Når et elektrisk varmeelement af titan er nedsænket i en jernchloridætsningsopløsning (42 grader Bé, 50 grader), hvilken overfladefinish (Ra-værdi) giver den længste induktionstid til grubetæring?

Byt-af for titanoverfladefinish i ferrichloridservice Ferrichlorid-ætseløsninger (FeCl 3 ) er stærkt oxiderende og stærkt ætsende for de fleste metaller. Typisk ferrichlorid er 42 grader Be (ca. 40 % FeCl 3 ). Grunden til, at titanium blev valgt på grund af dets modstandsdygtighed over for FeCl3, var det passive TiO2-lag. Lokaliseret ødelæggelse af den passive film forekommer imidlertid i mikroskala overfladedefekter, indeslutninger eller revner og kaldes pitting. Overfladepoleringen, defineret ved den gennemsnitlige ruhed Ra, påvirker direkte antallet og størrelsen af ​​potentielle kernedannelsessteder for grubetæring. Den glattere overflade (lav Ra) eliminerer små revner og reducerer antallet af steder, hvor kloridioner kan koncentreres. Meget glat overflade (Ra < 0,2 µm) kræver elektropolering eller mekanisk polering og øger omkostningerne. I det foreliggende arbejde blev forholdet mellem Ra-værdi og grubedannelsestid i 42 grader Bé FeCl3 ved 50 grader målt, og overfladefinishen, som resulterede i den længste tid til pitinitiering, blev identificeret. Virkninger af mekanisk integritet: Overfladeruhed og initiering af grubetæring Pitting af titanium i ferrichloridopløsning starter på steder, hvor den passive belægning er svagest, eller hvor sprækker letter akkumulering af chlorid. I en ru overflade (Ra > 1,0 µm) er dalene mikrofissurlignende. Disse trug har typisk en bredde på 5-20 um, og dybden er af størrelsesordenen Ra-værdien. I disse dale akkumuleres chloridioner på grund af diffusionsbegrænsninger, og den lokale pH-værdi falder på grund af hydrolyse af metalchlorider, som forårsager pitting. På en glat overflade (Ra < 0,4 µm) er dalene lavvandede (<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 måneder, da overfladeruhed og pitpotentiale har en eksponentiel sammenhæng. De mekanisk polerede overflader (Ra=0.2–0,6 µm) har mellemliggende induktionsperioder på 300–2000 timer, velegnet til mindre krævende applikationer. Elektropolering har ingen væsentlig varmestraf. Angiv varmelegemer til jernkloridætsning med elektropoleret overfladefinish verificeret Ra < 0,15 mikron og passivering i 20 % salpetersyre efterpolering. De højere efterbehandlingsomkostninger opvejes dog af undgåelse af grubetæringsrelaterede vanskeligheder og den længere levetid. Vælg den overfladefinish, der er mest passende for den forventede driftstid mellem vedligeholdelsesintervallerne. Elektropolering anbefales til enhver påføring, der overstiger 1.000 timer.