The decay of protective marine coatings by microbial attack occurs when the microorganisms are in contact with the coatings. The role of sulfate-reducing bacteria in this activity has been commonly recognised. They secrete hydrogen sulphide, resulting to generation of metal sulphides and sulphates when reacted with exposed metal substrates under disbonded coatings, and also form slimy biofilms on the outer coated surfaces of submerged substrates [1,2]. The presence of metal sulphides and sulphates on bare metals, and biofilms on coated substrates has been reported as one of the causes of microbial induced corrosion and biofouling. Thus, biofouling and corrosion due to marine sulfate-reducing bacteria has remained one of the major problems for underwater structures [[3], [4], [5]].

To solve this problem, a lot of efforts are being invested on the research and design of antibacterial/anticorrosion coatings for marine applications. As one of the design strategies, the use of non-toxic low-surface energy additives and nanoparticles comprising roughness features on multiple length scales, which have capability to resist microbial colonization has become a major thrust of recent research activities after copper and organo-tin compounds were banned [6]. With this research direction in mind, one of numerous design considerations remains the fact that surface topographic features present more attachment points that facilitate bacterial adhesion [7,8]. Also, it is well-known that the settlement of marine organisms on surfaces is governed by the surface topography and chemistry [9,10]. Based on these established facts, it has become difficult to precisely define the appropriate chemical composition and surface features of an effective antibacterial/anticorrosion coating. Nevertheless, numerous research activities are on-going to better understand the right additives and in what proportion that should be used for the design of effective antibacterial/anticorrosion coatings.

For example, Donmex et al. [11] designed different surface responsive antifouling coatings using polymeric materials with complex surface chemistry. The lower surface energy coatings inhibited both dead and living marine organisms in the range of 84%-91%. Similarly, Parra et al. [12] investigated the performance of engineered grapheme/SiO₂-poly(methyl methacrylate) coating film in reducing adhesion of biofilm-producing bacteria Halomonas spp. CAM2. The coating significantly reduced adhesion of the bacteria. Furthermore, Mincheva et al. [13] evaluated foul-release performance of nanostructured PDMS/SiO₂ coatings catalyzed using each of trifluoroacetic acid and dibutyltin dilaurate. Their observation showed that the coating with trifluoroacetic acid catalyst exhibited higher percentage removal of Ulva linza. Also, Ji et al. [14] examined antifouling performance of various carbon nanotubes-modified poly(dimethyl siloxane) (PDMS) nanocoatings which were exposed to seawater bacteria. Due to different observed topographies, the nanocomposite coatings demonstrated differences in adhesion by the bacteria.

Given the performance of low-surface energy polymeric resin on foul-release and anti-fouling efficacy of marine coatings, PDMS has continued to remain a material of choice in the formulation of effective nanocomposite barrier coatings, as described elsewhere [[15], [16], [17], [18]]. Though the ability of the PDMS-based coatings to optimally perform for anticorrosion and antibacterial applications depends on modification of the properties. In view of the above, the use of fluoro-chemical compounds and zinc oxide nanoparticles (ZnO NPs) in PDMS-based nanocoatings for anticorrosion and antibacterial potentials have further been investigated [[19], [20], [21], [22]]. Despite the antimicrobial advantages of ZnO NPs, some previous studies have highlighted their toxic effects to useful aquatic lives, such as fish and crustaceans lives [[23], [24], [25]]. In order to minimize the observed toxicity and improve biocompatibility, surface modification approach by treatment with phosphoric acid, among others has been reported [26].

Considering the modification effects of ZnO NPs, the low-surface energy additives and formulation chemistry, it is expected that design of low-surface energy nanostructured PDMS coatings containing phosphoric acid-treated ZnO NPs and fluorochemical additive should have the potential to reduce biofilm formation and corrosion of submerged structures. The target of this design strategy is usually to weaken the adhesive strength of the surface topographic features in order to reduce adsorption and attachment of bacteria and biofilm. Following this strategy, several polymeric coatings have been prepared and evaluated for anti-SRB effectiveness in marine environment [[27], [28], [29], [30]]. However, till date, there is no reported study, to the best of our knowledge, on the anti-SRB corrosion behaviors of nanocomposite PFDTS-modified PDMS/PA-treated ZnO coatings.

Therefore, in the present study, we have conducted experiments to assess the anti-biofouling and anti-corrosion performance of hydrophobic PDMS coatings modified with phosphoric acid-treated ZnO nanoparticles and FDTS, by exposing them to SRB/NaCl medium. The main objective was to assess the effect of low surface energy FDTS in inhibition of SRB induced corrosion and biofilms attachment to the PDMS-based coatings. The surface morphology and chemical interactions of the modified coatings before and after immersion in SRB/NaCl solution were analyzed by SEM and FTIR spectroscopy, respectively. The anticorrosion property of these coatings under exposure to SRB/NaCl solution was also monitored by use of electrochemical impedance spectroscopy (EIS).

Poly(dimethylsiloxane) and its curing agent, (CA) (trimethylsilyl capped dimethyl, methylhydrogen siloxane copolymer) purchased from Dow Corning chemicals, USA, were used as the coating resin. Zinc oxide nanoparticles with average diameter, 30 ± 10 nmand hexadecyltrimethoxysilane (HDTMOS) from Aladdin Industrial Corporation, Shanghai, China, were used as fillers and co-crosslinker, respectively. Ethyl acetate used as diluent was obtained from Sinopharm Chemical Reagent Co. ltd. China. A low-surface energy modifying agent, perfluorodecyltrichlorosilane (PFDTS) was obtained from Tokyo Chemical Ind. Co. Ltd Japan and phosphoric acid obtained from the FoodChem ltd., China were used as supplied without further treatment. Glass slides and Q235 steel coupons were used as substrates.

2.2.1. Substrate preparation

The glass slides that were used for water wettability measurements were first washed using de-ionized water, later cleaned with acetone, dried at room temperature and then stored in a desiccator. The surfaces of steel coupons with dimension, 0.2 cm × 1.5 cm × 1.5 cm were prepared by use of 1000 grit-size abrasive mounted on a metallographic polishing disc. The abraded steel surfaces were washed with tap water, rinsed in acetone, dried and then stored in a desiccator to be used as coating substrates for scanning electron microscopy (SEM) examinations. Similarly, square steel substrates with dimension 5.0 cm × 5.0 cm and thickness 0.2 cm, were prepared for use in anticorrosion evaluation of coatings.

Firstly, the surface of ZnO NPs was modified with phosphoric acid (PA). This was achieved by addition of 6.0 g of phosphoric acid into 394.0 g ethanol and stirred using magnetic stirrer for 15 min. Thereafter, 200 g ZnO NPs were introduced into the PA/ethanol solution and the entire mixture was agitated for 2 h at room temperature to obtain 1.0 wt% PA-treated ZnO NPs. The modified ZnO NPs were recovered after 3 min of centrifugation at 3000 rpm and then rinsed three times with deionized water to remove the residual chemicals around the nanoparticles. Thereafter, the PA-treated ZnO NPs were dried in a conventional oven set at 60 °C for 48 h, sieved and obtained as free-flowing zinc orthophosphate as shown in Eq. (1):

For the coating preparation, 2.50 g of PA-treated ZnO NPs was poured into 20.0 g of ethyl acetate and agitated for 15 min to disperse the nanoparticles. Thereafter, 20.0 g of ethyl acetate and 5.0 g of PDMS were mixed for 10 min by use of magnetic stirrer to obtain PDMS solution. Subsequently, the PA-treated ZnO NPs solution was poured into the PDMS solution and the entire mixture was agitated for 20 min at 30 °C in order to increase the kinetics of PDMS molecules and enhance chemical reaction of the constituent additives. Furthermore, 0.50 g HDTMOS and 0.50 g dimethyl, methylhydrogen siloxane copolymer were added into the coating mixture in quick succession. The cure reaction is shown in Scheme 1. To further reduce the surface energy of the coating mixture, 0.05 g PFDTS was incorporated into the coating formulation and agitated for additional 5 min. By use of similar preparation procedure, similar experiments were repeated with varying amount of PFDTS (0.10, 0.20 and 0.40 g), respectively. For comparative study, PDMS/PA-treated ZnO composite coating was prepared without PFDTS. Consequently, four PFDTS-modified PDMS/PA-treated ZnO composite coating formulations and one coating formulation without PFDTS (NPFDTS) were obtained and coated on glass and steel substrates by spin-coating method operated at 1000 rpm for 40 s. The coated surfaces were oven-dried for 7 d at 60 °C. Thereafter, the mean thicknesses of the dry coating film were measured using a thickness gauge (PosiTector 6000 of Defelsko, USA). The procedure for the coating preparation is illustrated as shown in Scheme 2.

The SRB used for this experiment were obtained from Bohai sea of China. As described elsewhere [31], the SRB were anaerobically incubated in the American Petroleum Institute Recommended Practice-38 medium which consists of MgSO₄.7H₂O (0.2 g/L), KH₂PO₄ (0.5 g/L), NaCl (10.0 g/L), ascorbic acid (1.0 g/L), sodium lactate (4.0 g/L), yeast extract (1.0 g/L), and Fe(NH₄)₂(SO₄)₂ (0.02 g/L) for enrichment. The culture solution has a pH value between 7.0 and 7.2 and was controlled by 1 mol/L NaOH. Before the experimental study, SRB (Desulfovibrio desulfuricans) strains were pre-cultured in an incubator for 12 h, and then 1000 mL of 3.5 wt% NaCl solution was inoculated with 20 mL of SRB enrichment. After 6 h, 100 mL of the SRB/NaCl solution was poured into testing devices wrapped with polymeric film before corrosion experiments.

2.4.1. Electrochemical impedance spectroscopy (EIS) analysis

The EIS study was used to evaluate the anticorrosion performance of the coatings during immersion in the SRB/NaCl solution. In the experiments, the coated substrates (working electrode) were clamped between rectangular polymeric block and glass tubes with an O-ring seals. The tube was filled with SRB/NaCl solution and sealed with a polymeric stopper that has an opening through which a reference saturated calomel electrode (SCE) and a platinum counter electrode were mounted. The entire setup was wrapped with transparent polymeric film to shield it from inlet of oxygen. EIS measurements were periodically performed at room temperature (23 ± 2 °C) with an Autolab electrochemical workstation, in the frequency range of 100 kHz to 10 mHz at open circuit potential, with a 20 mV AC perturbation for 1, 5, 10 and 15 d exposure times. To ensure reproducibility, each coating sample was evaluated three times, and the physical quantity of the EIS data were extracted using Zsimpwin software. The EIS results were presented in the form of Bode impedance modulus and phase angle.

2.4.2. Surface morphology and wettability measurements

In order to evaluate the effect of SRB on the surface properties of the nanocomposite coatings after 15 d of exposure to SRB/NaCl medium, the SEM was used to visualize the coatings after surface preparation. The coated substrates were fixed with 2.50% (vol.%) glutaraldehyde in a phosphate buffer solution (PBS, pH 7.4) for 30 min, then rinsed twice with the PBS and distilled water (5 min each), and dehydrated by use of ethanol gradient (at 50%, 70% and 100%), after which they were finally stored in a desiccator prior to use. A SEM was used to visualize the morphology of the coatings’ surfaces before and after immersion. For water contact angle measurements, a contact angle goniometer was used. 6 μL of water droplets were placed onto the coated substrates and were immediately captured by the CCD camera attached to the goniometer. Subsequently, static water contact angle values of the coatings before and after immersion in SRB/NaCl solution were determined. The measurements were taken on five different areas of each sample and the average values were reported.

2.4.3. FTIR spectroscopy

The FTIR spectra in the range of 400-4000 cm^-1 wavenumber was acquired by use of magan-IR560, Thermo Fitscher infrared spectrophotometer. Firstly, FTIR spectra of the phosphoric acid-treated ZnO NPs and the unmodified ZnO NPs were acquired. Secondly, FTIR spectra of the coatings whose surfaces were prepared by agitation and washing were acquired to evaluate the interaction of SRB with the coatings. The spectra of the coatings before immersion were also acquired for comparative analysis. The procedure involved the use of 4 mg of coating film from each of the samples and was mixed with 100 mg of pure anhydrous KBr. The mixture was ground to fine powdered form using a mortar and a pestle. The ground mixture was then placed onto a circular disk and pressed into a transparent flake. Subsequently, it was placed into the detection chamber IR spectrophotometer with a sample holder, and the IR spectra were recorded with the Omnic software at a resolution of 4 cm^-1, with 64 scans and a gain of 1.

Fig. 1 shows the FTIR spectra of unmodified ZnO and phosphoric acid-modified ZnO NPs samples. Both spectra exhibited similar trend of absorption peaks.

However, there are little observable differences especially in the intensity of the peaks and absorption bands at lower wavenumber, which indicate that treatment of ZnO with phosphoric acid actually modified the chemical properties of ZnO NPs. Further, the O__H stretching broad band at 3443 cm^-1 and sharp peaks at 1634 cm^-1 and 1109 cm^-1 can be observed. The higher intensity at 3443 cm^-1 implies the existence of larger amount of crystal water in the unmodified sample. Critical observation of the peaks in the modified ZnO NPs shows the intensities are reduced, which implies there was reduction in water after modification. The several sharp absorption bands from 1378-1561 cm^-1 correspond to the C₌₌O bonds. This implies that a certain level of carbon-related impurities exist in both the unmodified ZnO and zinc phosphate products obtained from acid-modification of ZnO. The absorption peak at 1042 cm^-1 is attributed to complex stretching and bending vibration of the PO43-group. These results have confirmed that zinc phosphate crystals were obtained after phosphoric acid modification of ZnO, and are consistent with some reports in the literature [26,[32], [33], [34]].

Prior to corrosion test, the thickness of the coatings was measured and 32 ± 4 μm dry film thicknesses were obtained. For the corrosion test results, Fig. 2 shows the trend of Bode impedance modulus and phase angle of the electrochemical processes that took place when the coating without PFDTS (NPFDTS) and PFDTS-based coatings were exposed to SRB/NaCl solution. The Bode impedance modulus value of the NPFDTS coating was found to be 3.474 × 10¹⁰ Ω cm² after 1 d exposure to SRB/NaCl solution and decreased to 4.182 × 10⁷ Ω cm² after 15 d. The impedance modulus (|Z|_{0.01 Hz}) of 0.05 g PFDTS-modified coating decreased from 1.660 × 10⁹ to 1.104 × 10⁷ Ω cm² during 1-15 d exposure to SRB/NaCl solution. Furthermore, the (|Z|_0.01 Hz) value of the 0.10 g PFDTS-based coating gradually decreased from 1.107 × 10¹⁰ after 1 d of exposure to 6.121 × 10⁶ Ω cm² after 15 d. Remarkably, the 0.20 g PFDTS-based coating showed slight decrease of 3.158 × 10¹⁰ Ω cm² impedance modulus after 1 d exposure to 7.997 × 10⁹ Ω cm² after 5 d exposure, and then increased to 6.301 × 10¹⁰ Ω cm² after 15 d. Besides, the impedance modulus of 4.600 × 10¹⁰ Ω cm² was observed on 0.40 g PFDTS-modified coating after 1 d exposure, and thereafter decreased to 3.918 × 10⁷ Ω cm² after 15 d. The decrease in values of the impedance modulus indicates that some corrosive species such as water, oxygen, ions and bacterial have penetrated the coatings, resulting to decrease in their protective functions, and could subsequently facilitate corrosion of the underlying metal. Again, the trend of values of impedance modulus associated with 0.20 g PFDTS-modified coating indicate the coating maintained high protective function against degradation of SRB and NaCl electrolyte.

Furthermore, the values of the impedance modulus at frequency of 10^-2 Hz (i.e. |Z|_{0.01 Hz}) are given in Table 1. The high impedance modulus |Z|_{0.01 Hz}, observed on NPFDTS coating could be attributed to the effect of phosphoric acid modification of ZnO nanoparticles and the hydrophobic property of the coating. For the PFDTS-based coatings in particular, the impedance modulus |Z|_{0.01 Hz}, increased with increasing amount of PFDTS up to 0.20 g, and later declined with further increase to 0.40 g of PFDTS. This indicates that the 0.20 g PFDTS-based coating displayed highest |Z|_{0.01 Hz} value. The observed result could be attributed to the synergy between phosphoric acid-modified ZnO nanoparticles and low surface energy property of PFDTS whose effect was optimum at 0.20 g. The highest anticorrosion performance experienced with 0.20 g PFDTS deviated from results of our previous studies [19,20] where the best anticorrosion performance was obtained with 0.10 g PFDTS. The observed variance could be attributed to the effect of phosphoric acid modification of ZnO on the performance of PFDTS.

Further observation of Table 1 shows the values of impedance modulus of the coatings decreased significantly with exposure time, but relatively slowly for the 0.20 g PFDTS-based coating. The observed results suggest that the contact of SRB with the coatings significantly influenced the coating’s properties, but showed relatively lower effect on the 0.20 g PFDTS-based coating. The observed decrease in |Z|_{0.01 Hz} values with time occurred due to existence of micro-pores in the coatings which might have provided some points of anchorage for SRB cells to cause some local changes in the physical properties as evidenced in the SEM images. Thus, the protective properties of the coatings were considerably affected. Furthermore, the Bode phase angle plots also depicted in Fig. 2 shows all the coatings except 0.20 g PFDTS-based coating exhibited two time constants after the end of 15 d immersion. The first time constant at high frequency is attributed to the combined effect of the biofilm and the coating’s polymeric film. The second time constant at lowest frequency is suggested to be due to the micro-porosity of the degrading coating film which allowed penetration of electrolyte to the metal surface under the coating. As shown in Fig. 2, after 1 d immersion, the Bode phase plot of the coatings (NPFDTS, 0.10 g PFDTS, 0.20 g PFDTS and 0.40 g PFDTS) showed one-time constant, which is attributed to the barrier characteristic of the coating. The 0.05 g PFDTS coating sample displayed two-time constant after 1 d immersion. With increase in immersion time to 5 d, the phase diagrams of coating samples, NPFDTS, 0.05 g PFDTS and 0.10 g PFDTS showed appearance of two-time constant. The formation of two-time constants on these three coating samples became very clear after 10 d of immersion, which could be attributed to the effect of penetration of the electrolyte into the coating. However, the 0.20 g PFDTS and 0.40 g PFDTS coatings still maintained one-time constant after 5 and 10 d immersion. After 15 d of immersion, the NPFDTS, 0.05 g PFDTS, 0.10 g PFDTS and 0.40 g PFDTS coatings still showed appearance of two-time constants. Interestingly, throughout the immersion times, the phase diagrams of 0.20 g PFDTS coating showed characteristics of one-time constant. This phenomenon may be attributed to the remarkable barrier property of the coating.

The EIS data of the coatings can be fitted well by use of equivalent electric circuits (EEC) given in Fig. 3. The values of physical parameters extracted from the EEC are presented in Table 1. After 1 d immersion, the EIS data of NPFDTS coating were fitted well using the model in Fig. 3(i). As a result of two-time constant in NPFDTS coating after 5 d immersion, the EIS spectra could be explained in terms of the model in Fig. 3(iii), in which C_diff is the diffusion CPE (constant phase element) and R_diff is the diffusion resistance. The EIS data after 10 and 15 d immersions were appropriately modeled using Fig. 3(ii), taking into account the appearance of Warburg (W) diffusion effect. The 0.05 g PFDTS coating showed two time constant throughout the immersion periods and the EIS data after 1, 5 and 10 d immersion were fitted well with Fig. 3(ii) while Fig. 3(iii) was used to model the EIS data after 15 d. After 1 and 5 d immersion, the EIS spectra for 0.10 g PFDTS coating were modeled with Fig. 3(i) and those of 10 and 15 d immersion were fitted with Fig. 3(ii). Fig. 3(i) model was used to model the EIS spectra of 0.20 g PFDTS coating acquired throughout the 15 d immersion. The same model was used to fit the EIS data obtained from 1, 5 and 10 d immersion of 0.40 g PFDTS coating while the data acquired after 15 days were appropriately modeled with Fig. 3(ii). The equivalent circuit in Fig. 3(i) consists of the solution resistance R_s, between the reference electrode and the working electrode, the coating capacitance C_c, the coating’s pore resistance R_c, the double-layer capacitance C_dl, and the charge transfer resistance R_ct. Taking the Warburg diffusion process into account, Fig. 3(i) is modified with Warburg resistance W to give Fig. 3(ii). When the diffusion process is beyond Warburg effect, a new circuit consisting of C_diff and R_diff is added in parallel to Fig. 3(i) to yield the model shown in Fig. 3(iii).

With the increase in the exposure time, the resistance (R_c) decreased, suggesting that electrolyte has found its way to the metal surface through micro-pores on the coating surface, resulting to corrosion. The diffusion of electrolyte containing SRB metabolites would accelerate corrosion due to metabolic activities with the bare metal [35,36]. Subsequently, the R_ct values of PFDTS-based coatings exposed to SRB decreased with exposure time, though without defined trend, due to instability of the corrosion products arising from the porosity of the coatings. It is also observed that the R_ct was highest with 0.20 g PFDTS-modified coating even after 15 d immersion. By this result, it could be reasoned that some of the corrosion products formed migrated to the coating surface through the micro-pores and then formed somewhat impervious film which increased the charge transfer resistance.

Before immersion in the SRB/NaCl medium, morphologies of the PDMS-ZnO coatings, with and without PFDTS were observed by SEM, as shown in Fig. 4A. The surface structures in the coating without PFDTS were influenced by the overlay of thin polymeric film (Fig. 4A(i)), leaving many micro-pores with inadequate and unstable air pockets. As for the PFDTS-modified coatings, there was evident formation of surface crystals and hierarchical structures arising from the interaction between the PFDTS and constituents of the PDMS-modified ZnO coating which increased with amount of PFDTS (Fig. 4A(ii) - (v)).

After 15 d of immersion in SRB medium, the coatings were retrieved and slightly rinsed in PBS solution. Subsequently, the scanning electron microscopy (SEM) was conducted and the images are shown in Fig. 4B. The results revealed presence of many micro-pores and high density biofilm on the coating without PFDTS (Fig. 4B(i)). Furthermore, the PFDTS-based coatings showed presence of high density biofilm on 0.05 g PFDTS coating, which decreased as the amount of PFDTS increased (Fig. 4B(ii) - (v)). Interestingly, the density of biofilm observed on 0.20 g PFDTS-based coating (Fig. 4B(iv)) was lowest, and could be attributed to effect of kinetic barrier owing to presence of low surface energy PFDTS. Further critical observation of SEM images in Fig. 4B suggests the biofilms interacted physiologically to variations in the surface properties of the coatings and thus influenced the nature and quantity of the adsorbed biofilms. The observation is consistent with the report of Little and Wagner [37]. When compared with the PFDTS-modified coatings, the higher density of biofilm observed on the coating without PFDTS clearly underscores the influence of low-surface energy property of PFDTS in reducing hydrophobic and electrostatic interactions.

The data obtained from wettability measurements are given in Fig. 5. The data show that the coatings are hydrophobic. The hydrophobicity was lowest in the unmodified coating, but gradually increased with addition of PFDTS.

Furthermore, Fig. 6 illustrates the influence of PFDTS on the static water contact angle of the coatings. The values of water contact angle increased with increase in the amount of PFDTS, before and after immersion in SRB/NaCl solution. Besides, observation of Fig. 6 clearly shows that values of water contact angle of the coatings after immersion in SRB/NaCl solution are lower than those of the coatings before immersion. This clearly indicates there was decrease in surface chemical composition and structures of the nanocomposite coatings after immersion in SRB/NaCl solution. Based on this result, it is evident that the contact of SRB biofilms with the coatings influenced their surface properties and thus lowered the hydrophobicity.

The potential of the PFDTS-based PDMS-modified ZnO hydrophobic coatings to resist substantial adsorption of SRB biofilm on the coatings’ surfaces during immersion in SRB/NaCl solution over a 15 d period was evaluated. Observation of the coatings’ surfaces in Fig. 6 shows there was no chemical adsorption of SRB cells and biofilms on the coatings’ surfaces after slight agitation. This implies that the presence of biofilms on the coatings (Fig. 4B) was due to physical adsorption. However, the morphology of the coatings suggests there was significant modification on the NPFDTS (Fig. 7A(i)) as well as 0.05 g and 0.40 g PFDTS-based coatings (Fig. 7A(ii) and A(v)), respectively. The suggestion is justified considering the numerous formations of micro-pores on the coatings’ surfaces. The 0.10 g and 0.20 g PFDTS-based coatings (Fig. 7A(iii) and A(iv)) also show presence of micro-pores, but fewer than those on the former coatings. Generally, the observed results also point to the effect of low-surface energy property of PFDTS in the coatings which is suggested to be the cause for decrease in hydrophobic and electrostatic interaction, resulting to physical adsorption (or weak adsorption strength) of the biofilms.

Additionally, to confirm that the biofilms were not chemically adsorbed on the coatings’ surfaces but physically, the FTIR spectroscopy was conducted for identification of bacteria adsorption [38]. The analysis was conducted with samples of coatings retrieved after 15 d immersion in SRB/NaCl solution, agitated in PBS solution, rinsed and dried. Fig. 8(a-e) shows the FTIR spectra of the coatings, before and after immersion. The absorbance peaks were compared to those of Naumann’s IR absorbance regions. The Naumann’s IR regions are the absorbance peaks in the range of 3000-2800 cm^-1 which are related to fatty acid region (region I); the peaks observed at 1700-1500 cm^-1 are considered to be associated to amide I and II of proteins and peptides (region II); those at 1500-1200 cm^-1 are attributed to a mixed region of fatty acid bending vibrations, proteins, and phosphate-carrying compounds (region III); 1200-900 cm^-1 indicates absorption bands of the carbohydrates in microbial cell walls (region IV); while the peaks at 900-700 cm^-1 is the ‘fingerprint region’ that correspond to specific bacteria (region V).

Based on the above reference, FTIR spectra of the coating without PFDTS (Fig. 8(a)) shows there are no new peaks indicating adsorption of bacteria on the coating after immersion. However, there are observable differences in the intensities despite similar peak positions. This suggests that the chemical surface was actually modified after the contact of SRB biofilm with the coating surface. The observation is consistent with the SEM examination of the coating surface shown in Fig. 7(a).

For the PFDTS-based coatings in Fig. 8(b)-(e), there are no obvious changes such as appearance of new peaks or shifts in the position of peaks after immersion in SRB medium. This suggests there was no adhesion of SRB biofilm on the coatings’ surfaces after agitation. Nonetheless, the coatings’ surfaces were slightly modified after immersion as indicated by the differences in the intensities of the spectral peaks. This could be due to physical interaction between the surface structures of the coatings and the SRB during their short residence time on the coatings’ surfaces before agitation. Furthermore, it is important to note that the coating in Fig. 8(d) displayed little difference in the intensity of the spectral pattern, suggesting the coating’s surface features were only slightly affected.

The effect of FDTS on the resistance of SRB-induced corrosion of hydrophobic PDMS/phosphoric acid modified ZnO coatings has been investigated. The SEM analyses showed that the SRB biofilm adsorbed physically on the coatings’ surfaces. The FTIR analysis confirmed that the interaction of the coatings with SRB biofilms was only physical. The corrosion resistance measurements by EIS technique showed the presence of PFDTS in PDMS/phosphoric acid modified ZnO coatings greatly inhibited the SRB induced corrosion. This effect was remarkable on the 0.20 g PFDTS-based coating, which remained virtually intact after 15 d exposure to SRB/NaCL medium, and also possess the potential to remain durable beyond the period of experimental study. The overall results indicate that the low-surface energy property of PFDTS (with optimum amount of 0.20 g) contributed significantly in limiting the interaction of SRB with PDMS-ZnO coatings. Thus, this information is very important as it would be useful in the design of effective anti-SRB coatings for marine corrosion application.

Authors declare there are no competing interests regarding the publication of this article.