damage mechanism

7/27/2019 Damage Mechanism

1/10

Descriptionmicrostructure change C,0.5 Mo,427-

593,strength,ductility,and/or creep

resi,carbide phase decompose to graphite

nodules

Form of dealloying in which iron

matrix is corroded leaving corrosion

products & porous graphite

change in micstru after exposure to

440 to 760 where carbide phase r

unstable and aggloremate into

spherioridal form.Loss in strength

&/or creep resi

Older vintage steel & C-0.5Mo

under combined effect of defor &

ageing,hardness incr,ductility ?&

toughness decr

affected material

CS,0.5mo

cast irons white cast iron is not

subjected to corrosion CS,LAS,

Pre 1980 STEEL WITH LARGE

GRAIN SIZE & C-0.5Mo

critical factors

chemistry,stress temp n time of

exposure,not normally found,add 0.7%cr to

eliminate,below 427 its very slow,types are

is in where nodules are distributed

randomly,doesnot usually lower creepresi,second results in chain or local planes

of conc noduleswhere there is signific loss

in load bearing cap + brittle

fracture.FORMS- haz corresponding to low

temp zone-eyebrow Non weld type is a

formof localised graphi where there is

plastic deform as a result of cold working

Microstructure environmentcombination,i.e temp,degree of

aeration,ph, time,occures in

presence of moisture below 93,much

damage during stagnant

condt,graphite is cathodic to iron

matrix.may affect adjacent

component by galvanic corr

Chemistry,microstr,exposure

time,& temp ...occures few hrs at

552 but may take several yrs at

454,Annealed steel more

resistant,Fine grained silicon killed

more resistant than Aluminium

steel composition& mfgprocess,bessemer & open hearth

have higher impurities than

BOF,fully killed Al with BOF not

suspectible,observed when cold

worked & without SR,is a major

concern for eqpt containing

cracks,

affected unitsFCC,CR,coker haz graphi reduses creep

str

Can occure in soft water,salt

water,dilute acids,feedwater

ppgs,pumps valves,

loss in strength is usally

accompained by increase in

ductility,Hot wall ppg in

FCC,CCR,COKER old and no pwht

appearance

metallography,microfissuring,microvoid,sub

surface crack or surface crack

may not be noticable even after

visual,soft & easily gouged by

knife,metallographic exam

Not easily nticable,metallography,

5%to9% CrMo it is the process of

transforming carbides from finely

dispersed to aggloramated

mitigation/inspection

use higher Cr content

Acoustic emission,UT,There is

reduction in hardness,can beprevented by coating,lining, difficult to prevent exceptminimising long term exposure apply PWHT and considerbuttering for old steel repairs

related mechanism spehoridization which occures

preferentially above 551 selective leaching graphitization

blue brittleness is another form of

strain ageing

Mechanism GRAPHITIZATION GRAPHITIC CORROSION STRAIN AGEINGSOFTENING (Spherodization)


2/10

Metallurgical change due to long term

expo to 343-593,causes upward shift in

ductile-brittle tran temp as measuredby charpy,may be suspectible to Brittle

fracture during startup & shutdown

characteristiced by surface initiated cracksoccur in piping exposed to caustic,primarly

adjacent to non pwht welds,

form of cracking when certain molten

metals come in contact with speceficalloys cracking can be sudden &

brittle

loss of ductility due to penetration of

H2 can lead to brittle cracking,can

occur during mfg,wldg,or service thatcan charge H2 in steel (gaseous or

aqueous,corrosive)

loss of tghness in alloyscontaining ferrite due to

exposure to 316 to 540

2.25Cr-1Mo,3Cr-1Mo (to less extent) &

hsla Cr-Mo-V rotor steel.

CS,LAS, & 300SS, (Nickel based alloys are

more resistant) CS,SS,LAS,HSS,Ni

CS,LAS,400SS,High strength nickle

base alloys

400 s.s,duplex,wrought & cast

300 ss,weld n weld overlay

alloy steel comp,thermal history,metal

temp & exposure time,Suspectibility is

largly determined by presence of

alloying elements Mg & SI,and tramp

elements like

phosphorous,tin,antimony,2.25Cr

developes more quickly at 482 than in

427-440 but long term ex

caustic strength,tempr,stress,cracking at

low caustic is possible if there is conc (50 -

100 ppm is sufficient to create

cracking)Generally accepted that stress

approaching yield r reqd for scc,conc can

occur as a result of wet n dry condt

localised hot sp

occures when speceficcombintion of

metals come in contact with low

melting point metals like

Zn,mercury,cadmium,Pb,Cu,Tin,High

tensile strengts promotes crack or

even if there is mutal contact,(SS-

Zn,Copper alloys- Mercury,Alloy 400-

Mercury,Al-Mercury,HSS

three condt must be present:H2 must

be present within critical conc

WITHIN steel,Strength level &

microstructure must be

suspectible,stress above threshold

for HE must be present,H2 can come

frm wldg,cleaning in acid soln,high

tempr h2 gas atm,wet h2s ser

cr content,amount of ferritephase, & operating

temp,increase in ductile to

brittle tran temp will

occur,primary consideration is

operating in critical temp

range,effect of tghness is not

prononcud at opt temp but is

significant at low temp

(shutdown/star

eqts mostly in hydroprocessing

units,ccr,fcc,

eqpts that handle caustics,including

mercaptan removal units,improperly heat

traced pipes,as a result of steam cleaning.

during fire galvanised zinc,cadmiumelectric housing may melt and come

in contact,S.S pipe contact with GI

pipe,some crudes and p[rocess instru

contain mercury

CS Piping in h2s service in

hydroproc,FCC,amine, sws,moresuspectible if PWHT is noy

done,Storage sphere due to

HSS,Bolts springs,reactors haz}

235BHN

most refining use ferritic steel

to non pressure boundary

can be confirmed thru impact

test,damage may eresult in

catastrophic brittle fracture,install

blocks of orignal heat& impact test

cracking II to weld in adj base metal but

also occur in weld deposit or HAZ,SPIDER

WEB,Intragranular,oxide filled surface

breaking in as welded CS,cracking in

300ss is transgranular & difficult to

distinguidh from chloride cc

appears as brittle cracks in ductile

mtl,intragranular,metallography,spect

rographic analysis may be reqd to

confirm presence

Mostly surface breaking but also

subsurface,occures at high

stresses& where microstructure is

conductive such as HAZ,

metallurgi change NOT

READILY seen by

metallography but can be

confir by bend/impact,increase

in hardness

cannot be prevented if impurities &

exposed to embrittling temp,

pressurization seq to limit system pr to

25% of dp for temp below min pr temp

(171-38 for new steel),carry pwht &

rapidly cool,limit acceptance level of

Mg,Si,Ph,Tn

PWHT,STEAM OUT OF NON pwht

WELDS SHD BE AVOIDED,WATER

WASH BEFORE STEAM OUT,USE LP

steam.Disperse caustic

properly.WFMT,EC,RT,surface prep is

usually reqd,PT not effective for finding

tight scale filled crack,AET can be used for

monitoring crack growth.

prevent contactno zinc or organic Zn

coating on S.S,Cracks by

MT,PT,Because of high den of

mercury RT can be used in excg

tubes,

Use lower strength & tempered

steel,reduce stresses,PWHT,Backing

of electrode,use dry

electrodes,control shutdown &

startup procedures,apply protective

lining,cladding,overlay,PT.MT,WFMT

,

use low ferrite & avoid

exposure can be reversible by

heat treatment followed by

rapid cooling.593,but re

embrittle will be fast,insp by

impact or bend test,increas in

hardness is another indication

NA Amine Cracking & carbonate cracking liquid metal cracking,

H2 flaking,underbead

cracking,delayed cracking,hic, not applicabl

475 EMBRITTLEMENTTEMPER EMBRITTLEMENT CAUSTIC EMBRITTLEMENT -CSCC HYDROGEN EMBRITTLEMENTLIQUID METAL EMBRITTLEM


3/10

Aggressive corrosion occuring inhydroprocessing effluent line & units

handling sour water

general or localised corrosion often pittingunder ammonium chloride or amine

salts,often in absence in free water phase

corrosion general and localised is

associated with dew point corrosion,

aqueous alkanolomine cracking in

systems used to remove h2sco2,form of alkaline scc,most found

adjacent to non pwht welds

general or localised in carbonsteel.HSAS is other

contaminant

cs,300ss,duplex,nickel base alloy are

more resistant

incr resis,cs,las,300ss,alloy

400,duplex,alloy 625 and titanium all common mtl c/s and las carbon steel

conc,velocity,PH,Temp,alloy

composition ,above 2% weight &

increase vel it is very

corrosive,nitrogen in feed is converted

to ammonia reacts with H2S to form

nh4hs,nh4 percipitates out of gasphase in reactor effluent stream at

temp below 66,it causes fouling &

plugging,nh4hs salt depozsit causes

underdeposit corrosion presence of

cyanides increases corrosion in

fcc,coker by normally destroying

sulfide scales

conc,temp and wateravailability,ammonium cl salts may

percipitate frm high temp stream as they

are cooled depending on temp and

conc,salts are hygroscopic,temp incr then

corr incr,when they deposit above water

dew point then water wash injection may

be reqd

conc,temp n alloy compi,first droplet

of water which condenses can be

very acidic(low ph)monel,titanium is

resistance,presence of oxidizing

agents incr corrosiontitanium fails in

dry HCL service

tensile stress,conc,temp, crack more

likely in lean MEA & DEA, cracking in

rich amine is associated with WET

H2S cracking.

corrosion depends on design n

operating practice,type of

amine,conc,temp & vel.CS is

suitable if designed n operated

properly.lean amine generallynon corrosive.HSAS above 2%

can significantly incr

corrosion.temp above 104 can

result in acid gas flashing &

severe localised corr if pr drop

is high.velocity limit to 3 to 6

fps

hydroprocessing,fcc unit,stripper

column ovhd sour

water,fcc,sws,amine(regenerator

ovhd,delayed oker downstream of

fractioning tower

crude tower top,top pump around

streams,reactor effluent streams,fccu and

coker fractioning ovhds,

CRUDE: Top of vaccum

tower,HYDROPROCESSING:

Chlorides can come frm make up gas

hydrocarbon feed,CATALYTIC

REFORMING

all non pwht welds in lean amine

including absorbers,regenerators,

and heat exchanger

amine

unit,regeneratorreboiler,regene

rator rich amine side of HE,

general loss as well as localised in

CS,low velocities leads to

underdeposit corr, heat excg may

show plugging,rapidly corrodes

aadmiralty brass tubes and other

alloys

whitish greenish or brownish

appearance,corrosion underneath salts is

localised,

CS,LAS general corrosion or

localised S.S may suffer cscc

primiraly in HAZ,parallel to weld.in

weld metal cracks are transverse or

longitudnal.at set on cracks are radial

in base mtl,at set in they are

parallelcracks more at external

attachm welds,metallography

uniform thinning,localised or

localised under deposit

attack.high velocity more

localised

good dsg with hydraulic balanced

flow,maintain velocities ( 10 to 20 fps

for cs)use resistant mtl ( alloy

825,duplex),design wash water

injection with low o2 content,AL tubes

are extremely suspect,use sampling of

fluid,ut,prt,ires,rfec & flux leakage,ec,

crude unit:limit salts,add caustic,filming

amine. HYDROPROCESSING: limitchlorides in hydrocarbon & make up

hydrogenwater wash.CATALYTIC

REFORMING: TREAT

EFFLUENTS,WATERWASH,FILMING

AMINERT OR UT ,monitor feed

streams,maintain temp above salt

deposition temp,probes n coupons

: esa ng,use an um

monel,water

wash,caustic,amineHYDROPROCESSING: minimise chlorides,use

resistant mtl ( no waterwash)

CATALYTIC REFO: Same as

hydroprocessing but use

waterwashchloride

treaters,UT,RT,PH

checkprobes,copon

pwht,use SS cladding or alloy

400.water wash befor wldg,pwht n

steamout.WFMT,ACFM,AET

SWUT,AET to monitor crack growth

and locn

proper operation.process temp

should not exceed designvalues.proper control of reboiler

temp.avoid build up of HSAS.

Control pressure drop upgrade

to SS,oxygen contributes to

HSAS,proper

filteration,inhibitors,UT,PRT,

Coupons,probes

erosion corrosion HCL corrosion ammonium chloride corrosion caustic scc,carbonate scc

amine stress corrosion

cracking

AMINE CORROSIONAmmonium Bisulfide corrosion AMMONIUM CHLORIDE CORROSION AMINE STRESS CORROSION CRCHYDROCHLORIC ACID CORRO


4/10

DEALLOYING DECARBURIZATION CARBIRIZATION METAL DUSTING NITRIDING

Selective corrosion in which one

or more constituent of alloy are

preferentialy attacked leaving less

density structure.mechanicalproperty are significantly degraded

steel loses strength due to removal

of carbon n carbides leaving only iron

matrix.occures during exposure to

high temp,fires

carbon is absorbed in mtl at

elevated temp while in contact with

carbonaceous materila.

FORM OF carburization resulting in

localised pitting. Which occures in

carburising gases andf/or process

streams containing carbon n

hydrogen.pits form on surface n

containsoot or graphite dust.

hard brittle surface layer due to

exposure to high temp streams

containing N2 compounds like

ammonia or cyanides PARTICULARLYin reducing condt

Primarly Copper alloy,as well as

ALLOY 400,and cast ironcarbon n low alloy steel CS,LAS,SS,nickel LAS,SS,Nickle CS,LAS,SS

composition,temp,deg ofaeration,PH n exposure

time.limited to very specefic ally

environment combination.

time,temp,carbon activity.material

must be exposed to gas phase that

has LOW carbon activity so thatcarbon in steel will diffuse at surface

to react with gas phase

constituent.loss in room temp tensile

strength and creep strength may

occur.

three condt must be

satisfied:exposure to carbonaceous

mtl,temp high enough for

diffusion,suspectible mtl.initially

carbon diffuses at high rate n then

decreases.300 series are more

resistant due to CHROMIUM

content.it results in loss of creep

ductility,ambient temp mech

prop,loss of weldability,corrosion

resis,

Process stream composition ,temp

,alloy composition.preceded by

carburization.it involves complex

series of reaction involving reducing

gas such as hydrogen, methane,

propane,482-816,saturation of metal

matrix by carbu,percipitation of

carbides on surface n grain

bound,deposition of graphite frm atm

onto metal carbides,decomposition of

metal carbides under graphite,further

decompositio of graphite catalyzesd by

metal particles.In high nickle alloys

metal dusting is without formation of

metal carbides

Temp,time partial pressure of N2 and

metal comp.temp shall be high enough

for thermal breakdown of n2 from

ammonia and for diffusion intometal.begins above 316 and is serious

above 482. corrosion resistance is

agffected.Alloys containing nickle are

more resistant.(30% -80%) loss of

creep ductility,ambient temp mech

prop,loss of weldability,corrosion resis,

component that are hot formedmay be affected

an occur in any equip exposed tohigh temp.pressure vessel

fired heater tubes most

affected.coke deposit are source ofcarbon.also encountered in

ethylene pyrolisis and steam

reformer surnaces.

fired heater tube,thermowells,catalyticreforming unit heater tubes,coker

heaters,gas turbines.

Ammonia synthesis plant,steammethane reformers,steam gas cracking

plants.

significant color change or deep

etched apperance.uniform or

localised attack.

damage verified by

metalography.carbon steel will be

pure iron.

depth of carburisation can be

confirmed by

Metallography.increase in

hardness loss of

ductility,volumetric increase in

affected component,incr in

ferromagnetism.

lin LAS wastage can be uniformbut

usually in form of pits.carbon dustin

SS attack is local,appearing as deep

round pits.metallography shows heavy

carburization

surface related and is dull,dark

greyinitial stages can only seen by

metallogrtaphy,at advanced stage very

high hardness,nitriding of LAS upto

12% cr is accom by incr in

volume.above 410 there is grain

boundary nitriding,

addition of alloying elements willhelp,dealuminification can be

prevented by heat treatment to

producr alpha n beta

microstructure.avoid partiucular

exposures,cathodic or barrier

coating may be effective.brass

become reddish

color,metallography ,reduced

can be controlled by chemistry of gas

phase n alloy selection.alloy steel

with cr and MO form atable carbides

and are More resistant.metallography

n replication.it results in softening

select alloys with strong surface

oxide or sulfide film

formers.SULFUR INHIBITS

carburization use metallography

when cracking is there use

RT,UT,MT

sulfur is good as it forms

film,aluminium diffusion

treatment.Compression wave UT,RT,

use nickle alloys,change in surface

color dull gray indicates

nitriding.hardness 400 to 500

bhn.nitrided layers are

MAGNETIC.metallography

selective leaching HTHASEVERE form is called metal

dustingcatastrophic carburizationb. crburization n metal dusting


5/10

Cracking due to stress relaxation

during PWHT or in service at elavatedtemp.most often OBSERVED in heavy

wall vessel.

at high temp metal component slowly

deform under load below yield pt.timedependant deformation is creep.it leads to

damage which eventaully leads to rupture

Permenant deformationoccuring at

relatively low stress level as a result

of localised overheating.usuallyresults in bulging eventaully leads to

rupture

Flow of heat energy across tube wall

will generate steam bubbles in

ID.when heaty flow balance is

disturbed steam bubbles jointogether(DNB).rupture due to short

term overheating

localised corrosion due to

caustic conc or alkaline salts

LAS,300SS,NICKLE BASE LIKE 800H all metal n alloys all fired heater tubes CS,LAS CS,LAS,SS

TYPE OF MTL ,GRAINSIZE ,stresses,thk,stress conc,strength,weldg n heat

treating condt.occures when creep

ductilyis insufficient to accomdate

strains reqd fro stress relief.Usually

cracks are confined to HAZ.cracks are

intragranullar n show little

deformation.occures at grain

boundaries.stress relief and

stabilization treatment can cause this

function of mtl,load,temp,time an increase

in 12 or increase in 15% stress can cut

remaining life in half.low creep ductility is

more severe for high tensile mtls,more

prevalent at low temp in creep range or low

stresses at upper creep range.more likely

in coarse grain.not evidenced by

detoriation of ambient temp prop.

temp time stress,usually due to flame

impignment.time to failure will

increase as internal pressure or

loading decr.corrosion contributes

due to loss of thk

Heat flux and fluid flow,improper

flaming,anything that restricts fluid

flow.failure occures as a result of

HOOP STRESS

Major factors are presence of

naoh,koh.used as

neutralizers.conc mechanism

must exist to build up caustic

strength.can become

concentrated by

DNB,evaporation and

deposition

occures in heavy wall vessel.HSLA

very suspectible.

heater tubes,low creep ductility failures

occurred in HAZ at nozzles.weld joining

dissimilar metals.

all fired tubes,furnace with coking

tendencies,

FIRED boilers,waste HE,INsuperheaters n reheaters during

startup when condensate blocks fluid

flow

associated with boilers and

steam generating equp.preheat

exch,furnace tubes,

intragranular but can be surface

breaking.mostly observed in coarse

grain str of HAZ

creep voids typically show at grainn

boundary fissuring and then cracks

(metallography)noticable deformation may

be observed.occures at stress

concentrators.

localised deformation or

bulging.FISHMOUTH failure

open burst edges drawn to knife

edge.microstructure shows severe

elongation of grain structure due to

plastic defor

localised typically grooves in

boiler tubes.in verticle tubes it

appears as circum groove.in

horizontal tube appears as

longitudal grooves.general corr

rates above 79 very high above

93

MINIMISE RETRAINT,PROPER

JOINT DSG,ADEQUATE PREHEAT

TO BE APPLIED.USE SMALL

GRAINED STR.Avoid stress conc.long

seam welds have mismatch and are

suspectible.UT,MT,PT

m n m se me a emp,avo s ress

conc,higher pwht temp may help.it is not

reversible.avoid localised

overheating.associated with microvoid and

fissuring ,1.25 cr 1mo & 1cr 0.5 mo are

particular PRONE to low creep

ductility.fired heater tubes:-vt,thk,diametric

growth with go nogo .

minimise localised tempr.proper

burner management n fouling.use

diffuse flame pattern.proprr

dsg.maintain refractory,ir

monitoring,thermocouples,ir scans

when DNB is developed tube rupture

will occur immediately.proper burner

management.avoid restriction of

flow.tubes to chk for bulging.

proper dsg,proper burner

management and minimise

ingress of alkaline salts.proper

mixing of caustic.alloy 400 is

quite resistant.UT,PROPER

DSG N USE OF INJECTION

PTS,RT,BOROSCOPE

Stress relief cracking reheat cracking,short term overheating creep stress rupture

steam blanketing can cause caustic

corr.SHORT term overheating

Caustic gouging or ductile

gouging,DNB

CAUSTIC CORROSIONREHEAT CRACKING CREEP STRESS RUPTUR STEAM BLANKATINGSHORT TERM OVERHEATING


6/10

corrosion of CS n other alloys resultinfrom reaction with sulfue compounds in

high tempin in absence of hydrogen

presence of h2 in high temp h2sstreamsat temp above 260.UNIFORM

LOSS inthk

general n localiused corrosionofCS,LAS,haz may experience

severe corrosion

hydrogen reacts with carbides in

steel to form methane. Whichcannot diffuse thru steel .loss of

carbide causes loss in strength

corrosion due to acidic sourwater containing H2S at PH

betn 4.5 to 7.0

CS,LAS,300SS,400SS,COPPER BASE

cs,las,400ss,300ss in order of incr

resis

incr resis..CS,316,ALLOY 20,high

silicon cast iron,high nickel cast

iron,alloy b2,alloy c276 incr resis cs,1cr-0.5 mo.. carbon steel

alloy composition,,temp n

conc,suspectibility is determined by

ability to form slfide scale,increasing cr

content incr resistance,PRIMIRALY

caused by H2S ,begins at metal temp

above 260

t5emp,presence of h2,conce of h2s

and alloy comp.hydrogen incr

corrosion,dhds has more corrosion

than nhtu,increasinin cr content decr

corrosion,

acid conc,temp,alloy

comp,velocity,contamination &

presence of oxidisers.corrosion of

cs increase if vel exceeds 2 to 3

fps or at acid conc below

65%,high corrosion when acid is

diluted

dependant on temp.hydrogen partial

pr,time n stress,

H2S content,ph

temp,vel,oxygen conc,h2s

partial pressure,h2s conc in

sour water decreases as temp

increases,h2s tends to

decrease ph.in some instance

ph above 4.5 forms thick sulfide

layer,this promotes pitting

crude,fcc,coker,vis

breaker,hydrotreater,heaters fired with

oil,gas depending on s contents,boilers

down stream of hydrogen injection

pointss,eqpts where high high temp

h2s streams are found

sulfuric acid alkalyation units

(effluent lines,reboilers)& waszte

water treat ment plant,acid usually

ends up in bottomof fraction

towersmostly

hydrotreaters,catalytic

reformer,boiler tubes in very high pr

service overhead system in FCC

uniform or localised due to erosion

corrosion

uniform loss of thk with formation of

iron sulfide scale,scale is in multiple

layers,shiny grey scale

general in nature but attacks cs

haz very fast,it attacks slag,

metallographic analysis,causes

surface DECARBURATION

intragranular n adjacent to pearlite

GENERAL

THINNING,LOCALISED,corrosi

on in co2 may be accompained

by carbonate stress cc

use higher cr alloy,clad,aluminium

diffusion treatment,process condt shd

be monitored,thermocouples,ut,prt,

use higher cr content,ut,vt,rtcheck

processs with

can be minimised my material sele

& proper velocity.acidified product

can be washed out with caustic to

neutralise acids,ut,rt,probes &

coupons

use alloy with cr-mo,UT,velocity

ratio & backscatter useful to detect

fissuring

300 ss below 60copper alloy

and nickleare generally not

suspectibility,ut,prt,corrosion

monitoring,sample

water,corrosion probes

high temp h2s corrosion in presence of

hydrogen

high temp h2s corrosion in absence of

hydrogen not applicable hydrogen damage wet h2s and carbonate ssc

SULFIDATION HIGH TEMP H2/H2S CORROSION HTHASULFURIC ACID CORROSION


7/10

primarly occures in crude n vaccum

units and downstream units

corrosion of carbon steel in plants usingphenol as solvent to remove aromatic

compound from lubricating oil feed stocks

LOCALISED PITTING OF CSdepending on water content.it is used

as catalyst in polymerization

general or localised cracking may beaccompained with

HIC/SOHIC,BLISTERING

CS,LAS,300SS. incr resis..CS,304L,316L,ALLOY 276 incr resis. CS,304L,316L ALLOY 20

cs,cuni,alloy 400,.las,ss are

generally NOT suitablefunctrion of naphthenic acid

content.temp sulfur content,velocity

and alloy comp.increasing acidity incr

corrr.TAN is a measure of acidity

(ASTM D-664).NAC is associated with

DRY hot hydrocarbon stream that do

not contain free water.nac is

determined by acidity of actual stream

not the crude charge coz it depends on

cuts.sulfur GOOD for nac.NAC

removes suffide film.occures above

218 but reported as low as 177.NAC is

destroyed by catalytic reactions in

hydroprocessing.increase MOLY to

reduce NAC.

temp,alloy chemistry,water content,and

velocity.corrosion is low below 121.CS and

LAS corrode rapidly in phenol service

above 232.dilute aqueous soln are very

corrosive.high vel promotes localised corr

acid conc temp and contaminant solid

phosphoric acid is non corrosive

unless water is present.corr incr with

incr temp,most corrosion during

water washing at

shutdown,contaminants increases

corr ( chlorides)

HF acid conc,temp,alloy comp, &

presence of contaminants like O2 n

sulfur compounds.carbon steel forms

protect film in dry conc acid.loss of

film due to high vel incr corrosion.

BELOW 66 CS can be used

corrosion rate increase with

INREASING water content.LIMIT

cR+Ni+Cu to 0.20 max.

heater tubes,transfer lines,vaccum

bottom piping,HVGO,LVGO.High

velocity causes problems.NAC found

upstream of hydrogen mix points phenol extract facilities in lube plants

piping and eqpt in polymerization

units,usually found in low velocity

areas

HF Alkaliation units,flare ppg,above66 high corr

rates,isostripper,depropaniser,flange

faces,he bundles,severe fouling may

result

localised,pitting,in high vel areas.

general or localised corr of CS.and in tower

overhead service general and local;ised thinning of CS

of CS.corr can be accompained by

blistering,HIC/SOHIC.fouling will be

there.non stress relieved alloy 400 is

suspectible to scc when contact with

moist HF vapours in presence of

oxygen

blend crudes upgrade metallurgy,utilise

chemical inhibitors use alloy with

higher moly,use inhibiytors.UT,RT

Monitor TAN and sulfur content

coupons, probes,HYDROGEN

PROBES

proper mtl selection.max vel of

30fps.recovery ovhd temp shallbe

maintained 17 above due pointtubes and

headers in extract furnace must be

316l.alloy c276 can be used in areas of

high VEL.UT,RT,PROBES,COUPONS

USE RESISTANT MTL TYPE 316L

SS AND ALLOY 20 are effective at

conc upto 85%,UT,RT,Probes n

coupons

carbon steel below 66 shall be

closely monitored,CAREFUL

OPERATION OF UNIT,maintain strict

control on water content,alloy 400

can be used is minimising blistiring

HIC/SOHIC problems.use alloy c276

if alloy 400 cracks.UT,RT.

SULFIDATION NOT applicable not applicable HIC/SOHIC

Naphthenic acid corrosion phenol (carbonic) acid corrosion hydrofluoric acid corr phosphoric acid corr


8/10

NORMALLY OCURING DURING

SHUTDOWN STARTUP or during opn

when air,moisture due to sulfur acids

formed frm sulfide scale on SENSITISEDss.USUALLY adj to weld.cracking may

within minutes.LOCALISED

surface breaking cracks adjacent to cs

WELDS under action of tensile stress ncorrosion in carbonate containing

system.ALKALINE SCC

Cracks develop under combinedeffect of cycilic loading n

corrosion.initiates at stress conc

Form of enviro cracking initiates atsurface of HSLA n CS with highly

localised zones of high hardness

300SS,Alloy 600/600h n alloy 800/800h

CARBON STEEL AND LOW ALLOY

STEEL all metals n alloys CS n LAS

combination of environment mtl n

stress,affected alloys may become

sensitizes during mfg welding or hightemp

service. Sensitization occures 400-

815.carbon content n thermal history have

significant effect on sensitization use L

grades but long term exposure to 399 will

sensitize L grade.residual stress promote

cracking

stress,water usually aoccures at welds or

cold worked areas,suspectibility increases

with incr PH and carbonate conce.if

equipment contains water phase with

50wppm of H2s at ph 7.6 or greater then

equipment is suspectible.

material,corrosive environ,cyclic

stresses n stress conc.there is no

fatigue load in this mechanism.

Hardness strength n stress.above 237

bhn.

all units using sensitized steel in sulfur

atmosphere.fired heaters,he,fcc

units(plenums,air rings,slide

valves,cuclone,exp joints) in

hydroprocessing unit,crude n coker

MOST PRELAVENT to fcc unit main

fractioner ovhd condensing reflux

system,ppg in potassium carbonate

catcarb and co2 removal facilities of

hydrogen mfg units

rotating equip: galvanic couples betn

impeller n pump shaft.DEAREATOR:

residual stress promote cracks.IN

CYCLING BOILERS due to start up

shutdownn differential expansion

all piping n eqpt exposed to HF acid

with hardness levels above

recommended limit.hsla BOLTS A193

are more suspectibleB7M also

suspectible if overtorqud

TYPICALLY occures next to weld but can

also occur in base metal.Cracking is

intragranular...corrosion or loss is

NEGLIGIBLE

parallel to weld away from TOE spider web

INTRAGRANULAR,oxide filled

cracks,surface breaking

brittle n transgranular BUT not

branched multiple parallel cracks.little

plastic deform,waterside of buckstay

attachment(RABBIT EARS)

only confirmed by

METALLOGRAPGY,cracking is

intragranular usually associated with

weldments

Flush eqpt with soda ash to neutralisesulfer .for furnace keep the firebox heated

above dew pointto prevent acid

forming.use L GRADES.thermal

stabilization 899 to stabilise after

wldg.ASTM A262.PT with sanding

use PWHT,USE CLADDING,(300ss,alloy

400),waterwash prior to steam out or heat

teratment.a metavanadate inhibitor can be

used in co2 units of hydrogen mfg

units,WFMT,SWUT,

minimise galvanic effecyts,modify

corrosive enviro,corr resis

mtls,minimise stress(pwht)minimiseweld reinforcement.startup slowly

with proper chemistry,UT,MT.IN

CYCLIC boilersa 1st damage is

pinhole leak on tube at buckstay

attachment

PWHT,CE less than 0.43Alloy

cladding.use alloy 400 WITH

pwht.WFMT,Hardness test,

intragranullar attack or corrosion

mechanical n vibration induced

fatigue hic/SOHIC/BLISTERING

POLYTHIONIC ACID SCC CARBONATE SCC HYDROGEN STRESS CRACKING-HFCORROSION FATIGUE


9/10

hydrogen diffuses in titanium n reacts to

form embrittling hydride phase.this can

result in complete loss in ductility with no

noticable sign of corrosion or loss of thk

sulfur n chlorine species in fuel will

form sulfur dioxide trioxide,hydrogen

chloride within combustion products

at low enough temp these gases will

condense to form acids

HIGH temp metal wastage wen contaminats

deposit n melt on metal surface of fired

heaters,boilers,GT.Fuel oil that is contaminated

with sulfer sodium potassium vanadium.slags

dissolve the surface oxide n enhance transport

of oxygen to form iron oxides.

titanium alloysmetal temp,solution chem,n

alloy composition.in specefic environment

above 74and at ph below 3.PH above 8 or

neutral p CS,LAS,SS all conventional alloys

metal temp,solution chem,n alloy

composition.in specefic environment above

74and at ph below 3.PH above 8 or neutral

ph with high H2s.Galvanic contact

promotes damage but can occur without

galvanic couple.beta alloys are more

tolerant to hydrogen than alph -beta alloys

conc oof contaminants,n operating

temp of flue gas metal surface.Dew

point temp of sulfuric acid is 138 n

hydrogen chloride is 54

conc of salt forming contaminants,metal

temp,alloy compcorrosion occures only if metal

temp is above the temp of liquid speciesformed.oil ash has vanadium pentoxide n

sodium sulphate melting point below 538 is

possible.for waterwall corrosion liquid speciies

are mixture of sodium n potasium polysulphates

melting point low as 371.for coal ash sodium n

potassium irons trisulphate with melting point

betn 544 -610.unburnt coal also adds carbon to

fly ash.corrosion rates in reducing condt will be

2 to 5 times faster than oxidizing condt

sour water stripper and amine unitsabove74.can also occur in hydrogen atm at temp

above 177 in absence of moisture or

oxygen

all fgired heaters which burn fuelscontaining sulfur in economiser

section n stacks.HRSGmay suffer

chloride SCC

fired heater,GT,FUELS WITH VANADIUM NSODIUM,tube hangers operate at high temp

than tubes n more suspec.coking of heater

tubes can cause high heat

Confirmed thut only metallograph n mech

testing.bend test of crush test.tubes remain

intact tillthe bundle is removed once

removed it cracks as bundle

flexces.Titanium shall not be used in amine

or sour water where possiblity of leak is not

acceptable

general wastage n broad shallow

pits.for SS cracking

severe metal loss associated with

slagging.Metallographic examination n deposit

analysis technic can be used.for oil ash corr of

superheater n reherter appearance will be in 2

distinct layers.dark grey black appearance

.ALLIGATOR HIDE.For waterwall cracks are

circumferential

Titanium shall not be used in amine or sour

water where possiblity of leak is not

acceptable,avoid galvanic couple,eddy

current,metallography n mech test

maintain temp above dew pt in HRSG

avois SS,sodium carbonate can be

added to final rinse to

neutralise,UT,VT,PT

blend fuel source,opoerate below the temp of

species.burner managementuse low excess

oxygen in firingUSE 50% cr-50% ni.Visual

insp,tubes need to be grit blasted

titanium n zirconium alloys only HCL CORROSION n chloride SCC hot corr, hot ash corr,oil ash corr

GALVANIC CORROSION TITANIUM HYDRIDING FUEL ASHFUEL GAS


10/10

SIGMA PHASE EMBRITTLEMENT

damage mechanism

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