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http://www.revistadechimie.ro REV.CHIM.(Bucharest)♦ 67♦ No. 7 ♦ 20161264

EIS Study on Biocorrosion of Some Steels and Copper inCzapek Dox Medium Containing Aspergillus niger Fungus

ANCA COJOCARU1*, PAULA PRIOTEASA (BARBU)2, ILONA SZATMARI1, ELENA RADU2, OANA UDREA2, TEODOR VISAN1

1 University Politehnica of Bucharest, Department of Inorganic Chemistry, Physical Chemistry and Electrochemistry, 132 CaleaGrivitei, 010737, Bucharest, Romania2 National Institute for Research and Development in Electrical Engineering ICPE-CA Bucharest, 313 Splaiul Unirii, 030138,Bucharest, Romania

The paper reports a study about biocorrosion of S235J2G3 low carbon steel, AISI 304 stainless steel and purecopper in Czapek-Dox media inoculated with Aspergillus niger fungus. A comparison of corrosion behaviorin media without/with sucrose (30 g L-1) was made using electrochemical impedance spectroscopy (EIS)technique. The influences of sucrose addition together with the immersion time on Nyquist and Bodediagrams for all three metallic materials were investigated. From the comparative EIS experiments it hasresulted that the corrosion behavior is different but more significant growth of fungus was recorded incoloidal solution with sucrose. Both Nyquist and Bode spectra indicated the formation of a thicker biofilmowing to the presence of sucrose as a carbon source, leading to much faster biocorrosion especially in thecase of copper. Values of circuit parameters for fitting EIS data were determined.

Keywords: biocorrosion, Czapek Dox culture medium, Aspergillus niger, carbon steel, AISI 304 stainlesssteel, copper

In all processes involving the use of metallic materialsin the presence of microorganisms like bacteria, themicrobial corrosion and degradation take place. This is whyin the last decades biocorrosion has become a significantarea of scientific research. The electrostatic chargeaccumulation on the surface of the metal and the surfaceroughness favor the formation of collonies as a biofilmcontaining bacterial cells [1-5]. The attachment of bacteria,release of metabolites and formation of biofilms, whichare corrosive, cause damage to the metal structure whichis termed ‘microbiologically influenced corrosion’ (MIC)[6-9]. There are no official documents for the cost of MIC,but studies have shown that at least 20% of the totalcorrosion costs are due to biocorrosion [10,11] and thedamages magnitude is influenced by the presence andactivities of microorganisms and their metabolites.

The detection, monitoring and prevention ofbiocorrosion have gained significance in recent years. Theproblem is complex because different microbial population(aerobic and anaerobic bacteria) in a system interactsduring the corrosion of metal in different ways. In general,MIC does not involve a new mechanism of corrosion. Therole of microorganisms on the metal surface is to carry outspecific biochemical reactions that alter the physical/chemical conditions and favor the maintenance ofcathodic and/or anodic half-reactions at the metal/solutioninterface [12-16]. However, few papers reported themethods to control the biocorrosion of various metals [17-21].

The biocorrosion of carbon steel and stainless steel isthe main cause of problems in the pipes of oil industry[22], affecting the costs of production and storage byvarious repairs and replacements (i.e., replacing pittedpipes, water heaters and other appliances affected bycorrosion products). Biocorrosion of mild carbon steel aswell as of iron was extensively studied in the presence ofdifferent species of sulphate-reducing bacteria [13,22-24].Owing to activity of Thiobacilli, sulfuric acid is producedfrom the oxidation of various inorganic sulphur sources

*email: [email protected]

such as thiosulphates. This acid causes corrosion of mildsteel and iron with formation of sulphates, which arenutrients for sulfate reducing bacteria [25,26].

Biocorrosion of low carbon steel due to various speciesof mitosporic fungi was studied by Lugauskas et al. inorganic media [27]. It was shown that micromycetes asArthrinium phaeospermum, Aspergillus niger andChrysosporium merdarium affected steel surface mostactively, whereas the products of the activity of bothPenicillium cyclopium and Cladosporium herbarum areveiled by the formed layers of Fe oxides and hydroxides.The biocorrosion of mild carbon steel have been studiedby us in conditions of synergistic action of both stray current(during polarization in a.c. voltage) and Aspergillus nigerfilamentous fungus [28]. Such corrosion type is commonin buried steel structures [29,30]. Also, the presence ofAspergillus niger in Czapek Dox medium led to citric acidas metabolite on S235J2G3 carbon steel surface [31].

Electrochemical behavior of stainless steels forbioprocessing industry, especially food industry, wasstudied intensively in recent years. The existence ofPseudomonas spp. or its biofilm on the stainless steelcauses formation of oxygen concentration cells, becauseon low oxygen concentration zones the metal acts asanode and the oxygen-rich zones become the cathode [16].In general, the pitting corrosion of metals by sulfatereducing bacteria is a result of the electron transfer frommetal surface to the bacterial sulphate reduction pathway.In a series of works, Stoica et al. [32-37] have investigatedthe biocorrosion of AISI 304 stainless steel immersed infungal suspensions (Aspergillus niger, Geotrichumcandidum) during the exposure at the disinfectant solutionsof ActiSEPT or Oxonia-Active.

Copper is a common material for tanks or pipes used inmany industries because of its robustness and malleability.Also, in various industrial and domestic cases, Cu pipesare used for the transport of cold or hot water. Althoughoften assumed to have inherent anti-microbial properties,Cu metal undergoes however degradation by biocorrosion.

REV.CHIM.(Bucharest)♦ 67♦ No. 7 ♦ 2016 http://www.revistadechimie.ro 1265

The corrosion of copper by microorganisms such asArthrobacter sulfureus bacterium causes blue-green water(BGW) problem 15,38-43]. Recently, Lugauskas et al.[44,45] have investigated the influence of micromyceteson the corrosion behaviour of Cu in environments pollutedwith organic substances.

With this in view, the objective of this study is to evaluatethe formation in Czapek Dox broth of Aspergillus nigerbiofilms and their effects for three metallic materials:carbon steel, AISI-304 austenitic steel and pure copper.Previously, we have investigated the biocorrosion processof S235J2G3 carbon steel in the same immersionconditions, the process being initiated by Aspergillus nigerfilamentous fungi [31]. In another paper [46] the researchwas focussed on polarization curves technique andmicroscopy in order to prove biocorrosion of AISI 304stainless steel and pure copper. In that work two Czapek-Dox media, without and with sucrose as carbon source forthe fungus, have been inoculated with Aspergillus nigerand the corrosion rate and micrographies were observedduring 14 days. The present paper extends theseinvestigations by considering the possibility of usingelectrochemical impedance spectroscopy (EIS) techniqueto better understand the action of fungus during corrosionof all three metallic materials, carbon steel, stainless steeland copper.

Experimental partAll three types of metallic samples consisted in sheets

with 0.4 mm thickness. The surface of coupons was firstsequentially polished with grit abrasive paper (from 800 to2000µm) and with aluminum oxide suspension to obtaina glossy surface, then was cleaned with distilled waterand dried before ever y measurement. Chemicalcompositions of S235J2G3 carbon steel for general use[47], AISI 304 austenitic steel with low carbon content[48] and copper with a purity for electrical purposes [49]were according to European standards. The used CzapekDox culture medium has prepared according torecommended chemical composition [50] by introducinginorganic salts (Merck) in water as solvent and adding excessagar-agar to form a gel. Prior to inoculation with spores ofAspergillus niger the coloidal solutions were sterilized byautoclaving for more than 30 min at 1100C. Tests wereperformed using Aspergillus niger strain (approx. 106

spores/mL) [51]. In order to evidence the influence ofsucrose on the growth of Aspergillus niger we used anddenoted two different Czapeck Dox culture media as: Amedium (without any carbon source) and B medium with30 gL-1 sucrose (Merck) as easily assimilated carbon sourcefor fungus.

Electrochemical impedance spectroscopy (EIS)measurements were carried out with an ac voltageamplitude of ±10 mV in the 10 mHz < f < 20 kHzfrequency range. The impedances of metallic sheets atopen-circuit potential were represented as Nyquist(imaginary component of impedance vs. real component)and Bode (both impedance modulus and phase angle vs.frequency) spectra. A VOLTALAB 40 type PGZ 301potentiostat/galvanostat provided with a frequencyresponse analyzer was used. The EIS experimental datawere processed with a VoltaMaster 4 graphical interface.Equivalent electrical circuits as electrochemical modelsof the metal/Czapek Dox solution interface wereestablished using the specialized software Zview 2.90c(Scribner Assoc.) and used for data fitting.

EIS studies were performed in an electrochemical cellwith one single compartment having the investigated

metallic sheet as working electrode, a platinum sheet asauxiliary electrode and a saturated calomel electrode(SCE) as reference electrode. The experiments have beencarried out in stationary conditions at room temperature(23±2oC). All samples and electrochemical cell prior tothe determination were sterilized by autoclaving for halfan hour to 110 oC and 5 bar pressure.

Results and discussionsCorrosion of carbon steel for general use

Figures 1 a-d show the experimental impedance dataobtained for the S235J2G3 steel samples in Czapek Dox Amedium after different immersion times. The first Nyquistcurve (fig. 1a) refers to an immersion of steel in a sterile Amedium, before introduction of Aspergillus niger, andtherefore it may be considered an initial time of immersion. Inthis figure the first capacitive semicircle at the highestfrequency, with very small diameter, may be neglectedbeing probably a short disturbation of electrical double layerdue to occurrence of Fe2+ ions by corrosion. As aconsequence, we have considered as the main effect onlythe large semicircle (inductive loop) having a diameteraround 800 Ω cm2. It is seen that Nyquist semicircles after3 days (fig. 1b) and 14 days (fig. 1c) of immersion ininoculated A solution differ significantly from data ofsterilized medium. The diameter value (which is in factthe polarization resistance, Rp) decreases drastically to100 Ω cm2 for the steel coupon immersed after 3 days,and to 60Ω cm2 after 14 days. Such changes indicate theintensification in time of corrosion process and the pitpropagation onto metal surface in the biotic medium. Takinginto account lower values of imaginary part of impedance(ZIm) than of real part (ZRe), all Nyquist plots presentdepressed semicircles, with the centre of semicircle underthe real axis at the higher frequencies. This fact is due toroughness, meaning inhomogeneity of the metal surface.Additionally, the straight line with angle of 45o at lowfrequency noticed in figure 1c is attributed to the diffusionresistance offered by the biofilm of the electroactivespecies, Aspergillus niger, from bulk solution to the metalsurface. This biofilm does not affect the corrosion rate,such features being due to the formation of porous andnon protective films on the surface.

The formation of biofilm is confirmed by Bode diagrams(fig. 1d), where the impedance modulus decreasesgradually in time. Evolution in time of maximum phaseangle is as following: it starts from -10o which ischaracteristic for an electrical conduction of metallicmaterial (steel), then has a value of -30o corresponding toa thin porous film, and finally reaches to -35o, this decreaseof angle being due to a thicker porous film. Biofilmformation of fungi on the surface may be responsible forreduced capacity of electrochemical system, as will beexplained further by simulating the electrical equivalentcircuit.

Experimental impedance data obtained for theS235J2G3 carbon steel samples in Czapek Dox B mediumafter different immersion times were represented in figures2 a-d. Immersion of steel in the sterile B medium, whichcontains an addition of 30 g L-1 sucrose before introductionof Aspergillus niger (fig. 2a), has led to a Nyquist semicirclewith a quite similar diameter of approx. 600 Ωcm2 ascompared with immersion in A medium (fig. 1a).Nevertheless, Nyquist not-fully semicircle recorded after 3days (fig. 2b) has significant higher diameter, of approx.1200 Ωcm2, suggesting the beginning of a passivationphenomenon with diffusion limiting. Similarly, a portion ofNyquist not-fully semicircle was recorded again after 14 days


of immersion (fig. 2c), but now it returns to a lower diameter,of approx. 600 Ω cm2. All Nyquist plots in figures 2 a-c.Also, on the basis of lower values of imaginary part ofimpedance (ZIm) than of real part (ZRe), it can say that allNyquist plots present depressed semicircles with thecentre under the real axis, which is due certainly toroughness and adsorbed cells of fungus. Moreover, figure2c shows that the capacitive semicircle is continued with astraight line proving the formation of a dense biofilm.

On the whole, the rate of growth for Aspergillus nigerfungus in Czapek Dox B medium, in the presence of sucrose,which is easily assimilated carbon source, is greater than inA medium, as expected. Bode diagrams for immersion thesteel in Czapek Dox B medium (fig. 2d) can also give anindication about the passivation produced by biofilm. Thus,the impedance modulus increases gradually in time. Themaximum phase angle starts from -10o in sterile B medium

that corresponds to a very conductive non-covered steel,then increases to -27o after 3 days in inoculated solutionand to -35o after two weeks in inoculated solution; thisfinal value suggests the formation of a thicker porous filmduring prolonged time.

Corrosion of AISI 304 stainless steelFigures 3 a,b show the experimental impedance data

obtained for AISI 304 austenitic steel samples in CzapekDox “A” medium after different immersion times. It is worthto mention that all impedance values are with one order ofmagnitude higher than the impedances for S235J2G3carbon steel. Our obtained impedances with values of tenskÙ cm2 are in very good agreement with values (40-440kÙ cm2) reported earlier for AISI 304 steel in suspensionsmedia with Aspergillus niger content [34]. In figure 3a thereare represented Nyquist spectra for three cases: immersion

Fig. 1. Comparative Nyquist spectra ofS235J2G3 steel in Czapek Dox A mediumobtained by immersion in sterile (a) and

inoculated with Aspergillus nigerafter 3 days (b) and 14 days (c). The

corresponding Bode spectra are alsoshown (d)

Fig. 2. Comparative Nyquist spectra ofS235J2G3 steel in Czapek Dox B medium

(with 30 g L-1 sucrose) obtained byimmersion in sterile (a) and inoculated

with Aspergillus niger after 3 days (b) and14 days (c). The corresponding Bode

spectra are also shown (d)


in sterile A medium, in inoculated with Aspergillus nigerafter 3 days and after 14 days. The non-depressedcapacitive semicircle occurred at high frequencies has thesmallest diameter for sample immersed in sterile mediumindicating an intense corrosion. In the following two casesalmost identical values of diameter are observed,suggesting a relatively constant corrosion rate in time.However, the formation of biofim is evidenced even at initialtime (case of sterile A medium) by the onset of a straightline at low frequency. This tendency is amplified after 3and 14 days of immersion, where the length of linear portionis prolonged and it maintains the same slope of 45o,meaning a diffusion-controlled process through porousbiofilm.

Bode spectra (fig. 3b) illustrate the correspondingchanges on the stainless steel surface. The impedancemodulus decreases gradually with immersion time. Themaximum phase angle also decreases in time, changingfrom -80o (electric insulator surface) to -70o after 3 daysand to -35o after 14 days, the last being a characteristicvalue for porous layer which is placed at low frequency. Itmay be noticed that in the case of AISI 304 stainless steelthe biofilm formed by Aspergillus niger cells may also includeiron and nickel oxides (or hydroxydes) resulted by corrosion ofsample material.

In figures 4 a,b there are represented Nyquist and Bodespectra for AISI 304 stainless steel in three cases ofimmersion in Czapek Dox B solution: in sterile mediumand in inoculated with Aspergillus niger after 3 days andafter 14 days. First, the large values of impedance of theorder of kiloohms × square centimeter can be observed,with one order of magnitude higher than for S235J2G3carbon steel. The shapes of Nyquist curves (fig. 4a) arenon-depressed semicircles continued with linear portionsinclined at 45o within lower frequency region. However,the diameters of semicircle decrease gradually withimmersion time clearly indicating the formation of biofilmand its continuous increase on the stainless steel surface.

We believe that a very porous layer is deposited duringmaintaining AISI 304 stainless steel sample in B medium.In spite of this, the Bode maximum phase angle keepsconstant its dielectric value (around -70o) along theexperiment together with a constant impedance modulus(fig. 4b). These results can be interpreted in terms of anelectrically inert layer composed from both attached funguscells and their bioproducts. It is probably to be included inthis biofilm inorganic ions (Fe2+, Ni2+) or salt molecules(MgSO4), taking into account that the chromium metalfrom stainless steel does not dissolved.

Corrosion of copper for use in electrical applicationsIt was shown in literature [43] that in spite of its anti-

microbial properties copper metal is however degradedby biocorrosion in aqueous or organic media. Our Nyquistand Bode impedance spectra recorded for copper samplesin Czapek Dox A medium after different immersion timesare presented in figures 5 a,b. It may be observed in figure5a that Nyquist capacitive semicircles are extremely flatcompared to stainless steel and even carbon steel. Thisver y depressed shape means a high degree ofinhomogeneity of copper surface in given experimentalconditions with both high roughness and adsorbed fungusspecies. It is known that the interception of the realimpedance axis with the Nyquist semicircle at highfrequency is exactly the uncompensated ohmic resistanceof the bulk solution [52]. As figure 5a shows, the start foreach semicircle is gradually moved toward high ZRe valueshowing an increase of solution resistance duringimmersion time, whereas the semicircle diameterincreases in time, too. Hence, the corrosion process ofcopper is intensified, correspondingly. The order ofmagnitude for impedance values obtained is of tens Ωcm2, correlated very well with the previous results ofcorrosion rate of copper determined by potentiodynamicpolarization experiments in the same system [46] as wellas by weight loss experiments [53]. It may observe for the

Fig. 4. Comparative Nyquist spectra(a) and Bode spectra (b) of AISI 304

stainless steel in Czapek Dox Bmedium (with 30 g L-1 sucrose)

obtained by immersion in sterile (1)and inoculated with Aspergillus niger

after 3 days (2) and 14 days (3)

Fig. 3. Comparative Nyquist spectra (a)and Bode spectra (b) of AISI 304 stainlesssteel in Czapek Dox A medium obtained

by immersion in sterile (1) and inoculatedwith Aspergillus niger after 3 days (2) and

14 days (3)


third Nyquist semicircle (two weeks immersion) acontinuation with a long straight line, inclined at 45o

attributed to biofilm formation.Bode diagrams (fig. 5b) show a gradual increase of

impedance modulus in time. Interestingly, in the samefigure we noticed very low values of maximum phase anglewhich first increases from -5o (in sterile medium) to -9o

(after 3 days in inoculated medium) and then decreasesagain to almost -60. The order of magnitude of these valuessuggest a very electrically conductive metal/mediuminterface which is probably correlated to occurrence of anon-adherent biofilm, meaning that the copper surfacealmost remains free and corrodes continuously duringexposure to the aggressive medium.

Figures 6 a,b show the experimental impedance dataobtained for pure copper samples in Czapek Dox B mediumafter different immersion times. One can see in the Nyquistrepresentation (fig. 6a) that the variation of solutionresistance during immersion time increases excessive,from initial value of 40 Ω cm2 reaching more than 350 Ωcm2 after two weeks of immersion and suggesting asignificant gelification of B medium compared to Amedium. Figure 6a shows only capacitive semicircles forall experiments and a gradually increase of diameter(polarization resistance, Rp) from initial time up to 14 days,i.e. a diminution of corrosion rate of immersed coppercoupons. The last semicircle (curve 3) has a final part likea snail in low frequency range that suggests a strongadsorption process. figure 6b presents only the variationwith ac frequency of phase angle as a part of Bode diagram;regarding impedance modulus no changes during exposureof copper surface to Czapek Dox B medium wereregistered. One can see in figure 6b a quite similar behavior

of phase angle, with maximum values in the range of smallvalues, from -7o to -10o. We suppose that in Czapek Dox Bmedium the atached biofilm includes predominantly in thefirst 3 days some conductive corrosion products consistingin copper oxide or hydroxide species, making on the wholean electrical conductive interface between electrodesurface and electrolyte. After this initial period theaccumulation of fungus metabolism products ispredominatly and these non-conductive products adsorbon copper surface, thus explaining the increase ofmaximum phase angle.

Fitting the impedance data by using equivalent electricalcircuits

The impedance spectra are analysed using equivalentcircuits, taking into account the contribution of eachphenomenon, such as the electrical double layer, electrontransfer during corrosion, film formation, and others. Theelectrical circuits used to model the interfaces of AISI 304stainless steel and pure copper are shown in figures 7 a,b.To obtain more precise fitting results, the capacitanceelements (of electrical double layer and of biofilm) in theelectrical equivalent circuits were both replaced withconstant phase elements (CPE) [54]. The components inthe equivalent circuit presented in figure 7a are thefollowings: Rs - the electrolyte solution ohmic resistance inseries with a parallel sub-circuit comprising CPE1 and Rp -the parameters used to describe the non-ideal capacitanceof electrical double layer and polarization resistance,respectively. In figure 7b, besides to solution resistance,the equivalent circuit contains two parallel sub-circuits,accounting for the behavior of electrode/solution interface(identical with that from fig. 7a) and, supplementary, for

Fig. 5. Comparative Nyquist spectra (a)and Bode spectra (b) of copper

in Czapek Dox A medium obtained byimmersion in sterile (1) and

inoculated with Aspergillus niger after3 days (2) and 14 days (3)

Fig. 6. Comparative Nyquist spectra(a) and Bode spectra (b) of copper inCzapek Dox B medium (with 30 g L-1

sucrose) obtained by immersion insterile (1) and inoculated with

Aspergillus niger after 3 days (2) and14 days (3)

Fig. 7. Electrical equivalent circuits for interfaces in sterile Czapek Dox media(a) and in Czapek Dox media inoculated by Aspergillus niger (b). Circuit

parameters: Rs – ohmic resistance of electrolyte solution; CPE1 – constantphase element for non-deal capacitance of electrical double layer;

Rp – polarization resistance; CPEf – constant phase element for non-dealcapacitance of biofilm; Rf – ohmic resistance of biofilm

a

b


the biofilm: non-ideal capacitance of biofilm (CPEf) andbiofilm ohmic resistance (Rf).

The impedance equation of CPE is defined by thefollowing equation:

(1)

where T is the magnitude of capacitance (Ω-1 cm-2 sn orapproximately µF cm-2) which is the time constantcomponent of CPE, ω is the angular frequency of ac voltage(ω=2πf, with frequency f in Hz), and j is the imaginaryunit, j2 = -1; p is the exponential part of the constant phaseelement. For p=1, the CPE can be considered to be anideal capacitor.

The EIS spectra for sterile Czape Dox media (initial timeof immersion) is represented by equivalent circuit fromfigure 7a. This result indicated that the corrosion at initialtime in both sterile A and B media was under chargetransfer control. EIS spectra for the inoculated A and Bmedia can be better simulated with the equivalent circuitpresented in figure 7b where the second sub-circuitaccounts for biofilm characteristics. Numerical values foreach parameter of circuit were calculated upon fittingthe model circuit to the experimental data and the obtainedvalues of fitted electrochemical parameters aresummarised in tables 1 and 2.

It can be observed from both tables that the Rs valuesare similar for both sterile and biotic systems being of the

order of tens Ω cm2 for stainless steel and copper. Pseudo-capacitances of double layer (CPE1-T) have values ofhundreds µF cm-2 for AISI 304 stainless steel and thousandsµF cm-2 for copper, while the p exponents (CPE1-P) were0.6-0.7 and 0.2-0.4, respectively; this means a morecomplex structure of the electric double layer in the copper/solution case which probably is due to adsorption andinclusion of inorganic corrosion products in biofim. Ingeneral, the polarization resistance Rp decreasessignificantly for longer duration of immersion (14 days)being correlated with increase of corrosion current by thegrowth of fungus population and its intensified activity. Thegrowth of the bacterial population may probably causepitting corrosion on the metal surface and this is a subjectfor future research. The corrosion is also accompanied bychanges in biofilm characteristics. CPEf-T values for casesof stainless steel in both A and B media are of usual orderof magnitude (50-60 µF cm-2) for the interface in aqueoussolutions and the corresponding CPEf-P is close to 1 (nearideal behaviour of capacitance). On contrary, CPEf-T valuesfor cases of copper in both A and B media have thousandsof ìF cm-2 and the corresponding CPEf-P is in the range of0.4-0.9. This may be explained by the damage to the biofilmformed, resulting in an increase in the active area and a lotof irregularities available for the corrosion process. All theseEIS results listed in tables 1 and 2 match well with thepotentiodynamic polarization results regarding bio-

Table 1VALUES OF CIRCUIT

PARAMETERSFOR DIFFERENT IMMERSION

PERIODSOF AISI 304 STAINLESS STEEL IN

CZAPEK DOX MEDIA

Table 2VALUES OF CIRCUIT

PARAMETERSFOR DIFFERENT IMMERSION

PERIODSOF COPPER FOR ELECTRICALAPPLICATIONS IN CZAPEK DOX

MEDIA


corrosion of stainless steel and copper in the same bioticmedia, reported earlier by us [46, 53].

ConclusionsThe results obtained using electrochemical impedance

spectroscopy suggest that the growth of Aspergillus nigerfungal in Czapek Dox solutions, especially in those withthe addition of sucrose, significantly facilitates corrosionprocess of all investigated metallic surfaces: S235J2G3low carbon steel, AISI 304 stainless steel and pure copper.For comparative Nyquist spectra the common feature is adepressed semicircle (at high frequency) followed by astraight line at lower frequency with a slope angle close to45°, all indicate the formation of a thicker biofilm owing tothe activity of Aspergillus niger colonies. The much morecorroded is copper surface and the least corroded is AISI304 stainless steel surface. We succeeded to modelizeEIS results by equivalent electrical circuits and to obtainvalues of circuit parameters.

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