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

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

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

Project Milestone Status July to September 2011

High temperature electrochemical corrosion

rate (ECR) probe for lab scale corrosion

experiments has been designed and

constructed.

Developed High Temperature Corrosion

Sensor

Working electrode 1 & 2

Reference electrode

Counter electrode

1 cm

Ag/Ag+/Fused-Quartz

Reference Electrode

Stability

Reproducibility

Reusability

Fused quartz tube

Ag wire

Tungsten wire

Mixture of Ag2SO4 and Na2SO4

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)

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

Laboratory Test Configuration

Electrochemical

measurement system

High temperature corrosion

system

Synthesis flue gas

exposure system

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

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

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

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

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

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

Electrochemical Techniques

Used to Study Corrosion

Open Circuit Potential (OCP) Measurement

Linear Polarization Resistance (LPR)

Electrochemical Impedance Spectroscopy (EIS)

Electrochemical Noise Analysis (ENA)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Corrosion Products in Coal Ash

from Deep Molten Coal Ash Corrosion of

IN740-1 Alloy at 800 C

1 mm

10 m

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

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

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.

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

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

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

Milestone Status Report

Milestone Status Report

Milestone Status Report

Thank You