When a Titanium Immersion Heater Is Installed in a 10% Sodium Metasilicate Solution at 80°C, Why Does the Silicate Ion Form a Protective, Non-Porous Film Only at pH >11, thiab Yuav ua li cas tshwm sim ntawm pH 9?
Tso lus
Hauv kev daws teeb meem ntawm sodium metasilicate (Na₂SiO₃) ntawm 80 degree, silicate ions polymerize ntawm titanium nto. Ntawm pH qhov tseem ceeb tshaj 11, ib qho txuas ntxiv, tsis yog - ntxeem tau, tiv thaiv zaj duab xis ntawm 0.5-2 µm thickness yog tsim los ntawm silicate. Ntawm pH 9, txheej txheej yog ntxeem tau, tawg thiab tsis yog - tiv thaiv thiab tso cai rau nkag mus ntawm chloride los yog lwm yam aggressive hom thiab pitting. Qhov sib txawv yog vim lub xeev polymerization ntawm silicate ions. Thaum pH siab, silicate tshwm sim raws li monomeric SiO₄⁴⁻ ions, uas tsim ib txheej tuab iav. Thaum pH qis, silicate polymerizes muab cov saw hlau colloidal uas ua rau cov qauv ntxeem tau tawg.
Lub Mechanism ntawm pH-Dependent Silicate Film Formation
The polymerization of silicate ions SiO4 4- + H2O HSiO4 3- + OH- At high pH (>11) qhov sib npaug txav mus rau monomeric ions uas tso ua ib txheej tuab amorphous silica. Ntawm pH 9-10 cov ions polymerize rau cov qauv zoo li cov saw hlau uas tsim kom muaj lub cev ntxeem tau. Cov yeeb yaj kiab kuj siv sodium ions los tsim cov iav sodium silicate. Lub peev xwm ntawm kev tiv thaiv yog txhais los ntawm qhov ntom ntawm zaj duab xis thiab qhov tsis muaj microcracks.
Quantitative Film Quality raws li kev ua haujlwm ntawm pH.
Cov nram qab no tau tsim los rau Qib 2 titanium los ntawm kev tswj xyuas hauv 10% Na2SiO3 ntawm 80℃rau 500 teev: Ntawm pH 8 lub thickness ntawm zaj duab xis yog 0.1-0.3 µm, lub porosity ntawm cov zaj duab xis yog ntau heev, tus nqi corrosion yog 0.15-0.30 hli nyob rau hauv ib xyoos. Ntawm pH 9, thickness yog 0.2-0.5 µm, porosity siab, corrosion tus nqi yog 0.08-0.20 hli ib xyoo, thiab pitting yog qee zaus. Ntawm pH 10, lub thickness yog 0.5-1.0 µm, lub porosity nruab nrab, tus nqi corrosion 0.04-0.10 mm / xyoo thiab pitting tsis tshua muaj. Ntawm pH 11, lub thickness yog 1.0-1.5 µm, porosity tsawg, tus nqi corrosion yog 0.02-0.05 mm / xyoo thiab pitting tsis tuaj. Lub thickness yog 1.5-2.5 μm, lub porosity tsawg heev, tus nqi corrosion yog 0.01-0.03 hli / xyoo thiab tsis muaj pitting tshwm sim ntawm pH 12. Ntawm pH 13, lub thickness yog 2.0-3.0 μm, lub porosity tsawg heev, tus nqi corrosion yog 0.005 mm / 0.
Cov nyhuv ntawm qhov kub thiab txias ntawm pH xav tau
Lub pH tseem ceeb rau cov yeeb yaj kiab tiv thaiv yog qhov kub thiab txias nyob ntawm. Qhov tsawg kawg pH yog 10.5 ntawm 60 o C thaum pom pH yog 11.0. Ntawm 80 degree, qhov tsawg kawg nkaus yog 11.0 thiab qhov pom zoo 11.5. Qhov tsawg kawg nkaus yog 11.5 ntawm 100℃thiab qhov pom zoo yog 12.0. Qhov tsawg kawg nkaus yog 12.0 thiab qhov pom zoo yog 12.5 ntawm 120 deg C. qhov kub ntau dua nyiam silicate polymerization thiab yog li xav tau ntau dua pH rau khaws silicates hauv daim ntawv monomeric.
Sodium Metasilicate Service pH Control Guide
Cov lus hauv qab no yog cov lus qhia rau pH tswj kom muaj kev tiv thaiv silicate zaj duab xis ntawm Qib 2 titanium rhaub hauv 10% sodium metasilicate ntawm 80 degree.
pH of Operation Film Quality Corrosion Rate (mm/year)Pitting Risk Recommended Action >11 Tiv thaiv<0.05 None None 10–11 Marginal 0.05–0.10 Low Increase pH with NaOH 9–10 Poor, porous 0.10–0.20 Moderate Add NaOH to increase to >11 8–9 Tsis yog- tiv thaiv 0.15–0.30 High Redesign daws<8 Unstable >0.30 Hnyav Tsis kam txais
Engineering Tshaj pH tswj
Qhov xav tau pH yog hais los ntawm qib titanium. Qib 7 (palladium-stabilized) ua kom muaj kev tiv thaiv silicate txheej ntawm pH 10.5 txog 11.0, qis dua Qib 2. Qib 2 yog tib yam li Qib 12. Phab ntsa thickness muaj kev tiv thaiv corrosion yog tias cov yeeb yaj kiab tsis tiv thaiv kiag li. Kev tsim cov yeeb yaj kiab yog cuam tshuam los ntawm cov ntsiab lus silicate . 5% ntawm Na 2 SiO 3 yuav tsum muaj pH ntawm 11.5. Ntawm 15% Na2SiO3 pH xav tau yog 10.5. Chlorides (los ntawm impurities) nce pH yuav tsum tau los ntawm 0.5 mus rau 1.0 pH unit. Lub pH meter calibrated rau siab pH yuav tsum tau siv los saib xyuas cov tshuaj.
Ua Spec Nrog Kev Paub
Ntxiv sodium hydroxide kom tswj pH siab tshaj 11.0 rau lub titanium immersion rhaub hauv 10% sodium metasilicate ntawm 80 degree. Nruab lub pH maub los nrog lub tswb ntawm 10.8 thiab cuam tshuam ntawm 10.5. Ntxiv NaOH rau cov tshuab uas twb muaj lawm nrog pH hauv qab 11 kom nce pH mus rau 11.5. Yog tias pH tsis tuaj yeem nce ntxiv, siv Qib 7 titanium lossis txo qhov kub ntawm 60 degree. Tshawb xyuas pitting ntawm lub tshuab cua sov thaum ua haujlwm. Tam sim ntawd nce pH yog tias pom pitting. Tus kws ua haujlwm tswj xyuas pH saum toj no 11 los tsim kev tiv thaiv, tsis yog - ntxeem tau silicate txheej saum titanium.
