Anba ki konbinezon de tanperati ak pH 316 Nerjaveèi aparèy chofaj djenn devlope molybdène rediksyon nan fim pasif ki mennen nan pitting nan klori ki ba -(<200 ppm) Water

Chanjman nan konpozisyon fim pasif kòm yon Mekanis Pitting kache

Pou 316 asye pur vlope tib chofaj elektrik, nan dlo ki ba klori (<200 ppm) where traditional pitting would not be expected, another failure mechanism can occur: molybdenum depletion from the passive film. Molybdenum is a critical alloying element in 316 (2.0-2.5 wt%) that is enriched in the passive film to 5-10% at the outer surface. However, in pH environments below 6.0 or above 9.0 at temperatures over 80°C, molybdenum will selectively dissolve from the passive film as molybdate ions (MoO4 2-) or as volatile MoO3. When the percentage of Mo in the passive film falls below about 1-2% the film is no longer resistant to chloride breakdown and pitting occurs at bulk chloride concentrations as low as 50-100 ppm. This mechanism is different from the bulk alloy molybdenum loss as it only affects the outer 1-5 nm of the passive layer . In this paper we quantify the pH-temperature requirements for molybdenum depletion and the corresponding pitting risk in low chloride water.

Mekanis nan disolisyon selektif nan Molybdenum

316 Nerjaveèi pwodui yon fim pasif bikouch ki gen ladan yon oksid enteryè-kwòm rich (Cr2O3) ak yon kouch ekstèn fè -ki rich ak molybdène kòm espès Mo(VI) (MoO4 2- oswa MoO3). Li estab (anrichi fim nan ak amelyore pitting rezistans) nan net pH (7.0-8.0) nan tanperati modere (<60o C). At pH <5.5 molybdenum is soluble as MoO4 2- and leaches out of the film under acidic circumstances. In alkaline settings (pH >9.0) molybdenum also becomes soluble as molybdate . The leaching rate increases exponentially at high temperature (>80 degre) akòz enèji aktivasyon an (~ 40-60 kJ / mol) pou yap divòse molybdène. Nan konsantrasyon molibdèn sifas ki ba (mwens pase yon konsantrasyon kritik, apeprè 1-2 nan%), fim nan pasif konpòte li tankou 304 asye pur (ki pa gen okenn molybdène) ak twou nan solisyon klori osi ba ke 50-100 ppm nan 80-90 degre.

Limit Quantified pou Molybdenum Appauvrissement ak Pitting sa yo

The electrochemical conditions that result in molybdenum depletion (>Yo te idantifye 50% rediksyon soti nan debaz 8-10% Mo nan fim nan pa kontwole kondisyon elektwochimik yo pandan tès 316 nan solisyon diferan pH ak tanperati, epi imedyatman mezire kontni an molybdène nan fim nan pasif lè l sèvi avèk X-ray photoelectron spèktroskopi (XPS).

Tanperati pH (degre) Tan pou rive 50% Molybdène Depletion nan fim pasif (h)Sifas Mo apre depwisyon (at.%Potansyèl Pitting nan 100 ppm Cl⁻ nan 80 degre (mV vs. SCE) Risk Pitting nan dlo klori ki ba (<200 ppm)
6.5-8.0 <60 >10,000 (no depletion) 8-10 >600 (pasif) 6.5-8.0 60-80 5,000-10,000 6-8 500-600 Trè ba
6.5-8.0 80-95 2,000-5,000 4-6 400-500 Ba 5.5-6.5 60-80 1,000-2,000 3-5 300-400 Modere 5.5-6.5 80-95 500-1,000 2-3 200-300 Segondè 4.5-5.5 60-80 200-500 1-2 150-200 Grav
4.5-5.5 80-95 100-200 <1 <150 Extremely severe
8.0-9.0 60-80 1,000-2,000 4-6 400-500 ba 8.0-9.0 80-95 500-1,000 2-4 250-400 modere 9.0-10.0 60-80 500-1,000 2-3 200-300 wo 9.0-10.0 80-95 200-500 1-2 150-250 grav<4.5 or >10 Nenpòt<100 <1 <150Very brutal
Efè sinèrjik nan klori sou sifas apovri

Konsantrasyon kritik nan klori pou pitting diminye drastikman lè fim nan pasif apovri nan molybdène. Yo rezime papòt klori pou pitting sou sifas ki apovri ak ki pa-nan 80 degre nan tablo ki anba a.

Kondisyon Sifas Sifas Mo Kontni (nan %) Klori Kritik pou Pitting nan 80 degre (ppm) 100 ppm Cl⁻ Tan pou Pitting Aksyon Sigjere
As-passivated (fresh) 8-10 >2,000 >10,000 èdtan Non
Yon ti kras apovri 4-6 500-1,000 5,000-10,000 èdtan Verifye
Modereman apovri 2-4 150-300 1000-5000 hrsBese pH oswa diminye tanperati
Gwo fatig 1-2 50-100 200 a 1,000 èdtan Aksyon korektif imedya
Konplètman apovri<1 <50 <200 hours Change heater, fix chemical
Espesifikasyon Chimi Dlo Pratik pou Sistèm klori ki ba-

Rediksyon Molybdène se danje prensipal pou 316 aparèy chofaj ki kouvri yo nan dlo ki ba-klori (<200 ppm). The following pH and temperature limitations are recommended:

Maximum Operating Temperature (°C) pH Range for >10,000-hr Passivity StabilitypH Range for >5,000 èdtan Sèvis Admisyon Rekòmande Aksyon si pH depase Ranje 60 5.0-9.5 4.5-10.0 Ajiste pH ak asid oswa baz 70 5.5-9.0 5.0-9.5 Kontwòl pH rekòmande 80 6.0-8.5 5.5-9.0 Kontwòl pH nesesè
90 6.5-8.0 6.0-8.5 Kontwòl pH aktif nesesè 100 7.0-7.5 6.5-8.0 316 pa rekòmande; ajou alyaj
Idantifikasyon Jaden Pitting Akòz Depletion Molybdène

Si yon aparèy chofaj 316 twou nan dlo klori ki ba (50-150 ppm Cl{-) nan 80-90 degre, si chimi an esansyèl kòrèk (2.0-2.5% Mo pa EDS oswa ICP), yo ka envestige depwsisyon molibdèn fim nan pasif. XPS analiz de yon echantiyon yo pran nan djenn ki echwe a pral montre yon konsantrasyon molybdène sifas ki mwens pase 2-3% (nòmalman 8-10%). Spèktroskopi elèktron Auger pral demontre ke rediksyon an lokalize nan 2-5 nm ki pi eksteryè yo. Twou yo jeneralman piti, souvan, pa fon, ak diferan de pi gwo, pi fon twou nan klori-induit pitting sou sifas fre. Chimi solisyon nan moman echèk la ta dwe egzamine pou randone pH deyò ranje 6.5-8.0, espesyalman si aparèy chofaj la te opere pou dire lontan nan pH <5.5 oswa > 9.0.

Konklizyon: pH ak tanperati kontwòl molybdène nan fim nan pasif

Pitting ka rive nan 316 aparèy chofaj asye pur anvlòp nan dlo ki ba klori (<200 ppm) at chloride levels which would ordinarily be safe, if the passive layer gets molybdenum depleted. Depletion happens at temperatures more than 80°C and pH less than 5.5 or greater than 9.0 where molybdenum is dissolved from the outer film layer. Once depleted, the surface behaves like a 304 without molybdenum and pits in 50-150 ppm chloride. For low-chloride high-temperature use, engineers designing 316 sheaths must control pH between 6.5 and 8.0, especially above 80°C. If pH control is not possible, upgrading to 316L (same Mo content but lower carbon) may not prevent depletion; a higher molybdenum alloy such as 317L (3.5% Mo) or 904L (4.5% Mo) may be necessary. The architecture proposed here connects pH, temperature and molybdenum depletion to the pitting danger in low-chloride water, and allows the customers to set restrictions on water chemistry that preserve the protective qualities of the passive layer.