4.2 Mech & Metal damage - Ind4.2. Mechanical and
Metallurgical Damages - All Industries Corrosion Mechanism
Description Temp. Range (F)Affected metallurgy Not AffectedAffected
EquipmentCritical Factors Prevention Inspection / MonitoringRemarks
Graphitization Change in microstructure after long term oper of CS
& 0.Mo steelLoss in strength, ductility & creep
resistanceFeC decompose into graphite nodules800-1100 F
Severe > 1000 in 5 yrSlight 850 in 30-40 yr
CS & .5Mo steel (upto 1% Mo) Bainitic grades less
susceptibleSi & Al have negligible effect1) hot piping and eqpt
in FCC, catalytic ref and coker unit.2) Few failures directly due
to graphitization.3) Severe eye brow GT lowers CRS.4) Seldom occurs
on boiling surface tubingRandom (nodule) graphitization does not
lower creep resistance. Chain type reduces load bearing capability
(brittle fracture). a- Weld HAZ graph occurs at low temp. edge (Eye
brow)b- Non weld graph occurs along planes of local
yeilding.Reporting is qualitative (NSMS-None, slight, moderate,
severe) Adding Cr +.7% to LAS 1) Can only be observed by
metallography.2) Evaluation by full thick sample for metallography
(Replica inadequate3) Surface or SSF cracks or microvoid (advanced
stage) difficult to detectCompeting:(Above 1025 --> SPHBelow
1025 --> GPH)Softening (Spheroidization)Change in microstructure
of steel after elevated temp exposure. In CS carbide agglomerate
from plate like to spheroidal form, in LAS (1-0.5Cr) from finely
dispersed to large agglomerated carbide.Loss in strength (up to 30%
usually accompanied by increase in ductility) and/ or creep
resistance.850-1400 F
at 1300F - few hoursat 850F - several yearsCS, LAS i.e. C-0.5Mo,
1-9% Cr-Mo Annealed steels more resistant than normalizedCoarse
grain more resistant than fine grainSi-killed more resistant than
Al-killedhot piping and eqpt in FCC, catalytic ref and coker
unitsRate of SP depends on temp and initial microstructureMnimize
long term exposure 1) Can only be observed thru field metallogprahy
or sample removal2) Reduction in tensile strength and hardness may
incidate spheroidization.Competing:(Above 1025 --> SPHBelow 1025
--> GPH) Temper EmbrittelmentReduction in toughness in certain
LAS due to metallurguical change due to long term operation. This
cause upward shift in ductile to brittle transition temp by CVN(May
result in BF during s/u and s/d)
650-1070(More quickly at 900, but more severe after LT at
850)Primarily 2.25Cr-1Mo (old gen prior to 1972 particular
suscepitble) and 3Cr-1Mo (lesser extent)Weld material more
affectedC-0.5Mo, 1Cr-0.5Mo and 1.25Cr-0.5Mo not much affected1) Few
failures directlu related to TE.2) Hydroprocessing units (Reactors,
hot feed / eff exchangers & hot HP seperators, FCC reactors 1)
Susceptiblity due to elements Mn, Si, P, Sn, Sb, As2) Significantly
reduces structural integrity of comp containing crack like flawFor
old material:1) Limit startup pressure to 25% of MDP below MPT
(safe oper. envelope).2) For weld repair, heating to 1150F and
rapid cooling to RT temp reverses TE. For new material:1) Best way
to Limit J for BM & X factor WM (100 to 15 for 2.25Cr-1Mo)2)
Newer less used Equivalent Phosphorus factor for WM & BM.1)
Confirmed thru impact testing (no effect on upper shelf energy).2)
Install test blocks of same heat no and impact test periodically3)
Ensure pressurization sequence to avoid BF4) SEM fractographs show
INTERGRANULAR cracking due to impurity segragation at GB.NONEStrain
Aging (Embrittlement)Combined effect of deformation and aging at
intermediate temp in old vintage CS and C-0.5MO steel results
increase in hardness, strenght and decrease in ductility and
toughness
Intermediate temp (with deformation)Old CS (pre 1980) with LARGE
GRAIN SIZE and C-.5 MoSteels made by BOF and fully killed with Al
not susceptibleMost likely in thk wall vessel made from susceptible
material not stress relieved.1) Steels made by Bessmer or open
hearth process, rimmed and capped steels with higher impurities (C
& N)2) Major concern for equipment having cracks (failure by
brittle fracture).3) Can occur when welding near cracks and notches
in susceptible matl.1) Not an issue for new steel with low
impuritie and Al > 0.015% to fully deoxidize stl.2) For old
eqpt, pressurize when temp > MDMT 3) PWHT or buttering of weld
repairs of old equipmentInspection NOT USED to control strain
agingWhen deformation is at intermediate temp it is Dynamic Strain
Aging. Blue brittleness also form of SA885oF Embrittlement Loss in
toughness due to metallurgical change in alloys containing FERRITE
PHASE due to high temp exposure600-1000 F (thousands of hrs may be
reqd at 600)400 SS (e.g. 405, 409, 410, 410S, 430 & 436), DSS
like 2205, 2304 and 25071) Refineries limit ferritic SS to non
press parts .e.g. fractionation trays in FCC, crude, coker units.
2) Cracks when weldings trays of 409, 4103) DSS HE tubes >
600F1) Increasing ferrite increases susceptibility (increase in
DBTT).2) Damage is cumulative due to precip of embrittling
intermetallic phase.1) Use of low or non ferrite alloys SS2)
Embrittlment is reversible by heat treatment to dissolve
precipitates. Use of PWHT 1100 followed by rapid cooling.1) Not
thru metallography but can be confirmed by Impact or bend test2)
Evaluation by increase in hardness3) Impact or bend test of samples
is positive indicator.4) Most embrittlement cracking during start
up & shutdown < 200FNONE Sigma Phase Embrittlement Formation
of sigma phase results in loss of fracture toughness in SS due to
high temp exposure 1000-1700 F300 SS wrought, welds and cast SS
(HK, HP having high 10 -40% ferrite content)400 Series (with 17 Cr
or more ) and Duplex SSsigmatized wrought materials still stuiable
at operating temp1) SS piping, cylcones, valves in HT FCC regen
service.2) 300 SS overalys, TTS welds during PWHT for Cr-Mo base
metal.3) SS tubes susceptible1) Sigma phase occurs in ferritic
& martensitic (Fe-Cr), austenitic (Fe-Cr-Ni) and DSS in given
temp range.2) Sigma can form in few hours in aust SS if subject to
PWHT (690 C)3) SS with sigma show complete lack of fracture
toughness by CVN < 500F4) Precipitation of hard, brittle
intermetallic compund1) Best way to use alloys resistant to sigma
formationsor avoid exp to temp range.2) 300 SS desigmatized by
solution annealing @1950F for 4 hrs & water quench.3) Control
ferrite to 5-9% for 347 ferrite and 304 still less.4) For SS clad
CrMo, exp time to PWHT to be limited1) Only confirmed by metall.2)
Cracking appears at welds during TA & startup below 500F3)
Testing sample removed from service is most positive indicator.NONE
Brittle Fracture Sudden rapid fracture under stress (R/A) with
little or no ductility or plastic deformation -- Older CS & LAS
prime concern (impurities) & 400 series SS also susceptible Sec
VIII-1 Eqpt subject to requirement of Ucs 66300 Series SSASME Sec 8
Div 1 vessels before Dec.1987 Add + Thk wall eqpt during start-up/
shutdown and Hydrotests at amb temp.Important 3 factors for BF:1)
Material Fracture Toughness by CVN, 2) Size, Shape & stress
conc. of flaw 3) Amount of R + A stresses on flawThicker matl have
lower resist to BF.In most cases, BF occurs only below the charpy
impact transition temp.1) For new eqpt best to use matl for low
temp (UCS 66 in sec VIII)2) engineering study with API 579 sec 3
level 1 or 2. 3) Some reduction in BF by PWHT if not done and for
weld repair + Warm hydrotest then cold HT to extend MSOT1) Cracks
are straight, non branching devoid of plastic deformation
(cleavage)2) Inspection normally NOT USED to mitigate BF.3)
susceptible vessels should be inspected for pre-existing
flaws/defectsTemper emb + Strain age emb + 885F emb + Ti hydriding
+ Sigma phase embrittlement Creep & Stress Rupture At high temp
metal can slowly deform under load below yield stress (time
dependent) leading to ruptureThreshold tempsAll Metals &
Alloys--1) HT Eqpt operating above creep range e.g. Heater tubes,
tube supports, hangers and furnace internals, FCC reactor, H2
reforming furnace tubes.2) Low creep ductility failures found in
weld HAZ at nozzles cat reformer.3) DSM welds (Fer to Aust) due to
TE stress.1) Damage (strain rate) is sensitive to load and temp.
Increase of about 25F or 15% of stress can cut remaining life in
half or more, depending on alloy.2) Life become nearly infinite
below Thresh temp3) Low creep ductility is more severe for high TS
matl and welds + more likely in coarse grain matl + promoted by
some carbides in CrMo steels.1) Little to do except Minimize metal
temp for FH tubes2) Higher PWHT to minimize creep cracking of metal
with LCD (1.25Cr-0.5Mo)3) Creep resitant alloys for heater tubes
for longer life + Minimuze hot spots on heater tubes and process
side deposits + VT and UT thk or strap measure to assess RL as per
API 579-1.4) For Eqpt, repair of creep damaged nozzles done by
grinding out affected area, rewelding and PWHT.1) Initial stage
(creep voids at GB) by SEM metallography2) Bulging of heater tubes
before final fracture.3) Creep damage micrvoid, fissures and dim
change not found by one technique but combination UT, RT, EC dim
measure & replica) used. 4) Remove sample & metallog to
confirm damage.4) For PV focus on welds of CrMo alloys in creep
range (check by VT, PT, WFTM, UT)5) Check FH tubes for diametric
growth, bulges, cracks, bows, blisters, wall thk measurementCreep
damage due to very HT + Reheat cracking Thermal Fatigue TF is the
result of cyclic stresses caused by variations in temp resulting in
cracking where DE is constrained (under repeated thermal cycling)No
set limit. As practical rule cracks may occur if temp swing exceeds
200FAll materials of construction--1) Mix points of hot and cold
streams e.g. H2 mix point in HP units, de-superheater or
attemporators.2) TF major problem in coke drum shells &
skirts.3) In steam gen common location at attachment bw SH and RH
tubes.4) Steam soot blowers if frist steam exit contain
condensate1) Key factor are magnitude of temp swing and number of
cycles2) Start-up & shutdown increase susceptibility to thermal
fatigue.3) Damage promoted by rapid changes in surface temp
(thermal shock).4) Notches (weld toe) and sharp corners act as
initiation sites1) Best prevented by proper Design & Operation
to minimize Thermal Stresses & Thermal Cycling. i.e.->
reduce stress conc, blend grind weld profile-> Controlled rate
of heating / cooling during startup / SD -> DE between DSM
material to be considered.2) Design should incorporate flexibility
e.g. avoid rigid attach + drain on soot blower1) Surface cracks,
wide and often oxide filled due to temp exposure, single or
multiple.2) Cracks propagate TRANSVERSE to stress and Dagger shaped
and TG.3) Cracks follow toe of fillet weld4) Water in soot blower
lead to crazing pattern5) VT, PT, MT, SWUT (for non intrusive) and
special UT for heavy wallCorrosion fatigue & Dissimilar metal
cracking Short Term Overheating Stress Rupture Permanent
deformation occuring at relatively low stress levels due to
localized overheating results in bulging & stress rupture--All
FH tubes and common materials of construction--1) All boilers &
FH tubes2) Furnaces with coking tendency crude, heavy oil HP and
coker units fired harder to maintain outlet temp prone to localized
heating.3) Refractory lined equpt in FCC (refractory damage)1)
Usually flame impingement or local overheating (above design
temp)2) Thk loss due to corrosion reduces Time to failure.1) in FH,
burner management and fouling / deposit control to minimize hot
spot.2) Use diffuse flame burner3) in HP unit, install TC in
reactors.4) Maintain refractory in serviceable condition. 1)
Localized deformation or bulging (3 to 10% or more) depend on temp,
alloy and stress level2) FISH MOUTH Failures by thinning at
fractured surface. 3) In FH, VI, IR montg, TST of tubes4) RLE
monitor by heat indic paingt or IR scan5) Reactor bed and skin
TCCreep/ Stress rupture Steam blanketing Normal heat flow results
in formation of discrete steam bubbles (nucleate boiling) on tubes
ID, when heat flow disturbs, bubbles join to form steam blanket
known as Departure from Nucleate Boiling (DNB). Tube rupture occurs
rapidly due to SHORT TERM OVERHEATING (in few minutes) -- Carbon
steel and Low alloy steel--All steam generating units (Fired
boilers + WHB, H2 reformers & FCC units), Failures occur in
super heaters and re-heaters during s/u when condensate blocks
steam flow.1) Heat flux & fluid flow critical factors 2) Flame
impingement from damaged or misdirected burners3) on Water side
anything that restricts flow (condensate, tube dent) cause DNB4)
Failure occurs from hoop stress due to steam pressure at HT1) When
DNB deveolpes tube rupture will quickly follow, proper burner
management to prevernt flame impingement. 2) BFW treatment to
prevent restricted fluid flow3) VI of tubes for bulging.1) STHTF
always show open bursts with fracture edges drawn to knife-edge
(ductile rupture)2) Microstructure always show severe elongation of
grain by plastic deformation at time of failure.3) Properly
maintain burners to avoid flame impingement.Steam blanketing can
cause caustic corrosion (Causitic Gouging) + Short term Over
Heating.DMW Cracking Cracking occurs in ferrtic (CS or LAS) side of
weld with Aust (300 SS or Ni alloy) at high temp resulting from
creep damage, fatigue cracking, Sulfide stress cracking or H2
disbonding (PWHT will not prevent cracking)Temp >510F (260C)
signifcant thermal expansion / fatigue stress in Ferr to Aust
jointCS & LAS welded to Aust SS.Any material comb having widely
differing thermal exp coeff--Overlayed CrMo nozzles to solid SS
pipe welds in HP reactor outlets + H2 reformer 1.25Cr-1Mo pigtails
to alloy 800 sockolet of tubes + alloy 800 o/l manifold to CS or
1.25CR transfer line + 300SS overlays in PV + all superheaters and
reheaters have DMW welds1) cracking occurs due different COE b/w
ferr and aust differ by 25-35% (leads to high stress at HAZ on ferr
side)2) stress on weld higher when Aust filler used than nickel
filler (COE closer to CS)3) DMW have narrow mixed zone of high
hardness near fusion line with ferr and susceptible to H2 cracking
or SSC.4) C diffusion from ferr HAZ into weld reduces creep
strenght (at 800 to 950F).5) In liq ash corrosion environ, ferr
will preferentially corrode.1) For HT, use NI base filler with COE
closer to CS & LAS can increase life of joint.2) If 300 SS
electrodes used DMW shuold be located in lower temp region.3)
Consider buttering on ferr (6.35mm) & PWHT to minimiz hardness
of mixed zone4) Install pup piece of intermediate thermal exp. 1)
Cracks form at weld toe in HAZ of the ferrtic material,2) Tubes
welds are problem area but support lugs of 300 SS to 400 SS are
also affected3) For new DMW welds, 100% PT after buttering, 100%
UT/ after PWHT, 100% UT/ RT & PMI.4) For FH tubes, RT and
SWUTsuch as top flangesThermal fatigue & Corrosion fatigue
& Creep and SSC. Thermal Shock A form of thermal fatigue
cracking - Thermal shock occurs when high and non-uniform thermal
stress develop in short time due to diff expansion or contraction,
usually when colder liquid contacts a warmer metal surface. --All
metals & Alloys--1) FCC cokers, catalyic reforming and HP units
in HT service.2)Material with lost ductility such as CrMo (temper
embr) more susceptible. 3) Equpt subjected to accelerated cooling
to minimize s/d time1) Temp cycling may initiate fatigue cracks (SS
have higher COE than CS or NI and more prone)2) Frature from stress
above YS result due to restraint thermal exp.3) Thick sections
develop high thernal gradient1) Prevent flow interruption in HT
line. 2) Minimize severe restraint. 3) Thermal sleeves to prevent
liquid impingement on PB4) Review hot/cold injection points 1)
Surface initiating cracks may also appear as CRAZE CRACKS2) PT and
MT to confirm cracking.3)Highly localized & difficult to
detect.Thermal fatigue Erosion / Erosion-Corrosion Erosion:
Accelerated mechanical removal of surface material by relative
movement, or impact from solids, liquids, vapor etc.
Erosion-Corrosion: when corrosion contributes to erosion to
remove protective film or scale expose metal to further
corrosion--All metals, alloys & refractories--1) All types of
eqpt exposed to moving fluid and/or catalyst are subject to EC2)
Gas or Liquid born particles (slurry) in pumps / compressor3) HP
effluent piping EC by ammonium bisulfide depends on vel.4) Crude
and VDU piping by naphthenic acid1) Metal loss depend on vel, conc
of impacting medium, size & hardness of particles, CR of matl
& Anlge of impact.Increasing hardness of substrate is common
approach, but not always good indicator of improved resistance to
erosion corrosion2) For each environment-material combination there
is often threshold veolocity for metal loss3) Increased corrosivity
by temp, PH reduce protective film stability and increase
susceptibilyt to E-C1) improve desing (increase pipe dia to reduce
velocity, streamline bends, wall thk)2) For Erosion: Hardfacing,
surface hardening, erosion resistant refractory3) For EC: Best
mitigated by more CR alloy or altering process (not by substrate
hardness only)4) higher Mo for NAC5) impingment plates and
ferrultes in HX1) Localized thk loss in form of pits, grooves,
gullies, waves, holes and valleys (directional pattern).2) VT, UT
and RT for metal loss3) Corrosion coupons and ER online probes4) IR
scan for refractory lossSpecialized terminology For various Forms
e.g. Cavitation, liquid impingement erosion, fretting etc.,
Cavitation Form of EROSION caused by the formation and
instanteneous collapse of tiny vapour bubbles, the bubble may
contain vapor phase of liquid, air or other gas, these collapsing
bubbles exert severe localised impact forces and result in metal
loss--Common materials e.g. Copper, brass, cast iron, CS, LAS, 300,
400 SS & Ni base alloys.--Pump casings, pump impellers (LP
side) and piping d/s orifices and CVs + restricted-flow passages by
turbulent flow (eg HX tubes, venturis, seals)1) Inadequate NPSH
(min head required to avoid cavitation).2) Temp approaching boiing
pt of liquid result in bubble formation.3) Presence of solid or
abrasive particles is not required for cavitaion but accelerate the
damage.1) Not significatnly improved by material change. Modify
design or operating change.2) Best prevented by avoiding abs
pressure to fall below vap press or change material properties
(changing to more CR or hihger hardness may not improve cavit
resist).1) Looks like sharp-edged pitting or gouged appearance in
low pressure zone2) Cavit pump sound like pebbles thrased.
Accoustic montg. turbulent areas for characteristic freq.3) VT, UT
& RT for thk loss.Liquid Impingement or ErosionMechanical
FatigueMachanical degradation that occurs when component exposed to
cyclic stresses for an extended period, resulting in sudden
failure. These stresses arise by mechanical loading or thermal
cycling well below yield strength.--All engineering alloys--1)
Thermal cycling: daily cycles in oper like coke drums + eqpt in
intermittent service as aux boiler + quench nozzles2) Mechanical
loading:pump rotating shafts at keyways + small dia piping in
vibration + high PD CV or steam reducing stations can cause
vibration problem in piping1) Design: FC initiate at surface
notches or stress raisers under cyclic loading. Design is most imp
for fatigue resist.2) Metallurg : For Ti, CS, LAS cracking will not
occur below Stress Endurance limit (ratio of EL to UTS is
40%-50%).3) Aust SS & Al dont have EL and FL is defined by num
of cyles at given stress.4) Finer grain have better fatigue resist
than coarse grain (HT such as quech + tempering improves FR).5) Max
cyclical stress ampliude is determined for 106-107 cylces (desired
in lifetime) 7) Inclusions in metal (dirty steel) have accelerating
effect on FC.1) Best defense is good design that minimize stress
conc in cyclic service.2) Select metal with FL for intended cyclic
life.3) Mimimize grinding marks, nicks, good fit-up and smooth
transition of welds, mimize weld defects, remove burrs or lips by
machining, use low stress stamps and marking tools.1) Signature
mark is clam Shell fingerprint that has concentric rings "Beach
marks" that results from Waves" of crack propagation occuring
during cycles above threshold loading (single for flaw with stress
conc and multiple w/o stress conc). 2) PT,MT & SWUT to detect
fatigue cracks at known pt.3) VT of small dia piping to detect
oscillation4) Vib montg of rot equpt5) In high cycle fatigue, crack
initiation difficult as crack initiation is majority of
FL.Vibration induced fatigue.Vibration Induced FatigueForm of
mechanical fatigue in which cracks are produced as result of
dynamic loading from vibration, water hammer or fluid flow.--All
engineering materials--1) Socket welds and small bore piping near
pumps & compressors.2) PSV subject to chatter, premat pop-off,
fretting, high pd CVs, HE tubes susceptible to vortex shedding.1)
high likelihood of cracking when input load synchronous to natural
freq.2) Lack of stifness or support allows vibration and cracking
initiated at stress raisers.1) VIF can be eliminated or reduced by
design and Supports & Vibrations dampening (matl upgrade not a
solution).2) Vortex shedding minimized at o/l of CV and PSV by side
branch sizing and flow stab.3) Vibration may be shifted when comp
anchored, unless source removed.1) Crack initiate at high stress
(thread or weld joint). 2) For Refractory damages, skin temp
measurement3) Check for visbile & audible signs of vibration,
during start up, s/d, upset. Measure vibration.4) check pipe
supports and spring hangers5) Damage to insul jacketing can cause
CUI.Mechanical fatigue & Refractory degradationRefractory
DegradationBoth thermal insul and erosion resist refractories
susceptible to mech damage (Cracking, spalling & erosion) and
corrosion due to oxidation, sulfidation, and other high temp
mechanisms--Refractory Matls inccl Insulating ceramic fibers,
castable, refractory brick and plastic refractories.--FCC Reactor
regenerator vessels, piping, cyclone, waste heat boiler, Boiler
firebox and stacks1) RLE should be designed for erosion, thermal
shock and expansion.2) Dry out and curing should be as per
manufacturer specs or ASTM req.3) Anchors resistant to condensing
sulf acids, HT oxidation and COE near BM.4) Refractory resistant to
erosion and abrasion and needles compatible with process env.Proper
selection of refractory, anchors and fillers and design &
Installation are the keys to min ref damage. 3) Excessive cracking,
spalling or lift-off from the substrate, softening or moisture
degradation.2) In erosive service, refract may be thinned exposing
anchors.2) Visual inspection during shut downs + IR scans
Oxidation, Sulfidation and Flue gas dew point corrosion.Reheat
CrackingCracking of metal due to stress relaxation during PWHT or
inservice at elevated temp, mostly in heavy wall sections.Above 750
FLAS (CR-Mo steels with V), 300 SS & Ni based alloys e.g Alloy
800 H--Mostly like to occur in heavy wall vessls in areas of high
restraint i.e. nozzle welds and heavy wall piping + HSLA steels are
susceptible1) RC requires high stresses and more likely in thick
sections2) RC occurs at ET where creep ductility is insufficient to
accomodate strain.3) Transverse cracks occured in 2.25Cr-1Mo-V in
SAW welds only traced to contaminated flux (cases in 2008)4) RC can
occur during PWHT or in service at high temp.5) Cracks are
INTERGRANULAR without deformation.6) Stablizing HT and SR of 300 SS
for CL-SCC and PTASCC can cause RC.1) Joints in heavy wall to be
designed to minimiz restraint during welding and PWHT.2) Large
grain size less result in less ductile HAZ making matl susceptible
to RC.3) Avoid stress conc e.g. long seam welds mismatch.4) For
2.25cr-1Mo-V Gleeble Tensile Screen Test reqd.5) For 800H,
inservice RC risk reduced using matching weld metal with AL+Ti 540C
matl requires stabiliz heat treatment and SR of welds6) for
thickwall SS piping, avoid PWHT1) RC is INTERGRANULAR and SB or
embedded mostly in COARSE-GRAINED sections of weld HAZ.2) Surface
cracks by UT, PT & MT, EMBEDDED cracks found only through UT.3)
Inspection for RC in 2.25Cr-1Mo-V reactor during fabrication
typically done with TOFD or manual SWUT.Also referred to as Stress
relief cracking and Stress relaxation crackingGaseous Oxygen
Enhanced Ignitiion and CombustionMany metals are flammable in
oxygen & enriched air (>25% O2) at low pressures which are
non fllammable in air. Spontaneous ignition of metals and
non-metals can cause fires and explosion in O2 rich gaseous
environments if not properly designed, operated or maintained.1) CS
& LAS are flammable in LP O2 > 15psig.2) Aust SS difficult
to below 200 psig.3) Al is avoided for flowing O2, if ignited burns
quickly.4) Easiest to ignite plastics, rubbers and HC lubricants.5)
Ti avoided in O2 rich service, can sustaintain comb at 1psiA O2
press.Copper (>50%) and Ni (>50%) alloys are fire resistant /
non flamm.Alloy 400 highly resistantO2 is used in sulfur recvory
units (SRU), FCCU, paritial oxidation units (POX) + O2 piping,
valves, regulators and impingement areas are vulnerable1) Primary
concern in high vel O2 flow particulate entrainement and impinging
on surface. O2 vel in CS & SS piping should comply with
industry limits (depending on impigement or non impingement).2)
Ignition temp for most alloys are near melt point in non flow
condition. Actual system can suffer ignitiion under flow at room
temp and lower due to particle impact.3) Contamination of O2 system
with metal fines or HC (oil or grease) can cause fire during
startup4) impingment areas, elbows, valves higher risk of ignition
in flowing O2.1) O2 fire are sudden event, best prevention to keep
system clean after insp or maint.2) Maintain velocity in recommend
limit. Avoid vel > 100 ft/sec 30 m/s).3) Use only O2 compatible
lubricants.4) Do not open unnecessary open O2 systems, for insp to
avoid contamination.5) Do not use plastic pipe, minimize person
during start up near O2 system.6) Thin SS ( 2-3 fps or acid conc
< 65%.2) Mix point with water release heat and high CR where
acid diluted.3) Oxidizers greatly increase CR.1) Corrosion
minimized thru materials selection and oper within design
velocities.2) Alloys such as Alloy 20, 904L and C-276 resist dilute
acid corrosion and form a protectiveiron sulfate film on the
surface.3) Acidic streams can be washed with caustic to neutralize
acid.1) General corrosion but attacks CS weld HAZ rapidly (attacks
weld slag).2) H2 grooving may occur in low flow or stagnant areas
such as in storge tanks or rail cars.3) If CR & vel high, there
will be no scale.4) Corr of steel by dilute acid is usually overall
metal loss or pitting, becomes severe with increasing temp &
vel.5) UT or RT inspection of turbulent zones and hottest areas.6)
Coupons and ER probes.Not ApplicableAqueous Organic Acid
CorrosionOrganic compounds in some crude oils decompose in crude
furnace to from low MW organic acids which condense in distillation
tower OH systems and contribute siginificantly to aq corrosionAll
grades of carbon steel are affectedMost CR alloys crude tower OH
system not affected1) All CS piping & eqpt in crude tower,
vacuum tower, and coker OH system incl HX, towers and drums.2)
Corrosion occur where water accumulates or water droplets impinge
and turbulent areas e.g. Botm of OH HX, boots of separtor drum,
elbows, tees, d/s of CV.1) Low MW acids incl formic, acetic,
propionic and butyric acid.2) Formic and acetic acid are most
corrosive, soluble in naptha, once water condenses lower the pH.3)
Presence may be masked by other acids HCL, H2SO4, H2CO3 etc.4) Type
and quantity of organic acid in OH system are crude specific, one
source is decompose of napthenic acid in crude.5) Light OA not as
corrosive as IOA. To calculate HCL equivalent factor, multiply OA
ppm-wt by the factor, result will be equivalent content of HCl (in
ppm).1) Corrosion by LOA in crude OH system can be minimized by
injecting neutralizer but problem with changes in crude blend.2)
Filming amines can prevent corrosion if it doesnt react with OA,
but it is not as effective as neutralization.3) Upgrading to CR
alloy1) Light OAC leaves surface smooth and difficult to
distinguish from HCL corr.2) In significant flow, surface smoothly
grooved.3) UT and RT inspection for thk loss, LRUT for long pipe
run.4) For CS, damage is general thinning or hihgly localized where
water condenses.5) Process monitoring for pH and water analysis in
crude tower OH drum for OA.6) Corrosion probes and/or
coupons.Damage difficult to differentiate from HCL corrosion. Amm
Chloride corrosion and Chloride SCC also related.
5.1.2 Env Asst Crack-Ref5.1.2. Environment Assisted Cracking -
Refining Corrosion Mechanism Description Temp. Range (F)Affected
metallurgy Not AffectedAffected EquipmentCritical Factors
Prevention Inspection / MonitoringRelated MechanismPolythionic Acid
Stress corrosion cracking (PASCC) SCC normally occuring during S/D,
S/U or during operation when air and moisture are present. Cracking
due to sulfur acids forming from sulfide scale, air & moisture
acting on sensitized Aust SS (adjacent to welds or high stress
areas). SCC may propagate rapidly (min or hrs).300 Series SS, Alloy
600/600H and 800/800H.1) HE tubes, furnace tubes and piping
sensitized and in sulfur environ.2) FH burning oil, gas, coke
depending on sulfur levels in the fuel.3) FCC, hydroprocessing
units (heater tubes, hot feed/effluent HX tubes, bellows), Crude
and coker units (piping).+ Boilers and HT eqpt exposed to sulfur
combustion products1) combination is required of:A-environment:
metal form sulfide scale exposed to sulfur comp. It reacts with
moisture and O2 to form sulfur acids (PA).B- metal must be in
sensited cond.C- stress Residual or applied2) Sensitization refers
to chromium carbide formation in grain boundary of metal in temp
range 750 to 1500F.3) Low C grades ( 750F.4) Improved resistance
with Ti & Nb stab grades e.g. SS 321, 347 and Ni alloys 825,
625.5) Thermal stab heat treatment at 1650F stab SS welds but
diffic in field.6) Susceptiblity by lab corr test as per astm A262
prac C.1) Typically occurs next to welds, but can also occur in
base metal (may not be evident until a leak appears during s/u or
in some cases, operation).2) Cracking is INTERGRANULAR, corrosion
or loss in thk is usually negligible.3) PT used to detect PASCC
after flpper disc sanding to remove tight deposit.4) PASCC is
inspection challene bec crack may not occur until shutdown.5) Montg
for PASCC during operation not practical bec condition not
present.Also know as Polythionic Acid Stress Corrosion Cracking
(PTA SCC), Intergranular Corrosion (IGC) andIntergranular Attack
(IGA).Amine stress corrosion crackingAmine cracking is common term
applied to cracking of steels in aqs alkanolamine systems used to
remove H2S and/or CO2 and mixtures from various gas and liquid HC
streams. It is most often found in non PWHT CS welds or cold worked
parts.Cracking has been reported at ambient temperatures with some
amines.
Increasing temperature and stress level incr the likelihood and
severity of cracking.Carbon steels & Low alloy steelsAll
non-PWHT carbon steel piping and eqpt in lean amine service
including contactors, absorbers,strippers, regenerators and heat
exchangers as well as any equipt subject to amine carryover.1)
Cracking more likely in MEA & DEA but also found in MDEA &
DIPA.2) API RP 945 for PWHT requirement for amine services.3)
Cracking most often in lean amine, rich amine cracking due to wet
H2S problem.4) Some refineries believe crack not occur below amine
conc 2-5% but steam out can reduce limit to 0.2%.1) PWHT all CS
welds, repair welds and attachment welds as per API RP 945. 2) Use
solid or clad SS Alloy 400 or other CR alloys in lieu of CS.3)
Water wash non-PWHT CS prior to welding, heat treatment or
steamout.1) Surface breaking cracks primarly in weld HAZ but also
in weld & high stress areas.2) Crack typically parelell to
welds, but inside weld transverse or long.2) At Set on Nozzles
cracks radial to BM and in set in nozzle parallel to weld.3)
Appearance similar to H2S cracking4) Positive ID by metallography
i.e. INTERGRANLUAR (Oxide filled, branched)5) Crack detection best
by WFTM, ACFM.6) PT not effective for tight, scale filled cracks.7)
SWUT for crack depth (not branched), and AET for crack growth.Amine
stress corrosion cracking is a form of Alkaline SCC. Caustic SCC
and carbonate SCC are two other forms of ASCC similar in
appearance.Wet H2S Damage (Blistering / HIC / SOHIC / SSC)Four
types of damage:a) H2 Blistering: . H atoms form during sulfide
corrosion, diffuse into steel, and combine to form molecule at
lamination or or inclusion, too large to diffuse out & press
builds to deform metal as surface bulges on ID, OD or within wall
thk.b) HIC: Adjacent blisters at different depths develop cracks,
that link them, have stair step appearance refer to as "stepwise
cracking".c) SOHIC: Similar to HIC but more damag form of cracking,
appears as array of cracks stacked on top of each other. Result is
thru thk crack perpendicular to to surface driven by high sress (R
or A). Appear in HAZ initiating from HIC or SSC.d) SSC: Form of HIC
result from absorption of atomic H2 produced by sulfide corr. It
can initiate on surface at high hardness zone in weld metal (cover
pass not temp) and HAZ. PWHT is beneficial to reduce hardness and
residual stress. Use preheat helps minimize hardness
problems.Blistering, HIC, and SOHIC damage have been found to occur
between ambient and 300F or higher.
SSC generally occurs below 180F (hihger if Aq phase of
H2S).Carbon steel and low alloy steels.1) 4 damage can occur in all
refinery where wet H2S present.2) in HP unit, incr conc of NH4HS
above 2% incr potential of Blistering, HIC and SOHIC in vapor
recovery unit of FCC and coker, fractionator tower, absorber &
stripper tower, comp interstage separator, ko drum, HX etc prone
bec of high NH4HS conc.3) SSC most likely in hard weld and HAZ, HS
comp such as bolts, relief valve springs, 400 SS valve trim,
compressor shaft, sleeves etc.1) pH: H2 diffusion minimal at pH 7,
HCN in water incr permeation in alkaline water. Fav cond for
damage:- >50 ppmw H2S dissolved in water- free water pH7.6,
20ppmw doslvd HCN and some dissolved H2S.- >0.0003 Mpa (0.05
psia) partial pressure H2S in gas phase- incr NH3 can >pH where
cracking can occur2) H2S:Arbitrary value 50 ppmw used for H2S conc
that cause problem but even 1 ppmw was found sufficient for H2
charging of steel. Susceptibility to SSC incr with incr H2S pp >
0.05 psi with TS about 90 ksi or hardness > 237HB.3)
Temperature:H2 charging potential incr with temp but SSC potental
max near amb temp.4) Hardness:Low CS should have weld hard < 200
HB as per NACE RP 0472 (not suscept to SSC unless hardness > 237
HB). Blistering, HIC and SOHIC not related to hardness.5) Steel
making:HIC often found in diry steel. Steel chemistry and manuf
tailored to make HIC resistant in NACE Pub 8X194 (but subsurface
SOHIC still possible)6) PHWT:PWHT will not prevent Blitering and
HIC but effective against SSC and SOHIC by reduce hardness and
residual stress. 1) Effective barrier that protect steel sruface
from wet H2S can prevent damage including alloy cladding and
coatings.2) A common practice to use wash water injection to dilute
HCN conc in FCC gas plant or convert HCN to harmless thio cynate by
inject ammonium polysulfie.3) HIC resistant steel to reduce
blistering & HIC per NACE Pub 8X194.4) SSC prevented by
reducing hardness of weld and HAZ to 200HB (Max) by preheat, PWHT,
WPS, and carbon equivalent (refer to NACE RP 0472).5) PWHT can
minimize SOHIC but not blistering and HIC.6) Special corrosion
inhibbitor.1) H2 blisters appear as bulges on surface of steel of
PV, found rarely on pipe and very rarely in middle of a weld. HIC
can occur wherever blistering or subsurface laminations are
present.2) In PV, SOHIC and SSC damage is most often associated
with welds. SSC also be found where zones of high hardness or in HS
steel components.3)Process condition evaluated by PE and Corro
specialist to identify eqpt prone to H2S damage.3) Insp for H2S
focusses on welds and nozzles (detection and repair outlined in
NACE RP 0296). 4) Crack detection best by WFMT, EC, RT or ACFM
technique. Surface prep usually not required for ACFM.5) SWUT
especially useful for insp and crack sizing. ER instruments not
effective for crack depth measuring.6) AET used to monitor crack
growth.7) Grinding or gouging crack another method to crack depht
measure.SSC is form of H2 Embrittlement + Amine cracking &
Carbonate crackign are simlair and also confused sometimes with wet
H2S crackingHydrogen Stress Cracking - HF Environmental cracking
that can initiate on surface of carbon steel and HSLA with
localized zones of high hardness in weld metal and HAZ as result of
exposure to aqueous HF acid environ.Carbon steels & Low alloy
steelsAlloy 400 not susceptible to cracking but prone to IG SCC in
non SR cond.1) Eqpt exposed to HF acid at any conc and hardness
above recomm limit.2) HSLA (ASTM A 193-B7) bolts and compressor
components.3) B7M bolts if overtorqued1) Susceptibility incr with
increasing hardness > 237 HB highly prone.2) Cracking may occur
rapidly in hours after exp to HF environment.3) Hard microstructure
may form in low heat input welds, HAZ, in LAS or inadequate PWHT1)
PWHT reduce residual stress and hardness.2) Low strength CS with
weld hardness < 200HB (NACE SP 0472 susceptible if >
237BH).3) Use CS with CE: 100 ppmw + with or without cyanides and
polysulfide3) FCCU feed quality and operation affect cracking
susceptiblity i.e. N2 higher in cases where ASCC occur + cracking
usualy in low sulfur feed + mostly N/S ratio in feed of 0 to 704)
In CO2 removal unit, cracking when CO2 content > 2% and temp
exceed 200F (93C).1) Post-fabrication SR heat treatment of about
1200-1225F as per WRC 452 proven method to prevent ACSCC (for
repair, int-ext attachment welds).2) Cracking can be eliminated
with barrier coatings, solid or clad with 300SS, use Alloy 400 in
lieu of CS.3) water wash non PWHT prior to steam out or heat
treatment in hot carbonate service.4) A Metavanadate inhibitor to
prevent cracking in hot carbon system in CO2 removal unit.1)
Cracking typically paralell to weld in HAZ or BM within 2" of
weld.2) Cracking may also occur in weld.3) Patternn is spider web
of small cracks + INTERGRANULAR + very fine oxide filled cracks
similar to caustic SCC, amine SCC.4) Cracks may be mistaken for SSC
or SOHIC but further away from weld toe.5) Montg of pH of FCC sour
water is fastest and cost effective method to locate areas prone to
ACSCC.6) Monitg of CO3- conc in sour water.7) Crack detect best by
WFMT or ACFM. PT cannot find tight, oxide filled cracks and should
not be used.8) SWUT for crack depth measure but ER instrument not
suitable due magnetic oxide in crack.AET for crack growth.9)
Grinding out crack is viable method to measure depth (cracks dont
extend by grinding)Amine cracking and caustic stress corrosion
cracking are to similar forms of ASCC.
5.1.3 Other Mechansim5.1.3. Other Mechanisms - Refining Indsutry
Corrosion Mechanism Description Temp. Range (F)Affected metallurgy
Not AffectedAffected EquipmentCritical Factors Prevention
Inspection / MonitoringRelated MechanismHigh temperature Hydrogen
Attack (HTHA)HTHA results from exposure to H2 at elevated
temperatures and pressures. The H2 reacts with carbides in steel to
form methane (CH4) which cannot diffuse through steel. Loss of
carbide causes an overall loss in strength. CH4 pressure builds up,
forming bubbles or cavities, microfissures and fissures that may
combine to form cracks.Failure can occur when cracks reduce the
load carrying ability of pressure part.Iincreasing resistance: CS,
C-0.5Mo, Mn-0.5Mo, 1Cr-0.5Mo, 1.25Cr-0.5Mo, 2.25Cr-1Mo,
2.25Cr-1Mo-V, 3Cr-1Mo, 5Cr-0.5Mo and similar steels.300 SS, 5Cr,
9Cr and 12 Cr alloys, not susceptible to HTHA at cond normally in
refinery units.HP units, such as hydrotreaters (desulfuriz) and
hydrocrackers, catalytic reformers, H2 producing and cleanup units,
such as pressure swing absorption units, are all susceptible. +
Boiler tubes in very high pressure steam service.1) HTHA preceded
with time period when no changes in properties is noted.2)
Incubation period is time during which damage is enough to be
measured with insp techniques.3) Cruves for temp, H2 pp and safe
oper limits for CS, LAS in API 941 (reasonably conservative for CS
up to 10,000 psi pp).1) Use LAS with Cr and Mo to increase carbide
stability and minimize methane formation, also W & V
stabilizer.2) Norma 25F to 50F safety factor approach when using
APIRP941 curves.3) Several failures of C-0.5Mo, so its curved
removed and not recommended for new eqpt in hot H2 service.4) 300SS
overlay or clad at H2 service if BM does not have adequate
sulfidation resist + decrease in partial pressures for outgassing
in shutdowns1) HTHA can be confirmed by special techniques incl
metallography.2) H2/C reaction can cause surface. If C diff
limited, then internal decarb, methan formation and cracking.3) In
early stage, bubbles / cavities can be detected by SEM (difficult
to distinguish with creep cavities). Early HTHA only be confirmed
by metallogrpahy.4) In later stage, fissures or decarb can be seen
by microscope of replica metall.5) Cracking and fissuring are
intergranular & occur adjacent to pearlite (Fe carbide).6) Some
blistering due to molecular H2 or methane in lamination visible by
VT.7) Damage occur in BM, weld and HAZ SO INSPECTION IS VERY
DIFFICULT.8) UT with vel ratio and ABUT succesful to find fissures
or cracking only if damage reached pt when microvoid visible at
1500X magnification by microscope.9) Bulging of cladding from BM is
tell tale sign that HTHA occured.10) In-situ metallog can detect
only micro fissure, fissureing and decarb near surface (but decarb
may be due to HT).11) Conventional WFMT and RT severely limited in
ability except advaced stage of cracking. AET not proven method.A
form of HTHA can occur in boiler tubes and is referred to by the
fossil utility industry as hydrogen damage.Titanium HydridingTi
Hydriding is metallurgical phenomenon in which hydrogen diffuses
into titanium and reacts to form an embrittling hydride phase,
result in complete loss of ductility with no noticeable sign of
corrosion or thickness loss.Occurs above 165F (74C) and at a pH
below 3, pH above 8 or neutral pH with high H2S content + above
350F in absence of moisture and O2Titanium AlloysPrimarily in sour
water strippers and amine units in OH condensers, HE tubes, piping
and Ti eupt operate > 165F & also above 350F (177C) in the
absence of moisture or O2 + CP equpt with potential values 1000f -
CS >1500 - 300SS Iron-based alloys Upgrading more resistant
alloy* UT Sulfidation Hydrogen accelerates corrosion- Uniform
thinning ( localized sometimes) >500, 1100F Iron-based alloys
Lower temp, higher O2&S partial pressure MG, hardness testing
Decarburization Requires low carbon-activity gas, CS will be pure
iron. --- CS, LAS Add Cr, Mo FMR, MG, hardness test Metal Dusting
Preceded by carburization-pits filled with crumbly residue of
oxides and carbides in LAS Deep round pits in SS 900-1500 All No
metal is immune-H2S forms protective sulfide layer VT* ,Heater
tubes-Compression wave UT* Fuel Ash Contaminants are S, Na,K,V-
Molten dissolves oxide layer & 50 Cr-50 Ni more resistant All
Injecting special additives VT Nitriding Its rare- Nitrogen
diffuses into surface forming needle-like particles of Fe3N and
Fe4N hard brittle surface layer dull gray dry Starts>600, severe
>900 CS, LAS, 300SS, 400SS 30-80% Ni Mg, hardness test, VT,
magnetism (300SS) Environment- Assisted Cracking Chloride SSC,
Surface initiated cracks under the combined action of tensile
stress, temperature and an aqueous chloride environment. >140 F
SS300 Use low chloride water PT, Phase analysis EC techniques *
Corrosion Fatigue Initiate @ pits, notches, surface defect. brittle
fracture ,Transgranular, ,not branched. ---- All Design UT, MTWFMT
Caustic SCC ( Embrittlement) Surface cracks adjacent to non-PWHT-
welds- Integranular spider web and filled w/oxides- Transgranular
in 300ss-50 to 100 ppm is sufficient for cracking ----- CS, LAS,300
SS PWHT, Ni alloys are resistant WFMT,EC,RT /ACFM,*PT Not Good
Ammonia SCC Copper alloys: aqueous, 8.5 pH, O2, zinc>15%, bluish
corrosion product, trans/intergranular cracksCS- anhydrous
ammonium, 3FPS and/or >65% concentration- General corrosion ----
All Ni alloys RT,UT, ER probes General- Environment-Assisted
Cracking Polythionic acid Stress CC (PASCC) Occurs during
shutdowns, startups, when exposed to air& moisture-HAZ L grade
SS is less susceptible to sensitization- Intergranular cracks.
750-150 for sensitization Sensitize austenitic steels Use
chemically stable steel (321ss, 347, alloy 825 and alloy625) PT
Amine Stress CC Occurs in lean amines- MEA, DEA mainly-
concentration is not a factor- initiate ID on welds ( transverse or
longitudinal) or adjacent to HAZ- Intergranular and filled with
oxides >ambient CS, LAS PWHT, Resistant alloys ACFM/WFMT, PT(not
good)* Wet H2S Damage (Blistering/HIC/SOHIC/SSC) Caused by atomic
hydrogen. H 2 is formed due to corrosion. 5Cr are not susceptible
to HTHA- may cause decarburization- Intergranular with blistering
sometimes. --- CS, C-Mo, Cr-Mo Add Cr&Mo UT techniques Titanium
hydriding Hydrogen diffuses into titanium to form hydride
(brittle)- PH8 >165f, > 350 in hydrogen atmosphere. Titanium
alloys Avoid titanium alloys in hydriding services EC
Techniques