Kev pauv -tawm rau Titanium Surface Finish hauv Ferric Chloride Service
Ferric Chloride (FeCl 3) etching daws yog heev oxidizing thiab aggressively corrosive rau feem ntau hlau. Hom ferric chloride yog 42℃Bé (li 40% FeCl 3). Titanium raug xaiv vim hais tias mus rau passive TiO₂ txheej, uas ua rau nws resistant rau FeCl₃. Txawm li cas los xij, pitting yog qhov kev puas tsuaj hauv zos ntawm cov txheej txheem passive, feem ntau ntawm qhov chaw tsis zoo, suav nrog lossis crevices ntawm microscale. Nto tiav, ntsuas los ntawm qhov nruab nrab roughness Ra, muaj kev cuam tshuam ncaj qha rau qhov ntau thiab qhov loj ntawm qhov chaw pib pitting. Ib tug smoother nto (tsawg Ra) tshem tawm cov fissures microscopic thiab txo tus naj npawb ntawm qhov chaw rau cov concentration ntawm chloride ions. Ib tug heev du nto (Ra <0.2 µm) yuav tsum tau electropolishing los yog mechanical polishing thiab ntxiv nqi. Txoj kev tshawb fawb tam sim no tau txiav txim siab qhov sib txheeb ntawm Ra tus nqi thiab pitting induction lub sij hawm nyob rau hauv 42℃Bé FeCl3 ntawm 50℃thiab tsim lub nto tiav uas muab lub sij hawm siab tshaj plaws ua ntej qhov pib pib.

Mechanical Integrity Impacts: Pitting Initiation thiab Surface Roughness
Pitting nyob rau hauv ferric chloride tov ntawm titanium pib ntawm qhov chaw uas cov zaj duab xis passive yog thinnest, los yog qhov twg fissures tso cai tsub zuj zuj ntawm chloride. Hauv qhov chaw ntxhib (Ra> 1.0 µm), cov kwj ha zoo li micro fissures. Cov troughs no feem ntau yog 5-20 µm dav thiab qhov tob yog tib yam li tus nqi Ra. Hauv cov kwj ha chloride ions sib sau vim muaj kev txwv tsis pub sib txawv thiab cov pH hauv zos txo qis vim hydrolysis ntawm cov hlau chlorides uas pib pitting. Ntawm qhov chaw du (Ra < 0.4 µm) cov kwj ha yog ntiav (<1 µm depth) and wide compared to their depth such that oxygen transport can maintain passivity. Electrochemical experiments in 42° Bé FeCl3 at 50°C showed that the pitting potential (Epit) of the Grade 2 titanium increased as the surface roughness decreased. For as-drawn surface (Ra = 1.5 µm) Epit = +0.65 V vs. Ag/AgCl. E_pit = +0.85 V for a mechanically polished surface (Ra = 0.4 µm). For electropolished surface (Ra = 0.1 µm) E_pit = + 0.95 V. In FeCl₃ the open-circuit potential is about +0.55 V. As-drawn surfaces are quite close to the pitting potential, whereas electropolished surfaces offer a safety margin of 400 mV. The induction time, which is the period from immersion to the first detectable pitting, shows an exponential dependence on the difference between Epit and the open-circuit potential. An increase of E_pit of 100 mV results in an increase of ~10 in induction time.

Thermal Performance: Cov nyhuv ntawm Deg tiav thiab cua sov hloov
Nto tiav kuj muaj kev cuam tshuam rau kev thauj mus los ntawm tshav kub, tab sis yog qhov thib ob rau pitting tsis kam. Qhov tseeb ntawm qhov chaw ntawm qhov rougher saum npoo yog ntau dua (2 mus rau 5 npaug ntawm qhov projected cheeb tsam rau Ra=1.5 µm, feem ntau) uas, nyob rau hauv txoj kev xav, txhim kho cov cua sov los ntawm kev ua kom thaj chaw sib cuag nrog cov tshuaj ferric chloride. Qhov tseeb txawm li cas los xij lub thickness ntawm lub convective ciam txheej (feem ntau yog 50-200 µm) yog ho loj dua qhov roughness nta thiab yog li ntawd lub tshav kub hloov coefficient yog yuav luag ywj siab ntawm Ra rau cov nqi qis dua 5 µm. Electropolishing (Ra=0.1µm) txo cov cheeb tsam tiag tiag los ntawm ~ 5% vs a mechanically polished nto, ua rau qhov tsawg (<<1%) reduction in heat transmission. This means there is no thermal penalty for specifying a smooth surface finish.

Synthesis of the Trade-off: Ra Value vs Pitting Induction Time
Surface Finish Ra Value (µm) Method E_pit (V vs. Ag/AgCl) Induction Time to First Pit (teev, 42℃Bé FeCl3, 50 degree) Tus nqi txheeb ze
Raws li -kos ( zeb tiav) 1.2 – 1.8 Tsis muaj +0.65 V 20 – 40 teev 1.0 × Pickled (acid descaled) 0.8 – 1.2 10% HNO₃ + 2% HF dip +0.70 V 50 – 100 teev (300 teev) 4 g polished 1. 0.6 Siv los yog log polishing +0.80 V 300 - 500 teev 1.5 ×
Mechanical polishing (600 grit) 0.2 - 0.3 Fine abrasive polishing +0.88 V 2.0 × 1, 000 - 2,000 teev
Electro polished (bright finish) 0.08 – 0.15 Electro chemical polishing+0.95 V >5,000 Hrs. 2.5x
The results indicates that electropolished surfaces (Ra < 0.15 µm) have a pitting induction time of > 5,000 hours (>6 lub hlis ntawm kev ua haujlwm tas mus li) thaum raws li - kos qhov chaw hauv 1-2 hnub. Qhov kev txhim kho loj hlob exponentially raws li Ra txo.

Engineering tom qab ua tiav: Passivation thiab Post-Kev kho polishing
Rau qhov zoo tshaj plaws pitting tsis kam, electropolished nto yuav tsum tau ua raws li nitric acid passivation kauj ruam (20% HNO 3 ntawm 50℃rau 30 feeb). Qhov kev kho no ua rau muaj kev sib haum xeeb, tsis xws luag-TiO2 txheej dawb uas yog tuab thiab ruaj khov dua li cov yeeb yaj kiab tsis zoo. Passivated electropolished titanium nyob rau hauv kev pab cuam nyob rau hauv ferric chloride qhia tsis muaj pitting tom qab 10,000 teev ntawm kev sim kuaj. Yog tias electropolishing kim heev rau daim ntawv thov, ces 600-grit mechanical polishing (Ra ≈ 0.25 µm) thiab passivation yuav muab lub sij hawm induction ntawm 1,000-2,000 teev uas yog tsim rau ntau batch etching txheej txheem qhov twg lub rhaub yog tshem tawm thiab ntxuav ntawm batches. Qhov ua kom yuam kev yog kom tsis txhob muaj cov kab mob ntawm qhov chaw (hlau hais, roj, los yog cov khoom sib txuas) uas tuaj yeem ua qhov chaw pib qhov chaw.

Cov ntsiab lus: Electropolished (Ra Tsawg dua lossis sib npaug ntawm 0.15 μm) Muab Lub Sijhawm Induction Ntev
The maximum induction time for pitting (> 5,000 hours of continuous service) was obtained for a titanium electric heating element immersed in 42° Bé ferric chloride etching solution at 50°C with an electropolished surface finish of Ra < 0.15 µm. Because of the exponential relationship between surface roughness and pitting potential, the improvement over as drawn surfaces ( Ra = 1.5 µm) is considerable, from 1-2 days to >6 hli. Lub tshuab polished nto (Ra=0.2-0.6 µm) ua rau nruab nrab induction lub sij hawm ntawm 300-2,000 teev, tsim nyog rau cov ntawv thov tsawg dua. Electropolishing tsis muaj qhov tseem ceeb thermal txim. Qhia cov cua sov rau ferric chloride etching nrog electropolished nto tiav pov thawj Ra Tsawg dua los yog sib npaug rau 0.15 μm thiab passivation hauv 20% nitric acid tom qab polish. Tab sis tus nqi ntau ntxiv yog them los ntawm kev zam ntawm pitting-cov teeb meem ntsig txog thiab lub neej ua haujlwm ntev dua. Xaiv qhov tsim nyog nto tiav raws li kev cia siab ua haujlwm tas li ntawm lub sijhawm txij nkawm. Rau ib daim ntawv thov tshaj 1,000 teev, electropolishing yog pom zoo.