Dab tsi tseem ceeb Crevice Gap Dav Ntawm 316 Stainless Steel Heater Sheath thiab Nws Mounting Flange Initiates Accelerated Pitting hauv 200 ppm Chloride Dej ntawm 80 degree

Lub geometric txwv ntawm oxygen depletion thiab hauv zos acidification

Qhov tawg ntawm cov sheath thiab cov mounting tivthaiv yog qhov chaw rhiab tshaj plaws rau qhov chaw corrosion ntawm 316 stainless hlau encased immersion heaters mounted los ntawm flanges los yog tank phab ntsa. Thaum qhov sib txawv ntawm qhov sib txawv yog txo mus rau qhov dav dav, feem ntau ntawm 0.1 hli thiab 0.5 hli, nyob ntawm chloride concentration thiab kub, qhov nkag ntawm cov dej ntau yog txwv, thiab cov pa oxygen depleted los ntawm qhov crevice. Tom qab ntawd, chloride concentration nce thiab pH txo, ua rau kom ceev pitting thiab crevice corrosion. A 316 sheath nrog qhov sib txawv ntawm qhov sib txawv tsawg dua 0.3 hli hauv 200 ppm chloride dej yuav pib crevice corrosion nyob rau hauv 6-12 lub hlis ntawm 80 degree, tab sis cov sheath zoo tib yam nrog qhov sib txawv ntawm ntau tshaj 0.5 hli yuav nyob twj ywm rau 5+ xyoo. Kab lus no kwv yees qhov tsim nyog qhov sib txawv ntawm qhov dav rau 316 sheaths nyob rau hauv cov xwm txheej kub kub thiab muab cov qauv tsim rau crevice dawb mounting.

Electrochemical Mechanism rau Pib ntawm Crevice Corrosion

Crevice corrosion ntawm 316 stainless hlau yog pib thaum lub geometry ntawm ib tug crevice hinders lub convective thiab diffusive hloov ntawm oxygen los ntawm tej kev daws rau cov hlau nto nyob rau hauv lub fissure. Cov pa oxygen tau noj los ntawm cov txheej txheem cathodic (cov pa txo qis) hauv qhov crevice thiab cov pa oxygen hauv zos poob qis rau zero theem thaum cov tshuaj ntau yog aerated. Qhov sib txawv muaj peev xwm tsim kom muaj galvanic cell. Cov ciam teb sab nraud ntawm qhov crevice (muaj oxygenated heev) ua cov cathode thiab sab hauv ntawm qhov crevice (deoxygenated) ua cov anode. Cov chloride ions tau nyiam mus rau hauv qhov sib cais kom tsis txhob muaj qhov cuam tshuam, ua tiav cov ntsiab lus 10-100 npaug ntawm cov khoom loj. Ib txhij hydrolysis ntawm yaj hlau ions (Fe 3+ , Cr 3+ ) ) tsim hydrogen ions thiab txo cov pH hauv zos mus rau 2-4. Cov nyhuv ntawm cov tshuaj chloride siab thiab pH qis ntawm kev tawg ntawm cov zaj duab xis passive ntawm 316 tau kawm los pib sai anodic dissolution. Qhov dav dav dav yog qhov dav dav tshaj plaws uas cov pa oxygen diffusion tsis tuaj yeem them nyiaj rau kev siv cov pa oxygen. Hauv qhov chaw zoo li qub lossis qis qis qhov sib txawv tseem ceeb yog feem ntau ntawm 0.3 thiab 0.5 hli. Qhov khoob loj dua 0.5 hli tso cai rau cov pa oxygen txaus los txhawb nqa kev ua haujlwm. Yog tias qhov sib txawv tsawg dua 0.3 hli dav, lawv tau deoxygenated nyob rau hauv cov sij hawm mus rau hnub thiab raug tawm tsam los ntawm crevice corrosion txawm nyob rau hauv cov dej uas tsis muaj chloride.

Qhov pib rau Crevice Corrosion ntawm 316 ntawm 80℃hauv 200 ppm Chloride Quantified.

Kev soj ntsuam zoo nrog cov neeg ua ntej nrog kev tswj hwm qhov dav dav ntawm 316 stainless hlau los xij sawv cev rau lub tshuab hluav taws xob sheath saum npoo hauv aerated 200 ppm chloride dej ntawm 80℃tau txiav txim siab qhov sib txawv tseem ceeb rau crevice corrosion pib. Qhov kev ntsuam xyuas yog 12 lub hlis ntev, nrog rau cov chav nyob ib ntus sib nrug thiab tshuaj xyuas kev tawm tsam. Cov txiaj ntsig tau pom qhov hloov pauv tam sim ntawm qhov dav ntawm qhov sib txawv ntawm 0.4 hli mus rau 0.5 hli. Crevice corrosion tau pib hauv txhua qhov qauv ntawm lossis qis dua 0.4 hli hauv 6 lub hlis. Npog .5 mm lossis loj dua tsis muaj kev tawm tsam rau tag nrho 12 lub hlis. Extrapolating rau kev pabcuam ntev, qhov khoob ntawm 0.5-0.8 hli tuaj yeem pib tawm tsam tom qab 3-5 xyoos nyob rau hauv lub xub ntiag ntawm hostile dej chemistry los yog thermal cycling ua rau concentration. Cov sij hawm nthuav dav rau crevice corrosion pib rau 316 sheaths tau muab rau hauv cov lus hauv qab no raws li kev ua haujlwm ntawm qhov dav dav, chloride concentration thiab kub.

Crevice Gap Width (mm) Chloride Concentration (ppm) Kub (degree) Lub Sijhawm pib ntawm Crevice Corrosion Lub Sijhawm rau Perforation (1.5 hli phab ntsa tom qab pib) Pom zoo rau 5-xyoo Kev Pabcuam<0.1 (tight contact) 200 80 <1 month 3-6 months No 0.1-0.2 200 80 1-3 months 3-6 monthsNo 0.2-0.3 200 80 3-6 months 6-12 months No
0.3-0.4 200 80 6-9 hli 9-18 hli Tsis muaj 0.4-0.5 200 80 9-12 hli 12-24 hli Marginal
0.5-0.7 200 80 >24 lub hlis (yuav tsis pib) -Yog lawm (nrog kev kuaj)
0.7-1.0 200 80 >60 lub hlis - Yes1.0 (qhib qhov sib txawv) 200 80 Tsis muaj crevice corrosion - Yog
0.5-0.7 500 80 6-12 months 12-18 months No 0.5-0.7 200 60 >60 hli Yes
0.3-0.5 200 60 12-24 lub hlis 24-36 Lub Hlis Pom zoo nrog kev saib xyuas Geometric Crevices hauv Cov Cuab Yeej Mounting Sib Tsoo

Rau flanged immersion heaters peb nquag tsim cov ntsiab lus ua rau muaj qhov sib txawv nrog qhov dav dav uas feem ntau qis dua qhov tseem ceeb. Thawj yog compression fittings los yog gasketed flanges qhov twg lub sheath hla los ntawm ib tug elastomeric los yog PTFE foob uas yog compressed tiv thaiv cov hlau. Kev sib tiv toj siab tuaj yeem txo qhov sib txawv yuav luag xoom ntawm cov chaw sib cuag, thaum cov chaw nyob ib sab ntawm cov ntsiab lus tiv tauj kuj tseem muaj qhov sib txawv ntawm 0.1-0.2 hli. Qhov thib ob yog welded los yog threaded bushings nyob rau hauv uas lub sheath yog muab tso rau hauv ib tug flange thaiv nrog ib tug clearance qhov ntawm 0.2-0.3 hli. Qhov kev tshem tawm no yog nyob rau hauv qhov tseem ceeb crevice ntau yam tseem xav tau rau kev sib dhos. The third is scale or debris that builds up between the sheath and a tight fitting tube sheet, reducing a bigger gap to a network of sub-critical fissures. Cov qauv tsim hluav taws xob tshiab qhia qhov sib txawv yam tsawg kawg nkaus ntawm 1.0 hli (siv lub bushing nrog clearance ntawm 1.0-1.5 hli) kom txo tau qhov crevice corrosion. Cov qauv tsim uas siv cov khoom flanges yuav tsum tau hloov los ntawm qhib kev txhawb nqa brackets los yog clamps, qhov twg lub sheath txuas ntawm cov chaw tsis sib xws (peb lossis plaub lub ntsiab lus sib cuag nrog qhib qhov khoob nyob rau lwm qhov), cuam tshuam qhov nruam nruam geometry.

Mitigation Strategies for Unavoidable Critical Gaps

Yog tias cov qauv tsim tsis tsim nyog yuav tsum muaj qhov sib txawv ntawm qhov sib txawv tsawg dua 0.5 hli, xws li hauv kev kub siab flange ntsaws ruaj ruaj uas yuav tsum tau ua nruj me ntsis, peb qhov kev txo qis yog siv los tiv thaiv crevice corrosion. Thawj thiab zoo tshaj plaws yog daim ntawv thov ntawm polymeric lossis elastomeric gaskets uas tag nrho txwv tsis pub dej los ntawm qhov tawg. Tsis muaj electrolyte nkag mus rau hauv qhov crevice, thiab li no chloride concentration thiab oxygen depletion yog zam los ntawm ib tug watertight kaw los ntawm compressed PTFE los yog EPDM gasket. Txoj kev no yuav tsum tau cov khoom siv gasket kom sib haum nrog cov kua dej kub (EPDM txog 120 degree, PTFE mus txog 250 degree). Qhov thib ob txoj hauv kev yog xaiv ib qho kev sib tw ntau dua - alloy resistant rau lub sheath ntawm qhov chaw mounting, siv welded hloov daim ntawm Alloy 625 los yog titanium. Cov ntaub ntawv no muaj qhov sib txawv tseem ceeb ntawm qhov tsawg dua 0.1 hli yog li txawm tias cov kev ceev ceev tsis tuaj yeem pib ua phem. Qhov kev xaiv thib peb yog kev tiv thaiv cathodic. Yog tias lub tshuab cua sov ua ke nrog cov zinc los yog txhuas anode, lub peev xwm hauv qhov crevice tuaj yeem txo qis hauv qab lub peev xwm pitting, tab sis qhov no yuav tsum tau ua tib zoo tsim kom tsis txhob muaj hydrogen embrittlement ntawm sheath.

Crevice Corrosion Identification ntawm Cov Cuab Yeej Nruab Nrab hauv Teb

Nyob rau hauv cov kev pab cuam heaters yuav pom crevice corrosion ntawm lub mounting flange raws li ib tug nplhaib ntawm pitting los yog dav nkim tam sim ntawd nyob rau hauv ib ncig ntawm lub flange los yog gasket, feem ntau 1 mus rau 5 hli los ntawm kev sib cuag. Qhov kev tawm tsam feem ntau yog qhov tob tshaj plaws thaum qhov sib txawv me me. Yog tias koj saib hauv qab lub flange disassembled koj yuav pom tias thaj chaw corroded yog ib qho tsaus nti pitted band uas tsis muaj cov hlau ci ntsa iab. Corrosion khoom feem ntau muaj xws li ntsuab nickel chloride ntsev. Yog tias lub tshuab cua sov tau ua haujlwm rau ntau tshaj 1-2 xyoos thiab cov dej muaj cov tshuaj chlorides siab tshaj 100 ppm, ces qhov kev xeb yuav tsum tau xav tias thaum twg lub rhaub flanged raug tshem tawm vim li cas. Rau kev hloov lub rhaub tshiab yuav tsum tau muab tso rau hauv qhov sib txawv qhib tsim (Ntau dua lossis sib npaug ntawm 1.0 hli tshem tawm) lossis tag nrho sealing gasket tsis tso cai rau cov khoom siv hluav taws xob.

Xaus: Kev pabcuam txhim khu kev qha nrog tsis muaj Crevice Mounting

Crevice corrosion pib nyob rau hauv 316 stainless hlau encased immersion heaters nyob rau hauv 200 ppm chloride dej ntawm 80℃rau crevice gap widths hauv qab no 0.5 hli nyob rau hauv 12 lub hlis thiab perforation tshwm sim nyob rau hauv 24 lub hlis. Yog tias qhov sib txawv ntau dua 0.7 hli, chav tsev yuav lav tau 5+ xyoo ntawm kev pabcuam dawb. Thaum xaiv flanged heaters, koj muaj peb txoj kev xaiv: tsim lub mount kom koj muaj tsawg kawg yog 1.0 hli ntawm clearance ntawm lub sheath thiab ib tug nyob ib sab hlau nto, siv ib tug tag nrho sealing gasket kom tsis txhob dej nkag mus rau hauv lub crevice cheeb tsam, los yog upgrade lub sheath khoom mus rau ib tug ntau crevice{11}}resistant alloy ntawm lub interface. Txoj hauv kev uas tau muab rau ntawm no tso cai rau cov neeg yuav khoom los qhia txog kev teeb tsa geometries uas zam qhov feem ntau ua rau ua tsis tiav ntxov nyob rau hauv flanged 316 immersion heaters ua haujlwm hauv chloride-cov dej kub los ntawm kev hais txog qhov sib txawv ntawm qhov dav mus rau lub sijhawm kom muaj nuj nqis los pib.