Vim li cas Titanium Immersion Heater hauv 50/50 Ethylene Glycol / Dej Sib xyaw nrog 500 ppm Chloride ntawm 90℃Fail sai dua los ntawm Crevice Corrosion dua los ntawm Uniform Thinning?

Ethylene glycol / dej sib xyaw ua ke (50/50 vol%) feem ntau yog siv los ua cov dej hloov hluav taws xob hauv lub hnub ci thermal, HVAC thiab lub cav txias. Ntawm 90 degree, 500 ppm chloride los ntawm deteriorated glycol los yog pleev xim dej yog hostile rau titanium. Txawm hais tias titanium muaj kev tiv thaiv zoo rau cov corrosion tsis sib xws hauv qhov sib xyaw no, nrog cov nqi corrosion qis dua 0.02 hli hauv ib xyoos, qhov tsis ua haujlwm feem ntau yog vim muaj kev sib txhuam ntawm cov raj txhawb nqa, flanges thiab gaskets. Lub crevice geometry ua rau muaj kev nce hauv chloride concentration thiab txo qis hauv oxygen concentration, thaum cov khoom ntawm glycol tawg (glycolic acid, formic acid) txo cov pH hauv zos. Crevice corrosion propagation tus nqi yog 0.3-1.0 hli / xyoo, 15-50 lub sij hawm sai tshaj li niaj hnub thinning. Qhov tsis ua haujlwm yog qhov tsawg heev thiab perforation tshwm sim thaum 95% ntawm lub raj nplaim sab laug tsis zoo.

Mechanism ntawm Crevice Dominance nyob rau hauv Glycol / Dej Mixtures

Hauv cov tshuaj ntau, ethylene glycol nce viscosity, uas txo cov pa oxygen diffusion. Kev ruaj ntseg ntawm cov zaj duab xis passive yog lav los ntawm cov pa oxygen txaus thiab cov tshuaj chloride nruab nrab. Nyob rau hauv ib tug crevice (xws li nyob rau hauv ib tug PTFE kev them nyiaj yug los yog nyob rau hauv lub gasket) oxygen yog sai sai depleted los ntawm cathodic txheej txheem thiab tsis muab. Chlorides diffuse mus rau hauv qhov crevice kom khaws hluav taws xob nruab nrab thiab ncav cuag qib ntawm 2,000 txog 10,000 ppm. Glycol thermally decomposes ntawm 90℃rau cov organic acids (glycolic, formic, oxalic). Cov kua qaub no txo ​​qis pH hauv zos ntawm qhov crevice mus rau 3-4. Chloride, pH tsawg thiab oxygen depletion ua ke degrade cov zaj duab xis passive. Corrosion ntawm crevice anode yog active, tsav los ntawm lub loj sab nraud cathode ntawm tus so ntawm lub raj nto. Yog hais tias lub galvanic tam sim no nyob nruab nrab ntawm lub crevice (anode) thiab sab nraud (cathode) yog sustained, lub dissolving tus nqi tseem ceeb. Thaum kawg, perforation tshwm sim.

Quantitative Comparison ntawm Uniform thiab Crevice Corrosion Rate

Kev tswj xyuas hauv 50/50 ethylene glycol / dej nrog 500 ppm chloride (pib pH 6.5) ntawm 90℃rau 2,000 teev tau qhia tawm qhov sib txawv ntawm corrosion. Qhov feem ntau corrosion tus nqi ntawm ib tug du raj tsis muaj fissure tsuas yog 0.008 mus rau 0.015 hli / xyoo. Qhov saum npoo yog ci ntsa iab thiab tsis muaj pitting, qhia tau tias muaj kev ruaj khov ua yeeb yaj kiab. Nrog rau qhov no, phab ntsa thickness ntawm 1.2 hli yuav muab lub neej kev pab cuam ntawm ntau tshaj 50 xyoo. Txawm li cas los xij, qhov crevice corrosion yog mus txog 0.30-0.70 mm / xyoo raws li kev txhawb nqa PTFE nrog qhov sib txawv ntawm 0.2 hli crevice. Qhov pib theem pib kav li ntawm 200 thiab 500 h, thiab ib tug zawj nrog qhov tob ntawm 0.3-0.6 hli tsim nyob rau hauv 1,000 h. Nrog rau cov roj hmab gasket uas muaj qhov sib txawv ntawm 0.1 hli, qhov crevice corrosion nce mus rau 0.50 mus rau 1.00 hli / xyoo, pib hauv 100 mus rau 300 teev thiab perforating 1.2 hli phab ntsa hauv 1200 txog 2400 teev. Nyob rau hauv ib lub raj rau tubesheet dov sib koom tus nqi ntawm crevice corrosion yog 0.20 mus rau 0.50 hli ib xyoo nrog pib nyob rau hauv 300 mus rau 800 teev.

Kev cuam tshuam ntawm Chloride thiab Glycol Degradation ntawm Crevice Severity

Qhov hnyav ntawm qhov kev tawm tsam yog qhov muaj zog ntawm glycol mob thiab chloride concentration. Fresh glycol (nrog rau inhibitors) ntawm 100 ppm chloride muab pH ntawm 5.5 tom qab 500 h nrog tus nqi corrosion ntawm 0.10-0.25 hli hauv ib xyoos. Fresh uninhibited glycol ntawm 500 ppm chloride poob rau pH 4.5 ntawm tus nqi ntawm 0.30-0.60 hli ib xyoo. Thermally decomposed glycol (500 teev ntawm 90 degree) nrog 500 ppm chloride muab pH 3.5. Tus nqi yog 0.60-1.00 mm / xyoo. Glycol deteriorated ntau tshaj 1,000 ppm organic acids nrog 1,000 ppm chloride, pH 3.0, crevice corrosion tus nqi 1.00-1.80 hli ib xyoo. Nyob rau hauv cov xwm txheej no, tus nqi tsis sib xws yog <0.025 hli / xyoo, qhia txog kev txhim kho corrosion raws li qhov tseem ceeb tsis ua haujlwm.

Glycol/Dej Heater Crevice Corrosion Prevention Measures

Cov lus hauv qab no muab cov lus qhia txog kev txo qis kev xeb hauv ethylene glycol / dej ua ke ntawm 90 degree, raws li kev teeb tsa lub rhaub thiab kev ntsuas kev pheej hmoo.

Crevice Hazard Heater ArrangementProposed Design ChangeExpected Crevice Corrosion Rate (mm/yr)
PTFE raj spacers (0.2-0.5mm qhov sib nrug) Siab Titanium welded txhawb kev hloov<0.02
Flanges nrog roj hmab gaskets siab heev Siv welded los yog braze connectors<0.02
Cov menyuam nthuav dav hauv tubesheet Nruab Nrab Weld raj- mus rau -tubesheet tom qab nthuav dav 0.05–0.10
High compression fittings Tshem tawm; siv cov welded fittings<0.02
Rov tsim kho cov crevices uas twb muaj lawm Tsis tuaj yeem rov tsim kho dua Siv PTFE daim kab xev los kho qhov sib txawv 0.15–0.30
Engineering Ntau tshaj Crack Elimination

Lub crevice corrosion kuj muaj zog cuam tshuam los ntawm qib titanium. Qib 7 (palladium stabilized) muaj 3-5 zaug lub sij hawm pib ntev dua thiab 50-70% qis tshaj tawm tshaj qib 2 hauv glycol/dej crevices. Phab ntsa thickness muab cov khoom pub dawb. Lub phab ntsa 2.0 hli nrog crevice corrosion ntawm 0.5 hli ib xyoo kav 4 xyoo, tab sis 1.2 hli phab ntsa kav tsuas yog 2.4 xyoo. Cov pob glycol inhibitor tseem ceeb; nitrite-based inhibitors txhawb titanium crevice corrosion, whereas molybdate los yog azole-based inhibitors yog benign. Cov qauv glycol tsis tu ncua rau pH, chloride thiab organic acid concentration ua rau kev saib xyuas kev kwv yees tau. Ntxiv monoethanolamine kom glycol pH saum toj no 8.0 txo qis kev tawm tsam.

Ua ib qho specification nrog kev pom

Tsim lub titanium immersion rhaub rau 50/50 ethylene glycol / dej nrog 500 ppm chloride ntawm 90C. Ua kom nws crevice-dawb: siv welded raj txhawb nqa, welded flanges, thiab welded raj-rau-tubesheet junctions. Yog tias tsis muaj qhov cuam tshuam raug tshem tawm, siv Qib 7 titanium, thiab xyuas txhua qhov chaw crevice txhua lub hlis twg nrog lub borescope lossis dye penetrant. Rau cov cua sov uas twb muaj lawm nyob rau hauv cov kev pab cuam uas muaj crevice corrosion, refit nrog PTFE daim kab xev los yog silicone sealant los npog qhov khoob thiab ntxiv sodium molybdate rau glycol ntawm 200 ppm raws li ib tug corrosion inhibitor. Txheeb xyuas glycol rau pH poob qis dua 6.0 thaum ua haujlwm; Yog tias pom pH poob, hloov glycol them tam sim. Crevice corrosion tuaj yeem ua rau lub tshuab cua sov tsis ua haujlwm 15-50 lub sij hawm sai dua li cov txheej txheem nyias. Paub txog qhov no, tus kws tshaj lij hais txog kev txo qis ntawm cov khoom siv qib hauv cov khoom siv ethylene glycol / dej kev pabcuam.