Þegar rafhitunarefni úr títan er sökkt í járnklóríð etslausn (42 gráður Bé, 50 gráður), hvaða yfirborðsáferð (Ra-gildi) veitir lengsta innleiðingartíma fyrir gryfju?

Skiptu-af fyrir títan yfirborðsáferð í járnklóríðþjónustu Járnklóríð (FeCl 3 ) ætarlausnir eru mjög oxandi og mjög ætandi fyrir flesta málma. Dæmigert járnklóríð er 42 gráður Be (um það bil 40% FeCl 3 ). Ástæðan fyrir því að títan var valið fyrir viðnám þess gegn FeCl3 var óvirka TiO2 lagið. Staðbundin eyðilegging á óvirku filmunni á sér hins vegar stað við yfirborðsgalla á smáskala, innfellingar eða sprungur og er kallað gryfja. Yfirborðslakkið, skilgreint af meðalgrófleika Ra, hefur bein áhrif á fjölda og stærð hugsanlegra gryfjukjarnastaða. Sléttara yfirborðið (lágt Ra) útilokar litlar sprungur og dregur úr fjölda staða þar sem klóríðjónir geta safnast saman. Mjög slétt yfirborð (Ra < 0,2 µm) krefst raffægingar eða vélrænnar fægingar og eykur kostnað. Í þessari vinnu var sambandið milli Ra gildis og innleiðingartíma gryfju í 42 gráðu Bé FeCl3 við 50 gráður mælt og yfirborðsáferð sem leiddi til lengsta tíma þar til hola byrjaði var auðkennd. Áhrif af vélrænni heilindum: Yfirborðsgrófleiki og upphaf gryfjunar. Grófa títan í járnklóríðlausn hefst á stöðum þar sem óvirka húðin er veikust eða þar sem sprungur auðvelda uppsöfnun klóríðs. Á grófu yfirborði (Ra > 1,0 µm) eru dalirnir örsprungur. Þessi trog eru venjulega 5-20 um á breidd og dýptin er af stærðargráðunni Ra. Í þessum dölum safnast klóríðjónir saman vegna dreifingartakmarkana og staðbundið pH lækkar vegna vatnsrofs málmklóríða sem veldur gryfju. Á sléttu yfirborði (Ra < 0,4 µm) eru dalirnir grunnir (<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 mánuðir, þar sem yfirborðsgrófleiki og gryfjunarmöguleiki hafa veldisvísistengingu. Vélrænt slípað yfirborðið (Ra=0.2–0,6 µm) er með 300–2000 klst aðlögunartíma, hentugur fyrir minna krefjandi notkun. Rafslípun hefur engin veruleg hitaviðurlög. Tilgreindu hitara fyrir járnklóríð ætingu með rafslípuðum yfirborðsáferð staðfestum Ra < 0,15 míkron og passivering í 20% saltpéturssýru eftir pólsku. Hins vegar er hærri frágangskostnaður á móti því að forðast gryfjutengda erfiðleika og lengri endingartíma. Veldu yfirborðsáferð sem hentar best fyrir áætlaðan keyrslutíma á milli viðhaldsbila. Mælt er með raffægingu fyrir alla notkun sem er lengri en 1.000 klst.