Yuav Ua Li Cas Thiaj Li Muaj Feem Ntau Ntawm Thawj Titanium Nitride (TiN) suav nrog hauv 316L rhaub Sheath Tubing Control Pitting Pib Qhov Chaw hauv Chlorinated Seawater ntawm 40 degree

Kev puas tsuaj ntau tshaj -Resistant Inclusion In Oxidising Conditions

The use of chlorine (normally 0.5-2 ppm residual) as a biocide results in highly oxidising conditions for 316L stainless steel sheathed electric heating tubes in chlorinated seawater service (e.g., marine heat exchangers, cooling systems for coastal power plants or offshore platform heaters). Under these conditions, the common pitting initiators, manganese sulphide (MnS) inclusions, are oxidised rapidly and dissolved, but primary titanium nitride (TiN) inclusions become the dominant pitting initiator sites. TiN inclusions are hard, angular and electrically conducting. During solidification , precipitation of chromium-rich carbides and nitrides occurs leading to depletion of chromium and molybdenum at their edge in the surrounding austenite matrix . TiN inclusions >5-10 µm nucleate pits hauv dej hiav txwv chlorinated ntawm 40℃hauv 500-1,000 teev. Khoom siv nrog TiN ntim feem<0.001% (essentially TiN free) remains pitting free for >10,000 teev. This paper assesses the statistical link between TiN inclusion content, the pit initiation density and the service life in chlorinated seawater.

TiN Induced Pitting Nyob rau hauv Oxidising Conditions: A Mechanistic Study

Titanium feem ntau ntxiv rau 316L raws li cov khoom ruaj khov (316Ti) lossis ua los ntawm -cov khoom ntawm titanium-deoxidized steelmaking. Kev ua kom muaj zog ntawm qhov kub thiab txias ua rau tsim cov titanium nitride (TiN) thawj cov koom nrog, uas feem ntau yog 2-20 µm hauv qhov loj thiab muaj cov duab angular cubic. Thaum MnS dissolves nquag TiN yog thermodynamically ruaj khov hauv austenitic matrix. TiN{10}}matrix interface, txawm li cas los xij, muaj ntau yam nta uas nyiam pitting. Ua ntej, thaum lub sij hawm solidification, titanium nitrides precipitate ntawm qhov kub thiab txias nyob ib puag ncig cov kua hlau ua depleted nyob rau hauv chromium thiab molybdenum, ua ib tug solute - cheeb tsam tsis zoo 0.5-3 µm dav nrog Cr tsawg li 12-14% (vs%{13}). Thib ob, TiN particle nws tus kheej yog ib qho kev coj ua thiab tuaj yeem ua raws li cathode, yog li ua rau anodic dissolution ntawm cov matrix nyob sib ze. Thib peb, TiN's angular daim ntawv tsim kom muaj qhov tawg ntawm qhov kev suav nrog-matrix interface. Hauv cov dej hiav txwv chlorinated (muaj peev xwm oxidation siab) TiN particle ua raws li cov cathode ua tau zoo thiab cov chromium-depleted matrix hauv nws cov zej zog dissolves nyiam.

Correlation ntawm TiN ntim feem thiab pitting corrosion nyob rau hauv chlorinated seawater

Kev tshuaj xyuas raug ntawm 316L tubing nrog ntau yam TiN suav nrog cov ntsiab lus (los ntawm cov txheej txheem sib txawv steelmaking) tau pom cov hauv qab no pib qhov ntom ntom hauv cov dej hluavtaws chlorinated seawater (dawb chlorine 1.5 ppm, 20,000 ppm Cl -, pH 40.0℃rau:

Volume Fraction of TiN Inclusion (％)TiN Inclusion Size Range (µm) TiN Morphology Pit Density tom qab 5,000 xuab moos (pits/cm2) Qhov siab tshaj plaws qhov tob tom qab 5,000 teev (µm) Lub Sijhawm rau Thawj Qhov Chaw ruaj khov (teev) Pom zoo rau Chlorinated Seawater Service<0.0005 (Ti-free 316L) None None <0.1 <10 >10,000 Best 0.0005-0.001 1-3 Few, fine 0.1-0.5 10-25 5,000-10,000 Excellent 0.001-0.002 2-5 Scattered 0.5-2 20-40 2,000-5,000 Acceptable 0.002-0.005 3-8 Moderate 2-5 30-60 1,000-2,000 Marginal 0.005-0.010 5-12 Frequent 5-15 50-80 500-1,000 Not recommended 0.010-0.020 8-15 Numerous, angular 15-40 60-100 200-500 Unacceptable >0.020 >15 Extensive, clusters >40 >100 <200 Unacceptable
Kev sib piv ntawm TiN thiab MnS suav nrog hauv chlorinated ib puag ncig

Chlorination drastic hloov cov txheeb ze aggressiveness ntawm ntau hom inclusions. Cov lus hauv qab no sib piv pitting pib los ntawm MnS piv rau TiN hauv unchlorinated piv rau chlorinated seawater ntawm 40C.

Hom Inclusion Volume fraction (%) Unchlorinated Sea Water Pitting Rate (mm/xyoo) Pitting Rate in Chlorinated Seawater (1.5 ppm Cl2, mm/year) Failure mode dominant MnS (sulphide) 0.01 0.5-1.0<0.05 (MnS oxidized/dissolved) MnS is protective (oxidised to MnO)
TiN (nitride) 0.001 0.05-0.10 0.3-0.8 TiN tau txais kev txhoj puab heev (galvanic)
Mixed (ob leeg) 0.005 txhua 0.5-1.0 0.5-1.0 (TiN yog qhov tseem ceeb) Hloov ntawm MnS mus rau TiN pib
Al2o 3<0.05 <0.05 0.005 Unchanged (inert)
Cov lus qhia tshwj xeeb rau 316L kev pabcuam hauv dej hiav txwv chlorinated

Rau 316L sheathed heaters siv nyob rau hauv chlorinated seawater kev pab cuam (0.5-2 ppm Cl₂, 40 degree), cov txheej txheem hauv qab no txo ​​qis TiN ntsig txog pitting:

Residual Chlorine (ppm) Seawater Temperature (oC) Max Tso Cai TiN Volume Fraction (%) Pom Zoo Steelmaking Practice Alternative Material 0.5 30 0.005 AOD + calcium treatment 316L tau txais 0.5-1.0 30 0.003 AOD + qis Ti (tsis txhob Ti deoxidation) 316L nrog kev saib xyuas {{7} rcum SR VAR (electroslag remelted) Duplex 2205 1.0-2.0 30-40 0.0005 (Ti-dawb) VAR + Ti-kev xyaum pub dawb Duplex 2205 lossis titanium2.0 Ib Ti-dawb (tsis pom zoo 316) N/A Titanium lossis Alloy 625
TiN Inclusion Content Verification

Daim ntawv no nthuav tawm cov txheej txheem pov thawj rau cov neeg yuav khoom rau qis -TiN 316L rau kev pabcuam dej hiav txwv chlorinated. Ua ntej, tshuaj ntsuam xyuas cov ntsiab lus titanium: Ti<0.005% (50 ppm) usually correlates to TiN volume fraction <0.001%. The second is the metallographic inspection (ASTM E45) with a scanning electron microscopy (SEM) for the direct counting of TiN inclusions (morphology-angular, cubic, gold-yellow colour in the reflected light). The acceptability threshold is less than 5 TiN inclusions per mm² greater than 2 µm. The third is an accelerated pitting test in which a sample is exposed to synthetic chlorinated seawater at 40°C for 1,000 hours. Acceptance criterion: No pits deeper than >20 μm.

Field Identification ntawm TiNIinduced Pitting

Yog tias cov pits nyob nruab nrab ntawm lub kaum sab xis, cov xim kub sib xyaw ua ke (pom nyob rau hauv lub tshuab ntsuas qhov ntsuas ntawm 500x), TiN suav nrog yog qhov yuav ua rau pitting ntawm 316L rhaub hauv chlorinated ntsev hauv 1-2 xyoos. EDS tsom xam xyuas Ti thiab N. Cov pits feem ntau me me (10-50 µm broad) tsis tau tob, feem ntau nrog TiN particle sab laug rau hauv qab ntawm lub qhov. Cov hlau uas nyob ib sab qhia tsis muaj pov thawj xeb ntxiv. Txoj kev daws teeb meem rau qhov ua tsis tiav yog xaiv Ti-dawb 316L (titanium<0.005%) for future purchases. Temporary mitigation can be achieved by electropolishing of the sheath surface, removing the surface layer with exposed TiN inclusions.

Xaus: Siv TiN-Dawb 316L rau Chlorinated Seawater Heaters

Qhov loj pitting pib qhov chaw rau 316L stainless hlau encased heaters nyob rau hauv chlorinated ntsev dej ntawm qhov kub ntawm 40℃yog tus thawj coj titanium nitride (TiN) suav nrog. TiN ntim feem ntau dua 0.002-0.005% ua rau lub qhov ntom ntom ntawm 2-15 pits ib cm2 hauv 1,000-5,000 teev, ua rau perforation hauv 2-5 xyoos. Rau 316L sheaths siv nyob rau hauv marine los yog ntug dej hiav txwv daim ntaub ntawv nrog tshuaj tua kab mob, Ti cov ntsiab lus hauv qab no 0.005% (Ti-dawb) yuav tsum tau, steelmaking kev xyaum uas tsis txhob Ti deoxidation (xws li Al los yog Si deoxidation) yuav tsum tau ua raws li, thiab yog hais tias tseem ceeb cov kev pab cuam, VAR los yog ESR remelting yog yuav tsum tau teev tseg rau hauv qab no 0.0 TiN pejxeem. Ntawm no, peb hais txog TiN ntim feem pua ​​​​rau qhov muaj peev xwm pib qhov tob thiab kev pab cuam lub neej hauv cov dej hiav txwv chlorinated, tso cai rau cov neeg yuav khoom xaiv 316L tubing uas tiv thaiv kev suav nrog-induced pitting nyob rau hauv feem ntau hostile oxidising marine chaw.