Hauv Titanium Electric Immersion Rhaub rau 10% Sodium Dichromate Solution ntawm 70 degree, Vim li cas Hexavalent Chromium Ion tsis Passivate Titanium Surface Txawm tias muaj zog Oxidizer?

Sodium Dichromate (Na2Cr2O7) yog cov tshuaj muaj zog oxidizing, 10% ntawm 70 degree. Hexavalent chromium (Cr⁶⁺) yog lub zog oxidizer, muaj peev xwm txo qis ntawm +1.33 V rau Cr₂O₇²⁻/Cr³⁺. Chromate ions yog siv rau passivating ob peb hlau, xws li stainless hlau. Txawm li cas los xij, hexavalent chromium ntawm titanium tsis tsim ib zaj duab xis ruaj khov. Cov chromium ions tsis tau muab tso rau hauv titanium dioxide lattice. Lub siab oxidation muaj peev xwm ua tau tsav titanium mus rau hauv cheeb tsam transpassive, uas ua rau kev tawg tas li ntawm cov yeeb yaj kiab passive. Tus nqi corrosion ntawm titanium nyob rau hauv 10% sodium dichromate ntawm 70℃yog 0.05-0.15 hli hauv ib xyoos, uas yog siab dua nyob rau hauv ntau cov tshuaj chloride nruab nrab. Mechanism ntawm Titanium Behavior hauv Hexavalent Chromium Solutions TiO2 zaj duab xis ruaj khov passivates titanium. Hauv cov tshuaj dichromate, cov txheej txheem txo Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O tshwm sim ntawm qhov chaw ntawm titanium. Vim yog qhov sib pauv siab tam sim no ntawm qhov kev tawm tsam no, qhov sib xyaw ua ke ntawm titanium hloov mus rau qhov muaj nqis siab dua +1.0 V vs cov pa hydrogen electrode. Nrog rau qhov muaj txiaj ntsig zoo li no, titanium mus txog thaj tsam transpassive uas TiO 2 txheej oxidizes ntxiv rau cov kua dej Ti 4+ hom. Cov zaj duab xis passive tsis ruaj khov thiab yaj tsis tu ncua. Chromate ions tsis tsim insoluble tebchaw nrog titanium, uas tej zaum yuav pab raws li ib tug barrier txheej, raws li lawv ua rau hlau thiab chromium, qhov twg chromate tsim tiv thaiv spinel oxides. Quantitative Corrosion Rates thiab Surface Characterization Tus cwj pwm corrosion ntawm Qib 2 titanium tau raug txheeb xyuas los ntawm kev ntsuas kev ntsuas rau 1,000 teev ntawm 70℃hauv 10% Na₂Cr₂O₇ li: Corrosion rate yog 0.05–0.15 mm / xyoo, piv txwv li{50} ntawm cov dej tshiab rau tib lub sijhawm. Qhov saum npoo yog xim daj-xim daj vim yog tso tawm chrome oxide lossis hydroxide. X-ray photoelectron spectroscopy tshawb nrhiav pom tias cov txheej txheej saum npoo yog qhov ntxeem tau, tsis yog- tiv thaiv kev sib xyaw ntawm TiO₂ thiab Cr(OH)₃. Qhov qhib -circuit muaj peev xwm stabilizes hauv cheeb tsam transpassive ntawm +0.6 rau +0.9 V vs. Ag/AgCl . Tsis muaj pitting pom tab sis niaj hnub thinning tau sau tseg. Rau qhov sib piv, tus nqi corrosion ntawm Qib 2 titanium hauv 3% NaCl ntawm 70℃yog 0.01–0.03 mm / yr nrog lub peev xwm passive ntawm +0.1 rau +0.3 V. Cov nyhuv ntawm Concentration thiab Kub ntawm qhov hnyav ntawm Corrosion Corrosion tus nqi nce nrog qhov nce hauv dichromate concentration thiab kub. Corrosion tus nqi ntawm 5% Na2Cr2O7 thiab 70℃yog 0.03-0.08 hli ib xyoo. Ntawm 10% thiab 70℃tus nqi yog 0.05-0.15 hli / xyoo. Ntawm 15% thiab 70℃tus nqi nce mus rau 0.08-0.20 hli / xyoo. Ntawm 10% thiab 50℃tus nqi txo rau 0.02-0.05 hli / xyoo. Ntawm 10% thiab 90℃tus nqi nce mus rau 0.10-0.25 hli / xyoo. Tus nqi yog 0.15-0.35 hli hauv ib xyoos ntawm 20% thiab 90 degree. Lub corrosion yog homogenous yam tsis muaj pitting, txij li thaum lub transpassive dissolution yog uniform saum npoo. Lub zog ua kom muaj zog rau cov txheej txheem yog nyob ib ncig ntawm 40 kJ / mol qhia txog kev sib xyaw kinetic thiab diffusion tswj. Corrosion cwj pwm ntawm titanium ntawm ntau qib nyob rau hauv dichromate kev daws Cov lus hauv qab no qhia txog corrosion ntawm ntau qib titanium hauv 10% sodium dichromate ntawm 70℃rau 1,000 teev thiab cov yam ntxwv ntawm lawv cov yeeb yaj kiab passive. Titanium Qib Corrosion Rate (mm/xyoo) Surface Film Composition Open-Circuit Potential (V vs. Ag/AgCl) Pitting Observed Qib 2 (kev lag luam ntshiab) 0.05-0.15 TiO2 + Cr(OH)3 (porous)+0.6 1} 5-Pde. 0.04–0.12 TiO2 + Cr(OH)3 + Pd +0.7 to +1.0 No Qib 12 (Ti-0.3Mo-0.8Ni) 0.06–0.18 TiO2 + Cr(2}OH){1} +0.8 Tsis Muaj Qib 5 (Ti-6Al-4V) 0.08–0.22 Mixed oxides +0.5 rau +0.8 Tsis Muaj Kev Tiv Thaiv thiab Kev Xaiv Cov Khoom Siv rau Dichromate Service Cov lus hauv qab no tau pom zoo rau kev xaiv cov cua sov titanium thiab kev txwv kev ua haujlwm rau cov ntawv thov uas xav tau cov cua sov ntawm sodium dichromate. Dichromate Concentration (%) Kub (degree) Pom zoo Titanium Qib Cia Siab Corrosion Rate (mm/xyoo) Lub Neej Cia Siab (1.5 hli phab ntsa)<5 <50 Grade 2 <0.03 >10 years 50–70 5–10 0.05–0.12 3–5 years Grade 7 10–15 70 Grade 7 0.08–0.20 2–3 years >15 >70 Not recommended >0.20 <2 years Engineering Grade Selection Beyond Wall thickness provides a corrosion allowance . A 2.0 mm wall corrodes at a rate of 0.10 mm per year for 20 years . The tube surface finish has little effect on transpassive dissolution. The inclusion of chlorides in the dichromate solution, which may be due to the presence of dirty water, raises the corrosion rate by 2-3 times. The addition of sulphate ions has a small inhibitory effect. Deaeration of the solution for oxygen removal slows the cathodic process and reduces the corrosion rate by 30-50%. Choosing Wisely. For a titanium electric immersion heater in 10 % sodium dichromate at 70 °C, expect a corrosion rate of 0.05 to 0.15 mm per year for Grade 2 or 0.04 to 0.12 mm per year for Grade 7. Use a minimum wall thickness of 2.0 mm for a service life of 10 to 20 years. Dichromate solution can be diluted, or the working temperature lowered for higher concentrations or temperatures. During operation, check the heater for variations in electrical resistance that may indicate thinning. If the solution is polluted with chlorides, lower the temperature to 50 °C. The engineer, knowing that hexavalent chromium does not passivate titanium but transpassively dissolves it, chooses the proper wall thickness and grade for predictable service life in sodium dichromate solutions.