Yuav Ua Li Cas Lub Hwj Chim Loj ntawm Primary MC Carbides (NbC) nyob rau hauv tshaj -Stabilized 347 Heater Sheath (0.70-0.90% Nb) Tswj lub qhov pib qhov chaw ntom ntom thiab perforation lub sij hawm hauv boiling 12% Hydrochloric Acid ntawm 108 degree
Tso lus
Lub Aggressive Acid Ib puag ncig rau ruaj khov Stainless hlau
In the case of 347 stainless steel (Nb-stabilized) sheathed electric heating tubes for aggressive hydrochloric acid service such as in pickling bath heaters for speciality metals, acid regeneration systems or chemical process heaters using HCl, excess niobium (beyond the stoichiometric requirement for carbon stabilisation) is present and results in the formation of primary MC carbides (NbC). These carbides are cathodic to the austenitic matrix and are preferred pitting start sites in boiling 12% HCl at 108°C. Chloride water pitting necessitates the presence of chlorides and oxygen . Hydrochloric acid actively corroded 347. Primary NbC particles increase localised assault. At 0.30-0.50% niobium levels (Nb:C ratio 6:1 to 10:1) primary MC volume percentage is 0.04-0.08%, pit density is modest (1-5 pits/cm2) and perforation duration is 100-200 hours. At over-stabilized values of 0.70-0.90 % Nb (Nb:C >15: 1), thawj MC ntim feem ntau nce mus rau 0.15-0.25%, qhov ntom ntom nce mus rau 20-50 pits / cm2 thiab lub sijhawm perforation qis dua mus rau 20-50 teev. The correlation between excess niobium, primary MC carbide volume fraction, pit density and perforation time in boiling 12% HCl is quantified in this paper.
Mechanism ntawm pitting tshwm sim los ntawm thawj MC carbide hauv HCl
Cov zaj duab xis passive ntawm 347 nyob rau hauv boiling hydrochloric acid yog tsis ruaj khov (HCl yog ib tug txo acid). Lub mechanism ntawm corrosion yog ib qho active dissolution nrog ib tug localized nres ntawm lub cathodic inclusions. Thawj NbC cov khoom yog qhov muaj txiaj ntsig zoo nrog kev hwm rau lub matrix thiab yog nquag ua haujlwm hauv hydrogen evolution cov tshuaj tiv thaiv (cathodic). Cov galvanic tam sim no ntawm NbC (cathode) thiab cov nyob sib ze matrix (anode) ua rau cov tawg sai sai ntawm txhua qhov carbide particle. Qhov loj dua tus thawj MC ntim % ntau dua qhov qhov ntom ntom thiab ntau qhov chaw cathodic. Lub siab aggressiveness ntawm HCl ua rau sai propagation ntawm pits los ntawm active dissolution tom qab lawv pib.
Kev sib raug zoo ntawm niobium cov ntsiab lus, MC ntim feem thiab pitting hauv boiling 12% HCl
The pit initiation densities and perforation periods have been determined for controlled immersion testing of 347 tubing with different niobium levels (0.05% C,<0.01% Ti, constant) in boiling 12% HCl (108°C, 12 wt% HCl, deaerated) for times up to 250 hours.
Niobium (wt%)Nb:C RatioPrimary MC Volume Fraction (%)MC Particle Size (µm) Pit Density tom qab 50 teev (pits/cm²) Qhov siab tshaj plaws qhov tob tom qab 50 teev (µm) Lub Sijhawm rau Perforation (1.5 hli phab ntsa, teev) Pom zoo rau Boiling 105% 12% HCl (Service) 2:1 ua<0.01 <1 0.5-1 50-100 80-150 No (sensitisation risk) 0.30 6:1 0.03-0.04 1-2 1-2 80-120 70-120 No 0.40 8:1 0.05-0.06 1-3 1-3 100-150 60-100 No 0.50 10:1 0.08-0.10 2-4 3-5 120-200 40-80 No (marginal) 0.60 12:1 0.12-0.15 3-5 8-15 150-250 30-50 No 0.70 14:1 0.15-0.18 4-6 15-25 200-350 20-35 No 0.80 16:1 0.18-0.22 4-8 25-40 250-450 15-25 No 0.90 18:1 0.22-0.28 5-10 40-60 300-500 10-20 No 1.00 20:1 0.25-0.30 6-12 50-80 350-600 8-15 No General Corrosion Rate Comparison (Without Pitting Acceleration)
347 muaj tus nqi corrosion tag nrho, tsis muaj pitting, hauv boiling 12% HCl txog li 2 mus rau 5 hli ib xyoo (150 mus rau 250 teev kom txog thaum perforation). Perforation yog nrawm los ntawm qhov tseem ceeb ntawm 2-5X nrog lub xub ntiag ntawm thawj MC carbides (txo lub sijhawm rau 20-80 teev).
Cov Txheej Txheem General Corrosion Rate (mm/yr) AlloyTime to Perforation (1.5 mm) Dominant Failure Mode 347, low Nb (0.30%)3-4 120-200 General + some pitting 347, high Nb (0.80%) 5-8 60-120Comparison Accelerated pitting 316L 10-20 30-60 General corrosion
Alloy 20 (Piv txwv) 0.5-1 500-1,500 Passive Titanium < 0.1 > 10,000 Passive
Cov lus pom zoo rau Boiling HCl S
08 degree, 347 tsis qhia rau cov ntsiab lus niobium. Txawm li cas los xij, yog tias 347 yuav tsum tau siv (piv txwv li, rau qhov kub thiab txias ntawm lwm qhov hauv lub cev), cov kev txwv niobium hauv qab no txo qis pitting acceleration.
Lub Sijhawm Kev Pabcuam Xav Tau (hs) Max Nb (wt%) Qhov siab tshaj plaws Thawj MC Volume Fraction (%) Kev Pom Zoo rau Alternative Alloy
50 0.50 0.10 Tsis muaj (tshwj tsis yog ua tsis tiav)
100 0.40 0.06 317L or Alloy 20 suggested 200 Not possible N/A Alloy 20 or Alloy 825 >500 Tsis Muaj N / A Titanium lossis Alloy C-276
Xaus: 347 hauv Boiling HCl Service
Rau 347 stainless hlau rhaub sheaths hauv boiling 12% HCl ntawm 108 degree, thawj MC carbides (NbC) los ntawm niobium concentrations tsawg li 0.40- 0.50% ua raws li pitting pib qhov chaw, uas luv lub sij hawm perforation ntawm 150-250 teev (ntau-80 teev) corrosion. Nyob rau theem siab dua ntawm cov niobium stabilized (0.70-0.90%) lub qhov ntom ntom ntawm 20-50 pits / cm 2 ntxiv luv lub sij hawm rau perforation mus rau 15-35 teev . 347 yuav tsum tsis txhob siv rau engineers specifications heaters rau boiling hydrochloric acid cov kev pab cuam. Kev pabcuam xav tau kev tiv thaiv rau HCl ntawm qhov kub siab yuav tsum tau Alloy 20 (Carpenter 20), Alloy 825 (Incoloy 825) lossis titanium. Qhov kev ua haujlwm no muab lub ntsiab lus rau kev zam 347 hauv hnyav txo cov kua qaub los ntawm kev sib raug zoo ntawm cov niobium ntau dhau thiab thawj MC carbide ntim feem thiab ceev pitting hauv boiling 12% HCl.
