Anba ki konbinezon espesifik nan konsantrasyon silfat ak pH 316L djenn aparèy chofaj nan dlo dous k ap koule nan 60 degre devlope pitting nan absans klori

Mekanis nan pitting nan asye pur san klori

Nan sistèm dlo dous ak klori ki ba anpil (< 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 AK 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.

Mekanis ki ba pH pwovoke silfat Pitting

Nan kondisyon net oswa alkalin, iyon silfat yo adsorbe sou fim pasif la epi anpeche klori -pwovoke twou. Sepandan, fim pasif la mwens estab nan pH ba (<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 degre, 1 m/s Flux Sulfat-pH Papòt Pitting Kantifye

Kondisyon yo kòmanse pitting yo te detèmine nan tès bouk koule kontwole nan 316L (elektwopoli, Ra{1}} µm) nan solisyon silfat (0-2,000 ppm SO42- kòm Na2SO4) ak pH reglemante pa H2SO4 (pa gen klori ajoute) nan 60 degre, 1 m / s 0000000 èdtan vitès.

Konsantrasyon Sulfat (ppm SO4 2-) pH (60 degre) Tan Inisyasyon Pitting (h) Pousantaj Pitting (mm/a)| Pit Dansite (twou/cm2)|Tan pou Perforation (1.5 mm miray, ane)|Rekòmande pou sèvis dlo dous k ap koule (ba Cl)||<500 | >4.0 | >10,000 | <0.02 | 0 | >75 | Yes | | 500-1,000 | 5.0 | >10,000 | <0.02 | 0 | >75|Wi|| 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|Pa rekòmande|| 1,000-2,000|4.0-4.5|2,000-5,000|0.05-0.15|1-5|10-30|Akseptab|| 1,000-2,000|3.5-4.0|800-2,000|0.15-0.40|5-15|4-10|majinal|| 1,000-2,000|3.0-3.5|300-800|0.40-0.80|15-40|2-4|Pa rekòmande|| 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 | |
Enfliyans vitès koule sou silfat pitting

Nan solisyon klori, pi gwo vitès koule diminye pitting, men yo ka amelyore pitting silfat lè yo retire pwodwi korozyon pwoteksyon.

Vitès Koule (m/s) Pousantaj Pitting nan 1,000 ppm SO4-2, pH 3.5, 60oC (mm/ane) Tan pou Dekouvèt (ane) Vitès Maksimòm Rekòmande pou Sèvis Silfat Asid<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

Pou aparèy chofaj 316L anvlòp nan dlo fre ak klori ki ba men sulfat ak pH asid nan 60 degre, paramèt sa yo aplike.

Kalite dlo pH nòmal Sulfat (ppm) klori (ppm) alyaj rekòmande lavi espere (ane)
Drenaj min asid (mode) 4.0-5.0 500-1,500<50 316L with monitoring 5-10 
Drenaj min asid (grav) 2.5-4.0 1,000-3,000<50 Duplex 2205 5-8
Refwadisman endistriyèl (asid) 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 
Nenpòt ki gen pH<3.0 <3.0 >500 Nenpòt pa 316L Amelyore nan alyaj 20 Idantifikasyon jaden nan silfat pwovoke Pitting

Silfat-pitting ka sispèk pou yon aparèy chofaj 316L ki twou nan dlo dous ak klori.<50 ppm but sulphate >500 ppm ak 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.

Konklizyon: Pa kite pH ak sulfat se kòz ki pa -klori pitting

Pitting nan djenn aparèy chofaj asye pur 316L nan dlo dous k ap koule (1 m/s) a 60 degre ak klori ki ba (<20 ppm) occurs at sulphate concentrations >500-1,000 ppm lè pH mwens pase 3.5-4.0. Pitting pousantaj de 0.4-0.8 mm/ane nan pH 3.0-3.5 ak 1000 ppm SO₄²⁻ pwodui pèforasyon nan 2-4 ane. Enjenyè yo bezwen teste pH ak konsantrasyon silfat, pa senpleman klori, pou defini 316L djenn pou rezèv dlo fre. Lè pH<4.0 and sulphate >500 ppm, swa netralizasyon pH oswa amelyorasyon alyaj nesesè. Fondasyon an devlope isit la, ki gen rapò kontni sulfat ak pH ak inisyasyon pitting ak pousantaj pwopagasyon nan 60 degre nan klori-dlo gratis, pèmèt achtè yo idantifye ak anpeche ki pa-klori twou nan djenn aparèy chofaj 316L nan sistèm dlo dous asid, ki gen gwo-sulfat.