P. Cristiani et al.

Corrosion Science 246 (2025) 112694

Fig. 19. Rp responses supporting weight loss trend in assessing enhanced biocide treatment of carbon steel MIC by an oilfield biofilm consortium (Modifed from
ref. [143]).

electrolyte conductivity or localized corrosion. Since the sum of Rp and 20 mV, with jcorr usually validated by gravimetric assessment [124,147].
the solution resistance (Rs) between electrodes is measured, relevant Fig. 20 provides an example of integrated corrosion monitoring of 70/30
noise can be introduced in the measurement when the electrolyte con­ cupronickel samples, biofilm growth using a BIOX probe, and other
ductivity is lower than about 0.5 mS cm− 1, depending on the probe parameters such as temperature and turbidity [147]. The corrosion
geometry [144]. Low conductivities can be found in oligotrophic trends, estimated in µm y–1 by LPR measurements and corroborated by
freshwaters, tap waters, boiler waters, and similar environments. For BIOX signal, highlight how corrosion increases, especially in case of new
highly conductive waters, the geometry of the monitored samples and (unpassivated) sample of the copper alloy, when the antifouling treat­
their surface finish can play a crucial role in MIC monitoring, as biofilm ment (chlorination) is interrupted in a cooling circuit supplied with
growth is strongly influenced by hydrodynamics and surface roughness. sediment-rich seawater (as highlighted by the increase in turbidity
In case of crevices, secondary electrochemical reactions which are signal).
controlled by diffusion of reaction species and passivation, are not For MIC studies accessing the Tafel slopes is not feasible, LPR can be
accounted for [145]. Conversely, Gonzalez et al. [146] have stated that used to track the corrosion rate evolution via 1/Rp without estimating
when the corrosion systems are under mass transfer control, even a small jcorr during the immersion test. This approach can also determine the
potential perturbation as low as 10 mV can cause changes in the mea­ role of microbial activity (i.e., biofilm and enzymes) in corrosion per­
surement. In this case, LPR cannot provide adequate information, formance. Fig. 21 illustrates how LPR is performed and how it can help
particularly regarding the initiation and growth of passivation or assess the influence of hydrogenase in the corrosion of mild steel [131].
passivation breakdown [145]. For high capacitance systems, such as In this case, it is clearly shown that hydrogenase increased the corrosion
carbon steel in anaerobic environment, LPR tends to overestimates the rate and induced grain boundary degradation, which was not observed
corrosion rate [76]. This high capacitance is often associated with in the control medium.
corrosion product deposits.
For active-passive metal alloys, since MIC is frequently accompanied
with pitting and localized corrosion, the LPR technique cannot, in
T [°C] Turbid.[NTU]

principle, provide accurate information about the actual weight loss.

Nevertheless, LRP is widely used to monitor corrosion rate in field and
BIOX [mV]

industrial applications, where corrosion phenomena are usually local­

ized and occur in near neutral pH environments. Indeed, commercial
corrosimeters are usually based on a variation of the LPR technique.
They scan the potential in the range of the Eocp ± 5–10 mV, close to the
open circuit potential of the corrosion process. Operating in a non-
destructive manner, the measurement provides an almost immediate
passivated samples
jcorr [µmy-1]

response (in a few seconds), allowing for frequent repeat measurements

jcorr [µmy-1]
new samples

over time. By monitoring the trend of collected data, it enables real-time

extraction of information linked to the overall corrosion process and the
effectiveness of anticorrosive treatments/mitigation strategies.
The practical application of LPR to monitor microbial corrosion of
copper alloys in industrial environments, such as marine cooling cir­
cuits, has demonstrated the usefulness of this technique for localized
corrosion as well [144]. Empirically estimated values of B (Tafel factor, Fig. 20. Integrated monitoring of corrosion of CuNi 70/30 samples by LPR,
biofilm (BIOX probe), temperature and turbidity (due to mud incoming) in a
see Eq. 25) for copper alloys usually falls within the range of 10 and
seawater cooling circuit of a power plant (from ref. [147]).

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

Fig. 21. Determination and evolution in time of the inverse of polarization resistance (1/Rp) for S235JR mild steel electrodes in 0.1 M Tris-HCl pH7 medium inside a
dialysis bag. Injection at t = 0 + of solution with or without hydrogenase. (a) Examples of polarization curves to determine 1/Rp (slope of the curve), obtained after
10 minutes of immersion with 75 µL hydrogenase solution (pink curve) and with 75 µL control solution bleu curve. (b) 1/Rp versus time for experiments performed
with 75 µL hydrogenase solution (pink point) and with 75 µL control solution (blue points). SEM images of the S235JR mild steel electrode surface after 24 hours of
immersion with hydrogenase (modified from ref. [131]).

4.4. Potentiodynamic polarization technique

The potentiodynamic polarization (PDP) technique is more intrusive

than the LPR technique. It scans a wide range of potentials, typically
± 0.5 to ± 1.0 V vs. OCP, to extrapolate Tafel curves, the pitting po­
tential Epit, and the protection potential Eprot. The analysis is generally
performed by considering the reactions at the steady state, can be
approximated using a low potential scan rate. For corrosion tests, this
means a scan rate of less than 5 mV s–1, and usually 1.666 mV s–1
(equivalent to 100 mV min–1).
Polarization curves provide an overview of the reaction kinetics for a
given corrosion system. including charge-transfer or mass-transfer
controlled reactions, passivity, transpassivity, and localized corrosion
phenomena [76]. Tafel curves offer mechanistic insights into the rates of
anodic and cathodic reactions, which can behave differently, especially
during when active-passive transition, thereby determining the limiting
step of the corrosion reaction. Under these conditions, passivation
properties can be evaluated based on the critical potential Ecrit, critical
(jcrit), and passive (jpass) current densities for passivation. The PDP
technique is widely used to study dynamic changes of anodic and
cathodic processes caused by microorganisms or a biofilm [1,148].
However, high-field polarization can alter the electrochemical condi­
tions at the metal-solution interfaces, affecting bacteria attachment and
biofilm formation. Therefore, anodic and cathodic polarization should Fig. 22. Potentiodynamic polarization curves for C1018 carbon steel MIC by
be performed separately, starting from the corrosion potential. Despite Desulfovibrio ferrophilus biofilm using continuous upward scan scheme (from
this, MIC researchers often use a continuous scan towards electroposi­ − 200 mV vs. OCP to +200 mV vs. OCP) and dual-half scan scheme with each
tive potentials on a single electrode instead of using two replicate half scan starting from OCP
electrodes to obtain separate cathodic and anodic curves. Indeed, an (from ref. [149]).
electropositive potential scan (Fig. 22 yellow curve A5) can result a
lower jcorr (often 50 % or more), along with significant compression of Regarding pitting corrosion performance, Epit (pitting potential or
the cathodic curve (in voltage range) and elongation of the anodic curve breakdown potential, Eb) can be determined when the anodic curve
when compared to the half-scan curves obtained separately (Fig. 22 shows a sharp increase, even in the presence of bacteria. Epit provides
black curve standard WEs) [149]. This is because when a living biofilm data on the tendency for pitting but not the pit propagation rate [76].
is suddenly exposed to the lowest voltage (e.g., an overpotential of Therefore, it is useful to assess the repassivation curve by reversing the
− 200 mV), the biofilm has no time to adapt to the external DC voltage, polarisation, allowing the current density to decrease to the passive
unlike the case when the scan pattern starts at OCP (no external current density at the repassivation potential (Er). Consequently, the Er
voltage). Moreover, when the dual-half scan scheme is adopted to scan a and the area under the repassivation curve give the extent of pit prop­
single electrode with a cathodic scan from OCP to − 200 mV over­ agation. For example, it was reported that the Geobacter species influence
potential and then an anodic scan from OCP to + 200 mV overpotential, the pitting behaviour of 304 L stainless steel differently depending on
the jcorr deviation is greatly reduced, resulting in more symmetric anodic the test medium in which the biofilms develop (Fig. 22). In
and cathodic curves (Fig. 22 curves A1 and A2). Furthermore, with the acetate-lacking media, pitting occurred with and without bacteria
mild dual-half scan scheme, the same electrode covered with a biofilm within the same potential range of, but the presence of bacteria mark­
can often be repeatedly scanned without severe deterioration (Fig. 22 edly increased the pit size [150]. In fumarate-lacking conditions
curves A3 and A4) [149]. (Fig. 22b), the biofilm exhibited a protective effect by shifting the Epit to

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

Fig. 23. Influence of medium composition on pitting curves (0.5 mV s− 1) in presence of Geobacter sulfurreducens for 304 L stainless steel electrode a) medium
containing 5 mM acetate (e- donor) and 25 mM fumarate (e- acceptor) (from ref. [148]) and b) after 20 days immersion in a medium containing 10 mM acetate and (○
blue) 25 mM fumarate or (Δ red) 0 mM fumarate. Closed and open symbols represent, respectively the presence of 5 % v/v G. sulfurreducens and the absence of
bacteria (from ref. [151]).

more positive potentials and improved the repassivation conditions

[151]. However, disadvantages of this technique can be highlighted as
follows:

1. Large polarizations alter the electrochemical conditions, which can

be deleterious to the biofilm microorganisms, especially when a large
potentials (high-field) are used or a continuous electropositive scan
is used [25,149].
2. This technique alone cannot provide information about the local
contribution of biofilm to corrosion.
3. The choice of potential scan rate is crucial in MIC studies to minimize
effects on biofilm structure and characteristics. Faster scan rates have
less the impact on microbial activities.
4. Slow potential sweep rates can affect localized conditions at the
metal-solution interface [87].

A fast potential scan usually leads to the formation of an electric

double layer of charge at the metal–solution interface. This non–faradaic
charging current constitutes a significant source of error in estimating
the faradaic current density, which influences the equilibria of the redox
reactions occurring at the metal surface [145]. Fig. 24. Schematic CV curves for reversible, quasi-reversible and irreversible
redox process.
4.4.1. Cyclic voltammetry
Cyclic voltammetry (CV) is a potentiodynamic technique to study the electropositive potentials, indicating the reduction catalysis by the
reversible, irreversible or quasi-irreversible behaviour of redox couples biofilm. CV is extensively used to identify the redox activity at the
at metal-solution interface. It involves applying a rapid triangular po­ biofilm/metal interface and to study the EET pathways of electroactive
tential scan to the working electrode, in a cyclic manner, inducing the bacteria and to calculate the standard EET rate constant and diffusion
oxidation of electroactive redox species followed by their reduction (or coefficients [154,156–162]. For example, numerous bacteria isolated
vice-versa). Single redox species generate peaks in the E-I curve both in from electro-active marine biofilms have shown high efficiencies in
the anodic and cathodic regions when the induced redox process is oxygen reduction catalysis through CVs performed on inert glassy car­
reversible. for irreversible processes peaks appear only in one region bon (Fig. 26). This activity was attributed not only to direct electron
(Fig. 24). In CVs, the scan rate (ν mV s–1) is the most important transfer between adhered cells and electrode but also to redox com­
parameter, as it affects the intensity and the shape of the redox peaks. pounds present in test medium [154]. Fig. 27 provides another example
Lower scan rates result in lower peak heights, while higher scan rates where CVs, obtained with a pencil graphite electrode covered by a
produce sharper redox peaks. For bioelectrochemistry systems and cultivated SRB biofilm (Desulfovibrio singaporenus) showed a reversible
consequently MIC studies, CV is useful for highlighting the electro­ peak due to the presence of redox mediator and the contribution of the
activity of a biofilm that forms on an electrode [24,152–154]. Some electroactive biofilm, as well as c-type cytochrome activity [158]. The
examples are shown in Fig. 25 with an anodic biofilm (a) formed from redox peak currents were enhanced by between 1.5 and 2.5 times with
garden compost, that oxidizes acetate [155]. In this case, the shape of riboflavin and cytochrome c, respectively, confirming the positive
the anodic curve is a wave with a jmax that indicates a limiting phe­ connection of the attached biofilm with soluble riboflavin and cyto­
nomenon, which can be attributed to the diffusion of substrates and/or chrome c in EET.
metabolites, the electron transfer rate and the microbial kinetics. These The CV technique can effectively highlight the catalytic effects of
curves can be fitted with the Nernst-Michaelis expression, see Eq. 7. enzymes and other molecules. Fig. 28 shows the irreversible wave of
Fig. 25b shows the CVs obtained with a cathode biofilm of Algo­ oxygen reduction catalysed by (a) laccase and (b) Hemin, an iron
riphagus yeomjeoni, a species isolated from a marine biofilm that catal­ protoporphyrin.
yses oxygen reduction [221]. It is interesting to note the shift of the However a possible disadvantage of CV, is that the faradaic current
cathodic curve, corresponding to the reduction of oxygen, towards can be masked by a high resistive and/or capacitive behaviour of the
electrode surface [163] as illustrated in Fig. 29. Although the different

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

a) b) Potential (V vs Ag/AgCl)
-0.6 -0.4 -0.2 0

Current density (A/m2)

-0.005
J/Jmax

-0.010

-0.015

Potential (V/SCE)
Fig. 25. CV curves (1 mV.s− 1) for a) microbial anodes, formed after 10 days’ polarization at 0.2 V/SCE in garden compost leachate, E-I curves drawn in a synthetic
medium with 20 mM acetate, 9-cm2 (continuous line) and 50-cm2 (dotted line) anodes made of flat carbon cloth [160] and b) microbial stainless steel cathodes,
formed after different times of polarisation at − 0.2 V vs Ag/AgCl in seawater inoculated with Algoriphagus yeomjeoni, an electroactive species extracted from
marine biofilm.(modified from ref. [221]).

Fig. 26. Roles of dissolved oxygen, adhered bacteria and released compounds (GC electrode and cell suspension of Roseobacter sp.). (a) CV1 and CV3 were recorded
in deoxygenated solution and CV4 after 10 minutes of air bubbling (Roseobacter sp. R-26140). (b) CV1 and CV3 were recorded following the standard procedure,
then the cell suspension (Roseobacter sp. R-28704) was replaced by fresh buffer that did not contain cells and, finally, the electrode was cleaned. (c) CV1 and CV3
(standard procedure) obtained with the filtrate of bacterial cell suspension (Roseobacter sp. R26140). Modified from ref. [154].

surface treatments (Acid – A, Electrochemical E, or the both A+E) of the the technique measures impedance of the entire electrochemical cell.
graphite electrodes enabled the generation of microbial bioanodes
delivering significantly higher current densities than the untreated 5. Alternating Current (AC) stimulation
control electrode (table in Fig. 29) after microbial colonization in do­
mestic wastewater (end of the 25-day of experiment), the CV graph of Methods based on the perturbation of an electrochemical system in
the electrode A+E is very different from the others. In fact, for the A, E equilibrium or steady state, through the application of a sinusoidal input (AC
and control electrodes, the CV graphs displayed a shape characteristic of voltage or AC current) over a wide range of frequencies, and the monitoring
bioanodes oxidizing organic matter with, however, a higher capacitive of the resulting sinusoidal response (current or voltage, respectively), are
current for the electrode E (C = 248 mF). In contrast, the A+E electrode powerful tools for investigating the electrical properties of interfaces, passive
presented a steady state current equal to that of the A one and yet the CV layers of corrosion products, and insulating coatings. These methods also
graphs showed a strongly capacitive (C = 627 mF) and resistive (incli­ provide detailed mechanistic insights into microbial corrosion and the role of
nation of the I/V curve) behavior that masked the oxidation phenome­ biofilms in corrosion processes. However, their successful application requires
non. Furthermore, while CV is mainly used to analyze solutions, it often advanced analytical techniques (now included in specific software) and a
struggles to clearly observe detect reactions in weakly concentrated thorough understanding of the electrical aspects of corrosion phenomena.
electroactive species in solution. Applying CV to solid samples, such as Unlike DC methods, where current flows only in one direction, in AC
the biofilm/metal interface, necessitates more advanced strategies, as methods current periodically reverses direction and changes its

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

stimulation can also strongly impact the process under investigation.

5.1. Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is an effective non-

destructive method for mechanistic studies of various corrosion phe­
nomena, not limited to MIC. In this method, the working electrode at the
corrosion potential is generally stimulated by a small amplitude sinu­
soidal voltage at variable frequencies. The response of the working
electrode to this perturbation, in terms of: current, signal amplitude, and
phase, is recorded [1]. By dividing the imposed voltage by the induced
current, the impedance of the system at all excited frequencies. The
applied voltage perturbation covers a wide frequency range, from mil­
lihertz (mHz) to megahertz (MHz),allowing the characterization of all
the phenomena occurring at the solid/solution interface [164]. Each
elementary phenomenon can be observed and characterized at a specific
Fig. 27. CV curves for the pencil graphite electrode covered by a cultivated frequency range that corresponds to its own time constant (the relaxa­
SRB biofilm in a 10 % carbon source medium for 15 days in different solutions: tion time to return to steady state). Rapid electrochemical phenomena
SRB solution (culture medium with planktonic cells), PBS medium (fresh abiotic (such as charge transfer) are revealed at high frequencies, whereas slow
medium) and PBS medium with addition of riboflavin or cytochrome c (from phenomena, such as diffusion (mass-transfer) or adsorption are observed
ref. [158]). at low frequencies. Experimentally, at high frequencies, only the ohmic
component due to the solution resistance is measured. When the
magnitude continuously with time. Similar to DC, AC stimulation can be measured |Z| increases inversely with frequency (on a log scale), it
a non-destructive technique or can influence the measured phenome­ demonstrates the characteristics of a pure capacitance. Thus, EIS enables
non, depending on the current intensity. The frequency of the the distinction between different phenomena that are stimulated at

a) b)
Poten�al (V/SCE)
-1.0 -0.5 0

20 mV/s

Current (mA)
-0.004
With Hemin
j (mA/cm-2)

Without Hemin

-0.008

Titanium
electrode
-0.012

E (V/SCE)

Fig. 28. Catalysis of oxygen reduction by a) laccase from Trametes versicolor on a titanium electrode in 0.1 M citrate-phosphate solution at pH 3.0, deaerated or
oxygen saturated at atmospheric pressure with and without Laccase confined near the electrodes and b) the iron protoporphyrin Hemin, adsorbed on a stainless steel
electrode in NaCl solution. Modified from ref. [107].

Fig. 29. CV performed in domestic wastewater on treated graphite surfaces (A+E, A, E) and non-treated commercial graphite surface (Control) after microbial
colonization (end of the 25-day experiment). The correspondent steady state current densities (at 25-days) are given in the table as well as the improvement over the
Control. Scan rate: 1 mV s− 1. A: acid, E: electrochemical. Modified from ref. [163].

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

different frequencies. EIS can be performed as a high-resolution tech­ interpretation of the EIS data is not straightforward and requires un­
nique to achieve kinetic parameters, determine reaction mechanisms, derstanding of the physical meaning of the electric components to model
and measure electrolyte and electrode conductivities, as well as the in­ an effective equivalent circuit of the corrosion process, including mi­
fluence of a biofilmson the metallic surface. EIS can thus be used to crobial interactions for MIC. To address this, an integrated approach
characterize non-conducting or semi-conducting passive layers, coating combining experimental observation, model development, and error
on the surface [45,165–170]. For example, Fedrizzi et al. reported that analysis has been proposed by Orazem and Tribollet [173]. Moreover,
high-frequency peaks were associated with coating performance, dedicated software packages are also available to mathematically
whereas the other peaks were related to activation of electrochemical generate equivalent electrical circuits from the raw EIS data. These
processes [45]. packages use a structural identification approach that involves the
EIS data can be interpreted using various plots such as Nyquist, Bode- generalized deconvolution of the impedance data without requiring
modulus, Bode-phase, and Cole-Cole plots. The Bode plots display in­ priori chosen assumptions. For example, Differential Impedance Anal­
formation on impedance, frequency, and phase angle [87]. For instance, ysis (DIA) is a technique that applies a structural identification approach
the Bode-phase plot can provide insights into pitting corrosion when to time constants [174]. It ensures both structural and parametric
combined with optical microscopy. It has been observed that when the identification based on a single experimental data. The method applies a
phase angle reaches a minimum at the lowest frequencies, pitting can be local operating model (LOM) for the parametric analysis by scanning
initiated, and the growth of active pits increases the capacitance [171, along the analytical coordinate frequency. The LOM describes a simple
172]. It must be noted that the breakdown of the passive layer, first-order inertial system extended with an additive term. A determin­
accompanied by pitting corrosion process, is critical to detect by EIS istic approach is used by extending of the initial set of data (Zreal and
since their initiation and development are unstable processes. In such Zimag components of the impedance and frequency ω) with two addi­
cases, EIS provides only qualitative, not quantitative, results. The tional terms – the derivatives of Zreal and Zimag with respect to the

Fig. 30. Nyquist (a) and Bode (b) plots of an aluminum bronze showing the most significant EIS spectra achieved during exposition in a control solution of 0.1 M of
NaCl solution (Circles); filtered biotic solution with metabolites (squares) and biotic solution containing Pseudomonas fluorescens bacteria (triangles) at different
exposition time; c) Trends of calculated film resistance Rf and d) trends of Charge transfer resistance Ct (from ref. [30]).

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

frequency. The number of plateaus found in the frequency range where surface [78,173]. Additionally, biofilms are also dynamic entities that
the LOM corresponds to the object’s behaviour, is the number of time cause short-term fluctuations in the electrochemistry at the metal/film
constants. The temporal analysis can qualitatively estimate frequency interface, making localized corrosion under the biofilm difficult to
distribution, indicated by deviations from the plateau-behaviour. detect. The period of these fluctuations is shorter than the time for
However, for precise quantitative analysis, i.e., for structural and para­ gathering EIS data through a full frequency sweep, meaning that not all
metric identification, another algorithm called Secondary DIA [175], frequencies of the applied signal respond to the same electrochemical
and complex non-linear regression least squares fitting procedure [176] conditions [25]. Nevertheless, many EIS reports monitoring bacterial
must be performed based on the results obtained by DIA. Despite the cell attachment [78,179] and biofilm formation [87,171,180,181],
complexity of this method, it was successfully applied recently in MIC bacterial electroactivity and conductivity of biofilm, often misinterpret
studies of copper alloys, supported by microscopy observations, Raman the equivalent circuits, and incorrectly attribute the double layer
data, and other electrochemical techniques [21], [220]. EIS techniques response of corrosion products to the biofilm.
also allowed us to demonstrate how the aerobic metabolism of Pseudo­ When analyzing EIS data linked to MIC or biofilm studies, it is
monas fluorescens promotes the formation of a stable and highly resistive important to remember that the biofilm is mostly composed of water
oxide film, which inhibits the corrosion of Aluminum bronze. Although [38] and is a good electrolyte. Therefore, it cannot exhibit a capacitive
this bacterium initially produced an electroactive meditator (pyocyanin) behaviour itself, although it can influence the charge transfer in several
that increases charge transfer, corrosion was ultimately inhibited. This is ways. For the study of MIC mechanisms, low frequency data are of
due to the enhanced metal dissolution being followed by the greater interest. In conclusion, the EIS technique can be a useful for
re-precipitation of metal ions in a more stable (higher capacitive and studying biofilm and its effects on corrosion if the experimental data are
resistive) passive layer compared to the sterile and metabolite control well analysed. EIS is non-destructive to the biofilm, and electrochemical
tests ([30], Fig. 30). conditions on electrodes change very little after the EIS measurements
Wharton et al. [177,178] report a study monitoring aerobic marine [1]. Several studies have demonstrated that the small signals required
biofilms on electrode surfaces, and the influence of nitric oxide attach­ for EIS do not adversely affect the numbers, viability, and activity of
ment control as an innovative method for detecting and monitoring microorganisms within a biofilm [76,78,181]. Conversely, cell pop­
marine bacterial biofilms using EIS. The study effectively demonstrates ulations can be significantly reduced on the polarized surface when
the use of microelectrodes to monitor the condition film, biofilm for­ subject to either PP or CV [182].
mation and the impact of nitric oxide on biofilm dispersal (Fig. 31). A Although the EIS cannot directly determine the corrosion current
key aspect of this investigation is its novel application of EIS for density and biofilm thickness, it can estimate the passive layer thickness
real-time biofilm monitoring, which is crucial for understanding biofilm under the biofilm. The high-frequency impedance range (40 MHz down
dynamics and developing mitigation strategies, which incorporated to 100 Hz) corresponds apparently to a pure capacitive behaviour [183].
confocal microscopy to validate the electrochemical data. However, the At infinite frequency, all resistances are negligible compared to the
study has limitations. The experimental setup, although detailed, may impedance corresponding to the capacitance [184]. Therefore, extrap­
not fully replicate the complex conditions of natural marine environ­ olating of the complex-capacitance diagram can reliably determine the
ments, potentially limiting the applicability of the findings. Addition­ thickness of the capacitive layer [184]. The complex capacitance (C) is
ally, a more thorough discussion on the long-term stability and calculated using the impedance data corrected for the electrolyte resis­
reproducibility of the sensor system under varying environmental con­ tance (Re) (Eq. 26).
ditions would have been useful. While the innovative use of micro­
1
electrodes offers high sensitivity and specificity, the potential scalability C(ω) = (26)
jω(Z(ω) − Re )
issues could limit practical applications. Overall, the study provided
insights into biofilm detection and control, although further studies are The physical meaning of this capacitance is linked with the dielectric
needed to address its limitations and improve its practical applications. properties of the electrochemical interface. Using the known dielectric
It is worth noting that equivalent circuit models often cannot accu­ constant of oxides, the capacitance estimated in the high-frequency
rately represent the corrosion process due to its complexity. Multiple domain using Eq. 27 allows the calculation of the oxide layer thick­
interfaces involving partial coverages, mixtures of corrosion products, ness δ as described in Eq. 28 (used for plate capacitors):
elements with different geometries, and porosity can co-exist on the

Fig. 31. EIS for abiotic and abiotic artificial seawater before and after nitric oxide dosing: (a) Nyquist (–Zimag vs. Zreal) and (b) Bode phase (deg. vs. f) and Bode
modulus (Zmod vs. f). From ref. [177,178].

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

Cox =
εε0
(27) under various test conditions. The positive slopes, found at potentials
δ more positive than about –0.5 V vs. SCE correspond to n-type behaviour.
Conversely, the negative slopes, observed at potentials more negative
Where ε0 is the permittivity of vacuum (ε0 = 8.85 × 10− 14 Fcm− 1 ) and
than around –0.7 V vs. SCE denote p-type behaviour. Additionally, there
ε is the dielectric constant for metal oxide. In practice, the high-
is a flat band potential (Efb) region between –0.504 V and –0.686 V vs.
frequency capacitance is determined at frequencies above 25 kHz [185].
SCE, which represents the transition between p-type and n-type con­
ductivity. Both Nd and Na calculated from the linear portions of the
5.1.1. Mott–Schottky plots to study passive films
slopes were of the order of 1021 and progressively decreased with the
Mott–Schottky (M-S) is another method that can be used to under­
development of biofilm and reduction of the light levels [189]. Alter­
stand how the biofilm affects passive layers of metallic surfaces from EIS
natively, another study shows that M-S analysis can elucidate why a
data. M-S plots show changes of capacitance at an electrochemical
G. sulfurreducens biofilm on 254SMO stainless steel exhibited higher
interface under voltage fluctuations and are typically used to determine
cathodic current densities compared to biofilms on 316 L stainless steel.
the semiconductor property of passive films. The Mott-Schottky analysis
It was found that at the cathodic potential used for biofilm formation
expressions are as follows (see Eqs. 28–29):
(below the EFB), 316 L displayed slight p-type behaviour, whereas
( )
1 2 kB T 254SMO did not show any semi-conductive behaviour. The p-type
= E − E − forn − type semiconductor (28)
C2SC εε0 eND
FB
q behaviour in 316 L indicates a lack of available electrons near the sur­
face, which is detrimental for the cathodic process. In contrast, the
( )
1 2 kB T absence of this p-type behaviour in 254SMO explains the superior
= E − EFB − forp − type semiconductor (29) electrochemical performance of biocathodes formed on it [193].
CSC εε0 eNA
2
q

Where e is the electron charge, ND and NA are the donor and acceptor 5.1.2. Dynamic electrochemical impedance spectroscopy
densities (cm− 3), ε is the dielectric constant of the passive film (ε = 12 Another potentially relevant technique for studying the dynamic
for iron/steel substrates), ε0 is the vacuum permittivity kB is the Boltz­ effects induced by a biofilm on protective oxides and anticorrosive
mann constant (1.38 × 10–23 J K–1), T is the absolute temperature and coatings is Dynamic Electrochemical Impedance Spectroscopy (DEIS).
EFB is the flat band potential. The interfacial capacitance, C is also DEIS is powerful for investigating mechanisms in surface electro­
explained in Eq. 30: chemical reactions by applying an AC signal while sweeping the po­
1 tential [194]. This novel approach provides impedance data from
C= − (30) non-stationary systems [194]. DEIS can be distinguishes from conven­
ωZimag
tional EIS through the use of advanced software, ensuring precise
where Zimag is the imaginary component of the impedance and ω = 2πf measurements in critical applications [195]. The technique achieves a
is the angular frequency [186]. In corrosion science, this technique is frequency limitation as low as 0.1 Hz, validated through the
often employed when the double layer capacitance is negligible. Under Kramers-Kronig transform and consistency tests [196]. The mechanisms
these conditions, the measured capacitance (C) is equivalent to the space of electrochemical systems can be analysed qualitatively and quantita­
charge capacitance (CSC). Consequently, a plot of 1/C2SC versus EFB is tively, enabling time-resolved studies of various physicochemical pro­
generated. In these plots, positive slopes indicate the presence of n-type cesses across different time scales. Different acquisition strategies and
semiconductor behaviour at all formation potentials. From the linear correction algorithms facilitate high-precision dynamic impedance
portion of the slopes, the donor density can be estimated. This method spectra and cyclic voltammetry measurements, addressing the challenge
calculates the dopant density near the alloy/passive film interface, of high-frequency artefacts caused by to non-ideal instrument behav­
where the concentrations of oxygen vacancies and metal interstitials are iour, which can lead to incorrect interpretations down to 0.1 Hz. By
expected to be the highest [186]. Thus, understanding the nature of the leveraging advanced computing to reduce computation time, DEIS can
passive film is crucial for comprehending the corrosion properties of effectively study corrosion, inhibitors, pitting, organic coating damage,
engineering metals and alloys [187]. For instance, M-S analysis is per­ cavitation and battery discharge processes [196]. This makes DEIS a
formed to investigate how the electronic properties of the passive film
change with an increase in solution temperature.
An important consideration in the M-S analysis is the influence of
frequency on the measured capacitance. To determine the optimized
frequency, capacitance should be measured across a range of fre­
quencies. The highest frequency at which the capacitance does not
significantly decrease should be selected as a reference frequency for
M–S analysis [188]. It is important to note that M-S analysis is not
suitable for analyzing biofilms, as they do not a capacitive character, a
fact sometimes overlooked in some publications. However, M-S analysis
can be used to study the impact of biofilms on the characteristics of the
passive layers beneath them [189–192]. For instance, a study presented
M-S plots for the passive films on 304 SS after 14 days of exposure to
both sterile and inoculated media. In both conditions, two linear regions
were observed. This confirmed that the biofilm did not change the
semiconductor properties of the passive film. However, the increase in
donor density (Nd) and acceptor density (Na) indicated enhanced elec­
tron transfer within the passive film, accelerating its dissolution [192].
Notably, the increase of these parameters also suggests that the passive
film became more defective. Thus, it is expected that the biofilm
compromise the barrier function of the passive film, increasing the Fig. 32. M-S plots obtained for biofilm formed on stainless steel surfaces
likelihood of pitting corrosion [192]. Fig. 32 shows an example of M-S immersed in coastal seawater for 37 d at different test conditions of ambient
plots with two distinct regions of semi-conductivity for stainless steel light (AL), moderate light (ML), low light (LL), and full darkness (D). From
ref. [189].

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

powerful tool for advancing the understanding and monitoring of compared to conventional electrochemical methods, there are only a
complex electrochemical phenomena. Although it has not yet been few reports on the application of EFM in MIC investigations [197,199].
applied to MIC scenarios, DEIS is valuable for analysing non-stationary In one study [200], iron corrosion in a culture media inoculated with
systems. It allows for time-resolved studies of various electrochemical Desulfovibrio sp. strain HS3 was investigated using EFM. A significant
processes, including those occurring in biofilms, to examine the dynamic deviation between the corrosion rates measured by EFM and LPR was
effects induced by biofilms on protective oxides and anticorrosive observed after five days of incubation, attributed to the onset of severe
coatings. Therefore, it shows promise for studying microbial corrosion in localized corrosion of the iron surface by the bacteria communities. In
real time for laboratory experiments. addition, localized colonization distorted the active surface area
measured by both EFM and LPR, leading to an underestimation of the
5.1.3. Electrochemical frequency modulation corrosion rate by LPR and an overestimation by EFM, compared with
Electrochemical frequency modulation (EFM) is a modern, non- average from long term incubation studies. The overestimation by EFM
destructive technique that applies a dual frequency potential perturba­ was attributed to a shift in the impedance of the corrosion system to­
tion to measure the current density response at sums, differences, and wards lower frequencies, causing the EFM input frequencies to fall into
multiples of the input frequencies [145]. Simpler and faster than EIS, the capacitive region [200]. However, EFM was able to detect significant
EFM provides data about corrosion parameters, such as corrosion rate changes in its causality factors CF–2 and CF–3 [145], indicating when
and polarization resistance of metals and alloys undergoing electro­ the MIC transitions from uniform to localized behaviour and when
chemical corrosion, without requiring prior knowledge of the Tafel corrosion rate monitoring breaks down. This capability of EFM could be
slopes. It validates the accuracy of results obtained using causality fac­ a distinct advantage over other electrochemical methods for MIC
tors (CF-2 and CF-3) [197]. The EFM approach can also interpret the monitoring [200].
corrosion mechanism as either activation, diffusion, or
passivation-controlled phenomena based on the potential-current rela­ 5.2. Localized Electrochemical Impedance Spectroscopy
tionship involved in the corrosion process [145]. Consequently, EFM is
suggested as a good alternative to the conventional electrochemical The localized application of EIS (LEIS) was firstly introduced by
techniques. Isaacs in 1992 [201]. In LEIS, the applied voltage is similar to the con­
Bosch and co–workers [198] modified the Stern–Geary equation and ventional EIS typically involving sinusoidal amplitudes of 10 mV and a
derived mathematical expressions for the theoretical calculation of DC potential scan rate of 1 mV s–1. However, LEIS measures the local
corrosion current density, Tafel slopes (constants) and causality factors potential difference using a moveable probe that scans the sample sur­
CF-2 and CF-3 from the current peaks at harmonic and intermodulation face. The potential difference between micro-probes is then used to
frequencies. (see Eqs. 31–34): accurately calculate the local current on the sample. This technique
involves a two-step process and can be customised as needed [202].
i2ω1,ω2
jcorr = √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅ (31) Initially, the probe is polarized at a fixed frequency and moved over the
2
8 iω1,ω2 i2ω2±ω1 − 3i2ω2±ω1 sample to identify the area of interest, for instance defects and discon­
tinuities in the passive film with low impedance. Subsequently, a sys­
ba iω1,ω2 ΔE tematic variable perturbation is applied above the area of interest,
βa = = √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅ (32) potentially using the second probe. In this configuration, complete
ln10 iω2±ω1+
8iω1,ω2 iω2±ω1 − 3i2ω2±ω1
impedance spectra over some frequencies can be acquired [203].
Three-dimensional graphs of the explored area, usually coloured as a
bc iω1,ω2 ΔE function of capacitance, are then produced. An example of such data
βc = = √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅ (33)
ln10 − ω2±ω1
8iω1,ω2 iω2±ω1 − 3i2ω2±ω1 processing is shown in Fig. 33a [223].
The spatial resolution of this technique is governed by several pa­
iω2±ω1 i2ω2±ω1 rameters, including size and distances between microelectrodes, as well
CF − 2 = and CF − 3 = (34) as the distance between the probe and substrate. To obtain reliable data,
i2ω1 i3ω1
it is crucial to consider the dependency of potential differences on the
where ω1 and ω2 are the (angular) perturbation frequencies in rad s− 1 distance of microelectrodes. In addition, the measured potential will be
(and ω = 2πƒ, with the frequency ƒ in Hertz), and ΔE is the perturbation smaller if the probe is positioned farther from the substrate during
potential. interface mapping [202].
Compared to conventional techniques such as LPR and PP, EFM of­ By equipping dynamic LEIS with two digital-analogue cards for
fers several advantages. Firstly, it is less susceptible to errors caused by measurement of current and voltage signals, EIS spectra can be deter­
the bulk medium in which the metal corrosion is analyzed, such as the mined over narrow time periods. This makes the method suitable for
solution resistance and the double layer charging during potential investigating pitting corrosion, as the breakdown of passive layers is
perturbation. Secondly, the causality factors help detect the initiation of usually accompanied by this corrosion type. A recent review on LEIS
phenomena like active-passive transition and localized pitting and [202] highlighted that the shape of the curves and the low frequency
crevice corrosion. EFM provides accurate values of Tafel constant and limit of the impedance diagrams were correlated with the potential at
corrosion current density, even when the metal is under passivated the bottom of the pit formed on stainless steel (see Fig. 33b [202]).
condition, pitting or diffusion control [145]. The experimental values of Hence, another feature of LEIS is its ability to measure the impedance
CF-2 and CF-3 obtained during an EFM measurement are usually during the propagation of a pit. However, this technique can be limited
compared with theoretically established values. If the theoretical and at low frequency ranges, where capacitive and inductive elements in the
experimental values are close, it means the EFM experiments are veri­ measurement setup and the electrochemical cell itself can introduce
fiable. It is important to use low frequencies in EFM experiments to artifacts, resulting in negative polarization resistances. To overcome this
avoid the influence of the capacitive behaviour of the electrochemical limitation, the conventional EIS and LEIS are usually conducted to
double layer. simplify interpretation of the LEIS data [202].
The Rp derived from EFM can be comparable to Rp obtained from EIS
and polarization techniques, but only when the metal corrosion is 6. Advanced localized electrochemical techniques
charge-transfer controlled. In the case of diffusion controlled and
passivation, these parameters are not comparable and Rp derived from Localized Electrochemical Impedance Spectroscopy (LEIS) and Scanning
EFM is generally more reliable [145]. Despite reliability of EFM Electrochemical Microscopy (SECM) are advanced techniques for studying

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

Fig. 33. Example of LEIS profiles and their projections in time (a): aluminum alloys under salt spray test from ref. [223]); (b): LEIS diagrams obtained above a
propagating pit on 316L steel in differnt media, from ref. [202]

MIC. LEIS maps corrosion sites by measuring local impedance, making it ideal 6.1. Scanning ElectroChemical Microscopy
for detecting pitting corrosion, although it has limitations at low frequencies.
SECM uses a microelectrode to scan surfaces and monitor localized corro­ Scanning ElectroChemical Microscopy (SECM) is highly effective
sion, biofilm activity, and redox reactions, providing detailed insights into technique for studying of a wide range of electrochemical processes,
biofilm-electrode interactions. Both techniques offer high spatial resolution including biological processes, surface reactivity, local corrosion, and
but face challenges such as slow scanning speeds and sensitivity to environ­ charge transfer mechanisms. The setup of SECM involves an ultra­
mental conditions. Combining SECM with Atomic Force Microscopy (AFM) microelectrode (UME) probe that scans the working electrode surface,
enhances both topographical and electrochemical analysis, offering a more while a two-channel potentiostat monitors the potentials of both the
comprehensive understanding of the corrosion/MIC processes. UME probe and the working electrode. The working electrode, where
electrochemical processes occur, perturbs the electrochemical response
of the UME probe. This perturbation provides information about the
nature and properties of the working electrode [204]. The distance

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

between the UME and the electrode surface is controlled by a the sample surface maintained at a micron scale. The current drop on the
three-dimensional positioning stepper motor and a piezoelectric motor UME revealed pit on anodic regions of the stainless steel surface
for precise adjustments. Results from SECM are presented by (Fig. 34a). These pits appear only in inoculated media, and their
three-dimensional graphs showing current versus distance. While the occurrence is intensified by the addition of catalase (Fig. 34c).
current of the working electrode does not depend on the UME Conversely, SECM images under sterile conditions showed no changes or
tip-to-electrode distance, the response of the UME probe highly depen­ current fluctuations (Fig. 34b).
dent on this distance. This is one of the most relevant factors that can In other studies, SECM has been widely used to investigate the for­
limit this technique. mation and conductivity of biofilms of various bacteria, including
Furthermore, the tip is often very fragile at dimensions of just a few Escherichia coli [209], Staphylococcus aureus [210], Salmonella typhimu­
hundreds of nanometres and is fabricated of glass; therefore probe/ rium [211], Rhodobacter sphaeroides [212] and Paracoccus denitrificans
substrate crashes are a regular occurrence in SECM analysis [205]. To [213]. In these cases, the early detection of biofilm formation is crucial,
allow the piezo to respond to height variations, it is necessary to slowly especially when microorganisms are present at relatively low densities
scan small areas Consequently, the time frame for SECM is relatively on metal surfaces. Under these conditions, the concentration of signal
long, with an area of just a few square millimetres taking over 10 h to molecules is low and can be easily disturbed by the background signals
fully scan. This extended scanning time can lead to issues such as sub­ from metal substrates and electrolytes [214], making detection chal­
strate fouling and aging, solvent evaporation, and irreversible chemical lenging. However, SECM uniquely enables the quantitative monitoring
reaction in the solution [205]. Therefore, SECM is ideal for small surface the redox-active molecules in biofilms [204]. SECM is also used to
areas on the microscale. Despite its limitation, SECM has become valu­ monitor the distribution and activity of enzymes immobilized on flat
able in corrosion science due to its ability to probe surface reactivity of surfaces, enhancing the sensitivity of this technique, through the use
wetted materials at microscopic scales. SECM can be used to study the alternating current mode (AC-SECM) [97].
MIC in real-time, detecting chemical reactions occurring at the interface Zhang et al. [215] investigated the EET mechanisms using SECM to
between two regions (such as metal/air or metal/electrolyte interfaces) examine the formation process of Shewanella biofilm on an electrode. As
in a corrosion process, providing insights into the pathways and speed of the bioelectrogenic process progressed, the SECM tip moved vertically to
such reactions with spatial resolution. In addition, SECM can probe the trigger positive feedback, while the feedback current decreased at each
diffusion layer of specific chemicals on the working electrode surface vertical point. This current change was influenced by two factors: the
and convert chemical signals into visual electrical signals [206]. increased electrochemical activity, which raised the current, and the
The application of SECM for MIC studies offers two significant ad­ shielding effect of the insulating biofilm, which lowered it. These find­
vantages: (1) it provides detailed data on localized corrosion, and (2) it ings demonstrated that SECM characterization can accurately reflect the
delivers insights into absorbed electroactive species (oxidation or biofilm thickness and conductive properties. Given the complexity of the
reduction enzymes) on the electrode. Thus, the technique has a bright biofilm and MIC reactions on metal surfaces, as well as the influence of
future to study MIC and EET-MIC mechanisms in particular [207,208]. metabolites, chemical signal molecules and electron shuttles on surface,
An MIC application example is illustrated in the Fig. 34 [206]. This modern techniques such as SECM are essential for detecting local elec­
work utilised SECM to study the influence of catalase on the MIC of 304 trochemical variations near the biofilm. This helps establish relation­
stainless steel in presence of archaeon N. tibetense. A platinum UME was ships between chemical species and the electrochemical corrosion
utilized as the SECM probe, with the distance between the probe tip and process at metal surfaces. When a potential is applied at the SECM tip,

Fig. 34. SECM image of the stainless-steel samples in the inoculated culture media (a without catalase, c with 0.3 mg/mL catalase) and sterile medium (b with
0.15 mg/mL catalase, d with 0.3 mg/mL catalase) on the 7th day (from ref. [206].

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

the oxidation or reduction of specific chemical species in the culture can become focal points in SECM-based MIC research [214]. The main
media or the biofilm can be observed. The resulting current map can limitation lies in the uncontrollable environmental conditions that
provide insights into the interactions within the biofilm [208]. hinder tracking variations in biological and electrochemical processes,
Li et al. [190] applied this technique to investigate the local con­ as samples are typically immersed in nutrient-rich culture media for
centration of riboflavin as a redox-active mediator and to monitor its extended periods. Another major limitation is the relatively low tem­
interactions with a stainless steel surface during the EET-MIC process. poral resolution, since the in situ detection of MIC is a time-dependent
Through starvation tests, one of common method to investigate the process. Improving the high-speed SECM mode [214] presents a real
mechanism of EET, Huang et al. [216] also applied SECM to reveal the challenge to overcome these limitations. In addition to AC-SECM mode,
acceleration of MIC for 304 stainless steel in the presence of Pseudo­ which is particularly well-suited bacterial behaviour and biofilm activity
monas aeruginosa and absence of an electron acceptor. Through starva­ in biological systems, there are several other operating modes. These
tion tests, a common method to investigate the EET mechanism, it was include Generation and Collection Mode, Redox Competition Mode, and
attributed to the bio-reduction of the passive film in micro-local regions. Potentiometric Mode, which utilize an ion selective electrode.
Multi-mode SECM was used to probe the environmental changes around The SECM approach can be successfully combined with atomic force
the biofilm (~30 µm) caused by respiratory metabolism, including Fe2+, microscopy (AFM-SECM) to probe the local electrochemical responses in
O2 and pyocyanin. liquid while studying the native topography of biological components
Aside from various enzymes, certain metabolic products and pH [217]. For instance, the AFM-SECM has been used to simultaneously
variations near or within the extracellular polymeric substances (EPS) investigate pit nucleation electrochemically and topographical imaging

Table 2
Methods without externally applied electrical stimulation.
Acronym: name Method Strengths Limitation Notes

OCP: Open Circuit Potential Measure of the voltage vs. a Non-destructive. Does not measure the corrosion For passivable alloys,
reference electrode. Simple to apply in both rate. anaerobic conditions
laboratory and field. A stable (reference) electrode is induce a negative shift,
necessary. while the cathodic effect
Different phenomena (oxygen of a biofilm induces a
conc., pitting, sulphides, etc.) positive shift.
can lead to similar variations.

EN: Electrochemical Noise Measure of the stochastic Non-destructive and allows for Requires complex mathematical The increase in current,
fluctuations of the current and the evaluation between approaches to characterize the along with drop in
potential between two nominally uniform and localized significance of the signal. The potential can be
identical sample surfaces, corrosion. noise induced by the setup can associated with localized
typically in the range of mV or be of the same order or higher microbial activity.
µA). than the measured parameter.

SVET: Scanning Vibrating Electrode Measure of the local ionic current Non-destructive and capable of For laboratory studies only. Provides qualitative and
Technique in an electrolyte near the surface mapping corrosion activity and The conductivity of the solution quantitative data about
of a sample using a vibrating surface morphology in real- must be low, and it can change if biofilms and their
microelectrode. This technique time. It allows the ions are released by the mapped component distribution.
maps the spatial distribution of identification of the active surface. It can also be
anodic and cathodic sites. catalytic centers, such as The microelectrode can displace corroborated with optical
enzymes, within the biofilm. and disrupt the biofilm, if it is microscopy.
too thick.

SKP: Scanning Kelvin Probe Measures the contact potential Non-destructive It cannot be employed immersed The SKP technique can be
difference between a sample and Provides high-resolution maps in aqueous solutions, and online combined with AFM
a vibrating reference electrode for the possible galvanic measurement of MIC is not (SKPFM) to visualize both
using a Kelvin probe. element distribution of possible. For laboratory studies topography and Volta
possible galvanic elements. only. surface potentials on
small surface area.

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

of pit formation on aluminum alloy AA1050 [218]. This method pro­ techniques generally do not specifically target MIC, they can monitor
vides pit distributions and detailed information about localized corro­ specific MIC conditions and corrosion rates. However, no single tech­
sion and intermetallic particles, offering real-time data with high spatial nique can fully address all aspects of the complex MIC process. The
resolution (in the micron or submicron range). diverse chemical and physical characteristics, along with the thermo­
dynamics of corrosion layers and their surface distribution, influence
7. Conclusive remarks metal corrosion behaviour and the impact of microorganisms. These
microorganisms form heterogeneous biofilms that interface with and
This review critically examined both conventional and advanced EC adapt to varying environmental conditions at the metal-solution
techniques used in-field and laboratory corrosion studies within the boundary. Consequently, environmental changes due to corrosion can
context of MIC investigations, as summarized in Tables 2 – 4. While EC affect the microbial communities within biofilms. Although the

Table 3
Direct Current (low/high perturbation) Methods.
Acronym: name Method Strengths Limitation Notes

CA: Chrono Amperometry Imposes a fixed potential It investigates electron transfer Does not measure the corrosion Industrial biofilm monitoring
and measures the current kinetics and allows online and rate. The conditions do not probes are based on this
response with time. long-time monitoring of the correspond to the pure technique. They measure biofilm
steady-state. spontaneous process. growth on a specific material
(stainless steel or titanium) that
can be different than on other
materials.

LPR: Linear Polarization Resistance Perform a potential sweep It is non-destructive and allows Issues in correlate Rp with Commercial corrosimeters for
within a narrow range of a the estimation of instantaneous corrosion current: suitable field and industry are most often
few millivolts around the corrosion rates both in the field coefficients from weight losses or based on the LPR technique,
corrosion potential. and in laboratory. The theory was Tafel plots are mandatory. despite its limitations.
validated for cases of uniform
corrosion, confirmed in
laboratory settings at neutral pH
and in field conditions.

PDP: Potentiodynamic Scan of a wide range of Provides an overview of corrosion The anodic sweep is invasive, The impact on microbial
Polarization potential ( ± 0.5 to ± 1 V reactions, charge transfer, particularly deleterious to activities decrease as the scan
vs OCP) at scan rates of diffusion-controlled reactions, microorganisms within the rate increases.
< than 5 mV s–1 passivity, transpassivity, and biofilm.
(typically, 1.666 mV s− 1). localized corrosion. It is unusable if system is under
diffusion control.

CV: Cyclic Voltammetry A rapid triangular The highest scan rate emphasizes Faradaic current can be masked It examines the reversible,
potential scan cyclically sharpe redox peaks, such as the by high resistive and/or irreversible or quasi-irreversible
applied to the working ones resulting to enzymatic capacitive characteristics of the behaviour of redox couples at the
electrode, at a fast scan activity, making it useful for surface. Weakly concentrated metal-solution interface.
rate (i.e., 10 − 50 mV s–1). highlighting biofilm electroactive species in solution
electroactivity. may not be detected.

SECM: Scanning Electro Chemical The surface of the working It is ideal for studying the surface Measurement can take over 10 h SECM can be combined with the
Microscopy electrode is scanned in at the level of individual microbes to fully scan, leading substrate atomic force microscopy (AFM-
three-dimensional manner and the activity of redox enzymes. fouling and aging, solvent SECM) to study in a liquid phase
using a polarized It can focus at nanometric scale evaporation, and irreversible the topography of biological
microprobe. (the most sensitive). chemical reactions in solution. components.

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P. Cristiani et al. Corrosion Science 246 (2025) 112694

Table 4
Sinusoidal Stimulation (Low Perturbation) Methods.
Acronym: name Method Strengths Limitations Notes

EIS: Electrochemical Impedance Spectroscopy The steady-state is Allows the characterization of the To analyze the data, While EIS cannot directly
disturbed by a low- electric phenomena (capacitance, specialized software and a determine the corrosion
amplitude sinusoidal resistance, inductance) occurring physical model are current density and biofilm
voltage at the solid/solution interface, essential. MIC can deviate thickness, it is capable of
(10 mV) at variable both in the laboratory and in the from a steady-state estimating the thickness of
frequencies ranging from field. condition during the the passive oxide layer
0.01 Hz – 106 Hz. measurement. The low thickness beneath the
frequency response is the biofilm.
most relevant for MIC.

EFM: Electrochemical Frequency Modulation A dual frequency potential Simpler and faster than EIS, it can The Rp derived EFM can be This method is well-suited for
perturbation is applied, provide data on corrosion rate and comparable with the Rp MIC because it is less prone to
and the current density polarization resistance, activation, derived from EIS and the errors caused by solution
response is traced. The diffusion, or polarization techniques, resistance and the double
generated spectrum of passivation–controlled only when the corrosion of layer charging during
multiples harmonics, is phenomena. metal is subject to an potential perturbation.
analyzed. activation-controlled
process.

LEIS: Localized Electrochemical Impedance A sinusoidal amplitude as Insights into local resistive, As for other EIS techniques, The output is a complex
Spectroscopy for traditional EIS. capacitive, or diffusional can be limited at low impedance 3D plots (Nyquist,
However, a local potential properties of the surface. frequency ranges, where Bode) or spatial maps of
difference is measured at a It is a two-step method that can be capacitive and inductive capacitance (presented in
moveable microelectrode suitably personalized. artifacts can cause a different colors).
that explore the sample polarization resistance with Suitable for complex
surface. negative values. Resolution electrochemical studies.
depends on the size of the
probe and its distance from
the surface.

evolution of chemical and microbial components over time effectively CRediT authorship contribution statement
assessed, the overall process remains largely unpredictable, making it
challenging to accurately replicate biofilm development and MIC at the Regine Basseguy: Writing – review & editing. Digby Macdonald:
laboratory scale. Ultimately, it is crucial to emphasize that combining Writing – review & editing. Tingyue Gu: Writing – review & editing.
electrochemical techniques with complementary methods -such as mi­ Dawei Zhang: Writing – review & editing. Pierangela Cristiani:
croscopy and advanced molecular analyses, including next-generation Writing – review & editing, Conceptualization. Masoumeh Moradi:
sequencing of specific genes - forms a powerful multi-evidence Writing – review & editing, Data curation, Conceptualization. Dake Xu:
approach. This integrative strategy can provide unparalleled insights Writing – review & editing, Supervision. Julian A. Wharton: Writing –
into the roles of biofilms, specific microbial groups, enzymes, and in­ review & editing.
dividual cells, offering a deeper understanding of the complex systems
across the taxonomic kingdoms of microorganisms involved in MIC. Declaration of Competing Interest
Despite present limitations, EC techniques offer innovative methods
for studying microbial corrosion across various scales. New online The authors declare that they have no known competing financial
probes can assist plant operators in preventing corrosion by optimizing interests or personal relationships that could have appeared to influence
biocide treatments. Additionally, the ability to visualize and electro­ the work reported in this paper.
chemically characterize surface regions as small as 1 µm enables more
precise analysis of MIC-induced localized degradation under controlled Acknowledgement
laboratory conditions. In conclusion, advanced EC techniques, com­
bined with microelectrodes and microscopy, now enable more effective Authors thanks the COST Action European MIC Network—New
exploration of MIC phenomena at the microscale. These methods can paths for science, sustainability and standards (Euro-MIC) [CA20130]
evaluate the effects of individual microorganisms rather than merely for supporting this work. RSE contribution to this work has been
integrating the impact of complex biofilms. Additionally, advanced financed by the Research Fund for the Italian Electrical System under the
statistical methods can efficiently process the large volumes of data Three-Year Research Plan 2025-2027 (MASE, Decree n.388 of
produced by these techniques, promising a deeper understanding of the November 6th, 2024), in compliance with the Decree of April 12th,
interactions between bacteria and metallic surfaces, as well as the 2024.
associated redox reactions, in both spatial and temporal contexts.

32