Characterization of plants and seaweeds based corrosion inhibitors against microbially influenced corrosion in a cooling tower water environment

2.1 Test organism

The corrosive bacterium of Pseudomonas stutzeri (2.2 × 10⁶ (approximately 10⁶ colony-forming unit/milliliter (CFU/mL))) was used in the experimental part of this study, and it was isolated from the cooling tower water collected from Ranipet Tannery Effluent Treatment ltd., Ranipet, Tamilndau. The isolated strain was cultured on the Luria-Bertani (LB) medium with the optimum condition (Kokilaramani et al., 2020).

2.2 Biochemical analysis

The identified bacterial culture was inoculated in the selective media of MacConkey agar and incubated for 16 h at 37 °C. After incubation, the isolated single colony was tested for biochemical tests (Kamil and Al-Hassani, n.d.).

2.3 Inhibitors preparation

The methanolic extraction of green inhibitors was extracted from various samples of plant leaf Artemisia pallens (MEAP) (Kokilaramani et al., 2020), Rhizophora mangle (MERM), Avicennia marina (MEAM) (Kokilaramani et al., 2022), and seaweeds Pandia povanica (MEPP), Sargassum tenerrimum (MEST) (Kokilaramani et al., 2021b). The green inhibitors were extracted by using the Soxhlet extraction method under optimal conditions. Following the extraction, it was filtered with a syringe filter (0.2 μm) before being dried in oven. The dried material was stored in the refrigerator for further examination (Zaher et al., 2022).

2.4 Gas chromatography-mass spectrometry (GC–MS) analysis

The inhibitor components were analysed by Agilent 7890A (GC) and 5975C (Mass Spectrometer) instrument, 1 μL of MEAP, MERM, MEAM, MEPP and MEST loaded in the column HP5-MS, injector volume of 1 μL of carrier gas (helium) with a 1.0 mL/min of flow rate. and the oven was preheated to 55 °C for 2 mins, increased to 70 °C/min, detector temperature of 225 °C, the overall running time is 35.5 mins and the compounds are identified by comparing the findings to the matching mass spectra in the library data (NIST05.LIB) (al Otaibi and Hammud, 2021; Mobin et al., 2020).

2.5 Antibacterial activity

The antibacterial activity of MEAP, MERM, MEAM, MEPP and MEST against SKR4 was determined using the agar diffusion technique. The bacterial strain was swapped on Muller Hinton Agar (MHA) plates and the agar was diffused with a gel puncher before adding the inhibitors, positive control (Ampicillin), and negative control (Double Distilled Water (D.D H₂O)). The plates were incubated at 37 °C for 24 h and to assess the antibacterial activity of inhibitors, a diameter of the bacterial inhibition zone in millimeters (mm) was determined (Odusote and Ajayi, 2013).

2.6 Minimal inhibition concentration assay

MIC assay was carried out in the broth dilution method on the different concentrations (10, 25, 50, 75 and 100 ppm) of green inhibitors (MEAP, MERM, MEAM, MEPP, MEST) were taken against the SKR4 bacterial strain in Mueller Hinton Broth (MHB). After the inhibition period, samples were examined in optical density (OD) values for each sample, the experiment was done in a triplet (Thakur and Kumar, 2021).

2.7 Enumeration of total viable count (TVC)

The bacterial count was performed by the serial dilution of incubators (MEAP, MERM, MEAM, MEPP, MEST) in different concentrations (10, 25, 50, and 100 ppm) on the MH Agar plates in spread plate method and the colony counts were done after the period of incubation for 24 h at 37 °C, it was denoted as CFU/mL (Feijoo-Siota et al., 2008).

2.8 Biofilm inhibition assay

The impact of green inhibitors at various concentrations on SKR4 for biofilm inhibition was assessed using a crystal violet (CV) test as earlier described (Pipattanachat et al., 2021). The log-phase bacterial culture SKR4 was resuspended in the MH broth and added to 96 well microtiter plate by following systems. A Total number of biofilm formation systems is 12 (n = 12) were placed on the 96-well microtitre plate with 1 mL of MHB (control) standard for all methods. System 1 (S1) - Control; System 2 (S2) - S1 + 100 μL of (0.500 OD) SKR4 culture; System 3 (S3) - S2 + 25 ppm of MEAP; System 4 (S4) – S1 + 25 ppm of MEAP; System 5 (S5) – S2 + 25 ppm of MERM; System 6 (S6) – S1 + 25 ppm of MERM; System 7 (S7) – S2 + 25 ppm of MEAM, System 8 (S8) – S1 + 25 ppm of MEAM, System 9 (S9) – S2 + 25 ppm of MEPP, System 10 (S10) – S1 + 25 ppm of MEPP, System 11 (S11) – S2 + 25 ppm of MEST, System 12 (S12) – S1 + 25 ppm of MEST. The plate was incubated for 24 h at 37 °C to induce biofilm growth. After incubation, wash the area with phosphate-buffered saline (PBS, pH-7.4) to remove any adhering microorganisms. The biofilms were treated for 20 mins at room temperature (RT) in 70 % ethanol and it was withdrawn from each well then 200 µL of 1 % crystal violet dye was added for staining in 20 mins. It was washed three times in PBS to get rid of the remaining color and each well received 500 µL of 30 % acetic acid to elute the excess dye. After 10 mins of incubation, the results were assessed at 570 nm using a UV–Visible spectrophotometer and the system of biofilm assay was done in triplicate (Karunakaran and Biggs, 2011).

2.9 Specimen preparation

The Mild Steel 1010 (MS 1010) coupons employed with a percent composition of Si- 0.002, S-0.026, P-0.01, Ni-0.04, Mo-0.002, Mn-0.16, Fe-99.64, Cu-0.093, C-0.03. For weight loss, it was cut into 2.5 cm² by 2.5 cm² with a 2 mm hole, SEM examination, 1 cm² by 1 cm² with a 1 mm hole at the top for hanging reasons and 1 cm² by 1 cm² for electrochemical study (working electrode (WE)). The surface of the specimen was polished with different grade sheets and cleaned in trichloroethylene, rinsed on D.D H₂O and sterilized with acetone and ethanol, before being kept in the desiccator, specimens were air-dried. The experimental coupons were treated under UV light exposure for 30 mins before starting the experiments (Kokilaramani et al., 2021b).

2.10 Weight loss (WL) method

A weighted sample (coupon − 2.5 cm2 × 2.5 cm2) of the MS 1010 was introduced into the SKR4 and different inhibitors. A Total number of biofilm formation systems is 12 (n = 12) were placed on a 500 mL conical flask with 400 mL of autoclaved CTW with 1 % NB medium (control) common for all methods. System 1 (S1) - Control; System 2 (S2) - S1 + 100 μL of (0.500 OD) SKR4 culture; System 3 (S3) - S2 + 25 ppm of MEAP; System 4 (S4) – S1 + 25 ppm of MEAP; System 5 (S5) – S2 + 25 ppm of MERM; System 6 (S6) – S1 + 25 ppm of MERM; System 7 (S7) – S2 + 25 ppm of MEAM, System 8 (S8) – S1 + 25 ppm of MEAM, System 9 (S9) – S2 + 25 ppm of MEPP, System 10 (S10) – S1 + 25 ppm of MEPP, System 11 (S11) – S2 + 25 ppm of MEST, System 12 (S12) – S1 + 25 ppm of MEST in triplicate with the immobile condition. The specimen was taken out of the flask after 21 days of exposure, then non-metallic scrapers were used to remove the biofilm layers. It was reweighed after being cleaned with a pickling solution (Narenkumar et al., 2017a) to get rid of all corrosion products. According to ASTM G1, the MS corrosion rate was assessed and reported in mils per year (mpy) and the weight loss calculation is converted as a Corrosion Rate (CR) as followed

The following formulae were used to obtain the % Inhibition Efficiency (IE):

$\%IE=\frac{W-W1}{W}\times 100$ where W and W1 stand for metal weight loss without and with inhibitors, respectively.

2.11 Fourier transform infrared spectrophotometers (FT-IR) analysis

The biofilm-formed coupons (12 samples) were adequately dried after the WL study. The dried samples were blended on KBr to make the shape into a pellet. The pellets were subjected to using FT-IR spectra with a range of 400–4000 cm⁻¹ wave number, 8 cm⁻¹ resolution and 64 scans/spectrum scan rate spectra (PerkinElmer Spectrum IR Version 10.6.0) (Rajasekar et al., 2008).

2.12 Scanning electron microscopy (SEM) analysis

SEM analysis was carried out Tescan VEGA 3SBH model make with high magnification and resolution. The MS samples were done for both with or without inhibitors and bacteria in a corrosive solution. After 21 days of exposure, the specimen surfaces S1, S2, S3, S5, S7, S9, S11 were examined in order to assess the surface microstructure and composition of biofilm formation (Huang et al., 2022).

2.13 X-ray diffraction (XRD) studies

The S1, S2, S3, S5, S7, S9, S11 systems scarped metallic dry powdered samples were analyzed by XRD on a Bruker D8 Advance made with a LynxEye & Scintillation Counter detector of 5° to 140° angular range and the power is 40KV;30KV (Muthukrishnan et al., 2017).

2.14 Electrochemical method

Electrochemical tests were performed on coupons with a diameter of 1 cm² were implanted in a specimen holder. A working electrode as MS 1010 coupon, a counter electrode a platinum mesh with low impedance, and a reference electrode as a saturated calomel electrode (SCE) was used on the system (12 systems). The Version of Nova 2.1.5 Metrohm Autolab electrochemical system was used to analyze the behavior at 30 ± 1 °C and each value of the studies is conducted in triplicate (Kokilaramani et al., 2021b).

3.1 Biochemical analysis

The fresh colonies on MacConkey agar plate, isolates were identified initially by their typical wrinkled, unusual shape, adherent, non-pigment, dry, light yellow color colony morphology. The Gram staining and biochemical test results shown in Table 1 were analyzed for specific bacterial confirmation. The results of gram staining are Gram-negative, rod-like structure, motile, and single polar-flagellated organism (Rossell6-Moral9 et al., 1994). Biochemical tests revealed that the strain is a P. stutzeri species: oxidase, catalase-positive bacteria, citrate utilization, nitrate reduction and gelation hydrolysis. It is an aerobic, nonfermenting, active, nonfluorescent denitrifying bacteria with respiratory metabolic activities using oxygen as the terminal electron receptor (Lalucat et al., 2006).

3.2 Bacterial identification

The corrosive bacterium of Pseudomonas stutzeri SKR4 was isolated from the CTW sample and it was confirmed by 16S rRNA sequencing and phylogenetic tree shows Fig. 1 that 99 % of sequences are similar to the P. stutzeri and it was submitted to NCBI GenBank the accession number as MT211552.1 (Kokilaramani et al., 2020).

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Fig. 1

Neighbor or Joining Phenogram Shows Phylogenic Position of Strain SKR4 within the Genus Pseudomonas Based on 16S rRNA Gene Sequence Analysis.

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3.3 Gas chromatography-mass spectrometry (GC–MS) analysis

The GC–MS analysis of green inhibitors MEAP, MERM, MEAM, MEPP and MEST have identified compounds provided in Table 2 Fig. 2. The GC–MS study showed a wide range of compounds are found as n-Hexadecanoic acid, Myo-inositol, 4-C-methyl and Lupeol, trifluoroacetate Heptasiloxane, 1,1,3,3,5,5,7,7,9,9,11,11,13,13- tetradecamethyl- substances to control the microbial activity. The obtained inhibitor substances prevent biofilm formation and control the corrosion process in the CTW (Khattab et al., 2012).

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Fig. 2

GCMS Analysis for Methanolic Extraction of Green Inhibitors.

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3.4 Antibacterial activity against P. Stutzeri

The results of antibacterial activity in the green inhibitors diameter of the inhibition zone (mm) were shown in Table 3, while 26 mm MEAP has a robust bacteriostatic impact, the rest of the inhibitors: 19 mm MERM, 15 mm MEAM, 11 mm MEPP and 8 mm MEST also show a decent zone of inhibition against SKR4 bacterial strain. As predicted, the sterile DDH₂O negative control was inactive against tested bacteria and the positive control Ampicillin as a clear zone is observed. All these five inhibitors show effective antibacterial properties, non-hazardous, easy to use and have no side effects. Therefore, they might be utilized as microbial corrosion inhibitors (Narenkumar et al., 2018; Obot et al., 2009).

3.5 Minimal inhibition concentration assay

MIC of SKR4 against the five green inhibitors of MEAP, MERM, MEAM, MEPP, MEST at different concentrations (10, 25, 50, 75 and 100 ppm) was evaluated and shown in Table 3, the OD values (μg/mL) of the inhibitors at 25 ppm of concentration manifested as a high bacterial resistance than other concentrations and MEAP 18.18 ± 1 is more efficient compared to others. As for the other inhibitors MERM 26. 47 ± 1, MEAM 29.25 ± 1, MEPP 42.51 ± 1, MEST 45.13 ± 1. Green inhibitors are the ability to act against microbial activities and control their growth (Okeniyi et al., 2019).

3.6 Enumeration of total viable count (TVC)

The impact of different concentrations (10, 25, 50, 75 and 100 ppm) of inhibitors on an SKR4 strain was evaluated (CFU/mL) and shown in Table 3. The results of this study showed that 25 ppm of the concentration has MEAP 1.0 × 10², MERM 2.5 × 10³, MEAM 1.8 × 10³, MEPP 2.5 × 10³, MEST 3.1 × 10³, gradually decreased colony formation on the plates when compared to other concentrations, and in that MEAP has higher inhibition efficiency (Kokilaramani et al., 2021b).

3.7 Biofilm inhibition assay

The crystal violet method was employed in the biofilm assay against SKR4. The results of 96 well microtiter plate wide range of color variations that are visible to the naked eye (Wang et al., 2017). The control S1, S4, S6, S8, S10, S12 wells have no color observation, SKR4 S2 well is highly color observation due to the biofilm formation and the inhibitors added system have significantly reduced biofilm formation. The results of 570 nm in a UV–Visible spectrophotometer shows S3-0.093; S5-0.321; S7-0.469; S9-0.684; S11-0.952. The biofilm inhibition efficiency was ordered by MEAP > MERM > MEAM > MEPP > MEST as shown in Table 3. The biofilm assay's major striking feature S2 is that the bacterium P. stutzeri developed deep biofilm layers due to an excess of EPS production by SKR4 and it shows that P. stutzeri has the ability to form high biofilm layers on the surfaces (Ding et al., 2019; Wilson et al., 2017).

3.8 Weightloss method (WL)

The weight loss measurement examined corrosion rates and calculated the percentage of inhibition efficiency of 12 samples in the absence and presence of 25 ppm concentration of MEAP, MERM, MEAM, MEPP, MEST inhibitors against SKR4 on the MS 1010 with CTW. The weight loss results are given in Table 4 and it reveals that the maximum inhibition efficiency of 25 ppm concentration of inhibitors is MEAP 75 %, MERM, MEAM, MEPP, MEST are 71, 69, 66, 63 % respectively. The massive pitting corrosion was recorded in the S2, because of the presence of bacterial culture. Compared to the S2, inhibitors added systems of S3, S5, S7, S9, S11 recorded low corrosion rates was observed which inhibits corrosion by developing a protective barrier on the metal surface and S1, S4, S6, S8, S10, S12 only the inhibitor systems are mild corrosion was noted due to the presence of some inorganic compounds of the CTW. The high solubility of corrosion products induces the metal to lose its stability, which causes it to be dissolved by a solution. The weight loss of specimens linearly decreases in the corrosion rate with the addition of inhibitors (Shalabi et al., 2014).

3.9 Fourier transform infrared spectrophotometers (FT-IR) analysis

Functional groups of the biocorrosion products were identified using FTIR spectra and the presence and absence of inhibitors on the metal surface results are shown in Table 5 and Fig. 3. The corrosion products indicate that inhibitor’s organic groups bind with the metal surface. The various bonds were identified in the dried corrosion samples. The OH stretch vibrations of the inhibitors have broadband in the range of 3839 cm⁻1. The weak intensity band of heteroaromatic = C—H stretch (Alkenes) was seen at 3280 cm⁻¹ and the intermolecular hydrogen bonding had wide bands and the C⚌C stretch, C—O stretch and C—O—C stretch phenols. Bond stretching was observed on 1621 cm⁻¹, 1152 cm⁻¹, and 1022 cm⁻¹ respectively. In the medium, strong intensity band at 1107 cm⁻¹, the iron oxide peak showed a strong peak at 483 cm⁻¹C-I stretch Alkyl halides. In the comparison of S2 and S3; S5; S7; S9; S11 the functional group peaks are shifted in the inhibitor’s metal surface absorption frequencies providing solid evidence that the inhibitor’s phytochemical components bind with the metal surface. It has been found that natural products containing phenolic compounds have strong antibacterial and corrosion inhibitions properties. In green inhibitors, the presence of aromatic substances such as oxygen, nitrogen, sulphur and lone-paired, π-electrons were shown to have a coordinating affinity for metal. As a result, electrons from the phenolic OH group create a thick layer for protection of the iron complex and unoccupied d-orbital irons are shared to link the inhibitor molecules to the MS surface (Odewunmi et al., 2020; Salih et al., 2021).

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Fig. 3

FTIR Analysis for Biocorrosion Systems.

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3.10 Scanning electron microscopy (SEM) analysis

The results of SEM micrographs are used to examine the MS 1010 surface morphologies of SKR4 with various inhibitors shown in Fig. 4 (S1, S2, S3, S5, S7, S9, S11). The close examination of S2 samples revealed that the specimen surface was severely scratched accompanied by an aggressive attack of pitting formation. The corrosion scale looks on the S3, S5, S7, S9, and S11 sample surfaces look to be over uneven, with corrosion products layered on top of a metal surface. The MS had a smooth surface with a few little notches in the presence of inhibitors (S4, S6, S8, S10, S12), indicating that it may prevent iron dissolving, lowering the corrosion rate and providing more excellent (protective layer) corrosion protection. The various organic cation and anions can dissociate in an aqueous solution, changes in inhibitory behaviour should be attributed and action of the molecular structure. It was confirmed on the metal surface, that the inhibitors create a great impact and prevent metal dissolution. The results suggest that the inhibitors added systems to slow down and restrict corrosion by being adsorbed on the sample surface (Rajasekar et al., 2005; Ramezanzadeh et al., 2014).

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Fig. 4

SEM Analysis for Biocorrosion Systems.

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3.11 X-ray diffraction (XRD) studies

The XRD results showed the film formation on MS in the absence and presence of inhibitors in CTW. The metal scratched sample of systems (S1, S2, S3, S5, S7, S9, S11) and inhibitor adsorbed on an MS surface confirmed the protective film formation. The MS specimen had undergone corrosion, leading to the formation of magnetite peaks due to the iron peaks that appeared in XRD spectra of MS surface SKR4 bacterial strain and the peaks are reduced containing an optimum concentration of the inhibitor extract as shown in Fig. 5 (Rajasekar et al., 2007). Carbon, nitrogen, oxygen and phytochemical compounds found in inhibitors may create a protective coating and reduce the iron peaks on metals.

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Fig. 5

XRD Analysis for Biocorrosion Systems.

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3.12 Electrochemical studies

3.12.1

3.12.1 Impedance spectroscopy

Nyquist plots of SKR4 bacterial solution on CTW in the absence of green inhibitors are shown in Table 6 and Fig. 6. The Nyquist plots generated depressed semicircles typically related to the solid roughness, inhomogeneity and inhibitor adsorption on a metal surface. The impedance figure exposed the semicircle form of curves that confirmed corrosion reaction regulates through charge transfer. The impedance data is analyzed using the proper equivalent circuits based on geometries of Nyquist plots. In the absence and presence of inhibitors systems of equivalent circuit, as illustrated in Table 7 and Fig. 7 is employed to match the Nyquist plot in SKR4 bacterial solution with CTW. The working electrode surface roughness capacitance is calculated by one constant phase element (CPE), charge transfer resistance (R_ct), polarization resistant (R_p) and solution resistance (R_s) then the double-layer capacitance (Cdl) to define the system’s heterogeneity and surface layer resistance (R²). The CPE impedance is calculated using the following formula

(2)

$$Z(CPE)=Y_0^{-1}{(j\omega )}^{-n}$$ where Y0 denotes the magnitude of the CPE, j is the imaginary number, w is the angular frequency, n is the deviation parameter of the CPE: −1 ≤ n ≤ 1. If n = 1, CPE is capacitor; if n = 0, CPE is resistor; if n = -1, CPE is inductor; if n = 0.5, CPE is Warburg. Y0 has a dimension of sⁿ/Ω, but the capacitance element (C) has s/Ω or F and the differentiation, Y0 is commonly applied as though it were the capacitance of the corroding system (Socorro-Perdomo et al., 2021). The values of R_ct show that inhibitor compounds are bounded on the surface of MS specimens to create a protecting barrier, although controlled and slow down the corrosion process. The metal surface and inhibitors are attached through exchanging of electrons in between aromatic rings of hetero atoms and iron or π electron interaction. The adsorption took place on negatively charged metal surfaces with positively charged inhibitor molecules, forming a coordinating kind of bond between electrostatic interaction or charge transfer or sharing (Preethi Kumari et al., 2017). The R_ct values of experimental system S2 represent the presence of SKR4 absorbing a significant amount of current, indicating that the bacteria may have produced high corrosion on the metal, after the addition of inhibitors systems S3, S5, S7, S9, S11 shows low current consumption. The addition of green inhibitors has reduced current consumption as well as corrosion rates and the protective layers are formed to minimize microbial adhesion on a metal surface. The green inhibitors blocked EPS secretion through a binding mechanism of lone pair electrons (Narenkumar et al., 2018) and the inhibitor adsorption electron atoms are important to create protective barriers on the metal surface (Deyab, 2016).

Table 6 Electrochemical Impendence Parameters for Biocorrosion Systems.

System	Rs (Ωcm2)	Rct (Ωcm2)
S1 – (Control − 500 mL flask with 400 mL of autoclaved CTW with 1 % NB medium)	−11.7	27
S2 - (S1 + 100 μL + SKR4 culture)	−2.49	67
S3 - (S2 + 25 ppm of MEAP)	−4.99	54
S4 - (S1 + 25 ppm of MEAP)	−3.71	75
S5 - (S2 + 25 ppm of MERM)	−8.96	438
S6 - (S1 + 25 ppm of MERM)	−4.76	226
S7 - (S2 + 25 ppm of MEAM)	−2.37	469
S8 - (S1 + 25 ppm of MEAM)	−4.74	218
S9 - (S2 + 25 ppm of MEPP)	−4.69	725
S10 - (S1 + 25 ppm of MEPP)	−3.72	158
S11 - (S2 + 25 ppm of MEST)	−4.97	951
S12- (S1 + 25 ppm of MEST)	−5.04	151

R_s – Solution Resistance; R_ct - Charge transfer resistance

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Fig. 6

Nyquist Plots for Biocorrosion Systems.

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Table 7 Equivalent Circuits for Biocorrosion System.

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Fig. 7

Equivalent Circuit for Biocorrosion Systems.

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3.12.2

3.12.2 Tafel polarization

Table 8 and Fig. 8 show Potentiodynamic Tafel polarization for MS 1010 coupons on CTW. The bacteria SKR4 was absence and presence of inhibitors on the specimen for anodic and cathodic polarization curves. The Tafel parameters are computed by corrosion potential (E_corr), corrosion current density (i_corr), cathodic slope (β_c), corrosion rate (CR) and percentage of inhibition efficiency (% IE). The value of MS in the absence of inhibitors S2 E_corr and i_corr value results in the potential to increase biofilm layer formation leading to pitting corrosion and high corrosion rate due to the SKR4 causing severe corrosion reaction and the presence of inhibitors controlling the corrosion current including the corrosion rate. At the optimal inhibitor concentration of 25 ppm, maximum efficiency in the range of S3, S5, S7, S9, S11 as 75, 71, 69, 66, 63 % respectively. As a consequence, lack of inhibitors exhibits a positive shift in the E_corr value, which indicates that the unbonded solution of inhibitors has the capacity to corrode by either an anodic or cathodic reaction. The presence of inhibitors shows an E_corr value at ± 85 mV indicating inhibitors act as a mixed type of inhibitors because it inhibits both hydrogen evolution reaction and metal hydrolysis. The atoms in chemical structures of inhibitors have lone pair electrons which helps to bind the unoccupied orbital of the metal ions, resulting from the covalent bond formation which reduces the corrosion rate and forms protective coatings. The impact of metal ion concentration on inhibitor sorption was investigated in this research. All the inhibitors have been identified to be efficient against biofilm formation, and act as good inhibitors and preventers (Kokalj and Peljhan, 2010). In this study confirmed that MEAP gives the best inhibition efficiency than the others MEAP > MERM > MEAM > MEPP > MEST.

Table 8 Tafel Potentiodynamic Polarization Parameters for Biocorrosion Systems.

System	Ecorr (V)	icorr (A cm2)	ba (V/dec)	bc (V/dec)	Rp (KΩcm2)
S1 – (Control − 500 mL flask with 400 mL of autoclaved CTW with 1 % NB medium)	−0.699	14.9251	0.1758	−0.1420	696.49
S2 - (S1 + 100 μL + SKR4 culture)	−0.972	83.5092	0.1286	−0.1477	851.14
S3 - (S2 + 25 ppm of MEAP)	−0.610	23.7639	0.2069	−0.4892	450.01
S4 - (S1 + 25 ppm of MEAP)	−0.602	2.1451	0.1887	−0.4373	394.52
S5 - (S2 + 25 ppm of MERM)	−0.613	27.8437	0.1475	−0.2130	482.60
S6 - (S1 + 25 ppm of MERM)	−0.580	3.3973	0.2329	−0.4817	550.97
S7 - (S2 + 25 ppm of MEAM)	−0.628	29.1091	0.2116	−0.3526	564.26
S8 - (S1 + 25 ppm of MEAM)	−0.603	5.4456	0.1703	−0.2652	533.44
S9 - (S2 + 25 ppm of MEPP)	−0.635	31.6555	0.1788	−0.2361	577.45
S10 - (S1 + 25 ppm of MEPP)	−0.512	7.3907	0.4380	−0.1539	669.23
S11 - (S2 + 25 ppm of MEST)	−0.634	35.1785	0.1833	−0.2866	676.38
S12 - (S1 + 25 ppm of MEST)	−0.618	9.53189	0.1969	−0.1208	920.87

E_corr - Corrosion Potential; i_{corr -} Corrosion Current Density; β_{a -} Anode Beta coefficient; β_{c -} Cathode Beta coefficient; R_P - Polarization Resistance; mV/dec – Millivolt/Decade

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Fig. 8

Tafel Polarization Plots for Biocorrosion Systems.

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