Anba ki konbinezon espesifik nan konsantrasyon oksijèn ki fonn dlo pou manje ak pH 316 Nerjaveèi aparèy chofaj asye soufri koule -Korozyon akselere (FAC) nan 200 degre nan gwo-Vitès Chodyè Dlo Alimentation

Disolisyon fim pasif la nan dlo pite segondè tanperati

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). Papye sa a evalye oksijèn ki fonn ak papòt pH pou inisyasyon FAC sou 316 djenn nan 200 degre.

Mekanis solubility tanperati ki wo nan fim pasif

Nan gwo -dlo tanperati (200 degre) fim nan pasif sou 316 se yon kouch oksid chromium (Cr₂O₃) ak fè chromium spinel (FeCr₂O₄). Ksid sa yo trè sansib sou pH ak oksijèn ki fonn pou solubilite yo. Lè deaere (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) Kwòm oksid fonn kòm chromite (CrO 2-) oswa chromat (CrO 4 2-). Nan pH ba (<6.0) the film dissolves into Cr3+ ions. High flow velocity (>3-5 m/s) retire espès yo fonn toujou epi anpeche repasivasyon an epi kenbe yon pousantaj dissolution segondè. Enèji aktivasyon pou FAC 316 se sou 40-60kJ/mol, sa vle di pousantaj korozyon an double chak 15-25 degre pi wo pase 150 degre.

Papòt FAC ki mezire 316 nan 200 degre nan dlo ki gen gwo vitès-

Anba a yo montre pousantaj korozyon ak konpòtman eklèsi miray pou asye pur 316 nan yon tès bouk kontwole nan 200 degre ak varyasyon nan oksijèn ki fonn, pH ak vitès koule.

Oksijèn ki fonn (ppb) pH nan 200 degre Vitès koule (m/s) Solibilite oksid dominan To FAC (mm/an) Tan a 0.5 mm Pèt mi (èdtan) Rekòmande pou sèvis 200 degre<5 (deaerated) 6.5-7.5 Any Minimal <0.02 >25,000 Wi<5 7.5-8.5 <3 Minimal <0.02 >25,000 Wi<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 Kwòmit disolisyon 0.15-0.40 1,250-3,500 pa akseptab<5 >10.5 Any Rapid chromite >0.40 <1,250 Unacceptable <5 5.5-6.5 >3 Cr3+ solubilite 0.10-0.30 1,700-5,000 Inakseptab<5 <5.5 Any Rapid Cr3+ >0.30 <1,700 Unacceptable
5-20 (low) 8.5-9.5 >3 Solibilite redwi 0.03-0.08 6,000-17,000 Marginal
20-100 (moderate) 8.5-9.5 >5 Fim pasif estabilize<0.02 >25,000 Yes 20-100 9.5-10.5 >5 Some stabilization 0.02-0.05 10,000-25,000 Acceptable >100Nenpòt Nenpòt Totalman pasif<0.02 >25,000 Wi
FAC a nan 316 depann anpil sou vitès koule.

Vitès koule se faktè dekale ki transfòme solubilite fim pasif nan eklèsi miray mezire. Tablo ki anba la a montre depandans vitès nan 200 degre C, kondisyon deaere, pH 9.2 (zòn majinal).

Vitès koule (m/s) estrès taye miray (Pa, estime) To FAC @ 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
Spesifikasyon Design ak Chimi Dlo Pratik nan aparèy chofaj Feedwater nan 200 degre

316 aparèy chofaj anvlòp nan 200 degre nan gwo vitès chodyè feedwater yo pwoteje kont koule akselere korozyon pa kondisyon sa yo.

Kalite Dlo Alimentation Sib Oksijèn Dissoud (ppb) PH Rekòmande, 200 degre Vitès Koule Maksimòm nan djenn (m/s) Vite Aparèy Chofaj Prevwa (ane)
Tout terapi temèt (AVT) - rediksyon 2-5 8.0-8.5 3.0 8-10
Tout -tretman temèt (AVT) - oksidasyon 50-100 8.0-8.5 5.0 10-15
Oxygenated therapy (OT) 100-300 8.0-8.5 6.0 >15 
Tretman fosfat 5-20 8.5-9.0 3.0 5-8 (moniteur)
Any treatment with pH >9.5 N/A >9.5 <2.0 5-10 (316 marginal)
Idantifikasyon jaden FAC sou 316 djenn chofaj

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 >9.5 oswa<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 (si konpatib ak sistèm an jeneral), oswa pou limite vitès koule lè w mete aparèy chofaj la nan yon bypass oswa yon pi piti. FAC sansiblite ka elimine pa chanje nan Titàn oswa Alloy 825.

Rezime: Feedwater Chimi ak Jesyon Vitès pou Prevni FAC

Koule a akselere korozyon nan 316 aparèy chofaj asye pur anvlòp nan 200 degre chodyè feedwater pran plas nan kondisyon deaerated (DO<5-10 ppb) in combination with alkaline pH >9.5 oswa pH asid<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 pral estabilize fim nan pasif epi evite FAC. Metodoloji yo bay isit la gen rapò oksijèn ki fonn, pH, ak vitès koule ak pousantaj FAC mezirab nan 200 degre, epi li pèmèt achtè a defini chimi dlo ak paramèt idwolik ki pral kenbe fim nan pasif sou 316 djenn aparèy chofaj nan gwo vitès chofaj dlo feedwater.