Kada je titanijumski električni grijaći element uronjen u otopinu za nagrizanje željeznog klorida (42 stepena Bé, 50 stepeni), koja površinska obrada (Ra vrijednost) pruža najduže vrijeme indukcije za piting?

Zamjena{0}}za završnu obradu titanijumske površine u servisu sa željeznim hloridom Rješenja za nagrizanje željeznog klorida (FeCl 3 ) su visoko oksidirajuća i vrlo korozivna za većinu metala. Tipični željezni hlorid je 42 stepena Be (približno 40% FeCl 3 ). Razlog zašto je titanijum izabran zbog njegove otpornosti na FeCl3 bio je pasivni sloj TiO2. Lokalizirano uništavanje pasivnog filma, međutim, događa se na mikrorazmjernim površinskim defektima, inkluzijama ili pukotinama i naziva se pitting. Poliranje površine, definirano prosječnom hrapavošću Ra, direktno utječe na broj i veličinu potencijalnih nukleacijskih mjesta pitinga. Glatkija površina (nizak Ra) eliminiše male pukotine i smanjuje broj lokacija na kojima se mogu koncentrirati joni klorida. Vrlo glatka površina (Ra < 0,2 µm) zahtijeva elektropoliranje ili mehaničko poliranje i povećava troškove. U ovom radu je izmjerena veza između vrijednosti Ra i vremena indukcije pittinga u 42 stepena Bé FeCl3 na 50 stepeni i identifikovana je završna obrada površine koja je rezultirala najdužim vremenom do inicijacije jame. Efekti mehaničkog integriteta: hrapavost površine i početak rupičastog udubljenja Titanijum u rastvoru željeznog hlorida počinje na mjestima gdje je pasivni premaz najslabiji ili gdje pukotine olakšavaju nakupljanje hlorida. Na hrapavoj površini (Ra > 1,0 µm), doline su nalik na mikropukotine. Ova korita su tipično širine 5-20 um, a dubina je reda veličine Ra. U ovim dolinama se akumuliraju ioni klorida zbog ograničenja difuzije, a lokalni pH opada zbog hidrolize metalnih klorida što uzrokuje rupice. Na glatkoj površini (Ra < 0,4 µm) doline su plitke (<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 mjeseci, jer hrapavost površine i potencijal udubljenja imaju eksponencijalnu vezu. Mehanički polirane površine (Ra=0.2-0,6 µm) imaju međuperiode indukcije od 300-2000 sati, pogodne za manje zahtjevne primjene. Elektropoliranje nema značajne toplotne kazne. Odredite grijače za jetkanje željeznim hloridom sa verifikovanim elektropoliranim površinskim slojem Ra < 0,15 mikrona i pasiviranjem u 20% azotnoj kiselini nakon poliranja. Međutim, veći trošak završne obrade nadoknađuje se izbjegavanjem poteškoća povezanih s udubljenjem i dužim vijekom trajanja. Odaberite završnu obradu površine koja je najprikladnija za očekivano vrijeme rada između intervala održavanja. Elektropoliranje se preporučuje za svaku primjenu koja prelazi 1000 sati.