Nyob rau hauv dab tsi tshwj xeeb kev sib xyaw ntawm Residual Chloramine Concentration thiab pH puas 316 Stainless Steel Heater Sheath Tsim Pitting hauv Municipal Kub Dej ntawm 55 degree

Mechanism of pitting corrosion of disinfectant in potable water systems Monochloramine (NH2Cl) is being increasingly used as a secondary disinfectant in municipal hot water systems (domestic hot water tanks, recirculation loops in hotels and hospitals, point of use heaters in commercial buildings) to replace free chlorine and minimise trihalomethane formation for 316 stainless steel sheathed electric heating tubes. Chloramine is less aggressive to stainless steel than free chlorine but can produce pitting at higher temperatures especially when the pH is not in the proper range for chloramine stability. 316 stainless steel is passive at chloramine levels below 2-3 ppm at a common residential hot water temperature of 55 °C and a pH of 7.5-8.5. However, below pH 7.0 chloramine breaks down into free chlorine and ammonia which leads to a more aggressive oxidising environment. Chloramine is unstable at pH > 9.0, and decomposes to dichloramine and nitrogen trichloride. Pitting begins at chloramine levels >4-5 ppm ntawm pH 7.0-8.0, los yog nyob rau hauv pH excursions. Qhov kev tshawb fawb no kwv yees pH qhov pib thiab chloramine concentrations uas pib pitting ntawm 316 sheaths ntawm 55 ⁰C. Chloramine-induced pitting mechanism thiab pH dependence Monochloramine (NH2Cl) yog ib tug tsis muaj zog oxidising tus neeg sawv cev tshaj li dawb chlorine (HOCl/OCl-) Chloramine, txawm li cas los xij, degraded los ntawm ntau txoj kev ntawm qhov kub siab tshaj 50℃. Ntawm nruab nrab mus rau me ntsis alkaline pH (7.0-8.5) decomposition muab ammonia thiab nitrogen nrog tsuas yog me me ntawm cov tshuaj dawb tso tawm. Ntawm pH < 7.0, qhov decomposition hloov mus rau dawb chlorine (HOCl) thiab dichloramine (NHCl 2), ob qho tib si yog cov muaj zog oxidants uas muaj peev xwm decompose lub passive zaj duab xis ntawm 316. Ntawm pH > 9.0 chloramine sai decomposes rau nitrogen trichloride (NCl3), uas yog ib tug volaisile thiab heev oxidants. Lub pitting mechanism yog cov dawb chlorine pitting. Localized tawg ntawm Cr 2 O 3 passive zaj duab xis thiab tom qab chloride-induced pit propagation. Tab sis chloramine xav tau qhov muaj zog ntau dua (kwv yees li 2-5x ntawm cov tshuaj dawb dawb) ua rau pitting ntawm qhov kub thiab txias. Kev txwv kev txwv ntawm pitting ntawm 55℃Tswj kev ntsuas immersion ntawm 316 stainless hlau (electropolished tiav, Ra=0.2 μm) nyob rau hauv cov dej hluavtaws (100 ppm Cl−, 80 ppm SO42−, 120 ppm CaCO3) ntawm ntau yam p. ciam teb rau pitting pib. pH (55 degree) Monochloramine (ppm as Cl2 sib npaug) Pitting Initiation Sijhawm rau Chloramine Decomposition Pathway (teev) Max qhov tob Tom qab 3000 Teev (micron) Lub Sijhawm rau Perforation (1.5 hli phab ntsa) 55℃Pom zoo Service<1.0 7.0-8.5 Stable >10,000 <10 >30 xyoo Tau 1.0-2.0 7.5-8.5 Ruaj khov 5,000-10,000 10-20 15-30 xyoo Tau 2.0-3.0 7.5-8.5 Ruaj khov 20-40 xyoo Feem ntau 8-15 xyoo 3,000-5,000 Txais tau 3.0-4.0 7.5-8.5 Nyob ib ncig ntawm 1,500-3,000 40-80 4-8 xyoo Marginal Decay} Moderate {{1} 800-1,500 80-120 2 mus rau 4 xyooTsis pom zoo 5.0-7.0 7.5-8.5 Kev puas tsuaj loj heev 400-800 120-200 1-2 xyoo tsis muaj 7.0 7.5-8.5 Kev lwj sai sai<400 >200 <1 year Unacceptable 2.0-3.0 6.5-7.0 (pH excursion)Free chlorine release 1,000-2,000 60-100 3-5 years Not recommended 2.0-3.0 <6.5 Free chlorine + dichloramine 300-800 100-180 1.5-2.5 years Not acceptable NCl3 formation 2.0-3.0 >9.0 500-1,200 80-150 2 - 3 yeras Unacceptable 3.0 < 7.0 Severe degradation < 500 > 150 < 1.5 years Temperature effect on chloramine aggressiveness Not acceptable The rate of decomposition of monochloramine rises with temperature, hence chloramine is more aggressive at higher temperatures. Water Temperature (oC) Maximum Monochloramine for 5-Year Life pH 7.5-8.0 (mg/L)Maximum Monochloramine for 10 Year Life at pH 7.5-8.0 (mg/L)Recommended Action 40 5.0 3.0 Standard practice 50 3.5 2.5 OK 55 3.0 2.0 Marginal above 2.5 ppm 60 2.2 1.5 Chloramine reduction if >2 ppm 70 1.5 1.0 316 not recommended, upgrade material 80 1.0 0.5 316 not appropriate Chloramine Treatment Practical Water Chemistry Specifications The following specifications will ensure long term pitting free service for 316 sheathed heaters in municipal hot water systems treated with monochloramine at 55oC. System Type Target Monochloramine (ppm) pH Range Max. Temp. (°C) Heater Life Expectancy (years)Hot water heating ideas for households1.0-2.0 7.5-8.5 55 10-15 Commercial (hotel/hospital) OK 1.5-2.5 7.5-8.5 55 8-12 Test for chloramine regularly. big circulation system 2.0-3.0 7.8-8.2 55 5-8Active pH regulation as required Any with chloramine >3.0 >3.0 7.5-8.5 55 2-4 Hloov mus rau 316L electropolish Txhua yam nrog pH excursions<7.0 <3.0 <7.0 56 2-5 pH control necessary Any at temperatures above 60°C 1.5 7.5-8.5 >60 2-5 2205 Duplex Upgrade Field qhia txog pitting tsim los ntawm chloramine Rau 316 lub rhaub dej kub hauv nroog nrog cov tshuaj dawb me me tab sis paub txog kev kho chloramine, cov pits feem ntau me me (0.1-}0.5 mm inch), nquag (5-20 pits/cm2), thiab tsis tu ncua. Cov hlau nto hauv lub qhov dej yog qhov qhia txog kev siv tshuaj chloride-tsav pitting morphology. Qhov sib txawv tseem ceeb yog tias tsis muaj cov tshuaj chlorine dawb hauv cov dej ntau (qhov qhia tau tias cov tshuaj chloramine tawg). Cov dej chemistry log yuav muaj cov txiaj ntsig chloramine siab dua 3-4 ppm thiab pH qis dua 7.0 lossis siab dua 9.0 ntawm lub sijhawm sib txawv. Kev ntsuam xyuas cov dej ntau rau cov tshuaj dawb thiab chloramine yuav paub meej tias hom tshuaj tua kab mob. Kev daws rau cov kev ua tsis tiav no yog txo qis cov tshuaj chloramine kom tsawg dua 2.5-3.0 ppm lossis khaws cov pH ze ntawm 7.5-8.5 lossis hloov mus rau duplex 2205 stainless hlau. Kev daws teeb meem rau cov tshuab chloramine siab yog tias tsis tuaj yeem txo cov chloramine concentration rau 3 ppm, cov txheej txheem hauv qab no yuav txo qhov kev pheej hmoo ntawm pitting: Electropolishing: Electropolished nto (Ra<0.2 µm) increases chloramine threshold by ~1 ppm. pH stability The pH should be maintained in the range 7.8 – 8.2 by adding a pH controller. Decomposes to free chlorine at low pH. Decomposes to NCl3 at high pH. Lower heater setpoint from 55C to 50C (40-50% reduction in chloramine decomposition rate) Alloy upgrade: Duplex stainless steel 2205 (higher pitting resistance equivalent number, PREN >35) lossis 316L nrog titanium nitride (TiN) suav nrog tswj. Cov ntsiab lus: 316 lub tshuab rhaub lub neej @ 55℃pH thiab Chloramine tswj 316 stainless hlau sheathed rhaub hauv cov dej kub hauv nroog ntawm 55℃qhia pitting pib saum toj ib ncig ntawm 3-4 ppm monochloramine ntawm pH 7.5-8.5 hauv 1,000-3,000 teev thiab perforation hauv 2-5 xyoos. Thaum ph poob qis dua 7.0 chloramine tawg kom dawb chlorine thiab txawm tias 2-3 ppm chloramine yog qhov hnyav heev thiab yuav ua rau 1-2 xyoos . 316 sheaths nrog cov ntaub ntawv ntawm chlorine thiab pH thiab pH ntau ntawm 7.5-8.5 thiab monochloramine.<2.5 ppm for 10 year life shall be specified by engineers for chloramine treated water. Electropolished 316L or duplex 2205 is suggested for chloramine levels over 3 ppm or where pH control is not reliable. The present method, connecting chloramine concentration and pH to measured pitting initiation times at 55°C, allows users to define water treatment programmes which prevent disinfectant-driven pitting in municipal hot water systems.