Pan fydd Elfen Gwresogi Trydan Titaniwm Yn Cael Ei Throchi mewn Ateb Ysgythru Clorid Ferric (42 gradd Be, 50 gradd ), Pa Gorffen Arwyneb (Gwerth Ra) Sy'n Darparu'r Amser Sefydlu Hiraf ar gyfer Pitting?

Masnach -i ffwrdd ar gyfer Gorffeniad Arwyneb Titaniwm mewn Gwasanaeth Clorid Ferric Mae hydoddiannau ysgythru fferrig clorid (FeCl 3 ) yn ocsideiddio iawn ac yn gyrydol iawn i'r rhan fwyaf o fetelau. Clorid fferrig nodweddiadol yw 42 gradd Be (tua 40% FeCl 3 ). Y rheswm y dewiswyd titaniwm am ei wrthwynebiad i FeCl3 oedd yr haen TiO2 goddefol. Fodd bynnag, mae dinistr lleol o'r ffilm oddefol yn digwydd ar ddiffygion arwyneb micro-raddfa, cynhwysiant neu graciau ac fe'i gelwir yn pitting. Mae'r sglein arwyneb, a ddiffinnir gan y garwder cyfartalog Ra, yn effeithio'n uniongyrchol ar nifer a maint y safleoedd cnewyllol tyllu posibl. Mae'r arwyneb llyfnach (Ra isel) yn dileu craciau bach ac yn lleihau nifer y lleoliadau lle gall ïonau clorid ganolbwyntio. Mae arwyneb llyfn iawn (Ra < 0.2 µm) angen electropolishing neu sgleinio mecanyddol ac yn cynyddu'r gost. Yn y gwaith presennol, mesurwyd y berthynas rhwng gwerth Ra ac amser sefydlu tyllu yn 42 gradd Bé FeCl3 ar 50 gradd a nodwyd y gorffeniad arwyneb a arweiniodd at yr amser hiraf i gychwyn pwll. Effeithiau Uniondeb Mecanyddol: Garweddrwydd yr Arwyneb a Chychwyn Pylo Mae gosod titaniwm mewn hydoddiant fferrig clorid yn cychwyn ar safleoedd lle mae'r gorchudd goddefol ar ei wanaf neu lle mae holltau'n hwyluso cronni clorid. Mewn arwyneb garw (Ra > 1.0 µm), mae'r dyffrynnoedd yn debyg i holltau micro. Mae'r cafnau hyn fel arfer yn 5-20 um o led ac mae'r dyfnder yn ôl trefn y gwerth Ra. Yn y dyffrynnoedd hyn mae ïonau clorid yn cronni oherwydd cyfyngiadau trylediad ac mae'r pH lleol yn disgyn oherwydd hydrolysis cloridau metel sy'n achosi pylu. Ar arwyneb llyfn (Ra < 0.4 µm) mae'r dyffrynnoedd yn fas (<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 mis, gan fod gan garwedd arwyneb a photensial tyllu gysylltiad esbonyddol. Mae gan yr arwynebau wedi'u sgleinio'n fecanyddol (Ra=0.2–0.6 µm) gyfnodau ymsefydlu canolradd o 300–2000 awr, sy'n addas ar gyfer cymwysiadau llai heriol. Nid oes cosb wres sylweddol gan electropolishing. Nodwch wresogyddion ar gyfer ysgythriad ferric clorid gyda gorffeniad arwyneb electropolished wedi'i wirio Ra < 0.15 micron a passivation mewn sglein post asid nitrig 20%. Fodd bynnag, mae'r gost gorffen uwch yn cael ei wrthbwyso gan osgoi anawsterau sy'n gysylltiedig â thyllu a'r bywyd gwasanaeth hirach. Dewiswch y gorffeniad arwyneb sydd fwyaf priodol ar gyfer yr amser rhedeg disgwyliedig rhwng cyfnodau cynnal a chadw. Argymhellir electropolishing ar gyfer unrhyw gais sy'n fwy na 1,000 o oriau.