Raws li kev sib xyaw tshwj xeeb ntawm Pub Dej Dissolved Oxygen Concentration thiab pH Ua 316 Stainless Steel Heater Sheath Suffer Flow -Accelerated Corrosion (FAC) ntawm 200℃hauv High-Velocity Boiler Feedwater

Dissolution ntawm Passive Zaj duab xis nyob rau hauv High Purity High Temperature Dej

The high velocity boiler feedwater systems operating at 200°C (typical for high-pressure boiler economisers or feedwater heaters) can be a problem for 316 stainless steel sheathed electric heating tubes due to flow-accelerated corrosion (FAC), which is thought to be immune to this phenomenon (which occurs mainly in carbon and low-alloy steels). The FAC of 316 is different from the traditional FAC of magnetite (Fe3O4) layers on carbon steel where it is a direct dissolution of passive film by the combined impacts of high flow velocity, high temperature and unique water chemistry. The passive film on 316 dissolves at 200°C, when the dissolved oxygen is less than 5 ppb (deaerated) and the pH is greater than 9.5 or less than 6.5. Under such conditions wall thinning rates of 0.1-0.5 mm/year have been seen in high velocity zones (>5 m / s). Daim ntawv no ntsuas cov pa oxygen thiab pH qhov pib rau FAC pib ntawm 316 sheaths ntawm 200 degree.

Mechanism ntawm kub solubility ntawm passive films

Nyob rau hauv siab - dej kub (200 degree) cov zaj duab xis passive ntawm 316 yog ib tug bilayer ntawm chromium oxide (Cr₂O₃) thiab hlau chromium spinel (FeCr₂O₄). Cov oxides no rhiab heev rau pH thiab yaj oxygen rau lawv solubility. Thaum tuag lawm (DO<5 ppb) the corrosion potential is in the active or passive-transition area where the passive film is less stable . At high pH (>9.5) chromium oxide yog yaj li chromite (CrO 2-) los yog chromate (CrO 4 2-). Thaum pH qis (<6.0) the film dissolves into Cr3+ ions. High flow velocity (>3-5 m / s) tshem tawm cov tsiaj yaj tas li thiab tiv thaiv kom tsis txhob rov ua dua thiab tswj tau tus nqi siab. Lub zog ua kom muaj zog rau FAC ntawm 316 yog hais txog 40-60kJ / mol, piv txwv li tus nqi corrosion ob zaug txhua 15-25℃saum toj 150 degree.

Quantified FAC Thresholds ntawm 316 ntawm 200℃hauv High-Veocity Dej

Hauv qab no yog qhia txog tus nqi corrosion thiab phab ntsa thinning tus cwj pwm rau 316 stainless hlau hauv kev ntsuas lub voj voog ntawm 200℃nrog kev hloov pauv hauv cov pa oxygen, pH thiab ntws nrawm.

Dissolved Oxygen (ppb) pH ntawm 200℃Flow Velocity (m/s) Dominant Oxide Solubility FAC Rate (mm/yr) Lub Sijhawm rau 0.5 mm Phab Ntsa Loss (teev) Pom zoo rau 200℃Kev Pabcuam<5 (deaerated) 6.5-7.5 Any Minimal <0.02 >25,000 Yog<5 7.5-8.5 <3 Minimal <0.02 >25,000 Yog<5 7.5-8.5 3-5 Slight 0.02-0.05 10,000-25,000 Acceptable <5 8.5-9.5 3-5 Cr(OH)3 solubility 0.05-0.15 3,500-10,000 Not recommended <5 9.5-10.5 >3 Chromite dissolution 0.15-0.40 1, 250-3,500 Tsis tau<5 >10.5 Any Rapid chromite >0.40 <1,250 Unacceptable <5 5.5-6.5 >3 Cr3+ solubility 0.10-0.30 1,700-5,000 Tsis txaus<5 <5.5 Any Rapid Cr3+ >0.30 <1,700 Unacceptable
5-20 (low) 8.5-9.5 >3 Txo solubility 0.03-0.08 6,000-17,000 Marginal
20-100 (moderate) 8.5-9.5 >5 Passive zaj duab xis stabilized<0.02 >25,000 Yes 20-100 9.5-10.5 >5 Some stabilization 0.02-0.05 10,000-25,000 Acceptable >100Txhua yam Passive tag nrho<0.02 >25,000 Yog
FAC ntawm 316 yog nyob ntawm qhov khiav ceev heev.

Flow velocity yog qhov ntsuas qhov ntsuas uas hloov cov zaj duab xis passive solubility rau hauv ntsuas phab ntsa thinning. Cov lus hauv qab no qhia tau hais tias qhov nrawm nrawm ntawm 200℃C, deaerated tej yam kev mob, pH 9.2 (marginal zone).

Flow Velocity (m/s) Phab ntsa Shear Stress (Pa, kwv yees) FAC tus nqi @ DO<5 ppb, pH 9.2 (mm/yr)Time to 50% of Original 1.5 mm Wall Loss(hours) Recommended Maximum Velocity for 10-Year Life <1 <10 <0.01 >75,000 Safe 1-2 10-20 0.01-0.03 25,000-75,000 Safe 2-3 20-35 0.03-0.06 12,500-25,000 Acceptable 3-5 35-60 0.06-0.12 6,000-12,500 Marginal (monitor) 5-8 60-100 0.12-0.20 3,750-6,000Not recommended 8-12 100-150 0.20-0.35 2,000-3,750 Unacceptable >12 >150 >0.35 <2,000 Not acceptable
Tsim Specification thiab Siv Dej Chemistry ntawm Feedwater Heaters ntawm 200 degree

316 encased heaters ntawm 200℃nyob rau hauv high-speed boiler feedwater yog tiv thaiv los ntawm kev khiav ceev ceev corrosion los ntawm cov hauv qab no yuav tsum tau.

Feedwater Hom Lub Hom Phiaj Dissolved Oxygen (ppb) Pom zoo pH, 200℃Qhov siab tshaj plaws ntws nrawm ntawm Sheath (m / s) Lub Neej Uas Muaj Peev Xwm (xyoo)
Txhua yam kev kho tsis haum (AVT) - txo 2-5 8.0-8.5 3.0 8-10
Txhua yam -Volatile Treatment (AVT) - oxidizing 50-100 8.0-8.5 5.0 10-15
Oxygenated therapy (OT) 100-300 8.0-8.5 6.0 >15 
Kev kho phosphate 5-20 8.5-9.0 3.0 5-8 (saib)
Any treatment with pH >9.5 N/A >9.5 <2.0 5-10 (316 marginal)
Field Identification ntawm FAC ntawm 316 Heater Sheaths

The FAC on a 316 heater will produce unique surface features that are different from conventional corrosion mechanisms in high temperature, high velocity feedwater. The surface is smooth, polished or "orange-peel" in appearance with no pitting or scale. The wall thinning is homogeneous at the high velocity zones (e.g. pipe entry, directly facing the flow). The thinning is often a steady drop of outside diameter throughout the heater length. Scanning electron microscopy (SEM) shows a smooth, featureless surface, with no attack at the grain boundaries. Water chemistry log will show pH >9.5o ib<6.0 and dissolved oxygen <5-20 ppb for prolonged durations. For such failures the answer is to adjust water chemistry to keep pH in the 8.0-8.5 range, and dissolved oxygen >20-50 ppb (yog tias ua tau nrog tag nrho cov kab ke), los yog txwv tsis pub khiav tawm los ntawm kev tso lub rhaub rau hauv ib qho bypass lossis tseem zoo. FAC susceptibility tuaj yeem raug tshem tawm los ntawm kev hloov mus rau titanium lossis Alloy 825.

Summary: Feedwater Chemistry thiab Velocity Management los Tiv Thaiv FAC

Cov dej ntws nrawm nrawm ntawm 316 stainless hlau encased rhaub hauv 200℃boiler feedwater yuav siv qhov chaw nyob rau hauv deaerated tej yam kev mob (DO<5-10 ppb) in combination with alkaline pH >9.5 los yog acid pH<6.0 and flow velocities >3-5 m/sec. Under these conditions, the passive film is dissolved as chromite or chromium ions, resulting in wall thinning rates of 0.1-0.5 mm/year. For high temperature feedwater service with 316 sheaths, engineers should stipulate pH of 8.0-9.0, dissolved oxygen >20 ppb (ideally 50-100 ppb) and a flow velocity below 3-4 m/s at the sheath surface. If high velocity (>5 m/s) cannot be avoided, oxidising water treatment (oxygenated treatment, OT) with DO >100 ppb yuav stabilize cov zaj duab xis passive thiab zam FAC. Cov txheej txheem muab ntawm no hais txog cov pa oxygen yaj, pH, thiab ntws nrawm rau ntsuas FAC tus nqi ntawm 200 degree, thiab tso cai rau tus neeg yuav khoom los txheeb xyuas cov dej chemistry thiab cov dej hydraulic uas yuav tswj tau cov zaj duab xis passive ntawm 316 lub rhaub dej kub hauv qhov siab tshaj plaws boiler feedwater ua haujlwm.