Thaum Titanium Hluav Taws Xob Hluav Taws Xob Hluav Taws Xob Hluav Taws Xob Hluav Taws Xob Hluav Taws Xob Hluav Taws Xob Hluav Taws Xob Hluav Taws Xob Hluav Taws Xob Hluav Taws Xob (42℃Bé, 50 degree), Dab tsi nto tiav (Ra Tus Nqi) muab lub sijhawm ntev tshaj plaws rau kev sib tsoo?

Kev lag luam -tawm rau Titanium Surface Finish nyob rau hauv Ferric Chloride Service Ferric chloride (FeCl 3) etching daws yog heev oxidizing thiab heev corrosive rau feem ntau hlau. Hom ferric chloride yog 42℃Be (kwv yees li 40% FeCl 3). Yog vim li cas titanium raug xaiv rau nws tsis kam mus rau FeCl3 yog lub passive TiO2 txheej. Kev puas tsuaj hauv cheeb tsam ntawm cov yeeb yaj kiab passive txawm li cas los xij, tshwm sim ntawm microscale defects, suav nrog lossis tawg thiab hu ua pitting. Cov polished nto, txhais los ntawm qhov nruab nrab roughness Ra, ncaj qha cuam tshuam tus naj npawb thiab qhov loj ntawm qhov chaw muaj peev xwm pitting nucleation. Qhov smoother nto (qis Ra) tshem tawm cov kab nrib pleb me me thiab txo qhov chaw uas cov chloride ions tuaj yeem mloog zoo. Qhov chaw du heev (Ra <0.2 µm) yuav tsum tau electropolishing lossis mechanical polishing thiab nce nqi. Hauv kev ua haujlwm tam sim no, kev sib raug zoo ntawm Ra tus nqi thiab pitting induction lub sij hawm nyob rau hauv 42℃Bé FeCl3 ntawm 50℃tau ntsuas thiab qhov chaw tiav uas ua rau lub sij hawm ntev tshaj plaws rau qhov pib pib. Cov teebmeem ntawm Mechanical Integrity: Surface Roughness thiab Initiation of Pitting Pitting of titanium in ferric chloride solution pib ntawm qhov chaw uas cov txheej txheej tsis muaj zog tshaj plaws los yog qhov chaw fissures pab txhawb nqa cov tshuaj chloride. Hauv qhov chaw ntxhib (Ra> 1.0 µm), cov kwj ha yog micro fissure zoo li. Cov troughs no feem ntau yog 5-20 um dav thiab qhov tob yog qhov kev txiav txim ntawm Ra tus nqi. Hauv cov hav no cov tshuaj chloride ions tau khaws cia vim muaj kev txwv tsis pub muaj kev txwv thiab cov pH hauv zos poob vim hydrolysis ntawm cov hlau chlorides uas ua rau pitting. Ntawm qhov chaw du (Ra < 0.4 µm) cov kwj ha yog ntiav (<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 lub hlis, raws li qhov chaw roughness thiab pitting muaj peev xwm muaj kev sib txuas exponential. Lub tshuab polished nto (Ra=0.2-0.6 µm) muaj lub sijhawm nruab nrab induction ntawm 300-2000 teev, tsim nyog rau cov ntawv thov tsawg dua. Electropolishing tsis muaj teeb meem kub kub. Qhia cov cua sov rau ferric chloride etching nrog electropolished nto tiav pov thawj Ra <0.15 micron thiab passivation hauv 20% nitric acid post polish. Txawm li cas los xij, tus nqi ntau dua tiav yog offset los ntawm kev zam ntawm pitting cov teeb meem cuam tshuam thiab lub neej ua haujlwm ntev dua. Xaiv qhov chaw tiav uas tsim nyog tshaj plaws rau lub sijhawm ua haujlwm uas xav tau ntawm kev saib xyuas lub sijhawm. Electropolishing raug pom zoo rau txhua daim ntawv thov tshaj 1,000 teev.