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Naing Naing Aung, Xingbo Liu
03-12-2012
Development of Self-Powered
Wireless-Ready High Temperature
Electrochemical Sensors
for In-Situ Corrosion Monitoring
of Boiler Tubes
WestVirginiaUniversity College of Engineering and Mineral Resources
DoE Award No. DE- FE0005717
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Project Objectives
To develop in-situ corrosion monitoring sensors
for corrosion of USC boiler tubes in next
generation coal-based power systems
To develop thermal-electric based energy
harvesting and telecommunication devices for
the self-powered wireless ready sensor system
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Current Milestones
July to September 2011 Initiate preliminary high-temperature electrochemical
corrosion rate (ECR) probe design
October to December 2011
To complete the design and construction of (ECR)
probe for lab scale corrosion experiments and to
complete laboratory test configuration
January to March 2012
To demonstrate the results of the corrosion tests as a
function of exposure time, temperature and various
simulated boiler exposure environments in lab-scale
setting
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Project Milestone Status July to September 2011
High temperature electrochemical corrosion
rate (ECR) probe for lab scale corrosion
experiments has been designed and
constructed.
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Developed High Temperature Corrosion
Sensor
Working electrode 1 & 2
Reference electrode
Counter electrode
1 cm
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Ag/Ag+/Fused-Quartz
Reference Electrode
Stability
Reproducibility
Reusability
Fused quartz tube
Ag wire
Tungsten wire
Mixture of Ag2SO4 and Na2SO4
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Time Dependency of the Potential of
Ag/Ag+/fused-quartz Reference Electrode in
Synthesis Coal Ash Mixture at 800 °C
0 100000 200000 300000 400000 5000000.00
0.05
0.10
0.15
0.20
0.25
Po
ten
tia
l (m
v)
Time (sec)
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Construction of custom-designed coal ash
exposure unit for lab-scale corrosion
experiments has been completed in WVU.
Project Milestone Status October to December 2011
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Laboratory Test Configuration
Electrochemical
measurement system
High temperature corrosion
system
Synthesis flue gas
exposure system
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Coal ash corrosion behaviour of nickel- based
Superalloy IN740-1 in synthetic coal ash
mixture at 800 C as a function of exposure
time
Project Milestone Status January to March 2012
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Corrosion in Coal-Fired Boilers
Low-NOX combustion produces H2S in the flue gas and
FeS in the deposit due to incomplete combustion of the
sulfur-bearing species in coal
Furnace Wall Corrosion on Waterwalls of the Boiler Tubes
Higher Steam Temperature and Pressure
Deposit-induced Liquid Phase Corrosion
Coal Ash Corrosion in Superheater/Reheater alloys
In the Upper Furnace
In the Lower Furnace
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Coal Ash Corrosion Mechanism
Ash
Oxide Scale
Metal
Ash deposition
Molten Salt
Formation of molten alkali iron sulfates
(Na, K)3Fe(SO
4)3
and fluxing away of protective oxide film Direct reaction between
bare metal and reduced
sulphate species
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IN 740-1 Ni-Base Superalloy
Ni Cr Fe Cu Co Mo Nb Al Mn Ta Ti W Si C Sn
42 35 30 17 6 5 5 5 5 5 1 0
20 m
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Intermetallic Phases in
IN 740-1 Ni-Base Superalloy
0 20 40 60 800
4000
8000
12000
16000
Ni3(AlTi)
Ni3(AlTi)
(Ti,Nb) C
(Ti,Nb) C, Ni3Ti
Ni3(AlTi), Ni
3Nb
Ni, Ni3(AlTi), Ni
3Ti
Ni, Ni3(AlTi), Ni
3Nb, (Ti,Nb) C
Ni, Ni3(AlTi), Ni
3Nb, Ni
3Ti
Inte
nsity
Angle (2 Theta)
0 500 1000 1500 2000
0
1000
2000
3000
4000
5000
6000
Ni3 (Ti,Nb)
Ni3Ti
(Ti,Nb)C
C
Matrix
Ni
Nb Ti
Ti
Ni Cr
Co
Cr
Co
un
ts (
Arb
ita
ry U
nit)
Energy (keV)
Ni
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Corrosive Media
For Coal Ash Corrosion
15 CO2 + 4 O2 + 1SO2 + 80 N2
Ash+10% Alkali Sulfates+1% NaCl mp=800 C
Ash - SiO2, Al2O3, and Fe2O3 in the ratio of 1:1:1 by weight mp=1600 C mp=2027 C mp=1566 C
Alkali sulfate mixture - Na2SO4 and K2SO4 in the ratio of 1:1
by weight mp= 880 C mp= 1067 C
Synthetic Flue Gas
Coal Ash Mixture
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Electrochemical Techniques
Used to Study Corrosion
Open Circuit Potential (OCP) Measurement
Linear Polarization Resistance (LPR)
Electrochemical Impedance Spectroscopy (EIS)
Electrochemical Noise Analysis (ENA)
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OCP for Deep Molten Coal Ash Corrosion
of IN740-1 Alloy at 800 C
OCP = -497 mV vs. Ag/Ag+ OCP = - 274 mV vs. Ag/Ag+
Without Synthetic Flue Gas With Synthetic Flue Gas
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Ecorr and Icorr Values for Deep Molten Coal
Ash Corrosion of IN740-1 Alloy at 800 C
OCP
(mV vs. Ag/Ag+)
Ecorr
(mV vs. Ag/Ag+)
icorr
(μA cm-2) a
mV dec-1
c
mV dec-1
Corrosion rate
(mm y-1)
-538 -592 4.71 198 217 0.11
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Electrochemical Impedance for Deep Molten
Coal Ash Corrosion of IN740-1 Alloy at 800 C
300 350 400 450 500 5500
20
40
60
80
Measurement
Simulation
Zim
(o
hm
cm
2)
Zre (ohm cm
2)
After 2 h
The transport of the oxygen to the alloy surface and the formation of a oxide scale
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220 240 260 280 3000
10
20
30
40
50
Measurement
Simulation
Zim
(o
hm
cm
2)
Zre (ohm cm
2)
500 600 700 8000
50
100
150
Measurement
Simulation
Zim
(o
hm
cm
2)
Zre (ohm cm
2)
700 800 900 1000 1100 1200 1300 14000
100
200
300
400
Measurement
Simulation
Zim
(o
hm
cm
2)
Zre (ohm cm
2)
After 24 h After 48 h After 72 h
Forming a porous scale in molten salts and the corrosion of the alloy is
controlled by diffusion of the oxidant in the melt
( )Z A i
Electrochemical Impedance for Deep Molten
Coal Ash Corrosion of IN740-1 Alloy at 800 C
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2000 2500 3000 3500 4000 4500 5000 55000
500
1000
1500
2000
Measurement
Simulation
Zim
(o
hm
cm
2)
Zre (ohm cm
2)
After 168 h
The corrosion of the alloy is controlled by diffusion of the ions through the scale
after forming a compact scale
Electrochemical Impedance for Deep Molten
Coal Ash Corrosion of IN740-1 Alloy at 800 C
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10-1
100
101
102
103
104
105
106
107
0
1000
2000
3000
4000
5000
6000
7000
2 h
24 h
48 h
72 h
168 h
Z (
oh
m c
m2)
Frequency (Hz)
10-1
100
101
102
103
104
105
106
107
0
5
10
15
20
25
30
35
40
2 h
24 h
48 h
72 h
168 h
Ph
ase
(D
eg
)
Frequency (Hz)
Bode Plots for Deep Molten Coal Ash
Corrosion of IN740-1 Alloy at 800 C
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Typical Potential Noise Signatures from Deep Molten
Coal Ash Corrosion of IN740-1 Alloy at 800 C
After 24 h
After 168 h
After 72 h
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Typical Current Noise Signatures from Deep Molten
Coal Ash Corrosion of IN740-1 Alloy at 800 C
After 24 h After 72 h
After 168 h
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Typical Noise Impedance from Deep Molten Coal Ash
Corrosion of IN740-1 Alloy at 800 C
After 72 h After 24 h
After 168 h
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Exposure Time
(h)
Noise Resistance (Rn)
(k)
Pit initiation
24
1.46
Pit propagation 72 30.13
Stable pit formation 168 112.183
Noise Resistance for Deep Molten Coal
Ash Corrosion of IN740-1 Alloy at 800 C
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Pitting Index for Deep Molten Coal Ash
Corrosion of IN740-1 Alloy at 800 C
PI = 0 , the individual current xi show only small deviations from the
mean value of current
PI = 1 , xi >> than the mean value of current
0 20 40 60 80 100 120 140 160 180
0.01
0.1
1
Uniform Corrosion
Mixed Corrosion
Pittin
g in
de
x
Time (h)
Pitting Corrosion
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Stern-Geary Linear Approximation
)())( ( 2.303
corrcorrca
ca
applied
npi
B
ii
ERR
Rp = Resistance obtained from the LPR and EIS techniques
Rn = Resistance obtained from the EN
B = Stern-Geary constant
a = Anodic Tafel constant
c = Cathodic Tafel constant
Icorr = Corrosion current density
n = Number of electrons freed by the corrosion reaction
M = Atomic mass
Corrosion rate (mm y1) = n
M28.3icorr
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Weight Loss Measurement
Wb = Weight of test sample before test, g
Wa = Weight of test sample after test, g
B = Weight loss of blank, g (the average weight loss from 3 unused and
clean sample was used as the blank correction)
A = Surface area of sample, cm2
t = Exposure time, day
= Density of alloy
274.0
1Corrosion rate (mm y-1) =
tA
BWW ab 1000)(
For thin coal ash film corrosion
For deep molten coal ash corrosion
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Weight Loss Rates for Deep Molten Coal
Ash Corrosion and Thin Coal Ash film
Corrosion of IN740-1 Alloy at 800 C
0.00
0.05
0.10
0.15
0.20
Co
rro
sio
n r
ate
(m
m y
-1)
672 h504 h336 h
Exposure time (h)
Thin coal ash film corrosion
Deep molten coal ash corrosion
168 h
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Corroded Surfaces for Deep Molten Coal Ash
Corrosion of IN740-1 Alloy at 800 C
After 1 weeks After 2 weeks After 3 weeks After 4 weeks
1 mm 1 mm 1 mm 1 mm
Without Synthetic Flue Gas
1 mm
Before Corrosion
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Oxide Layer Formation
on Corroded Surfaces for Deep Molten Coal
Ash Corrosion of IN740-1 Alloy at 800 C
100 m
1 m 1 m
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Oxides Formation
on Corroded Surfaces for Deep Molten Coal
Ash Corrosion of IN740-1 Alloy at 800 C
30 40 50 60 70
0
100
200
300
400
500
Co3O
4
Ni2Cr
2O
4
Ni2Cr
2O
4 Cr2O
3
Cr2O
3
Cr2O
3
Angle (2 Theta)
After 4 weeks
After 3 weeks
After 2 weeks
Inte
nsity
Before corrosion
After 1 week
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Corroded Surfaces for Deep Molten Coal Ash
Corrosion of IN740-1 Alloy at 800 C
After 1 weeks After 2 weeks
With Synthetic Flue Gas
1 mm 1 mm
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Molten Alkali Iron Sulfate Formation
During Deep Molten Coal Ash Corrosion of
IN740-1 Alloy at 800 C
30 40 50
0
1000
2000
3000
4000
5000
After corrosion
for 336 h
NaFe(SO4)
2
NaFe(SO4)
2
Ni, Ni3(AlTi)
Ni, Ni3(AlTi)
Ni, Ni3(AlTi)
Before corrosion
Angle (2 Theta)
Inte
nsity
NaFe(SO4)
2
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Pits Formation
on Corroded Surfaces for Deep Molten Coal
Ash Corrosion of IN740-1 Alloy at 800 C
Without Synthetic Flue Gas
After 1 week After 2 weeks
10 m
With Synthetic Flue Gas
1 m
1 m 10 m
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Corrosion Products in Coal Ash
from Deep Molten Coal Ash Corrosion of
IN740-1 Alloy at 800 C
1 mm
10 m
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1 mm 1 mm 1 mm
After 2 weeks After 3 weeks After 4 weeks
Corrosion Products in Coal Ash
from Thin Coal Ash Film Corrosion of
IN740-1 Alloy at 800 C
1 m 0 5 10 15 20
0
50
100
150
200
250
300
350
400
450
Globular phase
O
Si
O
Cr
Cr
Co
un
ts (
Arb
ita
ry U
nit)
Energy (keV)
Al
Needle phase
1 m
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Coal Ash Corrosion Mechanism
Initiation Stage
The transport of the oxygen to the alloy surface and the formation of an oxide scale
Propagation Stage
Formation of porous scale in molten salts and corrosion is
controlled by soluble diffusion of the oxidant in the melt
Stabilization Stage
Corrosion is controlled by diffusion of the ions through the
scale after forming a compact scale
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Conclusions
The preliminary results suggest that the developed high
temperature corrosion sensor allows accurate analysis of
the sample material via several electrochemical techniques.
Electrochemical and weight loss measurements show that
corrosion of IN740-1 alloy in synthesis coal ash mixture at
800 °C was due to localized or pitting corrosion behavior.
Three different stages of deep molten coal ash corrosion of
IN740-1 alloy in synthesis coal ash mixture at 800 °C have
been proposed.
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Commercially available hardware
• 1 Watt transmitter - frequency-hopping, spread spectrum technology in the 902-928 MHz ISM band
• Preliminary testing of the thermoelectric generator (TEG) shows a 5 Watt output potential when contacting a surface temperature in the 300° C to 350° C range
Self-Powered Wireless Communication
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Next Steps:
•Profile TEG power vs. temperature range
•Demonstrate wireless transmission capability with simulated signal
•Finalize specifications of signal converter for corrosion sensor input
•Complete lab scale demonstration
Completed:
•Purchased off-the-shelf TEG and wireless transmitter & receiver
•Demonstrated bench scale feasibility of TEG
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Future Work
Validate results with real-time USC boiler
systems
Extend test results to develop corrosion
model for USC boiler systems
Make alterations to sensor design to include
power source and transmitter
Test sensor reliability and sensitivity for in
situ applications
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Milestone Status Report
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Milestone Status Report
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Milestone Status Report
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