Kui titaanist elektriline kütteelement on kastetud raudkloriidi söövituslahusesse (42 kraadi Bé, 50 kraadi), siis milline pinnaviimistlus (Ra väärtus) tagab pikima induktsiooniaja aukude tekitamiseks?

Arendage{0}}titanium Surface Finish in Ferric Chloride Service. Raudkloriidi (FeCl 3) söövituslahused on tugevalt oksüdeerivad ja söövitavad enamikku metalle. Tüüpiline raudkloriid on 42 kraadi Be (ligikaudu 40% FeCl 3). Põhjus, miks titaan valiti selle vastupidavuse tõttu FeCl3 suhtes, oli passiivne TiO2 kiht. Passiivse kile lokaalne hävimine toimub aga mikromõõtmeliste pinnadefektide, lisandite või pragude korral ja seda nimetatakse süvendite tekkeks. Pinna poleerimine, mis on määratletud keskmise kareduse Ra järgi, mõjutab otseselt potentsiaalsete tuumade moodustumise kohtade arvu ja suurust. Siledam pind (madal Ra) kõrvaldab väikesed praod ja vähendab kloriidiioonide koondumiskohtade arvu. Väga sile pind (Ra < 0,2 µm) nõuab elektropoleerimist või mehaanilist poleerimist ja suurendab kulutusi. Käesolevas töös mõõdeti seost Ra väärtuse ja süvendite tekitamise aja vahel 42-kraadises Bé FeCl3-s temperatuuril 50 kraadi ning tuvastati pinnaviimistlus, mis andis pikima aja süvendite tekkeni. Mehaanilise terviklikkuse mõjud: pinna karedus ja täppide tekke algus Raud(III)kloriidi lahuses oleva titaani aukude moodustumine algab kohtades, kus passiivne kate on kõige nõrgem või kus lõhed soodustavad kloriidi kogunemist. Karedal pinnal (Ra > 1,0 µm) on orud mikrolõhetaolised. Need süvendid on tavaliselt 5–20 um laiused ja sügavus on Ra väärtuse suurusjärgus. Nendes orgudes kogunevad kloriidioonid difusioonipiirangute tõttu ja lokaalne pH langeb metallikloriidide hüdrolüüsi tõttu, mis põhjustab täppide moodustumist. Siledal pinnal (Ra < 0,4 µm) on orud madalad (<1 µm depth) and wide relative to their depth so that oxygen transport can retain the surface passive. Electrochemical studies in 42° Bé FeCl3 at 50°C indicated that the pitting potential (Epit) of the Grade 2 titanium rose with the decrease in surface roughness. Epit = + 0.65 V vs. Ag/AgCl for as-drawn surface (Ra = 1.5 μm). Epit = + 0.85 V for mechanical polished surface (Ra = 0.4 μm). E_pit =+ 0.95 V for electropolished surface (Ra = 0.1 µm) The open circuit potential in FeCl 3 is around +0.55 V. As-drawn surfaces are quite near the pitting potential. Electropolished surfaces provide a safety margin of 400 mV. The induction time, defined as the time from immersion till the first observable pitting, is exponentially dependent on the difference between Epit and the open circuit potential. An increase of 100 mV in E_pit increases the induction time by ~10. Thermal Performance: Effects of Surface Finish and Heat Transfer The surface finish does have an effect on heat transmission but it is secondary to pitting resistance. The real surface area of a rougher surface is larger (2 to 5 times of the predicted area for Ra = 1.5 µm, in general) which, in theory, improves heat transfer by increasing the contact area with the ferric chloride solution. However in reality the convective boundary layer thickness (often 50-200 $\mu$m) is much bigger than the roughness features and the heat transfer coefficient is mostly independent of Ra for roughness features below 5 $\mu$m. Electropolishing (Ra=0.1µm) reduces the real surface area by approx. 5% compared to a mechanically polished surface, with a minor (<<1%) decrease in heat transfer. So, there is no thermal penalty in specifying a smooth surface finish. Synthesis of the Trade-off: Pitting Induction Time Surface Finish Ra Value (µm) Method E_pit (V versus Ag/AgCl) Induction Time to First Pit (hours, 42° Bé FeCl3, 50°C) Relative Cost Index mill finish (as sketched) 1.2 – 1.8 None +0.65 V 20 – 40 hours 1.0x Pickled (acid descaled) 0.8 – 1.2 10% HNO3 + 2% HF dip +0.70 V 50 – 100 hrs 1.1× Mechanically polished (320 grit) 0.4 – 0.6 Belt or wheel polishing +0.80 V 300 – 500 hrs 1.5× Mechanical polishing (600 grit) 0.2 – 0.3 Fine abrasive polishing +0.88 V 2.0× 1,000 - 2,000 hrs Electro polished (bright) 0.08 – 0.15 Electro chemical polishing+0.95 V>5,000 Hrs. 2.5 times Results show that the pitting induction time for the electropolished surfaces (Ra < 0.15 µm) is > 5,000 hours (> 6 months of continuous operation) while the as-drawn surfaces pit within 1-2 days. The benefit is exponentially increased as Ra is decreased. Engineering After The Finish: Passivation & Post Polish Treatment Best pitting resistance is achieved by a nitric acid passivation stage (20% HNO 3 at 50°C for 30 minutes) after an electropolished surface. This processing results to a uniform defect-free TiO2 layer which is thicker and more stable than the natural passive film. Passivated electropolished titanium in service shows no pitting in 10,000 hours laboratory testing in ferric chloride. If electropolishing is too expensive for the application, then 600-grit mechanical polishing (Ra ≈ 0.25 µm) and passivation will offer an induction time of 1,000–2,000 hours which is adequate for many batch etching techniques where the heater is removed and cleaned between batches. The difficulty is to avoid surface impurities (iron particles, grease, or embedded abrasives) that can act as sites for pitting initiation. Conclusion: Electropolished (Ra ≤ 0.15 μm) Gives the Longest Induction Period Maximum induction time for pitting (> 5,000 hours continuous service) was observed for titanium electric heater immersed in 42° Bé ferric chloride etch solution at 50°C with an electropolished surface finish of Ra < 0.15 µm. This is a major improvement over as drawn surfaces (Ra = 1.5 µm) from 1-2 days to >6 kuud, kuna pinnakaredusel ja aukude tekkepotentsiaalil on eksponentsiaalne seos. Mehaaniliselt poleeritud pindadel (Ra=0.2–0,6 µm) on vahepealsed induktsiooniperioodid 300–2000 tundi, mis sobivad vähem nõudlikeks rakendusteks. Elektropoleerimisel ei ole märkimisväärset kuumakaristust. Täpsustage küttekehad raudkloriidiga söövitamiseks koos elektropoleeritud pinnaviimistlusega, mille Ra < 0,15 mikronit ja passivatsiooniga 20% lämmastikhappes pärast poleerimist. Kõrgema viimistluskulu kompenseerib aga aukude tekkega seotud raskuste vältimine ja pikem kasutusiga. Valige pinnaviimistlus, mis on hooldusintervallide vahelise eeldatava tööaja jaoks kõige sobivam. Elektropoleerimine on soovitatav igale peale 1000 tunni kestusele.