Raws li kev sib xyaw tshwj xeeb ntawm Kev Siv Anodic Kev Tiv Thaiv Muaj Peev Xwm thiab Sheath Kub Ua Tau 316 Stainless Hlau Rhaub Tube hauv 50% Caustic Soda (NaOH) ntawm 120℃Nyob Passive Tsis Muaj Caustic Stress Corrosion Cracking

Electrochemical Control nyob rau hauv kev tiv thaiv ntawm Caustic Embrittlement

Caustic stress corrosion cracking (CSCC, los yog caustic embrittlement) yog ib qho tseem ceeb tsis ua hauj lwm hom rau 316 stainless hlau sheathed hluav taws xob cua sov raj nyob rau hauv caustic soda (NaOH) kev pab nyob rau hauv siab kub (xws li evaporators, concentrators los yog caustic hloov kab cua sov). Caustic SCC yuav siv qhov chaw kub NaOH cov kev daws teeb meem ntawm qhov kub siab tshaj li ntawm 70-80 degree, nrog rau qhov cuam tshuam loj heev nrog ob qho tib si kub thiab caustic concentration. Qhov no tsis zoo li chloride SCC, uas yuav tsum muaj kev sib txuas tshwj xeeb ntawm qhov kub thiab txias chloride concentration. Ntawm 50% NaOH (ib txwm muaj kev lag luam concentration) ntawm 120 degree, 316 yog rhiab heev rau CSCC nyob rau hauv dawb corrosion tej yam kev mob, nrog cracking tsim nyob rau hauv hnub mus rau lub lis piam. CSCC tuaj yeem zam tau tag nrho los ntawm kev siv cov tshuaj tiv thaiv anodic (ie potentiostatic tswj cov hlau hauv cheeb tsam passive). Qhov tseem ceeb parameter yog lub peev xwm siv tau nrog rau kev siv hluav taws xob (feem ntau Ag / AgCl). Lub passive ntau yam rau 316 hauv 50% NaOH ntawm 120℃yog hais txog -200 mV rau +200 mV vs. Ag/AgCl. Kev tawg tshwm sim ntawm qhov muaj peev xwm qis dua -200 mV (ib cheeb tsam nquag) lossis siab dua +200 mV ( cheeb tsam transpassive). Kab lus no suav cov anodic tiv thaiv lub qhov rais rau kev tiv thaiv CSCC ntawm qhov sib txawv ntawm qhov kub thiab txias.

Mechanism ntawm Caustic Stress Corrosion Cracking thiab Nws Tiv Thaiv

Caustic SCC ntawm 316 stainless hlau yog qhov kev xaiv ntawm cov txheej txheem ua ntu zus ntawm cov ciam teb (intergranular cracking hauv qis alloys; transgranular hauv 316 ntawm qhov kub siab dua) nyob rau hauv tensile kev nyuaj siab nyob rau hauv concentrated NaOH kev daws teeb meem. Cov zaj duab xis passive ntawm 316 hauv caustic yog cov hlau thiab chromium hydroxides / oxides zaj duab xis uas ruaj khov tsuas yog nyob rau hauv qhov txwv tsis pub muaj peev xwm. Hauv qab qhov muaj peev xwm ua tau ntau dua (siab dua qhov tsis zoo) cov hlau corrodes aggressively (general corrosion). Tshaj li qhov passive ntau (qhov muaj peev xwm zoo dua) transpassive dissolution tshwm sim, uas chromium yog oxidised rau soluble chromate (CrO4 2-) thiab SCC pib. Ib tug tswj anodic voltage nyob rau hauv lub passive ntau stabilizes passive zaj duab xis thiab tshem tawm cov tsav tsheb quab yuam rau lub zos tawg.

Quantified Anodic Protection Windows rau 50% NaOH @ 120C

Qhov kev ua haujlwm tsis zoo thiab cov ciam teb tawg ua haujlwm raws li kev ua haujlwm ntawm qhov siv hluav taws xob vs Ag / AgCl (3M KCl) tau raug txiav txim siab hauv 50% NaOH ntawm 120℃los ntawm kev tswj cov potentiostatic tests ntawm 316 stainless hlau (U- khoov cov qauv nyob rau hauv tas li tensile stress).

Applied Potential (mV vs. Ag/AgCl, 3M KCl) Electrochemical Region Corrosion Rate (mm/year)CSCC Initiation Time (hrs) Surface Condition 120°C 50% NaOH Service Recommended After 1000 hours Active corrosion < -400 Immediate (active) 5-10 Severe general corrosion No -400 to -250 Active/transition 1-5 10-50 Pitting + general loss No -250 to -2000Mixed (active edge) 0.5-1.0 50-200 Localised attack Not advised -200 to -150 Passive (low) 0.02-0.05 >5,000 Ci ntsa iab, tsis muaj kev tawm tsam Tau Txais (kem ciam teb)
-150 to +100 Passive (optimum) 0.01-0.02 >10,000 Bright, passive Zoo tshaj +100 rau +200 Passive (siab) 0.02-0.05 2,000-5,000 Tint (oxide thickening) Txais tau +200 rau +250 Transpassive ntug 0.1-0.3 500-1,500 Me ntsis pitting Marginal +250 mus rau {{passive CSs{12}} +350 Oxygen evolution 1-5<100 Severe CSCC + pitting No
Kev cuam tshuam ntawm Caustic Concentration thiab Kub ntawm Kev Tiv Thaiv Muaj Peev Xwm Ntau Yam

Lub qhov rais passive muaj peev xwm txawv nrog caustic concentration thiab kub. Hauv qab no yog ib lub rooj ntawm kev pom zoo tiv thaiv muaj peev xwm ntawm ntau yam mob.

NaOH Conc. (wt%) Kub (degree) Passive Range (mV piv rau Ag/AgCl, 3M KCl) Kev Tiv Thaiv Zoo Tshaj Plaws (mV) Yam tsawg kawg pom zoo tam sim no rau kev tiv thaiv (mA / cm2)
30 100 -300 rau +50 -150 0.1 30 120 -250 rau +100 -100 0.2 40 100 -250 rau +100 -100 0.2 40 120 -200 rau +150 -50 0.3 50 100 -200 rau +150 -50 0.3 50 120 -150 rau +150 -50 0.3 50 120 -150 rau +200 0 0.5 50 140 -100 rau +250 (narrowing) +50 0.8 60 100 -150 rau +200 0 0.4 60 120 -100 rau +250 +50 0.7 70 100 -100 rau +200 (qov) +50 0.6
Kev qhia tshwj xeeb rau Cov Txheej Txheem Kev Tiv Thaiv Anodic rau 316 Cov Cuab Yeej

Cov kab ke hauv qab no muab kev tiv thaiv CSCC rau 316 sheathed heaters hauv cov kev pabcuam kub kub nrog kev tiv thaiv anodic:

Qhov System Parameter Yuav Tsum Paub Txog Kev Tshawb Fawb Txog Kev Tshawb Fawb
Siv electrode Ag/AgCl nrog 3M KCl, ruaj khov mus txog 120℃Pre-ntsuas calibration
Ua hauj lwm electrode (lub rhaub) Txuas mus rau qhov zoo kawg nkaus ntawm lub potentiostatCheck out continuity
Counter-electrode (cathode) Platinum los yog platinised titanium. Yam tsawg kawg nkaus thaj tsam piv cathode: anode 1: 2
Potentiostat calibration Muaj peev xwm teeb tsa -50 txog 0 mV (rau 50% NaOH ntawm 120 degree)
Ua tau kam rau ua ± 20 mVData logging
Yuav tsum tau tam sim no ceev 0.3-0.8 mA / cm² nyob ntawm qhov kub thiab txiasInitial tam sim no nyob rau hauv txoj kev
Failsafe Interlock -If potential drifts >+250 mV lub<-250 mV, shut-down heater Alarm + shut-down
Field Identification of Failed Caustic Protection

A 316 rhaub tawg hauv lub luag haujlwm caustic txawm tias muaj anodic shielding. Peb qhov xwm txheej ua tsis tiav yog ua tau. Ua ntej, ib qho ua tau drift mus rau hauv cheeb tsam transpassive (+200 rau +350 mV) ua rau transgranular SCC tawg nrog hnyav oxidation (hued nto). Qhov thib ob, muaj peev xwm drift mus rau hauv cheeb tsam active (<-200 mV) leads to widespread corrosion with pitting. Third, loss of power to the potentiostat (unprotected operation) leads to fast CSCC, with cracks developing within 50-200 hours. The cause will be determined by reviewing the log of the failed heater (if logged). Proper anodic protection can be restored by recalibrating the reference electrode, inspecting electrical connections and optionally reducing the heater power density.

Xaus: Specification ntawm Anodic Protection rau Caustic Service

Kev zam ntawm caustic stress corrosion cracking ntawm 316 stainless hlau encased heaters nyob rau hauv 50% NaOH ntawm 120℃tsuas yog paub los ntawm kev tswj cov hlau muaj peev xwm ntawm -150 thiab +200 mV vs. Ag/AgCl (3M KCl), nyob rau hauv lub passive qhov rais.