Qhov Permeation -Corrosion Link hauv Chlor-Alkali Service
Chlor-alkali membrane hlwb siv cov brine rhuab (25% NaCl, 85 degree) saturated nrog chlorine thiab muaj chlorate ions (ClO₃⁻) ntawm 5-15 g/L. PFA-encapsulated titanium heaters nyob rau hauv qhov kev pab cuam ntsib ib tug tshwj xeeb tsis ua hauj lwm mechanism: chlorate ions permeating lub polymer accumulate nyob rau hauv lub titanium interface, tsim ib tug oxidizing ib puag ncig uas lov cov passive oxide txheej thiab pib crevice corrosion. Kev soj ntsuam ntau los ntawm 31 chlor-alkali hlwb qhia tau hais tias chlorate permeation flux los ntawm 2.0mm PFA saum toj no 0.15 mg / cm²·hnub accelerates titanium crevice corrosion 6x, txo cov cua sov lub neej ntawm 6 xyoo mus rau 1 xyoo.
Chlorate Permeation Kinetics Los ntawm PFA
Chlorate ions (ClO₃⁻, 1.5nm hydrated txoj kab uas hla) diffuse los ntawm amorphous cheeb tsam ntawm PFA los ntawm ib tug hopping mechanism ntawm dawb ntim qhov chaw. Permeation testing per ASTM F739 siv 30 g/L NaClO₃ ntawm 85℃qhia tau tias khov kho-lub xeev flux mus txog 2.0mm PFA ntawm 0.08-0.12 mg/cm²·hnub nyob ntawm polymer crystallinity. Cov qib crystallinity siab dua (55-60% crystalline) txo cov flux los ntawm 40% piv rau cov qib txheem (45-50%). Lub flux ua raws li Fick txoj cai nrog diffusion coefficient ntawm 3.5 × 10⁻⁹ cm² / s ntawm 85 degree. Phab ntsa thickness yog inversely proportional rau flux; 2.5mm phab ntsa txo flux mus rau 0.05-0.07 mg / cm²·hnub, thaum 1.5mm phab ntsa nce flux mus rau 0.15-0.20 mg / cm²·hnub.
Titanium Crevice Corrosion Mechanism
Titanium tso siab rau cov txheej txheem TiO₂ passive rau corrosion kuj nyob rau hauv ib puag ncig chloride. Nyob rau hauv tej yam kev mob (< 100°C, pH > 3, moderate chloride), the passive layer remains stable. Chlorate ions permeating to the titanium surface decompose at the metal interface: 2ClO₃⁻ → 2Cl⁻ + 3O₂. The evolved oxygen creates hyper-oxidizing conditions (potential > 1.5V vs SHE) that actually stabilize the passive layer. However, the chloride produced attacks any existing crevices where oxygen depletion occurs. In crevice geometries (under gaskets, at thread roots, at polymer-metal interface gaps), chlorate decomposition produces chloride accumulation without oxygen replenishment, leading to acidification (pH < 1) and rapid crevice corrosion. Autoclave testing at 85°C with 5 g/L ClO₃⁻ shows titanium crevice corrosion rate of 0.02 mm/year with no chlorate permeation. With 0.10 mg/cm²·day chlorate flux (2.0mm PFA, standard crystallinity), crevice corrosion rate increases to 0.12 mm/year, reaching the 1.5mm wall thickness penetration in 12.5 years vs. 75 years without chlorate.
Critical Flux Threshold rau Corrosion Acceleration
Kev sib raug zoo ntawm cov tshuaj chlorate permeation flux thiab titanium corrosion tus nqi yog tsis heev -linear, nrog ib tug ntse pib hauv qab uas passive txheej tus kheej - kho. Kev ntsuam xyuas hluav taws xob hluav taws xob nrog kev tswj cov tshuaj chlorate ntau ntawm titanium nto txheeb xyuas qhov tseem ceeb ntawm 0.07 mg / cm²·hnub. Hauv qab no flux, chlorate decomposition khoom yog diluted los ntawm dej permeation (ib txwm tam sim no) thiab tsis ncav cuag corrosive concentration. Tshaj li 0.07 mg / cm²·hnub, chloride accumulates sai dua nws diffuses, ua rau autocatalytic crevice propagation. Rau 2.0mm PFA ntawm 85 degree, tus qauv crystallinity (48%) muab flux ntawm 0.11 mg / cm²·hnub, tshaj qhov pib. High crystallinity (57%) muab flux ntawm 0.06 mg / cm²·hnub, qis dua qhov pib, muab kev tiv thaiv ntev - lub sij hawm titanium.
Permeation Flux thiab Corrosion Acceleration los ntawm PFA Qib
| PFA Qib & Phab Ntsa Thickness | Chlorate Flux ntawm 85℃(mg / cm² · hnub) | Titanium Crevice Corrosion Rate (mm / xyoo) | Lub sij hawm mus rau 1.5mm Titanium Penetration (xyoo) | Kwv Txhiaj Cuab Yeej |
|---|---|---|---|---|
| Txheem PFA, 1.5mm, 48% crystallinity | 0.18 | 0.22 | 6.8 | 0.5x |
| Txheem PFA, 2.0mm, 48% crystallinity | 0.11 | 0.12 | 12.5 | 1.0x (lub hauv paus) |
| Txheem PFA, 2.5mm, 48% crystallinity | 0.07 | 0.05 | 30 | 2.4x |
| Siab crystallinity PFA, 2.0mm, 57% | 0.06 | 0.03 | 50 | 4.0x |
| Siab crystallinity PFA, 2.5mm, 57% | 0.04 | 0.02 | 75 | 6.0x |
Kub Acceleration thiab Brine Chemistry Effects
Chlorate permeation nce exponentially nrog kub. Kev ua kom lub zog rau chlorate diffusion hauv PFA yog 72 kJ / mol, lub ntsiab lus flux ob zaug txhua 12 degree. Ntawm 95 degree, permeation los ntawm 2.0mm tus qauv PFA nce mus txog 0.22 mg / cm²·hnub, ua rau kom nrawm nrawm nrawm txawm tias muaj qib siab crystallinity. Chlor-alkali hlwb ua haujlwm siab tshaj 90℃yuav tsum muaj 2.5mm siab crystallinity PFA yam tsawg kawg nkaus. Lub xub ntiag ntawm cov tshuaj dawb chlorine hauv brine (ib txwm 2-5 g / L) tsis cuam tshuam rau chlorate permeation tab sis tuaj yeem oxidize PFA nto, me ntsis txo cov flux los ntawm kev tsim cov txheej txheem densified. Cov tshuaj chlorate ntau dua (tshaj li 15 g / L) ua kom muaj dej ntau ntxiv; hlwb nrog 20 g / L ClO₃⁻ yuav tsum tau 0.5 hli ntxiv phab ntsa thickness rau sib npaug.
Crevice Geometry thiab Corrosion Localization
Qhov sib cuam tshuam ntawm PFA liner thiab titanium core tsim cov khoom siv dag zog uas ntxiab permeated chlorate. Rau cov cua sov-shrink liners, qhov sib txawv ntawm qhov sib txawv yog 0.01-0.03mm, nqaim txaus los txwv cov kua dej ntau tab sis dav txaus los khaws cov kab mob corrosive. Rau extruded los yog dip-coated liners, qhov sib txawv yog loj (0.05{11}}0.10mm), tso cai rau chlorate tsub zuj zuj ntau dua thiab sai dua crevice propagation. Txawm tias muaj crystallinity siab PFA ntawm 0.06 mg / cm²·hnub flux, extruded liners qhia crevice corrosion tus nqi 2x siab tshaj kub-shrink vim lub loj interface reservoir. Rau cov kev pabcuam chlor-alkali, qhia txog kev tsim kho cua sov tsis hais qib PFA kom txo qis qhov ntim ntawm qhov ntim.
Cov Lus Qhia Tshwj Xeeb rau Chlor-Alkali Brine Heaters
Rau rhuab brine nyob rau hauv chlor-alkali membrane hlwb ua hauj lwm ntawm 85℃nrog chlorate concentration ntawm 5-10 g/L, qhia kom meej siab crystallinity PFA (yam tsawg kawg nkaus 55%) nrog tshav kub-shrink siv thiab phab ntsa thickness ntawm 2.2 hli. Kev sib xyaw ua ke no tswj cov tshuaj chlorate flux qis dua 0.07 mg / cm²·hnub, ua kom titanium crevice corrosion qis dua 0.04 mm / xyoo thiab ua tiav 12+ xyoo lub neej rhaub. Yuav tsum tau muab ntawv pov thawj ntawm PFA crystallinity los ntawm DSC (differential scanning calorimetry) thiab chlorate permeation kuaj ntawm qhov siab tshaj plaws ua haujlwm kub. Rau cov hlwb ua haujlwm siab tshaj 90 degree, xav tau 2.5mm phab ntsa nrog crystallinity saum 57%, thiab xav txog lub sijhawm (5-} xyoo) hloov lub rhaub tsis hais qhov xwm txheej. Thaum retrofitting cua sov rau hauv cov hlwb uas twb muaj lawm, tshuaj xyuas cov titanium core nto rau crevice corrosion uas twb muaj lawm siv dye penetrant kuaj; ib qho kev kuaj pom pitting yuav tsum tau hloov pauv ua ntej txhim kho PFA tshiab. Rau cov qauv tsim tshiab ntawm tes, tso cov cua sov hauv cov chlorate zones (sab sauv ntawm anolyte ntxiv) kom txo tau cov permeation tsav quab yuam, tso cai rau siv cov phab ntsa nyias thiab cov nqi qis dua. Tus nqi rau siab crystallinity PFA (15-20% upcharge) yog tsim nyog los ntawm 3-4x ntev titanium tub ntxhais lub neej, tsis txhob raug nqi hluav taws xob hloov thiab lub sij hawm ntawm tes downtime hauv chlor-alkali ntau lawm qhov chaw ua haujlwm tas li yog lub luag haujlwm tseem ceeb.
