TY - RPRT
T1 - Microbial Corrosion and Cracking in Steel
T2 - Fundamental studies on the Electrochemical Behaviour of Carbon
Steel Exposed in Sulphide and Sulphate-reducing Environments
AU - Hilbert, Lisbeth Rischel
PY - 1998
Y1 - 1998
N2 - The aim of the report is to give a fundamental understanding of
the response of different electrochemical techniques on carbon
steel in a sulphide environment as well as in a biologically
active sulphate-reducing environment (SRB). This will form the
basis for further studies and for recommendations in regards to
electrochemical monitoring of MIC. The work presented here and
further studies are also planned to lead to a Ph.D. thesis on "MIC
monitoring based on mechanisms of corrosion".The results of
laboratory experiments conducted in the period 1995 to 1997 are
summarised. Conclusions will be based on results from the entire 3
year period, but only selected experimental data primarily from
the latest experiments will be presented in detail here.Microbial
corrosion of carbon steel under influence of sulphate-reducing
bacteria (SRB) is characterised by the formation of both biofilm
and corrosion products (ferrous sulphides) on the metal surface.
Experiments have been conducted on carbon steel exposed in near
neutral (pH 6 to 8.5) saline hydrogen sulphide environment (0 to
100 mg/l total dissolved sulphide) for a period of 14 days.
Furthermore coupons have been exposed in a bioreactor for a period
of up to 120 days in sulphide-producing environment controlled by
biological activity of (SRB).Electrochemical studies have been
conducted in order to characterise the electrochemical response of
the biofilm / ferrous sulphide / metal interface and clarify
whether the tested electrochemical techniques (LPR, Tafel, EIS,
stepped LPR) are actually applicable to this system.Exposure of
carbon steel in the bioreactor environment leads to:· formation of
very porous sulphur rich surface films· mild grain boundary
corrosion attacks Electrochemical studies from the bioreactor can
be summarised to:· apparent corrosion rates are very high ( Rp
appr. 100 Ohm*cm2 ~ 3 mm/yr) which is highly in contradiction with
the visual inspection indicating little corrosion· large
interfacial capacitances (up to 300 mF/cm2) are obtained·
hysteresis dominates DC polarisations· for purely capacitative
hysteresis a corrected corrosion rate can be estimatedExposure of
carbon steel in hydrogen sulphide atmosphere results in:·
formation of sulphur rich surface films with varying degree of
porosity· indications that high film sulphur content correlate
with low polarisation resistanceElectrochemical studies in
hydrogen sulphide atmosphere can be summarised to:· Polarisation
resistance estimated with four different techniques show a good
correlation in solutions with a sulphide concentration below ~10
mg/l sulphide· Polarisation resistance increases with increasing
sulphide concentration and time· Corrosion rate estimation is in
the range of 3 to 30 kOhm*cm2 (0.1 to 0.01 mm/yr) which does not
contradict with visual inspection and weightloss measurements.· A
threshold value can be found at 5 to 10 mg/l sulphide above which
interfacial capacitance increases almost two decades· Large
interfacial capacitances (up to 50 mF/cm2) are obtained· Stepwise
Tafel polarisations indicate that hysteresis is not only caused by
high capacitative current contributions but probably highly
dominated by diffusionThree different electrochemical
characteristics can be suggested:· Formation of a uniform, dense
and not electrochemically active film. The polarisation resistance
increases with the film resistance and an small underestimation of
corrosion rate is possible, if film resistance is large.· An
electrochemically reactive film (ferrous sulphides) results in
current contributions that will be added to the metal dissolution
reaction current. A very reactive film will therefore increase the
apparent corrosion rate, even if the corrosion rate of steel is
low. In this case corrosion rate estimation will fail.· Porous
film formation gives an increased surface area and induces large
effects of diffusion. The interfacial capacitance and the
polarisation resistance are distributed over a huge area. The
result is an increased apparent corrosion rate and capacitance.
Corrosion rate estimation fails - especially if porosity is
combined with a reactive film.The conclusion is therefore that
corrosion rate estimation by electrochemical techniques is
possible. However, a very large risk exists that the true metal
dissolution rate is totally masked and that the operator will not
know when to trust data and when to discard them. It does not seem
likely that corrosion rate can be estimated for long term exposure
in a biological environment. Whether the corrosion rate
estimations in the hydrogen sulphide environment can be trusted is
still not settled, but possibly corrosion rate estimation by
electrochemical techniques is possible below a sulphide threshold
value.Identification of a high interfacial capacitance determined
by EIS is a very strong indication of sulphide. The high
capacitance is primarily caused by the formation of a reactive
porous layer with a rapid enhancement of surface area. The effect
is increased in the biological environment as compared to the
hydrogen sulphide solutions.· Tafel polarisations are only
relevant for mechanistic studies. The technique is, however,
destructive, time consuming and fails, when things become
complicated.· LPR and stepped LPR are applicable only in simple
cases and gives little information on when and why the
measurements may be unreliable.· EIS gives the opportunity to
recognise characteristic changes such as increasing capacitance
and indications of porous behaviour, but analysis is somewhat
complicated.To conclude electrochemical measuring techniques must
be used with caution and a direct corrosion rate measurement (ER,
weightloss) applied in order to obtain actual corrosion rate.
However, electrochemical techniques still represent a diagnostic
tools and e.g. the identification of high interfacial capacitance
might be a way of recognising the initiation of conditions leading
to MIC. Future studies will cover other electrochemical techniques
(electrochemical noise, galvanic coupling) and direct corrosion
rate estimation for mechanistic studies.
AB - The aim of the report is to give a fundamental understanding of
the response of different electrochemical techniques on carbon
steel in a sulphide environment as well as in a biologically
active sulphate-reducing environment (SRB). This will form the
basis for further studies and for recommendations in regards to
electrochemical monitoring of MIC. The work presented here and
further studies are also planned to lead to a Ph.D. thesis on "MIC
monitoring based on mechanisms of corrosion".The results of
laboratory experiments conducted in the period 1995 to 1997 are
summarised. Conclusions will be based on results from the entire 3
year period, but only selected experimental data primarily from
the latest experiments will be presented in detail here.Microbial
corrosion of carbon steel under influence of sulphate-reducing
bacteria (SRB) is characterised by the formation of both biofilm
and corrosion products (ferrous sulphides) on the metal surface.
Experiments have been conducted on carbon steel exposed in near
neutral (pH 6 to 8.5) saline hydrogen sulphide environment (0 to
100 mg/l total dissolved sulphide) for a period of 14 days.
Furthermore coupons have been exposed in a bioreactor for a period
of up to 120 days in sulphide-producing environment controlled by
biological activity of (SRB).Electrochemical studies have been
conducted in order to characterise the electrochemical response of
the biofilm / ferrous sulphide / metal interface and clarify
whether the tested electrochemical techniques (LPR, Tafel, EIS,
stepped LPR) are actually applicable to this system.Exposure of
carbon steel in the bioreactor environment leads to:· formation of
very porous sulphur rich surface films· mild grain boundary
corrosion attacks Electrochemical studies from the bioreactor can
be summarised to:· apparent corrosion rates are very high ( Rp
appr. 100 Ohm*cm2 ~ 3 mm/yr) which is highly in contradiction with
the visual inspection indicating little corrosion· large
interfacial capacitances (up to 300 mF/cm2) are obtained·
hysteresis dominates DC polarisations· for purely capacitative
hysteresis a corrected corrosion rate can be estimatedExposure of
carbon steel in hydrogen sulphide atmosphere results in:·
formation of sulphur rich surface films with varying degree of
porosity· indications that high film sulphur content correlate
with low polarisation resistanceElectrochemical studies in
hydrogen sulphide atmosphere can be summarised to:· Polarisation
resistance estimated with four different techniques show a good
correlation in solutions with a sulphide concentration below ~10
mg/l sulphide· Polarisation resistance increases with increasing
sulphide concentration and time· Corrosion rate estimation is in
the range of 3 to 30 kOhm*cm2 (0.1 to 0.01 mm/yr) which does not
contradict with visual inspection and weightloss measurements.· A
threshold value can be found at 5 to 10 mg/l sulphide above which
interfacial capacitance increases almost two decades· Large
interfacial capacitances (up to 50 mF/cm2) are obtained· Stepwise
Tafel polarisations indicate that hysteresis is not only caused by
high capacitative current contributions but probably highly
dominated by diffusionThree different electrochemical
characteristics can be suggested:· Formation of a uniform, dense
and not electrochemically active film. The polarisation resistance
increases with the film resistance and an small underestimation of
corrosion rate is possible, if film resistance is large.· An
electrochemically reactive film (ferrous sulphides) results in
current contributions that will be added to the metal dissolution
reaction current. A very reactive film will therefore increase the
apparent corrosion rate, even if the corrosion rate of steel is
low. In this case corrosion rate estimation will fail.· Porous
film formation gives an increased surface area and induces large
effects of diffusion. The interfacial capacitance and the
polarisation resistance are distributed over a huge area. The
result is an increased apparent corrosion rate and capacitance.
Corrosion rate estimation fails - especially if porosity is
combined with a reactive film.The conclusion is therefore that
corrosion rate estimation by electrochemical techniques is
possible. However, a very large risk exists that the true metal
dissolution rate is totally masked and that the operator will not
know when to trust data and when to discard them. It does not seem
likely that corrosion rate can be estimated for long term exposure
in a biological environment. Whether the corrosion rate
estimations in the hydrogen sulphide environment can be trusted is
still not settled, but possibly corrosion rate estimation by
electrochemical techniques is possible below a sulphide threshold
value.Identification of a high interfacial capacitance determined
by EIS is a very strong indication of sulphide. The high
capacitance is primarily caused by the formation of a reactive
porous layer with a rapid enhancement of surface area. The effect
is increased in the biological environment as compared to the
hydrogen sulphide solutions.· Tafel polarisations are only
relevant for mechanistic studies. The technique is, however,
destructive, time consuming and fails, when things become
complicated.· LPR and stepped LPR are applicable only in simple
cases and gives little information on when and why the
measurements may be unreliable.· EIS gives the opportunity to
recognise characteristic changes such as increasing capacitance
and indications of porous behaviour, but analysis is somewhat
complicated.To conclude electrochemical measuring techniques must
be used with caution and a direct corrosion rate measurement (ER,
weightloss) applied in order to obtain actual corrosion rate.
However, electrochemical techniques still represent a diagnostic
tools and e.g. the identification of high interfacial capacitance
might be a way of recognising the initiation of conditions leading
to MIC. Future studies will cover other electrochemical techniques
(electrochemical noise, galvanic coupling) and direct corrosion
rate estimation for mechanistic studies.
M3 - Report
BT - Microbial Corrosion and Cracking in Steel
ER -