Study on Corrosion Characteristics of Aluminum Alloy Al3102 in Sulfate-reducing Bacteria Environment and Investigating the Use of Azadirachta indica Leaves Extract in its Control

An effort to understand the corrosion characteristics of aluminium Al3102 alloy in a neutral medium in the presence of Desulfovibrio desulfuricans was made. The corrosion rate increased up to tenfold in the bacterial medium in comparison to the control (abiotic) medium. Scanning electron microscopy (SEM) studies showed substantial sulfate-reducing bacteria (SRB) adhesion on the alloy surface and pit formation. Supplementation of 1000 ppm Azadirachta indica leaves extract decreased corrosion by 82%. Sulfide analysis, SEM, and X-ray photoelectron spectroscopy (XPS) studies showed a significant effect of neem leaf extract (NLE) on bacterial metabolic activity and confirmed the adsorption of the inhibitor molecules on Al3102 surface. Electrochemical impedance spectroscopy studies showed that the NLE formed a highly resistive external layer which protected the alloy surface from the corrosive effects. The gas chromatography-mass spectroscopy studies (GC-MS) showed appreciable


Introduction
Damage to the metal structures by the activity of microorganisms has been an issue of concern for many decades.The activity of microorganisms and the subsequent damage to the metal structures has been termed microbially influenced corrosion (MIC).Unlike corrosion caused by corrosive chemical activity, the occurrence of MIC is widespread ranging from simple wastewater-carrying lines, wastewater treatment plants, cooling towers, and heat exchangers to sophisticated nuclear reactor coolant systems, parts, and machinery of ships and aircraft.The damages caused by the MIC sometimes cause serious consequences such as loss of lives [1].Microbe-induced corrosion is always the consequence of the activity of various types of microbes.Biofilm on the metal surface houses various types of bacteria with an aerobic population occupying the top layer where oxygen is available in plenty.At the inner zones of the biofilm which is closer to the metal surface and where oxygen is scarce, the anaerobic population predominates.Though the MIC is a contribution of different types of bacterial strains, a significant portion of the damage is contributed by the activity of the anaerobic bacterial population most importantly, sulfate-reducers.The sulfate-reducing bacteria (SRB) are known to attack metals and concrete structures.SRB participates in the electron transfer processes of the metal system and produces the respective metal sulfides.These metal sulfides have been identified as corrosion promoters due to the formation of local concentration cells [2].
Aluminum is one of the industrially preferred metals due to its excellent corrosion-resistant properties.Aluminum and its alloys are used in the manufacture of cooking vessels, storage containers, condenser tubes, fins, and as construction material in marine and aviation industries.In contact with air, Al forms a corrosion-resistant oxide film on the surface.However, the native film has been found to be less resistant to corrosion under adverse conditions.The response of Al and its alloys varies with respect to the medium in which they are present.In fact, the composition of the alloys and the nature of the medium in which the SRB and the metal are present play a major role in dictating corrosion characteristics.In the saline medium, SRB was found to protect Al 5052 from the corrosive effects of the halide ions [3].However, studies on Al 5052 in Postgate C medium containing SRB reported severe pitting corrosion [4].In another study with Al 5052 and Al alloy containing zinc, cadmium, and indium, in Postgate C medium containing SRB, the Al-Zn-Cd-In alloy presented more corrosion damage due to the SRB activity when compared to Al 5052 which showed relatively less pitting [5].It was suggested that the addition of other elements to Al during alloy formation helps in enhancing the SRB electron transfer activity by breaking the passive layer [5].It was also reported that though the role of Al in the electron transfer activity of SRB is not dominant as in the case of iron, it does contribute to a little extent leading to metal dissolution.The corrosion products were found to consist of Al(OH) 3 , Al 2 O 3 , and Al 2 S 3 [5].With limited reports available on SRB interaction with Al and its alloys, investigation of the same has become important considering the widespread applications of Al and its alloys.There are very limited reports on the control of SRB-caused corrosion damage in Al and its alloys [6,7].Porous self-assembled monolayers of 1H,1H,2H,2H-perfluorodecyltriethoxysilane were formed on anodized Al (99.9 % pure) into which lubricant perfluoropolyether was filled.Studies over a seven-day period in SRB containing Postgate medium showed that the coating prevented the SRB adhesion leading to increased corrosion resistance [7].Similarly, Al (>99.99%)coated with antimicrobial agents triclosan and cetyl trimethyl ammonium bromide containing tannin-FeIII complex effectively decreased SRB adhesion up to 14 days [6].Lam et al. reported that the amino acids, such as d-methionine, d-leucine, d-tyrosine and d-phenylalanine, have the tendency to control the biofilm formation and reduce the biocorrosion [8].Reports on the use of an environmentally friendly corrosion inhibitor /biocide to control the SRB-caused damages on Al and its alloys are still lacking.Neem leaves extract was found to be effective in control of microbial corrosion of copper caused by Arthrobacter sulfureus [9].The present study involves understanding the corrosion characteristics of aluminum alloy Al3102 alloy in a modified Baar's medium.In addition, the work also investigates the efficacy of Azadirachta indica, the Indian neem leaves extract (NLE) in the control of SRB-induced corrosion.Desulfovibrio desulfuricans (DD) belong to SRB group and has been used in this work.

Experimental 2.1 Medium and D. desulfuricans strain
Analytical-grade chemicals from Loba Chemie (India) were used in the study.The nutrient medium used for the study was modified Baar's medium consisting of calcium and magnesium sulfates as the sulfate source, ammonium chloride, di-potassium hydrogen phosphate as the phosphate nutrient source, sodium lactate as the carbon provider, and ferrous ammonium sulfate as the iron source, the composition of which is mentioned elsewhere [10].The SRB strain, Desulfovibrio desulfuricans was purchased from NCIM, Pune, India.

Al specimen preparation
Aluminum 3102 sheets (Al3102) of 1 mm thickness were purchased from the local market and were used for corrosion studies.The Al3102 sheet was made into small coupons (1 cm × 1 cm), polished with grit paper from coarse to fine grade.After polishing, the coupons were cleaned in acetone and deionized water, dried, and stored in an airlock container.For both the weight loss and electrochemical experiments, the sides and one face of the coupons were properly sealed using sticky Teflon tape.It was made sure that only one surface of the sample is in contact with the corrosive medium and that the available area is 1 cm 2 .For the weight loss studies, a polymeric thread was fixed to the sealed side of the coupons to keep them hanging in the corrosive environment.For the electrochemical trials, a copper wire was connected to the unexposed surface before sealing it with Teflon tape.

Inhibitor preparation
Neem leaves were powdered after proper cleaning and drying.The powder was soaked overnight in methanol and the filtrate was subjected to 5 h reflux at 50 °C [11].The neem leaf extract (NLE) was collected after vacuum evaporation of the solvent.The concentration of the NLE used for the studies was 1000 ppm.

Weight loss and electrochemical investigations
Both the weight loss and electrochemical investigations were conducted for four weeks.The details of the nutrient medium used for the studies and their notations are shown in Table 1.
For monitoring the weight loss of the Al3102 sample in different media, pre-weighed samples were kept hanging inside the test tubes containing different media and incubated at 37 °C in dark.All the samples and the medium were properly sterilized and bubbled with nitrogen to ensure anaerobic conditions.Four samples were taken out every week, washed to remove the external films formed [12], and dried and weight loss was noted.The weight loss values were utilized for the calculation of the corrosion rates (C R ) and efficiency of corrosion inhibition (η).The C R was calculated using the standard method reported elsewhere [9].The efficiency of the NLE in controlling the DD/chemical-induced corrosion in Al3102 samples is estimated by comparing weight loss observed in uninhibited (MB / MC) and inhibited (MBI / MCI) samples using the standard formula reported in our earlier report [9].
For the electrochemical investigations, along with the prepared Al working electrodes, platinum counter and Ag/AgCl reference were used.The electrochemical investigations were performed using a CH6091E electrochemical analyzer (CH instruments, USA).For the electrochemical runs, the corresponding nutrient medium at 37 °C was the electrolyte.The potentiodynamic polarization (PDP) studies were made in the voltage range ± 250 mV from the open circuit potential (OCP) at a step rate of 1 mV s −1 .The electrochemical impedance spectroscopy (EIS) was performed between 10 5 and 0.1 Hz with a sine wave of rms amplitude 10 mV.Prior to each electrochemical run, the system was maintained at the open circuit potential for 30 min.Tafel extrapolation calculations from the PDP runs were performed by employing the CH-electrochemical analyzer software to find out the corrosion current ( I c ) and potential ( E c ).The data from EIS study were validated using Kramers Kronig Transformation (KKT) and the appropriate electrical circuit for the system was obtained using ZSimpWin software [13].

Results and discussion 3.1 Weight loss studies
The results of the weight loss studies for Al3102 in different media for every week are shown in Fig. 1 (a) and (b).The extent of Al3102 corrosion in the MC and MCI media has been observed to be very less.In the MC media, the C R for the first two weeks was almost the same 0.013 mmpy which decreased to 0.001 mmpy in the fourth week.This trend can be attributed to the deposits of corrosion reaction products with prolonged immersion that helps to suppress corrosion.With the addition of NLE, the corrosion was found to be completely stopped in the MCI (abiotic) media.
In the presence of Desulfovibrio desulfuricans, the C R increased to 0.05 mmpy which is 3.8 times higher than that of the first week C R observed with MC medium.The C R decreased slightly till the third week and remained stable thereafter.The biofilm produced by bacterial metabolism prevents the passage of the corrosive entities from reaching the Al3102 surface thus minimizing the C R .With the addition of the NLE, the C R decreased to 0.01 mmpy during week 1 and further dropped to 0.004 mmpy in week 4. Thus, the NLE plays the role of an antibacterial agent which controls the activity of the DD and helps in minimizing the C R .A zero C R value observed for the samples immersed in the MCI medium shows that apart from the antibacterial action, the NLE also functions as corrosion inhibitor by adsorption on the metal surface.On an average, the corrosion decreased by 82% for the Al3102 samples immersed in DD-containing media.Fig. 1 (c) shows the dissolved sulfide concentration at different immersion periods in MB and MBI samples.It can be seen that the NLE significantly lowers the sulfide concentration which shows that it influences the metabolic process of the DD.

Morphological studies
The surface images from SEM analysis for the Al3102 coupons in MB and MBI medium at different immersion periods are given in Figs. 2 and 3. Fig. 2 (a) and (b) show the SEM results for the Al3102 coupons with an external film formed on it, after one week immersion period in MB and MBI medium, respectively.In the MB sample,  Table 2 shows the elemental analysis of the Al3102 samples after the first and third-weeks immersion in MB and MBI media.Increased oxygen content in the MBI sample immersed for one week period could be due to the adsorbed inhibitor molecules.Similarly, the third week MBI sample shows increased bromine content which could be due to the adsorption of molecules from the NLE extract.The I c values of the MBI samples do not show significant variation with the immersion period.A significant drop in the I c values with NLE shows that it is effective against DD-influenced corrosion of Al in the neutral pH.Fig. 5 shows the PDP plots for the MC and MCI samples.It is obvious that the I c values observed in the MCI samples are much less when compared to the MC samples indicating that the NLE works as a corrosion resistor against the corrosive action of the medium.

Electrochemical impedance spectroscopy (EIS)
Fig. 6 (a) and (b) show the modulus of impedance (|Z|) variation with frequency for Al3102 coupons in MB and MBI media respectively at different immersion periods.The phase angle (φ) variations are shown in Fig. 6 (c) and (d).The reliability, linearity, and causality of the EIS data are performed by conducting the KKT analysis [9].In Fig. 6 ((a)-(d)), the symbols represent the values obtained in the experiment, and the continuous lines refer to the KKT simulation results.The experimental and the KKT fit data match well indicating the data set is from a causal, linear, and stable system.In Fig. 6 (a), the |Z| remains stable in the high-frequency (HF) zone from 10 5 to 10 3 Hz.It increases gradually for the subsequent frequencies.In the mid to low-frequency (LF) region, a change of slope is observed for the immersion periods 2, 3, and 4 weeks, which are characteristic of pitting corrosion [15].The slope change observed shifts towards the LF region with respect to the immersion period.The LF region's |Z| for the first week was 27900 Ω cm 2 , which significantly decreases to 10000 Ω cm 2 for the subsequent weeks.These observations reveal that significant damage is caused to the material in the presence of DD.Evidence of pitting corrosion has also been observed [15].
For the MBI medium in Fig. 6 (b), the |Z| in the LF region significantly increases for the samples with NLE inhibitor.The slope change observed in the middle-frequency (MF) and the LF regions in the case of MB samples, is not observed in the MBI sample.However, |Z| in the LF zone for the third week shows a value closer to 10000 Ω cm 2 which could be due to the changing thickness of the protective films.The value increases again for the fourth week indicating an improvement in the corrosion protection effect.
In Fig. 6 (c), shows the φ variation in MB medium, and the plots show the presence of two-time constants, one extending from HF to MF and the other in the LF region.These are attributed to the corrosion activity and double-layer capacitance [16].The φ peak in the MF region is broader during the first week indicating the biofilm or external layer formed is well adsorbed on the metal surface [16].The MF region's φ decreases during the second week and its width decreases.A slight shift towards the LF region is also observed.For the subsequent weeks also the peak in the MF region shifts to the LF side which indicates the progress of corrosion and damages caused by the external film [17].Moreover, the peaks in the LF region for the third and fourth weeks move towards very low values and the |Z| plot for the same period (MB samples) shows a change in slope.Such a feature of decreased |Z| with a change in slope combined with a very low φ in the LF region corresponds to pitting corrosion [15].Fig. 6 (d) shows the φ variations in the MBI sample present peaks spreading over wide frequency ranges indicating adsorption of the NLE inhibitor molecules [18].The φ curve during the first week shows a maximum phase angle value of 77° at the highest frequency and shows a broader MF peak indicating good adsorption of the NLE molecules.The φ at the highest frequency presents a maximum value of 85° for the second week which shows that the maximum thickness of the film is attained in the second week [19].For the subsequent weeks, the HF region φ drops which indicate the degradation of the protective film and a drop in the corrosion protection effect.Fig. 7 (a) and (b) show the Nyquist plots of Al3102 samples in MB and MBI media respectively at different immersion periods.In both cases, two depressed semi-circular patterns, which are difficult to distinguish, appear along with the diffusion barrier.The semi-circle pattern present in the HF region is usually ascribed to the external film formation and the LF capacitive trend occurs due to the electron transfer reactions at the metal and the electrolyte interface [20].With MB sample, the two semi-circular patterns can be easily identified for the third and fourth weeks.The depressed semi-circular pattern is associated with the constant phase element (CPE), a non-ideal form of the capacitor [9].The diameter of the semi-circle decreases with immersion time owing to the constantly changing biofilm depth profile, ionic concentration, and the products of corrosion reactions on the metal surface.A progressive drop in the impedance values with immersion time shows the progress of corrosion.
The Nyquist plot of MBI samples shows a significant increase in the impedance values.The plot shows two depressed semi-circles one in the HF region and the other in the LF zone.The presence of Warburg diffusion resistance is also obvious in the Nyquist plots.The diameter of the semi-circle increases till the second week and then drops for the third week.This again increases during the fourth week.This fluctuating trend is the result of the changes observed in external films as already mentioned.Thus, the NLE serves to protect the corrosion of the Al sample till the second week, and its effect drops for the subsequent weeks.
In Eq. ( 1), σ is the Warburg coefficient ( Ω cm 2 s −0.5 ) and the angular frequency ω ( rad s −1 ) is given by 2 πf.Table 4 shows the best-fit parameters of MB and MBI systems corresponding to the EEC.The Warburg coefficient (σ) helps in measuring the resistance of the external porous films.A higher value of σ shows higher resistance to the flow of corrosive medium through the film.In Table 4, the values of σ are higher for the MBI samples indicating higher diffusion resistance to the flow of the corrosive medium [21].
The total resistance for the MBI system is high for all the immersion periods.The capacitance values keep on changing with the immersion period in both cases due to the constantly varying thickness, composition, and conducting nature of the layers formed on the metal surface.

XPS investigations
Fig. 9 gives the full spectrum view of XPS analysis of the Al3102 samples immersed in MB and MBI samples after three weeks immersion period with the film formed on them.The MB coupon shows the presence of the sulfide group whereas, the MBI sample does not show the presence of sulfide.The presence of the O1s group is identified from the peaks around 530 eV.The peak at 531.8 eV in MB   sample corresponds to ketones [22].Desulfovibrio desulfuricans is known to produce certain forms of ketones during its metabolic activity [23].The presence of a peak at 532.8 eV in the case of the MBI sample shows the presence of hydroxyl and epoxy functional groups [24].Table 5 shows    earlier reports by other groups.These peaks are a contribution from the bio-polymeric substances of the biofilm [32].
In addition, N1s peaks at 402.8 eV and 403.9 eV present in the MBI sample (Fig. 12) are due to the adsorbed nitrogen-containing NLE molecules [32].

GC-MS studies of NLE
The results from the GC-MS investigation of NLE are presented in Table 6 and it shows the presence of significant quantities of esters, heterocyclic sulfur compounds, ketones, and terpenoids.The O1s and the C1s plots for the MBI sample show the existence of adsorbed ester compounds.Table 6 for NLE shows the presence of various steroid esters at 38 min, 50.25 min, and 53.08 min.These three esters constitute about 46.74% of NLE.The Al2s spectra and the EDX analysis of the third-week MBI sample show the presence of bromine which could have come from the heterocyclic nitrogen compound -acridinedion derivative which comes out at 43.42 min.The attachment of these functional groups on Al3102 surface helps to protect it against corrosion.Apart from these, these compounds are also known for their antibacterial activity [33] which helps in the control of the DD population.

Mechanism
Fig. 13 shows the mechanism of corrosion caused by DD in the Al3102 sample with and without NLE.Results from different studies have shown that supplementation of 1000 ppm NLE has protected the aluminum metal from the corrosive effects of D. desulfuricans to a significant extent.
From the studies, the extract offers better corrosion control for two weeks, after which the effect slightly decreased.The XPS analysis confirms the presence of an external layer comprising of aluminum oxide, hydroxides, and sulfides on the MB specimen.Whilst the MBI specimen shows no sulfide which affirms the NLE effect of DD.The presence of esters, amines, and other organic groups along with Al 2 O 3 is observed in MBI specimen.Based on the above results the following mechanism shown in Fig. 13 (MB) can be proposed [4,5].
The corrosive environment leads to the generation of aluminum ions by anodic dissolution given by Eq. ( 2) [4,5]: Water dissociation gives rise to the formation of H + and OH − radicals (Eq.( 3)) which further take part in the reactions: The H + ions move towards the cathode and take part in the hydrogen generation reaction given by Eq. ( 4) [4,5]: The hydrogen uptake facilitates electron transfer and reduction of SO 4 2− to sulfide by the DD given by Eq. ( 5): The produced HS − undergoes a reversible reaction which results in the production of H 2 S (Eqs.( 6) and ( 7)) [34]: HS H H S l 2 .
The HS − ion further reacts with Al 3+ ions which gives rise to the formation of Al 2 S 3 (Eq.( 8)): Aluminium ions also react with hydroxyl ions to form Al(OH) 3 which further forms Al 2 O 3 (Eq.( 9)): The Al 2 S 3 formed dissolve in the corrosive medium to give raise to the formation of Al(OH) 3 (Eq.( 10)): In the presence of neem leaf extract, the cathodic reaction rate is highly affected as observed from the potentiodynamic polarization results.The metabolic activity of DD is decreased which can be justified by the absence of sulfide in the MBI sample in the XPS studies.The SEM analysis also shows no biofilm formation in the MBI sample.However, the corrosive effects of the medium have caused a little corrosion which causes Al 2 O 3 and Al(OH) 3 to form.Adsorbed compounds from NLE offer protection against the chemical effects of corrosion which can be verified from the presence of many functional groups representing compounds that have been identified in the NLE.

Conclusions
The effect of Desulfovibrio desulfuricans on Al3102 was investigated in modified Baar's medium.Though a thicker biofilm formed on the exterior of Al3102, the occurrence of DD caused severe pitting corrosion on the alloy.The DD attack caused a five to ten times increase in the C R of Al3102 with reference to the control.SEM analysis showed significant pitting corrosion and more sessile bacteria on the alloy's surface.The addition of 1000 ppm of NLE effectively controls the DD-influenced corrosion.The average inhibition efficiency of 82% and 80% were observed in the weight loss and PDP studies, respectively.Electrochemical investigations reveal that the effectiveness of the inhibitor lasts for two weeks and decreases slightly for the third and fourth weeks.

Fig. 1
Fig. 1 Effect of immersion time on (a) corrosion rate (b) inhibition efficiency and (c) sulfide concentration

3. 3
Electrochemical tests 3.3.1 Potentiodynamic polarization (PDP) Fig. 4 (a) and (b) present the PDP plots for Al3102 samples in MB and MBI media respectively at various immersion periods.It is noticed from Fig. 4 (b), that the addition of NLE does not change the shape of the potentiodynamic polarization plots.This shows that the mechanism is not

Fig. 2
Fig. 2 SEM images of Al3102 coupons after one week immersion period in (a) MB and (b) MBI medium with the external film (c) MB and (d) MBI medium after removing the external film significantly altered with the addition of the NLE.In the MB samples, the anodic branches move towards the right with an increasing immersion period indicating a surge in the corrosion process with the immersion period.With NLE, both the anodic and cathodic current densities decrease indicating a drop in the respective reaction rates.The E c values of the MB sample show a negative shift from −230 mV to −320 mV in the second week which shows an increased corrosion rate and matches well with the behavior of the weight loss experiment results.For the third week, it moves in the positive direction and decreases again in the fourth week.Such a fluctuating behavior of E c can be attributed to the variations occurring in the depth and composition of the external film.In the MBI samples, the E c values show a positive shift till the third week and show a negative shift in the fourth week.This trend could also be due to the fluctuations in the vicinity, changes in the charge of the biofilm, and its thickness.The non-monotonic trend observed in the E c values with the immersion period shows that D. desulfuricans plays a major role in the electrochemical processes[14].The I c values attained by extending the Tafel part of the cathodic and anodic branches are shown in Table3along with other Tafel parameters.The I c values of the MB samples increase continuously till the third week and decrease slightly for the fourth week.A maximum I c value of 3.1 μA cm −2 is observed for the third week.On the other hand, the I c values for all four weeks in MBI samples are less when compared to the I c values of the MB samples.

Fig. 4 Fig. 5
Fig. 4 Potentiodynamic polarization plots for Al3102 in (a) MB and (b) MBI medium at different immersion periods

Fig. 6
Fig. 6 Bode impedance modulus plot for Al3102 in (a) MB and (b) MBI medium at different immersion periods.Bode phase angle plot for Al3102 in (c) MB and (d) MBI medium at different immersion periods

Fig. 7 Fig. 8
Fig. 7 Nyquist plot for Al3102 coupon in (a) MB and (b) MBI medium at different immersion periods

Fig. 9
Fig. 9 Wide range XPS spectra for Al3102 coupons immersed in MB and MBI media after three weeks of immersion periods with an external film formed on it the atomic percentage of the elements in MB and MBI samples in XPS analysis.The absence of sulfur in the MBI sample shows that the NLE has minimized the Desulfovibrio desulfuricans metabolic activity.The Al2p and Al2s de-convoluted spectra for the Al samples in MB and MBI media are shown in Fig. 10 (a)-(d).The Al2p spectrum of the MB sample (Fig. 10 (a)) shows the presence of Al 2 S 3 (74.3eV) and Al 2 O 3 (74.7 eV)[25].The sulfide ions formed due to the sulfate reduction step react with the Al 3+ to produce Al 2 S 3[5].The Al2p spectrum of the MBI sample (Fig.10 (b)) shows the presence of oxides and hydroxides of aluminum formed by the reaction of the metal ions with the environment.It also shows the presence of Al-N group which could be due to the adsorption of nitrogen-containing groups from NLE.The Al2s spectrum for the MB sample (Fig.10 (c)) confirms the presence of Al 2 S 3[26].Further, the Al2p spectrum of the MB sample (Fig.10 (a)) also shows the presence of Al 2 S 3 .The peaks at 123.6 eV and 126 eV in the Al2s plot of the MBI sample (Fig.10 (d)) could be due to the adsorbed halogen-containing compounds from NLE[27].The C1s spectra for the MB samples (Fig.11 (a)) show a peak at 284.8 eV related to the C-C group which was observed in many studies related to DD biofilm and has been attributed to the presence of amino acids[28,29].The carbon spectra for the MBI sample (Fig.11 (b)) show a prominent signal at 288.9 eV due to the carboxylic or ester groups.This confirms the adhered NLE molecules on the Al surface.O1s spectra of MB sample (Fig.11 (c)) shows a peak at 532 eV corresponding to the C-O groups of the biofilm[30].The deconvoluted O1s (Fig.11 (d)) spectra for the MBI sample show the presence of esters (533.3 eV) and C-OH (534.7 eV) groups adsorbed onto the Al3102 surface[31].The N1s spectra for the MB sample (Fig.12) show peaks at 398.5 eV and 400 eV which confirms the metal-N bonds (N-Me) and C-N/C-N-O groups individually, and these findings match well with the

Fig. 10
Fig. 10 Deconvoluted Al2p spectra of Al3102 samples in (a) MB and (b) MBI media after three weeks of immersion.Al2s spectra for Al samples in (c) MB and (d) MBI media after three weeks of immersion

Fig. 11
Fig. 11 Deconvoluted C1s plot of Al3102 samples in (a) MB and (b) MBI media after three weeks of immersion.O1s spectra for Al samples in (c) MB and (d) MBI media after three weeks of immersion o Investigations using XPS and EDAX confirm the adsorption of NLE molecules on the sample.Corrosion products composed of Al 2 S 3 , Al(OH) 3 and Al 2 O 3 .

Fig. 13
Fig. 13 Mechanism involved in the biocorrosion of Al3102 with and without NLE

Table 1
Details of the nutrient medium used in the study

Table 2
Elements of the external films formed on the Al3102 samples in MB and MBI media at the end of the first and third weeks of immersion

Table 3
Tafel fit parameters for Al3102 in MB and MBI medium at different immersion periods and the inhibition efficiency (IE)

Table 4
EEC fit parameters for Al3102 system in MB and MBI media at different immersion periods

Table 5
The atomic concentration of the elements from XPS analysis

Table 6
Name of a few compounds present in significant quantities in neem leaf extract identified from GC-MS analysis