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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.