electrochemical noise intro

7/27/2019 Electrochemical Noise Intro

1/23

Electrochemical Noise


2/23

Electrochemical Noise Measurement

-0.3644

-0.3642

-0.364

-0.3638-0.3636

-0.3634

-0.3632

-0.363

-0.3628

P

o

te

n

tia

l

(V

/S

0

5E-08

1E-07

1.5E-07

2E-07

2.5E-07

C

u

rre

n

t

(A

)

0 400 800 1200Time (secs)

Potential

Current

Steel in 0.05M Ca(OH)2 + 0.1M NaCl


3/23

Potential Noise

Measured as potential difference between twonominally identical electrodes, or relative to areference electrode (usually assumed to be

noiseless).


4/23

Measurement Methods

1 - Potential Noise

V

Reference

ElectrodeWorking

Electrode


5/23

Terminology - Potential Noise

Instantaneous noise potential,

Potential noise power (= variance of signal)

Potential noise standard deviation

E E En2

2

E E En

E E En22


6/23

Potential Noise

First shown to be related to corrosion rate byIverson in 1968.

Recent theoretical work suggests that the potential

noise is larger for less frequent, and hence morelocalized events, such as metastable pittingcorrosion


7/23

Current Noise

Usually measured as galvanic current flowingbetween two nominally identical electrodes,although potentiostatic measurements can be

made.


8/23

Measurement Methods2 - Current Noise

A

Working

Electrode 2Working

Electrode 1

Zero Resistance

Ammeter


9/23

Terminology - Current Noise

Instantaneous noise current,

Current noise power (= variance of signal)

Current noise standard deviation

I I In

2 2 ( )

I I In

I I In22


10/23

Current Noise

Standard deviation of current is usually found toincrease as corrosion rate increases.


11/23

Noise Resistance

Obtained by dividing standard deviation of potentialnoise by standard deviation of current noise,measured at the same time, to produce Rn.


12/23

Measurement Methods3 - Noise Resistance

A

Working

Electrode 2

Working

Electrode 1

Zero Resistance

Ammeter

V

Reference

Electrode


13/23

Noise Resistance

Rn is expected to be equivalent to the linearpolarization resistance, Rp.

Hence often gives good quantitative estimate of

corrosion rate using the Stern-Geary equation.There are questions about the effective

measurement frequency.


14/23

Sources of Noise

Active dissolution (or other electrochemicalreaction)

Pit nucleation, propagation and death

Hydrogen bubble evolution

Mass transport fluctuations

Mains frequency interference

Electronic noise

Vibration


15/23

Crevice Corrosion

-0.3644

-0.3642

-0.364

-0.3638

-0.3636

-0.3634

-0.3632

-0.363

-0.3628

P

o

ten

tia

l

(V

/S

0

5E-08

1E-07

1.5E-07

2E-07

2.5E-07

C

ur

re

n

t

(A

)

0 400 800 1200Time (secs)

Potential

Current

Mild steel in 0.05 M Ca(OH)2 + 0.1 M NaCl


16/23

Pitting Corrosion

-0.276

-0.2755

-0.275

-0.2745

-0.274

-0.2735

-0.273

-0.2725

-0.272

P

o

ten

tia

l

(V

/S

C

3E-08

4E-08

5E-08

6E-08

7E-08

8E-08

9E-08

C

ur

re

n

t

(A

)

0 200 400 600 800 1000 1200Time (secs)

Potential

Current

mild steel in 0.05 M Ca(OH)2 + 0.025 M NaCl


17/23

Correlation of E and I

-0.3643

-0.3642

-0.3641-0.364

-0.3639

-0.3638

-0.3637

-0.3636

-0.3635

P

otential(V

/S

C

E

)

5E-08

1E-07

1.5E-07

2E-07

2.5E-07

Cu

rrent(A

)

0 10 20 30 40 50 60

Time (s)

Vol tage Current

-0.276

-0.2755

-0.275

-0.2745

-0.274

-0.2735

P

otential(V

/S

C

E

)

3E-08

4E-08

5E-08

6E-08

7E-08

8E-08

Cu

rrent(A

)

100 110 120 130 140 150 160

Time (s)

Potential Current


18/23

Bidirectional Transients in Pitting

-0.388

-0.386

-0.384

-0.382

-0.38

-0.378

P

o

te

n

tia

l

(V

/S

2E-07

3E-07

4E-07

5E-07

6E-07

7E-07

8E-07

C

u

rr

e

n

t

(A

)

0 200 400 600 800 1000 1200Time (secs)

Potential

Current


19/23

Pitting Corrosion

-0.04

-0.035

-0.03

-0.025

-0.02

-0.015

P

o

te

n

ti

a

l

(V

/S

-1.8E-06

-1.6E-06

-1.4E-06

-1.2E-06

-1E-06

C

u

rre

n

t

(A

)

0 200 400 600 800 1000 1200

Time (s)

Voltage

Current

Mild steel in 0.01 M NaNO2 + 0.05 M NaCl


20/23


21/23


22/23

Shot Noise Analysis

Noise results from random variation in number ofpulses arriving within each sample interval

Current noise has constant power per unit of

bandwidth (white noise)

tmeasuremenofbandwidth=current=

pulseincharge=where

2= 2

bI

q

qIbIn


23/23

Shot Noise Analysis

We can estimate the charge in each event from

Similarly we can estimate the frequency of eventsfrom

B

b

qEI

=

2

2

==E

corr

nb

B

q

If

electrochemical noise intro

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