Meta Element tat-Tisħin Elettriku tat-Titanju Huwa Mgħaddas f'Soluzzjoni ta 'Inċiżjoni tal-Klorur Ferriku (42 grad Bé, 50 grad), Liema Finish tal-wiċċ (Valur Ra) Jipprovdi l-Itwal Ħin ta' Induzzjoni għall-Pitting?

Skambju-off għall-Finitura tal-wiċċ tat-titanju fis-Servizz tal-Klorur Ferriku tas-soluzzjonijiet tal-inċiżjoni tal-klorur tal-ħadid (FeCl 3 ) huma ossidanti ħafna u korrużivi ħafna għall-biċċa l-kbira tal-metalli. Klorur ferriku tipiku huwa 42 grad Be (madwar 40% FeCl 3 ). Ir-raġuni li t-titanju ntgħażel għar-reżistenza tiegħu għal FeCl3 kien is-saff TiO2 passiv. Distruzzjoni lokalizzata tal-film passiv madankollu, isseħħ f'difetti tal-wiċċ fuq mikroskala, inklużjonijiet jew xquq u tissejjaħ pitting. Il-lustrar tal-wiċċ, definit mill-ħruxija medja Ra, jaffettwa direttament in-numru u d-daqs ta 'siti potenzjali ta' nukleazzjoni tal-pitting. Il-wiċċ aktar lixx (Ra baxx) jelimina xquq żgħar u jnaqqas in-numru ta 'postijiet fejn il-joni tal-klorur jistgħu jikkonċentraw. Wiċċ lixx ħafna (Ra < 0.2 µm) jeħtieġ electropolishing jew illustrar mekkaniku u jżid l-ispiża. Fix-xogħol preżenti, ġiet imkejla r-relazzjoni bejn il-valur Ra ​​u l-ħin ta 'induzzjoni tal-pitting f'42 grad Bé FeCl3 f'50 grad u ġiet identifikata l-finitura tal-wiċċ li rriżultat fl-itwal żmien għall-bidu tal-fossa. Effetti ta 'Integrità Mekkanika: Ħruxija tal-wiċċ u Bidu ta' Pitting Pitting ta 'titanju f'soluzzjoni ta' klorur ferriku jibda f'siti fejn il-kisja passiva hija l-aktar dgħajfa jew fejn xquq jiffaċilitaw l-akkumulazzjoni tal-klorur. F'wiċċ mhux maħdum (Ra > 1.0 µm), il-widien huma mikro fissuri simili. Dawn il-ħwat huma tipikament ta 'wisa' 5-20 um u l-fond huwa ta 'l-ordni tal-valur Ra. F'dawn il-widien il-joni tal-klorur huma akkumulati minħabba limitazzjonijiet tad-diffużjoni u l-pH lokali jonqos minħabba l-idroliżi tal-kloruri tal-metall li tikkawża pitting. Fuq wiċċ lixx (Ra < 0.4 µm) il-widien huma baxxi (<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 xhur, peress li l-ħruxija tal-wiċċ u l-potenzjal ta 'pitting għandhom konnessjoni esponenzjali. L-uċuħ illustrati mekkanikament (Ra=0.2–0.6 µm) għandhom perjodi ta' induzzjoni intermedji ta' 300–2000 siegħa, adattati għal applikazzjonijiet inqas impenjattivi. L-elettropolixxjar m'għandux penali sinifikanti tas-sħana. Speċifika ħiters għall-inċiżjoni tal-klorur ferriku b'finitura tal-wiċċ elettropolished verifikata Ra < 0.15 mikron u passivazzjoni f'20% lustrar ta 'wara tal-aċidu nitriku. Madankollu, l-ispiża ogħla tal-irfinar hija kkumpensata mill-evitar ta 'diffikultajiet relatati mal-pitting u l-ħajja ta' servizz itwal. Agħżel il-finitura tal-wiċċ li hija l-aktar xierqa għall-ħin ta 'tħaddim mistenni bejn l-intervalli ta' manutenzjoni. L-elettropolixxjar huwa rakkomandat għal kull applikazzjoni li taqbeż l-1,000 siegħa.