1-fac-what is[1]
TRANSCRIPT
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FAC PRESENTATION
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PIPE THINNING
Corrosion
Erosion Erosion corrosion
Flow assisted
corrosion
Cavitations
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Factors necessary for corrosion
Requirements for corrosion Anode & cathode Electrolyte & electron flow
Anode Cathode
Electrolyte H2O ,4 OH-
O2
2Fe
2Fe+2
4e-
Anodic dissolution:
Fe Fe+2
+ 2e-
Cathodic reduction2 e- + 1/2O2 + H2O 2OH
-
OH-OH
-
or oxideCorrosion products
Metal ions dissolves
+e-
Anode
Typical anodic reactions:
Fe ( Metal) = Fe++
+2e -
Fe+2
+ 2OH-
= Fe(OH)2
4Fe(OH)2 + O2 +2H2O =4Fe(OH)3
Typical cathodic reactions:
1/2 O2 + H2O + 2e- = 2OH
-
Fe
+3
+ e- = Fe
+2
2H+
+ 2e- = H2
Cathod
e -
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Erosion-Corrosion : EPRI defines
this as:
Degradation of material caused by bothmechanical and chemical processes.
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Flow Accelerated (or Assisted)
Corrosion: EPRI defines this as:
A process whereby the normally protective oxidelayer on carbon or low ally steel dissolves into astream of flowing water or a water steammixture.
It can occur in single phase and in two phaseregions. EPRI has stated that the cause of FACis water chemistry.
Identified by TIGER STRIP or CHEVRONS
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FAC
DEFINITION :
FLOW ASSISTED CORROSION IS A
DEGRADATION PHENOMENA THATATTACKS PIPING AND VESSELS
EXPOSED TO MOVING WATER OR
STEAM RESULTING IN THE LOSS OFMATERIAL AND WALL THINNING.
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Factors affecting Flow Assisted Corrosion
Sl.No. Parameters FAC rate
01 Low velocity Lower
02 Low pH Higher
03 Low dissolvedoxygen
Higher
04 Low temperature Higher (80-230oC),Maximum (130-150oC)
05 Geometry Turbulence
06 Low chromium Higher
07 Low molybdenum Higher
08 Low copper Higher
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FAC is the chemical dissolution of surface oxide and
metal, accelerated by flow and flow impingement.
Flow Assisted Corrosion (FAC):
Diagram of the interaction of key controllable factors thatjointly lead to flow-assisted corrosion
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EnvironmentEnvironment Water Coolant (Single phase or Two Phase)Water Coolant (Single phase or Two Phase) TemperatureTemperature pHpH Oxygen ContentOxygen Content
HydrodynamicsHydrodynamics
VelocityVelocityTurbulenceTurbulenceFlow impingement such as at bendsFlow impingement such as at bendsFlow discontinuities such as at bends and teesFlow discontinuities such as at bends and teesTurbulence promoters such as irregular weld or changesTurbulence promoters such as irregular weld or changes
in pipe diameterin pipe diameter
MaterialMaterialCarbon Steels containing some amount of Cr, Cu and MoCarbon Steels containing some amount of Cr, Cu and Mo
reduce FAC ratereduce FAC rate
Factors Affecting FACFactors Affecting FAC
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Corrosion as a function of velocity
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Schematic representation of the mechanism of flow-assisted corrosion of steel.
Mechanism of flow assisted corrosion of steel
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Simplified mechanism for FACSimplified mechanism for FAC
Start with water
at surface of
base metal
Reaction betweendissolved oxygen
and metal surface
forms a protective
oxide layer
High flowvelocity
removes oxide
and carries it
downstream
New oxidelayer forms on
exposed
patches of bare
metal
Continualremoval and
reforming of
oxide results in
wall thinning
Flow
1 2 3 4 5
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FAC induced Scallops
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Fluid motion in the vicinity of a Scallop
At point 1 the main flow separates in the form of a jet above the recirculating flow
At point 2 the jet flow becomes irregular and turbulent
At point 3 it causes mixing between the jet fluid and and the fluid in the lee eddy
At point 4 the jet flow impinges most directly on the surface where the rate ofdissolution is the highest.
How Scalloped Surface is Formed ?
FAC induces Scalloped or Orange Peel surface appearance.
It is formed due to surface irregularity:
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Effect of temperature on Flow Assisted
Corrosion
Single phase FAC damage: within 80-
230oC.
Maximum: ~130-150oC (within O2 free
environment.
With increasing temperature
Fe+2 concentration at the water -oxide interface decreases linearly.
Fe+2 diffusivity into the coolant increase,resulting in a mass transfer
coefficient that increase almost linearly.
The resulting FAC rate variation with temperature is a bell shaped
curve.
Effect of temperature on FAC
Temperature (oC)
38 65 93 122 150 178 205 232 260 288
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Effect of temperature on Flow Assisted Corrosion
Both the pH at temperature and the
level of dissolved oxygen in the
stream influence the stability and
solubility of the magnetite oxide
layer.
The solubility increases withincreasing temperature and
decreasing pH.
The solubility of Fe(OH)2 rises with
increasing temperature to amaximum at around 150oC, then
decreases with a steep drop to the
solubility of magnetite between 200-
250oC.
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Effect of pH on Flow Assisted Corrosion
US NRC Information Notice No. 86-106
FAC wear rate:High between pH 7.0
and 9.0 or below pH 5.0.
FAC rates: Drop sharply at pH > 9.2. At Surry-2: pH levels were between pH
8.8 and 9.2.
Korean literature,
Corrosion rate decreases In the pH range from 8.0 to 9.5.
Corrosion rate increases in the pH range from 9.5 to 10.0.
Effect of pH on FAC
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Effect of oxygen on Flow Assisted Corrosion
Oxidising conditions (10-50 g/l O2)
Makes the OPR positive, Favours formation of FeOOH, and
Restricts diffusion of oxidant (O2/H2O)
Overall effect: Reduction in FAC rate.
Haematite more stable than magnetite.
Haematite is less soluble than magnetite.
FAC rates come down when dissolved oxygen is increased over this
threshold values.
Concept is called as Oxygenated Water Chemistry (OWC).
Effect of dissolved O2 on FAC
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Effect of flow on Flow Assisted Corrosion
Increase in flow velocity: Increases the FAC rate.
Local flow velocities: Can be 2-3 times higher than bulk velocities.
Areas prone to FAC: Down stream of restricting orifices, flow control
valves, reducers, and in elbow and tees.
FAC can occur in both low and high velocity area where there is achange in flow pattern (laminar to turbulence).
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Effect of flow on Flow Assisted Corrosion
At low velocity: FAC rate controlled by the rate of mass transfer. At higher velocity: FAC rate controlled by chemical reactions at the
metal-oxide and oxide-water interfaces. Above critical velocity: Metal will be removed by mechanical
processes.
Turbulent
flow
regime
Laminar
flow
regime
Vc Flow Velocity
Erosion
co
rrosion
ra
te
Effect of velocity on FAC
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Effect of alloy on Flow Assisted Corrosion
As per US NRC Information Notice No. 86-106
Carbon steel: More vulnerable to erosion/corrosion under certainunfavorable conditions.
Resistance to erosion corrosion can be increased significantly by
increasing the alloy element.
2 Cr-1 Mo steel improves piping resistance to erosion corrosion bya factor of four.
Surry 2 failed pipe elbow: Unusually low amounts of these elements,
particularly chromium (less than 0.02%).
Chemical composition (% by weight)
(ASTM) C Mn P S Si Cr Mo Fe
A106 Gr.B 0.30 0.29~1.06 0.048 0.058 0.1 Balance
A336 P11 0.15 0.3~0.6 0.03 0.03 0.5~1.0 1.0~1.5 0.44~0.65 Balance
A335 P22 0.15 0.3~0.6 0.03 0.03 0.5 1.9~2.6 0.87~1.13 Balance
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2.25% Cr & 1% Mo Material
Properties similar to Carbon Steel. No significant
Design Change required. No separate design analysis required.
Solubility of FeCr2O4 is less compared to Fe3O4.
Disadvantage Require special welding, preheat& post weld heat Treatment.