Nyob rau hauv dab tsi tshwj xeeb sib xyaw ntawm Sulfate Concentration thiab pH Ua 316L Rhaub Sheath hauv ntws dej ntshiab ntawm 60℃Tsim Pitting thaum tsis muaj chlorides

Mechanism ntawm Pitting ntawm Stainless hlau tsis muaj chloride

Hauv cov dej tshiab uas muaj chloride tsawg heev (< 20 ppm), but high to moderate sulphate concentrations (SO4 2-) and acidic pH such as in acid mine drainage, some industrial cooling waters or agricultural runoff, pitting corrosion can occur even in the absence of chloride ions, with 316L stainless steel sheathed electric heating tubes. For flowing conditions (1 m/s), sulphate ions are typically thought to be non-aggressive or inhibitive to pitting. However, at pH values below 4.0-4.5, sulphate can attack and create soluble iron sulphates, breaking down the passive film. The essential combination for pitting is: sulphate >500-1,000 ppm thiab pH<4.0. Rates of pitting of 0.2-0.5 mm/yr occur at pH 3.0 and 1,000 ppm SO4 2-. Rates > 1 mm/year, pH 2.5, 1,000 ppm SO 2 4 . The sulfate-pH-pitting threshold for 316L in flowing freshwater at 60°C in the absence of chlorides is quantified in this article.

Mechanism ntawm Low pH Induced Sulphate Pitting

Nyob rau hauv nruab nrab los yog alkaline tej yam kev mob sulphate ions yog adsorbed rau ntawm passive zaj duab xis thiab tiv thaiv chloride -induced pitting. Txawm li cas los xij, cov zaj duab xis passive tsis ruaj khov ntawm pH qis (<4.0). Sulphate can form soluble complexes with ferric ions ( FeSO4+ ), and can also take part in the anodic dissolution reaction. Local acidification in growing pits (pH as low as 1-2) permits sulphate to behave as an aggressive anion like chloride but under more acidic conditions. In sulphate solutions, the pitting potential (Epit) decreases with decreasing pH and increasing sulphate concentration .

60℃, 1 m/s Flow Sulfate -pH Pitting Thresholds Quantified

Cov nram qab no pitting pib tej yam kev mob tau txiav txim los ntawm kev tswj ndlwg voj kev ntsuam xyuas ntawm 316L (electropolished, Ra=0.2 µm) nyob rau hauv sulphate daws (0-2,000 ppm SO42- as Na2SO4) nrog pH tswj los ntawm H2SO4 (tsis muaj chlorides ntxiv) ntawm 60℃.2 , 1 octee m.

Sulphate Concentration (ppm SO4 2-) pH (60 degree) Pitting Initiation Sijhawm (h) Pitting Rate (mm/y)| Qhov ntom ntom (pits/cm2)|Lub sij hawm rau Perforation (1.5 hli phab ntsa, xyoo)|Pom zoo rau Flowing Freshwater Service (low Cl)||<500 | >4.0 | >10,000 | <0.02 | 0 | >75 | Yes | | 500-1,000 | 5.0 | >10,000 | <0.02 | 0 | >75|Yog|| 500-1,000|4.0-4.5|5,000-10,000|0.02-0.05 |<1 | 30-75 | Yes | | 500-1,000 | 3.5-4.0 | 2,000-5,000 | 0.05-0.15 | 1-5 | 10-30 | Acceptable | | 500-1,000 | 3.0-3.5 | 1,000-2,000 | 0.15-0.40 | 5-20 | 4-10 | Marginal | | 500-1,000 | <3.0 | 200-1,000 | 0.40-1.00 | >20|1.5-4|Tsis pom zoo|| 1,000-2,000|4.0-4.5|2,000-5,000|0.05-0.15|1-5|10-30|Txais|| 1,000-2,000|3.5-4.0|800-2,000|0.15-0.40|5–15|4–10|Marginal|| 1,000-2,000|3.0-3.5|300-800|0.40-0.80|15-40|2-4|Tsis pom zoo|| 1,000-2,000 |<3.0 | 100-300 | 0.80-2.00 | >40 | 0.75-2 | No | | >2,000 | <4.0 | <200 | >1.00 | >50 | <1.5 | No | |
Kev cuam tshuam ntawm kev khiav ceev ntawm sulphate pitting

Hauv kev daws teeb meem chloride, cov dej ntws ntau dua yuav txo qis pitting, tab sis tuaj yeem txhim kho sulphate pitting los ntawm kev tshem tawm cov khoom tiv thaiv corrosion.

Flow Velocity (m/s) Pitting Rate ntawm 1,000 ppm SO4-2, pH 3.5, 60oC (mm/xyoo) Lub Sijhawm Ua Haujlwm (xyoo) Pom Zoo Tshaj Tawm Tshaj Tawm rau Kev Pabcuam Acidic Sulphate<0.5 0.15-0.25 6-10 Acceptable
0.5-1.0 0.25-0.40 4-6 Marginal 1.0-1.5 0.40-0.60 2.5-4 Not recommended 1.5-2.5 0.60-0.90 1.7-2.5 No >2.5 >0.90 <1.7 No Practical Recommendations for Low-Chloride, High-Sulfate, Acidic Freshwater Service

Rau 316L encased rhaub nyob rau hauv dej tshiab nrog tsawg chlorides tab sis sulphates thiab acid pH ntawm 60 degree, cov nram qab no tsis siv.

Hom dej li qub pH Sulfate (ppm) Chloride (ppm) Pom zoo Alloy Lub neej ntev (xyoo)
Acid mine kua (mob) 4.0-5.0 500-1,500<50 316L with monitoring 5-10 
Acid mine kua (hnyav) 2.5-4.0 1,000-3,000<50 Duplex 2205 5-8
Muaj cua txias (acidic) 3.5-4.5 500-1,000<20 316L (acceptable) 8-12 
Agricultural runoff (acidic) 4.0-5.0 100-500 10-50 316L >10 
Txhua yam nrog pH<3.0 <3.0 >500 Ib Tsis Muaj 316L Hloov Kho rau Alloy 20 Field Identification ntawm Sulphate Induced Pitting

Sulfate-induced pitting tej zaum yuav xav tias yog ib tug 316L rhaub uas pits hauv dej tshiab nrog chlorides<50 ppm but sulphate >500 ppm thiab pH<4.0. The pits are usually tiny (50-200 µm) and frequent and no chloride was discovered in the pit residue (EDS). The corrosion product may contain sulphur. Water analysis will show low chloride, moderate to high sulphate and low pH. The remedy is to elevate the pH (neutralise) or go to Alloy 20 or duplex 2205.

Xaus: Tsis txhob cia pH thiab sulfate ua rau tsis yog -Chloride Pitting

Pitting ntawm 316L stainless hlau rhaub sheaths hauv ntws (1 m / s) dej tshiab ntawm 60℃nrog tsawg chloride (<20 ppm) occurs at sulphate concentrations >500-1,000 ppm thaum pH tsawg dua 3.5-4.0. Pitting tus nqi ntawm 0.4-0.8 mm / xyoo ntawm pH 3.0-3.5 thiab 1000 ppm SO₄²⁻ tsim perforation hauv 2-4 xyoos. Cov kws ua haujlwm yuav tsum tau kuaj pH thiab sulfate concentration, tsis yog cov tshuaj chlorides xwb, txhawm rau txheeb xyuas 316L sheaths rau cov dej tshiab. Thaum pH<4.0 and sulphate >500 ppm, pH neutralization lossis kev txhim kho hlau yog xav tau. Lub moj khaum tsim los ntawm no, hais txog cov ntsiab lus sulphate thiab pH rau pitting pib thiab nthuav tawm tus nqi ntawm 60℃hauv chloride- dej dawb, ua rau cov neeg yuav khoom txheeb xyuas thiab tiv thaiv tsis yog -chloride pitting ntawm 316L rhaub sheaths hauv acidic, siab -sulfate dej qab zib.