Ketika Elemen Pemanas Listrik Titanium Dicelupkan ke dalam Larutan Etsa Ferri Klorida (42 derajat Bé, 50 derajat ), Lapisan Permukaan Apa (Nilai Ra) yang Memberikan Waktu Induksi Terpanjang untuk Pitting?

Pengorbanan{0}}untuk Pelapis Permukaan Titanium dalam Layanan Ferri Klorida Larutan etsa besi klorida (FeCl 3 ) sangat mengoksidasi dan sangat korosif terhadap sebagian besar logam. Besi klorida yang khas adalah 42 derajat Be (kira-kira 40% FeCl 3 ). Alasan titanium dipilih karena ketahanannya terhadap FeCl3 adalah lapisan TiO2 yang pasif. Namun, penghancuran film pasif yang terlokalisasi terjadi pada cacat permukaan skala mikro, inklusi atau retakan dan disebut pitting. Pemolesan permukaan, yang ditentukan oleh kekasaran rata-rata Ra, secara langsung mempengaruhi jumlah dan ukuran lokasi nukleasi lubang yang potensial. Permukaan yang lebih halus (Ra rendah) menghilangkan retakan kecil dan mengurangi jumlah lokasi di mana ion klorida dapat terkonsentrasi. Permukaan yang sangat halus (Ra <0,2 µm) memerlukan pemolesan elektro atau pemolesan mekanis dan meningkatkan biaya. Dalam penelitian ini, hubungan antara nilai Ra dan waktu induksi pitting pada 42 derajat Bé FeCl3 pada 50 derajat diukur dan permukaan akhir yang menghasilkan waktu terlama untuk memulai pit diidentifikasi. Pengaruh Integritas Mekanis: Kekasaran Permukaan dan Inisiasi Pitting Pitting titanium dalam larutan besi klorida dimulai pada lokasi dimana lapisan pasifnya paling lemah atau dimana celah memfasilitasi akumulasi klorida. Pada permukaan yang kasar (Ra > 1,0 µm), lembahnya berbentuk seperti celah mikro. Palung ini biasanya memiliki lebar 5-20 um dan kedalamannya sesuai dengan nilai Ra. Di lembah ini ion klorida terakumulasi karena keterbatasan difusi dan penurunan pH lokal akibat hidrolisis logam klorida yang menyebabkan lubang. Pada permukaan halus (Ra <0,4 µm) lembahnya dangkal (<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, karena kekasaran permukaan dan potensi lubang memiliki hubungan yang eksponensial. Permukaan yang dipoles secara mekanis (Ra=0.2–0,6 µm) memiliki periode induksi menengah selama 300–2000 jam, cocok untuk aplikasi yang tidak terlalu menuntut. Pemolesan listrik tidak memiliki penalti panas yang signifikan. Tentukan pemanas untuk etsa besi klorida dengan permukaan akhir yang dipoles secara elektro yang diverifikasi Ra <0,15 mikron dan pasivasi dalam 20% asam nitrat pasca pemolesan. Namun, biaya penyelesaian yang lebih tinggi diimbangi dengan menghindari kesulitan terkait pitting dan masa pakai yang lebih lama. Pilih penyelesaian permukaan yang paling sesuai dengan perkiraan waktu pengoperasian di antara interval perawatan. Pemolesan listrik direkomendasikan untuk aplikasi apa pun yang melebihi 1.000 jam.