Raws li kev sib xyaw ua ke ntawm Sulfate yog dab tsi-rau-Chloride Ratio thiab Kub Ua 316 Stainless Hlau Rhaub Sheathing tseem Passive hauv Simulated Cooling Tower Dej

Inhibitive nyhuv ntawm Sulphate Ions ntawm Pitting Induced los ntawm Chloride

Rau 316 stainless hlau clad hluav taws xob cua sov raj hauv cov dej txias ntauwd, qhov piv ntawm sulphate ions (SO4 2-) rau chloride ions (Cl-) nyob rau hauv cov dej recirculating feem ntau cuam tshuam ntau dua li cov tshuaj chloride kiag li hauv kev txiav txim siab qhov pib ntawm pitting corrosion. Sulphate ions sib tw nrog chloride ions rau qhov chaw adsorption ntawm cov zaj duab xis passive ntawm 316 stainless hlau. Ntawm sulfate / chloride ratios tshaj ib tug tseem ceeb tus nqi, feem ntau 3: 1 mus rau 5: 1 los ntawm qhov hnyav nyob ntawm qhov kub thiab txias, cov zaj duab xis passive yog ruaj khov txawm tias muaj cov tshuaj chloride ntau ntau uas yuav ua rau muaj kev cuam tshuam pitting. Hauv qab no theem, chlorides dominate thiab pib pitting. Daim ntawv no kwv yees qhov tsim nyog sulfate - rau - chloride piv rau 316 sheaths ntawm ib txwm txias ntauwd kub (30-60 degree) thiab muab lub hauv paus rau dej chemistry specification rau kev ua tau zoo ntawm lub tshuab cua sov.

Kev sib tw Adsorption Mechanism ntawm Passive Film.

The passive coating on 316 stainless steel (mostly chromium oxide with enrichment of molybdenum) has a positive surface charge in neutral to slightly alkaline solutions. Chloride ions are tiny and highly polarisable and are strongly adsorbed on the film surface. Once adsorbed, chlorides locally decompose the film by generating soluble chloro-complexes with chromium and iron ions and act as initiation sites for pitting. Sulphate ions are also adsorbed on the passive film but do not induce film rupture. Instead they compete for adsorption sites with chlorides thereby preventing the access to chlorides. The competitive adsorption is described by a Langmuir isotherm: the effective chloride concentration at the surface of the film is lowered by a factor which depends on the sulphate concentration. The critical sulphate to chloride ratio is the threshold at which the sulphate adsorption exceeds the chloride adsorption to the point of inhibiting the chloride driven film breakup. For 316 stainless steel at 50 0C the crucial ratio is about 3:1 (by weight three parts sulphate to one part chloride). The ratio may be as low as 1.5:1 at 30 °C. At 70 °C the ratio must be >5:1 ua. Tshaj li 80 degree, txawm tias qhov sib piv ntawm cov sulphate siab yuav tsis tuaj yeem tiv thaiv pitting, vim tias cov kinetics ntawm chloride adsorption sai dua li cov sulphate adsorption.

Quantifying Thresholds los tiv thaiv Pitting hauv Dej Txias Ntauwd

Kev ntsuam xyuas electrochemical ntawm 316 stainless hlau hauv simulated txias ntauwd dej (pH 7.5-8.5, alkalinity 100-300 ppm li CaCO₃, hloov cov tshuaj chloride thiab sulfate concentrations) tau qhia qhov kub thiab txias thiab sulphate rau chloride piv ua ke uas yuav tiv thaiv pitting rau {{5} xyoo. Cov lus hauv qab no muab thaj chaw ua haujlwm nyab xeeb thiab thaj chaw muaj kev pheej hmoo pitting.

Kub (degree) Sulfate-rau-Chloride Ratio (wt/wt)Chloride Concentration (ppm) Pitting Risk (5 Xyoo Kev Pabcuam) Pom Zoo Ua Haujlwm
30 >1.5:1 <1000 No pitting Acceptable 30 0.8-1.5:1 200-1000 Low risk, mild pitting possible Monitor every 2 years 30 <0.8:1 Any High risk Add sulphate or decrease chlorides
40 >2.0:1 <800 No pitting Acceptable 40 1.0-2.0:1 200-800 Moderate risk Add sulphate or upgrade to 316L 40 <1.0:1 Any High risk Not recommended 50 >3.0:1 <500 No pitting Acceptable 50 1.5-3.0:1 200-500 Moderate-high risk Requires treatment 50 <1.5:1 Any High risk 316 unsuitable 60 >5.0:1 <300 No pitting Acceptable with monitoring 60 2.0-5.0:1 100-300 High risk 316 marginal; consider duplex 60 <2.0:1 Any Very high risk 316 not suitable 70 >8:1 <200 Marginal Not recommended for 316 70Any Any High riskUpgrade to Duplex 2205 or Titanium >80 Txhua Yam Kev pheej hmoo siab 316 tsis tsim nyog tsis hais qhov piv txwv
Tswv yim Dej Chemistry Kho kom zoo los tiv thaiv cov cua sov

Kev ntsuas dej txias ntauwd qhia tau hais tias sulfate / chloride piv hauv qab qib kev nyab xeeb. Peb cov tshuaj hloov pauv tuaj yeem tiv thaiv 316 lub tshuab cua sov. Txoj kev tseem ceeb tshaj plaws thiab nrov tshaj plaws yog kev sib ntxiv ntawm sodium sulphate (Na2SO4) los txhim kho cov sulphate concentration. Rau lub kaw lus nrog 400 ppm chloride ntawm 50 degree, qhov tsawg kawg nkaus piv ntawm 3: 1 yuav tsum muaj lub hom phiaj sulphate concentration ntawm 1200 ppm. Txhawm rau ua kom tiav lub hom phiaj no, sodium sulphate (48% sulphate los ntawm qhov hnyav) yog ntxiv rau qib ntawm 1,800 ppm. Tus nqi pheej yig thiab sulphate tsis yog -corrosive thiab ib puag ncig siv tau. Qhov kev hloov kho thib ob yog qhov siab ntawm pH (los ntawm 7.5 mus rau 8.5-9.0 los ntawm sodium hydroxide), uas ua rau kom muaj kev ruaj ntseg ntawm cov zaj duab xis passive thiab txo qhov xav tau sulfate - rau-chloride piv los ntawm ~ 30%. Qhov kawg hloov kho yog qhov sib ntxiv ntawm zinc los yog phosphate raws li corrosion inhibitors. Cov inhibitors adsorb rau cov zaj duab xis passive thiab muab kev tiv thaiv ntxiv, txo qhov kev xav tau ntawm 40-50%.

Field Diagnosis of Pitting Ua los ntawm Sulfate -Chloride Imbalance

316 encased heaters nyob rau hauv cov kev pab cuam txias ntauwd. Pitting tej zaum yuav tshwm sim txawm tias muaj cov tshuaj chloride nruab nrab (200-500 ppm) yog tias sulfate- rau-chloride piv qis. Kev ntsuas dej yuav tsum yog rau cov tshuaj chloride (los ntawm ion chromatography lossis nyiaj nitrate titration) thiab sulphate (los ntawm txoj kev gravimetric lossis turbidimetric). Yog tias qhov sib piv tsawg dua qhov pib ntawm lub rooj rau qhov kev khiav hauj lwm kub, ces pitting yuav tshwm sim vim chloride dominance. Cov pits lawv tus kheej feem ntau me me (0.1-1.0 hli), ntiav (0.1-0.3 hli sib sib zog nqus) thiab ntau indentations es tsis yog qhov tob, dav spaced pits pom nyob rau hauv high-chloride ib puag ncig. Qhov no yog vim lub fact tias cov zaj duab xis passive lov ntawm ntau cov ntsiab lus tseem yog ib feem rov qab los ntawm kev sib tw sulphate adsorption, ua rau ntau lub qhov me me ntau dua li ob peb lub loj. Nyob rau hauv xws li cov tshuab siv sulphate los tsim kom muaj kev nyab xeeb piv yuav tiv thaiv ntxiv pitting kev vam meej tab sis cov pits uas twb muaj lawm yuav nyob twj ywm raws li qhov chaw muaj kev ntxhov siab.

Xaus: Tswj Sulfate- mus rau -Chloride Ratio rau Consistent Cooling Tower Heater Service

Cov sulphate rau chloride piv yog ib qho tseem ceeb ntawm cov dej chemistry parameter rau 316 stainless hlau clad heaters nyob rau hauv lub txias ntauwd dej systems los ntsuam xyuas yog hais tias cov passive txheej tseem nyob twj ywm los yog pitting pib. Ntawm 50℃qhov tsawg kawg nkaus piv yog 3: 1 (sulfate: chloride los ntawm qhov hnyav). Rau kev pabcuam tshaj 5 xyoos. Qhov sib piv yuav tsum tau nce ntau ntawm qhov kub thiab txias tshaj 70℃316 tsis qhia ntawm ib qho piv txwv. Cov kws tshaj lij qhia txog cov cua sov rau kev ua haujlwm txias ntauwd yuav tsum ua kom tiav cov dej chemistry tsom xam, ntsuas qhov kev ua haujlwm kub, thiab kho cov qib sulphate lossis inhibitor cov kev pab cuam kom tswj tau qhov xav tau piv. Txoj hauv kev uas tau muab ntawm no tuaj yeem siv los ntawm cov neeg yuav khoom los qhia txog cov kev kho dej uas txo cov kev pheej hmoo ntawm kev ua tsis tiav ntxov ntawm 316 sheaths los ntawm kev txheeb xyuas cov sulfate - rau - chloride piv thiab qhov kub thiab txias rau qhov kev pheej hmoo pitting.