Nyob rau hauv dab tsi kua PH thiab kub sib xyaw ua ke puas 316 Stainless Steel Sheath nyob rau hauv ib tug submerged rhaub hloov los ntawm Passive mus rau Active Corrosion nyob rau hauv Sulfuric Acid Solutions

pH-Qhov kub ntawm qhov tsis tuaj yeem cuam tshuam ntawm Passive Film

Tus cwj pwm corrosion ntawm 316 stainless hlau qhwv cov hluav taws xob cua sov hauv cov tshuaj sulfuric acid (H2SO4) tsis yog ib qho kev ua haujlwm ncaj nraim ntawm cov kua qaub lossis pH ib leeg. Hloov chaw, nws muaj qhov ntsuas tau zoo uas qhov kev tiv thaiv chromium oxide txheej hloov pauv los ntawm kev ruaj khov passivation mus rau active dissolution nrog ceev thiab uniform hlau poob. Qhov kev hloov pauv no yog nyob ntawm qhov kev daws teeb meem kub thiab pH (lossis ntau dua qhov tseeb ntawm cov tshuaj dawb sulfuric acid). Hauv qab qhov pib (feem ntau pH> 1.5 ntawm 25 degree, nce mus rau pH> 2.5 ntawm 80 degree), 316 nthuav tawm tus cwj pwm tsis zoo nrog cov nqi corrosion tsawg dua 0.1 hli hauv ib xyoos. Tshaj qhov pib (ntau dua corrosive tej yam kev mob los yog kub siab dua) cov zaj duab xis passive hauv zos yaj thiab cov corrosion nce mus rau 1-10 hli ib xyoo, perforating ib tug 1.5 hli sheath phab ntsa nyob rau hauv lub hlis los yog lub lis piam. Daim ntawv no suav cov passive-rau- thaj tsam hloov pauv mus rau 316 sheaths hauv kev pabcuam sulfuric acid, muab cov txheej txheem kev txiav txim siab rau cov ntsiab lus ntawm cov cua sov hauv cov kua qaub ib puag ncig thiab txhais cov qhov rais ua haujlwm nyab xeeb rau kev ua haujlwm ntev ntev.

Electrochemical Mechanism ntawm Passivity Breakdown hauv Sulfuric Acid

Cov zaj duab xis passive ntawm 316 stainless hlau nyob rau hauv sulfuric acid yog ib tug chromium -enriched oxide zaj duab xis (feem ntau Cr2O3 nrog ib co oxides ntawm hlau thiab molybdenum). Nws tsim spontaneously ntawm qhib Circuit Court nyob rau hauv cov kev daws teeb meem nrog txaus oxidising zog. Hauv dilute sulfuric acid (pH> txog 2.0) yaj cov pa oxygen muab cov tshuaj tiv thaiv cathodic (oxygen txo) uas tswj lub peev xwm nyob rau hauv ntau yam passive. Cov yeeb yaj kiab feem ntau yog 2-5 nm tuab thiab nws qhov ruaj khov nyob ntawm qhov sib npaug ntawm cov yeeb yaj kiab yaj (favoured los ntawm pH qis thiab kub siab) thiab zaj duab xis repassivation (favoured los ntawm muaj cov pa yaj los yog lwm yam oxidisers). Yog tias pH qis dua ib qho nqi lossis qhov kub thiab txias tau nce, qhov kev sib cais ntawm chromium oxide nce exponentially, txij li cov solubility ntawm Cr 3+ hom nyob rau hauv acidic daws nce ho. Yog hais tias tus nqi dissolving ntau tshaj qhov repassivation tus nqi, cov txheej yog nyob rau hauv lub zos thinned thiab liab qab hlau yog raug uas dissolves ceev (active corrosion). Cov molybdenum concentration (2.0-2.5%) yog qhov tseem ceeb rau kev ruaj ntseg nyob rau hauv kev txo cov acids xws li sulfuric rau 316. Molybdenum tsim nyob rau hauv cov zaj duab xis passive thiab txhawb txoj kev loj hlob ntawm ib tug ruaj khov molybdate txheej uas hinders active dissolution. Tab sis txawm nrog molybdenum muaj qhov txawv pH-kub barrier . 316 yog ib tug passive corrosion resistant alloy hauv qab qhov pib. Saum lub thaiv nws coj zoo li cov pa roj carbon zoo tib yam.

Quantification ntawm Passive-rau-Txoj Kev Hloov Ciam Teb rau 316 hauv Sulfuric Acid

Passive to active transition of 316 stainless steel in sulphuric acid has been established over the temperature range 25-100 °C by systematic corrosion testing employing weight loss measurements and electrochemical potentiodynamic polarisation. 316 is passive up to ~10 wt% H2SO4 (pH ~0.5) at 25 °C. At 50 °C the passive limit is reduced to 5 wt% H₂SO₄ (pH about 0.8). At 80 °C the limit is reduced to 1 wt% H2SO4 (pH ~1.2). The limit at 100 °C (boiling dilute acid) is around 0.5 wt % H2SO4 (pH about 1.5). Below these concentrations corrosion rates are less than 0.1 mm/year. Corrosion rates are very high above these concentrations, reaching >2 hli / xyoo hauv 5 wt% H2SO4 ntawm 80℃thiab perforating 1.5 hli phab ntsa hauv<9 months. In high concentration sulphuric acid (over 90 wt%) 316 does indeed become passive again due to the production of a new layer. But such concentrations are rarely found in immersion heating applications. The most hostile situation for 316 is the intermediate concentration range (0.5-15 wt% depending on temperature) when the passive film is unstable. The detailed corrosion rates for 316 sheaths in sulphuric acid are shown in the table below as a function of concentration, temperature and time to perforation.

Concentration H2SO4 (wt%)Estimated pH Solution Temperature (°C) Corrosion Rate (mm/yr)Time to Perforation (1.5mm wall) Recommended for Continuous Service 0.1 1.5 25 0.02 >75 years Yes 0.1 1.5 80 0.05 >30 years Yes 0.1 1.5 100 0.15 10 years Marginal (monitor) 0.5 1.2 25 0.03 >50 years Yes 0.5 1.2 80 0.20 7-8 years Marginal, upgrade to 316L or Alloy 20 0.5 1.2 100 0.80 1.5-2 years Not recommended 1.0 1.0 25 0.05 >30 xyoo Tau 1.0 1.0 60 0.30 5 xyoo Txais nrog kev tshuaj xyuas
1.0 1.0 80 2.0 9 Lub hliTsis pom zoo 2.0 0.8 25 0.15 10 xyoo Tau txais kev siv qee zaus
2.0 0.8 50 1.0 1.5 xyoo Tsis pom zoo 5.0 0.5 25 0.50 3 xyoo marginal (kho dua tshiab)
5.0 0.5 50 4.0 4-5 hli Tsis tau
10.0 0.2 25 1.5 1 xyoo Tsis tsim nyog rau 316 tshwj tsis yog tias txias thiab sib cuam tshuam
15.0 0.0 25 3.0 6 lub hlis Txais tsis tau
Qhov Extension ntawm Passive Range los ntawm Oxidising Agents

Cov kev txwv tsis pub dhau los saum toj no yog siv tau rau cov tshuaj aerated (xws li oxygen yaj hauv cov tshuaj). Nyob rau hauv deaerated los yog txo cov xwm txheej (xws li sulfuric acid tsis muaj oxygen thiab nrog hydrogen los ntawm corrosion tam sim no) lub passive ntau yam narrows considerably vim muaj kev txwv ntawm cov cathodic cov tshuaj tiv thaiv (oxygen txo) . 316 yuav active corroded nyob rau hauv cov sij hawm es tsis txhob ntawm lub hlis ntawm 80℃nyob rau hauv 0.5 wt% H2SO4 nrog deaeration. Txawm li cas los xij, qhov sib ntxiv ntawm cov tshuaj oxidising, xws li nitric acid, ferric (Fe3 +) lossis cupric (Cu{13}}) ions, nthuav dav cov kab mob mus rau qhov siab dua thiab kub dua. Hauv sulfuric acid uas muaj 1000 ppm Fe3+ 316 nyob twj ywm mus txog 10 wt% ntawm 80℃-kev txhim kho kaum npaug . Nyob rau hauv cov ntawv thov cua sov qhov twg cov txheej txheem kev daws teeb meem suav nrog oxidising hom raws li ib qho kev tivthaiv (xws li, pickling da dej uas muaj ferric sulphate, los yog khib nyiab acid ntws los ntawm cov hlau tiav), 316 tej zaum yuav haum txawm nyob rau hauv tej yam kev mob uas yuav ua rau active corrosion nyob rau hauv ntshiab acid. Qhov teeb meem nrog nyob ntawm qhov muaj nyob ntawm oxidisers yog tias cov txheej txheem cuam tshuam tuaj yeem txo lawv cov concentration thiab ua kom nrawm nrawm ntawm corrosion. Lub passive - thaj tsam ua haujlwm nyob rau hauv cov lus ua ntej yuav tsum tsis meej meej txaus siab nyob rau hauv qhov phem tshaj - rooj plaub (tsawg kawg oxidising peev xwm) rau cov engineers los qhia 316 rau sulfuric acid kev pab.

Kev kuaj pom tseeb ntawm kev ua haujlwm corrosion hauv cov cua sov ua haujlwm

Peb daim teb qhia tau hais tias kev ua haujlwm corrosion tau pib rau 316 sheaths ua haujlwm nyob rau hauv sulfuric acid daws ze rau qhov passive- thaj tsam nquag. Thawj yog kev hloov xim ntawm cov tshuaj: kev sib zog ua kom tawg ntawm 316 tsim ntsuab Fe 2+ thiab Ni 2+ ions, hloov cov xim tsis muaj xim lossis xim daj me ntsis rau xim daj lossis xiav -ntsuab ntsuab. Qhov thib ob yog evolution ntawm hydrogen gas. Lub active corrosion ntawm stainless hlau nyob rau hauv cov kua qaub yog nrog los ntawm zus tau tej cov hydrogen npuas nyob rau hauv sheath nto, uas yuav pom tau raws li ib tug kwj ntawm zoo npuas nce los ntawm lub rhaub. Nyob rau hauv lub xeev passive, tsis muaj hydrogen evolution, vim hais tias cov txheej txheem cathodic yog txo cov pa oxygen. Qhov kos npe thib peb yog qhov poob qis hauv lub tshuab rhaub hluav taws xob kub rau tib lub zog tawm tswv yim. Cov hluav taws xob tsis kam ntawm cov hluav taws xob sab hauv hluav taws xob tseem zoo ib yam. Cov phab ntsa sheath tau dhau los ua thinner thiab muaj thermal tsis kam. Yog tias lub zog ntom ntom ntom ntom ntom ntom ntom ntom ntom ntom, sab nraud qhov kub yuav raug txo. Cov phab ntsa loj heev thinning yog pov thawj los ntawm qhov ntsuas kub ntawm qhov txo qis ntawm 5-10℃hauv 1-2 lub hlis. Yog tias lub tshuab cua sov tuaj yeem raug tshem tawm rau kev tshuaj xyuas, phab ntsa poob tuaj yeem ua raws lub sijhawm nrog lub ntsuas ntsuas ultrasonic thickness rau kev soj ntsuam ntau. Pib npaj rau kev hloov yog tias tus nqi corrosion ntau tshaj 0.3 hli hauv ib xyoos.

Alternative Alloys Thaum 316 mus dhau qhov kev txwv Passive

For the sheathed heater, if the necessary sulphuric acid concentration and temperature are outside the passive range for 316, various more resistant alloys are available. Alloy 20 (Carpenter 20) comprises 32-38 % Ni, 19-21 % Cr and 2-3 % Mo plus copper and provides excellent resistance to dilute sulphuric acid up to 10-15 wt% at boiling. Similar performance with a little less nickel is Incoloy 825 (Alloy 825). Alloy C-276 (Hastelloy C-276) or Alloy B-3 (Hastelloy B-3) are required for larger concentrations (15-50 wt% at moderate temperatures), but are considerably more expensive. For very dilute acid (less than 1 wt%, pH>1.0) mus txog 80℃lub neej txaus (5-10 xyoo) yog muab los ntawm 316L nrog cov pa roj carbon tsawg thiab zoo polished nto. Nyob rau hauv txhua daim ntawv thov qhov twg qhov kub thiab txias sib xyaw ua ke yog ze rau thaj tsam nquag, qhov kev pheej hmoo corrosion raug tshem tawm tag nrho los ntawm kev xaiv lub rhaub nrog lub tshuab hloov pauv lossis cov hlab ntsha nrog cov cua sov sab nraud, tshem tawm cov cua sov los ntawm kev sib cuag nrog cov kua qaub.

Xaus: Kev txwv kev nyab xeeb rau 316 hauv Sulfuric Acid Services

Passive corrosion tus nqi ntawm 316 stainless hlau encased immersion heaters yog 0.02-0.2 mm / xyoo, tab sis nquag dissolution tus nqi yog 1-10 ​​mm / xyoo nyob rau hauv sulfuric acid daws. Lub pH-kub qhov pib rau kev hloov pauv los ntawm kev tiv thaiv corrosion mus rau kev ua kom nrawm nrawm yog feem ntau nyob ntawm cov kua qaub concentration. 316 tuaj yeem tiv taus txog li 10 wt% H2SO4 ntawm qhov kub thiab txias. Ntawm 80 C qhov kev txwv tau raug txo rau 0.5-1 wt%. Txawm tias ntawm 0.5 wt% tus nqi corrosion yog me me ntawm 100 degree. Thaum cov engineers qhia 316 sheathed heaters rau cov kua qaub ua haujlwm, lawv yuav tsum txiav txim siab txog cov txheej txheem tiag tiag acid concentration thiab qhov siab tshaj plaws ua haujlwm kub, tom qab ntawd muab cov txiaj ntsig sib piv rau ntau thaj tsam ntawm cov lus saum toj no. Yog tias qhov sib xyaw ua ke poob rau hauv qhov marginal lossis qhov tsis txaus ntseeg, lub rhaub yuav tsum tau derated mus rau qhov kub thiab txias (los ntawm kev txo lub zog ceev), tus qauv yuav tsum tau hloov mus rau ntau dua corrosion resistant alloy, los yog cov txheej txheem cua sov yuav tsum tau hloov mus rau indirect lossis sab nraud cua sov. Los ntawm kev khi qhov kev hloov pauv hloov mus rau qhov muaj txiaj ntsig ntawm cov tshuaj thiab cov yam ntxwv thermal, cov txheej txheem tau hais los ntawm no ua rau cov neeg yuav khoom xav tias yuav tau txais kev pabcuam lub neej ntawm 316 sheaths nyob rau hauv sulfuric acid nqis thiab zam qhov kev nruj nruj uas tshwm sim tom qab cov yeeb yaj kiab tsis ua haujlwm.