A magnetite layer forms on metal surfaces in contact with water or steam in steam generators. This layer protects the base metal from corrosion. However, under certain conditions like intermittent loading or system draining and refilling after outages, the protective magnetite layer can break down and dissolve in flowing water. This leads to flow accelerated corrosion that reduces the protective oxide layer and removes base metal material. Suggested actions to address this include reducing the dose of oxygen scavenger added to boiler feedwater to avoid thick magnetite buildup, analyzing deposit samples, and properly flushing and refilling HRSG systems after outages.

2. 2 /
GE /
April 3, 2015
Magnetite Layer
• A protected layer of magnetite (Fe3O4) is formed on water touched
metal surfaces in the steam generator
• Magnetite layer acts as a coherent media that stops further
corrosion of the base metal
• Magnetite is the combination of FeO and Fe2O3
• Oxygen scavenger, Carbohydrazide, converted to Hydrazine at
temperature >135 degree C, Hydrazine passivate oxidized areas
of piping & tube material by the following reaction.
N2H4 + O2 = N2 + 2H2O
6Fe2O3 + N2H4 = 4Fe3O4 + N2 + 2H2O
. 4Fe3O4 + O2 = 6Fe2O3
Formation of Magnetite Layer

3. 3 /
GE /
April 3, 2015
Flow accelerated corrosion
• Flow-accelerated corrosion is essentially a process whereby the
normally protective magnetite (Fe3O4) layer on carbon steel is
dissolved in a stream of flowing water and wet steam.
• This phenomenon reduces the protective oxide layer (magnetite)
that may cause the removal of the base steel material.
Causes of flow accelerated corrosion
• The magnetite layer may be thicker by using the oxygen
scavenger for a long period of time and is dissolved in stream of
water/steam during intermittent loading conditions .
• During long shut down and outage when we drain, open and cool
down the HRSG systems, the magnetite layer breaks due to
environmental changes.
• When refill the system with water break magnetite layer which is
in powder form is mixed with water.

4. 4 /
GE /
April 3, 2015
HP2 Drum
18/2/2015
BFWTank
18/2/2015

5. Action taken and suggestion to avoid the FAC problem
• The dose of Oxygen scavenger/metal passivater
(carbohydrazide) in boiler feed water has been reduced from 100
ppb to 50 ppb to avoid the thick magnetite layer
• Sending deposit sample collected from boiler feed water pumps
strainers for analysis to GE water technologies lab Belgium.
• The HRSG system should be drain and refill after long shut down
and after the outage flush with demin water before refilling.

6. 6 /
GE /
April 3, 2015
Thanks

7. 6 /
GE /
April 3, 2015
Thanks