O dan Ba ​​Gyfuniad Penodol o Crynodiad Sylffad a pH Sy'n Gwneud Gwain Gwresogydd 316L mewn Dŵr Croyw sy'n Llifo ar 60 gradd Datblygu Pyllau yn Absenoldeb Cloridau

Mecanwaith Gosod Dur Di-staen heb Glorid

Mewn systemau dŵr croyw gyda chlorid isel iawn (< 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 A 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.

Y Mecanwaith o Dyllu Sylffad a Anogir â pH Isel

Mewn amodau niwtral neu alcalïaidd mae ïonau sylffad yn cael eu hamsugno ar y ffilm oddefol ac yn atal tyllu a achosir gan glorid. Fodd bynnag, mae'r ffilm goddefol yn llai sefydlog ar pH isel (<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 gradd , 1 m/s Sylffad Llif-pH Trothwyon Taro Wedi'u Meintoli

Mae'r amodau cychwyn tyllu canlynol wedi'u pennu o brofion dolen llif rheoledig o 316L (electropolished, Ra=0.2 µm) mewn hydoddiannau sylffad (0-2,000 ppm SO42- fel Na2SO4) gyda pH wedi'i reoleiddio gan H2SO4 (dim cloridau wedi'u hychwanegu) ar 60 gradd, 1 m/s, cyflymder 1 m/s, cyflymder 000 am awr.

Crynodiad Sylffad (ppm SO4 2-) pH (60 gradd ) Amser Cychwyn Pitting (h) Cyfradd Taro (mm/y)| Dwysedd Pwll (pyllau/cm2)|Amser i Berforation (wal 1.5 mm, blynyddoedd)|Argymhellir ar gyfer Gwasanaeth Dŵr Croyw Llif (Cl isel)||<500 | >4.0 | >10,000 | <0.02 | 0 | >75 | Yes | | 500-1,000 | 5.0 | >10,000 | <0.02 | 0 | >75|Ydw|| 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|Heb ei argymell|| 1,000-2,000|4.0-4.5|2,000-5,000|0.05-0.15|1-5|10-30|Derbyniol|| 1,000-2,000|3.5-4.0|800-2,000|0.15-0.40|5-15|4-10|Ymylol|| 1,000-2,000|3.0-3.5|300-800|0.40-0.80|15-40|2-4|Heb ei argymell|| 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 | |
Dylanwad cyflymder llif ar dyllu sylffad

Mewn toddiannau clorid, mae cyflymder llif uwch yn lleihau tyllu, ond gallant wella tyllu sylffad trwy gael gwared ar gynhyrchion cyrydiad amddiffynnol.

Cyflymder Llif (m/s) Cyfradd Bwrw ar 1,000 ppm SO4-2, pH 3.5, 60oC (mm/blwyddyn)Amser torri trwodd (blynyddoedd)Cyflymder Uchaf a Argymhellir ar gyfer Gwasanaeth Sylffad Asidig<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

Ar gyfer gwresogyddion caeedig 316L mewn dŵr ffres gyda chloridau isel ond sylffadau a pH asid ar 60 gradd, mae'r paramedrau canlynol yn berthnasol.

Math o Ddŵr pH Sulfad arferol (ppm) Clorid (ppm) Alloy a Argymhellir Oes (blynyddoedd)
Draeniad mwynglawdd asid (ysgafn) 4.0-5.0 500-1,500<50 316L with monitoring 5-10 
Draeniad mwynglawdd asid (difrifol) 2.5-4.0 1,000-3,000<50 Duplex 2205 5-8
Oeri diwydiannol (asidig) 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 
Unrhyw â pH<3.0 <3.0 >500 Unrhyw Ddim 316L Uwchraddiad i Aloi 20 Maes Adnabod Pytiau a Achosir gan Sylffad

Mae'n bosibl bod amheuaeth o bylu a achosir gan sylffad ar gyfer gwresogydd 316L sy'n pylu mewn dŵr croyw â chloridau<50 ppm but sulphate >500 ppm a 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.

Casgliad: Peidiwch â gadael i pH a sylffad fod yn achos pydru heb glorid.

Gosod gwainiau gwresogydd dur gwrthstaen 316L mewn dŵr croyw sy'n llifo (1 m/s) ar 60 gradd gyda chlorid isel (<20 ppm) occurs at sulphate concentrations >500-1,000 ppm pan fydd pH yn llai na 3.5-4.0. Mae cyfraddau tyllu o 0.4-0.8 mm y flwyddyn ar pH 3.0-3.5 a 1000 ppm SO₄²⁻ yn cynhyrchu trydylliad mewn 2-4 blynedd. Mae angen i beirianwyr brofi crynodiad pH a sylffad, nid cloridau yn unig, i ddiffinio gwain 316L ar gyfer cyflenwad dŵr croyw. Pan fydd pH<4.0 and sulphate >Mae angen 500 ppm, naill ai niwtraliad pH neu welliant aloi. Mae'r fframwaith a ddatblygir yma, sy'n cysylltu cynnwys sylffad a pH â chyfraddau cychwyn tyllu a chyfraddau lluosogi ar 60 gradd mewn-dŵr rhydd o glorid, yn galluogi prynwyr i nodi ac atal tyllu heb glorid o wain gwresogyddion 316L mewn systemau dŵr croyw asidig, uchel{-sylffad.