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