チタン電熱体を塩化第二鉄エッチング液(42度Bé、50度)に浸漬した場合、孔食の誘導時間が最も長くなる表面仕上げ(Ra値)は何ですか?

-塩化第二鉄サービスにおけるチタン表面仕上げとのトレードオフ 塩化第二鉄 (FeCl 3 ) エッチング液は酸化性が高く、ほとんどの金属に対して腐食性が高くなります。典型的な塩化第二鉄は 42 度 Be (約 40% FeCl 3 ) です。 FeCl3 に対する耐性のためにチタンが選ばれた理由は、不動態の TiO2 層でした。しかし、不動態皮膜の局所的な破壊は、ミクロスケールの表面欠陥、介在物、または亀裂で発生し、孔食と呼ばれます。平均粗さ Ra によって定義される表面研磨は、潜在的な孔食核形成サイトの数とサイズに直接影響します。より滑らかな表面 (低 Ra) により、小さな亀裂がなくなり、塩化物イオンが集中する場所の数が減ります。非常に滑らかな表面 (Ra < 0.2 µm) には電解研磨または機械研磨が必要であり、費用が増加します。今回の研究では、42度BéFeCl3、50度でのRa値と孔食誘発時間の関係を測定し、孔食開始までの時間が最も長くなる表面仕上げを特定した。機械的完全性の影響: 表面粗さと孔食の開始 塩化第二鉄溶液中のチタンの孔食は、不動態皮膜が最も弱い場所、または亀裂が塩化物の蓄積を促進する場所で始まります。粗い表面（Ra > 1.0 µm）では、谷は微細な亀裂のようなものになります。これらの谷の幅は通常 5 ～ 20 um で、深さは Ra 値程度です。これらの谷では、拡散限界により塩化物イオンが蓄積され、金属塩化物の加水分解により局所的な pH が低下し、孔食の原因となります。滑らかな表面 (Ra < 0.4 μm) では、谷は浅くなります (<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 >表面粗さと孔食の可能性は指数関数的な関係があるため、6 か月。機械的に研磨された表面 (Ra= 0.2 – 0.6 µm) は 300 ～ 2000 時間の中間誘導期間を持ち、要求がそれほど厳しくない用途に適しています。電解研磨には重大な熱損失はありません。 Ra < 0.15 ミクロンであることが確認された電解研磨表面仕上げと研磨後の 20% 硝酸による不動態化を備えた塩化第二鉄エッチング用のヒーターを指定します。ただし、仕上げコストが高くても、孔食に関連する問題が回避され、耐用年数が長くなることで相殺されます。メンテナンス間隔の予想される実行時間に最も適した表面仕上げを選択します。 1,000 時間を超える用途には電解研磨をお勧めします。