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MODERN METHODS OF MONITORING BIOCORROSION 1

ELECTROCHEMICALELECTROCHEMICALMONITORINGMONITORING

P. CRISTIANI, CESI S.p.A, Italycristiani@cesi.it

BIOCOR - European Summer School on Biologically influenced Corrosion, Portsmouth, 7 - 14 July 2002

BIOFOULING IS A SEVERE PROBLEM IN POWER PLANTSBIOFOULING IS A SEVERE PROBLEM IN POWER PLANTSBIOFOULING IS A SEVERE PROBLEM IN POWER PLANTS

Roughly 50% of the damages on condenser tubes might be prevented improving the cleaning during the plant operation.

Utilities spend millions dollars per year for:

- the reduction of condenser performancescaused by fouling

- the prevention of biofouling and microbial corrosion.

CESI

MIC IN POWER PLANTSMIC IN POWER PLANTS

The most significant phenomena are present in:

? condenser tubes? cooling water intake tunnels and culverts? close loops? cooling towers? emergency (safety) service water systems? service water systems

( stagnant water flow, for short periods also)

CESI

Microbial activity is present in a large part of corrosion phenomena in industrial equipments using natural waters ...

Biofilm grows practically on all kind of materials...

On copper alloys of condenser tubes of seawater cooling circuits in a power plant,for example...

CESI

Condenser tubes - seawater cooling

CESI

CESI

WATER FLOW

C W R W C

TO CORROSION METER

CESI Electochemical Probe for sea water

CESI

Characteristics of CESI system for LPR measurements in sea water:

Galvanostatic configuration• Battery source for LPR measurements

(to avoid grounding problems and A.C. interference)• Computer control on-line Data Acquisition

Stern & Geary theory

Vcorr=B/(Rp*Area)

Rp+ Rs=? E/? I (8 mV ? ? E ? 10 mV)

(constant B was experimentally determined by W.L.)

If Conductivity is very high: Rp=? E/? I

Linear Polarisation Resistancemeasurements

CESI

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250

Vcor [? m/year]

Err

or

%2 ? S/cm

10 ? S/cm

50 ? S/cm

250 ? S/cm

100 ? S/cm

1000 ? S/cm

Error due to ohmic drop in the determination of corrosion rate (Vcor) by LPR technique.For low conductivities the cell constant must be measured in order to correct Vcor for ohmic drop contribution.

CESI

0

50

100

150

200

250

300

350

400

04/04 14/04 24/04 04/05 14/05 24/05 03/06

Vco

r [ ?

m/y

]

Al BrassCu70 Ni30

LPR TECHNIQUECondenser sea waterwith intermittent antifouling treatment

Al Brass CuNi

CESI

0

5

10

15

20

25

30

35

40

45

50

25 m

ag 0

0

26 m

ag 0

0

27 m

ag 0

0

28 m

ag 0

0

Vco

r [ ?

m/a

nn

o]

Al BrassCuNi

Details during intermittentantifouling treatment

CESI

0

20

40

60

80

100

120

140

160

15/03/00 04/05/00 23/06/00 12/08/00 01/10/00 20/11/00 09/01/01

Vco

r ( ?

m/y

) Al BrassCuNi

Al Brass CuNiCESI

Effect of filming amine treatment

0

50

100

150

200

250

300

15 s

et 0

0

17 s

et 0

0

19 s

et 0

0

21 s

et 0

0

23 s

et 0

0

25 s

et 0

0

27 s

et 0

0

Vco

r A

l Bra

ss ( ?

m/y

)

Al Brass

Treatmentwith filming amine

Probable tendency of Vcor without filming amine

CESI

Comparison between gravimetric and LPR(CESI data)

0

5

10

15

20

25

30

35

40

45

04 a

pr 0

0

24 m

ag 0

0

13 lu

g 00

01 s

et 0

0

21 o

tt 00

10 d

ic 0

0

Wei

gh

t L

oss

(m

g/c

m2)

Al Brass: Time Integral of VcorCuNi: Time Integral of VcorAl Brass W.L.CuNi W.L.

Corrosion-erosionof Al Brass CESI

OCP(Open Circuit

Potential)

whichreference

electrode ?

-90

-70

-50

-30

-10

10

30

07 a

go

27 a

go

16 s

et

06 o

tt

26 o

tt

15 n

ov

05 d

ic

25 d

ic

Po

ten

zial

e [m

V/A

g-A

gC

l]

SILVER

-960

-940

-920

-900

-880

-860

-840

-820

07 a

go

27 a

go

16 s

et

06 o

tt

26 o

tt

15 n

ov

05 d

ic

25 d

ic

Po

ten

zial

e [m

V/A

g-A

gC

l]ZINC

CESI

OCP measurements (vs. Ag-AgCl)

-250

-200

-150

-100

-50

009

apr

00

29 m

ag 0

0

18 lu

g 00

06 s

et 0

0

26 o

tt 00

15 d

ic 0

0

Po

ten

zial

e [m

V/A

g-A

gC

l]

CuNi

Al Brass

CESI

Effect of Taprogge balls

0

5

10

15

20

259

apr 0

0

10 a

pr 0

0

11 a

pr 0

0

12 a

pr 0

0

13 a

pr 0

0

14 a

pr 0

0

15 a

pr 0

0

16 a

pr 0

0

17 a

pr 0

0

18 a

pr 0

0

19 a

pr 0

0

Vco

r (

m/y

)

-250

-240

-230

-220

-210

-200

Eco

r (m

V/A

g-A

gC

l)

Al Brass

Ecor

Vcor

Probe cleaning by Taprogge balls

CESI

BIOCORROSION - Thematic NetworkTask 5 : Biofilm and Corrosion Monitoring

MIC ON-LINE MONITORING TECHNIQUES

From partner’s experience good performance by following traditional techniques:Weight lossesElectric Resistance (ER) Open Circuits Potential (OCP)Linear Polarisation Resistance (LPR)Electrochemical Impedance Spectroscopy (EIS)

http://www.corr-institute.se/english/Web_DT/Dom_index2.html

BIOCORROSION - Thematic NetworkTask 5 : Biofilm and Corrosion Monitoring

OPEN CIRCUITS POTENTIAL MEASUREMENT (OCP)

Necessary for stainless steel, useful for steel especiallycompared with corrosion rate, to follow the biological activityin aerobic-anaerobic conditions

Not very important information if copper alloys in aerobiccondition

Reference electrode:zinc and commercial Silver/Silver chloride electrode are OKin sea water.Copper/copper sulphate in soils is OK.

BIOCORROSION - Thematic NetworkTask 5 : Biofilm and Corrosion Monitoring

Linear polarisation resistance (LPR)Direct current technique

• Good results for active-passive alloys (copper alloys) and carbon steel if aerobic conditions: localised corrosion has been detected and emphasized

• Contradicting results for stainless steel

• Bad results if anaerobic conditions and sulphide (5 ppm or more) on carbon steel due to formation of porous corrosion products affecting the response from the interface

The corrosion element is analysed as electrical circuits

Rs

R

C

0.00E+00

2.00E+03

4.00E+03

6.00E+03

0.0E+00 2.0E+03 4.0E+03 6.0E+03

Zreal (ohm.cm2)

-Zim

a (o

hm

.cm

2)

Rp+RsRs

I

E

Rs Rp

Rp=B/ikorr

Rp+Rs

LPR EIS

BIOCORROSION - Thematic NetworkTask 5 : Biofilm and Corrosion Monitoring

Electrochemical ImpedanceSpectroscopy (EIS)

Better than LPR if low conductivity and low sulphide.

The technique is not sensitive enough for detecting biofilm growth ofthickness less than about 200 micron (From CEA Experience)

Mainly applied in laboratory experiments, provides better information thanLPR, for mechanistic studies in particularly.

Data interpretation is not simple and requests specialist knowledgenevertheless, if the system characteristics are known, the analyses could besimplified.

For rough estimating corrosion rate the technique has shown useful with thesame limitations given for carbon steel in anaerobic sulphide media as givenconcerning LPR. High capacitances can occur for steel in sulphideenvironment leading to very long measurement time, in the range of hours(From DTU experience).

SRB gives two decades increased C and reduced

Rp (Tech. Univ. of Denmark data)

0.1

1

10

100

1000

10000

100000

0 7 14 21 28 35 42 49 56

Days

Cap

acit

ance

mF

/cm

2 , sul

phid

e m

g/l,

Res

ista

nce

Oh

m*c

m2

Interfacial capacitanceSulphidePolarisation resistance

SRB inoculated sea water

When is corrosion rate difficult to measure by electrochemical techniques ?

Difficult analysis of data if:

Thick or porous deposits

Other electrochemical reactions like in sulphide media

Low conductivity

Combined sulphide, slightly alkaline pH and biofilm

(Tech. Univ. of Denmark data)

Microbially influenced corrosion: localised corrosion

Dense protective film

Porous film of biofilm and corrosion products

All known techniques give an average corrosion rate instead of alocalised corrosion rate

(Tech. Univ. of Denmark data)

Weight loss (coupons) and electrical resistance ER- direct measure of the physical metal loss

? simple? widely used techniques? standardised? applicable in all media? visual examination of

coupons

? sensitivity normally low

? historical data? sudden changes cannot

be detected ? weight loss is

destructive technique

AL

R??

?tAmm

V afterinitialcorr ??

??

sensitivity of ER can be improved by design and better instrumentation

A

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 50 100 150 200 250 300 350 400

Time (hours)

RC-R

R (

??)

1200

2200

357,5 358,5

Time (hours)

RC-R

R (

??)

135,5

136,5

347,8 348,8

Time (hours)

RC-R

R ( ?

?)

2300 ?m/y

1.3 ?m/y

0.025M Borax Buffer solution pH 8.5 flushed with 50 ppmH2S/N2 for 355 hours (14 days) then flushed with O2

EIS1 EIS2 EIS3 EIS4

(Tech. Univ. of Denmark data)

BIOCORROSION - Thematic NetworkTask 5 : Biofilm and Corrosion Monitoring

Electric Resistance (ER)The new improved and patented (from DTU) device is sensitive as LPR and couldbe used instead of weight losses

It works in all environments (aerobic and anaerobic) without limitations regardinge.g. conductivity.

Tests have been conducted in laboratory studies in soil and sea water media withpromising results for MIC monitoring (DTU).

This technique could be potentially very useful for MIC and will be interesting toconfirm the results (obtained from prototype) from other researcher also.

BIOCORROSION - Thematic NetworkTask 5 : Biofilm and Corrosion Monitoring

MIC MONITORING TECHNIQUES discussed:

? Weight losses? Electrical Resistance? Linear Polarisation Resistance ? Open Circuit Potential measurement ? Redox Potential? Field Signature method ? Electrochemical Impedance Spectroscopy ? Electrochemical Noise

http://www.corr-institute.se/english/Web_DT/Dom_index2.html

BIOCORROSION - Thematic NetworkTask 5 : Biofilm and Corrosion Monitoring

Electrochemical Noise (EN)Only for laboratory applications

Good results have been obtained by combining this technique with corrosion ratemeasurements in laboratory application to study the effect of SRB and otherorganisms on the formation of iron sulphide film contributing to pitting corrosion ofreinforcing steel and concrete (data from USA)

Redox potentialThe reduction-oxidation (redox) reactions could be used mainly to establish ifcorrosion processes are developing in aerobic or anaerobic conditions, evaluating theredox potential of the solution.(OCP give more information on corrosion processes than Redox potential).

Field signature method (FSM)Too much expensive, for dept localised corrosion in pipelines only.1/1000 of wall thickness sensitivity

?? OnOn--line monitoring is line monitoring is necessary in any casesnecessary in any cases

? Several traditional techniques for corrosion monitoring could be employed to detect MIC, but it does not exist a technique specific for MIC only

? The right approach could be very different in function of the type of material and the environment at the metal-biofilm interface

CONCLUSIONCONCLUSION

On line monitoring of corrosion rateOn line monitoring of corrosion rate??

OnOn--line monitoring of biofilm growthline monitoring of biofilm growth

it is possible to monitor MIC but It is difficult to separate MIC effect from other

corrosion phenomena

Suggested solution:

CESI