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Flow Accelerated Corrosion (FAC) (Environmental Conditions (Temperature…
Flow Accelerated Corrosion (FAC)
Corrosion mechanism in which a normally protective oxide layer on a metal surface dissolves in a fast-flowing water. The underlying metal corrodes to re-create the oxide, and thus the metal loss continues
Involve dissolution of normally poorly soluble oxide by combines electrochemical, water chemistry and mass transfer phenomena
Mechanism of FAC
Corrosion Process
Production of soluble Fe2+ and their accumulation at the oxide water interface
Mass Transfer Process
Flowing water remove soluble ferrous ions by a convective mass transfer mechanism
Causes wall thinning of piping, vessels, and components
Contributory Factors
Water chemistry (PH, hyrazine)
Temperature
Velocity
Oxygen Content
Alloy composition
Environmental Conditions
Temperature
Threshold temperature 212 - 482 F (100-250C)
pH
Rapid reduction in FAC as pH is increased from 9.0 to 9.5
Dissolved Oxygen
Corrosion rates high at low levels of dissolved oxygen
Velocity
Metal loss increase with velocity and turbulence
Material ( Cr, Mo Cu)
Water must present, FAC does not occur in dry stream
Area most often affected
Components
Feedwater Control Valve By Pass Line
Feedwater Heater Tube Sheet
Feedwater Heater Tubes- Inlet Ends
Feedwater Piping (Bends, Valves, Tees)
Feedwater Heaters
Pumps
Cross Under Piping
System
Condensate
Feedwater
Blowdown
Deaerator
Accident = Mihama-3 NPP
accident occurred in building housing turbines for Mihama 3 reactor
Hot water and steam leaking from a broken pipe killed 5 workers and six injured