Raws li kev sib xyaw tshwj xeeb ntawm pH thiab Saturation Index Puas Yog 316 Stainless Steel Heater Sheath tsim Calcium Carbonate Scale uas ua rau hauv qab -Txaus Corrosion ntawm 90 degree

Lub Scaling Threshold Los Ntawm Kev Tiv Thaiv rau Corrosive

For 316 stainless steel encased electric heating tubes immersed in hard water at 90°C, the precipitation of calcium carbonate scale on the sheath surface is not a simple thermal insulation problem-it can also lead to localised corrosion referred to as under-deposit corrosion (UDC). At high supersaturation (Langelier Saturation Index, LSI, between 0.5 and 1.5), the scale layer is thin (20-100 µm) and porous, so that corrosive species (chlorides, oxygen) can reach the metal surface below the scale, generating a differential aeration cell that speeds up pitting. At high supersaturation (LSI >2.0) scale is thick (>200 µm) thiab tsis tu ncua nres cov pa oxygen txav tag nrho thiab tsim cov txheej txheem corrosive heev. Tsis muaj daim ntawv teev hauv qab LSI 0.5, thiab 316 yog passive. Qhov sib xyaw tseem ceeb rau UDC yog pH 7.5 txog 8.5 (qhov twg bicarbonate-carbonate equilibrium yog qhov zoo rau kev tsim cov nplai) thiab LSI 0.8 txog 1.8. Daim ntawv no suav cov pH -LSI qhov pib rau hauv qab - tso tawm corrosion ntawm 316 sheaths ntawm 90℃.

Mechanism of Under -Deposit Corrosion

Thaum cov txheej txheem calcium carbonate ntxeem tau tsim ntawm 316 stainless hlau hauv dej kub, cov hlau hauv qab ntsuas yuav ua rau cov pa oxygen tshaib plab vim tias cov pa nkag mus los ntawm cov nplai thiab cov txheej txheem nyob ruaj khov qeeb. Cov nplai dawb qhov chaw (yog tias muaj) lossis cov cheeb tsam sab nraud ntawm qhov ntsuas tau ua kom muaj oxygenated ua ib qho kev sib txawv aeration ntawm tes, cov pa oxygen deprived hlau nyob rau hauv lub teev yog anode thiab cov pa nplua nuj liab qab hlau cathode. Chloride ions ntws mus rau thaj tsam anodic kom khaws cov nqi nruab nrab thiab qis pH los ntawm cov hlau hydrolysis nyob rau hauv cov nplai inhibits repassivation. Tus nqi corrosion tuaj yeem ncav cuag 0.5-2 hli hauv ib xyoos, uas yog piv nrog tus nqi ntawm active corrosion nyob rau hauv acid. Lub nplai nws tus kheej yuav raug tso tseg, tab sis cov hlau hauv qab nws yaj, tsim cov kab noj hniav uas perforates lub sheath.

Quantified thresholds rau qis dua - deposit corrosion ntawm 90 degree

Cov pH-LSI ua ke ua rau muaj kev cuam tshuam tsis zoo nyob rau hauv 1,000 teev tau txiav txim siab rau 316 sheaths (Ra=0.4 µm) hauv cov dej hluavtaws (300 ppm CaCO3 hardness, 200 ppm alkalinity, 200 ppm Cl{5}, 9, p. W / cm2).

Langelier Saturation Index (LSI) pH ntawm 90℃(kwv yees) Scale Thickness (tom qab 1,000 teev) (microns) Scale Porosity (%) Hauv qab -Deposit Corrosion Rate (mm/yr) Lub Sijhawm rau Perforation (1.5 hli phab ntsa) Pom zoo<0 (undersaturated) <7.0 No scale N/A 0.02 (no UDC) >75 xyoo Yes 0-0.5 7.0-7.5 0-20 N/A 0.02-0.05 30-75 xyoo Yog
0.5-0.8 7.5-7.8 10-30 30-50% 0.05-0.10 15-30 xyoo Txais tau
0.8 txog 1.2 7.8 rau 8.1 20 rau 50 20 txog 30% 0.10 txog 0.25 6 mus rau 15 xyooMarginal 1.2-1.5 8.1-8.4 30-80 10-20% 0.25-0.50 3-6 xyoo Tsis qhia
1.5-1.8 8.4-8.7 50-120 5-10% 0.50-1.00 1.5-3 xyoo Tsis tau
1.8-2.0 8.7-8.9 80-150 <5% 0.20-0.40 (protective scale) 4-8 years (limited UDC) Transition2.0 >8.9 >150 <3% (dense) <0.05 (sealed, no UDC) >30 xyoo (tab sis kev pheej hmoo ntawm overheating) kev pheej hmoo ntawm overheating, tsis yog UDC
Kev cuam tshuam ntawm chloride ntawm qhov kev siv ntawm hauv qab - deposit corrosion

UDC tau nrawm nrawm los ntawm chloride concentration. Kev cuam tshuam ntawm chloride ntawm qhov tseem ceeb LSI qhov pib rau UDC ntawm 90℃yog pom hauv cov lus hauv qab no.

Chloride Concentration (ppm) LSI Threshold to Initiate UDC (5-xyoo lub neej) nyiam tshaj LSI rau 5-xyoo Lub Neej Pom Zoo Ua Haujlwm<50 >1.5 1.8 Scale inhibitor acceptable 50-100 >1.2 1.5 Scale inhibitor preferred 100-200 >1.0 1.2 Softening lossis inhibitor yuav tsum tau
200-500 >0.8 1.0 Water softening strongly recommended >500 >0.5 0.8 Softening yuav tsum tau; txhim kho kev txiav txim siab
Tswv yim UDC Tiv Thaiv Dej Chemistry Specifications

Txhawm rau kom tsis txhob nyob rau hauv - tso nyiaj rau corrosion, cov txheej txheem dej hauv qab no rau 316 sheathed heaters ntawm 90℃raug pom zoo:

Langelier Saturation Index (LSI): keep below 1.0 (goal 0.5-0.8) if chloride is >200 ppm; khaws cia hauv qab 1.2 yog tias chloride yog 100-200 ppm; khaws cia hauv qab 1.5 yog tias muaj chloride<100 ppm

pH @ 90C: Tswj 7.0 - 7.8 (qis pH=qis dua, kev phom sij ntau dua corrosion Siab dua pH=ntau nplai, ntau dua UDC)

Alkalinity: Tsawg dua 200 ppm li CaCO3 los txo cov peev txheej loj hlob

Scale inhibitor - ntxiv 2-5 ppm phosphonate lossis polyacrylate yog LSI> 1.0

Hauv qab -Kev Tshawb Fawb Kev Tshawb Fawb Txog Kev Txhaum Cai

UDC hauv qab calcium carbonate nplai ua rau cov kab noj hniav hemispherical ntawm lub plhaub, feem ntau yog 2-10 hli hauv txoj kab uas hla, nrog cov nplai tseem txuas lossis tshem tawm ib nrab. Tshem tawm cov nplai nthuav tawm qhov ci, glossy, pitted nto ntawm hlau, es tsis txhob npub, scaled nto. Cov pits feem ntau muaj cov npoo qis uas qhov corrosion tau txav mus tom qab hauv qhov teev. Nyob rau hauv cov ntaub ntawv ntawm scale build- nrog rau cov cua kub tsis ua hauj lwm tab sis tsis muaj pov thawj ntawm pitting nyob rau hauv scale - dawb seem, UDC yuav tsum tau txiav txim siab. Kev ntsuas ntsuas los ntawm X-ray diffraction feem ntau qhia calcite lossis aragonite (CaCO₃), qee zaum ua ke nrog cov khoom siv corrosion (Fe₂O₃, Fe₃O₄). Cov lus teb yog kom txo qis LSI (los ntawm pH txo qis los yog dej softening) los yog ntxiv cov nplai inhibitor uas hloov cov siv lead ua zoo li tsis yog - teev ua raws.

Xaus: Tswj LSI thiab pH kom tsis txhob muaj nyob rau hauv -Deposit Corrosion

316 stainless hlau encased rhaub nyob rau hauv 90℃nyuaj dej. Tsim kom muaj qhov ntxeem tau ntawm calcium carbonate nplai, uas tsim cov hlwb sib txawv, tshwm sim thaum Langelier Saturation Index (LSI) nyob nruab nrab ntawm 0.8 thiab 1.8 thiab pH yog nyob nruab nrab ntawm 7.5 thiab 8.5 . Saum LSI 1.8 qhov ntsuas yog ntom thiab tiv thaiv UDC (txawm hais tias muaj teeb meem). Ntawm LSI <0.8, UDC tsis tshwm sim los ntawm kev tsim cov nplai. LSI yuav tsum tau muab xam ntawm qhov kub thiab txias. Hauv cov neeg ua haujlwm kho dej nyuaj uas tau teev tseg 316 sheaths yuav tsum khaws LSI qis dua 1.0 thaum cov tshuaj chloride tshaj 200 ppm lossis suav nrog cov nplai inhibitors uas cuam tshuam rau cov nplai morphology. Cov txheej txheem muab rau ntawm no muab kev sib txuas ntawm pH, LSI thiab ntsuas tus nqi ntawm UDC, uas tso cai rau cov neeg siv khoom xaiv dej chemistry uas yuav tiv thaiv qhov ntsuas -induced localized corrosion.