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MODERN METHODS OF MONITORING BIOCORROSION 1
ELECTROCHEMICALELECTROCHEMICALMONITORINGMONITORING
P. CRISTIANI, CESI S.p.A, [email protected]
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