde-aeratorperformance

downtimecorrosion

pfeedwaterquality

condensatepoor external

in leakage

scavenger underfeed

downtimecorrosion

boilercorrosion

contamination treatment

poor boilerfeedwater concentratingfeedwaterquality

condensatecontamination

poorexternal poor chemical

feed/control

concentratingmechanism stressed

area

deposition stress corrosion cracking

treatmentinadequateblowdowncontrol

feed/control embrittling watercharacteristics

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deposition stress corrosion cracking

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Corrosion vs pHCorrosion vs. pH

Corrosion rateCorrosion rate

Safe range8.5 12.7

4 6 8 10 12pH

at 25ºC

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pHpHThe pH is an important factor influencing either scale formation and

i t d i f th t corrosion tendencies of the water. Water with a low pH will result in corrosion of the feed water lines, boiler materials and heat exchangers (Acid corrosion)materials and heat exchangers (Acid corrosion).A high pH may lead to excessive scaling.

Boiler feed water : pH controlled to around 9 - IOWBoiler water : pH between 9.5 and 11- IOW

pH Control:Non-volatile - Sodium phosphate & Caustic for BFW and BWNon volatile Sodium phosphate & Caustic for BFW and BWVolatile - Ammonia, Morpholine, Hydrazine for BFW mainly

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Oxygen corrosionOxygen corrosion

High Oxygen levels destroy the protective Magnetite (Fe O ) by the High Oxygen levels destroy the protective Magnetite (Fe3O4) by the formation of soft Fe2O3.

Localized corrosion (pitting) occurs in oxygen corrosion cells.(p g) yg

Oxygen corrosion can occur throughout the system.Oxygen corrosion is influenced by [O2] pH and temperatureOxygen corrosion is influenced by [O2], pH and temperature

General strategy/approach to mitigate: gy pp gDecrease Oxygen levels by mechanical deaeration and chemical dosing (e.g. Hydrazine, Hydroquinone) – IOWO l l h ld b k t b l 20 b IOWOxygen levels should be kept below 20 ppb - IOW

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Oxygen corrosionOxygen corrosion

Attack in FW and economizer sections where magnetite is damagedAttack in FW and economizer sections where magnetite is damagedUnder depositsBy thermal cyclingBy thermal cycling

Deposit + porous Fe-oxides

F F + e++ +++ -→

O + 2 H O + 4e 4 OH2 2- -→

O2OH-

Cathode Anode

Fe Fe + e++ +++→OH

Cathode Anode

Fe Fe + 2e+ + -→

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Oxygen corrosionToo much oxygen (> 20 ppb dissolved)

Reaction on magnetite:4 Fe3O4 + O2 6 Fe2O3

Reaction on bare surface:4 Fe + 2 n H2O + 3 O2 2 (Fe2O3.nH2O)

⇒ Hematite - not protective

⇒ Pitting⇒ Pitting

Fe O th dOOFe3O4

Fe

cathode

anodeFen+

O2O2

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Oxygen pitting corrosionOxygen pitting corrosionCorrosion product

presentpresentremoved

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Oxygen corrosionOxygen corrosionOxygen corrosionOxygen corrosion

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Oxygen removalMechanical de-aeration

Chemical oxygen scavengingChemical oxygen scavenging

Sulphite ⇒ sulphate

d ( )Hydrazine (N2H4)⇒ N2, H2O ⇒ s pports passivation⇒ supports passivation ⇒ NH3 (neutralisation of condensate)

C b h d idCarbohydrazides⇒ hydrazine, CO2

O hOthers

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High alkalinity attackHigh alkalinity attackCaustic attack / Gouging

Local concentration of caustic under deposits and in crevices causing a high pH.g g pDissolution of protective magnetite layer and the steel below as ferrates.

Caustic embrittlement♦ form of stress corrosion cracking, along grain boundaries in

metal♦ high alkalinity in crevice / deposit

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Caustic CorrosionCaustic Corrosion

U ll f d i hi h b ilUsually found in high pressure boilersProblem mainly caused by depositsLocalized corrosionAlso known as crater attack or caustic gougingAlso known as crater attack or caustic gouging

ConditionsConditionsPresence of caustic soda in the boiler waterPresence of a concentrating mechanism

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Caustic CorrosionCaustic corrosion can occur in the presence of

Porous metal oxide depositsPorous metal oxide depositsOperation above rated capacityExcessive localized heat inputExcessive localized heat inputLocalized pressure differentialsRestrictions in the steam generating tubesRestrictions in the steam generating tubes

MitigationLimit the presence of “free” causticCoordinated phosphate treatmentsCongruent sodium phosphatePhosphate-low hydroxide

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Caustic CorrosionCaustic CorrosionSteam

Boiler waterPorous Iron Deposit

Na+ local increase of pH

MagnetiteOH-local increase of pH

Caustic attacks iron SteelSteel

Fe + 4OH FeO + 2H O2+ -2

2-2→ 2 2

→4OHFe - 2FeO + 2H O2-

2 FeO22- +O 3 4 +

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Caustic CorrosionCaustic Corrosion

Removal of magnetite film at high pHRemoval of magnetite film at high pHFe3O4 + 4 NaOH ⇒

2 NaFeO2 + Na2FeO2 + 2 H2O,2 NaFeO2 + Na2FeO2 + 2 H2O,soluble products

Attack of bare steel at high pHAttack of bare steel at high pHFe + 2 NaOH ⇒ Na2FeO2 + H2,

soluble productsp

Attack of bare steel at moderately high pH3 Fe + 4 H2O ⇒ Fe3O4 + 4 H2,3 e 2O ⇒ e3O4 2,

porous oxide layer

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Co-ordinated phosphate effectCo-ordinated phosphate effectBuffer: HPO4

2- + OH- ↔ PO43- + H2O

Ph h hid i i k

Boiler water

Phosphate hide-out is a risk

SteamBoiler water

Porous Iron DepositHPO4

2-OH-

Magnetite4 OH-

Na+

Phospate pneutralises OH Steel- No attack

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Caustic Corrosion

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Caustic gougingCaustic gouging

ref: The NALCO Guide to Boiler Failure Analysis (ISBN 0-07-045873-1)

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Caustic cracking / embrittlementCaustic cracking / embrittlementC diti i dConditions required

Concentrating mechanismMetal under high stressesMetal under high stressesTemperature (rare below 150 ºC)

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Flow Assisted CorrosionThinning corrosion associated with high purity, low oxygen condensate

(or boiler feed water) caused by the relative movement of a fluid against ( ) y gthe metal surface.

Metal loss results from the dissolution of the protective oxide film by localized turbulencelocalized turbulence

Type of Corrosion-Erosion

Feed water velocities above 2.1 m/s can remove oxide filmsDamage is aggravated by local turbulence Low oxygen concentration destabilizes the magnetite layer pH < 9.3 (note: pH or feed water controlled between 8.5 – 9.3)Maximum damage inMaximum damage in o one-phase flow @140 deg Co two-phase flow @ 180 deg C

C i t b t 0 1 10 /Shell Global Solutions

Corrosion rates between 0.1 – 10 mm/year35

Flow Assisted CorrosionFlow Assisted CorrosionToo low oxygen concentrations with too high liquid velocities oo o o yge co ce t at o s t too g qu d e oc t escan lead to Flow Assisted Corrosion

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Flow Assisted CorrosionFlow Assisted CorrosionFeed water line inside a boiler drum Feed water contained <5 ppb O<5 ppb O2.

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Flow Assisted CorrosionFlow Assisted CorrosionMitigationg

Modification of the water chemistry(usually minimum changes allowed)(usually minimum changes allowed)

Materials selection:

- Resistance improves with small additions of Cr, Cu and Mo

- Use of 1.25 Cr or 2.25 Cr steels

- Use of 12Cr stainless steels provides best protection

Ch i l i h hi h iChange material at areas with high corrosion rates

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Carry-OveryCarry-over is any contaminant in the solid-, liquid- or vapour form, which leaves the boiler with the steam due to incomplete separation of the steam in the steam drum. It is related to steam quality.Carry-over may lead to superheater failures, sticking govenor valves in turbines, erosion of turbine parts, reduced turbine efficiency.

Causes for carry-over:yMechanical

Pressure or level surges inadequate design overloading Pressure or level surges, inadequate design, overloading, high drum level operation

ChemicalChemicalFoaming, formation of bubbles (accumulation of solids), vapour

carry over (silica)Shell Global Solutions

carry-over (silica)39

Cracking due to deposits from carry-over in superheaterCracking due to deposits from carry over in superheater

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Erosion - Corrosion/ErosionCombined action of corrosion and erosionM t l l t d d Metal loss rates depend on

Velocity and concentration of impacting mediumSize and hardness of impacting particlesHardness/corrosion resistance of material.Hardness/corrosion resistance of material.

Factors increasing corrosivity of the environment ( temperature, pH) can increase susceptibility to metal losscan increase susceptibility to metal loss.Prevention

D i i t dif h / t / t i l l tiDesign improvements – modify shape/geometry/materials selection

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Erosion - Corrosion/Erosion

Major locations for ErosionSoot BlowersSteam cutting from adjacent tube failuresSteam cutting from adjacent tube failuresFly-Ash Erosion in eco, superheater, reheater & rooftubing(higher sensitivity for lower temperatures: harder particles)(higher sensitivity for lower temperatures: harder particles)

Erosion on water side is comparatively rarep yLocations with increased turbulence(internal surface discontinuities & tube ends: Corrosion-Erosion)(internal surface discontinuities & tube ends: Corrosion Erosion)

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Corrosion/Erosion: tube endCorrosion/Erosion: tube end

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Chelant CorrosionChelant Corrosion

Only when Feed Water is chelant controlledOnly when Feed Water is chelant controlledAttacks the protective magnatite layer of the boiler (drum, economizer tubes lines)economizer, tubes, lines)Mechanism

Fe3O4 + Fe + 8H+ 4Chelant 4Fe(II)Chelant + 4H2OMitigation

Close control of Chelant levelsControl of dissolved Oxygen levelsControl of dissolved Oxygen levelsReduce fluid velocity (turbulent flow)

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Scaling Scaling Silica

The presence of Silica can promote the formation of Sodium-iron-silicates. The solubility of Silica is dependent of the lk l f halkalinity of the water.

The higher the alkalinity, the higher the solubility.For high pressure steam boilers the Silica content in the BFWFor high pressure steam boilers, the Silica content in the BFW should be limited to 20 ppb.Cycles of concentration raises the Silica concentration to yppm levels e.g. 60 cycles x 20 ppb > 1.2 ppm

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ScalingScalingIron

Can form a scale in boiler tubes, particular in the presence of Silica thru the formation of Sodium iron silicateIron levels should be limited to 50 ppb (commonly required by boiler manufacturers)(co o y equ ed by bo e a u actu e s)

CopperppThe presence of copper is thought to accelerate on-going corrosion processesprocessesRecovered condensate (unpolished) is the main source of copper

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ScalingScaling

Mud drum scalesMud drum scales

Riser tubeRiser tube

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Corrosion fatigue/mechanical stressCorrosion fatigue/mechanical stressDamage to protective Fe3O43 4

mechanical damage (hammer testing, etc.):

tirusting

tube vibration or thermal cycling :

cyclic stress with repetitive breakage/formation of Fe3O4corrosion fatigue cracking

water hammer at de-aerator inlet

i i f h t d t t ith ld k tmixing of hot condensate steam with cold make-up water

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De aerator crackingDe-aerator crackingCracking has been observed in de-aerator vesselsg

The root cause of failure is corrosion fatigue

Si th k ti ht it ll l bSince the cracks are very tight, it can usually only be found with (wet) fluorescent magnetic particle examinationexamination

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Overheating & Stress RuptureOverheating & Stress Rupture

Localized overheating (short term/long term) causes permanentLocalized overheating (short term/long term) causes permanent deformation (bulging) and eventual failure/rupture at normal operating stress levelsp g

Critical factors: Temperture & Time (stress)Loss in thickness reduces the time to failureAvoid overheating

Burner managementPrevent scalingPrevent scaling

Main sources of fouling in boiler tubes are silica, iron and copper

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Creep

Time dependent deformation of stressed components under load at high temperaturesCritical factors: Material, Load/Stress and TemperatureBeyond threshold temperatures damage/cracking/failure is a concernBeyond threshold temperatures damage/cracking/failure is a concernMitigation: Operate equipment within design limits, Inspect & Monitor

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Creep damageCreep damage

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Failure from creep voidsFailure from creep voidsMembrane tube from boilerOverheated at 750 °C after voids had developed

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CreepCreepAllowable stress

SS 321time dependent

CS LAtime dependentcreep strength

time independent800H

time independentstrength

200 400 600 800

T t i °C

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Temperature in °C54

Creep Threshold TemperaturesCreep – Threshold Temperatures

Carbon Steel : 410 ºCCarbon Steel : 410 CC - ½Mo : 490 ºC1¼C ½M 500 ºC1¼Cr - ½Mo : 500 ºC2¼Cr - 1Mo : 480 ºC5Cr - ½Mo : 470 ºC9Cr – 1Mo : 515 ºC9Cr 1Mo : 515 C304H : 580 ºC347H 595 ºC347H : 595 ºC

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Fire-side CorrosionOxidation

Firing with excess of Oxygen (1 2 %)Firing with excess of Oxygen (1-2 %)

Oxide scale (protective)

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Fire-side CorrosionOil-Ash Corrosion

Metal temperature range: 593 – 816 CPresence of Vanadium and Sodium Compounds in Fuel

MechanismFormation of V2O5 & Na2O by oxidation (in flame)Ash particles stick to the metal surface (Na2O as binder)V2O5 + Na2O forms a liquid (eutectic)Liquid fluxes the protective oxide resulting in rapid corrosion

MitigationMetal temperature < 593 ºCAdditive: Mg-compounds, when fuels with very low quantities of V and Na can not

be specified

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Flue Gas Corrosion (Cold-End Corrosion)Flue Gas Corrosion (Cold-End Corrosion)Sulfuric acid corrosion if

Steel temperature is below 150°C 1)

ANDAND

Fuel contains sulphur♦ natural gas: [S] > 20 ppmv ( 30 mg/Nm3)♦ natural gas: [S] > 20 ppmv (~30 mg/Nm3)

♦ oil: [S] > ~ 0.1 %w

1) or the actual acid dew point of the flue gas, calculated on the basis of the SO3 concentration, estimated from the radiant cell temperature and the excess air.

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Condensate corrosion – COCondensate corrosion – CO2

CO2 is formed by the dissociation 2 yof carbonates in the boiler

NaHCO3 ⇒ NaOH + CO2

Mechanism:

CO2 + H2O ⇒ H+ + HCO3-

low pH at carbon steel

large patches of attack with steep edges

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Condensate corrosion – OCondensate corrosion – O2

O iO2 corrosion sources:

Oxygen ingress

DDe-aerator

Mechanism:Mechanism:

Oxygen pitting Fe

Worse: CO2 & O2 combined

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Condensate corrosion – NHCondensate corrosion – NH3

Sources:Sources:NH3 from boiler

M h iMechanism:Cu & alloys Stress Corrosion Cracking

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Prevention Condensate CorrosionPrevention Condensate CorrosionReduce …

carbonates BFW

oxygen ingressoxygen ingress

vacuum systems

vents

condensate pumps / traps / valvesp p / p /

intermittently operated systems

ammoniaammonia

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Condensate TreatmentCondensate TreatmentDe-oiling (oil from turbines)g

Oil removed by skimming tanks, clarifiers and filters. Alternatively by coalescers and absorbentsy y

Condensate PolishingOil-free condensate is treated in a condensate

polishing treater (ion exchangers)

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Once a boilerOnce a boiler ...

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Thank you for your attentionThank you for your attention

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