Titanli elektr isitish elementi temir xlorid eritmasiga (42 daraja Bé, 50 daraja) botirilganda, qanday sirt qoplamasi (Ra qiymati) chuqurlash uchun eng uzun indüksiya vaqtini ta'minlaydi?

Temir xlorid xizmatida{0}}titan sirtini pardozlash Temir xlorid (FeCl 3) bilan ishlov berish eritmalari koʻpchilik metallar uchun yuqori oksidlovchi va yuqori korroziv xususiyatga ega. Odatda temir xlorid 42 daraja Be (taxminan 40% FeCl 3) dir. Titanning FeCl3 ga chidamliligi uchun tanlanganiga sabab passiv TiO2 qatlami edi. Passiv plyonkaning lokal ravishda yo'q qilinishi mikro miqyosdagi sirt nuqsonlari, qo'shimchalar yoki yoriqlarda sodir bo'ladi va chuqurlik deb ataladi. O'rtacha pürüzlülük Ra bilan belgilangan sirt jilo, potentsial chuqurchalar yadrolari hosil bo'ladigan joylar soni va hajmiga bevosita ta'sir qiladi. Yumshoqroq sirt (past Ra) kichik yoriqlarni yo'q qiladi va xlorid ionlari to'planishi mumkin bo'lgan joylar sonini kamaytiradi. Juda silliq sirt (Ra < 0,2 mkm) elektropolishing yoki mexanik polishingni talab qiladi va xarajatlarni oshiradi. Ushbu ishda Ra qiymati va 42 daraja Bé FeCl3 50 gradusda chuqurchalar paydo bo'lish vaqti o'rtasidagi bog'liqlik o'lchandi va chuqurning boshlanishi uchun eng uzoq vaqtga olib keladigan sirt qoplamasi aniqlandi. Mexanik yaxlitlikning ta'siri: Sirtning pürüzlülüğü va chuqurlikning boshlanishi Temir xlorid eritmasida titanning chuqurlashishi passiv qoplama eng zaif yoki yoriqlar xlorid to'planishini osonlashtiradigan joylarda boshlanadi. Qo'pol sirtda (Ra > 1,0 mkm) vodiylar mikro yoriqlarga o'xshaydi. Bu oluklarning kengligi odatda 5-20 um, chuqurligi esa Ra qiymatiga teng. Bu vodiylarda xlorid ionlari diffuziya cheklovlari tufayli to'planadi va metall xloridlarning gidrolizlanishi tufayli mahalliy pH pasayadi, bu esa chuqurchaga olib keladi. Silliq yuzada (Ra < 0,4 mkm) vodiylar sayoz (<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 oy, chunki sirt pürüzlülüğü va chuqurlik potentsiali eksponensial aloqaga ega. Mexanik jilolangan yuzalar (Ra=0.2–0,6 mkm) 300–2000 soatlik oraliq indüksiyon davrlariga ega bo'lib, kamroq talab qilinadigan ilovalar uchun javob beradi. Elektropolishingda sezilarli issiqlik jazosi yo'q. Ra < 0,15 mikron boʻlgan elektropolyetlangan sirt qoplamasi va 20% nitrat kislotadan keyingi jiloda passivatsiyaga ega boʻlgan temir xlorid bilan ishlov berish uchun isitgichlarni belgilang. Biroq, yuqori tugatish xarajati chuqurchalar bilan bog'liq qiyinchiliklarning oldini olish va xizmat muddatini uzaytirish bilan qoplanadi. Xizmat oraliqlari orasidagi kutilgan ish vaqti uchun eng mos bo'lgan sirt qoplamasini tanlang. 1000 soatdan ortiq har qanday dastur uchun elektropolishing tavsiya etiladi.