Apabila Elemen Pemanas Elektrik Titanium Direndam dalam Penyelesaian Goresan Ferrik Klorida (42 darjah Bé, 50 darjah ), Apakah Kemasan Permukaan (Nilai Ra) Membekalkan Masa Aruhan Terpanjang untuk Pitting?

Tukar{0}}untuk Kemasan Permukaan Titanium dalam Perkhidmatan Ferrik Klorida Larutan etsa Ferrik klorida (FeCl 3 ) sangat mengoksida dan sangat menghakis kepada kebanyakan logam. Ferrik klorida biasa ialah 42 darjah Be (kira-kira 40% FeCl 3 ). Sebab titanium dipilih untuk rintangannya terhadap FeCl3 adalah lapisan TiO2 pasif. Pemusnahan setempat bagi filem pasif bagaimanapun, berlaku pada kecacatan permukaan skala mikro, kemasukan atau retak dan dipanggil pitting. Pengilat permukaan, yang ditakrifkan oleh purata kekasaran Ra, secara langsung mempengaruhi bilangan dan saiz tapak nukleasi pitting yang berpotensi. Permukaan licin (Ra rendah) menghilangkan retakan kecil dan mengurangkan bilangan lokasi di mana ion klorida boleh menumpukan. Permukaan yang sangat licin (Ra < 0.2 µm) memerlukan penggilap elektro atau penggilap mekanikal dan meningkatkan perbelanjaan. Dalam kerja ini, hubungan antara nilai Ra dan masa aruhan pitting dalam 42 darjah Bé FeCl3 pada 50 darjah telah diukur dan kemasan permukaan yang menghasilkan masa yang paling lama untuk permulaan pit dikenal pasti. Kesan Integriti Mekanikal: Kekasaran Permukaan dan Permulaan Pitting Pitting titanium dalam larutan ferik klorida bermula di tapak di mana salutan pasif paling lemah atau di mana rekahan memudahkan pengumpulan klorida. Dalam permukaan kasar (Ra > 1.0 µm), lembah adalah seperti fisur mikro. Palung ini biasanya berukuran 5-20 um lebar dan kedalaman adalah mengikut susunan nilai Ra. Di lembah ini ion klorida terkumpul kerana had resapan dan pH tempatan menurun akibat hidrolisis logam klorida yang menyebabkan pitting. Pada permukaan licin (Ra < 0.4 µm) lembah adalah cetek (<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 bulan, kerana kekasaran permukaan dan potensi pitting mempunyai sambungan eksponen. Permukaan yang digilap secara mekanikal (Ra=0.2–0.6 µm) mempunyai tempoh aruhan pertengahan 300–2000 jam, sesuai untuk aplikasi yang kurang menuntut. Penggilap elektrik tidak mempunyai penalti haba yang ketara. Nyatakan pemanas untuk goresan ferik klorida dengan kemasan permukaan yang digilap elektro disahkan Ra < 0.15 mikron dan pempasifan dalam 20% pengilat selepas asid nitrik. Walau bagaimanapun, kos penamat yang lebih tinggi diimbangi dengan mengelakkan kesukaran yang berkaitan dengan pitting dan hayat perkhidmatan yang lebih lama. Pilih kemasan permukaan yang paling sesuai untuk jangka masa berjalan antara selang penyelenggaraan. Penggilapan elektrik disyorkan untuk sebarang aplikasi yang melebihi 1,000 jam.